Extreme tooth wear 3

[louiecski]

Hey all, I have what appears to be a motor problem that may actually be a gear problem. I have visually inspected the gear and the teeth are worn nearly half-way through the original thickness. Since the motor driving this gear struggles and draws a lot more current than it should, I'm wondering if the worn teeth could be creating enough friction to cause my troubles? Since the gear teeth are hobbed into the motor armature, my interest is mainly out of curiousity at this point. The entire motor will need to be replaced regardless. Also, the mating gear seems to be in good shape and the center distance appears to be correct.

I guess what I'm wondering is, can a worn tooth surface cause a significant amount of friction? Perhaps because it's no longer an involute profile? Any information will be appreciated.

Thanks,
Lou

 

[dinjin]

It is probably a bearing problem and not a gear problem.

 

[BobM3]

I don't understand the mechanism but with our product, if the worm and gear teeth are wearing badly the motor torque does go up. So yes, badly wearing teeth can cause the torque to go up. I'm not sure if it's the friction though that is the sole cause.

 

[plasgears]

Wearing of a motor pinion is a bad sign. By pressing on a HT pinion, you may avoid this problem. Don't know the full details, but generally the pinion runs at a stress higher than the gear. See that the motor pinion has a greater face width than the driven gear. A hard pinion running against a softer gear will run quieter than steel to steel. I hope you don't have a 6 tooth pinion. The most reliable small pinion is 9 tooth, if you can allow it. Report fuller details.

 

[louiecski]

Ok, thanks for the replies so far. To answer the calls for more info: the motor is a starter motor on a motorcycle. This really isn't work related, but, being an engineer - I'm very curious about the tooth wear I'm seeing. Here's what I know:

The motor turns extremely slowly (judging by sound and difficulty of starting). It also draws the battery voltage down to about 5-7 Volts from it's normal state of 12VDC when cranking. It sometimes "jams" and won't turn the engine unless the starter is released then retried. The battery is new and holding charge just fine. The solenoid is working properly. In fact, the motor has been jumped directly from a truck battery with jumper cables bypassing the solenoid with no difference in behavior. The motor has new brushes and o-rings, and spins freely when no-load tested outside the bike. The bearings look like sintered bronze, and don't appear to have excessive wear. The commutator looks excellent, and a continuity test of opposite bars tested good all the way around the commutator.

The pinion teeth are cut into the end of the armature shaft, making the gear and armature one unit. I don't have an exact count of teeth, but 9 sounds about right and the driven gear in contact with it would be around 40-50 teeth. There is another pinion on that shaft, which then drives the flywheel. The flywheel is clutched to prevent backdriving of the starter when it's not in use.

What I noticed about the pinion teeth is that they are all uniformly worn to about 1/2 their original thickness on one side only (makes sense, given that they only transfer torque in one direction). The mating gear looks to be in good shape with no visible wear. None of the teeth are missing from any of the gears. They are not lubricated except by oil dripping past the crank seals and slinging around the flywheel housing.

Thanks again,
Lou

 

[Matt51]

Its a good guess the gear wear is the problem. Install a new pinion and you will know for sure.

 

[MikeHalloran]

I'm wondering if the tips of the mating gear teeth are jamming into a 'shelf' at the base of the worn sides of the teeth, adjacent the tooth roots, presumably still holding their original shape.

If so, moving the driven gear a few thou away might free it up for a few more miles.





Mike Halloran
Pembroke Pines, FL, USA

 

[plasgears]

OK, we now know it is a starter motor. Have experience there.
One of the problems of starter motors is lack of synch during engagement. The pinion slams into the gear out of phase, and bad things happen. Experience on yard equipment shows that an attached pinion with 'wobble' clearance on the driven shaft works well to allow desired engagement. We came up with this because mfrs of engines did not want to 'manicure' the gear with lead-ins. The gear was rough without any entry shaping or smoothness. The burden was placed on the starter motor people.
For your purposes, you can enhance the starting engagement by removing sharp edges on the driven gear and adding as large a radius as possible. An hourglass-shaped Dremel tool would do it.

 

[louiecski]

Thanks again for all the comments. Let me respond.

Plasgears:
Unlike most automotive or large engine starters, the pinion teeth in this case stay in contact with the mating gear always. There is a clutch in the flywheel to prevent backdriving of the starter motor once the engine is running, so no bendix or solenoid slamming the pinion into its mate. If I understand you correctly, you're suggesting that smoothing out the sharp edges on the axial edge of the teeth (tooth to face corner). This idea has me thinking I might take some of MikeHalloran's idea about the "shelf" having formed in the tooth root and smoothing that off with a dremel. Unfortunately, the center distance is fixed so I can't increase that.

I still may end up buying a new starter, but I'm curious enough to try these ideas just to see if it's the teeth causing the problem. Thanks again!

 

[dinjin]

Without knowing the full specs on the two gears,
it is hard to know if it is a design problem.
Is this happening on other starters for other
motorcycles or is this just happening with your
motorcycle? Who is the manufacturer of both
the starter and motorcycle?