The factor of safety (FoS) is used in engineering design to provide a margin between the expected load and the strength of a component. It accounts for uncertainties in materials, loading conditions and manufacturing.
Factor of Safety (FoS) is the ratio of a component’s strength to the applied load. It is used in engineering to ensure reliability and account for uncertainties in materials, design and operating conditions.
The factor of safety also known as Safety Factor, is used to provide a design margin over the theoretical design capacity to allow for uncertainty in the design process. The uncertainty could be any one of a number of the components of the design process including calculations, material strengths, duty, manufacture quality. The value of the safety factor is related to the lack of confidence in the design process. The simplest interpretation of the Factor of Safety is
FoS = Strength of Component / Load on component
If a component needs to withstand a load of 100 Newtons and a FoS of 4 is selected then
it is designed with strength to support 400 Newtons...
The selection of the appropriate factor of safety to be used in design of components
is essentially a compromise between the associated additional cost and weight and the benefit of
increased safety and/or reliability. Generally an increased factor of safety results from a heavier component or a component
made from a more exotic material or / and improved component design
The factors of safety listed below are based on the yield strength..
| Factor of Safety | Application |
| 1.25 - 1.5 | Material properties known in detail. Operating conditions known in detail Loads and resultant stresses and strains known with with high degree of certainty. Material test certificates, proof loading, regular inspection and maintenance. Low weight is important to design. |
| 1.5 - 2 | Known materials with certification under reasonably constant environmental conditions, subjected to loads and stresses that can be determined using qualified design procedures. Proof tests, regular inspection and maintenance required |
| 2 - 2.5 | Materials obtained for reputable suppliers to relevant standards operated in normal environments and subjected to loads and stresses that can be determined using checked calculations. |
| 2.5 - 3 | For less tried materials or for brittle materials under average conditions of environment, load and stress. |
| 3 - 4 | For untried materials used under average conditions of environment, load and stress. |
| 3 - 4 | Should also be used with better-known materials that are to be used in uncertain environments or subject to uncertain stresses. |
A convenient method of ensuring safe confident design is to use design codes;
A good standard used by mechanical engineer is
BS 2573-Pt 1:1983 Rules For Design of Cranes.
Specification for Classification, stress, Calculations and design criteria for structures.
This standard (together with BS 2573 part 2) includes rules for completing calculations and applying factors and the relevant
allowable stresses to be used for the different grades of materials. This standard is
primarily used for design of cranes and associated equipment but it is used widely for
design of similar mechanical systems. When designing systems based using the rules from this standard
it is not generally necessary to include additional margins of safety.
.
When design engineering structures using structural steel section a useful standard is..
BS 5950-1:2000-Structural use of steelwork in building. Code of practice for design. Rolled and welded sections.
This standard together with BS 5950-Part 2,3-1,4,5,6,7,8 & 9 provide service factors and design stresses
relevant to structural design.
In designing many equipment items including vessels, pumps, valves, piping systems there are equivalent
standards and codes which should be followed. These documents generally identify the necessary
design procedures and the safety margins to be included.
Factor of safety is the ratio of a component’s strength to the applied load, used to ensure safe and reliable design.
FoS = Strength of component ÷ Applied load.
Typical values range from 1.25 to 4 depending on material reliability, loading conditions and design uncertainty.
It ensures components can withstand unexpected loads, material variations and uncertainties in design.