Factor Of Safety Used in Machine Design
While designing the component it is necessary to ensure that sufficient reserve strength is kept to deal with an uncertain situation. The factor of safety is taken into account for this 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 the 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 the 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 the ultimate stress.
Factor of safety = Ultimate stress / Working or design stress
This relation may also be used for ductile materials.
factor of safety
Equipment | Factor of Safety |
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 |
Applications | Factor of Safety |
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 |
Factors based on which FOS is selected are given below.
1.The reliability of the properties of material and change of these properties during service.
2.The Reliability of test results and accuracy of the application of these results to actual machine parts.
3.The Reliability of the applied load.
4.The certainty as to the exact mode of failure.
5.Extent of simplifying assumptions.
6.The extent of localized stresses.
7. The extent of initial stresses set up during manufacture.
8.The extent of loss of life if a failure occurs. A higher factor of safety is chosen if there is more risk of life.
1) Where the magnitude and exact nature of forces is unknown, higher FOS is selected.
2) Where there is possibility of Impact or unseen sudden loads, higher FOS is selected
3) Where there are chances of residual or initial stress in the component, higher FOS is selected.
4) Where there is a very high risk of life/property in case of failure, higher FOS is selected.
5) When very high reliability is expected from the components, higher FOS is selected.
6) Where the components are non-accessible for repairs, or failure of that component causes a lot of inconvenience higher FOS is selected.
Factor of safety is a ratio of maximum stress withstand by an object to applied stress. Whenever a Factor of safety is greater than or equal to, then the applied stress is less than or equal to the maximum stress so the object can withstand load. But when the ratio is equal to
1, the object tough enough to withstand load.
Whenever a Factor of safety is less than 1, the applied stress is greater than maximum stress then the object can’t withstand the stress applied it leads to failure. The high factor of safety results in unnecessary risk of failure.
There are many factors as
1- materials strength (if the materials is a brittle or ductile)
2-possible misuse: the designer must consider any responsible of for foreseeable use
3-loading (static. impact, repeated).
4- complexity of stress analysis
5-cost
6-environment and temperature, and this factor affect also to choose the materials. and we can say also each factor from the previous factors affects on another one its like a network and the designer person must know this Entanglement perfectly