31 May 2016

What is factor of safety

In machine design, while designing the component, it is necessary to provide sufficient reserve strength in case of an accident so this is achieved by taking a suitable factor of safety.

fs = Failure stress / Allowable stress

fs = Failure load / Working load

The magnitude of the factor of safety depends upon the following factors:
  • Effect of failure 
  • Types of load
  • A degree of accuracy in force analysis
  • Material of component
  • Reliability of component
  • Cost of component
  • Testing of the machine element
  • Service conditions
  • Quality of manufacture
Points mention below the following condition where a higher factor of safety is chosen :
  • Magnitude and nature of external forces acting on the machine component cannot be precisely estimated.
  • The material of the machine component has a non-homogeneous structure.
  • The component of the machine is subject to the force of impact in service.
  • There is a possibility of residual stresses in a machine component.
  • The machine part is subjected to a high temperature during operation.
  • In applications such as aircraft components, higher reliability is required.
  • There is a possibility if abnormal variation in external load on some occasions.
  • The machine part's manufacturing quality is poor.
  • There is stress concentration in a machine component.
A higher factor of safety increases the component's reliability.