What Is Factor Of Safety? In What Cases Factor Of Safety Is Taken High Or Low? - SKengineers

 

WHAT IS FACTOR OF SAFETY?

·        There are number of parameters, which are difficult to evaluate accurately in design analysis such as,

a)      Uncertainty in magnitude of external forces acting on the component.

b)     Variations in the properties of material like yield strength or ultimate strength.

c)      Variations in the dimensions of component due to imperfect workmanship.

·        In order to ensure the safety against such circumstances factor of safety is used.

·        FACTOR OF SAFETY – is defined as the ‘ratio of maximum stress to working stress’.

Mathematically,

           Factor of safety (F.O.S) = maximum stress / working or design stress.

·        In case of ductile materials, e.g. mild steel, where yield point is clearly defined, factor of safety is based on yield point stress.

Factor of safety = yield point stress / working stress.

·        In case of brittle materials e.g. cast iron, where yield point is not well defined, factor of safety is based on ultimate stress .

Factor of safety = ultimate stress / working stress.

Ø  FACTORS TO BE CONSIDERED WHILE SELECTING FACTOR OF SAFETY –

1.      Reliability of properties of material and change in these properties during service.

2.      Reliability of applied load.

3.      No doubt about exact type of failure.

4.      Extent of stress concentration.

5.      Extent of initial stresses set up during manufacture.

6.      Extent of loss of life, if failure occurs.

7.      Extent of simplifying assumptions.

8.      Reliability of test results carried out on actual machine parts.

Ø  IN WHAT CASES, FACTOR OF SAFETY IS TAKEN HIGH?

1.      Analysis of loads with respect to magnitude and nature is difficult to estimate.

2.      Impact loading.

3.      Non-uniformity in material.

4.      High temperature applications.

5.      Reliability of parts.

6.      Materials likely to corrode (corrosion).

7.      In case of possibilities of accidents, leading to damage property, risk towards lives of workers.

Ø  IN WHAT CASES, FACTOR OF SAFETY MAY BE TAKEN LOW?

1.      No damage to property and lives of workers in case of design failure.

2.      Less frequently used parts.

3.      Analysis of loads with respects to magnitude and nature is easy to estimate.

4.      Past knowledge of working conditions and strength of similar parts is known.

5.      In case of failure, little time and low money is needed for rectification.

Ø  Why factor of safety is high in design subjected to uncertain environments?

·        There may be different environmental conditions at the time of use than those assumed at design stage.

·        For example – a machine component is designed and manufactured with reference to standard atmospheric conditions. When this machine component is actually put to operate, then it may be subjected to high temperature surrounding and corrosive atmosphere.

·        In such circumstances, factor of safety chosen for machine component should be increased considering the difference between assumed or standard environmental conditions and actual environmental conditions, otherwise, it will reduce the strength of component.

Ø  FACTOR OF SAFETY ACCORDING TO TYPE OF LOAD –

TYPE OF LOAD	FACTOR OF SAFETY
Static load -	1.5 to 3
Variable load -	3 to 6
Impact / shock load	6 to 12