393 Motor "Popping"

We have been driving our robot around recently pretty often. We were driving yesterday and we started hearing a clicking or popping sound coming from one of our drive motors. It is a 393 motor GEARED FOR SPEED and is connected directly to the wheel. When we turn the motor by hand the motor hits some stopping points like some of the 269 motors. We have recently installed the new INTEGRATED ENCODERS but we are certain they are installed correctly.

The motor is driving a back 4.25" omni wheel and has a lot of load on it. I have also programmed, using RobotC, functions to prevent stress on the motors (linear ramping the motor to the desired position).

Any help would be appreciated.

Have you even investigated this problem (opened the motor)? It could be something with the teeth broken or the gear sheared. From what I saw with your robot, I think you have a few missing teeth in one of the gears, and the popping is these teeth that popped off and are now inside the motors somewhere.

Yes we did open up the motor and inspected the gears and nothing was visibly wrong with them. We did however find a piece of plastic that seemed to have got shaved off from the casing.

Did you open the back side? When the gear sheared for our motor, it was only visible from the green side.

We did open the “green” side and that is where we found the piece of shaved plastic.

Have you tried overlaying the old gear and the encoder gear on top of each other to make sure the new ones aren’t slightly smaller for whatever reason?
If they are how they should be, then I can think of 2 cases:

  1. The axle of the encoder gear isn’t seated well on one side (encoder casing side or the motor side)
  2. The casing isn’t attached tight enough on all 4 screw locations (if you have stripped screws, this might be the case); I’m sure 3 screws is sufficient to hold the plat/encoder on there, but I’m not entirely sure on some of the dynamics like slight warping (especially when I don’t have the motor in front of me)

btw, any idea from which part of the enclosure the plastic was shaved off of (actual motor enclosure, the encoder cover, or motor’s mounting-side cover)?

The piece was hanging off of the black plastic right under the gears on the back plate. I had to pull it off. I think the backplate is seated right as I took it off and put it back on like twice. The first time I pulled it off, the shaft for the encoder gear actually fell out.

How heavy is your robot? If you do not have enough torque you will break the internal gears. Make sure you have enough torque to get your bot moving. I made a tutorial to find out what ratio you should be using. The link is attached below.


I am certain. We havent had problems since the encoders were added. Our robot is 15 pounds without the battery.

That may be the problem. My team does not have the new encoders and we run 4 393 motors geared for speed and our robot is 17LBS and we run just fine.

The new encodes shouldn’t add any resistance though. Sorry we have 393 on all wheels (4)

Feels as though the shaft is either bent or the hole on the encoder end is mis-aligned for whatever reason
Should’ve asked earlier, but is this “popping” causing any driving problems?
If it’s just the sound, it may not happen anymore after few more hours/days of driving (and have the same functionality)
Of course, if it is causing driving problems, I’d suggest switching to the green cover or creating a replica (from some kind of clear plastic) and do some test runs to see if the “popping” still happens; if you use some clear plastic and make a replica, you should be able to see how and when they fail

It isnt makeing any drive problems. We are going to swap it with a front motor so that if it does fail, it will be easier to change at world.

Good decision
Just be sure to have some backup motors with you (unless you want to buy them at the Vex store, which also isn’t a bad idea; getting in line and start talking about random stuff with other teams or their mentors, since they’re just as bored as you are while waiting)