DESIGN OF MACHINE MEMBERS

UNIT I :: Introduction, Design for Static and Dynamic loads

Mechanical Engineering Design: Design process, design considerations, codes and
standards of designation of materials, selection of materials.
Design for Static Loads: Modes of failure, design of components subjected to axial,
bending, Torsional and impact loads. Theories of failure for static loads.
Definition

The subject Machine Design is the creation of new and better machines and improving

the existing ones. A new or better machine is one which is more economical in the

overall cost of production and operation. The process of design is a long and time

consuming one. From the study of existing ideas, a new idea has to be conceived. The

idea is then studied keeping in mind its commercial success and given shape and form in

the form of drawings.

Classifications of Machine Design

The machine design may be classified as follows :

1. Adaptive design.

In most cases, the designer’s work is concerned with adaptation of existing

designs. This type of design needs no special knowledge or skill and can be

attempted by designers of ordinary technical training. The designer only makes

minor alternation or modification in the existing designs of the product.

2. Development design.

This type of design needs considerable scientific training and design ability in order

to modify the existing designs into a new idea by adopting a new material or

different method of manufacture. In this case, though the designer starts from the

SRI VENKATESWARA INSTITUTE OF TECHNOLOGY – S.V.I.T – 9F Page 1

 DESIGN OF MACHINE MEMBERS

existing design, but the final product may differ quite markedly from the original

product.

3. New design.

This type of design needs lot of research, technical ability and creative thinking.

Only those designers who have personal qualities of a sufficiently high order can

take up the work of a new design. The designs, depending upon the methods used,

may be classified as follows:

(a) Rational design.

This type of design depends upon mathematical formulae of principle of
mechanics.
(b) Empirical design.
This type of design depends upon empirical formulae based on the practice
and past experience.
(c) Industrial design.
This type of design depends upon the production aspects to manufacture
any machine component in the industry.
(d) Optimum design.
It is the best design for the given objective function under the specified
constraints. It may be achieved by minimizing the undesirable effects.
(e) System design.
It is the design of any complex mechanical system like a motor car.
(f) Element design.
It is the design of any element of the mechanical system like piston,
crankshaft, connecting rod, etc.

(g) Computer aided design.

SRI VENKATESWARA INSTITUTE OF TECHNOLOGY – S.V.I.T – 9F Page 2

 DESIGN OF MACHINE MEMBERS

This type of design depends upon the use of computer systems to assist in
the creation, modification, analysis and optimization of a design.

General Considerations in Machine Design

Following are the general considerations in designing a machine component:

1. Type of load and stresses caused by the load.

2. Motion of the parts or kinematics of the machine.

The successful operation of any machine depends largely upon the simplest

arrangement of the parts which will give the motion required.

The motion of the parts may be :

(a) Rectilinear motion which includes unidirectional and reciprocating

motions.

(b) Curvilinear motion which includes rotary, oscillatory and simple harmonic.

(c) Constant velocity.

(d) Constant or variable acceleration.

3. Selection of materials

4. Form and size of the parts.

5. Frictional resistance and lubrication.

6. Convenient and economical features.

7. Use of standard parts.

8. Safety of operation.

9. Workshop facilities.

10. Number of machines to be manufactured.

SRI VENKATESWARA INSTITUTE OF TECHNOLOGY – S.V.I.T – 9F Page 3

 DESIGN OF MACHINE MEMBERS

11. Cost of construction.

12. Assembling.

SRI VENKATESWARA INSTITUTE OF TECHNOLOGY – S.V.I.T – 9F Page 4

 DESIGN OF MACHINE MEMBERS

General Procedure in Machine Design:

The flow chart for the general procedure in machine design is shown in Fig. 1.1.

FIG. 1.1. GENERAL PROCEDURE IN MACHINE DESIGN.

SRI VENKATESWARA INSTITUTE OF TECHNOLOGY – S.V.I.T – 9F Page 5

 DESIGN OF MACHINE MEMBERS

In designing a machine component, there is no rigid rule. The problem may be

attempted in several ways. However, the general procedure to solve a design problem is

as follows :

1. Recognition of need.

First of all, make a complete statement of the problem, indicating the need,

aim or purpose for which the machine is to be designed.

2. Synthesis (Mechanisms).

Select the possible mechanism or group of mechanisms which will give the

desired motion.

3. Analysis of forces. Find the forces acting on each member of the machine and

the energy transmitted by each member.

4. Material selection. Select the material best suited for each member of the

machine.

5. Design of elements (Size and Stresses). Find the size of each member of

the machine by considering the force acting on the member and the permissible

stresses for the material used. It should be kept in mind that each member

should not deflect or deform than the permissible limit.

6. Modification. Modify the size of the member to agree with the past experience

and judgment to facilitate manufacture. The modification may also be

necessary by consideration of manufacturing to reduce overall cost.

7. Detailed drawing. Draw the detailed drawing of each component and the

assembly of the machine with complete specification for the manufacturing

processes suggested.

SRI VENKATESWARA INSTITUTE OF TECHNOLOGY – S.V.I.T – 9F Page 6

 DESIGN OF MACHINE MEMBERS

8. Production. The component, as per the drawing, is manufactured in the

workshop.

Fundamental Units

The measurement of physical quantities is one of the most important operations in

engineering. Every quantity is measured in terms of some arbitrary, but internationally

accepted units, called fundamental units.

Derived Units

Some units are expressed in terms of other units, which are derived from

fundamental units, are known as derived units e.g. the unit of area, velocity,

acceleration, pressure, etc.

System of Units

There are only four systems of units, which are commonly used and universally

recognized. These are known as :

1. C.G.S. units,

2. F.P.S. units,

3. M.K.S. units, and

4. S.I. units.

Since the present course of studies are conducted in S.I. system of units, therefore,

we shall discuss this system of unit only.

Rules for S.I. Units

The eleventh General Conference of Weights and Measures recommended only the

fundamental and derived units of S.I. units. But it did not elaborate the rules for the

SRI VENKATESWARA INSTITUTE OF TECHNOLOGY – S.V.I.T – 9F Page 7

 DESIGN OF MACHINE MEMBERS

usage of the units. Later on many scientists and engineers held a number of meetings

for the style and usage of S.I. units. Some of the decisions of the meeting are :

1. For numbers having five or more digits, the digits should be placed in groups of

three separated by spaces (instead of commas)** counting both to the left and

right of the decimal point.

2. In a four*** digit number, the space is not required unless the four digit number

is used in a column of numbers with five or more digits.

3. A dash is to be used to separate units that are multiplied together. For example,

newton × metre is written as N-m. It should not be confused with mN, which

stands for milli newton.

4. Plurals are never used with symbols.

For example, metre or metres are written as m.

5. All symbols are written in small letters except the symbol derived from the

proper names.

For example, N for newton and W for watt.

6. The units with names of the scientists should not start with capital letter when

written in full.

For example, 90 newton and not 90 Newton.

SRI VENKATESWARA INSTITUTE OF TECHNOLOGY – S.V.I.T – 9F Page 8

 DESIGN OF MACHINE MEMBERS

Classification of Engineering Materials

The engineering materials are mainly classified as :

1. Metals and their alloys, such as iron, steel, copper, aluminium, etc.

2. Non-metals, such as glass, rubber, plastic, etc.

The metals may be further classified as :

(a) Ferrous metals, and

(b) Non-ferrous metals.

The *ferrous metals are those which have the iron as their main constituent,

such as cast iron, wrought iron and steel.

The non-ferrous metals are those which have a metal other than iron as their

main constituent, such as copper, aluminium, brass, tin, zinc, etc.

Selection of Materials for Engineering Purposes

The selection of a proper material, for engineering purposes, is one of the most

difficult problem for the designer. The best material is one which serve the desired

objective at the minimum cost. The following factors should be considered while

selecting the material :

1. Availability of the materials,

2. Suitability of the materials for the working conditions in service, and

3. The cost of the materials.

The important properties, which determine the utility of the material are physical,

chemical and mechanical properties.

SRI VENKATESWARA INSTITUTE OF TECHNOLOGY – S.V.I.T – 9F Page 9