The selection of a proper factor of safety to be used in designing any machine component depends upon a number of considerations, such as the material, mode of manufacture, type of stress, general service conditions and shape of the parts. Before selecting a proper factor of safety, a design engineer should consider the following points.
- The reliability of the properties of the material and change of these properties during service ;
- The reliability of test results and accuracy of application of these results to actual machine parts ;
- The reliability of applied load ;
- The certainty as to exact mode of failure ;
- The extent of simplifying assumptions ;
- The extent of localised stresses ;
- The extent of initial stresses set up during manufacture ;
- The extent of loss of life if failure occurs ; and
- The extent of loss of property if failure occurs.
Each of the above factors must be carefully considered and evaluated. The high factor of safety results in unnecessary risk of failure. The values of factor of safety based on ultimate strength for different materials and type of load are given in the following table:
A Textbook of Machine Design by R.S.Khurmi and J.K.Gupta