Factor of Safety :

It is defined, in general,as the ratio of the maximum stress to the working stress, Mathematically

Factor of safety = Maximum stress / Working or design stress

In case of ductile materials e.g. mild steel, where the yield point is clearly defined, the factor of safety is based upon the yield point stress, In such cases,

Factor of safety = Yield point stress / Working or design stress

In case of brittle materials e.g. cast iron, the yield point is not well defined as for ductile materials. Therefore, the factor of safety for brittle materials is based on ultimate stress.

Factor of safety = Ultimate stress / Working or design stress

This relation may also be used for ductile materials.

Introduction to Factor of safety :

Equipment
Factor of Safety
- FOS
Aircraft components1.5 - 2.5
Boilers3.5 - 6
Bolts8.5
Cast-iron wheels20
Engine components6 - 8
Heavy duty shafting10 - 12
Lifting equipment - hooks ..8 - 9
Pressure vessels3.5 - 6
Turbine components - static6 - 8
Turbine components - rotating2 - 3
Spring, large heavy-duty4.5
Structural steelwork in buildings4 - 6
Structural steelwork in bridges5 - 7
Wire ropes8 - 9

### General recommendations

Applications
Factor of Safety
- FOS
For use with highly reliable materials where loading and environmental conditions are not severe and where weight is an important consideration1.3 - 1.5
For use with reliable materials where loading and environmental conditions are not severe1.5 - 2
For use with ordinary materials where loading and environmental conditions are not severe2 - 2.5
For use with less tried and for brittle materials where loading and environmental conditions are not severe2.5 - 3
For use with materials where properties are not reliable and where loading and environmental conditions are not severe, or where reliable materials are used under difficult and environmental conditions3 - 4