[FTC]: TETRIX DC Motor failure analysis

[JesseK]

I just got some info back from yesterday's build session. The students discovered that the slowdowns were due to the excessive use of rubber bands that over stiffened the contact between the shooter wheels and the ball. They were adding more rubber bands throughout the day in order to increase consistency of the shots. Everything's back up to normal speeds now.

4 teams with a total of 22 motors on the bots and 0 broken after 1 competition. Just a different environment and different luck I guess. Last year I think our 4 teams burned through 12 or so.

[aklego]

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.

[PhilBot]

Originally Posted by aklego

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.

[aklego]

One of the (many) failure modes I've seen among our local teams stems from the encoder set screw. When this screw is tightened into the soft bushing, the bushing deforms and binds on the output shaft. This binding causes a torque to be transmitted to the bushing every time the motor is rotated. At best there will be accelerated wear and the bushing will survive. At worst, we send another $30 to Pitsco. The set screw torque needed to cause this failure is quite low and I consider it to be a fault of the design rather than of the installer.