Mechatronics in Computer Numerical

Control (CNC) machines

Prepared by

Dr. K. Dileep
Professor and Head
Department of Mechanical Engineering
Jawaharlal College of Engineering and Technology
Downloaded from Ktunotes.in
 MECHATRONICS is the science of motion control
A mechatronic system consists of a mechanical structure where actuators generate the
required motions, where sensors measure the resulting motion, and where a control
algorithm cancels out the differences between the resulting motion and the desired motion
specified by the task programming system.

Designing mechatronic systems entails the design of an

optimal mechanical structure and of optimal motion control
systems.

The problem is that both the mechanical and the control

behavior interact with each other.

There is a need for a concurrent engineering approach,

where mechanical and control behaviors are simultaneously
optimized.
This calls for a so-called mechatronic compiler, where high-level design specifications are
(semi)-automatically translated into an optimal mechatronic system and whereby the
mechanical structure and motion controller are considered to belong to one single system
to be optimized.
Downloaded from Ktunotes.in
 Importance of Mechatronics in automation
Today’s customers are
demanding more variety and
higher levels of flexibility in the
products.

Mechatronics concurrently
employs the disciplines of
mechanical, electrical, control
and computer engineering at
the stage of design itself.

Mechatronics based automated

systems such as automatic
inspection and quality
assurance, automatic
packaging, record making, and Automation in the machine tools has reduced the human
automatic dispatch help to intervention in the machining operation and improved the process
efficiency and product quality. Therefore it is important to study the
expedite the entire
principles of mechatronics and to learn how to apply them in the
manufacturing operation. automation of a manufacturing system.
Downloaded from Ktunotes.in
 Mechatronics system

A spring-force system Constituents of a mechatronics system

Microprocessor based
equivalent spring mass system

Downloaded from Ktunotes.in

 Mechatronics and manufacturing automation

Mechatronics has a variety of applications as products and systems in the area of

‘manufacturing automation’. Some of these applications are as follows:

1. Computer numerical control (CNC) machines

2. Tool monitoring systems
3. Advanced manufacturing systems
a. Flexible manufacturing system (FMS)
b. Computer integrated manufacturing (CIM)
4. Industrial robots
5. Automatic inspection systems: machine vision
systems
6. Automatic packaging systems

CNC machine is the best and basic example of application of

Mechatronics in manufacturing automation
Downloaded from Ktunotes.in
 Tool monitoring systems
Uninterrupted machining is one of the challenges in front manufacturers to meet the
production goals and customer satisfaction in terms of product quality. Tool wear is a critical
factor which affects the productivity of a machining operation.

Mechatronics based cutting tool-wear

condition monitoring system is an integral
part of automated tool rooms and
unmanned factories

Off-line and on-line tool monitoring system for tool edge grinding
Downloaded from Ktunotes.in
 Advanced Manufacturing Systems
Flexible Manufacturing System (FMS) Benefits of an FMS
• Flexibility to change part variety
• Higher productivity
• Higher machine utilization
• Less rejections
• High product quality
• Reduced work-in-process and inventory
• Better control over production
• Just-in-time manufacturing
• Minimally manned operation
• Easier to expand

FMS is a manufacturing cell or system consisting of one or more CNC machines, connected
by automated material handling system, pick-and-place robots and all operated under the
control of a central computer. Downloaded from Ktunotes.in
 Computer Integrated Manufacturing (CIM)
In the life cycle of a product or a system, a number of activities and operations viz. designing,
analyzing, testing, manufacturing, packaging, quality control, etc. are involved.

Application of principles of automation to each of these activities enhances the productivity

only at the individual level. These are termed as ‘islands of automation’.

Integrating all these islands of automation into a single system enhances the overall
productivity. Such a system is called as “Computer Integrated Manufacturing (CIM)”.

‘CIM is the integration of the total manufacturing enterprise through the use of integrated
systems and data communications coupled with new managerial philosophies that improve
organizational and personal efficiency’.

CIM basically involves the integration of advanced technologies such as computer aided
design (CAD), computer aided manufacturing (CAM), computer numerical control (CNC),
robots, automated material handling systems, etc.

Downloaded from Ktunotes.in

 CNC
Computer numerical control (CNC) is the automated control of machining tools
(drills, boring tools, lathes) by means of a computer. A CNC machine processes a piece of
material (metal, plastic, wood, ceramic, or composite) to meet specifications by following
a coded programmed instruction and without a manual operator directly controlling the
machining operation.

Downloaded from Ktunotes.in

 Types of CNC
Two types of CNC are used according to
the controller design.

Hybrid CNC;
Straight CNC.

In a hybrid CNC, the controller comprises

soft-wired and hard-wired logic circuits. In
a straight CNC, hard wire is used to
interface the computer with the
machine.

The various types of CNC machines are :

CNC machining center.

CNC turning center.
CNC lathes.
CNC milling/drilling machines.
CNC special-purpose machines.
Downloaded from Ktunotes.in
 Major Elements of CNC Systems
Hardware: It includes a microprocessor, machine tools, actuators and other
peripheral devices.

Software: CNC software includes various instructions, programming languages, I/O control
and graphics.

Information: Information is nothing but the data required for cutter location, programming,
machining process.

Downloaded from Ktunotes.in

 Design of modern CNC machines

The design and construction of computer numerically controlled (CNC) machines differ
greatly from that of conventional machine tools.

The important constituent parts, and aspects of CNC machines to be considered in their
designing.

(a) Machine structure

(b) Guide ways
(c) Feed drives
(d) Spindle and spindle bearings
(e) Measuring systems
(f) Controls, software and operator interface
(g) Gauging
(h) Tool monitoring

Downloaded from Ktunotes.in

 Machine structure
The machine structure is the load carrying and
supporting member of the machine tool.
All the motors, drive mechanisms and other
functional assemblies of machine tools are
aligned to each other and rigidly fixed to the
machine structure.

The machine structure is subjected to static and

dynamic forces.

It is essential that the structure does not deform or

vibrate beyond the permissible limits under the
action of these forces.

The basic design factors involved in the design

of a machine structure are

Static Load
Dynamic Load
Thermal Load
Downloaded from Ktunotes.in
 Static load
The static load of a machine tool results from the weights of slides and the workpiece, and
the forces due to cutting.

To keep the deformation of the structure due to static loading within permissible limits, the
structure should have adequate stiffness and a proper structural configuration.

Generally there are two basic configurations of machine tools as depicted.

MECHATRONICS is the science of motion control

Downloaded from Ktunotes.in
 Dynamic load
Dynamic load is a term used for the constantly changing forces acting on the structure
while movement is taking place.

These forces cause the whole machine system to vibrate.

The origin of such vibrations is:

(a) Unbalanced rotating parts

(b) Improper meshing of gears
(c) Bearing irregularities
(d) Interrupted cuts while machining (like in milling)

The effect of these vibrations on the machine performance is reduced by:

(a) Reducing the mass of the structure

(b) Increasing the stiffness of the structure
(c) Improving the damping properties

Downloaded from Ktunotes.in

 Thermal Load
In a machine tool there are a number of local heat sources which set up thermal gradients
within the machine.

Some of these sources are:

(a) Electric motor
(b) Friction in mechanical drives and gear boxes
(c) Friction in bearings and guideways
(d) Machining process
(e) Temperature of surrounding objects

Steps to reduce thermal deformation.

(a) External mounting of drives, i.e. motors and gear boxes

(b) Removing frictional heat from bearings and guideways by a proper lubrication system
(c) Efficient coolant and swarf removal system for the dissipation of heat generated from the
machining process
(d) Thermo-symmetric designing of the structure

Downloaded from Ktunotes.in

 Guideways

Guideways are used in machine tools to:

(a)Control the direction or line of action of the carriage

or the table on which a tool or a work piece is held

(b) To absorb all the static and dynamic forces

The shape and size of the work produced depends on

the accuracy of the movement and on the geometric
and kinematic accuracy of the guideway.

The geometric relationship of the slide (the moving part)

and the guideway (stationary part) to the machine base
determines the geometric accuracy of the machine.

Kinematic accuracy depends on the straightness, flatness

and parallelism errors in the guideway.
Downloaded from Ktunotes.in
 Points to be considered while designing guideways
Rigidity
Damping capability
Geometric and kinematic accuracy
Velocity of slide .
Friction characteristics
Wear resistance
Provision for adjustment of play .
Position in relation to work area
Protection against swarf and damage
The drive mechanism should be placed in such a manner that the reaction and hence the
frictional forces are uniform in the guiding system.

This will ensure uniform wear on guideways.

Guideways are primarily of three types.

(a) Friction guideways - The vee , Flat, Dovetail and Cylindrical Guideways
(b) Antifriction (with rolling friction) linear motion (LM) guideways
(c) Frictionless guideways
Downloaded from Ktunotes.in
 Friction Guideways

Friction guideways are most widely applied in

conventional machine tools due to their low
manufacturing cost and good damping properties.

These guideways operate under conditions of sliding

friction and do not have a constant coefficient of
friction. The coefficient of friction varies with the sliding
velocity.

The coefficient of friction is very high when the

movement commences and as the speed of the slide
increases, it rapidly falls and beyond a certain critical
velocity it remains almost constant. Therefore to start the
movement, the force to overcome friction has to be
correspondingly high.

Downloaded from Ktunotes.in

 Stick-slip phenomenon
As the speed increases the friction decreases and a greater amount of movement than that
intended for the slide takes place.

There is a possibility of this cycle of events repeating itself and resulting in errors in positioning
and consequently in a jerky motion.

This phenomenon is known as the stick-slip phenomenon.

To reduce the possibility of stick-slip, there should be a minimum but constant friction between
the surfaces in contact.

V- Drive
R- Elasticity
M- Mass

Downloaded from Ktunotes.in

 Materials for guideways

Stick slip reduction is achieved in friction guideways by using strips of material such as poly
tetra fluoro ethylene (PTFE) or turcite lining at the guideway interface.

Turcite is a special type of plastic with particles of graphite embedded on its surface.
These materials have a low and constant coefficient of friction.

Cross-section of coated guideways

Downloaded from Ktunotes.in

 The vee or inverted vee
• It is widely used on machine tools, especially on
lathe beds.

• The advantages is that the parallel alignment of

the guideway with the spindle axis is not affected
by wear.

• There is a closing action as the upper member

settles on the lower member, and this
automatically maintains the alignment.

The majority of lathes have a combination of vee and

flat guideways to prevent the twisting of the slide.

Gibs are not required with the vee guideway to take

up the clearance caused by wear.

Downloaded from Ktunotes.in

 Flat and Dovetail Guideways

Tapered gibs and screws

• The flat guideways have better load-bearing capabilities than the other guideways.
• After a period of use, wearing may occur owing to the sliding of the surfaces over each
other.
• Gibs are used to ensure accurate fitting of the slide to both the flat and dovetail guideways.
• The gibs are tapered and can be adjusted to reduce excessive clearance caused by wear.
• Flat and dovetail types of guideway is normally cast iron to cast iron.
• The cast iron may be heat treated (flame hardened) to increase its hardness, and the
surfaces ground to obtain the required accuracy.

Downloaded from Ktunotes.in

 Cylindrical Guideways

• In cylindrical guideways, the bore in the carriage housing provides support all around the
guideway.

• For relatively short traverses and light loads, cylindrical guideways are very efficient.

• A limitation on the use of these guideways for long traverses is that if the guide bar is
supported only at each end.

Downloaded from Ktunotes.in

 Antifriction Linear Motion (LM) Guideways
Antifriction linear motion guideways are used on CNC machine tools to:

(a) Reduce the amount of wear

(b) Improve the smoothness of the movement
(c) Reduce friction
(d) Reduce heat generation

They use rolling elements in between the moving and the stationary elements of the
machine.

The main disadvantage of these guideways as compared to friction guideways is their lower
damping capacity.

There are essentially two categories of antifriction guideways:

Recirculating ball bushing;

Linear bearing with ball and rollers.

Downloaded from Ktunotes.in

 Recirculating ball bushing

The linear ball bush consists of recirculating balls. Ball bushes may be of the closed type or open
type, with or without seals.
Normally the shaft is made with a high precision with the application of a linear bearing to offer
a very smooth movement.

Downloaded from Ktunotes.in

 Linear Bearing with Ball and Rollers

The rollers are in contact with the guideways and are

guided between shoulders of a supporting element
having a very close tolerance.

The machine bed surfaces coming in contact with rollers

are hardened.

Advantages compared with friction guides.

(a) Low frictional resistance

(b) No stick-slip
(c) Ease of assembly
(d) Commercially available in ready-to-fit condition
(e) High load carrying capacity
(D) Heavier pre loading possibility
(g) High traverse speeds

Downloaded from Ktunotes.in

 Frictionless Guideway

Frictionless guideways fall into two categories:

Hydrostatic guideways: In these a very thin layer of fluid at a pressure of about 250-300 bar
is applied between the slide and the guide to separate them.

As metal-to-metal contact is completely eliminated, there is no stick-slip effect.

The bulk property of fluid has an effect on damping, and dynamic stiffness will minimize
the vibration problem and improve machinability.

Aerostatic guideways: Here, the separation is obtained by a cushion of compressed air.

Their application is limited due to low dynamic stiffness.
Downloaded from Ktunotes.in
 Aerostatic bearings
Aerostatic bearings are also named as
externally pressurized air bearings,
considering that the pressure of air film is
generated by an external air supply
system. Pressurized air is fed into the gap
between two bearing surfaces through a
specific restrictor and then discharged to
the surrounding ambient from the exit
edges of bearing clearance.

The thin film acts as the lubricant in the

clearance between stationary parts and
moving parts. During the working state,
the moving and stationary surfaces of air
bearing do not contact, not only
avoiding many problems of conventional
bearings, such as wear and friction but
also offering distinct merits for precision
positioning
Downloaded from Ktunotes.in
Downloaded from Ktunotes.in
 Selection of Guideways
Requirement of the load carrying capacity;

Damping and dynamic stiffness (Ratio of force to response)

Traverse speed.
Feed Drives
On a CNC machine the function of feed drive is to provide motion to the slide as per the
motion commands.

Since the degree of accuracy requirements is high, the feed drive should have high
efficiency and response.

The feed drive consists of:

(a) Servomotor
(b) Mechanical transmission system

Hydraulic power is also used to achieve slide motion.

The most common power utilized is the electric motor as well as DC motors which are
very popular:
Downloaded from Ktunotes.in
 Servo motor & stepper motor
A servomotor is a rotary actuator or linear actuator that allows for precise control
of angular or linear position, velocity and acceleration.
It consists of a suitable motor coupled to a sensor for position feedback.
A stepper motor or step motor or stepping motor is a brushless dc electric motor
that divides a full rotation into a number of equal steps.

The motor's position can then be commanded

to move and hold at one of these steps
without any position sensor for feedback (an
open-loop controller), as long as the motor is
carefully sized to the application in respect to
torque and speed.
Commonly used feed drive motors for CNC
machines are direct current (dc) servomotors
and alternating current (ac) servomotors.
AC servomotors provide a constant torque
over their entire speed range, require less
maintenance due to brushless operation, have
a better response and dynamic stiffness, and a
higher reliability compared to dc servomotors
Downloaded from Ktunotes.in
 Mechanical Transmission System
The mechanical transmission system of a feed drive comprises all the components which
are in the force and motion transmission paths from drive motor to the slide.

Classification of transmission systems.

(a) Elements to convert the rotary motion to a linear motion (recirculating ball screw nut or
rack-and-pinion system)

(b) Torque transmitting elements (gear box or timing belt and couplings)
The main criterion to be considered in the design of a mechanical transmission system is to
keep the transmission errors to a minimum.

The essential requirements are:

(a) High natural frequency

(b) High stiffness
(c) Sufficient damping
(d) Low friction
(e) Backlash free operation
Downloaded from Ktunotes.in
 Elements Used to Convert the
Rotary Motion to a Linear Motion

Several actuating mechanisms are used in CNC

machines to convert the rotational movement to a
translational movement.

The efficiency and responsiveness of the actuating

mechanism have the greatest influence on the accuracy
of the work produced.

The actuating mechanisms used for the slides of CNC

machines are screw and nut, rack and pinion.

Downloaded from Ktunotes.in

 Screw and nut

The screw and nut system is effective for medium

traverses.

With longer traverses the screw sags under its own

weight.

Longer the screw length, lower is the upper limit of

traverse rates due to reduction in the critical speed.

There are two types of screw and nut systems used on

the CNC machine tools recirculating ball screws roller
screws.

Downloaded from Ktunotes.in

 Ball Screws or ball bearing screw or
recirculating ball screw
A ball screw is one of the most efficient methods of power
transmission with a minimum of friction.

It uses the rolling friction instead of the sliding friction because

rolling friction is much less as compared to the sliding friction.

It converts rotary motion into linear motion.

Ball screw assemblies consist of a screw with a precision

ground helical groove, a nut (the outer race) with an internal
groove and a circuit of precision steel balls in the grooves
between the screw and the nut.

Downloaded from Ktunotes.in

 Recirculating ball screw

The flanged nut is attached to the moving part of CNC

machine tool.

As the screw rotates, the nut translates the moving part

along the guide ways.

However, since the groove in the ball screw is helical, its

steel balls roll along the helical groove, and, then, they may
go out of the ball nut unless they are arrested at a certain
spot.

Thus, it is necessary to change their path after they have

reached a certain spot by guiding them, one after another,
back to their “starting point” (formation of a recirculation
path). The recirculation parts play that role.

Downloaded from Ktunotes.in

 Recirculating Ball screws
There are two thread groove designs that have been
developed for ball screws
Circular arc design;
Gothic arc design.

Gothic arc design offers a greater advantage

because if any foreign material enters the effective
ball groove, it will be pushed aside by the balls into
the space provided by the divergence arcs.

The load-carrying capacity of the ball screws can be

increased by increasing either the ball diameter or
the screw diameter or the number of bearing balls

Downloaded from Ktunotes.in

 Advantages of recirculating ball screws

Low frictional resistance

Low drive power requirement
Little temperature rise
Less wear and hence longer life
No stick-slip effect
High traverse speed
High efficiency

Downloaded from Ktunotes.in

 Roller screws
It is used for converting a rotary torque into a linear motion.
It can carry heavy loads for thousands of hours in the most
arduous conditions.

This makes roller screws the ideal choice for demanding

continuous-duty applications.
The features of the roller screws are:
1. They provide minimum backlash with a high efficiency of
transmission.
2. Better position control with simpler electronic circuitry,
attributed to the smaller pitch of the screw as compared to
pitch of the ball screw.
3. Roller screws are more expensive than ball screws.
There are two types of roller screws: Planetary roller screws
and Recirculating roller screws.
Roller screws have grooved roller elements which make the
physical contact with the threads of the nut and screw to
engage thread from inside the nut and on the outside of
the screw.
Downloaded from Ktunotes.in
 Comparison Between Roller and Ball Screws

Loads and stiffness: The numbers of contact points in a ball screw are limited by the ball size.
However, roller screw designs provide many more contact points than possible on a
comparably sized ball screw. Therefore, roller screws have a higher load-carrying capacity
and improved stiffness.

Travel life: With higher load capacities, the roller screws deliver major advantages in the
working life and roller screw will have an expected service life that is 15 times greater than
that of ball screws.

Speeds: Typical ball screw speeds are limited to 2000 rpm and less, due to the interaction of
the balls colliding with each as the race rotates. In contrast, the rollers in a roller screw are
fixed in a planetary fashion by journals at the ends of the nut and therefore can work at 5000
rpm or higher.

Downloaded from Ktunotes.in

Roller screw actuators have
several advantages over
hydraulic or pneumatic actuators
for many applications, especially
those involving heavy loads and
fast cycles. For example, the
controls are simpler, eliminating
the complex system of valves,
pumps, filters and sensors that
make up hydraulic and
pneumatic systems.

Downloaded from Ktunotes.in

 Rack-and-pinion
For longer strokes, a ball screw needs to be supported at
intermediate points to minimise deflection due to its own weight
over the length and a large diameter has to be used to reduce
torsional deflection.

In addition, there is a limitation on the operating speed of a balls

crew due to its lower critical speed.

These factors restrict the use of ball screws for machines with
Critical speed is the lowest
longer strokes. Rack-and-pinion drives are particularly suitable for
rotational speed at which
longer strokes. the ball-screw shaft is in
resonance. In applications
A slide operated by a rack-and-pinion drive has the following with rotating shafts, it limits
advantages the screw's rpm. Variables
that influence critical speed
The stiffness of the drive is independent of the length of the stroke. are shaft diameter,
The rack-and pinion system is cheaper as compared to the unsupported length, and the
balls screw system. configuration of support
There are special pinions which provide for a minimum backlash bearings.

Downloaded from Ktunotes.in

 Torque Transmission Elements
The torque is transmitted from a prime-mover shaft to an output shaft
through torque transmission elements.

The output shaft may be a pinion or a ball screw.

Various elements are used on CNC machines to transmit the torque,

viz., gears, timing belts, flexible couplings, etc.

Gear Box

Depending on the requirement, the drive to the ball screw may be

through a gear box or a timing belt.

A gear box is required to reduce the high motor speed to a speed

A CNC gear Box
suitable for the feed drive, to reduce the inertia of load referred to
on the motor shaft and to reduce the torque on the motor shaft.

They are more frequently used where the reduction is required

between the shafts which are not coaxial or parallel.
Downloaded from Ktunotes.in
 Timing Belts
These are endless toothed belts.

The teeth engage with a timing pulley having teeth on its periphery.

The teeth profiles on the belt and the pulley are compatible
with each other.

They are becoming more popular due to their inherent advantages

of low cost, less noise, elimination of lubrication, less maintenance,
and higher efficiency.

Downloaded from Ktunotes.in

 Flexible Couplings
These couplings are used when the driver and driven shafts are
coaxial, but difficult to align the driver and driven shafts
perfectly on the same axis.

Further, heat and elastic deformation cause additional

misalignments between the two coaxial shafts.
A certain amount of flexibility is built into the couplings to
compensate for these errors.

Three kinds of errors can be compensated by using the flexible

couplings.

(a) Radial misalignment (λ)

(b) Angular misalignment (α)
(c) Axial shift (δ)

Run-out or runout is an inaccuracy of rotating

systems, specifically that the tool or shaft does
not rotate exactly in line with the main axis.
Downloaded from Ktunotes.in
Flexible couplings are used for connecting a ball screw and a servomotor.

These couplings behave like a rigid element in the direction of rotation.

In the axial and angular directions, however, they have elastic properties.

Misalignments caused by fitting errors or other influences can be compensated by these elastic
properties.

Downloaded from Ktunotes.in

 Taper Lock Bushes
These elements are used to couple a shaft and a hub of a gear, timing pulley, etc.
They are built in the form of taper rings with self-releasing tapers.
Both male and female tapers are supplied by the supplier of these elements.
For the purpose of assembly, the user has to machine only cylindrical shafts and bores.

The taper lock bushes are capable of

transmitting a torque from the shaft to the hub
(or vice versa) without any backlash.

For assembling the bushes, the male and

female tapers are forced onto each other by
tightening the screws axially.

This expands the bushes and generates an

enormous radial force due to small taper angle
which locks the hub and the shaft, and the
torque is transmitted through friction.

Downloaded from Ktunotes.in

 Bearings
Bearing is a stationary element which support
rotating shaft and confines their motion.
A bearing is a machine element that constrains
relative motion to only the desired motion, and
reduces friction between moving parts.

It offer minimum frictional resistant to moving

parts.

The main purpose of bearings is to prevent direct

metal to metal contact between two elements
that are in relative motion. This prevents friction,
heat generation and ultimately, the wear and
tear of parts. It also reduces energy consumption
as sliding motion is replaced with low friction
rolling.

Downloaded from Ktunotes.in

 Classification of Bearings
1. Based on type of load

Radial bearing
The load act perpendicular to the direction of
motion of moving parts

Thrust bearing
The pressure act along or parallel to the axis of the
shaft.

2. Based on nature of contact

Sliding contact bearing

No interposed elements between shaft and bearings.

Rolling contact or Antifriction bearing

The steel balls are provided in between shaft and
bearings.
Downloaded from Ktunotes.in
3.Depending upon the nature of lubrication

Thick film type: Here the surfaces of the bearings are

completely separated from each other by the lubricant.

Thin film type: Here although a lubricant is present, surfaces

are partially in contact with each other. It is also termed as
boundary lubrication.

Hydrostatic bearings: Here the fluid film pressure is obtained

by supplying the lubricant under high pressure such that the
force exerted by the pressure supports the loaded shaft at
all points.

Hydrodynamic bearings: In this type, the fluid film pressure is

generated only by the rotation of the journal.

The position of the journal gets adjusted in such a manner

that a force supporting the journal load is produced due to
film pressure.
Downloaded from Ktunotes.in
 Journal bearing

A journal bearing is a sliding contact bearing which gives

lateral support to the rotating shaft. It consists of two main
parts:

1. A journal (the part of the shaft which runs in a sleeve or

bushing).

2. A hollow cylinder (sleeve or bushing).

In a journal bearing, the diameter of the journal is kept less

than the diameter of the bearing to allow the flow of lubricant
between the surfaces.

Downloaded from Ktunotes.in

 Spindle bearings

When a workpiece holder (such as a chuck) is mounted on the spindle, the accuracy of
rotation is extremely important as it affects the roundness of the components produced.

The rotational accuracy of the spindle is dependent on the

quality and design of the bearings used and the pre loading.
The bearings should support the spindle radially and axially.

The accuracy and the quality of the work produced depends

directly on the geometrical accuracy, running accuracy and
the stiffness of the spindle assembly.

The various types of spindle bearings used in the design of a

spindle for machine tools are:

(a) Hydrodynamic beatings

(b) Hydrostatic bearings
(c) Antifriction bearings
Downloaded from Ktunotes.in
Material removal using single point or multipoint workpiece
requires rotational speeds of the order of 30-6000 rpm and
even higher.

All work or tool carrying spindles rotating at these speeds

are subjected to torsional and radial deflections.

They are also subjected to thrust forces depending on the

nature of the metal cutting operation being performed.
To increase the stiffness and minimise torsional strain on the
spindles they are designed to be as stiff as possible with a
minimum overhang.

Also, the final drive to the spindle should be located as

near as possible to the bearings

Downloaded from Ktunotes.in

 Hydrodynamic bearing
In hydrodynamic bearing lubrication systems, a thin film of lubrication is created between
shaft and bearing.

Initially, the shaft is at rest and it sinks at the bottom of the

clearance space under the action of load W.

The surfaces of the journal and bearing touch during" rest'.

When the journal starts to rotate, it will climb the bearing
surface gradually as the speed is further increased; it will then
force the fluid into the wedge-shaped region.

When more and more fluid is forced into a wedge-shaped

clearance space, pressure is generated within the system.
Since the pressure is created within the system due to the
rotation of the shaft, this type of bearing is known as a self-
acting bearing.
The pressure generated in the clearance space supports the external load (W).

Downloaded from Ktunotes.in

In a fluid film bearing the shaft is supported by a thin layer of lubricating fluid, and the
rotating and stationary parts are not in direct contact with one another. There are two
main type of fluid film bearings used in industry; hydrostatic and hydrodynamic. Hydrostatic
bearings are externally pressurized with a lubricant and do not rely on rotating of the shaft
to develop a fluid film. Conversely, hydrodynamic bearings rely on the speed of the shaft
to pressurize the fluid in the bearing and lift the shaft off the bearing. The following image
shows how a spinning shaft in a fluid film bearing generates a pressure wedge which lifts
the shaft off the bearing surface and prevents metal on metal contact. This separation
between rotating and stationary surfaces allows for fluid film bearings to be designed with
theoretically infinite life.

Downloaded from Ktunotes.in

Hydrodynamic bearings are are used where the load carrying capacities are low and frequent, starting
and stopping of the spindle is not required; for example in grinding machines.

The main limitation in a hydrodynamic bearing is that a definite clearance must be provided for the oil
film to be maintained between the spindle and the bearing.

This clearance may result in the centre of a spindle in the bearing to change its position owing to
variation in the applied force.

Clearances normally provided between the spindle and the bore of the bearing for the oil film vary
from 50 μm to 200 μm depending upon the diameter of the journal.

The essential features of these bearings include simplicity, good damping properties, and a good
running accuracy.

The pressure of the oil is created within the bearing by the rotation of the spindle. As the spindle rotates,
the oil in contact with the spindle is carried into wedge-shaped cavities between the spindle and the
bearing.

The oil pressure is increased as the oil is forced through the small clearances between the bearing and
the spindle.

Downloaded from Ktunotes.in

 Hydrostatic bearing
In hydrostatic bearing lubrication systems, a thick film of lubrication is created between the
journal and bearing by supplying lubricant under pressure with an external source such
as a pump.

Since the lubricant is supplied under pressure, this type of bearing is called externally
pressurized bearing.
For a hydrostatic bearing, the spindle is supported by a
relatively thick film of oil supplied under pressure, similar to that
used in the bearings for linear movements.

The load carrying capacity of this type of bearing is

independent of the rotational speed.

They have high damping properties, high running

accuracy, high wear resistance, but are very expensive.

Downloaded from Ktunotes.in

These are used only where temperature effects cause problems in the part accuracy as in
the case of grinding machines and fine boring machines.

As compared with hydrostatic bearing, hydrodynamic bearings are simple in construction,

easy to maintain and lower in initial as well as maintenance cost.

Downloaded from Ktunotes.in

 Antifriction/ Roller Bearings
These are suitable for high speeds and high loads.

They are often used in preference to hydrodynamic bearings because of their low friction,
moderate dimensions, lesser liability to suffer from wear or incorrect adjustment, ease of
replacement, and high reliability.

Types of ball and roller bearings

1. Ball bearings
(i) deep groove ball bearings
(ii) angular contact ball bearings

2. Roller bearings
(i) cylindrical roller bearings
(ii) cylindrical roller bearings (double row) with tapered bore
(iii) tapered roller bearings

Downloaded from Ktunotes.in

 Types of ball and roller bearings
The selection of a particular type of bearing for the spindle depends on the requirements of the
particular machine, like speeds of operation, accuracy of the spindle and stiffness of the spindle

Downloaded from Ktunotes.in

 Bearing Materials
Bearing material should have the following
properties:

– high compressive strength;

– low coefficient of friction;
– high thermal conductivity;
– high resistance to corrosion;
– sufficient fatigue strength;
– it should be soft with a low modulus of
elasticity;
– bearing materials should not weld easily to the
journal material.

The materials which are commonly used as

bearing materials are lead base babbit, tin base
babbit, leaded bronze, copper-lead alloy.

Downloaded from Ktunotes.in

 System modelling and
Mechatronics in Robotics

Downloaded from Ktunotes.in

 Basic System Model
Consider the following situation. A microprocessor switches on a motor.

How will the rotation of the motor shaft vary with time?

The speed will not immediately assume the full-speed value but will only attain that
speed after some time.

Consider another situation. A hydraulic system is used to open a valve which allows
water into a tank to restore the water level to that required.

How will the water level vary with time?

The water level will not immediately assume the required level but will only attain
that level after some time.

Downloaded from Ktunotes.in

 Mathematical models
In order to understand the behavior of systems, mathematical models are needed.

These are simplified representations of certain aspects of a real system.

Such a model is created using equations to describe the relationship between the input
and output of a system and can then be used to enable predictions to be made of the
behavior of a system under specific conditions, e.g. the outputs for a given set of inputs,
or the outputs if a particular parameter is changed.

A mathematical model of a system is a description of it in terms of equations relating

inputs and outputs so that outputs can be predicted from inputs.

Downloaded from Ktunotes.in

In devising a mathematical model of a system it is necessary to make assumptions and
simplifications and a balance has to be chosen between simplicity of the model and
the need for it to represent the actual real-world behaviour.

The basis for any mathematical model is provided by the fundamental physical laws
that govern the behavior of the system.

Range of systems considered, includes mechanical, electrical, thermal and fluid

examples.

In lumped parameter system each parameter, i.e. Property or function, is considered

independently.

Downloaded from Ktunotes.in

 Mechanical system building blocks
The models used to represent mechanical systems have the basic building blocks
of springs, dash pots and masses.

Springs represent the stiffness of a system, dashpots the forces opposing motion, i.e.
frictional or damping effects, and masses the inertia or resistance to acceleration .

The mechanical system does not have to be really made up of springs, dashpots
and masses but have the properties of stiffness, damping and inertia.

All these building blocks can be considered to have a force as an input and a
displacement as an output.

Downloaded from Ktunotes.in

Downloaded from Ktunotes.in
In devising a mathematical model of a system it is necessary to make assumptions
and simplifications and a balance has to be chosen between simplicity of the model
and the need for it to represent the actual real-world behavior.

The basis for any mathematical model is provided by the fundamental physical laws
that govern the behavior of the system.

Range of systems considered, includes mechanical, electrical, thermal and fluid

examples.

In lumped parameter system each parameter, i.e. Property or function, is considered

independently.

Downloaded from Ktunotes.in

 Springs represent the stiffness of a system

The stiffness of a spring is described by the

relationship between the forces F used to extend or
compress a spring and the resulting extension or
compression x.

F = kx

where k is a constant, The bigger the value of k, the

greater the forces have to be to stretch or compress
the spring and so the greater the stiffness.

Downloaded from Ktunotes.in

 The dashpot
The dashpot building block represents the types of forces
experienced when we endeavor to push an object through a
fluid or move an object against frictional forces.

The faster the object is pushed, the greater the opposing forces
become.

The damping or resistive force F is proportional to the velocity v

of the piston.
v = dx/dt
Thus F = cv where c is a constant. The larger the value of c, the
greater the damping force at a particular velocity.

Since the velocity is the rate of change of displacement x of the

piston, i.e. v = dx/dt.

The output (the displacement ) and the input (force) are related
thus.
Downloaded from Ktunotes.in
 The mass
The masses represent the inertia or resistance to acceleration.

The mass building block exhibits the property that the bigger
the mass, the greater the force required to give it a specific
acceleration.

The relationship between the force F and the acceleration a

is (Newton's second law) F = ma, where the constant of
proportionality between the force and the acceleration is the
constant called the mass m.

Downloaded from Ktunotes.in

 Energy
Energy is needed to stretch the spring, accelerate the mass and move the piston in
the dash pot.

However, in the case of the spring and the mass we can get the energy back but
with the dashpot we cannot.

The spring when stretched stores energy, the energy being released when the spring
springs back to its original length. The energy stored when there is an extension x is
Since F = kx

Energy stored in the mass when it is moving with a velocity v

There is no energy stored in the dashpot. The dashpot dissipates energy rather than
storing it, the power P dissipated depending on the velocity v and being given by

Downloaded from Ktunotes.in

 Rotational systems
Three building blocks are a torsional spring, a rotary
I
damper and the moment of inertia, i.e. the inertia of
a rotating mass.

With such building blocks the inputs are torque and the
outputs angle rotated.

With a torsional spring the angle ϴ rotated is proportional

to the torque T. Hence
T= k ϴ
With the rotary damper a disc is rotated in a fluid a resistive torque T is proportional to the
angular velocity ω, and since angular velocity is the rate at which angle changes, i.e.
dϴ/dt,

The moment of inertia building block has the property that the greater the moment of inertia
I, the greater the torque needed to produce an angular acceleration :

Downloaded from Ktunotes.in

Thus, since angular acceleration is the rate of change of angular velocity, i.e. dω/dt,
and angular velocity is the rate of change of angular displacement, then

Downloaded from Ktunotes.in

 Rotational systems, contd..

The torsional spring and the rotating mass store energy; the rotary damper just dissipates
energy. The energy stored by a torsional spring when twisted through an angle θ is
and since T = kθ this can be written as

The energy stored by a mass rotating with an angular velocity ω is the kinetic energy E,
where

The power P dissipated by the rotatory damper when rotating with an angular
velocity ω is

Downloaded from Ktunotes.in

Table summarizes the equations
defining the characteristics of
the mechanical building blocks
when there is, in the case of
straight line displacements
(termed translational), a force
input F and a displacement x
output and, in the case of
rotation, a torque T and
angular displacement θ.

Downloaded from Ktunotes.in

Downloaded from Ktunotes.in
Downloaded from Ktunotes.in
Downloaded from Ktunotes.in
Downloaded from Ktunotes.in
Downloaded from Ktunotes.in
Downloaded from Ktunotes.in
 Fluid system building blocks

In fluid flow systems there are three basic building blocks which can be considered to
be the equivalent of electrical resistance, capacitance and inductance.

Fluid systems can be considered to fall into two categories:

Hydraulic, where the fluid is a liquid and is deemed to be incompressible; and

Pneumatic, where it is a gas which can be compressed and consequently shows a
density change.

Downloaded from Ktunotes.in

Hydraulic resistance is the resistance to flow which occurs as a result of a liquid
flowing through valves or changes in a pipe diameter (Figure 17.14(a)). The
relationship between the volume rate of flow of liquid q through the resistance
element and the resulting pressure difference (p1-p2) is
p1-p2 = Rq
where R is a constant called the hydraulic resistance. The bigger the resistance, the
bigger the pressure difference for a given rate of flow.

Downloaded from Ktunotes.in

 Hydraulic and Pneumatic building blocks

Hydraulic capacitance is the term used to describe energy storage with a liquid
where it is stored in the form of potential energy. A height of liquid in a container
(Figure 17.14(b)), i.e. a so-called pressure head, is one form of such a storage. For
such a capacitance, the rate of change of volume V in the Fluid system
building blocks container, i.e. dV/dt, is equal to the difference between the
volumetric rate at which liquid enters the container q1 and the rate at which it
leaves q2

Downloaded from Ktunotes.in

 Hydraulic and Pneumatic building blocks
Hydraulic inertance is the equivalent of inductance in electrical systems or a spring in
mechanical systems. To accelerate a fluid and so increase its velocity, a force is
required. Consider a block of liquid of mass m (Figure 17.14(c)). The net force acting
on the liquid is

Downloaded from Ktunotes.in

Hydraulic and Pneumatic building blocks

Downloaded from Ktunotes.in

 Thermal building blocks
There are only two basic building blocks for thermal systems: resistance and capacitance. There
is a net flow of heat between two points if there is a temperature difference between them.

The electrical equivalent of this is that there is only a net current i between two points if there is a
potential difference v between them, the relationship between the current and potential
difference being i = v/R, where R is the electrical resistance between the points. A similar
relationship can be used to define thermal resistance R.

If q is the rate of flow of heat and (T1 - T2) the temperature difference, then

The value of the resistance depends on the mode of heat transfer. In the case of conduction
through a solid, for unidirectional conduction

and

Downloaded from Ktunotes.in

Thermal capacitance is a measure of the store of internal energy in a system. Thus, if
the rate of flow of heat into a system is q1 and the rate of flow out is q2, then
rate of change of internal energy

or

where C is the thermal capacitance and so C = mc.

Downloaded from Ktunotes.in

Thank you

Downloaded from Ktunotes.in