What type of steel are gears commonly made from?

[Andrew]

Originally Posted by Matt Adams

I've been trying to hunt all over the place to find out what sort of steel that boston gear, martin, or SDP makes their gears out of. I'm trying to perform some calculations on gear teeth strength, and I'm looking to find the yield strength for the steels that are commonly used.

I've assumed thus far that 40 KSI is probably a conservative estimate, but I'm pretty sure I've heard that the yield strength on gears could be even twice or three times that. I've also heard that you don't want to assume that sort of strength with gears in calculations, and want to use half or even a third of the true yield strength?

Any thoughts? Even just dropping a few materials would be fine- I'm aware of http://www.matweb.com

Thanks!

Matt

When I last needed gear material information from Boston, I just emailed them and they responded with the type of material and basic specs. I asked about their brass, so I cannot give you the numbers for their steel.

In my experience, yield strength is not a reliable number. It can depend on how the material has been worked. In fact, when the gear is made, it will be worked and therefore the yield point will vary.

Therefore, in calculations, you probably want to use the yield strength as a minimum guaranteed number. This is why you would pick a lower number than that quoted for the material. In other words, if yield strength varied +- 10%, you would want to choose the -10% value for failure calculations.

Other than that, the conditions that the gear tooth is going to experience are highly variable and the stress formulas include a number of approximations. You normally assume a safety factor in your calculations to account for both of these variations. Numbers from 1.5 to 3 are not uncommon.

As for variations in yield strength by material, some steels have 40 ksi. Without looking it up, I remember that 4140 steel is twice the "generic steel" yield. I seem to remember also that stainless steel tends to have lower yield strength than the non-stainless, high strength steels. That doesn't mean that there isn't a stainless, high strength steel out there.

You also need to make sure you mean "yield" and not "ultimate." At yield, the tooth will deform. At ultimate, the tooth will break.

Not knowing what you are trying to accomplish, I would recommend using 30 ksi for yield and a safety factor of 2. If your numbers say this isn't going to be good enough, I'd start to look deeper into the type of material and actual loading conditions.