Design of Machine Members-I
Lecture Notes – 1
(d) Optimum design. It is the best design for the given objective function under the specified
constraints. It may be achieved by minimising 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. This type of design depends upon the use of computer systems to
assist in the creation, modification, analysis and optimisation 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. The load, on a machine component, may act
in several ways due to which the internal stresses are set up. The various types of load and
stresses are discussed later.
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
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. It is essential that a designer should have a thorough knowledge of
the properties of the materials and their behaviour under working conditions. Some of the
important characteristics of materials are: strength, durability, flexibility, weight, resistance to
heat and corrosion, ability to cast, welded or hardened, machinability, electrical conductivity,
etc. The various types of engineering materials and their properties are discussed later.
4. Form and size of the parts. The form and size are based on judgment. The smallest
practicable cross-section may be used, but it may be checked that the stresses induced in the
designed cross-section are reasonably safe. In order to design any machine part for form and