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Can M42 be used to manufacture exceptionally hard gears

Joshua Nicoll

Plastic
Joined
Dec 19, 2020
Hello, recently when talking with someone, they mentioned they had very small beleved spur gears (metric module .8mm) and due to the high operating speeds (up to 25K RPM) the gears did not last too long. The gears in question are used to rotate a shaft 90° in a quite normal fashion, same tooth count just a direction and angle change, but due to have to be a small size, and with such wear, apparently they do not last long. The current gears in the technical drawing specify 65 HRC but knowing M42 can be between 68-70 got me wondering, would M42 be too brittle for such a use? My bandsaw uses M42 and I never noticed it being anymore brittle than high carbon blades, actually I thought it was less brittle, so would M42 be suitable for quite small high speed gears? I understand that making such small gears from M42 would be a nightmareish challenge but I like problem solving so wondered if in theory it would even work.
 
It would help if you indicated the wear and failure modes. Is it abrasive wear, and the teeth grind themselves to death, or brittle failure and teeth crack off, or ??

You may have issues of improper mesh, bad bearings, or geometry that doesn't support the gears properly. Bad lubrication, overheating - all sorts of things could lead to premature gear failure.

Can you take some pictures of worn gears and the gearbox and post them?

Once the failure modes are better understood, materials recommendations will follow.
 
Hello, recently when talking with someone, they mentioned they had very small beleved spur gears (metric module .8mm) and due to the high operating speeds (up to 25K RPM) the gears did not last too long. The gears in question are used to rotate a shaft 90° in a quite normal fashion, same tooth count just a direction and angle change, but due to have to be a small size, and with such wear, apparently they do not last long. The current gears in the technical drawing specify 65 HRC but knowing M42 can be between 68-70 got me wondering, would M42 be too brittle for such a use? My bandsaw uses M42 and I never noticed it being anymore brittle than high carbon blades, actually I thought it was less brittle, so would M42 be suitable for quite small high speed gears? I understand that making such small gears from M42 would be a nightmareish challenge but I like problem solving so wondered if in theory it would even work.

More to gear alloys than "High Carbon"!

M42 is actually pretty resilient as hard alloys go. See drills, taps, dies, and milling cutters with an age-old reputation as being less likely to just snap than solid carbides. More challenging materials by far have found their way into speciality gearing.

That said, "if you have to ASK".. you need a gear specialist familiar with bleeding-edge challenges ...already.. to do the do ... with what THEY know to JF work.

Not just advice off the 'net. Gears are not easy.

Exotic high stress problem-solvers are a whole 'nuther level of doing the impossible affordably and well. "Live" tooling uses a lot of that in tiny spaces.

"Find the experts". It's their Day Job. They didn't learn it in just the one day.

Life can be a tad short to "become one" at yer own R&D and expense.
 
My first thought is the lubricant is being flung off causing the gears to operate dry. I would conduct a test with a constant supply of oil being injected between the gears just as they roll together (not when rolling apart). Use a recirculating system with an inline filter.

An oil bath has little chance of working at those RPMs.
 
It would help if you indicated the wear and failure modes. Is it abrasive wear, and the teeth grind themselves to death, or brittle failure and teeth crack off, or ??

You may have issues of improper mesh, bad bearings, or geometry that doesn't support the gears properly. Bad lubrication, overheating - all sorts of things could lead to premature gear failure.

Can you take some pictures of worn gears and the gearbox and post them?

Once the failure modes are better understood, materials recommendations will follow.

Ok, I checked quickly with him and he said the gears suffer from abrasive wear. He mentioned that he used some other slighly different ones that would wear themselves in and got hot quickly, but he doesn't use those anymore. Apparently also sometimes axial pressure causes failures which to me sounds like the gearbox housing, which is a 90° drill head basically (the gearbox housing drill head "thing" is used in equin denistry, as this is a tooth drill type device), could be the culprit at play. I've talked with him and he's gonna drop off apparently a lot of failed units for me to tear apart and inspect, so I'll be able to post photos and failure modes of many units then.

For now lets assume the gears really are the only problem, would a harder material actually do anything, since these gears at 65 HRC are already very hard, would making them even harder actually solve the issue or only make them last a bit longer instead of 3x or 4x longer?
 
Ok, I checked quickly with him and he said the gears suffer from abrasive wear. He mentioned that he used some other slighly different ones that would wear themselves in and got hot quickly, but he doesn't use those anymore. Apparently also sometimes axial pressure causes failures which to me sounds like the gearbox housing, which is a 90° drill head basically (the gearbox housing drill head "thing" is used in equin denistry, as this is a tooth drill type device), could be the culprit at play. I've talked with him and he's gonna drop off apparently a lot of failed units for me to tear apart and inspect, so I'll be able to post photos and failure modes of many units then.

For now lets assume the gears really are the only problem, would a harder material actually do anything, since these gears at 65 HRC are already very hard, would making them even harder actually solve the issue or only make them last a bit longer instead of 3x or 4x longer?

?? WTH?? Dental heads are OLD as such challenges go.

Smaller mouths of humans drove the technology to water-and air powered turbines ages ago.

How much meat is there likely to be, chasing this bone?

Bound to be a solid "industry" out there, and already well-established?

What are THEY doing? Could was accepting limited-life is the cheapest route -no MONEY for better, even if it was to be had?
 
?? WTH?? Dental heads are OLD as such challenges go.

Smaller mouths of humans drove the technology to water-and air powered turbines ages ago.

How much meat is there likely to be, chasing this bone?

Bound to be a solid "industry" out there, and already well-established?

What are THEY doing? Could was accepting limited-life is the cheapest route -no MONEY for better, even if it was to be had?

To be clear, these are huge diamond pads they use to drill the whole tooth down, they use modified die grinders with speed controllers to power these things and they're over 400mm long, these drills are *nothing* like a human denistry drill, as they're for horses which have huge teeth pretty far in.
 
My first thought is the lubricant is being flung off causing the gears to operate dry. I would conduct a test with a constant supply of oil being injected between the gears just as they roll together (not when rolling apart). Use a recirculating system with an inline filter.

An oil bath has little chance of working at those RPMs.

There would be very little room to have a recirculating oil system, as it stands they seem to use food grade grease in these things since they're for use in a horses mouth, and the gear module being .8 will tell you how small they are. I did suggest using molybdenum disulphide powder instead of a grease but not sure if it'd be any better. Perhaps adding tiny oil ports that inject then suck the oil would be a better option than grease that could be shed quickly. He said if he doesn't keep it well greased it dies even quicker.
 
Here are some of the technical drawings related to the gearbox-drill head item in question and the gears, as you can see they are small and fairly average, I made a mistake when I thought they called for 65 HRC when it clearly says 60 there. Obviously there is some challenge just from the small size and the forces they are subject to. It seems like the diamond grinding disk like device mounts directly to one of the gears and that also seems like a bad design decision but space is at a premium so maybe they don't have an option.
 

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Maybe pneumatics and an inline lubricator using a food grade oil (castor oil?) if it can be exhausted to the outside. Also DLC coating on the gears.

I like your idea of using tiny ports possibly with a lossy oil system or recirculating (on the outside) if practical.
 
Joshua, when you do get the failures it’s important to match the gear sets. A 1:1 is the worst gear ratio that you can possibly have for wear properties. 1:1 means that any one tooth on the driver will only mate with one particular tooth on the driven gear exclusively. Look to see if one tooth set has more wear/distress than the others on a matched pair gears. You may have to ask for a set that has not failed yet but that is only a bit noisy if those you receive are beyond analysis.

If this is the case your solution would be to lap or grind the teeth after heat treat.

As an example, Auto and truck axles also perform a 90° power transfer. Their ratios are out to 2 decimal places like 4.11:1 meaning no two individual teeth make contact with each other again for many, many revolutions. This is why thy last so long.
 
Joshua, when you do get the failures it’s important to match the gear sets. A 1:1 is the worst gear ratio that you can possibly have for wear properties. 1:1 means that any one tooth on the driver will only mate with one particular tooth on the driven gear exclusively. Look to see if one tooth set has more wear/distress than the others on a matched pair gears. You may have to ask for a set that has not failed yet but that is only a bit noisy if those you receive are beyond analysis.

If this is the case your solution would be to lap or grind the teeth after heat treat.

As an example, Auto and truck axles also perform a 90° power transfer. Their ratios are out to 2 decimal places like 4.11:1 meaning no two individual teeth make contact with each other again for many, many revolutions. This is why thy last so long.

Yes I know when I get the heads to make sure I never get the gears mixed up, I'll be doing them one at a time and bagging all the components and such for each head. I'll pay particular attention to the bearings and the wear patterns. I have not seen these gears before and not sure what the surfaces are like, but I'd imagine being small I might have to get a microscope. I do not think it'd be possible to change the gear ratio but maybe adding 1 tooth and removing one from the other gear wouldn't effect functionality but extend life.
 
Joshua, when you do get the failures it’s important to match the gear sets. A 1:1 is the worst gear ratio that you can possibly have for wear properties.

This was on my mind too, I was waiting to find out if there was any relevance to the ratio before suggesting a tooth count change. If using the original gear cases then a custom gear geometry may be needed to change the ratio, but that's doable if you're making new ones.

I would strongly suggest looking at either a DLC coating (as mentioned), or plasma nitriding to improve surface tribological properties.

I'd also try to figure out if there's a way to induce grease to move towards the gear mesh, like having some thread or auger shapes on the cylindrical bodies of the gears to bring lube back into the mesh zone.

But given the tight confines, I'd really like to have a remote pump and ingress/evacuation plumbing into the gear housing, not just for lubrication of the gears, but for heat removal. The oil sump would either have an oil to air heat exchanger, or be of sufficient volume that normal service would not be enough to significantly raise the oil temps.

The lube could be a pure mineral oil, there's versions suitable for animal/human exposure and ingestion if a little leaks out.

Other (more expensive) options include changing to a spiral gear tooth form.

When you get the sample parts, try to get accurate measurements of the housing bearing centers, if they're closely controlled that's a good start. If they're wandering significantly then you've got to address that before making up new gear sets.
 
Maybe pneumatics and an inline lubricator using a food grade oil (castor oil?) if it can be exhausted to the outside. Also DLC coating on the gears.

I like your idea of using tiny ports possibly with a lossy oil system or recirculating (on the outside) if practical.

I was thinking along the lines of some kind of diamond or titanium nitride coating on the gears as well as either another solution or maybe in conjunction with the tool steel but at that point that might add even more insane cost to these, while these currently are not cheap devices, they're not overly expensive too for the niche that they fill. Origionally I want just contracted to make the shafts lighter in weight but the more we talked about them the more complaints I heard and it got my problem solving half of my brain convinced there where better ways to do this.
 
I feel like the gears aren't held solidly enough axially. Forcing them together while spinning at high speed will kill them for sure, and spreading them apart isn't much better.
 
Is there any opportunity to lubricate with an oil mist conveyed by compressed air?

Interesting conversation... And NO politics..... Yet..

I was thinking that grease seems like a bad idea. They spin up, it gets tossed off and doesn't
flow back. Oil will at least flow back into the gears and re-lubricate them. Somebody must make
a food safe 80W.
 
Eliminate the gears. Vane motor at the end. Just like a real dental drill.

None of the dimensions are ligible for me, but I did see 12.8mm pitch diameter or some such so this thing is like 2-3 times the size of a human dental drill and 1/3 the RPM.

build a 3X scale dental drill. Done deal.
 
To be clear, these are huge diamond pads they use to drill the whole tooth down, they use modified die grinders with speed controllers to power these things and they're over 400mm long, these drills are *nothing* like a human denistry drill, as they're for horses which have huge teeth pretty far in.

Grew up on a farm still powered by horses. Not "human", but right nice "people" nonetheless when treated well.

You think even a Brabant has large teeth? See Elephant molars!

:D

The point stands. You can only work a size-limited gear but so hard, and the "experts' actually ARE in CNC machine-tool "live tooling" head bizness. Their tightly-size-constrained heads have to do more work in an hour than a whole herd of horses need in their lifetimes.

Wouldn't be "impossible" that what you need for the high-RPM right-angle solution is a stock item. "Small" is OFTEN on their spec sheet. Look around.

Annnnd. Turbine tech is NOT limited to uber-tiny, either. See high-speed supplemental spindle milling and drilling goods, Japanese and not-only.

BTW: Castor oil may be "vegetable". Also a powerful emetic. Even toxic.

Horses are not just funny-looking dogs or humans, have their own physiology and tolerances. Vets understand and respect that.

Oil mist and its exhaust sound would not be on my dance card. Water could be.
BOTH might be expected to need collection and draw-away right at the exhaust, as well as silencing. Water is, however, of use cooling the tooth and flushing away the grinding debris.
 








 
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