The factor of safety is a design margin used in structural applications to account for uncertainties. It is the resistance of a component divided by the load on it and varies depending on materials and use. A high safety factor means a heavier component and improved design.
The factor of safety is a figure used in structural applications that provides a design margin over the theoretical design capability. Also known as a factor of safety, it allows for uncertainties in the design process, such as calculations, material strength, service and quality. It is equal to the resistance of the component divided by the load on the component. For example, if a machine is to support a load of 22 pounds of force (97.86 Newtons) and the safety factor is chosen out of four, the strength of the component will be 88 pounds (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 using an arbitrary factor of safety. This is because these factors can be misleading and are known to imply greater security than the case. A safety factor of two does not mean that a piece of equipment can carry twice as much as it was designed to.
While each part of the appliance has the same factor, the appliance as a whole does not necessarily equal it. If a part is stressed beyond its maximum force, the distribution may be changed throughout the structure and its ability to function may be impaired. Determining the safety factor is a balancing act between cost reduction and safety. This number helps engineers determine the facts about the appliance’s design structure and structural capacity.
In general, a high safety factor means a heavier component, a more refined material and an improved design. A factor of one means that the stress is at the allowable limit. Less than one means probable failure. A factor of safety of three is used when the strength of the material is known to be within a specific limit, and four or greater is used when some portion of the load on the fixture is variable.
Five or six are typical safety factors when the load will be alternately taken off and on, as with suspension rods. Six or greater is used when stresses are reversed from tension to compression, and ten or greater is used when appliance components experience repeated shock loads. The number can reach a value of 40 or more when the stress is complicated and the amount uncertain, as in the crankshaft of a reversing engine.
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