What you mean by FOS in a design?

 

FACTOR OF SAFETY - FOS

While designing a component, it is necessary to ensure sufficient reserve strength in the case of an accident.  It is ensured by taking a suitable factor of safety (fs)

FOS can be defined as: [s_all = allowable stress]

Ratio of  failure stress to allowable stress…..

             Or

It is defined as the ratio of failure load to working load….

             The allowable stress is the stress value which is used in design to determine the dimensions of the component.  It is considered as a stress which the designer, expects will not exceed under normal operating conditions.

             For ductile material, the allowable stress (s_all) is given by,

             Ratio of Syt to F.S    

For brittle material:

             RATIO OF Sut to F.S  Syt = yield tensile strength, Sut = ultimate tensile stress

The FOS ensures against

-           Uncertainty in the magnitude of external force acting on the component.

-           Variations in the properties of materials like yield strength or ultimate strength.

-           Variations in the dimensions of the component due to imperfect workmanship.

The magnitude of FOS depends upon following conditions:

1.          Effect of failure:

             Failure of the ball bearing in gear box.

             Failure of valve in pressure vessel

2.          Types of Load

             -          When external force acting on the m/c element is static - FOS is low.

             -          Impact load – FOS is high

3.          Degree of Accuracy in force analysis.

             When the force acting on the m/c element is precisely determined low FOS can be selected. Where as higher FOS is considered when the m/c component is subjected to a force whose magnitude or direction is uncertain and unpredictable.

4.          Material of Component

             When the component is made of homogenous ductile material, like steel, yield strength is the criterion of feature.  FOS is small in such cases.  Cast Iron component has non-homogenous structure and a higher FOS based on ultimate strength is chosen.

5.          Reliability of the component:

             FOS & Reliability

             -          Defense

             -          Power stations

6.          Cost of Components:

             FOS & Cost

7.          Testing of Machine element

             Low FOS – when m/c comp. are tested under actual condition of service and operation.

8.          Service conditions:

             Higher FOS – when m/c element is likely to operate in corrosive atmosphere or high temp. environment.

9.          Quality of Manufacture:

             Quality is inversely proportional to FOS.