ENGINEERING DESIGN 1
1.4 Factor of Safety. A machine or machine element at times has to operate under various conditions which cannot be assessed correctly beforehand. A factor of safety which is a ratio of
ultimate stress of the material and the
working stress is to be provided in the design. The value of safety factor in designing
a member of the machine, by whose failure there is danger of loss of life or valuable property, must be kept higher. Alternatively, the factor of safety of a component
maybe defined in terms of load, stress, deflection, wear etc, as the ratio of the magnitude of any of the listed parameters resulting in failure (found
from experimental testing, to its working value (actually value sustained in service. Mathematically, Factor of safety =
F a ilu res tress iia llo w able stress i. (1) or
=
F ail u reload iiw or king load i. (The failure stress for ductile materials is the yield point stress and for brittle materials, it is the ultimate strength of the material. In mechanical design calculation, by counting errors in estimating the load, dimensions and material properties, design factor of safety or allowable safety Factor is introduced as in equation
[2]. Other factors that require the need fora suitable factor of safety in design include
uncertainty in the level of external force acting on the component.
variations in the properties of materials e.g. yield strength or ultimate strength variations in the dimensions of the components due to imperfect workmanship. etc,
ENGINEERING DESIGN
2 The determination of the design factor of safety depends upon the application and the uncertainties associated with a particular design. It is mainly based on consequence of failure such as human safety or costly repair. Other factors need taking into account is the degree of uncertainty about loading or material properties etc. The recommended values of the safety factor values can be found in design codes and engineering handbooks.
Example. In the design of a steel shaft, the ultimate shear stress for the steel maybe taken as 300 MPa and a factor of safety as 5. Determine the allowable stress for the shaft. Solution. We know that the allowable stress, τ = τu / F.S. = 300 / 5 = 60 N/mm2 
