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Learn the definition of Factor of Safety, its examples, and its importance in ensuring safety in design and use in the overall safety of people and workplaces.
Published 26 Jun 2023
The higher the number of FoS, the safer the product or structure is. An FoS of 1 indicates that a structure or component will fail immediately when the design load is reached and will not be capable of supporting any extra load. Structures or components with FoS less than one are not acceptable. If the consequences of failure are severe, such as loss of life or physical injury, a higher FoS will be required either by design or by law.
Simply put, structures or devices must be able to bear more weight than they would under normal use to be safer. FoS indicates by what factor the design is safe.
Below is a table of typical FoS of equipment, provided by Engineering ToolBox:
Equipment
Factor of Safety
– FOS –
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 (specified in the design code)
Turbine components – static
Turbine components – rotating
2 – 3
Spring, large heavy-duty
4.5
Structural steel work in buildings
4 – 6
Structural steel work in bridges
5 – 7
Wire ropes
Applications
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
All of the calculations basically measure the same thing—how much additional stress beyond the designed load that a structure can withstand. A simple calculation for safety factor is:
Many Factor of Safety calculators are available online for calculating FoS values when the maximum stress and allowable stress values are known.
Factor of Safety varies according to the situation. Systems are intentionally designed to be way stronger than they need to be for normal settings. This increases the likelihood that they will continue functioning even under extreme conditions such as emergency situations, added loads, overuse, or degradation caused by wear and tear. Moreover, below are more reasons why using FoS in design is important:
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According to A Textbook of Machine Design by R.S.Khurmi and J.K.Gupta, the selection of the appropriate Factor of Safety to be used in the design of any mechanical system is based on a variety of considerations, including the following:
The following are two instances of how the Factor of Safety is used:
Boilers and pressure vessels, as well as nuclear power plant systems, are subject to the American Society of Mechanical Engineers (ASME) International Boiler and Pressure Vessel Code safety guidelines, which control the design, manufacturing, and inspection of boilers and pressure vessels during the construction process. By their very nature, pressure vessels are potentially hazardous. It necessitates the addition of safety factors to protect against failure—uncertainty in design, materials used, manufacture, inspection, and operation.
Personal Fall Arrest Systems (PFAS) and other fall protection equipment must be built with a high safety factor. The Occupational Safety and Health Administration (OSHA) standard 1915.159 outlines the criteria for connectors and anchorage to be capable of sustaining a minimum tensile load of 3,000 to 5,000 pounds (22.24 Kn) per employee, and a requirement of a complete personal fall arrest system which maintains a safety factor of at least 2. If the equipment is going to be used in tough circumstances, it may require an even higher safety factor.
The Factor of Safety exists as a safety measure that is designed to make a product, a system, or a structure safe. SafetyCulture (formerly iAuditor) can help manufacturers, engineers, and inspectors conduct external and internal inspections of equipment and assess their safety factor to determine their capacity to withstand maximum allowable stress. With SafetyCulture, you can:
Check out other helpful checklists for safety factor:
Loida Bauto
Loida Bauto is a content contributor for SafetyCulture. An Interior Designer by training, she began to pursue her passion for writing in 2017. Her interests involve a diverse range of topics such as Disability, Universal Design, and Sustainability, among other matters that aim to improve the world we live in. She is a self-published book author in 2018 and 2021.
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