Factor of Safety in Design and Engineering: The Basics

Good design engineers must consider so many factors when designing a part or component. Design for assembly, cost, logistics, manufacturability, reliability, and other qualities all require forethought and creativity. Perhaps one of the most important qualities to be considered when creating parts or products is safety...and naturally, an entire industry has cropped up around the need to manufacture safe products and structures for consumer use. Most commonly, you’ll hear the terms “Factor of Safety” (FoS) or “Safety Factor (SF), but there are several definitions and calculations that may be referred to. Let’s look at the basics of FoS for design and engineering.

What’s the big picture when it comes to Factor of Safety (FoS)?

“Factor of Safety” usually refers to one of two things: 1) the actual load-bearing capacity of a structure or component, or 2) the required margin of safety for a structure or component according to code, law, or design requirements. A very basic equation to calculate FoS is to divide the ultimate (or maximum) stress by the typical (or working) stress. A FoS of 1 means that a structure or component will fail exactly when it reaches the design load, and cannot support any additional load. Structures or components with FoS < 1 are not viable; basically, 1 is the minimum. With the equation above, an FoS of 2 means that a component will fail at twice the design load, and so on.

Different industries have different ideas on what a required margin of safety should be; one of the difficulties associated with using a FoS or SF is some measure of ambiguity. But there are some general rules of thumb across multiple verticals. Obviously, if the consequences of failure are significant, such as loss of life, personal harm, or property loss, a higher FoS will be required by design or by law. Another consideration is cost: how much extra does it cost per part to achieve a certain FoS, and is that a viable business model?

Other factors to consider when designing parts with safety in mind

Depending on the intended use for a product, both the overall design and individual components need to be assessed as accurately as possible for anticipated conditions. The following considerations can help design engineers account for real-world conditions, and create a better product.

·         Intensity of stress concentrations; which components will be subjected to more intense stress, more often?

·         Is thermal cycling or extreme temperature exposure an issue? How does this impact function, or the materials to be used?

·         Is scheduled maintenance likely to occur? Will it happen regularly, and be of similar quality each time?

·         Does the combination of materials used weaken or strengthen the overall design?

·         Is it likely that wear and tear will be accelerated through consumer use (say, regularly exceeding rated capacity, etc)? Are there controls in place to help prevent this?

·         Is the structure or component subject to deterioration through corrosion?