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What is Factor of Safety?

By Rachel Burkot
Updated Feb 26, 2024
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Factor of safety is a figure used in structural applications that provides a design margin over the theoretical design capacity. Also known as the safety factor, it allows for uncertainty in the design process, such as calculations, strength of materials, duty and quality. It is equal to the strength of the component divided by the load on the component. For example, if a machine needs to support a load of 22 pound force (97.86 Newtons), and the safety factor is chosen to be four, the strength of the component is 88 pound force (391.44 Newtons).

The number chosen as the safety factor depends on the materials and use of the item. Industry standards for design and engineering usually specify the allowable stress, or ultimate strength of a given material divided by the factor of safety, rather than use an arbitrary safety factor. This is because these factors can be misleading and have been known to imply greater safety than is the case. A safety factor of two does not mean that an appliance can carry twice as much as it was designed for.

Even if each part of the appliance has the same factor, the appliance as a whole does not necessarily equal it. If one part is stressed beyond its maximum force, the distribution might be changed throughout the entire structure, and its ability to function could be affected. Determining the safety factor is a balancing game between cost reduction and safety. This number helps engineers determine facts about the appliance’s design structure and structural capability.

In general, a high factor of safety means a heavier component, more upscale material and an improved design. A factor of one means that the stress is at the allowable limit. Less than one means likely failure. A safety factor of three is used when the strength of the material is known to within a specific limit, and four or greater is used when a portion of the appliance’s load is variable.

Five or six are typical factors of safety when the load will alternately be taken off and put back on, like with suspension rods. Six or greater is used when stresses are reversed from tension to compression, and ten or greater is used when components of the appliance experience repeated shock loading. The number can reach a value of 40 or more when the stress is complicated and the amount uncertain, like in the crankshaft of a reversing engine.

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Discussion Comments

By Valencia — On Apr 17, 2011

I remember becoming interested in this topic when my high school bought exercise bikes for the gym. They came with a maximum weight load warning sticker that couldn't be removed. One of the teachers took this opportunity to lecture us on the factor of safety formula - talk about glazed eyes!

To be honest I didn't really grasp the way it works until I read this article.

By Bakersdozen — On Apr 15, 2011

@Acracadabra - the car salesman may not have known the exact figures you were looking for. It sounds like he was following company policy, which is based on sound testing and knowledge.

I have heard that tractors have a low factor of safety against overturning if they are used for towing. Perhaps it is caused by the same principle.

By Acracadabra — On Apr 14, 2011

I'm not sure how car manufacturers decide the factor of safety calculation, but when I bought a compact car the salesman was convinced that towing anything would be impossible! He couldn't produce any stats to back this up though. I felt like I was trying to get classified information from him!

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