Should I overdesign my product? If so, by how much?

As product design engineers we always face the question of whether we should overdesign any product. Well, the short answer is yes you should! But the most important questions are: by how much? and against which criterion?

Factor of Safety

Factor of Safety (FoS) is a well established concept in mechanical engineering to calculate how safe a given design is in terms of its internal mechanical stresses. Most of the time we want to design a part so it never reaches Yield Stress under certain loading conditions.

The formula for Factor of Safety is just the ratio of a calculated stress to the stress criterion we are using in our design.

Obviously, having a Factor of Safety below 1 literally means that the part will fail under the maximum loading circumstances. A Factor of Safety of just 1 means that our design barely withstands the maximum load and thus there is still a high chance that it will eventually fail from multiple cyclic loading (fatigue). Therefore, the best practice is that our FoS should always be above 1.

Wouldn't it be great if we could overdesign products to have FoS of 2, 3 or even 10? After all, all we are doing is protecting our design against failure. Well, overdesigning by too much is not exactly the best practice. Why? Because it means that your product will have much more material than necessary, thus making it expensive, heavy and more difficult to manufacture. Moreover, overdesigning means that the product will have a long life cycle and it may not be cost-effective in the long run since inventory turnaround will be slow.

Failure criteria

So, what criteria is out there to compare our design against?

In Mechanical Engineering we have several Failure Criteria proposed by different material failure theories. Those theories are based from the principal stresses derived from normal stresses in a stress tensor. Normal stresses are three-dimensional representations of tensile, compressive and shear stresses in a given point of our design. These theories use principal stresses in their calculations in order to give a number that we can compare our design against to. The most common theories used in Finite Element Software are Von Misses stress and Maximum Shear stress (Tresca). There are many more criteria (see Wikipedia link) but the most commonly used of them all is the Von Misses criterion as it works adequately well in ductile materials (e.g. metals and plastics).

Once you have determined which is the best criterion to use and you have calculated its value you can now compare your design against it.

So then, what is the best Factor of Safety?

This really depends on your application but if low weight of the design is important (e.g. aerospace applications) your design should have a Factor of Safety of 1.25 to 1.5. This applies only if you know the material properties, loading conditions and the calculated stresses with a high degree of accuracy.

If you don't know your material very well, or the loading conditions are fluctuating within a range somewhat known then your design should be around 2 - 3.

Moreover, any design where your material properties are not well known or where the loading conditions are poorly understood should be around 3-4 or above.

There are, of course, exceptions to this where certain things are required to have a large FoS by design codes, such as elevator cables, which require a FoS of 11.

In essence, choosing an appropriate Factor of Safety really depends on the application, material properties, and how accurate your calculations and loading conditions are.

To learn more about product design please visit GSE Biomedical and contact us with your product design needs.

Article written by Ramses Galaz