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Unread 21-12-2009, 11:24
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AKA: Phil Malone
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Re: [FTC]: TETRIX DC Motor failure analysis

Quote:
Originally Posted by aklego View Post
For those of you who have lost a motor due to the failure of the main bronze bushing, how many had an encoder installed? The encoder collar set screw, installed per the instructions, seems tailor made to crush the bushing.
We had the main bushing fail, and it did have an encoder mounted, but I don't think the set screw was the problem. The bushing pulled free of the motor housing.

It's likely that the motor shaft had excess side load because the encoder-mount forces you to have the drive gear out at the far end of the shaft.

It seems like the bushing quickly develops some slop where it mounts to the end of the gearbox. (It's only crimp fitted to the gearbox). It's possible that since the encoder mounting ring is wider, it converts the side-to-side torque into a lifting motion on the bushing. Thus pulling it out of the housing.

Even on our wheel drives, the losening up of the bushing enabled the encoder mount to spin freely, with only the wires holding it into place. It seems that the support ring really needs to be attached to the gearbox housing, rather than the bushing.

Note to FIRST... it would be great to be able to epoxy the encoder ring to the end of the gearbox. This would have the dual effect of protecting the bushing AND making the encoder more secure. It wouldn't have any impact on robot performance other than to prevent common failures.
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Phil Malone
Garrett Engineering And Robotics Society (GEARS) founder.
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