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Worm Drive material selection

Worm Drive material selection

Worm Drive material selection

(OP)
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

RE: Worm Drive material selection

Metman..

Perhaps a combination of 440 at high strength + passivation + shot-peening + wear-chrome-plating [+0.0020" Thick) + grinding-to-finish [8Ra-to-4Ra =polished] would be effective.

Note: 4340 Alloy steel [etc] + shot-peening + wear-chrome-plating [+0.0020" Thick) + grinding-to-finish would also be effective.

NOTE: Be very careful suggesting H-11 steel parts. FAA AC20-127 has lots to say regarding H-11 steel bolts [mostly why NOT to use!!!]. Check it out at the following web-site.

http://www1.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/MainFrame?OpenFrameSet  ;[enter ac20-127 into "search"].

The combo of a polished-chrome-surfaced worm drive-shaft and bronze gearing [suggest VHS AMS4590] might work best [weight/performance ratio].

Regards, Wil Taylor

RE: Worm Drive material selection

(OP)
Wil,

Thanks for your input.  I am interested to see the FAA info on H11 but the link you provided led to "This page cannot be dispayed."

Jesus is The TRUTH

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