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KidsWheels
Need new motors? Grind a gearbox? Adding teeth to a pinion?
#147730
Ok, so I've got two 21T 7R gear boxes with metal gears from MLTOYS. I'm using Robinson racing 8721 hardened pinions. In one gearbox, the metal gear and pinion match up just fine. In the other, it is so misaligned that it's causing slippage. So, in my troubleshooting I decided to swap the parts from the "ok" gearbox to the "misaligned gearbox" to see if it fixed the problem. What I swapped was, the metal gear, bearings, gear shaft, and motor to keep variables the same. It didn't fix the problem, the gears were still misaligned. ...Put the same parts back in the other gearbox and they line up just fine. So... I ruled out all of those parts-- must be the gearbox, right?? Maybe deformed slightly due to heat & usage?

So, I order a new empty gearbox from ML Toys recently. Two actually, in case I need a backup. Well, both of those don't allow the metal gear and pinion to line up properly either. Same exact issue. So, only 1 of my original gearboxes align properly... but 3 others (two of which are BRAND NEW) don't line up with the same parts?

Scratching head here... Any ideas? Anyone have a similar issue? Surely MLTOYS can't be selling several defective gearboxes???
#147737
Yes, here's two pictures. One labeled "ok" and the other "bad". Now, the gear alignment on the "ok" one doesn't look ideal to me but it's been fine enough to work without any issues. The gear alignment on the "bad" one is much worse.

Part of me is wondering if I should just run 22T pinions and hopefully that will align correctly? But honestly, I don't really want to pay for more $$ hardened pinions and reduce my torque any more than it is...
Attachments
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#147740
I didn't compare because the original plastic gear is too damaged for comparison. yea, I've got two other robinson racing 8721's I've compared no difference. based on how bad the gear mesh is right now in the "bad" picture...do you think a 22T would be large enough to tighten the gap ??
#147741
Are you using a centering spacer/sleeve (MLTOYS or otherwise) or just relying on the screws to pull the motor into position?

The 'bad' fit is about what I'd expect for a 21T gear in a fitting that expects a 22T gear -- the size difference means you'll have 0.016" more gap than a 22T (the "correct fit"). The factory seems to only set 0.030-0.035" mesh to account for thermal expansion, run-to-run variation, and other factors. Even the "OK" fit is looser than I've been adjusting my quad-damage gearboxes for (on those I can move them to exactly where I want with the gearbox open, then tighten down the screws).

You can you think of the difference with a 22T gear as adding about 25% of a gear tooth's depth (the tooth is ~0.060" from root to tip and the radius of the 22T is 0.0155" larger than a 21T). So in the bad one, it would probably close up almost half of the remaining gap (and would probably be perfect). On the "OK" one it may be too tight, though the steel expands less as it heats up vs. the plastic, so you don't need as much gap to start with. Since you have 3 "tight" gearboxes, this would be no problem. Seems like the one that is working probably just left the factory on the tight side.

I was able to cleanly use standard 15T pinions on a 16T gearbox by making an offset sleeve and drilling out the screw holes very slightly. If you have a drill press and some 1/2" PVC pipe and some time , that may be an option rather than buying new pinions. (I posted about this in the Dune Racer thread - http://forum.modifiedpowerwheels.com/viewtopic.php?f=9&t=21263)
#147753
I 'm using the MLTOYS centering sleeves. I had to enlarge the hole to fit over the hub of my motors though.

I did wonder about making offset sleeves...I like that idea, I'll leave it as an option to use the 21T pinions later if I don't like the performance with the 22's.

I'm glad it seems that the 22T pinion will tighten the gap correctly. I am wondering though...not to bash, but why doesn't MLTOYS state to use at least the "middle" pinion tooth # that your gearbox is rated for? They say you can use either of the 3 with no problems.

I'll have to use your spreadsheet to see the torque difference between 21T/22T. ...waiting for the new pinions to come in
#147755
I'm not sure why they don't state that. Using a 23T would almost certainly result in a ton of binding/grinding and cause problems. I tried putting in a 17T on my 15-16-17 gearbox with a standard non-offset sleeve and it was pretty obvious it was going to bind.

That information about the hub size on the MLTOYS spacer is useful; it's not stated anywhere on the website and the RS-775 series motors come in either 15mm or 17.5mm hub diameters. It wasn't clear which one they had tailored it for and I didn't see any specs from Titan after a brief search.

I uploaded a newer version of the spreadsheet -- it's still beta, but adds a bit more flexibility (allows you to pick any # of gearboxes in parallel, for instance).

https://drive.google.com/open?id=0B7sByZqUm8XxLThkYk44RDJUYjA

Torque difference will be just proportional to the # of teeth, so you'll have 21/22 (95.5%) as much torque as you had before.
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