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Fixing oversized gear issues?

I am assembling some complex drive gearboxes for a 150 lb competition robot. The gearboxes use 24 DP gears from Martin Sprocket, Boston Gear, and Stock Drive, all high quality components. The one part that I did not supply myself is the drive pinion from the 12 volt motor, which is an 8mm shaft with a 2mm keyway. Prior to this year, I have always used supplied pinions from Martin Sprocket (reamed and broached by a third party supplier), which have been the same high quality as the rest of the gears . However, this year, due to availability, I am using pinions from a different supplier, which are of much lower quality, most likely machined from gear bar stock.

The main problem is manifesting itself in that the low quality pinions seem to be slightly over the correct pitch diameter, making proper meshing difficult. Furthermore, the pitch diameter seems to vary along their length, which makes it difficult to be consistent across the 8 different gearbox parts.

My question is what is the best way to reduce the size of just one gear. If the poor meshing was due to an undersize center-to-center distance, I would simply apply some fine abrasive compound and run in the gearbox, dissasemble, clean, and lubricate. However, in this situation, I want to target only the low quality gear, and not the high quality gear with which it meshes. Additionally, two pinions actually drive this gear (two motors feed into a single gearbox), and I need to target them individually to get the least binding with as little backlash and slop as possible. I could individually locate the high spots on each gear and attend to them with needle files and scrapers, but this is extremely labor intensive for all 8 pinions, not to mention the time and labor to disassemble and reassemble the gearboxes several times.

Any suggestions would be much appreciated Thanks ww88

Reply to
woodworker88
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First option - bite the bullet and order some good ones, even if it means boring them out yourself.

Second - set up a sacrificial good gear and the 8 pinions in a dedicated grinding setup, apply abrasive, and run for a while - but this requires disassembly, reassembly, "cut and try" and building a new fixture, and getting another good gear. If I understand grinding theory correctly, a softer "good gear" material should embed the abrasive and grind the "bad gears".

Reply to
Ecnerwal

Alright, I was thinking about doing that. I can bore out new ones, but what would you suggest doing about the 2mm keyway? The new pinions are a press fit onto the motor shafts (the old ones had setscrews) and they seem to be holding, although those are still also keyed as well. Thanks, ww88

Reply to
woodworker88

Enco has a 2mm broach for $38.63 McMaster wants $40.93 MSC wants $43.34

Bushing looks to be another $10 or so. Add another couple of bucks worth of stuff and use the February free shipping code from Enco.

Members of the crude expedient club have been known to file a keyway, in a pinch. Not that anyone else in this high-falutin' newsgroup would admit to stooping so low...

Reply to
Ecnerwal

Depending on the torque being transmitted, you could file off the keyway and produce a flat on the motor shaft. The pinion set screw would press on the flat.

I have bought good quality gears from QTC in New Jersey; price was quite reasonable: For M 1 module pinions (just a little smaller teeth than 24 DP) the price was less than $10. Enlarging the pinion bore without affecting the concentricity between PD and bore took some thought but can be done if you have a lathe and face plate.

I bolted a piece of aluminum plate to the face plate and drilled/ bored a counterbore for a light press fit on the gear OD. Pressed the pinion into the bore using the tailstock. Added 2 small finger clamps to hold the pinion tight in the bore... Since I was curious about the concentricity between the gear OD and its bore I checked with a dial indicator: The indicator needle did not move perceptively and I was happy. Then drilled to 7/16" and bored to 12 mm. Had to buy a 3 mm broach for the keyway. Since these were change gears for a hobbing machine the keyway was necessary.

You can lap the gears for improved running smoothness. But you should use an appropriate non-embedding abrasive that breaks down after a short period of use. Powdered turkey bone is one such material, another is the old domestic cleaning powder named "BON AMI". I found some at a flea market.

Trust this helps.

Wolfgang

Reply to
wfhabicher

"woodworker88" wrote: (clip) the low quality pinions

^^^^^^^^^^^^^^^^^^^^^^ The pitch diameter of a gear is not a fixed number. It is determined by the center-to-center distance of the gears and the gear ratio. IOW, the two pitch circles adjust so they roll on each other. If the pinion is slightly too large, you might be able to solve the problem by just moving the drive motor out a little. This will work, but it will depend on how the motor is mounted whether it is practical.

Reply to
Leo Lichtman

Ecnerwal wrote in news:LawrenceSMITH- snipped-for-privacy@news.verizon.net:

I've file one or three in my years of working with mechanical things, in a pinch. If you have good files, and patience, you can actually get a very good keyway with this method, getting it started straight is the worst part.

Reply to
Anthony

Alright, I may do that. Lots of good suggestions, I'll try them out and see what works the best. Thanks again, ww88

Reply to
woodworker88

snipped-for-privacy@hotmail.com wrote in news:868ea18e-fe16-44a0-9026- snipped-for-privacy@i12g2000prf.googlegroups.com:

Timesaver lapping compound is a commercial product with the properties you describe:

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No affiliation. It was just the result of a search I did about a month ago when I needed to deal with some tight gears for a project.

Reply to
Bruce Spainhower

Looks like good stuff to me. Did you buy it directly from them or through someone else?

Reply to
woodworker88

Auto parts stores. Another brand to ask for is Clover. Ask for valve grinding paste or valve grinding compound.

Cheers Trevor Jones

Reply to
Trevor Jones

Be careful. The "valve grinding compound" one gets in auto parts stores is carborundum grit in some kind of water-based gel, and does not break down into harmless stuff. The standard maker is Permatex. One must carefully wash all traces of the grit away.

Joe Gwinn

Reply to
Joseph Gwinn

woodworker88 wrote in news:58ca7768-f27a-4064-80f0- snipped-for-privacy@l16g2000hsh.googlegroups.com:

Got it through a Model T parts house, the yellow stuff, for lapping softer metals. Pretty common for fitting babbit bearings in old engines. I needed it for both the cam bearings and the cam gear.

Reply to
Bruce Spainhower

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