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View Full Version : Meshing of spur gears.



brian Rupnow
06-01-2013, 04:31 PM
Last year, or possibly two years ago, I read a post where someone had machined a set of mating spur gears, and the meshing of the gears left a lot to be desired..---Sticky, binding spots as they were rotated. He solved the problem by putting some carborundum paste on them and letting them run that way for a bit. He claimed that it fixed things right up. I thought at the time that it sounded a bit crude, but I filed it away for future reference if I ever needed it. Today I needed it. The gears on my "something old Something older" project were very nasty---some spots turned freely, other spots were very "tight" during the rotation. The gears are pretty lousy as gears go---they were reclaimed from an old TV antenna rotor. I happen to have a full jar of automotive valve grinding paste that I purchased when I built my first I.C. engine. I found out that it was far to coarse for grinding small engine valves, but I kept it anyways. Since I didn't want to invest a lot of time in this project, I thought, "What have I got to lose?"--I coated the gear teeth with some of this compound and ran the crankshaft for about 5 minutes with my variable speed drill. WOW---What amazing results. Those gears now mate like as smooth as can be. The method may be a bit crude, but Hey!!!-Who am I to argue with success? I thought I would post about it.---Might help somebody out of a BIND! (Groan.)

Black_Moons
06-01-2013, 05:11 PM
Hah, the problem is its totaly uncontroled, It might be ok at making 'totaly unfit' gears mate 'better in the tight spots', but realise such gears are not going to last long, and the tooth profile is wrong now.

That said, Maybe you only need low RPM's and it will only see a few thousands revolutions over the next 100 years. Not all gearsets need to last 100,000km. Sometimes 'smoothness' of apparent fit is more important then backlash, life, jitter, noise, etc.

Also with machined gears, its likey he had burrs/surface finish issues, He could of had them just grind enough to remove the burrs without altering the tooth form much.

The main problem is that gear teeth don't 'rub' in a motion thats conductive to fixing the tooth form.

Norman Bain
06-01-2013, 05:18 PM
I also recall the post re use of grinding paste. Think the objections were mostly around the "embedding" of hard paste grains into the metal being worked; with result the grinding action would continue ... in the minds eye of the anal home hobbyist; indefinitely.

I can appreciate that concern but am sure that it could be minimized with a rag and brush cleanup then spin the gears again with a good "wash mix" late in the cycling (grinding) process. Perhaps a wash mix of kerosene and ??? something magical ... flour perhaps.

brian Rupnow
06-01-2013, 05:34 PM
I wouldn't recommend this for the gearbox in your Lamborghini, but for something as simple as a model, it works very well. A good wash with Varsol gets rid of the mess.

Toolguy
06-01-2013, 05:37 PM
Some gears just weren't very good quality to begin with. If they can be improved and made to run smoother with lapping compound, then that's the way to fix them. Some things (like Brian's project) simply don't require gears with "perfect" tooth profile, just teeth that will make the parts go round.

David Powell
06-01-2013, 06:26 PM
Some gears just weren't very good quality to begin with. If they can be improved and made to run smoother with lapping compound, then that's the way to fix them. Some things (like Brian's project) simply don't require gears with "perfect" tooth profile, just teeth that will make the parts go round.

I was told that you can get a better job of grinding the gears by moving one of them sideways while they are turning at a reasonable speed. I think my informant suggested that they be slowly be brought almost out of mesh to one side, then moved slowly sideways so they were almost out of mesh on the other side until they run smoothly. Hope this is of interest David Powell.

The Artful Bodger
06-01-2013, 06:58 PM
Logic suggests the method would work. Gears of perfect form would roll against each other while those of imperfect form would rub, obviously then imperfectly formed gears will abrade faster than perfect formed gears until they too are perfect in form.

Furthermore, I have shown that running a soft plastic blank against the teeth of a revolving gear results in well formed (I am unable to claim perfection) teeth on the plastic blank.

My vote is that it would work, in fact I have one poorly shaped change gear on a lathe and I would try the idea except that I have only 'good' gears to run it against and I fear destroying one of them.

J. R. Williams
06-01-2013, 10:03 PM
Lapping gears was a common practice for the automotive business for many years. It works.

J Tiers
06-01-2013, 10:29 PM
Lapping gears was a common practice for the automotive business for many years. It works.

Yep.

I suppose the main difference is that a special-purpose, specially-sized lap was used, instead of the other gear. No reason to suppose it wouldn't be OK for many less-than-critical purposes. If you use the Clover brand (IIRC) paste that breaks down as it works, it might be even better.... satisfying some of the more anal folks.

J. R. Williams
06-02-2013, 12:22 AM
I found my reference for lapping gears. It is in the book "Gear Cutting Practice" by Colvin and Stanley. The photos show three gears all running on the gear being lapped. The rotation is changed after a few minutes. The lapping gears are usually made of cast iron. The three driving gears are larger in diameter than the one being lapped.

wierdscience
06-02-2013, 01:12 AM
Yep.

I suppose the main difference is that a special-purpose, specially-sized lap was used, instead of the other gear. No reason to suppose it wouldn't be OK for many less-than-critical purposes. If you use the Clover brand (IIRC) paste that breaks down as it works, it might be even better.... satisfying some of the more anal folks.

http://www.rexnord.com/sites/Process/ringgears/Documents/Maintenance%20-%20Lubrication%20made%20easier%20with%20Lapping%20 Compound%20Timesaver.pdf

http://www.newmantools.com/lapping/time.htm

No affiliation,just a happy customer.I've used it on large frame gear reducers,especially preloaded helical units.Use of the compound means the run in period is reduced along with harmonics and heat.