Answered: 393 Motor Gears Shearing

We just encountered a strange one tonight. Both sets of the internal gears of the high strength configuration of the 393 motors sheared on us tonight. Not stripping, not grinding - shearing. (see pictures from links)

It was working just fine and then blammo! they broke. Anyone else see this before? And it was not just one, they both went. Maybe one went and we did not notice since it was vibrating and maybe not turning the axle?

So came home and opened up the puppy. We had not one, but two gears shear!

It was working just awesome this weekend at two different events. So it has worked well previously.

A bit more about the robot:

  1. It’s a 4 level scissor lift in a “pantograph configuration” as we call it. So it has lots of joints. But each of those rubbing points has teflon washers. Scissors are two parallel sets of 1x25 aluminum pieces with 1/2" standoffs in between (a la 3192A from worlds last year)
  2. Gears/motors are in line with each other driving both lifts simultaneously
  3. it’s a pretty massively heavy robot so there is a bit to lift so rubber bands help
  4. The rubber bands were left on there for a few days so they may be worn out now. So the motors were probably working kind of hard tonight.
  5. Batteries were fresh off the chargers rearin to go
  6. Motors are plugged into ports 1 and 10 respectively and one is reversed as you’d expect.
  7. Code has not been downloaded since the previous day of running just fine so no reversals in there.
  8. The lift was running fine a minute before and no one touched the wires to reverse ports causing torque against each other
  9. These motors have not tripped the thermistor at all previously
  10. Motor runs off joystick values from port 2 of joystick 2 directly 1:1

Did not want to do a full on reveal but here you go…

We swapped out the drive gears tonight for speed 84 on the motors --> 60 on the wheels (and tripped a 393 almost instantly so I guess we’ll be going to a neutral gear state or back to the previous torque configuration 60 on the motors --> 84 tooth on the wheels). But that has nothing to do with the lift at all.

So anyone else have this gear shearing happen to them?

Whoa. Haven’t seen that one before. Are there any shock-loads on the gears during the motion of the lift? Maybe there is something abnormal about the loading…? If this happens again, please try to reduce the load on the motor (maybe use surgical tubing to reduce the force required to actuate the lift?).

Any chance you’d be willing to send those gears in for us to take a look at?

Shoot an email to and they’ll arrange a swap.


Sure will! Also, I forgot to add these were new out of the box at the beginning of the school year. So they were not abused on any of last year’s robots (just this year’s!).

There were not a lot of shock loads per se. At the upper end of th elift, before we limited to the potentiometer reading, we have a mechanical stop via a standoff. Bumping into that is the toughest shock hit we got so far.

But we have given these poor motors plenty of strain over the past few months. There is the usual strain when the scissor is collapsed and a bit larger than you’d like. (In the folded up config as with any scissor, there’s not much vertical lift force and it’s mainly in the horizontal load at that point)

A few configurations during development were much tougher on the motors, but still, not shock/jerk impacts. We also had some heavier strains and a steel scissor without washers before the aluminum. And too tight bolts, etc etc. We could not lift for many days. Could that previous strain cause the wear and this was just waiting to happen? However, we ran in over 15-20 matches in two days straight this past weekend without issue.

The rubber bands really help out the lifting. Maybe they need to be changed out more often as they tend to stretch reducing their effect. ($8 a pound of #2 rubber bands at our Office Max is a nice disposable part!)