The broad objective of designing a machine element is to ensure that it preserves its operating capacity during the stipulated service life with minimum manufacturing and operating costs. In order to achieve this objective, the machine element should satisfy the following basic consideration:
(i) Strength: A machine part should not fail under the effect of the forces that act on it. It should have sufficient strength to avoid failure either due to fracture or due to general yielding.
(ii) Rigidity: A machine component should be rigid, that is, it should not deflect or bend too much due to forces or moments that act on it.
(iii) Wear Resistance: Wear is the main reason for putting the machine part out of order. It reduces useful life of the component. Wear also leads to the loss of accuracy of machine tools. There are different types of wear such as abrasive wear, corrosive wear and pitting. Surface hardening can increase the wear resistance of the machine components, such as gears and cams.
(iv) Minimum Dimensions and Weight: A machine part should be sufficiently strong, rigid and wear resistant and at the same time, with minimum possible dimensions and weight. This will result in minimum material cost.
(v) Manufacturability: Manufacturability is the ease of fabrication and assembly. The shape and material of the machine part should be selected in such a way that it can be produced with minimum labor cost.
(vi) Safety: The shape and dimensions of the machine parts should ensure safety to the operator of the machine. The designer should assume the worst possible conditions and apply ‘fail-safe’ or ‘redundancy’ principles in such cases.
(vii) Conformance to Standards: A machine part should conform to the national or international standard covering its profile, dimensions, grade and material.
(viii) Reliability: Reliability is the probability that a machine part will perform its intended functions under desired operating conditions over a specified period of time. A machine part should be reliable, that is, it should perform its function satisfactorily over its lifetime.
(ix) Maintainability: A machine part should be maintainable. Maintainability is the ease with which a machine part can be serviced or repaired.
(x) Minimum: Life-cycle Cost: Life-cycle cost of the machine part is the total cost to be paid by the purchaser for purchasing the part and operating and maintaining it over its life span. It will be observed that the above mentioned requirements serve as the basis for design projects in many cases.
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