Re: Material for gear

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Posted by Bill S. on October 05, 2002 at 00:56:46:

In Reply to: Material for gear posted by Mike Tate on October 04, 2002 at 08:56:56:

Hi Mike,
If 8620 is not standing up, you would have to go to 4140 at 42 Rc max, or some higher chrome-moly alloy, which is going to be bad news on hobbing cutters.

I think you are seeing a misalignment problem, probably due to high torque loading. High loads can cause the gear shafts to separate or flex away from each other, causing two bad things: All the contact shifts to the tooth face edge nearest to the bearing. Also teeth "slide" as they mesh resulting in high friction, heating, wear, pitting, etc. rather than the normal conjugate rolling tooth action.

The sensible remedy is to stiffen shaft and bearings and to back off the gear load imposed by the machine. You want to prevent the loaded contact pattern from shifting to the tooth's face edge. (But, knowing that customers often operate production machines at or above design loads, you probably won't win points by telling him to ease up ;^).

There may be a few things that might help, if you haven't tried 'em already:

45 deg chamfer all the way from tip to root of tooth. This moves tooth loading away from the extreme face edge which is the weakest part of the tooth. Probably a small improvement at best.

Cutter designed to leave a fillet in tooth root, also slight chamfer at tooth tip. A modest fillet in the tooth root avoids stress concentration in the otherwise sharp corner due to beam deflection of the tooth.

Oversize the pinion by between a half to one tooth, if the set is not on fixed centers. There are formulas that help optimize the oversize amount, but that would involve an engineering analysis of the loads, deflections, impact, etc.
Oversizing widens tooth base adding to tooth strength.
I'm guessing there is not enough room to go to a larger module.
What is the mating gear size? does it last about the same amount of time? Or does the 42T gear always fail first?

Tooth crowning is one of the ways to make a gearset more forgiving of misalignment, but requires special equipment. The teeth are slightly "fatter" halfway between the faces, which helps keep contact from shifting to a face edge.

Another design element in a helical set is to increase face width and/or helical angle to bring more teeth into simultaneous contact. Distributing the load amongst 3 or more teeth has obvious advantages. Probably won't help much if shaft flexure or misalignment is severe.

In some applications where load is high, same direction and constant, gears can be designed with an "offset" in the helix angle. I've been told this is common in large marine gear boxes. This offset is small of course, and not going to help in cases where high impact loading is a problem.

Also, it might be that this is the fuse, shear pin, or "weak link" in this machine. Fix this problem, and something else in the train may start failing prematurely. Ah, well....

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