I have a task to choose a material and gear module for a double step up gearbox with equal ratios

The first pinion has a max bending stress of 296 MPa and the gear has 239 MPa. The permissible being stress for my material is 345 MPa. So both are acceptable for my chosen material. But how do I consider the factor of safety in my choice and work it out?

  • $\begingroup$ Look at the difference between the values. How much safety margin do you want? $\endgroup$
    – Solar Mike
    Dec 30, 2022 at 12:59
  • $\begingroup$ Hi, from general research I think spur gears are around 1.2 sf. It’s more of a you decide than you require this if you get me. So am I correct in saying 345/296 gives a min sf of 1.17 for my chosen material? $\endgroup$
    – Efan
    Dec 30, 2022 at 15:34
  • $\begingroup$ If you have a permissible stress, isn't the safety factor already in that value? $\endgroup$ Dec 30, 2022 at 22:24

1 Answer 1


There are several ways to come up with a safety factor. These are some of the considerations:

  1. use existing industry standards, such as NASA-STD-5017, which define safety factors for mechanisms. In this example, you'd use something like 1.4 against yield strength, and 2.00 against bearing strength.
  2. come up with one on your own, after having considered ramifications of failure, fatigue, difficulty or cost to replace/repair
  3. contact the manufacturer of gears, and ask them for recommendations.

You will have widely different allowable stresses for bearing and bending. You should consider each one, and consider appropriate FoS for each.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.