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Factor of Safety, FOS

Factor of Safety

The factor of safety also known as the Safety Factor is used to provide a design margin over the theoretical design capacity to allow for uncertainty in the design process. The factor of safety is recommended by the conditions over which the designer has no control, that is to account for the uncertainties involved in the design process.

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Importance of Factor of Safety

It is common practice to size the machine elements so that the maximum design stress is below the UTS (Ultimate Tensile Stress) or yield stress by an appropriate factor – the Factor of Safety, based on UTS(Ultimate Tensile Stress) or Yield Strength.

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How the factor of safety affects the engineering design

The uncertainties include (but are not limited to),

  • Uncertainty regarding the exact properties of the material. For example, the yield strength can only be specified in between a range.
  • Uncertainty regarding the size. The designer has to use the test data to design parts that are much smaller or larger. It is well known that a small part has more strength than a large one of the same material.
  • Uncertainty due to machining processes.
  • Uncertainty due to the effect of assembly operations like riveting, welding, etc.
  • Uncertainty due to the effect of time on strength. Operating environments may cause a gradual deterioration of strength, leading to premature and unpredictable failure of the part.
  • Uncertainty in the nature and type of load applied.
  • Assumptions and approximations made in the nature of surface conditions of the machine element.

Selection of factor of safety

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The selection of the appropriate factor of safety to be used in the design of components is essentially a compromise between the associated additional cost and weight and the benefit of increased safety or/and 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.

An appropriate factor of safety is chosen based on several considerations. Prime considerations are the accuracy of load and wear estimates, the consequences of failure, and the cost of over-engineering the component to achieve that factor of safety.

For example, components whose failure could result in substantial financial loss, serious injury, or death usually use a safety factor of four or higher (often ten). Non-critical components generally have a safety factor of two. Extreme care must be used in dealing with vibration loads, more so if the vibrations approach resonant frequencies.

The vibrations resulting from seismic disturbances are often important and need to be considered in detail. Where higher factors might appear desirable, a more thorough analysis of the problem should be undertaken before deciding on their use.

  • 1.25 – 1.5

    Material properties are known in detail. Operating conditions are known in detail. Loads and resultant stresses and strains are known with a 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 are required.
  • 2 – 2.5

    Materials obtained from reputable suppliers to relevant standards are 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 the environment, load, and stress.
  • 3 – 4

    For untried materials used under average conditions of the environment, load and stress. It should also be used with better-known materials that are to be used in uncertain environments or subject to uncertain stresses.

Applications of the factor of safety

Usually, the factor of safety is kept larger, except in the aerospace and automobile industries. Here safety factors are kept low (about 1.15 – 1.25) because the costs associated with structural weight are so high. This low safety factor is why aerospace parts and materials are subject to more stringent testing and quality control.

Now computers are being used to provide a more accurate simulation of stresses that occur in components, particularly in the case of high-value products where safety and saving weight are essential.

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