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 components 1.5 – 2.5
Boilers 3.5 – 6
Bolts 8.5
Cast-iron wheels 20
Engine components 6 – 8
Heavy duty shafting 10 – 12
Lifting equipment – hooks .. 8 – 9
Pressure vessels 3.5 – 6
Turbine components – static 6 – 8
Turbine components – rotating 2 – 3
Spring, large heavy-duty 4.5
Structural steelwork in buildings 4 – 6
Structural steelwork in bridges 5 – 7
Wire ropes 8 – 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 consideration 1.3 – 1.5
For use with reliable materials where loading and environmental conditions are not severe 1.5 – 2
For use with ordinary materials where loading and environmental conditions are not severe 2 – 2.5
For use with less tried and for brittle materials where loading and environmental conditions are not severe 2.5 – 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 conditions 3 – 4

Spread the love