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41L40 for gear 1

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dan21703

Mechanical
Oct 2, 2005
16
I changed the material for a gear without appropriate due diligence. When reviewing the project I realized the oversight.

The original gear was 4615 heat treated Rc 55-60, with an unknown process. It is driven by a 4615 carburized idler gear. I changed the material to 41L40 pre-hardened Rc28-32. The original design would run 40 years without noticable wear. The gear is shock loaded. During initial tests the part appears to perform adequately. It is a little difficult to get a handle on the load which will be on this gear as it is driving a cutting wheel.

I fear the gear may fail by wear due to the carburized material being much harder.

It would be convenient to nitride the gear and stay with 41L40. Or should I be considering another material. The gear is part of a long shaft nominally 2" diameter.

I may be in trouble with this customer if I am not able to proactively address this problem if there is one.
 
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From the hardness range you quote, I suspect that the part you replaced was also carburized .
Without knowing the operating conditions it is impossible to know what problem you may have. The old gearset may have been way over-designed and the 30 Rc replacement might be OK but without all the details it is impossible to say.
 
I prefer to do my due diligence. Would you know how to analyze the relative wear of the 30c against the 50 c material? If I could approach this analytically it may work out better. I do have a way to back into the gear load.

Dan
 
Use AGMA standard to analyse the gears for bending and pitting
 
I agree with Carburize that the 4615 part was likely carburized. This means that both the bending resistance and the surface pitting resistance will be higher for the 4615 part vs. the 4140 part. I also agree with israelkk that you should perform some analysis using AGMA or other gear standards. The following information appeared in the March/April 2003 issue of Gear Technology ("Heat Treat Process and Material Selection for High Performance Gears" by Gerald J. Wolf, pp. 26-30):

Grade 1 (carburized & hardened to minimum 55 HRC) has an allowable design stress of 180 ksi for contact and 55 ksi for bending. The steel grades listed are 8620 & 9310, but 4615 would be similar for this type of hardness.

4140 that has a core hardness of 300 HB (~ 32 HRC or 145 ksi tensile strength) and nitrided to 84.5 HR15N (~ 48 HRC) has an allowable design stress of 155 ksi contact and 37 ksi in bending. All of this data is based on ANSI/AGMA 2001-C95.
 
I checked a random gearset for AGMA 2001 ratings and got pitting resistance power rating 3 times as high for a carburized pinion as for a 28 Rc pinion. Bending strength power rating was 1.6 times as high.

Nitriding is particularly UNsuited for shock loading - it is brittle.
 
Good point about the nitride option Philrock - have a star -
 
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