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Machine Design Factors


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1. What Device or Mechanism to be used: 
This is best judged by understanding the problem thoroughly. Sometimes a particular 
function can be achieved by a number of means or by using different mechanisms and the
designer has to decide which one is most effective under the circumstances. A rough 
design or layout diagram may be made to crystallize the thoughts regarding the relative 
arrangement of the elements. 
2. Material: 
This is a very important aspect of any design. A wrong choice of material may lead to failure, 
over or undersized product or expensive items. The choice of materials is thus dependent 
on suitable properties of the material for each component, their suitability of fabrication 
or manufacture and the cost.
3. Load: 
The external loads cause internal stresses in the elements and these stresses must be
determined accurately since these will be used in determining the component size. 
Loading may be due to: 
• Energy transmission by a machine member 
• Dead weight. 
• Inertial forces. 
• Thermal effects. 
• Frictional forces. 
• Environmental Conditions 
In other ways loads may be classified as: 
Static load- Does not change in magnitude and direction and normally increases 
gradually to a steady value. 
4. Dynamic load:
- Changes in Magnitude- for e.g. traffic of varying weight passing a bridge. 
- Changes in Direction- for e.g. load on piston rod of a double acting cylinder. 
5. Size, Shape, Space Requirements and Weight: 
Preliminary analysis would give an approximate size but if a standard element is to be
chosen, the next larger size must be taken. 
Shapes of standard elements are known but for nonstandard element, shapes and space 
requirements must depend on available space in a particular machine assembly. A scale 
layout drawing is often useful to arrive at an initial shape and size. 
Weight is important depending on application. For example, an aircraft must always be
made light. This means that the material chosen must have the required strength yet it
must be light. Similar arguments apply to choice of material for ships and there too light 
materials are to be chosen. Portable equipment must be made light. 
6. Manufacture: 
Care must always be taken to ensure that the designed elements may be manufactured with 
ease, within the available facilities and at low cost.
7. How will it Operate: 
In the final stage of the design a designer must ensure that the machine may be operated 
with ease. In many power operated machines it is simply a matter of pressing a knob or
switch to start the machine. However in many other cases, a sequence of operations is to
be specified. This sequence must not be complicated and the operations should not 
require excessive force. 
8. Reliability and Safety: 
Reliability is an important factor in any design. A designed machine should work effectively 
and reliably. The probability that an element or a machine will not fail in use is called 
reliability. Reliability lies between 0 ≤ R < 1. To ensure this, every detail should be
examined. Possible overloading, wear of elements, excessive heat generation and other 
such detrimental factors must be avoided. 
Safety has become a matter of paramount importance these days in design. Machines must 
be designed to serve mankind, not to harm it. Industrial regulations must be followed to
avoid any damage or harm arising out of a defective product.