What is Factor of Safety in Design and How to Calculate it?

In design and engineering, the Factor-of-Safety defines how strong a design or product is from the required specification. In a layman language safety factor is the ratio of actual capacity and the demand value.

This image shows the formula for safety factor.
Safety Factor Formula

Factor of Safety value depends on the end use of a product. For example, Value of factor of safety is kept higher if a design failure has a greater impact on the product performance or safety of humans.

For example if the requirement is to design a car with a sitting capacity of 4 peoples. Considering the safety of people if you design the car for 12 people. That means the car has a safety factor of 3.

Formula for Safety Factor

In mechanical engineering, mathematically Factor of safety is the ratio of material strength and allowable stress. 

this image shows Factor of Safety Formula in Mechanical Engineering
Factor of Safety in Mechanical Engineering

For example, if the required specification of a shaft is to withstand 100 kg load. But if we design the shaft to withstand 200 kg load. Safety factor for the Shaft is “two”.

Why Factor-of-Safety is Required?

Safety Factor is used in product design due to following reasons.

  • To consider uncertainty in applied forces, design specifications and material properties.
  • Consider the impact of manufacturing variations.
  • To ensure product function even after some wear and tear.
Factors affecting the value of Safety Factor

Here is the list of factors that have an impact on the value of the factor-of-safety.

  • Criticality of the Component / part.
  • Impact of environmental conditions on product life.
  • Impact on user safety due to product failure.
  • Unknown design parameters.
  • Variations in user behavior.
  • Cost Factor.
  • Variation in manufacturing Process.

Difference between Safety Factor and Design Factor

Design factor value is driven from standards, regulatory bodies or industry specific. It defines how much force a product should be able to withstand. Whereas value of safety factor is derived from actual load requirement and how much actual load a body can withstand.

For example, if the standard requirement for critical automotive components is to have a minimum 2 design factor. Critical automotive components should have minimum FOS equal to or more than 2. In other words, the value of safety factor is always greater than or equal to the design factor.

What is the difference between Factor of Safety and Margin of Safety?

Margin of safety is used in industries to measure if the requirements are fulfilled and for how much additional load a product can withstand or reserve strength of a product. 

Mathematically Margin of safety is equal to the factor of safety minus one.

This image shows the formula for margin of safety.

For example if you are working on the design of bench with 4 seating capacity. Bench will be designed in such a way that it can take the load of more than four peoples. This margin is known as margin of safety.

If the value of safety margin is zero, that means product is not designed to take any additional load. But if the value of safety margin is negative. Design modification are advisable because the product will fail before reaching design load.

We will keep adding more information on safety factor in design and engineering. Please add your comments, questions or suggestions on the factor-of-safety in mechanical engineering in the comment box. We suggest you also read this article on Mechanical Stress and Engineering Strain.

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