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

What is Factor of Safety?

Factor-of-Safety in design and engineering defines how safe a design or product is from the required specification. In layman’s language, the Safety Factor is the ratio of actual capacity and the demand value.

This image shows factor of safety formula.
Safety Factor Formula

The Factor of Safety value depends on the end-use of a product. Engineers consider the high factor of safety value if the design failure can impact product performance or safety.

For example, a company designs a car considering loads from 12 passengers even if the sitting capacity is for four passengers. Vehicle factor of the safety will be (12/4 = 3) three.

Why Factor-of-Safety is Required?

Engineers use the Safety Factor in product design due to the 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 wear and tear up to acceptable limits.
Factors affecting the value of Safety Factor

The following factors affect the value of the Factor of Safety.

  • Criticality of the component or 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.
  • Manufacturing process variation.

Factor of Safety Equation

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

this image shows Factor of Safety equation in Mechanical Engineering
Factor of Safety Equation in Mechanical Engineering

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

Safety Factor Calculator

Difference between Safety Factor and Design Factor

Standards, regulatory bodies, or industries drive the value of the Design factor. The design factor defines how much force a product design should withstand. Whereas the value of the safety factor is derived from actual load requirement and how much actual load a body can withstand.

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

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

The margin of safety has the following applications.

  • Measure if product design fulfills the product design requirements.
  • How much additional load a product can withstand
  • 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.
Margin of Safety Calculation Equation

For example, if you are working on four seating capacity bench design. You need to design a bench that will not fail even if more than four peoples sit on it. This margin is known as the margin of safety.

The zero safety margin means that the product design can not take any additional load. But if the value of the safety margin is negative. Design modifications are advisable because the product will fail before reaching the design load.

Commonly asked Questions on a Factor of Safety

Value of factor-of-safety less than one indicates the product design is not as per requirements and product may fail in extreme working conditions.

The Factor of Safety value depends on the end-use of a product. Engineers consider the high FOS if the design failure has a greater impact on the product performance or human safety.

The following factors have an impact on the value of FOS:

  1. Application of the product or Working conditions.
  2. Manufacturing method
  3. Material used
  4. Types of loads and stresses acting on the product etc.

We will keep adding more information on safety factor in design and engineering. 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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