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Mrowka

Pinion gear bafflement

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Took one of my Javelins out yesterday for a bit of light bashing.

All went according to plan, when I started getting a whining noise.The drivetrain was sticking whenever the pinion was at a particular spot, but when I disconnected the motor, everything was smooth again. I took off the pinion gear (30T 45p Robinson) and saw no obvious damage or wear.

The motor worked fine with the pinion gear off. Nor could I find anything wrong with the spur or counter gears. But the noise quickly started up again when I put everything back together.

Having no good ideas, I took the car home and installed another Robinson pinion (29T) and everything worked fine again. Go figure.

Looking at the original Robinson gear, I noticed that the grub screw is *really* loose and wobbly in the hole (no matter how far down it is) and the hex in the grub is a bit stripped. I compared with another pinion to see if it was just my imagination.

The only thing I can think of is that maybe the grub screw was so wobbly that it allowed the pinion to move, relative to the flat spot on the motor shaft, with the result that the pinion was no longer centered, relative to the motor shaft. But not so wobbly that it would spin on the shaft.

Does this make any sense? Or does anyone have a better explanation?

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RR pinions use imperial grub screws (Last time I bought one). Tamiya use metric screws. Perhaps you've put a metric grub screw in an imperial hole. The pinion might be slightly eccentric. (Motor shaft hole might not be exactly in the centre of the gear, causing tight spots when it rotates)

 

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Your guess is as good as mine, although this particular pinion seemed to be working just dandy, until it wasn't.

So I am not sure what could turn a circular pinion eccentric as part of normal operation. For that matter, it doesn't explain how a grub screw could suddenly get wobbly, either.

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11 hours ago, Mrowka said:

The only thing I can think of is that maybe the grub screw was so wobbly that it allowed the pinion to move, relative to the flat spot on the motor shaft, with the result that the pinion was no longer centered, relative to the motor shaft. But not so wobbly that it would spin on the shaft.

Does this make any sense? Or does anyone have a better explanation?

That would be my first guess . 
mynfirst step in troubleshooting would be to try with a different pinion ( which you did ) 

I don’t know why the RR pinion would have worn out like that. May actually be the case the hole is imperial and the grub screw is metric or vice versa.

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