Stainless steel gears

[mgoldburt]

We are researching servo planetary gearboxes and recently had presentation from one company. Mechanical design is probably the best and most impressive we've seen, there is only one concern: they are using nitrided stainless gears (unfortunately, I do not know what stainless steel they are using). I newer thought of stainless being a material of choice for gears. Any comments?

 

[onestout]

My comments are just based on my experience (just a warning). Most gears in order to last are used with oil which prevents rusting. The more stainless you make SS the softer it gets (which is usually undesirable for gears). Gears are usually made from 8620 or 9310 when HT or sometimes 4140. Without knowing the application it's tough to guess why they designed it this way.

Michael

 

[diamondjim]

Ask them why they chose stainless steel for
your application. They probably had a good
reason for specifying it. Always get a second
source for your application and have them
specify the expected life. You are entitled
to know what you are getting for the cost
specified.

 

[EnglishMuffin]

There is an old caveat about using mating parts made from stainless steel - they have a tendency to "pick up". However, nitrided parts in general seem to have a resistance to this phenomenon, so I would imagine that with stainless they would also tend to perform OK. For nitriding, you need the presence of alloying elements such as aluminum or chromium to create the nitrides, so I would imagine that stainless, with its high chromium content , would nitride quite well. With gas nitriding, you generally can't get the case much deeper than about .02" to .03", and that can take several days, but with relatively small teeth that's an adequate depth to get good pitting resistance. They could be using stainless because it is cheaper and more common than nitralloy, which has a high aluminum content. Also, I have found that nitrided nitralloy gear teeth are so hard that they tend to chip extremely easily. In Germany, they have a special grade of steel that is just right for nitriding gears (I can't remember the designation) - it produces a case that is hard but not too brittle, and is chromium based, but it is not easy to get in this country - so maybe they have found that they can get similar results with stainless, which is of course readily available and not too expensive. The nearest steel to that german grade that I know of in this country would be something like H13 tool steel, which nitrides well becuse of its chromium content, but again, it's more expensive than stainless.

 

[Carburize]

The 12% Cr family of stainless steels (i.e. 410) nitride pretty well and will have good core hardness however I would be a bit concerned about low ductility.

 

[plasgears]

The only SS gears I have used were made by the PM process and heat treated either by air hardening out of the sintering process or conventional heat treat and tempering. Contact Keystone in PA for information.