Worm Drive material selction

[metman]

We are designing a linear actuator for driving the flaps on an airplane. The gearbox is a worm drive at 20:1 reduction using Aluminum Bronze (UNS C95400 equivalent to Ampco 18) for the gear. The design engineer has seen similar applications where the worm was 440C S.S. or 420 SS. Since I am a metallurgist, I was considering the callout for heat treat of the worm and while talking to the metallurgist at our heat treat vendor learned the following: 440C with its extra Carbon, forms carbides that protrude above the Martensite matrix approximately 0.0001” which tend to abrade a much softer material like Aluminum Bronze. I therefore began to consider other materials and processes and wondered if we need corrosion resistance in this application since the worm drive is enclosed and lubricated. The worm operates at 3000 rpm max.

I am suggesting we consider AISI 1144 which is direct hardening free machining steel. This will provide adequate tempered surface hardness of HRC 55 min which should wear good against Aluminum Bronze at HRC 37. I am assuming the stresses in the worm are low enough that fatigue is not a problem even considering that it is resulfurized steel but of course this must be verified.

If 1144 is unacceptable for any reason, my next suggestion will be nitrided Type 520 (AISI H11) tool steel. This seems somewhat overkill but the parts are relatively small so that material cost is not a factor. Root diameter approximately ½” X about 3” long. My suggestion of 1144 is primarily for ease of processing (machining and heat treat). Again if 1144 is unacceptable, nitrided H11 has a lot of positive processing and service history.

I am interested in your comments and suggestions for alternate material selection.
Jesus is The TRUTH

 

[GregPerry]

In the valve industry there are a lot of worm boxes used. The ones I have seen have brass or cast iron worms and BS 970 709M40 for the worms. The 709M40 is hardened and tempered. It is a very common material.