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DESIGN AND FABRICATION OF BED FOR INCAPACITATED PATIENTS

Thesis · December 2012

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DESIGN AND FABRICATION OF BED FOR
INCAPACITATED PATIENTS

Dissertation submitted to
Visvesvaraya National Institute of Technology, Nagpur
In partial fulfillment of requirement for the award of
degree of

M. Tech. (Research)
by

Anil M. Onkar
(Enrolment No. RT09MEC012)

Guide

Dr A.B. ANDHARE

Department of Mechanical Engineering

Visvesvaraya National Institute of Technology,
Nagpur 440010 (India)

December 2012
© Visvesvaraya National Institute of Technology (VNIT), 2012
 INTELLECTUAL PROPERTY RIGHT NOTICE
Part of this thesis may be protected by one or more of Indian Patent (pending)
by Mr. Anil Onkar, Dr. Atul Andhare and Dean, Research and Development,
Visvesvaraya National Institute of Technology (VNIT), 2012. VNIT restricts the use,
in any form, of the information, in part or full, contained in this thesis ONLY on
written permission of the Competent Authority : Dean (R&C), VNIT, Nagpur, Mr.
Anil Onkar, 175, Bajaj Nagar, Nagpur, Dr. Atul Andhare, VNIT, Nagpur.
 Declaration
I, hereby declare that the project titled “Design And Fabrication of Bed for
Incapacitated Patients” submitted herein for the award of Degree of Master of
Technology (Mechanical Engineering) has been carried out by me in the Department
of Mechanical Engineering of Visvesvaraya National Institute of Technology,
Nagpur. The work is original and has not been submitted earlier as a whole or in part
for the award of any degree / diploma at this or any other Institution / University.

Anil M. Onkar
Date:

Certificate
The research titled “Design And Fabrication of Bed for Incapacitated
Patients” submitted by Anil M. Onkar for the award of Degree of Master of
Technology (Mechanical Engineering) has been carried out under my supervision at
the Department of Mechanical Engineering of Visvesvaraya National Institute of
Technology, Nagpur. The work is comprehensive, complete and fit for evaluation.

Dr. Atul B. Andhare

Associate Professor,
Department of Mechanical Engineering,
VNIT, Nagpur

Forwarded by -
Head,
Department of Mechanical Engineering,
VNIT, Nagpur
Date:
 Certificate of Course Work Completed
This is to certify that Mr. Anil M. Onkar (Enrolment No. RT09MEC012) has
completed necessary course work as under for the award of Degree of Master of
Technology (Mechanical Engineering) at the Department of Mechanical Engineering
of Visvesvaraya National Institute of Technology, Nagpur.

Course Title Grade Earned Cumulative

Code Obtained Grade Grade point
Points Average
HML 601 Technical Communication AA 10
MEL 523 Computer Integrated Manufacturing AA 10
MEL 431 Advanced mechanisms BB 8
9.00
MEL 432 Computer Graphics & Solid Modeling AB 9
MEL 436 Mechatronics AB 9
MEL 439 Product Design & development BB 8
Total 54

Dr. Atul B. Andhare

Associate Professor,
Department of Mechanical Engineering,
VNIT, Nagpur

Forwarded by -
Head,
Department of Mechanical Engineering,
VNIT, Nagpur
Date:
 Acknowledgements
I gratefully acknowledge the invaluable guidance and support of my
guide, Dr. A. B. Andhare throughout the project. He encouraged for the timely
completion of every stage of project. He visited the factory during
manufacture of prototype and supported for improvements. He took great
pains for presentation of papers on the subject. He also guided a lot for
preparation of thesis through frequent and long discussions. His sincerity and
dedication has enabled me to complete the post graduation after a gap of 28
years. I am proud to have such resourceful guide. I am deeply thankful for his
enthusiastic approach, without which the project would not have been
conceivable.

I would like to express deep gratitude towards Dr. P.M. Padole for
encouraging me to register for post graduation even after a long gap. He took
special interest in resolving difficulties in my study as well as project. He also
provided valuable guidance for development of the product. I am
overwhelmed by his positive approach and interest in developments in
engineering. Once again I would like to thank him for his unconditional and
voluntary support.

I also wish to thank Dr. Animesh Chatarjee & Dr. I. K. Chopde, Head
of the department, Mechanical Engineering, and all faculties of the department
for their able guidance.

I am also thankful to all staff of M/s Onkar Furnitech, Nagpur for

extending help in fabrication of model within stipulated time.

Anil M. Onkar

Date:
 List of Publications
1) “Design of Bed for Bedridden Patients: Determination of Design Criteria” in
4th International Conference on Advances in Mechanical Engineering, SVNIT,
SURAT, September 23-25, 2010, pp. 1-5.
2) “Design of Bed for Bedridden Patients: Analysis and Synthesis of
Mechanisms” 15th National Conference on Machines and Mechanisms
(NaCoMM 2011), IIT, Madras, Nov 30 & Dec 1-2, 2011, pp. 309-317.
 CONTENTS
Chapter No Title Page No
Abstract I

List of Tables II

List of Figures III

List of Images VI

List of Videos VII

Nomenclature VIII

1 INTRODUCTION 1
1.1 Background 1
1.1.1 Methodology 2

1.2 SURVEY OF STAKE HOLDERS 3

1.2.1 Scope of survey 3
1.2.2 Methodology of survey 3
1.2.3 Questionnaire and results of survey 4
1.2.4 Conclusions from survey 8
Facilities required / expected by Patients &
1.2.4.1 8
relatives
1.2.4.2 Requirement of product 8
1.2.5 Objective of the work 9

1.3 LITERATURE REVIEW 10

1.3.1 Review of publications 10
1.3.2 Review of patents 16
1.3.3 Review of available products 27

1.4 FUNCTIONAL REQUIREMENTS OF THE PRODUCT 29

2 KINEMATIC DESIGN OF MECHANISMS 33

 2.1 Type synthesis for actuators 33
2.2 Synthesis of mechanism for raising and lowering commode 34
2.3 Synthesis of mechanism for platform 36
2.4 Dimensional synthesis 42
2.4.1 Dimensional synthesis for platform 42
2.4.2 Dimensional synthesis for commode 44
2.4.3 Dimensional synthesis for actuator 45
2.4.4 Dimensional synthesis for connecting mechanism 46

3 MECHANICAL DESIGN OF THE SYSTEM 49

3.1 Mechanical design for commode 52
3.1.1 Design of links and pins 53
3.2 Mechanical design for puller 59
3.2.1 Design of bar, stopper and pipes 59
3.3 Mechanical design for platform 62
3.3.1 Design of pins, roller, link and members 63
3.4 Mechanical design for telescopic screw 72
3.4.1 Design of screws, nuts and accessories 73
3.5 Mechanical design for backrest 84
3.5.1 Design of members and links 85
3.6 Mechanical design for legrest 89
3.7 Mechanical design for board cum table 90
3.7.1 Design of members and pins 90
3.8 Design of basin stand 93
3.8.1 Design of members 93

4 CAD MODELLING, FABRICATION AND FIELD TRIALS 96

4.1 CAD modeling 96
4.2 Fabrication of prototype 98
4.3 Validation of prototype 102
4.3.1 Testing of prototype 102
4.3.2 Feedback of user 103
 4.4 Results 104
4.5 Conclusion 108

5 SCOPE OF FURTHER WORK 111

6 REFERENCES 112

7 ANNEXURES 115
7.1 Annexure -A – Feedback 115
 ABSTRACT
It was observed that managing bedridden patients is an important issue as
many people are involved in it and patients need round the clock assistance. Any
reduction in the number of people involved and the amount of effort required is going
to benefit a large section of the society. A survey was conducted to know the
problems of such patients and their relatives and to identify the need for any product.
The survey confirmed need of a bed with special arrangements for such patients.
Therefore, an attempt is made by the author, to reduce the amount of assistance
required in managing these patients by designing a new bed.

A bedridden patient is unable to move or sit, has to take bedpan for normal
physiological functions such as defecation & urination. Using bedpan is cumbersome
& uncomfortable. Therefore, a new bed was designed and fabricated. This bed is
integral with commode and patients can carry out the physiological functions by using
this commode attached to bed. The bed with commode is convertible from bed to
chair and facilitates the sitting of patient without shifting the patient.

The arrangement in bed consists of a suitable hole in the bed top & mattress
which is normally covered by platform with mattress during idle position. This
platform slides out by manually operated screw to clear the hole. Simultaneously a
commode pot rises through the hole & is ready to use. A commode pot with flushing
arrangement, cleaning jet, odor trap and drainage is used. All controls are within
patient’s reach. After use, arrangement is reversed and bed is brought back to its
normal position. The other facilities like a storage rack, an adjustable wash basin, and
a height adjustable and foldable utility table are provided to make other daily
activities easy.

The bed was tested and results are satisfactory.

I
 LIST OF TABLES
TABLE PAGE
TITLE
NO. NO.
1.1 Description of patient’s problem 5
1.2 Difficulties faced by the support staff / assistants 5
1.3 Difficulties faced by the patients 6
1.4 Type of furniture used 6
1.5 Present method of management 7
1.6 Major needs highlighted by the survey 7
1.7 Expectations from the new design 7
2.1 Results of three position synthesis for four bar mechanism 41
3.1 Names of various parts of commode and platform
51
mechanism
4.1 Results of testing of model 102

II
 LIST OF FIGURES
FIGURE PAGE
TITLE
NO. NO.
1.1 Break up of surveyed population 4
1.2 Types of patients covered in the survey 4
1.3 Structure of bed surface 10
1.4 Main Bed, nursing bed, main bed with positions 11
1.5 Chair cum bed 11
1.6 Hospital bed with auxiliary functions 12
1.7 Marionette bed 13
1.8 Method of manoeuvring of bed sheet for repositioning 13
1.9 Repositioning of a rigid body with a flexible sheet and its 14
application to an automated rehabilitation bed
1.10 Bed to prevent bed sores 14
1.11 Patient mover system for transfer of patients 15
1.12 Surface wave distributed actuation method 15
1.13 Bedside toilet for incapacitated patients 16
1.14 Hospital bed 17
1.15 Convertible bed 17
1.16 A bed arrangement 18
1.17 Bed with built in commode 19
1.18 Invalid’s bed adjustable to sitting up position and having 20
toilet facilities
1.19 Bed Intended for use by sick and invalid persons 21
1.20 Bed Intended for use by sick and invalid persons 22
1.21 Hospital bed with toilet facility 22
1.22 Excretion apparatus combined into a bed for bedridden 23
person
1.23 Convertible bed / chair with waste disposal 23
1.24 Nursed person’s bed with flush toilet 24
1.25 A bed cum chair with commode for patient 25
1.26 Arrangement of bed showing various features and 31

III
 dimensions in cm
1.27 Arrangement of bed showing commode up and platform 32
down
2.1 Two toggle jack arrangement to move commode up and 34
down
2.2 Three lead screw arrangement to move commode up and 35
down
2.3 Parallel link arrangement to move commode up and down 35
2.4 Hinged platform arrangement to open and close hole in bed. 36
2.5 Platform with parallelogram links to open and close hole in 36
bed.
2.6 Platform with wheels and sloping guides to open and close 37
hole in bed
2.7 Platform on four bar mechanism (Mo, No, M1, N1) to open 38
and close the opening in bed for position 1.
2.8 Modified platform mechanism 42
2.9 Two positions of linkages for platform mechanism 43
2.10 Linkages for commode mechanism 44
2.11 Location of Actuator mechanism 45
2.12 Distances of mechanisms for commode 47
3.1 Arrangement of various parts for commode up position 49
3.2 Arrangement of various parts for platform up position 50
3.3 Arrangement of various parts of commode mechanism 52
3.4 Arrangement of telescopic puller 59
3.5 Arrangement of platform mechanism 62
3.6 Forces on platform mechanism 63
3.7 Forces on supports to pin 17 and pipe 22 68
3.8 Cross section of channel 18 71
3.9 Arrangement of telescopic screw 72
3.10 Cross section of pipe at pin 79
3.11 Arrangement of various parts backrest mechanism 84
3.12 Cross section of rectangular pipe 86

IV
3.13 Arrangement of foot board cum utility table 90
3.14 Arrangement of basin stand 93
4.1 CAD model of bed showing sleeping position- backrest 97
down, legrest up.
4.2 CAD model of bed showing commode position- backrest 97
up, legrest down, commode up,
4.3 CAD model of bed showing chair position- backrest up, 97
legrest down, platform up, utility table up.

V
 LIST OF IMAGES
IMAGE TITLE PAGE
NO. NO.
1.1 Dignity Bed 27
1.2 YG bed 28
2.1 Image of calculation program in worksheet for position 1 39
2.2 Image of calculation program in worksheet for position 11 40
4.1 Product at sleeping position- backrest down, legrest up, 99
platform up.
4.2 Product at chair position- backrest up, legrest down, 100
platform up.
4.3 Product at commode position- backrest up, legrest down, 100
commode up.
4.4 Product at with chair position- utility table up. 100
4.5 Product at chair position with basin. 101
4.6 Telescopic screw arrangement. 101

VI
 LIST OF VIDEOS (INCLUDED IN CD)
VIDEO TITLE
NO.
1 Video of product showing raising backrest
2 Video of product showing lowering backrest
3 Video of product showing lowering legrest
4 Video of product showing raising legrest
5 Video of product showing lowering platform and raising
commode
6 Video of product showing lowering commode and raising
platform
7 Video generated in CAD modeling showing lowering
platform and raising commode
8 Video generated in CAD modeling showing lowering
commode and raising platform

VII
NOMENCLATURES
SR. NO NOMENCLATURE SYMBOL
1 End condition coefficient C
2 Modulus of elasticity E
3 Moment of inertia I
4 Radius of gyration K
5 Bending moment M
6 Bearing pressure p
7 Yield Strength in compression Syc
8 Yield Strength in shear Sys
9 Yield strength in tension Syt
10 Torque T
11 Modulus of section Z
12 Slenderness ratio λ
13 Bending stress σb
14 Torsional stress τ

VIII
CHAPTER – 1
 INTRODUCTION
1.1 BACKGROUND
The population of India was estimated to be 1.21 billion by April 2012. About
64 millions of people of this population are above the age of 65 and a significant
number of them are bedridden due to age related problems. Apart from this, many
people are forced to be bedridden because of various reasons like – paralysis,
orthopedic problems, vehicle & other accidents, etc. These bedridden people need
almost round the clock assistance and care from their relatives, care takers, doctors
etc. This problem is compounded as the system of joint family is almost nonexistent
now and round the clock assistance is not available in the family, to take care of
bedridden patients. Any reduction in the number of people involved and the amount
of effort required in managing such patients is going to benefit a large section of the
society. In addition to the above, it is the experience of the author as an entrepreneur
engaged in manufacture and marketing hospital furniture / equipment that there is
good demand for such health care products. This was experienced through interaction
with various doctors, patients and their relatives, etc. Therefore, it is necessary to
develop some kind of healthcare system by which such patients can be managed
easily. Therefore, an attempt is made by the author, to reduce the amount of assistance
required in managing these patients by designing a new bed for such patients.

The author is in the business of manufacture & marketing of hospital furniture

since last 24 years. During this period he had interaction with doctors about their
requirement of hospital furniture and he always tried to meet their requirement by
modifying the product. It was highlighted by Dr R.M. Chandak (M.S. Ortho), Nagpur
that numbers of patients have problems in managing themselves after any accident or
surgery. Moreover the staff and relatives also face problems while nursing such
patients.

A few years back one of the family members of the author met with an
accident and suffered backbone injury. At that time the author had firsthand
experience of the problems faced by such patients. At that time a normal bed with
backrest in which a porcelain commode was fixed below a hole in the bed was used.
 2

However, it was a temporary and crude arrangement but was used as there was no
other option available. A market search was also carried out at that time and it was
found that no such bed was available in India and models available in countries like
U.S. were very costly.

When the author set up a showroom in Nagpur, the first showroom of hospital
furniture & equipment in central India, number of domestic users approached to
purchase a bed for patient to be maintained at home. After having interaction with
them it was realized that there is a need of such furniture / equipment badly.
Therefore, it was decided to take up a task of design and fabrication of a bed for
bedridden patients. It was thought of making a bed which can be useful for bedridden
patients. To ascertain the need and requirements of such a bed a survey was carried
out. To study the earlier work in this area extensive literature (published papers) and
patent reviews were performed. In addition to this the market was also surveyed for
the types of bed available, if any, for such patients.

Following paragraphs presents the details and outcome of these surveys of

stake holders, papers, patents and products.

1.1.1 METHODOLOGY
Once, it was decided to design a bed for such patients, the following
methodology was decided.
1) To conduct a survey of patients to crystallize the problem.
2) To conduct literature survey and study the various solutions suggested for
managing the said problem. To study their merits and demerits.
3) To decide the requirement of the product along with size and arrangement of
components based on above literature survey and actual need of patients.
4) Identify possible mechanisms and selection of mechanism.
5) Analysis, synthesis and selection of mechanisms for the product.
6) Decide the design requirement.
7) Decide the sizes of important components based on design calculations.
8) To prepare CAD model and check the motions needed with arrangements.
9) Fabrication of model and testing for load, motions and positions desired.
 3

1.2 SURVEY OF STAKE HOLDERS

1.2.1 SCOPE OF SURVEY
In this context, a survey was carried out to find the requirements of such a
system for bedridden healthcare for the patients, who need management at home /
hospital. The patients were of following type.
1) Bed-ridden patients having restricted movements due to old age, at home.
2) Patients having limbs in plaster at home / hospital due to accidents.
3) Patient with backbone (spine) diseases advised rest at home / hospital.
4) Patient not allowed sitting for medical reasons.
5) Patient advised strict bed rest for various reasons.
6) Diarrhoea patients.
7) Heart patients.
8) Patients with paralysis
These patients were unable to move limbs or advised not to move the limbs or
not allowed to sit. The management of the normal physiological functions like
defecation & urination were difficult for them.

1.2.2 METHODOLOGY OF SURVEY

Bedridden patients need to be managed in hospitals or at homes and this
involves number of people like family members of the patient, doctors, nursing staff
and other caretakers. In bedridden patients, the management of the normal
physiological functions like defecation & urination is a difficult and sensitive issue.
Therefore, it was decided to take opinion of all the concerned people to find out the
design requirements. A questionnaire was designed for this purpose and was
circulated among all the concerned people and feedback was obtained from them.
In all, 52 persons were interviewed and the questionnaire was filled. Details of
the persons contacted are given in Figure 1.1 and the types of patients covered,
including those patients who were covered through their doctors, caretakers, etc., are
given in Figure 1.2.
 4

Figure 1.1: Break up of surveyed population

Figure 1.2:
1. Types of patients covered in the survey

1.2.3 QUESTIONNAIRE AND RESULTS OF SURVEY

The questionnaire along with the answers received is as under. Each
respondent has conveyed multiple difficulties and suggestions for improvements.
Therefore, the number of total responses the difficulties/ suggestions are more than 52
(the total number of persons contacted) whereas, the percentage is based on total
number 52.

1) Description of patient’s problem - left leg disabled / right leg disabled

disable / both leg
disabled / left hand disable
disabled / right hand disabled / both
th hand disabled / cannot sit
/ all four limbs disable
disabled.
 5

The various disabilities as listed in table 1.1 were found which made them to
spend more time on bed and they were dependent on somebody for most of their
activities.
Table 1.1: Description of patient’s problem
Sr. No. Patient’s Problem No. of responses % of total
1 Cannot sit 34 65%
2 4 Limbs disabled 4 8%
3 Left leg disabled 5 9%
4 Right leg disabled 4 8%
5 Both hand disabled 1 2%
6 Both leg disabled 2 4%
7 Left hand disabled 1 2%
8 Right hand & Right leg disabled 1 2%

2) Describe the problems you face while handling, nursing, maintaining the patient –
Urination / Toilet / Cleaning / Sponging / Sitting / Bad Smell.
The various difficulties faced by assistants are as listed in table 1.2. The most
significant problem was related to urination & toilet, followed by its cleaning, sitting,
sponging and bad smell. Thus the physiological functions are most problematic for
the attendant.

Table 1.2: Difficulties faced by the support staff / assistants

Sr. No. Activity / Difficulty No. of responses % of total
1 Toilet (defecation) 39 75%
2 Urination 36 69%
3 Cleaning 27 52%
4 Sitting 15 29%
5 Sponging 12 23%
6 Bad smell 9 17%
 6

3) What are problems faced by patient – Awkward position due to bed pan / bed pan
causing minor injury/ Have to lift the hip for the bed pan/ Difficulty to wash after
pan use.
The various difficulties faced by patients are as listed in table 1.3. All
problems were prominent in most of the cases and are mostly due to physiological
functions.

Table 1.3: Difficulties faced by the patients (as reported by patients / assistants)
Sr. No. Difficulty faced by patient No. of responses % of total
1 Awkward position due to bed 31 59%
2 Difficulty in washing of pan 27 52%
3 Lifting the hip for using bed pan 19 37%
4 Minor injuries due to bed pan 15 29%

4) What type of Bed / chair / furniture is used? Write facilities available in it.
The type of facility available to patient are as listed in table 1.4. Most patients
were nursed on the domestic bed / could not tell anything specific about facility while
few had provision of manual / handle operated backrest. Few used commode chair.
However no provision of commode in bed was available.

Table 1.4: Type of bed / facility used

Sr. No. Type of furniture used No. of responses % of total
1 Domestic bed 15 29%
2 Chair with commode 2 4%
3 Bed with Backrest and handle 11 21%
4 Chair 4 8%
5 Manual backrest 2 4%
6 Walker 1 2%
7 No Answer 13 25%
 7

5) How you manage to meet the problem now? – Use bed pan for urine & toilet /
Sitting by support of pillow.
The various methods of management are as listed in table 1.5. Most used bed
pans for physiological functions. Sitting position is mostly given by a pillow support.

Table 1.5: Present method of management

Sr. No Management of Problem No. of responses % of total
1 Use bed pan for Urine & Toilet 46 88%
2 Sitting by support of pillow 15 29%
3 Backrest assisted sitting 9 17%
4 Did not use pillow/Backrest 7 13%

6) Is there a need of any bed/furniture / equipment which shall solve or ease the
situation?
The Major needs highlighted are as listed in table 1.6. It was strongly felt that
if such bed is available, it will be preferred by the attendants and patients.

Table 1.6: Major needs highlighted by the survey

Sr. No. Type of need No. of responses % of total
1 Special bed 48 92%
2 Commode attached to bed 8 15%

7) What facility you expect from such furniture / equipment- No body movement for
pan / easy washing / no smell / user friendly / other________.
Expectations from the new design are listed in table 1.7. All the expectations
are related to physiological functions and ease for handling patient.

Table 1.7: Expectations from the new design

Sr. No. Expectation from new design No. of responses % of total
1 No body movement for pan 33 63%
2 Easy washing 34 65%
3 No smell 18 35%
4 User friendly 18 35%
 8

8) How much such facility should Cost?

It was found that the surveyed population expected a cost between Rs 5,000/-
to 25,000/- for a newly made bed.

1.2.4 CONCLUSIONS FROM SURVEY

From the survey conducted it was found that there was a need to design a new
bed for bedridden healthcare. This bed should be designed as a single unit with
facility of attaching commode, having various features.

1.2.4.1 Facilities Required / Expected By Patients & Relatives

• The design should reduce the amount of work / assistance required.
• Facility of commode attachable to bed.
• Sitting position by moving backrest up.
• Chair position for easy urination and defecation.
• Design of bed should be simple and easy to operate.
• Movement of patient should be minimum.
• Design should take care of the bad smell.
• It should provide additional facility like racks to store medicines, wash basin,
folding table etc.
• New bed should be affordable – cost limited to @ Rs. 25,000/-.

Hence the need of design and development of such facility was confirmed. It
was decided to do research on design and fabrication of bed for incapacitated patient
for overcoming their own difficulties & problems faced by the attendant while
managing the patient.

1.2.4.2 Requirements of Product

The requirements of the product were decided as under
1) A bed with an opening in its structure and mattress. The opening is to be covered
by a strong matching part with piece of mattress on it.
 9

2) The piece of mattress should move down in order to avoid friction to the body of
the patient. A suitable mechanism to move the piece of mattress in suitable
direction below the bed level to clear the opening.
3) A light weight commode to minimize power requirement, attached to bed which
shall come up through the opening in bed to support buttocks of the patient.
4) Sitting position by moving backrest up by mechanism.
5) Chair position achieved by moving leg part downwards.
6) Mechanisms to be operated by handle / electric motor / both.
7) Flushing cistern for flushing the toilet and cleaning water jet in commode to help
washing of anus / buttocks operated by a valve with suitable water connection.
8) Commode with anti odour water trap required to prevent foul smell. The anti
odour trap is to be adjustable in direction as per site need for connection to sewer
line by flexible pipes.
9) The controls should be within patients reach & positions should be adjustable
according to the patient.
10) A basin with water connection for use of patient.
11) A height adjustable multipurpose table in front of patient.
12) Minimum number of drives to be used in order to reduce cost and easy operation.
13) Storage rack for needs like medicine, water, towel, tissue paper, toothbrush,
toothpaste, glass, spoon, spittoons etc.
14) Cost of such equipment should be affordable in general.

1.2.5 Objective of Work

The objective was to design and develop a bed with above facilities at
minimum initial cost and low maintenance cost in order to benefit a large section of
society.
In view of above objective it was necessary to study various attempts made
earlier in this direction.
 10

1.3 LITERATURE REVIEW

In view of product requirements based on survey the various attempts to solve
the problem were studied.

1.3.1 REVIEW OF PUBLICATIONS

In the area of equipment or system design for bedridden healthcare, few
published works are available. As bed is the most important unit in this healthcare
system, major focus is on the design of beds or related systems so that the patients can
be managed without much assistance. Various researchers have tried different
approaches for designs of such healthcare systems. These approaches can be broadly
classified in following three categories.
The first approach is that of a robotic or automated bed / wheel chair which
can be operated by the patients themselves.
One such approach was presented by Lin Tan et. al [1] [Fig. 1.3]. They have
designed a robotic nursing bed which has the functions of raising back, curling legs
and side turning over from left to right separately, and voice recognition control,
according to paralyzed patient’s requirements.

Fig. 1.3: Structure of bed surface [1]

Shih-Wei Peng, and Feng-Li Lian [2] designed a mechanism and a

mechatronic control of a test bed. Their test bed consists of a set of two robotic beds:
one main bed and one nursing bed [Fig. 1.4]. The main bed is designated for posture
 11

changing and the nursing bed is for transportation. Both beds are also equipped with a
belt system for transferring the body between them.

Fig. 1.4: (a) Main Bed, (b) Nursing Bed, (c) main bed with positions [2]

Stephen Mascaro et. al [3] developed a hybrid wheel chair cum bed system for
bedridden patients [Fig. 1.5]. The powered wheelchair could be attached to the bed
and reconfigured in flat shape to match with the bed. The main aim was to have easy
movement of patient from the bed to wheel chair and vice versa. This design was a
wheelchair cum bed [Fig. 1.5 (a)]. To detach the wheelchair from the bed portion, the
back of the chair is fist raised, the whole body of the chair is then slid off from the end
of the bed portion as the foot rest is folded down and the arm rests are raised [Fig.
1.5(b)].

Fig. 1.5: Wheel chair cum bed [3]

 12

The wheelchair can be docked to a toilet directly and automatically so that the
bedridden patient can use a toilet without changing the seating [Fig. 1.5(c)]. The toilet
body can move right beneath the seat of the wheelchair. The toilet is a wall-mounted
type that comes out of a bathroom wall clearing the floor. The toilet is equipped with
a shower and dryer for automatic cleaning, while the seat of the wheelchair has a
small window which would be engaged with the toilet bowl [Fig. 1.5(d)]. After the
toilet has been used, the wheelchair is undocked from the toilet and can go back.
To move the chair back to the bed configuration, the procedure is to be reversed.

The second approach is focussed on the positioning of patients within the bed
for transportation or preventing bed sores. Wei Ching-Hua et. al [4] proposed a
hospital bed with auxiliary functions of lateral positioning and transferring of
immobilised patients. Transfer of patient from main bed to auxiliary bed is shown in
[Fig. 1.6(a)]. Change of lateral position of patient is shown in [Fig. 1.6(b)]

(a) (b)
Fig. 1.6: Hospital bed with auxiliary functions [4]
It has a two different mechanisms, one for assisting patient to rotate laterally that
facilitate patient to change lateral position either right or left within the maximum
angle by 800 and helps nurses to easily provide massage on the back of patients and
second mechanism for assisting to transport patient laterally from main bed to the
other bed or a moving carrier such as wheelchair and mechanism on moving carrier to
 13

transform into wheelchair that helps patients to easily get off the main bed and
maneuver to locations, where the medical treatment or recreation purpose are desired.

Arin Basmajian et. al [5] presented the concept of Marionette bed - for
automated rolling and repositioning of bedridden patients. Fig. 1.7(a) shows person
lying flat waiting to be rolled by Marionette Device. Fig. 1.7(b) shows person tilted
by sheet action. Fig. 1.7(c) shows person rolled by motion of left roller towards the
right. Fig. 1.7(d) shows horizontal translation of the patient.

Fig. 1.7: Marionette bed [5]

Continuing on the same lines, Binayak Roy et. al [6] proposed a method of
manoeuvring of bed sheet for repositioning of bedridden patient [Fig. 1.8]. This
method was applied by them to a rehabilitation bed for turning and transferring a
bedridden patient to alleviate bedsores and other ailments

Fig. 1.8: Method of manoeuvring of bed sheet for repositioning [6]

 14

Further Binayak Roy et. al [7] evaluated its application to an automated

rehabilitation bed [Fig. 1.9].

Fig. 1.9: Repositioning of a rigid body with a flexible sheet and its
application to an automated rehabilitation bed [7]

Goeran Fiedler et al. [8] also focussed on the problem of bed sores. They
designed a bed [Fig. 1.10] with adaptable bedding surface, especially for prophylaxis
and treatment of decubitus ulcers. This bed surface helps in reducing the average
contact pressure peaks in prolonged supine bed rest while significantly reducing the
onset and progression of pressure sores.

Fig. 1.10: Bed to prevent bed sores [8]

The third approach focuses mainly on the transportation of patients from bed
to wheel chair and vice versa. Hangbo Wang and Fumino Kasagami [9] designed a
patient mover system for transfer of patients [Fig. 1.11] within the hospital. It was
claimed that the patient could be transferred easily from bed to stretcher and vice
versa.
 15

Fig. 1.11: Patient mover system for transfer of patients [9]

Joseph Spano and H. Asada [10] used a surface wave distributed actuation
method [Fig. 1.12] for transporting bedridden patients. They have discussed design
tradeoffs and guidelines for developing a feasible and practical surface wave bed
based on the prototyping and experiments.

Fig. 1.12: Surface wave distributed actuation method [10]

From the above, it was observed that all the published literature indicate
design approaches focused on the use of automated devices/systems, consisting of
sophisticated sensors with electronic or computer controls. As a result, the cost of
 16

such beds will be quite high and also the patient or caretaker needs to become familiar
to the sophisticated controls for using such systems. Most of the products are designed
for use in hospitals.
Thus, these beds are not suitable in the Indian context and there is a need to
develop a simple, low cost alternative for bedridden healthcare for the Indian
population which can be easily used at home also. Till today, no equipment is
available in India at affordable cost, having simple mechanisms.

1.3.2 REVIEW OF PATENTS

A search about various patents was also conducted. Following patents related
to bedridden healthcare were found and are discussed below.
The first approach was to provide a toilet which can be attached to bed when
required. Havens Thomas [11] had a US Patent No 5384920. [Fig.1.13] The system
has a flat (2) having one tapered end (3). Flat (2) has toilet opening (4). Easily
removable bed pan is mounted underneath the toilet opening. At the end (5), the toilet
is supported by two folding adjustable legs at corner (1). Flat (2) has four rail support
openings (6). The back support rail assembly comprises a back rail (7), sized to
exactly fit into either pair of two adjacent rail support openings (6) . Two L shaped
lengths of tubing form the folding side rails (8). In use, the legs at corner (1) are
unfolded, adjusted to the height of the patient's bed and locked in position. The
tapered end is then placed on the bed. The patient is taken on toilet by opening handle
(8). After use the process is reversed.

Fig. 1.13: Bedside toilet for incapacitated patients [11]

 17

The second approach was to cut a hole in the bed top surface and mattress and
attach a pot when required. Weronica Dry [12] had US Patent No 1589377 which
used a bed with an opening as shown in Fig 1.14. It has flaps (1) covering the
opening.. Below the flap there is a cavity (2). A container (3) is kept in the cavity for
the collection of waste. The container is fixed with the bed by strap (4) by clamps (5)

Fig. 1.14: Hospital bed [12]

Violante Anthony [13] in US Patent No 4282613 [Fig. 1.15] has a 3 part table
which has a hole (1) in mattress (2) in which normally a piece of mattress (3) is kept.
The piece is removed by shifting patient to side. A toilet pot (4) is kept in the hole.
After use it is removed. Again
Aga mattress is kept, chair position is given by fixing
latches (5) in hooks (6)

Fig. 1.15: Convertible bed [13]

 18

The third approach is to shift the patient on the toilet in the bed
Di Matteo Paul [14] had US Patent No 4023218 [Fig.1.16]. In this device the
t legs
are lifted first by rotation about pins ( 1 and 2)
2). Simultaneously seat (3) slides down.
The commode cover (4) rotates about pin (5) to touch the buttocks of patient. The
patient is secured at position by belt(6). Then the patient is turned by turning backrest
(8) about pin (5) in chair position on toilet (7)

Fig. 1.16: A bed arrangement [14]

 19

The fourth approach was to cut a hole in the bed top surface and mattress. The
hole is covered by expandable cushion. A commode fixed below hole is available
when this cushions contracts. Tokunaga Kenki et. al [15] in US Patent No 5077845
[Fig. 1.17] used a fixed toilet (1) in bed. The opening (2) above toilet is covered by
balloons (3). The balloons are inflated by compressed air in normal position. The
balloons are deflated as and when required to open the toilet for use. The bed has
provision of backrest and legrest for chair position.

Fig. 1.17: Bed with built in commode [15]

The fifth approach was to cut a hole in the bed top surface and mattress. The
mattress piece moves to clear the hole and the fix toilet pot below hole is ready for
use. Barnett Forest [16] in US Patent No 3503083 [Fig. 1.18] provided foldable wash
basin and bath tub in bed. An opening (1) in bed is covered by pieces of mattress (2)
fixed from lower side. The pieces of mattress are fixed on brackets (3). The brackets
are able to move in guide ways (4) by pin (5). They are moved down by mechanism.
Another pin (6) of mattress piece (2) is fixed on belt (7). The belts are moved by
motor and rollers (8) to move the pieces of mattress down. The toilet seat (9) is
available for use. The sitting position is available by raising backrest and lowering
legrest.
 20

Fig. 1.18: Invalid’s bed adjustable to sitting up position and having toilet
facilities [16]

Rene Bucher [17] in US Patent No 3965501 [Fig. 1.19] used the following
arrangement. The pieces of mattress (1) are mounted on pins (2) moved away by
mechanism. This mechanism is operated by the cables at foot end. The toilet seat (3)
is available for use.
 21

Fig. 1.19: Bed Intended for use by sick and invalid persons [17]

The sixth approach was to cut a hole in the bed top surface and mattress. The
mattress piece moves to clear the hole and the pot is moved in the position of use.
Yoshitaka Ishikawa [18] had US Patent No 3943583 [Fig. 1.20]. The bed has backrest
(1) which is lifted by air assisted mechanism. Mattress piece (8) is removed by sliding
it first downward and then below the backrest on guides (11) by air bag (7), blower
(10) and mechanism (9). Toilet (2) comes out by linkages (3) pushed by link (4) by air
bag (5) and mechanism associated. After use air bag (5) flattens & commode comes
down by gravity. The air bag (7) also flattens and the mattress piece is moved first
horizontally and then vertically by spring (6) to close hole (12).
 22

Fig. 1.20: Bed Intended for use by sick and invalid persons [18]

Fugett Mary [19] US Patent No 4631762 [Fig. 1.21]. The Piece of mattress (1) along
with part holding it (2) is moved out by geared mechanism (3) powered by motor. The
toilet (4) is moved up by another geared mechanism (5) powered by motor.

Fig. 1.21: Hospital bed with toilet facility [19]

 23

Sumiyo Kanai [20] in US patent No 5513404 [Fig. 1.22] has vertical type of
toilet (1) which comes up by compressed air, and the material is removed by suction.
The piece of mattress (2) is fixed on toilet.

Fig. 1.22: Excretion apparatus combined into a bed for bedridden person [20]

Hargest Thomas et. al in US Patent No 5842237 [21] and 6009570 [22]

developed an intensive care bed with two piece hip support section (1). Below these
two pieces a toilet seat (2) is fixed. These pieces (1) moves away by mechanism and a
toilet come up for waste collection. The bed is provided with backrest and legrest for
chair position. It has wheels for motion.

Fig. 1.23: Convertible bed / chair with waste disposal [21, 22]
 24

Kiyoshi Okamoto and Hironobu Nakayama [23] in US PATENT No 5926875

[Fig. 1.24] used commode covered (1) by a rolling shutter (2) which opens
horizontally. The commode is covered with a part (3) which is lifted up to buttocks on
hinges at its side (4) by mechanism

Fig. 1.24: Nursed person’s bed with flush toilet [23]

Patwardhan Bhaskar [24] in EP 2182907 [Fig. 1.25] has following

arrangement. A hydraulic cylinder (1) operates a lever (2) which operates set of
strings (3) moving on pulleys (4) to lift the patient at certain height to change the bed
sheet. The bed pan (5) is provided below the hole (6) in mattress (7). The pot is fixed
on hinge (8) and is locked to bed by lever (9). When lever is operated the pot lowers
itself by gravity. The piece of mattress (10) can be taken out and pot is locked again
by lever. After use the pot is to be cleaned and refitted with mattress. Backrest and
legrest is provided for sitting position.
 25

Fig. 1.25: A bed cum chair with commode for patient [24]

These patents have various problems associated with them as under

• The patient has to move from the bed for which lot of assistance is required and it
may not be suitable for patients recommended bed rest for various reasons. [Fig.
1.13]
• The container is manually kept and removed. The handling of pot with odour is
not comfortable for assistant. [Fig. 1.13, 1.14, 1.15, 1.23, 1.25]
• To fix pot from lower side an attendant has to bend down which is uncomfortable.
[Fig. 1.13, 1.14]
• The flaps in [Fig 1.14] or the piece of mattress in [Fig. 1.15, 1.23] cannot be
opened without lifting the patient which involved higher level of effort. Again the
opening is to be closed by lifting patient resulting in double handling.
• The legs are lifted first then patient is turned in chair position. It needs more time
and more mechanisms are involved. [Fig. 1.16]
• Only vertical sitting position is obtained it may be uncomfortable for patient. [Fig.
1.16]
 26

• Patient is likely to get jerks when turned due to gap between hip & seat. The
turning in this manner may not be allowed for many patients. [Fig. 1.16]
• The Intricate mechanisms/ air assisted mechanisms/ motors results in high initial
cost and high maintenance costs. [Fig. 1.16, 1.17, 1.18, 1.20,1.21, 1.22, 1.23]
• The balloons have certain life and needs maintenance and replacement. [Fig 1.17]
• Operating costs are high for pneumatic systems as balloons are normally inflated.
[Fig. 1.17]
• Level of toilet pot is below mattress level which creates gap in buttock & toilet.
This may result in splashing of waste material. [Fig. 1.18, 1.19, 1.24]
• The part of mattress is moved against patient’s weight which will generate
rubbing action on hips. Alternatively the patient has to be shifted to reduce the
load on mechanism and avoid rubbing. The shifting of patient needs assistance.
[Fig. 1.19, 1.21, 1.24]
• No provision of backrest hence, sitting position is not available for patient. [Fig.
1.19]. The backrest is not coming up to sitting position. [Fig. 1.20]
• Toilet has certain width. The legs are spread widely leading to uncomfortable
position. Less spread results in gap between buttocks and toilet. Alternatively, legs
are to be kept vertical without any support leading to strain on legs. [Fig. 1.20,
1.22]
• Legs cannot be bent for chair position. [Fig. 1.20]
• The movement of mattress piece at its place is on vertical guides supported by
spring pressure. The direction of spring force is not suitable to retain mattress
piece at normally closed position. Slight loosening of spring will not allow
mattress to rise at level. [Fig. 1.20]
• The piece of mattress is also removed and placed manually. To fix mattress piece
from lower side an attendant has to bend down. This is uncomfortable. [Fig. 1.25]
• The lifting of patient on ropes needs the ropes to be always placed below the body
of patient. This can be uncomfortable. Even lifting on rope may be uncomfortable.
[Fig. 1.25]

Important fact is that none of these patents has been commercialised

because one or the other problems in the suggested designs.
 27

1.3.3 REVIEW OF AVAILABLE PRODUCTS

A search of various sites of manufacturers in India was also conducted but no
such equipment is found to be manufactured by them. Till today no such equipment is
available in India.
During the search of various manufacturers outside India, following products
related to bedridden healthcare were found and are discussed below.
Dignity Bed [25] [Image 1.1] is an electronic four section bed with a built-in
electronic dry toilet facility. The working of the built-in dry toilet is as under. All
functions are electronically controlled via a hand-set.
• The bed frame lowers until the wheels of the built-in dry toilet are on the floor.
• The dry toilet electronically slides out from under the bed frame.
• The bed frame lowers until the top of the mattress is level with the top of the dry
toilet.
• The toilet lid is lifted and the patient attends the built-in dry toilet facility.
• After toilet use the used waste bag, including the used toilet paper, is
electronically sealed.
• The bed rises back up and the dry toilet electronically slides back in under the bed
frame.

Image 1.1: Dignity Bed [25]

This bed has problems associated with it as under

• The patient has to move from the bed to toilet for which lot of assistance is
required and it may not be suitable for patients recommended bed rest for various
reasons.
 28

• The Intricate mechanisms / motors results in high initial cost and high
maintenance costs.
Another bed called yg bed [26][Image 1.2] is available in which a scissor
mechanism is used to lift the platform. The scissor mechanism is lifted by handle
driven screw. The piece of mattress closes the hole in the bed in normal condition.
When platform is lowered, a commode pot can be kept manually. After use the
process is reversed.

Image 1.2: yg bed [26]

This bed has problems associated with it as under

• The container is manually kept and removed. The handling of pot with odour
is not comfortable for assistant.
• To fix pot, the patient has to be shifted. The shifting of patient needs
assistance.

Thus the findings from the survey indicated that, there is a need of simple new
bed design for managing bedridden patients, at an affordable cost.
The literature review and product review, both confirmed that there is need
to have a bed in which all features like -
1) The piece of mattress should be removed from lower side of the bed to eliminate
shifting of patient and to reduce the friction due to body weight.
2) The commode should touch the buttocks to avoid splashing due to gap between
commode and buttocks.
 29

3) The commode should move upwards through hole in bed in order to touch the
buttocks.
4) Odour to be reduced by sealing the drainage.
5) The sitting position is to be achieved by raising backrest and lowering legrest.
6) The operating mechanisms should have minimum joints, easy in operation and be
powered by hand to reduce initial cost, running cost and maintenance.
7) Number of drives to be reduced by combining the mechanisms.
8) Accessories like utility table, wash basin, storage rack to be provided.

Based on the survey, literature review and existing manufacturing practices

product specifications were decided.

1.4 FUNCTIONAL REQUIREMENTS OF THE PRODUCT

Based on the currently used hospital beds, commodes available in the market,
expectations of stakeholders, users, literature review and past experience, the product
specifications were decided and are given below.
The dimensions of the bed are as shown in Fig. 1.26
1) The inner length of bed is selected 1830 mm to accommodate the patient. The
width of bed is selected 760 mm which is minimum, to accommodate the patient
while turning. The normal height of bed is 510 mm which is easy for climbing as
well as nursing.
2) The bed provided with backrest of 760 mm length to accommodate back of tall
patient which will move upwards for sitting position.
3) The leg part of 510 mm length equal to bed height will be moved downwards for
chair position.
4) The bed top to have an opening of 360 mm length and 300 mm width having
shape similar to commode.
5) The commode of size of 340 mm length and 280 mm width.
6) Width of platform needed is 400 mm to cover 300 mm width of the opening in
bed and 50 mm on both sides for lifting mechanism and length 375 mm to cover
length of opening 360 mm.
 30

7) Space of 100 mm is left between the backrest and start of opening to allow space
for the thickness of mattress.
8) Similarly 100 mm space is left after the opening to provide sitting depth of 560
mm.
9) The thickness of mattress is 65 mm which shall have 50 mm high density base and
15 mm foam covered by waterproof cloth.
10) Mattress to have an opening matching to the opening in the bed. A piece of
mattress matching to the opening in mattress to be fitted on platform
11) This platform will cover the opening from lower side in order to open the opening
by moving platform down without disturbing patient. This platform will clear the
opening in the bed in such a way that it does not interfere raising of commode
through the opening in bed.
12) A commode seat with water sealed odour trap to be provided below bed. This
commode shall be raised through the hole in the bed to touch the buttocks of the
patient.
13) The direction of trap shall be adjusted according to position of drainage.
14) The maximum height of commode with trap is to be 350 mm.
15) All handles & screw mechanism to be provided at head end which provides free
space below one piece leg rest to fold it.
16) The leg side legs of bed shifted towards head end in order to reduce the size of
main frame and to allow leg bow to turn upwards to make utility table.
17) Storage rack for medicines, glass, towel, etc
18) The mechanisms are required to
a) Move backrest for sitting position.
b) Move legrest for sitting Position.
c) Opening and closing of the opening in bed top.
d) Move commode up to the mattress level and retract it.

Normally a square threaded screw operated mechanisms are used to move

backrest and legrest. They are simple and well known in the field of hospital
equipment. Hence our discussions are focused on the remaining two mechanisms.
Once these are finalized, it is easy to incorporate the first two mechanisms.
 31

Based on the requirements listed above, tentative arrangement of the bed was
proposed and is shown in Fig. (1.26). It shows a commode (1) attachable to bed,
which shall come up through an opening (2) in bed when required. Sitting position is
achieved by moving the backrest (3) up. Chair position is achieved by moving leg part
(4) downwards for easy urination and defecation. The commode can be raised to the
top level of mattress (5) through the opening (2). Normally the opening in mattress is
covered by a matching piece of mattress mounted on platform (6) which closes the
opening when not in use and moves out when commode is needed.

Fig. 1.26: Arrangement of bed showing various features and dimensions in mm

 32

Fig. (1.27) shows the arrangement of bed when commode is lifted up and
platform is moved down for use of commode.

Fig. 1.27: Arrangement of bed showing commode up and platform down

Based on design considerations above, the kinematic design of mechanisms is

discussed in next chapter.
CHAPTER – 2
 33

KINEMATIC DESIGN OF MECHANISMS

Based on the product specifications the various mechanisms for providing motions
to backrest, legrest, commode and platform are synthesized and analyzed. This chapter
presents the details about the synthesis and analysis.

2.1 TYPE SYNTHESIS FOR ACTUATORS

The options available for actuators are :-
1) Handle driven screw & nut mechanism with square threads to transmit power. The
mechanism is simple and cheap. It is also easy for maintenance. It does not fail
suddenly. Power requirement is zero as it works on human power. It has long trouble
free life. Only disadvantage is attendant is needed to drive it.
2) Hydraulic cylinder powered by hand operated power pack is also a option without need
of power. But initial cost is higher and spares are not easily available. Moreover
maintenance needs skilled mechanic.
3) Hydraulic cylinder powered by motorized power pack is having still higher cost than
that with hand operated power pack. It has higher cost of maintenance due to cost of
spares and skill needed. Non -availability of spares may lead to long downtime of
equipment. It has advantage that the human labour is eliminated in daily use. It can be
controlled remotely with additional investment. The cost of power needed is a running
expenditure.
4) Air operated cylinder needs air receiver to store the compressed air and air compressor
to generate compressed air. This makes the system costlier than mechanical screw. The
maintenance is easy if simple manually operated valves are used. If remote controlled
system is used then cost of valves and switches needs high initial investment. The
availability of spares will also be less resulting in downtime of equipment. Cost of
power consumption is a running expenditure. The motion of pneumatic cylinder may
give jerks to mechanism.
5) Electrical Actuators which drives a square threaded screw through nut is good option
for simplicity of automation and costs much lesser than the pneumatic and hydraulic
system. Though the spares are not easily available, it has long life and reliability over
hydraulic and pneumatic options. They need very small space and are easy to install /
 34

replace. The operation is silent and requires less power. The running voltage of
actuators is 24V which is safe for human being.

Considering all the factors it was decided to develop the product with most simple,
easy to operate, economical option of mechanical screw & nut mechanism. It also has
further possibility of providing motor.

Following paragraphs presents a discussion on synthesis of mechanical linkages for

the commode, platform, and other mechanisms.

2.2 TYPE SYNTHESIS OF MECHANISM FOR RAISING AND

LOWERING COMMODE

Fig. 2.1: Two toggle jack arrangement to move commode up and down

Fig. (2.1) shows two toggle jacks (1) connected to each other by chain and
sprocket (2). It can raise the toilet seat (3) to the required height. It can be seen that
the handle does not have fixed position and it moves upwards when commode is
raised. The commode needs vertical guides to match the hole in bed. Also, the
operator has to sit on ground or be in bent position to operate the handle. This
arrangement is not convenient for operation. Therefore, to maintain a suitable fixed
height of handle, another arrangement as shown in Fig. (2.2) is considered.
 35

Fig. 2.2: Three lead screw arrangement to move commode up and down

Fig. (2.2) shows arrangement of three lead screws (1) driven by a bevel gear
(2) chain (3) on sprockets (4). Over a period of time there may be play in chain drive
due to sprocket wear and chain elongation which can result in lag in motion of the
screw and the commode top may not remain horizontal. The system is complicated
and costly as compared to toggle jack. Hence, one more arrangement as shown in Fig.
(2.3), is considered for evaluation.

Fig. 2.3: Parallel link arrangement to move commode up and down

Fig.(2.3) shows two parallel links (1and 2). One link (1) is a ‘L’ type lever
which is activated by a horizontal screw. When horizontal force is applied at lower
end (3) of link (1), the horizontal link rotates around pin (4) resulting in up down
motion of commode. The upward motion of commode is guided by link (2) moving
on pins (5 and 6). This mechanism is simple and needs negligible maintenance. It also
keeps the commode in level as well as it guides the motion vertically. Hence this
mechanism can be used if it does not obstruct the mechanism needed for operation of
moving platform, which is discussed in the next paragraph.
 36

2.3 TYPE SYNTHESIS OF MECHANISM FOR PLATFORM

Fig. (2.4) shows a platform (1) on hinge (2) below bed, pulled by a drive (3)
through link (4) which will open the hole in bed. This mechanism is simple and has
low initial cost. However, it needs space below it for its movement.

Fig. 2.4: Hinged platform arrangement to open and close hole in bed.

The length of cover is 400 mm. The height of bed is 510 mm. The remaining
height available for commode is 110 mm. It is not possible to accommodate any
commode seat with trap in this height. If at all this mechanism is to be used then the
mechanism earlier selected for commode (Fig. 2.3) cannot be used. Then the
commode has to be fitted on the other side of mechanism. The commode will have to
be shifted horizontally towards left first and then vertically upward. Considering the
weight and volume of commode and pipes attached to it, it shall need heavy
mechanism and more power. Therefore, this arrangement of Fig. 2.4 is not suitable.
Hence, to use the mechanism of Fig. (2.3) for commode motion, the platform
should have mechanism which shall move the cover first vertically down and then
`horizontally to clear the hole as shown in Fig. (2.5).

Fig. 2.5: Platform with parallelogram links to open and close hole in bed.
 37

Fig. (2.5) shows cover (1) fixed on platform (2) with parallel links crossed to form
two parallelograms. One joint (3) is a pin joint whereas other joint (4) is a roller
contact joint. The links (8 and 9) are with 4 wheels (6) resting on fixed channel (10).
When force is applied at joint 5, the total parallelogram
parallelogram structure moves below the
hole in bed till wheels (6) stop at stopper 10. Further application of force will raise the
platform to close the hole in bed. To lower the platform, force in opposite direction
has to be applied. Till the cover (1) is in contact with mattress, the platform will move
down and as soon as the cover (1) moves below the mattress the mechanism will start
moving horizontally away from hole. This shall lead to high wear and tear of mattress.
To avoid this, it is required to hav
havee a guide for vertical motion of platform which shall
disengage after mattress comes below the bbed
ed level. The system shown in Fig. (2.5)
(2.5
shall consume minimum vertical space below the hole and allows space for selected
commode mechanism. Th
This mechanism may be used after exploring other alternative
as shown in Fig. (2.6)

Fig. 2.6:
6: Platform with wheels and sloping guides to open and close hole in
bed.
Fig. (2.6)) shows cover (1) fixed on platform (2) having two front wheels (3)
and two rear wheels (4). The front wheels (3) move on upward turning guides (5)
which are close to platform. The rear wheels (4) move on upward turning guides (6)
which are away from platform
platform by distance more than width of wheel or equal to width
of guide (5). When Force (F) is applied to link (7) the platform moves first on
 38

horizontal part of guides and then on sloping part to close the hole. The above
mechanism also consumes minimum vertical space below hole and allows space for
mechanism for commode. However, the force required to raise the platform on slope
is considerably high as compared to that required in mechanism of Fig. (2.5). This
mechanism may also be used after considering other option of a four bar mechanism
for three positions of the platform.
Fig. (2.7) shows a four bar mechanism for three positions- closed, lowered and
extreme side position of the platform. The mechanism was synthesized by using
analytical (matrix) method. The location of fixed pivots has to be below the bed level
and below the backrest as backrest does not come below bed level. Initially, the fixed
pivots are assumed at M0 (5, 45) and N0 (15, 45). All positions of platform were
considered to be horizontal. Positions of moving pivots were obtained and are M1 (48,
30) and N1 (58,30). For getting other positions, the X coordinate of No was
incremented to increase the distance between Mo and No. The results are tabulated at
Sr. No 1 to 5 in Table 2.1. For next five positions, the Y coordinates of Mo and No
were lowered by 5 cm and the X coordinate of No was incremented to increase the
distance between Mo and No. The results are tabulated at Sr. No 6 to 10 in Table 2.1.
For further points, platform was considered to be tilted by 50 and 100 with respect to
its closed position. Keeping all the other data same as in 1-10, positions 11-20 of
Table 2.1 were obtained.

Fig. 2.7: Platform on four bar mechanism (Mo, No, M1, N1) to open and close
the opening in bed for position 1.

To facilitate the calculations a calculation program as under was developed in

worksheet and results were obtained by substituting various parameters.
 39

Three Position Synthesis By Analytical Method - For Platform

1 Assumptions - M0, N0 are Fixed Pivots with coordinates Position - 1
M0x M0y N0x N0y
5 45 15 45
50 mm Horizontal Travel
2 The positions needed will be between 1&3
30 mm Vertical Travel
Position Px Py Qx Qy
Sin Angle between 1&3
0.00 mm Tilt at position 3 w.r.t
3 37 21 71 21.0
0 0 1
0.00 mm Tilt at position 2 w.r.t
2 82 41 116 41.0
0 0 1
10 mm Vertical Travel
1 87 51 121 51
0 between 2&3
3 For Positions 1&2 Rotational Matrix 12
1.00 0.00 -5.00
0.00 1.00 -10.00
0 0 1
4 From Rotational Matrix 12
d11 x d13 = -5 d12 x d13 = 0 d13 x d13 = 25
d21 x d23 = 0 d22 x d23 = -10 d23 x d23 = 100
1-d11 = 0.00 1-d22 = 0.00
5 Substituting the values in Equation
(d11xd13+d21xd23+(1-d11)M0x-d21xM0y)M1x + (d12xd13+d22xd23+(1-d22)M0x-d12xM0y)M1y
= d13xM0x+d23xM0y-1/2(d13xd13+d23xd23)
-5 M1x + -10 M1y = -537.5 A
-5 N1x + -10 N1y = -587.5 B
6 For Positions 1&3 Rotational Matrix 13
1.00 0.00 -50.00
0.00 1.00 -30.00
0 0 1
7 From Rotational Matrix 13
d11*d13 = -50 d12*d13 = 0 d13*d13 = 2500
d21*d23 = 0 d22*d23 = -30 d23*d23 = 900
1-d11 = 0.00 1-d22 = 0.00
8 Substituting the values in Equation
(d11xd13+d21xd23+(1-d11)M0x-d21xM0y)M1x + (d12xd13+d22xd23+(1-d22)M0x-d12xM0y)M1y
= d13xM0x+d23xM0y-1/2(d13xd13+d23xd23)
-50 M1x + -30 M1y = -3300 C
-50 N1x + -30 N1y = -3800 D
9 Solving the Equations A & C
250 M1x + 500 M1y = 26875
250 M1x + 150 M1y = 16500
0 M1x + 350 M1y = 10375
M1y = 29.64
10 Substituting Values in Equation A
-5 M1x + -296 = -537.5
M1x = 48.21
11 Solving the Equations B & D
250 N1x + 500 N1y = 29375
250 N1x + 150 N1y = 19000
0 N1x + 350 N1y = 10375
N1y = 29.64
12 Substituting Values in Equation D
-50 N1x + -889 = -3800
Result N1x = 58.21
The Moving pivotes for closed position of platform ( position 1) are
M1 ( 48.21 , 29.64 ) N1 ( 58.21 , 29.64 )

Image 2.1 : Image of calculation program in worksheet for position 1

 40

Three Position Synthesis By Analytical Method - For Platform

1 Assumptions - M0, N0 are Fixed Pivots with coordinates Position - 11
M0x M0y N0x N0y
5 45 15 45
50 mm Horizontal Travel
2 The positions needed will be between 1&3
30 mm Vertical Travel
Position Px Py Qx Qy
Sin Angle between 1&3
6.25 mm Tilt at position 3 w.r.t
3 37 21 71 14.7
0.17 10 1
3.14 mm Tilt at position 2 w.r.t
2 82 41 116 37.9
0.09 5 1
10 mm Vertical Travel
1 87 51 121 51
0 between 2&3
3 For Positions 1&2 Rotational Matrix 12
0.98 -0.17 5.18
0.17 0.98 -24.33
0 0 1
4 From Rotational Matrix 12
d11 x d13 = 5.0991 d12 x d13 = -0.9 d13 x d13 = 26.8
d21 x d23 = -4.225 d22 x d23 = -24 d23 x d23 = 592
1-d11 = 0.02 1-d22 = 0.02
5 Substituting the values in Equation
(d11xd13+d21xd23+(1-d11)M0x-d21xM0y)M1x + (d12xd13+d22xd23+(1-d22)M0x-d12xM0y)M1y
= d13xM0x+d23xM0y-1/2(d13xd13+d23xd23)
-6.864396 M1x + -16.9719 M1y = -1378.52 A
-6.712473 N1x + -16.81998 N1y = -1326.742 B
6 For Positions 1&3 Rotational Matrix 13
1.00 -0.09 -45.22
0.09 1.00 -37.39
0 0 1
7 From Rotational Matrix 13
d11*d13 = -45.05 d12*d13 = 3.94 d13*d13 = 2045
d21*d23 = -3.259 d22*d23 = -37 d23*d23 = 1398
1-d11 = 0.00 1-d22 = 0.00
8 Substituting the values in Equation
(d11xd13+d21xd23+(1-d11)M0x-d21xM0y)M1x + (d12xd13+d22xd23+(1-d22)M0x-d12xM0y)M1y
= d13xM0x+d23xM0y-1/2(d13xd13+d23xd23)
-52.21351 M1x + -29.36364 M1y = -3630.156 C
-52.17545 N1x + -29.32559 N1y = -4082.396 D
9 Solving the Equations A & C
358.4142 M1x + 886.1626 M1y = 71977.34
358.4142 M1x + 201.5636 M1y = 24918.82
0 M1x + 684.599 M1y = 47058.52
M1y = 68.74
10 Substituting Values in Equation A
-6.864396 M1x + -1167 = -1378.52
M1x = 30.87
11 Solving the Equations B & D
350.2263 N1x + 877.5902 N1y = 69223.36
350.2263 N1x + 196.8472 N1y = 27402.97
0 N1x + 680.7429 N1y = 41820.38
N1y = 61.43
12 Substituting Values in Equation D
-52.17545 N1x + -1802 = -4082.396
Result N1x = 43.71
The Moving pivotes for closed position of platform ( position 1) are
M1 ( 30.87 , 68.74 ) N1 ( 43.71 , 61.43 )

Image 2.2 : Image of calculation program in worksheet for position 11

 41

Table 2.1: Results of three position synthesis for four bar mechanism

X. Y Tilt Tilt Y
Travel Travel at 3 at 2 Travel
S. Mo No M1 N1
betn 1- betn 1- wrt 1 wrt 1 betn 2-
No. (x,y) (x,y) (x,y) (x,y)
3 in 3 in in in 3 in
mm mm mm mm mm
1 5 45 15 45 50 30 0.00 0.00 10 48 30 58 30
2 5 45 20 45 50 30 0.00 0.00 10 48 30 63 30
3 5 45 25 45 50 30 0.00 0.00 10 48 30 68 30
4 5 45 30 45 50 30 0.00 0.00 10 48 30 73 30
5 5 45 35 45 50 30 0.00 0.00 10 48 30 78 30
6 5 40 15 40 50 30 0.00 0.00 10 48 25 58 25
7 5 40 20 40 50 30 0.00 0.00 10 48 25 63 25
8 5 40 25 40 50 30 0.00 0.00 10 48 25 68 25
9 5 40 30 40 50 30 0.00 0.00 10 48 25 73 25
10 5 40 35 40 50 30 0.00 0.00 10 48 25 78 25
11 5 45 15 45 50 30 6.25 3.14 10 31 69 44 61
12 5 45 20 45 50 30 6.25 3.14 10 31 69 50 58
13 5 45 25 45 50 30 6.25 3.14 10 31 69 56 54
14 5 45 30 45 50 30 6.25 3.14 10 31 69 63 51
15 5 45 35 45 50 30 6.25 3.14 10 31 69 69 47
16 5 40 15 40 50 30 6.25 3.14 10 32 60 45 53
17 5 40 20 40 50 30 6.25 3.14 10 32 60 51 50
18 5 40 25 40 50 30 6.25 3.14 10 32 60 57 47
19 5 40 30 40 50 30 6.25 3.14 10 32 60 63 44
20 5 40 35 40 50 30 6.25 3.14 10 32 60 69 41

From Table 2.1 it is observed that the four bar mechanism of Fig. (2.7) is not useful
due to following reasons.-
1. From the comparison of readings (1-10) with (11-20) it is observed that by tilting
the platform in positions 2 and 3, the positions of moving pivots go above the bed
level. This is totally unacceptable.
2. Comparison of readings (1-5) with (6-10) shows that lowering of fixed pivots
lowers the position of moving pivots. There is no change in x coordinate.
3. Assuming all the positions of platform in horizontal plane, while using the four
bar mechanism, location of fixed pivots needs to be on the body of the bed and
below the backrest. The moving pivots are far away from the platform. The load
due to platform is like a cantilever on the moving and fixed pivots. This demands
accurate pins without any play. Moreover, little clearance in pins will not allow
platform to reach closing position.
 42

4. The force required to keep platform in closed position will be continuous and high
as weight of patient body will be continuously acting on it.
5. If the four bar mechanism is a used, separate mechanism for commode and
platform with two separate drives are needed.
6. The path of extreme point of platform is interfering with the commode at lowered
position
7. Thus, the alternatives available for platform mechanism are mechanism from Fig.
(2.5) or Fig. (2.6). The mechanism in Fig. (2.6) needs higher force to raise
platform on the slope. Moreover, there is tendency of platform to roll down.
Hence, the mechanism in Fig. (2.5) is selected as a single drive can operate
commode and platform both.

2.4 DIMENSIONAL SYNTHESIS

Now the dimensions of various mechanisms are to be decided based on our
considerations in Fig. 1.27. Following paragraph shows the synthesis procedure.

2.4.1 DIMENSIONAL SYNTHESIS FOR PLATFORM

It was observed that, in Fig. 2.5 the platform will have the tendency to tilt due
to patient weight at up position as both the supports (one fixed and one rolling) were
coming on one side of platform. Hence it was decided to modify mechanism as shown
in Fig. (2.8). In this mechanism, the fixed support (1) to platform (4) has to be moved
in center and two rolling supports (2) be provided to balance the load. The lower side
was provided with three rollers (3) which move in channel (5).

Fig. 2.8: Modified platform mechanism

 43

It was considered that

1) The mattress (6) to have thickness of 65 mm.
2) Structural member (7) of bed to mount mattress be of 25 mm height.
3) The platform will have to move down by sum of mattress thickness and structural
member thickness. Hence the vertical travel of platform to be 90 mm.
4) Considering Length of link AD as 70 mm, and minimum and maximum angle
made by link with vertical as 180 and 720 as shown in Fig. 2.9.

Fig. 2.9: Two positions of linkages for platform mechanism

 44

5) The maximum height AE comes as 133 mm and minimum height A’E’ as 43 mm.
This gives required vertical stroke of 90 mm. Hence size of the link AB is
confirmed as 70 mm.
6) The diameter of wheels at 2 and 3 of Fig. 2.8 is considered as 25 mm.
7) The thickness of platform 4 of Fig. 2.8 has to be 30 mm to accommodate the
wheels of 25 mm dia. in channel below it.
8) The outer height of channel 5 of Fig. 2.8 to be 30 mm to accommodate the wheels
of 25 mm dia.
9) The total height of mechanism with channel and platform will be 133 mm
(maximum mechanism height) + 15 mm (½ of platform thickness) + 15 mm (½ of
channel thickness) = 163 mm.
10) Adding 12 mm clearance above mattress during horizontal movement of
platform, the channels for platform movement to be fitted below the bed at
distance of 175 mm (163 + 12) from top of bed to bottom of channels.

2.4.2 DIMENSIONAL SYNTHESIS FOR COMMODE

Fig. 2.10: Linkages for commode mechanism

1) The commode should rise 35 mm above bed level in order to support the body of
patient.
2) The vertical travel of commode to be 175 mm to clear the channel + 35 mm above
bed = 210 mm.
3) For vertical stroke of 210 mm needed for the commode by link ratio 2:1,
horizontal stroke needed has to be 105 mm.
 45

4) We have assumed length of commode as 350 mm. considering shape of commode

and that the weight of patient will be on wider side, the pin hanging the commode
is shifted by 30 mm towards head end.
5) The length of horizontal part of bell crank lever = 175 mm (½ commode length) +
30 mm (shift of pin) + 55 mm (space for fixing bell crank lever) = 260 mm.
6) The vertical part of bell crank lever has to be inclined by 65 mm with vertical in
order to have travel on both side of vertical.
7) The supporting link can be at distance of 75 mm below horizontal part of bell
crank lever.

2.4.3 DIMENSIONAL SYNTHESIS FOR ACTUATOR

1) The length of platform is taken as 375 mm in order to cover 360 mm length of

hole.
2) Horizontal stroke needed for lifting platform will be 90 mm (same as vertical
stroke).
3) The total requirement of screw stroke is 570 mm. ( 375 mm platform length +
105 mm for commode travel + 90 mm for platform lift)
4) The normal length of backrest is 760 mm. The space above and below backrest is
15 mm and 50 mm respectively. When platform is at the center, the screw moves
115 mm below the platform. Thus maximum space available for screw is 940 mm
when open as shown in Fig. 2.11.

Fig. 2.11: Location of Actuator mechanism

 46

5) For screw and nut the stroke is limited to 50% of open length less nut length.
Therefore the stroke for 940 mm open length of screw cannot be more than 450
mm. However our requirement of stroke is 570 mm. Thus the required closed
length of screw is 370 mm. Hence a telescopic screw can be used to solve the
problem.
6) The number of rotations of handle for screw per minute are limited by human
capacity. A human being can rotate the handle at 60 rpm without strain. The
operation of platform lowering and commode rising is to be performed within 2
minutes so as to make it ready to use. The number of revolutions will be 120.
Hence the pitch of the screw has to be 4.75 mm (say 5 mm)

The availability of screw, air cylinder, hydraulic cylinder and actuator is to be

checked for above purpose.
1) Hand operated screw can be designed for this purpose. It will have telescopic
double screw which shall have closed length of 370 mm and stroke of 570 mm.
2) The hydraulic cylinders are available. The cost of cylinder is Rs 40,000/- & the
Power Pack manually operated is Rs 26,000/- whereas, motorized power pack
costs Rs 82,000/- which is not in budget of common man.
3) The air cylinders available are single stroke and closed length of 370 and stroke of
570 mm is not available. If available, it will be costly like above.
4) The electric actuators are not available for closed length of 370 mm and stroke of
570 mm as they have only single screw.
Hence the telescopic screw and nut is only option for moving commode and
platform.
Then the suitability of this platform mechanism with earlier selected commode
mechanism from Fig. (2.3) was checked. It needs connecting mechanism.

2.4.4 DIMENSIONAL SYNTHESIS FOR CONNECTING MECHANISM

The two parallelogram mechanisms (Fig. 2.5) need to be connected to the

commode mechanisms. The puller can be attached to roller near to commode. It will
create a problem as the distance of this roller from the commode was changing when
platform rises. Hence the puller has to be attached to middle roller, as it was not
changing its place while lifting and lowering the platform.
 47

The Fig. 2.12 shows various distances of the platform mechanism and
commode mechanism for commode up position and commode down position.

Fig. 2.12A: Distances of mechanisms for commode up position.

Fig. 2.12B: Distances of mechanisms for commode down position.

1) Considering the above dimensions the maximum distance between pulling point
(1) of commode link and center of lower middle roller (2) will be 590 mm (200
mm for 1½ parallelogram center distance + 15 mm for width of parallelogram link
+ 30 mm extra travel of platform to clear the hole in bed + 360 mm length of hole
 48

+ 50 mm distance of pin (3) for bell crank lever – 65 mm inclination of pulling

lever with vertical) as shown in Fig. 2.12A.
2) The minimum distance between pulling point (1) of commode link and center of
lower middle roller (2) will be 290 mm (180 mm for ½ length of hole in bed + 50
mm distance of pin (3) for bell crank lever + 40 mm inclination of pulling lever
with vertical + 20 mm extra travel of pulling lever to lower commode below the
level of channel) as shown in Fig. 2.12 B.
3) As the distance was varying from 290 mm to 590 mm, it was not possible to have
a simple link to pull the commode. This generated requirement of a link which can
collapse / fold during that travel. A telescopic link was thought as a solution as the
link has to pull the commode up. There was no need to push the commode down
as it comes down by gravity. Hence the telescopic link can be used. For maximum
distance of 590 mm we need to provide flats with hole to connect it to platform
mechanism and commode mechanism. This needs 80 mm space approximately. In
addition to this 40 mm space is required for overlap of outer cylinder and bar
inside it to maintain the axial alignment. Hence the stroke available was 235 mm
(½ (590-80-40)). Hence the closed length of that puller was limited to 355 mm.
This was obstructing the motion of the platform while raising platform. Hence
double telescopic puller having reduced closed length was used.

Based on these mechanisms and calculations the design calculations are

discussed in next chapter.
CHAPTER – 3
 49

MECHANICAL DESIGN OF THE SYSTEM

After selecting the appropriate mechanisms for commode and platform

motions, design calculations were performed and sizes of various parts were decided.
Calculations of other mechanisms like backrest, legrest etc. are not performed as sizes
of the components are common.
Fig. 3.1 and 3.2 shows the positions of commode up and platform up positions
respectively. The various parts in these mechanisms are numbered and description of
the parts is shown in Table 3.1.

Fig. 3.1: Arrangement of various parts for commode up position

The Fig. 3.1 shows the position of mechanism where commode is at up

position and platform is at down position. When handle 29 is rotated to close
telescopic screw mechanism, the screw mechanism pulls bar at 17. The bar at 17
connects screw mechanism to two multiple scissor mechanisms. These scissor
mechanisms are attached below platform at it's center. Hence the platform is at it's
extreme position to clear the hole in bed. This force also keeps the platform at lower
level in order to pull it on the channel 18 below the bed. Two telescopic pullers are
 50

attached to pins of lower central rollers of each platform mechanism at one end and at
pins 9 of both commode mechanisms. The puller is pulled by the rollers at open
condition. It exerts pull on commode mechanism at pin 9. This force is used to lift the
commode by bell crank lever made of 2 and 3. The screw keeps it lifted at the highest
position.

Fig. 3.2: Arrangement of various parts for platform up position

The Fig. 3.2 shows the position of mechanism where commode is at lowered
position and platform is at up position. When handle 29 is rotated to open telescopic
screw mechanism, the screw mechanism pushes bar at 17. The bar at 17 connects
screw mechanism to two multiple scissor mechanisms. These scissor mechanisms
carry the platform away. As the screw mechanism opens, the two telescopic pullers
allows the commode to move down due to gravity clearing the hole in bed and moves
below the channels to clear the path of platform. When the platform touches the
stopper at extreme end, further travel of platform is restricted. As rotation of screw is
still continued the scissor mechanisms operate and platform gets lifted. The scissor
mechanism is attached to platform at central top pin. Other two rollers of scissor
mechanisms come closer and keeps platform balanced at upper height.
 51

Table 3.1: Names of various parts of commode and platform mechanism

Name of the Part Number
Pin on commode pot connecting pot with link 2 1
Horizontal link joined with link 3 to act as bell crank lever 2
Vertical link joined with link 2 to act as bell crank lever 3
Pin on commode pot connecting pot with link 5 4
Horizontal link connecting fixed pivot 6 and moving pivot 4 5
Pin at fixed pivot 6
Vertical fixed link to form fixed pivot 6 and 8 7
Pin at fixed pivot 8
Pin at pulling point for commode mechanism 9
Inner rod of telescopic puller 10
Stopper at the end of inner rod 10 11
Middle pipe of telescopic puller 12
Outer pipe of telescopic puller 13
Pin for Platform mechanism 14
Roller for platform mechanism 15
Flat for platform mechanism 16
Bar connecting screw mechanism with platform mechanism 17
Channel for movement of roller 18
Inner screw of telescopic screw mechanism 19
Nut for inner screw of telescopic screw mechanism 20
Hollow screw of telescopic screw mechanism 21
Pipe to hold nut for hollow screw of screw mechanism 22
Thrust bearing 23
Bolt to hold thrust bearing 24
Outer pipe of telescopic screw mechanism 25
Locking pin for screw 26
Bolt to hold pipe 25 27
Locking Bush 28
Handle 29
 52

3.1 MECHANICAL DESIGN FOR COMMODE

Fig. 3.3 shows closer view of commode mechanism only. When force at point
9 is applied by the screw mechanism, it pulls link 3 about fixed pivot 8. The link 3 is
welded to link 2 to form a bell crank lever. Movement of link 3 moves link 2 by same
degree about pin 8. The link 2 lifts commode pot as it is attached by pin 1. Link 5 is
fixed at parallel position to link 2 by pin 4 on commode pot and pin 6 on fixed link 7.
During the upward motion of commode pot link 5 does not allow it to rotate it about
pin 1 due to unbalanced load on commode pot. Once the commode pot is raised it
remains at the highest position due to force applied by screw mechanism.

Fig. 3.3: Arrangement of various parts of commode mechanism

 53

3.1.1 DESIGN OF LINKS AND PINS

Following paragraph shows the design procedure for various components of

commode mechanism.

Design of Pin 1
The pin is considered as made of SAE 1015 hot rolled steel. The pin [part No.
1 of Fig. 3.3] is subjected to the load of commode and person on commode pot. It is
subjected to shear force. Hence it is designed for shear. Following paragraph shows
the design procedure.

Assume Mass of commode (C) 10 kg

Acceleration due to gravity (g) 9.81 m/sec2
Assume Weight of person (W) 120 kg
Effective weight (E) on commode is assumed 2/3 of total
weight of the person (W) as other body parts are supported. 80 kg
Total weight on the commode= E+C 90 kg
Vertical Force acting = (E+C) x g 882.9 N -- (A)
Total vertical force on links (2) 882.9 N
No of pins on which above vertical force is acting 2 No

Actual force on each pin =  × vertical force 441.5 N -- (B)
Considering factor of safety 1.2
Maximum force (F) on pin 1 529.7 N
Allowable Yield Strength in shear (S ) 140 Mpa [27]
F
S =
A
Yield Strength in shear

Required cross-sectional area (A) for pin 1 3.8 mm2

= 
4
Cross-sectional area of round pin

Required diameter of pin 1 2.2 mm

 54

Design of flat 2
The flat is considered as made of SAE 1015 hot rolled steel. The flat [part No.
2 of Fig. 3.3] is supporting the commode using pin 1 and it is the link responsible for
movement of the commode. It is subjected to bending load. Hence it is designed for
bending strength. Following paragraph shows the design procedure.

Allowable bending stress for selected material (σ ) 310 Mpa [27]
Force on each flat 2 is same as on pin 1 441.5 N from (B)
Considering factor of safety 1.2
Maximum force (F) on flat 2 at pin 1 529.7 N
Length (L) of flat 2 260 mm
Bending Moment (M) at pin 8 due to force (F).
M= F×L 137732.4 N-mm
M
σ =
Z
Bending stress

Required modulus of section (Z) 444 mm3

bd
Z=
6
Modulus of a rectangular section

Consider thickness (b) of flat 2 5 mm

Required width (d) of flat 2 23

Design of flat 3
The flat is considered as made of SAE 1015 hot rolled steel. The flat [part No.
3 of Fig. 3.3] is a part of bell crank lever made by joining it to flat 2. When force is
applied at the end of this flat it rotates bell crank lever about pin 8 and lifts the
commode. The bending moment at pin 8 as same as in flat 2. Hence size of the flat
has to be same as flat 2.

Design of pin 4
The pin is considered as made of SAE 1015 hot rolled steel. The pin [part No.
4 of Fig. 3.3] is a moving pivot for flat 5. It is subjected to shear force due to any
unbalanced load of person on commode pot. Any load which is not acting vertically
on pin 1 will produce turning moment at pin 1 and flat 5 will be subjected to tension
 55

or compression. The pin 4 is subjected to shear force due to tensile or compressive

force on flat 5. Hence it is designed for shear. Following paragraph shows the design
procedure.

Vertical force responsible for bending moment

about pin 4 441.5 N from (B)
Considering this vertical force (V) acts between
pin 1 and edge of the commode. Assuming
distance of vertical force from pin 1as 80.00 mm
Bending moment at pin 1 due to force (V)
M=V×80 35316 N-mm
This bending moment is balanced by force in flat
5 and distance of this force from pin 1 is 75 mm
By equating bending moment about pin , force
along flat 5 acting on pin 4 is 470.9 N ---- (C)
Considering factor of safety 1.2
Maximum force (F) 565.1 N
Allowable Yield Strength in shear (S ) 140 Mpa [27]
F
S =
A
Yield Strength in shear

Required cross-sectional area for pin 1 4.0 mm2

π 
A= d
4
Area of round pin

Required Diameter of pin 4 2.3 mm

Design of flat 5
The flat is considered as made of SAE 1015 hot rolled steel. The flat [part No.
5 of Fig. 3.3] will be subjected to tension or compression. Hence it is designed for
compression. Following paragraph shows the design procedure.

Force on flat 5 470.9 N From (D)

Considering factor of safety 1.2
Maximum force (F) 565.1 N
 56

Allowable yield strength in compression (Syc) 310 Mpa [27]

F
S =
A
Yield strength in compression

Required area of Cross-section 1.8 mm2

Area of Cross-section of a flat A= b x t
Assuming thickness (t) of flat 5 mm
Required width (b) of flat 0.4 mm
It is not possible to use flat of above size hence
width of flat considered same as flat 2 in order to
enable fixing of pins 25 mm
Checking flat 5 for buckling
Length of flat 5 (L) 260 mm
tb
I=
12
Moment of inertia (I) of a flat

Moment of inertia (I) of the flat 5 260.4 mm4

Cross-sectional area of a flat A= b x t
Cross-sectional area (A) of flat 5 125 mm2

I
K=
A
Radius of gyration

Radius of gyration (K) of flat 5 1.4 mm

L
λ=
K
Slenderness ratio

Slenderness ratio (λ) of flat 5 180.1

If λ>90 the we have to use Euler's formula , if 10<λ<90 then we have to use Johnsons
formula
End fixidity coefficient ( C ) for both ends fixed 1.0
Modulus of elasticity (E) 206000.0 Mpa [27]
S# λ
W = AS !1 − '
4cπ E
Buckling load by Euler's formula

Safe Buckling load (W) for flat 5 is 26002.7 N

As the actual load is much less than safe buckling load. The size of flat 5 is confirmed
to be 25 x 5 mm.
 57

Design of pin 6
The pin is considered as made of SAE 1015 hot rolled steel. The pin [part No.
6 of Fig. 3.3] is subjected to same force along the flat 5. As the force acting on pin 4
and 6 is same, pin 6 has to be of same size that of pin 4.

Design of flat 7
The flat is considered as made of SAE 1015 hot rolled steel. The flat [part No.
7 of Fig. 3.3] is fixed to body of bed. It is subjected to force by horizontal flat 5. It is
subjected to bending moment due to force in flat 5. Hence it is designed for bending.
Following paragraph shows the design procedure.

Allowable bending stress for selected material

(σ ) 310 Mpa [27]
Force along flat 5 470.9 N From (C)
Considering factor of safety 1.2
Maximum force (F) 565.1 N
Length of flat 7 (L) 65 mm
Bending Moment (M) at pin 8 due to above force.
M=F×L 36728.64 N-mm
M
σ =
Z
Bending stress

Required modulus of section (Z) of flat 7 118.5 mm3

bd
Z=
6
Modulus of rectangular section

Assuming thickness (b) of flat as 5 mm

Required width of flat (d) 11.9 mm
It is not possible to use flat of above size hence width of flat considered same as flat 2
in order to enable fixing of pins

Design of pin 8
The pin is considered as made of SAE 1015 hot rolled steel. The pin [part No.
8 of Fig. 3.3] is a fixed pivot for bell crank lever. It is subjected to shear force due to
 58

reaction at the pin. Hence it is designed for shear. Following paragraph shows the
design procedure.

Vertical Force on flat 2 at pin 1 441.5 N from (B)

Considering factor of safety 1.2
Maximum force (F) 529.7 N
Length of flat 2 260 mm
Bending Moment at pin 8 (M) 137732.4 N-mm
Length of flat 3 130 mm
By equating bending moment Force (P) at pin 9 1059.5 N ---- (D)

Reaction at pin 8 for forces F and P R + = ,F  + P 

Reaction at pin 8 1147.8 N
Allowable Yield Strength in shear (S ) 140 Mpa [27]
F
S =
A
Yield Strength in shear

Required cross-sectional area of pin 8 8.2 mm2

π 
A= d
4
Cross-sectional area of round pin

Required diameter of pin 8 3.2 mm

Design of pin 9
The pin is considered as made of SAE 1015 hot rolled steel. The pin [part No.
9 of Fig. 3.3] is connecting pin between flat 3 and pulling bar 10. It is subjected to
shear force due to load of commode and person on commode pot transferred through
bell crank lever. Hence it is designed for shear. Following paragraph shows the design
procedure.

Force on pin 9 (P) 1059.48 N from (D)

Considering factor of safety 1.2
Maximum force (F) 1271.4 N
Allowable Yield Strength in shear (S ) 140 Mpa [27]
F
S =
A
Yield Strength in shear
 59

Required cross-sectional area of pin 9.1 mm2

π 
A= d
4
Area of round pin

Required diameter of pin 3.40 mm

3.2 MECHANICAL DESIGN FOR PULLER

In order to move the commode up, force has to be applied at pin 9.

Considering a common screw mechanism for commode and platform, the puller has
closed length of 290 mm and stroke needed was 300 mm. Hence telescopic puller as
shown in Fig. 3.4 is to be used. This puller is subjected to same tensile load on the pin
9. Following paragraphs describes the design procedure for the same.

Fig. 3.4A: Arrangement of telescopic puller (closed)

Fig. 3.4B: Arrangement of telescopic puller (open)

3.2.1 DESIGN OF BAR, STOPPER AND PIPES

Following paragraph shows the design procedure for various components of

telescopic puller mechanism.
 60

Design of Bar 10
The bar is considered as made of SAE 1015 hot rolled steel. The bar [part No.
10 of Fig. 3.4] is subjected to tensile force. Hence it is designed for tension.
Following paragraph shows the design procedure.

Axial Force on rod 10 (P) 1059.48 N from (D)

Considering factor of safety 1.2
Maximum force (F) 1271.4 N
Modulus of elasticity (E) 206000 Mpa [27]
Assume allowable elongation dl 0.02 mm
F×L
dl =
A×E
Elongation

Length of rod (L) 145 mm

Required cross-sectional area of bar 44.7 mm2
π 
A= d
4
Area of bar

Required diameter of rod 7.5 mm

Design of stopper 11
The stopper is considered as made of SAE 1015 hot rolled steel. The stopper
[part No. 11 of Fig. 3.4] is subjected to shear force. Hence it is designed for shear.
Following paragraph shows the design procedure.

Outer diameter of rod 10 7.5 mm

Circumference of rod 10 23.7 mm
Axial force on rod 10 (P) 1059.48 N From (D)
Considering factor of safety 1.2
Maximum force (F) 1271.4 N
Allowable Yield Strength in shear S 140 Mpa [27]
F
S =
A
Yield Strength in shear

Required shearing area 9.1 mm2

Area = circumference × thickness of stopper
Required Thickness of stopper 0.4 mm
 61

Design of pipe 12
The pipe is considered as made of SAE 1015 hot rolled steel. The pipe [part
No. 12 of Fig. 3.4] is subjected to tensile force. Hence it is designed for tension.
Following paragraph shows the design procedure.

Axial Force pipe 12 (P) 1059.48 N from (D)

Considering factor of safety 1.2
Maximum force (F) 1271.4 N
Modulus of elasticity (E) 206000 Mpa [27]
Assumed allowable elongation (dl) 0.02 mm
F×L
dl =
A×E
Elongation

Length of pipe (L) 183 mm

Required cross-section area of pipe 12 56.5 mm2
π 
A= (D − d )
4
Cross-section area of pipe

Assuming outer diameter (D) 16 mm

Required inner diameter (d) 13.6 mm

Design of pipe 13
The pipe is considered as made of SAE 1015 hot rolled steel. The pipe [part
No. 13 of Fig. 3.4] is subjected to tensile force. Hence it is designed for tension.
Following paragraph shows the design procedure.

Axial Force pipe 13 (P) 1059.48 N from (D)

Considering factor of safety 1.2
Maximum force (F) 1271.4 N
Modulus of elasticity (E) 206000 Mpa [27]
Allowable elongation assumed (dl) 0.02 mm
F×L
dl =
A×E
Allowable elongation

Length of rod (L) 225 mm

Required area of cross-section (A) 69.4 mm2
 62

π 
A= (D − d )
4
Cross-section area of pipe

Assuming outer diameter (D) 25.4 mm

Required inner diameter (d) 23.6 mm

3.3 MECHANICAL DESIGN FOR PLATFORM

The platform lowering / lifting mechanism works on the principle as explained

below. The two extreme positions of the mechanism – platform at lowest and highest
positions are shown in Fig. 3.5A and 3.5B respectively. The platform mechanism is
attached to the screw mechanism by pin 17. The platform mechanism consists of set
of parallelogram links on both sides of platform. The lower three joints are provided
with rollers 15. These rollers are free to move in channel 18. Out of upper three joints
middle one is fixed to center of platform side along the length of the bed and other are
provided with rollers. The upper rollers are free to move in a channel below platform.
When the screw is opened to push the platform towards the hole it the bed, the
platform is free to move towards the hole by rollers 15 on channel 18. The horizontal
travel of platform is restricted by a stopper when it reaches below the hole in the bed.
Further force applied by screw mechanism activates parallelograms and raises the
platform. When the direction of screw rotation is reversed the platform lowers first
and then moved away to clear the hole. Following paragraphs describes the design
procedure for the same.

Fig. 3.5A: Arrangement of platform mechanism (down position)

 63

Fig. 3.5B: Arrangement of platform mechanism (up position)

3.3.1 DESIGN OF PINS, ROLLER, LINK AND MEMBERS

Following paragraph shows the design procedure for various components of
platform mechanism.

Design of pin 14
The pin is considered as made of SAE 1015 hot rolled steel. The pin [part No.
14 of Fig. 3.5] is subjected to shear force due to load of platform and patient. Hence it
is designed for shear. The forces for two extreme positions of the mechanism –
platform at lowest and highest positions are shown in Fig. 3.6A and 3.6B respectively.
Following paragraph shows the design procedure.

Fig. 3.6A: Forces on platform mechanism (down position)

 64

Fig. 3.6B: Forces on platform mechanism (up position)

Assumed mass of platform 5 Kg

Acceleration due to gravity (g) 9.81 m/sec2
Maximum weight of person (W) 120 Kg
Effective weight on platform is assumed 2/3 of total
weight (W) as other body parts are supported. 80 Kg
Total weight on the platform mechanism 85 Kg
Total force on links = g × Weight of platform mechanism 833.9 N
No of points supporting load 2 No
Force on each point (F') [Fig. 3.6] 416.9 N ----(E)
Considering factor of safety 1.2
Maximum force (F) 500.3 N
Allowable Yield Strength in shear (S ) 140 Mpa [27]
F
S =
A
Yield Strength in shear

Required cross-sectional area of pin 3.6 mm2

π 
A= d
4
Area of pin

Required diameter of pin 2.13 mm

 65

Design of roller 15
The roller is considered as made of SAE 1015 hot rolled steel. The roller [part No. 15
of Fig. 3.5] is moving at very low speed on pin14. Hence it is designed for bearing
pressure. Following paragraph shows the design procedure.

Force on each point (F') [Fig. 3.6] 416.9 N from (E)

Considering factor of safety 1.2
Maximum force (F) 500.3 N
Diameter of shaft (d) (Pin 14) 2.13 mm
Bearing pressure (p) 60 N/mm2 [27]
F
p=
td
Bearing pressure

Thickness of roller required (t) 3.9 mm

Design of flat 16
The flat is considered as made of SAE 1015 hot rolled steel. The flat [part No. 16 of
Fig. 3.5] is subjected to compressive load due to weight of platform and person.
Hence it is designed for compressive load. Following paragraph shows the design
procedure.

Effective force coming on mechanism at top center

pin (F') [Fig. 3.6] 416.9 N from (E)
Referring to Fig. 3.6A
Maximum angle (θ1) 72
F2
F2343 =
cos (θ1)
Force along A'B'

Force along A'B' when angle=72 1346.6 N ----(F)

Force along C'B' F8343 = F2 × tan(θ1)
Force along C'B' when angle=72 1280.4 N ----(G)
Referring to Fig. 3.6B
Minimum angle (θ2) 18
F2
F24 =
cos (θ2)
Force along AB
 66

Force along AB when angle=18 438.4 N

Force along CB F84 = F2 × tan(θ2)

Force along CB when angle=18 135.4 N

Taking maximum Force along flat 16 1346.6 N
Considering factor of safety 1.2
Maximum force (F) 1615.9 N
Yield strength in compression (Syc) 310 Mpa [27]
F3
S =
A
Yield strength in compression

Required area cross-section 5.2 mm2

Area of cross-section A= b × t
Assuming thickness (t) of flat 3 mm
Required width (b) of flat 1.7 mm
This size of flat is not available. The diameter of pin
14 is 2.13 mm. This diameter has to be increased to
about 5 mm for availability. Hence width of flat
15 mm
considered as

Checking flat for buckling

Length of flat (L) 70 mm
tb
I=
12
Moment of inertia of flat

Moment of inertia of flat (I) 33.8 mm4

Cross-sectional area of flat 16 A= b x t
Cross-sectional area (A) of flat 16 45 mm2

I
K=
A
Radius of gyration

Radius of gyration (K) = 0.9 mm

L
λ=
K
Slenderness ratio

Slenderness ratio (λ) 80.8

we have to use Euler's formula for λ>90 and Johnsons formula for 10<λ<90.
 67

by Johnsons formula
End fixidity coefficient ( C ) for both end hinged 1.0 [27]
Yield strength in compression (S ) 310 Mpa [27]
Modulus of elasticity (E) 206000 Mpa [27]
S# λ
W = AS !1 − '
4cπ E
Buckling load

Allowable buckling load (W)= 10472.4 N

As effective load coming on flat is much less than buckling load. So selected flat is
confirmed

Design of pin 17
The pin is considered as made of SAE 1015 hot rolled steel. The pin [part No. 17 of
Fig. 3.5] is subjected to bending stress between two cross link mechanisms on sides of
the platform. Hence it is designed for bending load. Following paragraph shows the
design procedure.

Allowable bending stress for selected material 310 Mpa [27]

Force acting horizontally at the pin due to commode
on each side of mechanism 1059.5 N from (D)

Force acting horizontally at the pin due to platform 1280.4 N from (G)

Since the force due to commode mechanism and

platform mechanism are not acting simultaneously,
the maximum force is considered 1280.4 N
Total force acting at centre of beam will be twice
the above force as two mechanisms are exerting
above force on ends of simply supported beam 2560.8 N ---- (H)
Length of simply supported beam (bar) is equal to
distance between two mechanisms or width of
platform 400 mm

As above force is applied at the centre of beam,

distance for moment will be half of length of pin 200 mm
 68

Bending Moment (M) at a fixed point A 512154.9 N-mm

M
σ =
Z
Bending stress

Modulus of section (Z) 1652.1 mm3

π 
Z= d
32
Required modulus of section

Required diameter of pin (d) 25.6 mm

It is not possible to insert a bar of 25 mm diameter in flat 16 of width 15 mm.

The width of flat can be increased to 25 mm considering OD of roller. In this flat max
size of bar can be fixed is 12.7 mm. In order to strengthen bar 17, two supports AC
and BC are proposed as shown in fig. 3.7. This structure is to be checked and
diameter of bar AC/BC is to be determined.

Fig. 3.7: Forces on supports to pin 17 and pipe 22

Design of supports to Bar 17 [Fig. 3.7]

Forces developed while raising platform
Force (F') 2560.8 from (H)
Reactions developed at end A & B as RA & RB
respectively = F'/2 1280.4 N
assume angle (θ) as 60
Forces developed on bar AC (FAC) = reaction/sinθ 1478.9 N In compression
Forces developed on bar AD (FAD)= reaction/cosθ 2558.4 N In tension

If truss is in equilibrium, then each of the joint must be in equilibrium. Hence ∑Fx=0,
 69

∑Fy=0 and moment at any point =0

Taking moment at A and equating to zero FCD × AD – RB × AB = 0

FCD= 2560.8 In tension
As truss is symmetric force developed in link CB and BD is equal to force developed in
AC and AD respectively

Forces developed while raising commode

The forces will be same but members which are in tension will be subjected to
compressive force and vice versa. Hence all members have to be checked for buckling
load.

Member AC and BC
Assume diameter (d) of bar AC 10 mm
πd<
I=
64
Moment of inertia (I) of bar

Moment of inertia (I) 490.6

π
A = d
4
Cross-sectional area of bar

Cross-sectional area (A) 78.5 mm2

I
K=
A
Radius of gyration

Radius of gyration (K) = 2.5 mm

Length of bar (L) 400
L
λ=
K
Slenderness ratio

Slenderness ratio (λ) 159.9

If λ>90 the we have to use Euler's formula , if 10<λ<90 then we have to use Johnsons
formula

End fixidity coefficient ( C ) for both ends fixed 4.0 [27]

Modulus of elasticity (E) 206000 Mpa [27]

C π EL
W=
λ
Buckling load by Euler's formula
 70

Safe Buckling load (W) for bar 17A 3179.4 N

It is more than actual load hence it is confirmed

Member AD and BD
Assumed diameter (d) of bar AC 12.7 mm
πd <
I=
64
Moment of inertia (I) of bar

Moment of inertia (I) 1276.3

π
A = d
4
Cross-sectional area of bar

Cross-sectional area (A) 126.6 mm2

I
K=
A
Radius of gyration

Radius of gyration (K) = 3.175 mm

Length of bar (L) 200 mm
L
λ=
K
Slenderness ratio

Slenderness ratio (λ) 63.0

If λ>90 the we have to use Euler's formula , if 10<λ<90 then we have to use Johnsons
formula
by Johnsons formula
End fixidity coefficient ( C ) for both end hinged 4.0
Yield strength in compression (S ) 310 Mpa [27]
Modulus of elasticity (E) 206000 Mpa [27]
S# λ
W = AS !1 − '
4cπ E
Buckling load

Buckling load (W) 18403.2 N

As effective load coming on flat is much less than buckling load. So selected flat is
confirmed

Design of channel 18
The channel is considered as made of SAE 1015 hot rolled steel. The channel
[part No. 18 of Fig. 3.5] is subjected to bending stress between two supports at the
 71

ends and load of the platform and patient. Hence it is designed for bending load.
Following paragraph shows the design procedure.

Fig. 3.8: Cross section of channel 18

Force on channel, a simply supported beam 833.9 N from (J)

Considering factor of safety 1.2
Maximum force (F) 1000.6 N
Length of channel (L) 500 mm
Modulus of elasticity 206000 Mpa [27]
bh − b h 
I=
12
Moment of inertia (I)

b1 11 mm Fig. 3.8
B 13 mm Fig. 3.8
H 30 mm Fig. 3.8
h1 26 mm Fig. 3.8
Moment of inertia (I) 13138.7
Assume allowable deflection (Y ) 1 mm
FL
Y =
48EI
Allowable deflection

Force (F1) 1039.3 N

It is more than maximum force hence it is confirmed
 72

3.4 MECHANICAL DESIGN FOR TELESCOPIC SCREW

The telescopic screw is used for achieving the motions of platform by using
mechanism shown in fig. 3.5. For applying load F, at the left end of mechanism, this
screw is used. It is also used to lift the commode as shown in Fig. 3.1. It is operated
mannually by rotating a lever connected to the screrw. Arrangement of screw in fully
closed position and fully open position ia shown in Fig. 3.9A and 3.9B respectively.
The screw mechanism is subjected to compressive load while lifting platform and
subjected to tensile load while lifting the commode. Following paragraphs show the
design of various elements of this teleswcopic screw. The dimensions were obtained
from preliminary CAD model.

Fig. 3.9A: Arrangement of telescopic screw (closed position)

Fig. 3.9B: Arrangement of telescopic screw (open position)

 73

Fig. 3.9C: Cross-section of telescopic screw

3.4.1 DESIGN OF SCREW, NUT AND ACCESSORIES

Following paragraph shows the design procedure for various components of
telescopic screw mechanism.

Design of screw 19
The screw is considered as made of SAE 1015 hot rolled steel. The screw [part
No. 19 of Fig. 3.9] is subjected to tensile and compressive load alternatively. Hence it
is designed for compressive load. Following paragraph shows the design procedure.

Allowable Yield Strength in compression (S ) 310 Mpa [27]

Allowable Yield Strength in Tension (Syt) 310 Mpa [27]
Allowable Yield Strength in shear (Sys) 140 Mpa [27]
Force on pin 9 of each commode mechanism 1059.5 from (D)
Total force for lifting commode is double 2119.0 N
Force for moving platform 1280.4 N from (G)
Force on screw maximum of above two as
loads are not simultaneous 2119.0 N ---- (K)
Considering factor of safety 1.2
Maximum force (P) 2542.8 N
P1
S = π 
4 d
Compressive stress (yield)

Core diameter (dc) 3.2 mm

 74

The pitch required for the screw is 5 mm.

Hence depth of the thread will be 2.5 mm
Above diameter will not be able to withstand
cutting forces for required depth and will
buckle during manufacture. Hence The core
diameter is assumed to be equal to that of
normally used screws. 20 mm

Major diameter or outer diameter of nut (do) 25 mm

dE − d
dABCD =
2
Mean diameter of screw

Mean diameter of screw (dmean) 22.5

For single start screw lead (L)=pitch 5 mm
Force on screw (P) 2119.0 N from (K)
Considering factor of safety 1.2
Allowable force 2542.8 N
Coefficient of friction (μ) 0.2 [27]
P × dABCD L + μπdABCD
T= G I
2 πdABCD − μL
Torque required (T)

Torque required (T) to rotate the screw 7856.9 N-mm ---- (L)
The screw is subjected to direct compressive stress and torsional shear stress

P
σ = π
4 dABCD
Compressive stress 

Compressive stress on screw (σ ) 6.4 Mpa

T
τ = π
4 dABCD
Torsional stress 

Torsional stress on screw (τ ) 79.1 Mpa

Principle stresses generated due to torsional and compressive stress are as under
σ 1
σACK = + Lσ  − 4τ 
2 2 
Maximum compressive stress

Maximum compressive stress on screw 82.3 Mpa

1
τACK = Lσ  − 4τ 
2 
Maximum shear stress
 75

Maximum shear stress on screw 79.1 Mpa

The principle stresses are below the allowable stresses. Hence the design is safe.
Checking the screw 19 for buckling
Length of screw 380 mm
Nominal screw diameter or mean diameter 22.5 mm
πdABCD <
I=
64
Moment of inertia of screw

Moment of inertia of screw (I) 12574.2

π
A= dABCD 
4
Cross-sectional area

Cross-sectional area (A) 397.4

I
K=
A
Radius of gyration

Radius of gyration (K) = 5.625 mm

L
λ=
K
Slenderness ratio

Slenderness ratio (λ) 67.6

End fixidity coefficient ( C ) 1.00 [27]
Modulus of elasticity (E) 206000.0 Mpa [27]
S λ
W = AS !1 − '
4cπ E
Buckling load

Buckling load (W)= 101742.7 N

As buckling load is greater than actual load. Hence the screw is safe.

Design of nut 20
The nut is considered as made of SAE 1015 hot rolled steel. The nut [part No.
20 of Fig. 3.9] is subjected to crushing and shear at teeth. Hence it is designed for
both. Following paragraph shows the design procedure.
The cross section of telescopic screw is shown in Fig. 3.9C. The core diameter
of screw 19 is 20 mm. Outer diameter of screw 19 is 25 mm. The nut 20 will have
root diameter of 25 mm. We need to keep minimum 2.5 mm wall thickness for nut.
Hence outer diameter of nut will be 30 mm. This nut will match with core of hollow
screw 21 having outer diameter of 30mm and inner diameter of 25mm. The hollow
 76

screw 21 will have core diameter of 30mm and outer diameter of 35 mm. The nut for
hollow screw will have inner diameter of 30mm and root diameter of 35 mm. Adding
for wall thickness of 2.5 mm makes outer diameter of nut to 40 mm. This nut will
match with pipe 22.hence the pipe 22 will have inner diameter of 35 mm and outer
diameter of 40 mm.

Allowable Bearing pressure (Pb) 10.0 N/mm2 [27]

P1
P = π
4 MdE − d Nn
Bearing pressure  

No of threads (n) required 1.4

In order to maintain the screw and nut in the same axis.
No of threads (n) selected 5
Length of nut = n x pitch
Length of nut(H) 25 mm
The nut is subjected to crush and shear
P1
σOPQRDS = π
4 MdE − d Nn
Crushing stress  

Crushing stress in compression on nut (σOPQRDS) 0.35 Mpa

As crushing stress is less than yield strength in compression. Hence the nut is safe in
crushing.
Checking for teeth failure of nut
P1
σDP# =
pitch
πdE n 2
Shear stresses on threads of nut

Shear stresses on threads of nut 2.6 Mpa

As shear stress is less than yield strength in shear. Hence the nut is safe in shear.

Failure of teeth in screw

P1
σOBU =
pitch
πd n 2
Shear stresses on threads of screw

Shear stresses on threads of screw 3.2 Mpa

As shear stress is less than yield strength in shear. Hence the screw is safe in shear.
As all the conditions are satisfied. Hence design of screw 19 and nut 20 is safe.
 77

Design of hollow screw 21

The screw is considered as made of SAE 1015 hot rolled steel. The screw [part
No. 21 of Fig. 3.9] is subjected to tensile and compressive load alternatively. Hence it
is designed for compressive load. Following paragraph shows the design procedure.
This screw has same threading as that of screw 19. Both are subjected to same
force. Hence same threads can be used safely. The nut for the screw is bigger in dia
than nut 20. Hence it is also safe to use same pitch of nut. The screw has to be
checked for buckling.
The outer dimeter of nut 20 is 29 mm. This will be core diameter for hollow
screw. Adding depth of thread on both sides, outside diameter of hollow screw will be
34 mm.

Checking hollow screw 21 for Buckling [Fig. 3.9]

Length of screw 330 mm
Outer diameter (D) 30 mm
Inner diameter (d) 25
π(D − d )
I=
64
Moment of inertia of screw

Moment of inertia of screw (I) 20575.6

π
A = (D − d )
4
Cross-sectional area

Cross-sectional area (A) 215.9

I
K=
A
Radius of gyration

Radius of gyration (K) = 9.8 mm

L
λ=
K
Slenderness ratio

Slenderness ratio (λ) 33.8

End fixidity coefficient ( C ) 1.00 [27]
Yield strength in compression 310 Mpa [27]
Modulus of elasticity (E) 206000.0 Mpa [27]
 78

S# λ
W = AS# !1 − '
4cπ E
Buckling load

Allowable buckling load (W) on screw 64003.7 N

As allowable buckling load is greater than actual load. Hence the screw is safe.

Design of pipe 22
The pipe is considered as made of SAE 1015 hot rolled steel. The pipe [part
No. 21 of Fig. 3.9] is subjected to tensile and compressive load alternatively. Hence it
is designed for compressive load. Following paragraph shows the design procedure.
Outer diameter of hollow screw is 34 mm. Adding 2 mm wall thickness of nut
on both sides. The outer diameter of pipe 22 has to be 38 mm. As standard 40 mm
diameter is available it can be used here.

Buckling of pipe 22 [Fig. 3.9]

Length of pipe 315 mm
Outer diameter (D) 40 mm
Inner diameter (d) 35
π(D< − d< )
I=
64
Moment of inertia of screw

Moment of inertia of screw (I) 51975.6

π
A = (D − d )
4
Cross-sectional area

Cross-sectional area (A) 294.4

I
K=
A
Radius of gyration

Radius of gyration (K) = 13.3 mm

L
λ=
K
Slenderness ratio

Slenderness ratio (λ) 23.7

End fixidity coefficient ( C ) for both end hinged 1.0 [27]
Allowable yield strength in compression (S ) 310 Mpa [27]
Modulus of elasticity (E) 206000.0 Mpa [27]
 79

S λ
W = AS !1 − '
4cπ E
Buckling load

Allowable buckling load (W) on the pipe 89299.4 N

As allowable buckling load is greater than actual load. Hence the pipe is safe.

Fig. 3.10 Cross section of pipe at pin

Crushing of pipe 22 at Pin 17 [Fig. 3.10]

Force on pipe 2119.0 from (K)
Considering factor of safety 1.2
Maximum force on pipe (F) 2542.8 N
Allowable yield strength in compression
(S) 310 Mpa [27]
Thickness of pin (t) 12.7 mm Fig. 3.10
Outer diameter (d1) 40.0 mm Fig. 3.10
Inner diameter assume (d2) 35.0 Mm Fig. 3.10
π
F = V W(d ) − (d ) X − (d − d )tY S
4
Allowable Force

Allowable Force 71571.25 N

As allowable force is greater than actual force on the pipe. Hence the pipe is safe.

Design of thrust bearing 23

The thrust bearing is considered as made of SAE 1015 hot rolled steel. The
thrust bearing [part No. 23 of Fig. 3.9] is subjected to thrust load. Hence it is designed
for bearing pressure. Following paragraph shows the design procedure.

Force transmitted over bearing surface (F) 2119.0 Mpa from (K)
 80

Considering factor of safety 1.2

Allowable force 2542.8 N
Assuming number of collars (n) 1 No
Outer radius of collar ( R ) 25 mm
Inner radius of collar ( r ) 20 mm
Cross-sectional area of bearing surface A = nπ(R − r  )
Cross-sectional area of bearing surface (A) 706.5
Allowable bearing pressure for low speed 10 N/mm2 [27]
F
p=
A
Bearing pressure

Bearing pressure (p) 3.6 N/mm2

As actual bearing pressure is less than allowable bearing pressure the bearing is safe

Design of bolt 24
The bolt is considered as made of SAE 1015 hot rolled steel. The bolts [part
No. 24 of Fig. 3.9] two in quantity, are holding thrust bearing in pipe 25. They are
subjected to shear load due to axial load on screw. Hence it is designed for shear.
Following paragraph shows the design procedure.

Force coming on bolt 2119.0 from (K)

Considering factor of safety 1.2
Maximum force 2542.8 N
No of bolts taking force 2 No
Force on each bolt (F) 1271.4
Allowable Yield Strength in shear (S ) 140 Mpa [27]
F
S =
A
Yield Strength in shear

Required cross-sectional area of bolt 9.1 mm2

π 
A= d
4
Cross sectional Area of bolt

Required diameter of bolt 3.4 mm

 81

Design of pipe 25
The pipe is considered as made of SAE 1015 hot rolled steel. The pipe [part
No. 25 of Fig. 3.9] is subjected to crushing at bolt 24. Hence it is designed for
crushing. Following paragraph shows the design procedure.

Force (F) 2119.0 from (K)

Considering factor of safety 1.2
Maximum force (F) 2542.8 N
Allowable Yield Strength in compression
(S )
310 Mpa [27]

Thickness of pin (t) 3.4 mm Fig. 3.10

Outer diameter (d1) 50.0 mm Fig. 3.10
Inner diameter assume (d2) 45.0 Fig. 3.10

F = V< W(d ) − (d ) X − (d − d )tY S

[
Force

Allowable Force 110319.3 N

As allowable force is greater than force on the pipe. Hence the pipe is safe.

Design of lock pin 26

The pin is considered as made of SAE 1015 hot rolled steel. The pin [part No.
26 of Fig. 3.9] is used to lock bush 28 on screw shaft. It is subjected to shear force due
to axial load on screw. Hence it is designed for shear. Following paragraph shows the
design procedure.

Force (F) 2119.0 from (K)

Considering factor of safety 1.2
Maximum force 2542.8 N
No of bolts taking force 1 No
Force on each bolt (F) 2542.8
Allowable Yield Strength in shear (S ) 140 Mpa [27]
F
S =
A
Yield Strength in shear

Required cross-sectional area of pin 18.2 mm2

 82

π 
A= d
4
Cross-sectional area of pin =

Required diameter of pin 4.81 mm

Design of bolt 27
The bolt is considered as made of SAE 1015 hot rolled steel. The bolt [part
No. 27 of Fig. 3.9] is used to fix the pipe 25 on bed frame. It is subjected to shear
force due to axial load on screw. Hence it is designed for shear. Following paragraph
shows the design procedure.

Force (F) 2119.0 from (K)

Considering factor of safety 1.2
Maximum force 2542.8 N
No of bolts taking force 1 No
Force on each bolt (F) 2542.8
Allowable Yield Strength in shear (S ) 140 Mpa [27]
F
S =
A
Yield Strength in shear

Required cross-sectional area of pin 18.2 mm2

π 
A= d
4
Cross-sectional area of pin

Required diameter of pin 4.81 mm

Design of bush 28
The bush is considered as made of SAE 1015 hot rolled steel. The bush [part
No. 28 of Fig. 3.9] is fixed to the screw shaft by pin 26. It is subjected to crushing
force due to axial load on screw. Hence it is designed for compression. Following
paragraph shows the design procedure.

Force (F) 2119.0 from (K)

Considering factor of safety 1.2
Maximum force (F) 2542.8 N
Allowable Yield Strength in compression
(S ) 310 Mpa [27]
 83

Thickness of pin (t) 6 mm

Outer diameter (d1) 25.0 mm
Inner diameter assume (d2) 17.0
π
F = V W(d ) − (d ) X − (d − d )tY S
4
Force

Allowable Force 66885.6 N

As allowable force is greater than force on the pipe. Hence the pipe is safe.

Design of handle bar 29

The handle is considered as made of SAE 1015 hot rolled steel. The handle
[part No. 29 of Fig. 3.9] is used to rotate screw manually. It is subjected to bending
force. Hence it is designed for bending. Following paragraph shows the design
procedure.

Normal Length of handle for hospital beds 140 mm

Torque coming on shaft 7856.9 N-mm from (L)
Considering factor of safety 1.2
Maximum torque (T) 9428.3 N-mm
Force to be applied on handle 67.3 N
This force can be easily applied by hand. Hence the length of handle is confirmed.
The diameter of handle is to be calculated on basis of torsional load.
π <
J= d
32
Polar moment of inertia

Torsional stress (τ) 50% of shear strength 70 N/mm2

T×r
τ=
J
Torsional stress (τ)

16 × T
τ=
π × d
Torsional stress

Required diameter of handle bar 8.8 mm

Now the diameter of handle is to be calculated on basis of bending load.
Allowable bending stress (σ ) 310 Mpa [27]
Force applied but a person during rotation of
67.3 N
handle
Maximum length of handle 140 mm
 84

Bending Moment at a fix point (M) 9428.3 N-mm

M
σ =
Z
Bending stress

Required modulus of section (Z) 30.4 mm3

π 
Z= d
32
Modulus of section

Required diameter of shaft (d) 6.8 mm

Considering maximum diameter required 8.8 mm

3.5 DESIGN OF MECHANISM FOR BACKREST

The backrest is raised by a screw and nut mechanism, which when rotated by
handle pulls the flat plates 33 towards head. These flats are attached to pipe 32 by pin
34. The pipe 32 is attached to a U shaped pipe 30. The backrest is lifted up by turning
about pin 31. This mechanis is shown in Fig. 3.11. Following paragraphs show the
design of various elements of this backrest mechanism.

Fig. 3.11: Arrangement of various parts backrest mechanism

 85

3.5.1 DESIGN OF MEMBERS AND LINKS

Following paragraph shows the design procedure for various components of
backrest mechanism.

Design of pipe 30
The pipe is considered as made of SAE 1015 hot rolled steel. The pipe [part
No. 30 of Fig. 3.11] is a frame of backrest with sheet on it. It is free to move up on
two pins 31. When backrest raises from level the pipe on both sides are subjected to
bending force. Hence it is designed for bending. Following paragraph shows the
design procedure.

Acceleration due to gravity 9.81 m/sec2

Assumed weight of a person (W) 120 Kg
Effective weight on backrest is considered as
2/3W, as part of body is supported by backrest. It
is considered to be acting at the center of backrest. 80 Kg
Force acting on backrest (F) 784.8 N ----(M)
When the backrest is lifted from horizontal, No of
pipes on which force F is acting 2 No
Actual force on each pipe = F/2 392.4 N ---- (N)
Considering factor of safety 1.2
Maximum force on a pipe 470.9 N
Modulus of elasticity (E) 206000 Mpa [27]
length of cantilever beam (L) = ½ backrest length 380 mm
Assuming allowable deflection (Yb) 5 mm
FL
Y =
3EI
Deflection at free end

Required moment of inertia (I) for pipe 8361.9 mm4

π(D< − d< )
I=
64
Moment of inertia of round pipe

(D4-d4) 170432.7
Outer diameter (D) is selected as 25.4 mm
Required Inner diameter (d) of pipe 22.3 mm
 86

Design of pin 31
The pin is considered as made of SAE 1015 hot rolled steel. The pin [part No.
31 of Fig. 3.11] is subjected to shear force due to the load of person. Hence it is
designed for shear. Following paragraph shows the design procedure.

Force (F) 392.4 N from (N)

Considering factor of safety 1.2
Maximum force 470.9 N
F
S =
A
Yield Strength in shear

Allowable Yield Strength in shear (Sys) 140 Mpa [27]

π 
A= d
4
Area of round pin

Required diameter of pin 2.07 mm

Design of rectangular pipe 32

The pipe is considered as made of SAE 1015 hot rolled steel. The pipe [part
No. 32 of Fig. 3.11] is subjected to torsion due to the load of patient. Hence it is
designed for torsion. Following paragraph shows the design procedure.

Force (F) acting on backrest 784.8 kg from (M)

Considering factor of safety 1.2
Maximum force 941.8 N
Distance of force (F) from pipe 32 380 mm
Torque (T) acting on the pipe 357868.8 N-mm
This pipe is assumed to be rectangular in shape for easy manufacture. The dimensions
of pipe are considered as under.

Fig. 3.12 Cross section of rectangular pipe

 87

H 50 Fig. 3.12
Hi 44 Fig. 3.12
W 25 Fig. 3.12
Wi 19 Fig. 3.12
radius ( r ) 28.0 mm Fig. 3.12
H×W  Hi × Wi
J= (H + W  ) − (Hi + Wi )
12 12
Polar moment of inertia

Polar moment of inertia (J) 165496.5

Allowable torsional stress (τ) 50% of shear
stress 70 N/mm2
T×r
τ=
J
Torsional stress (τ)

Torsional stress coming on material (τ) 60.4

As allowable torsional stress is greater than torsional stresses on the pipe. Hence the
pipe is safe.

Design of flat 33
The flat is considered as made of SAE 1015 hot rolled steel. The flats [part
No. 33 of Fig. 3.11] are subjected to bending stress due to the force applied by screw
mechanism. These flats are kept inclined at 300 in order to get optimum transmission
angle for total angular motion of 750-800. Hence it is designed for bending. Following
paragraph shows the design procedure.

Force on backrest 784.8 N From (M)

Considering factor of safety 1.2
Maximum force 941.8 N
Distance between hinge to a point at which force is
applied on backrest (l) 380 mm
The standard screw assembly available has travel
of 150 mm. The angular motion of the flat is
similar to angular motion of backrest. Hence
150 mm
length of flat is
Equating moment about hinge (31) of backrest
 88

Force (F) acting at pin 34 2385.8 N ----(O)

Force acting on each flat 1192.9
Bending moment (M) on flat 178934.4 N-mm
Allowable bending stress for selected material 310 Mpa [27]
M
σ =
Z
Bending stress

Required modulus of section (Z) of flat 577.2 mm3

bd
Z=
6
Modulus of section

Considering thickness of flat (b) = 5 mm

Required width of flat (d) is 26.3 mm

Design of pin 34
The pin is considered as made of SAE 1015 hot rolled steel. The pins [part No.
34 of Fig. 3.11] are subjected to shear due to the force applied by screw mechanism.
Hence it is designed for shear. Following paragraph shows the design procedure.

Angle of inclination (θ) 30 Degree

Force (F) 2385.8 N From (O)
Considering factor of safety 1.2
Allowable force 2863.0 N
Force F = FK × cosθ
Fx 3305.3 N ----(P)
This is the axial force required along x direction to pull the mechanism of backrest.
Same force is coming on the pin (34)
Force (W) 3305.3 N from (P)
Allowable Yield Strength in shear
(Sys = W/A) 140 Mpa [27]
2
Area 23.6 mm
π 
A= d
4
Area

Diameter (d) required 5.5 mm

 89

Design of pipe 35
The pipe is considered as made of SAE 1015 hot rolled steel. The pipe [part
No. 35 of Fig. 3.11] is subjected to tensile load due to the force applied by screw
mechanism. The pipe is also subjected to crushing at pin 34. Area subjected to
crushing is less than area subjected to tension. Hence it is designed for crushing at pin
34. Following paragraph shows the design procedure.

Force 3305.3 From (P)

Considering factor of safety 1.2
Maximum force (F) 3966.4 N
Yield Strength in compression 310 Mpa [27]
Thickness of pin 34 3.8 mm Fig. 3.10
Consider outer diameter d1 from
available size 32 mm Fig. 3.10
Consider Inner diameter d2 from
available size 29.5 mm Fig. 3.10
π
F = V W(d ) − (d ) X − (d − d )tY S#
4
Allowable force

Allowable force (F) 33164.8 N

As allowable force is greater than force on the pipe. Hence the pipe is safe.

Design of screw 36
The screw is considered as made of SAE 1015 hot rolled steel. The screw [part
No. 36 of Fig. 3.11] is subjected to tensile load due to the weight of backrest and
person on it. Hence it is to be designed for tension. However we have designed screw
19 for compressive load which is more than the tensile load on this screw. Hence
same size of screw, nut, bolts and handle can be used safely.

3.6 DESIGN OF MECHANISM FOR LEGREST

The legrest is lowered and raised by a screw and nut mechanism, which is
similar to that of backrest. As the legrest is subjected to lesser load than backrest same
design can be used for legrest for ease of manufacture.
 90

3.7 DESIGN OF FOOT BOARD CUM TABLE

The foot board is capable to turn about pin 40. It can be used as table for
various applications while sitting. The height of table can be adjusted by telescopic
arrangement of bar 38 and pipe 39. It can be lockes at desired height by knob.
Following paragraphs show the design of various elements of this table.

Fig. 3.13: Arrangement of foot board cum utility table

3.7.1 DESIGN OF MEMBERS AND PINS

Following paragraph shows the design procedure for various components of
foot board cum utility table.

Design of horizontal pipe 37

The pipe is considered as made of SAE 1015 hot rolled steel. The pipe [part
No. 37 of Fig. 3.13] is subjected to bending load due to the force applied by person.
Hence it is designed as cantilever beam. Following paragraph shows the design
procedure.

Weight of person (W) 120 kg

Effective weight on table is assumed as W/3, as major part
of body weight is on bed. 40 kg
Force acting 392.4 N
No of pipes on which force F is acting 2 No
 91

Actual force on each pipe 196.2 N ----(Q)

Considering factor of safety 1.2
Maximum force acting on each pipe (F) 235.4 N
Modulus of elasticity (E) 206000 Mpa [27]
Length of beam (L) 360 mm
Deflection at free end (Yb) 5 mm
FL
Y =
3EI
Deflection at free end

Required moment of inertia (I) for pipe 3554.9 mm4

π(D< − d< )
I=
64
Moment of inertia

(D4-d4) 72456.9
Outer diameter (D) selected from available sizes 25.4 mm
Required inner diameter (d) 24.2 mm

Design of vertical bar 38

The bar is considered as made of SAE 1015 hot rolled steel. The bar [part No.
38 of Fig. 3.13] is subjected to bending load due to the force applied by person. Hence
it is designed as cantilever beam. Following paragraph shows the design procedure.

Force acting (F) 196.2 N from (Q)

Considering factor of safety 1.2
Maximum force 235.4 N
Modulus of elasticity (E) 206000 Mpa [27]
Length of beam (L) 300 mm
Considering deflection at free end (Yb) 10 mm
FL
Y =
3EI
Deflection at free end

Required moment of inertia (I) 1028.6 mm4

πd<
I=
64
Moment of inertia

Required outer diameter (D) required 12.0

 92

Design of vertical pipe 39

The pipe is considered as made of SAE 1015 hot rolled steel. The pipe [part
No. 39 of Fig. 3.13] is subjected to bending load due to the force applied by person.
Hence it is designed as cantilever beam. Following paragraph shows the design
procedure.

Force acting (F) 196.2 N from (Q)

Considering factor of safety 1.2
Maximum force 235.4 N
Modulus of elasticity (E) 206000 Mpa [27]
Length of beam (L) 435 mm
Deflection at free end (Yb) 10 mm
FL
Y =
3EI
Deflection at free end

Moment of inertia (I) 3135.9 mm4

π(D< − d< )
I=
64
Moment of inertia

(D4-d4) 63916.0
Outer diameter (D) selected from available
size 25.4
Required inner diameter (d) of pipe 24.4 mm

Design of pin 40
The pin is considered as made of SAE 1015 hot rolled steel. The pin [part No.
40 of Fig. 3.13] is subjected to shear force due to the force applied by person. Hence it
is designed as shear. Following paragraph shows the design procedure.

Force at the end of cantilever 196.2 N from (Q)

Considering factor of safety 1.2
Maximum force 235.4 N
Length of vertical from lock point beam (l) 435 mm
Distance of lock pin from bolt 40 mm
Shear force coming on pin (F) 2560.4 N
 93

Allowable Yield Strength in shear 140 Mpa [27]

F
S =
A
Yield Strength in shear

Required cross-sectional area of pin 18.3 mm2

π 
A= d
4
Cross-sectional area of round pin

Required diameter of pin 4.8 mm

3.8 DESIGN OF BASIN STAND

The the basin stand is fixed in holder along the bed length. It can be turnrd on
bed as and when required. The height of stand can be adjusted by telescopic
arrangement of bar 41 and pipe 42. It can be lockes at desired height by knob.
Following paragraphs show the design of various elements of this stand.

Fig. 3.14: Arrangement of basin stand

3.8.1 DESIGN OF MEMBERS

Following paragraph shows the design procedure for various components of
basin stand.
 94

Design of horizontal bar 41

The bar is considered as made of SAE 1015 hot rolled steel. The bar [part No.
41 of Fig. 3.14] is subjected to bending load due to water in basin. Hence it is
designed as cantilever beam. Following paragraph shows the design procedure.

Effective weight on basin 15 kg Assumed

Force acting (F) 147.15 N ----(R)
Considering factor of safety 1.2
Maximum force 176.6 N
Modulus of elasticity (E) 206000 Mpa [27]
Length of beam (L) 370 mm
Considering deflection at free end (Yb) 10 mm
FL
Y =
3EI
Deflection at free end

Required moment of inertia (I) for bar 1447.3 mm4

πd<
I=
64
Moment of inertia

Required outer diameter (D) of bar 13.1

Design of vertical pipe 42

The pipe is considered as made of SAE 1015 hot rolled steel. The pipe [part
No. 42 of Fig. 3.14] is subjected to bending load due to the weight of basin. Hence it
is designed as cantilever beam. Following paragraph shows the design procedure.

Force acting (F) 147.2 N from (R)

Considering factor of safety 1.2
Maximum force 176.6 N
Modulus of elasticity (E) 206000 Mpa [27]
Length of beam (L) 350 mm
Deflection at free end (Yb) 10 mm
FL
Y =
3EI
Deflection at free end
 95

Required moment of inertia (I) 1225.1 mm4

π(D< − d< )
I=
64
Moment of inertia

(D4-d4) 24969.4
Outer diameter (D) selected from available sizes 25.4
Required inner diameter (d) for pipe 25.0 mm

The mechanican design obtained is to be used to create a CAD model, which

is discussed in next chapter.
CHAPTER – 4
 96

CAD MODELLING, FABRICATION

AND FIELD TRIALS

4.1 CAD MODELING

A CAD model based on the design calculations and mechanisms selected was
prepared. The normal components of bed like frame, legs, bows, hardware were taken
as they are in use. The components were also selected from list of available
components. The sizes of designed components were modified for various aspects.
PRO-Engineer software was used for the modeling. All components were
drawn initially. Components were assembled to get subassemblies like, backrest,
legrest, bed frame, screw assembly, platform mechanism, commode mechanism. All
subassemblies were assembled for final product. The assembled product was checked
for matching of dimensions, any interference or gaps were eliminated.

The mechanisms were given motions in order to check the resultant positions
required. It was observed that
1) The stopper to the lower front roller was not feasible. The roller was travelling to
extreme right position before platform starts lifting and then again coming back to
raise the platform. To stop the platform a vertical stopper touching at center of
platform was introduced to stop horizontal motion of platform when it reaches at
its extreme right position.
2) In lowering stroke of platform two vertical stoppers were added in order to avoid
horizontal motion of platform till it reaches its lowest vertical position. These
stoppers were also used to restrict vertical motion of platform before it reaches at
the extreme right position. This does not allow motion of platform in a curved
path and fouling of mattress piece on platform with bed can be avoided.
The parts were modified accordingly and CAD model was completed. Fig. 4.1
to 4.3 shows the various views of CAD model.
 97

Fig. 4.1: CAD model of bed showing sleeping position- backrest down, legrest up.

Fig. 4.2: CAD model of bed showing commode position- backrest up, legrest
down, commode up,

Fig 4.3: CAD model of bed showing chair position- backrest up, legrest down,
platform up, utility table up.
 98

4.2 FABRICATION OF PROTOTYPE

Based on the design calculations and CAD Model the fabrication of prototype
was taken up. During fabrication following problems were faced and overcome

1) While manufacturing the telescopic screw the middle part which has internal and
external threading posed a challenge of boring to such long length from one side
and internal threading from other side. This was resolved by using two separate
pieces one with internal bore and another with internal threads. Both pieces were
welded together. The joint was machined for finish and the external threads were
cut to get the middle part.

2) The small size commode pot with washing jet, flush and arrangement for
attaching lifting links was a challenge. Making it in a proper shape without any
die or tools was very difficult. A tube was bent in desired shape. It was drilled at
various locations which, provided as outlet for flushing water. One big hole was
drilled and a tube at right angle was welded as inlet for water from cistern. Pieces
of sheet were hand bent and welded to achieve rough shape of commode pot.
They were welded by MIG welding to avoid leakage. A 3 mm dia. pipe was
inserted in tubular ring just below top level of commode to work as washing jet
and the pipe was bent down to avoid interference with hole in bed when the pot
moves up through the hole. A PVC trap was attached by means of a clamp on a
metal ring welded to bottom of commode pot. A seal out of a rubber sheet was
provided in between PVC trap and metal ring to avoid water leakage.

3) Instead of manufacturing of thrust bearing a roller contact thrust bearing which

was available was used in the screws.

4) In initial drawings the screw has telescopic cover. However need of support to bar
(17) during design did not allow for telescopic cover. Hence it could not be
provided.
 99

5) The vertical pipe of dining table was not designed for crushing at stopper. A 5 mm
thick flat piece was welded to pipe to avoid crushing.

6) The cost of fabrication of the model was about Rs. 50,000/-. This is higher than
the expectations of the users as revealed from the survey presented in chapter 1.
This cost was on the higher side as it was a development of working model as per
the design. The fabrication involved number of modifications and was done
without any special tools and equipments to produce special components. This
cost will be significantly lowered if the product is manufactured on a large scale
by using special tools and equipments, and it is expected to meet the expectations
of the users.

7) For the fabrication of the bed, time and work study was not performed as it was a
development work. This study can be taken up at the time of manufacturing the
bed on large scale. This study will help to further bring down the cost of the bed.

The Images 4.1 to 4.6 shows actual product adjusted at various positions

Image 4.1: Product at sleeping position- backrest down, legrest up,

platform up.
 100

Image 4.2: Product at chair position- backrest up, legrest down,

platform up.

Image 4.3: Product at commode position- backrest up, legrest down,

commode up.

Image 4.4: Product at chair position with utility table up.

 101

Image 4.5: Product with chair position with basin.

Image 4.6: Telescopic screw arrangement.

Videos of the product showing the actual working are available in a C.D. at
the end of this dissertation.
 102

4.3 VALIDATION OF PROTOTYPE

It was decided that the prototype is to be tested for the design load and
motions. The prototype may be tested by trial use by a patient to get user’s feedback.

4.3.1 TESTING OF PROTOTYPE

The manufactured prototype was tested on actual load and various

observations were noted as shown in Table 4.1.

Table 4.1: Results of testing of prototype

Sr. As per Design / CAD
Component Observed in product
No model
Screw No. Thread Screw No. Time Weight
travel of pitch in travel of in of patient
in mm Rev. mm Rev. sec. in Kg
1 Backrest 150 30 5 145 29.5 15-20 120
2 Legrest 150 30 5 150 30.5 15-20 120
Commode &
3 platform 570 114 5 587 116 80-95 90

1) It was observed that the motions are similar as in the CAD model and are suitable
for use.
2) It was observed that more number of revolutions / more screw travel is required
for the commode and platform mechanism. It was due to the clearances in joints in
commode mechanism and platform mechanism.
3) The actual load was applied on the commode and platform mechanism was lesser
than design load for non availability of person of that weight.
4) The utility table was tested by applying load while person is in sitting position.
The deflection noted was within acceptable range.
5) The basin was checked for suitability of position of use and it was found
satisfactory.
 103

6) It was observed that, when backrest is lifted the mattress is pushed towards leg
side. This resulted in fouling of mattress with the commode pot and piece of
mattress on platform. In order to eliminate the problem backrest has to be rotated
in such a way that the mattress is not shifted. For this the hinge of backrest is to be
shifted above bed level by mattress thickness. This solved the problem.

4.3.2 FEEDBACK OF USER

The prototype was given for use to a patient. It was installed at his house. All
piping and drainage connections were completed. During initial use it was observed
that there is water leakage. It was noticed that the leakage is from the joint between
sheet pieces and the tube used for top ring of the pot. As the water from flush was
injected from the holes in the top ring it was splashing out through gaps. This gap was
sealed by sealant and problem was solved.
The motions and mechanisms were working properly giving position required
by patient. The effort required to handle the patient is reduced to great extent.
The user is satisfied with the product as he has used it for 5 months and shows
that the patient has received good relief from the product. There is great reduction in
the effort required by the assistant and the time of assistance is also reduced.
The user has specifically pointed out about the quality of the commode pot. It
is expected that the commode pot should have smooth surface for cleanliness, easy
flushing and should not have any leakages. The manufacture of commode was a
challenge for prototype. This problem can be resolved by making dies for commode
for mass production.
The feedback was received from the user and is available in annexure-1.
 104

4.4 RESULTS

1) From the survey conducted it was found that there was a need to design a new
bed for bedridden healthcare. This bed should be designed as a single unit with
facility of attaching commode, having various features like commode,
backrest, basin, table, rack etc.
The requirements of the product were
• A bed with an opening in its structure and mattress.
• The opening is to be covered by a strong matching part with piece of mattress
on it.
• The piece of mattress should move down in order to avoid friction to the body
of the patient. A mechanism to move the piece of mattress in suitable direction
below the bed level to clear the opening.
• A light weight commode to minimize power requirement, attached to bed
which shall come up through the opening in bed to support buttocks of the
patient.
• Sitting position by moving backrest up by mechanism.
• Chair position achieved by moving leg part downwards.
• Mechanisms to be operated by handle / electric motor / both.
• Flushing cistern for flushing the toilet and cleaning water jet in commode to
help washing operated by a valve with suitable water connection.
• Commode with anti odour water trap to prevent foul smell. The trap is to be
adjustable as per site need for connection to sewer line by flexible pipes.
• The controls to be patients reach & positions adjustable according to the
patient.
• A basin with water connection for use of patient.
• A height adjustable multipurpose table in front of patient.
• Minimum number of drives to be used in order to reduce cost and easy
operation.
• Storage rack for needs like medicine, water, towel, tissue paper, toothbrush,
toothpaste, glass, spoon, spittoons etc.
• Cost of such equipment should be affordable in general.
 105

2) From literature review it was observed that the design approaches focused on
the use of automated devices/systems, consisting of sophisticated sensors with
electronic or computer controls. As a result, the cost of such beds will be quite
high and also the patient or caretaker needs to become familiar to the
sophisticated controls for using such systems. Most of the products are
designed for use in hospitals.
Thus, these beds are not suitable in the Indian context and there is a need
to develop a simple, low cost alternative for bedridden healthcare for the
Indian population which can be easily used at home also. Till today, no
equipment is available in India at affordable cost, having simple mechanisms.

3) The patent search shows that there are various problems associated with them
as under
• The patient has to move from the bed for which lot of assistance is required
and it may not be suitable for patients recommended bed rest for various
reasons. [Fig. 1.13]
• The container is manually kept and removed. The handling of pot with odour
is not comfortable for assistant. [Fig. 1.13, 1.14, 1.15, 1.23, 1.25]
• To fix pot from lower side an attendant has to bend down which is
uncomfortable. [Fig. 1.13, 1.14]
• The flaps in [Fig 1.14] or the piece of mattress in [Fig. 1.15, 1.23] cannot be
opened without lifting the patient which involved higher level of effort. Again
the opening is to be closed by lifting patient resulting in double handling.
• The legs are lifted first then patient is turned in chair position. It needs more
time and more mechanisms are involved. [Fig. 1.16]
• Only vertical sitting position is obtained it may be uncomfortable for patient.
[Fig. 1.16]
• Patient is likely to get jerks when turned due to gap between hip & seat. The
turning in this manner may not be allowed for many patients. [Fig. 1.16]
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• The Intricate mechanisms/ air assisted mechanisms/ motors results in high

initial cost and high maintenance costs. [Fig. 1.16, 1.17, 1.18, 1.20,1.21, 1.22,
1.23]
• The balloons have certain life and needs maintenance and replacement. [Fig.
1.17]
• Operating costs are high for pneumatic systems as balloons are normally
inflated. [Fig. 1.17]
• Level of toilet pot is below mattress level which creates gap in buttock &
toilet. This may result in splashing of waste material. [Fig. 1.18, 1.19, 1.24]
• The part of mattress is moved against patient weight which will generate
rubbing action on hips. Alternatively the patient has to be shifted to reduce the
load on mechanism and avoid rubbing. The shifting of patient needs
assistance. [Fig. 1.19, 1.21, 1.24]
• No provision of backrest hence, sitting position is not available for patient.
[Fig. 1.19]. The backrest is not coming up to sitting position. [Fig. 1.20]
• Toilet has certain width. The legs are spread widely leading to uncomfortable
position. Less spread results in gap between buttocks and toilet. Alternatively,
legs are to be kept vertical without any support leading to strain on legs. [Fig.
1.20, 1.22]
• Legs cannot be bent for chair position. [Fig. 1.20]
• The movement of mattress piece at its place is on vertical guides supported by
spring pressure. The direction of spring force is not suitable to retain mattress
piece at normally closed position. Slight loosening of spring will not allow
mattress to rise at level. [Fig. 1.20]
• The piece of mattress is also removed and placed manually. To fix mattress
piece from lower side an attendant has to bend down. This is uncomfortable.
[Fig. 1.25]
• The lifting of patient on ropes needs the ropes to be always placed below the
body of patient. This can be uncomfortable. Even lifting on rope may be
uncomfortable. [Fig. 1.25]
Important fact is that none of these patents has been commercialised
because one or the other problems in the suggested designs.
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4) The available products also had problems associated with them as under
• The patient has to move from the bed to toilet for which lot of assistance is
required and it may not be suitable for patients recommended bed rest for
various reasons. [Image 1.1]
• The Intricate mechanisms / motors results in high initial cost and high
maintenance costs. [Image 1.1]
• The container is manually kept and removed. The handling of pot with odour
is not comfortable for assistant. [Image 1.2]
• To fix pot, the patient has to be shifted. The shifting of patient needs
assistance. [Image 1.2]

The literature review and product review, both confirmed that there is
need to have a bed in which all features like -
• The piece of mattress should be removed from lower side of the bed to
eliminate shifting of patient and to reduce the friction due to body weight.
• The commode should touch the buttocks to avoid splashing due to gap
between commode and buttocks.
• The commode should move upwards through hole in bed in order to touch the
buttocks.
• Odour to be reduced by sealing the drainage.
• The sitting position is to be achieved by raising backrest and lowering legrest.
• The operating mechanisms should have minimum joints, easy in operation and
be powered by hand to reduce initial cost, running cost and maintenance.
• Number of drives to be reduced by combining the mechanisms.
• Accessories like utility table, wash basin, storage rack to be provided.
Based on the requirements listed above, tentative arrangement of the bed was
proposed and is shown in Fig. (1.26).

5) The synthesis of mechanisms resulted in selection of mechanism shown in Fig.

(2.3) for commode, mechanism shown in Fig. (2.8) for Platform, Telescopic
screw for actuator and telescopic puller for connecting mechanism.
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6) The mechanisms designed as above were checked in CAD model and it was
found that the design is suitable for manufacture.
7) The prototype manufactured was tested and it was found that the working the
prototype is similar as in the CAD model and is suitable for use.
8) The prototype was used by user and it shows satisfactory results.

4.5 CONCLUSION

1) A bed for incapacitated patients was developed as a new product for providing
comfort to the patients, making various facilities available near them and
reducing the efforts and time required for assisting such patients. This product
was designed and manufactured to provide facilities such as
• A Backrest to support back for propt up position
• Backrest and legrest for achieving sitting position and providing comfort.
• A Commode with flush and water jet which can be operated either by patient
or assistant.
• Easy attachment to the drainage system to avoid manual handling of pot.
• A platform which closes the commode hole when not in use.
• A mechanism to operate the commode and platform eliminating the need of
shifting the patient for providing pot.
• Thereby avoiding uncomfortable position of patient during urination and
defecation.
• Reducing the efforts required to handle the patient for sitting, cleaning,
sponging, urination and defecation.
• A height adjustable utility table to provide for dining, reading, support for
sitting.
• A height adjustable and position adjustable basin for brushing, face washing.
• A position adjustable storage rack to keep glass, medicine, napkin, tissue
paper etc.
• Provision for railing along length for safety of patient.
Thus a totally new bed for patient care at home and hospital has been designed,
manufactured, tested and put to use.
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2) The project leads to development of a new product and a PATENT has been
filed.

3) The kinematic design of a new mechanism for movement of platform and

commode was developed. This mechanism is a combination of two separate
mechanisms for movement of platform and commode. The platform
mechanism is actuated by a telescopic screw and nut mechanism. The platform
mechanism is connected to commode mechanism through a telescopic puller
mechanism. When the handle attached to screw is rotated the platform moves
down as its horizontal motion is restricted by a stopper. During this downward
motion of platform the commode mechanism does not operate as the
telescopic puller mechanism does not open completely till platform reaches its
lowest position. On further rotation of screw the platform starts moving
horizontally so as to clear the hole. The commode mechanism does not operate
as the telescopic puller mechanism is going to open completely only when
platform clears the hole. As soon as platform clears the hole the telescopic
puller mechanism achieves open condition and starts pulling the commode
mechanism, thereby lifting the commode to required top position. When the
handle is rotated in reverse direction, the telescopic screw mechanism pushes
the platform towards the hole. This motion allows the telescopic puller
mechanism to close. As at other end of puller mechanism there is commode
mechanism and the commode is now free to move down due to gravity, it
moves down. The commode reaches its lowest position and clears the space
for platform. Once the commode has reached its lowest position further
motion of screw allows platform to move horizontally towards the hole and
the telescopic screw mechanism starts closing as it does not have any load due
to commode. When platform reaches at its position below the hole further
horizontal movement is restricted by a stopper and the telescopic puller
mechanism is at its closed position. Further movement of screw elevates the
platform to close the hole.
Thus kinematic design of new mechanisms for movement of commode
and platform was developed.
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4) The total mechanism consisting of platform mechanism, commode mechanism

and puller mechanism is a new development and can be used with any type of
actuator like mechanical, electrical, hydraulic and pneumatic for automation of
machine or material handling.
This resulted in development of new mechanism for automation of
machine.

5) The platform mechanism developed for horizontal and vertical movement of

platform is also a new development of scissor mechanism or lazy tongs
mechanism on wheels. This mechanism along with a standard screw and nut
mechanism can be used for material handling in workshops to move heavy
loads.
This resulted in development of new mechanism for material handling.

6) The product was developed with available resources and facilities and actually
tried by patients. The results are satisfactory and the mechanisms are working
properly.
7) The cost incurred for manufacture of sample product was high (Rs 50,000/-
approx.). The cost was high due to extra labour, single piece manufacture, non
availability of proper tools and dies. The product can be commercialized with
further developments and at a reduced cost due to mass production. The
expected cost of the product is about Rs 20000-25000/- for user. The required
work / time study is possible only in mass production to reduce the
manufacturing cost.
The product has wide market in India as well as abroad in developing /
undeveloped countries where health care is a developing concern.
CHAPTER – 5
 111

SCOPE OF FURTHER WORK

The product was tested. The problems faced during the test and manufacture
suggests further improvement in product.

1) Improvement in commode
a) A light weight commode with smooth surface is required. Dies / special tools
required for large scale production can be designed.
b) A water seal odour trap suitable for commode avoiding leakage.
c) The flexible pipes material properties to be selected for strength and flexibility
at optimum level for mass production.
d) Intensify the pressure of washing jet if head available is low.
2) Improvement in telescopic screw
a) Modification for drive from a motor preferably of 24 volts for safety keeping
options of handle in case of power failure or motor failure.
3) Removable commode pot mounted on steel skeleton of commode can be
developed which can be manually removed from lower side without disturbing the
patient in case flush is not required.
4) The rusting of hardware and links are to be avoided, hence stainless steel
components can be used. Cost of small components will increase cost marginally.
5) Presently the handle driven screws needs time to shift from one position to
another. This can be reduced by introduction of electrical actuators and combining
the activation of number of actuators in single function switch on control pendent.
6) The work and time study can be taken up for large scale production and reducing
the manufacturing cost.
 112

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ANNEXURES

ANNEXURE-A
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