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Worm Drive material selection

Worm Drive material selection

Worm Drive material selection

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

I agree with your heat treater, in that 440 C stainless is not a good suggestion for use with an aluminum bronze mating component.  

My first question is why does the steel part need to have a surface hardness so much higher than the bronze part?  

My next question is what is the proposed method for manufacturing the initial blank for the worm?  Machined entirely or forged then machined for final dimensions?

Next question, have you considered the desired hardness profile, and the required heat treat process to produce it?  It sounds like you are favoring direct hardening over induction hardening, case hardening, carburizing, etc.  If this is definitely the direction you want to proceed in, then an unalloyed, medium carbon steel may be the best choice.

I am rather biased against SAE 1144 in general, and certainly when considering dynamically loaded components like gears.  The Sulfur content is just absurdly high.  I would think that SAE 1045 would be adequate for the hardness profile you require, which can be produced by a conventional direct quench & temper process, or by induction hardening.  The Sulfur content of 0.050 % max should be sufficient for low volume manufacturing.

For further information, I recommend you look at the 2-part series called GEAR HEAT TREATMENT that just appeared in the June 2002 and July/August 2002 issues of Heat Treating Progress (a publication by the Heat Treating Society section of ASM International).  It discusses current state of the art, and was quite thorough.  Your heat treater should be able to provide a free copy if you don't receive it yourself.

RE: Worm Drive material selection

Thank you for the input.
1)  I don't know that the steel needs to be that much harder.  Apparently 440C is used in some similar applications and would not normally be applied unless high hardness is wanted.
This prompts the question; why not make worm and gear from hardened steel?

2)  Worm machined entirely.

3)  You read me correctly here but after looking  more closely at heat treat response of 1144 it requires water quench and with worm geometry, I fear quench cracking.

4)  My understanding is that the loads are relativley light and that it is more of wear problem but I should have pinned down the stress level at the outset.

420 SS has also been applied in a similar app but again why a need for SS?  Now I'm thinking 4140 or 41L40 depending on the stress level.

Jesus is The TRUTH

RE: Worm Drive material selection

If you need some type of corrosion resistance, then the 420 SS is a good choice-- good hardenability, not excessive carbide formation like 440C, etc.

4140 or 41L40 sound like very good choices for minimizing quench cracking, distortion, etc.  

My guess on the hardness is because of wear resistance on another application where the mating component was steel, not bronze.  Obviously you have a good understanding of where you are at with respect to materials and processes, so good luck on finalizing this project.

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