When should you replace motors?

I’ve been taking inventory for the upcoming year, and I’ve noticed a large number of our motors sound weird, and they’ve been weaker. How do I tell if a motor is “healthy”, what should a motor sound like? I’m hesitant to buy a new set of motors since I know they will become obsolete, but I also don’t want to be at a disadvantage by using worn down motors. I haven’t been able to tell if I’m being paranoid about the motors or if they are legitimately bad.

Good question. I’m curious to see how people reply. Normally, we don’t replce a motor until it actually dies (stops moving). Sometimes, the gears can be replaced and oiled and the nasty sounds go away - so I would try that before throwing away a motor. Also, if we’ve noticed degradation in a motor, that motor is used in a less strenuous application. But, like I said, others may have much better answers. We, too, do not want to buy one more motor than we need this years - so we are really trying to conserve, when possible!

I recall few years back, there was an article that mentioned about 393 motors has a lifespan of about 100hrs.
We still stick to this guideline. But of course, if there is any obvious degradation in performance (or some times, weird noise), then it is a clear sign to change.

I’ve tried re greasing the motors. The main thing I’ve noticed is an odd buzzing noise from the tiny gear in the back that cannot be replaced, and the second shorter gear in the back makes a noise that sounds like its touching something. The tiny gear has an odd range of noises, none of which sound good. I mainly feel like the motors are kind of weak and have tons of unnecessary friction. Since I’ve been using these motors for my entire time on a VEX team, I don’t remember what a new motor sounds like or how much power it should have.

We are sorting our motors right now. We started with a high level triage phase. We set up a cortex with two buttons. One button runs the motor in one direction, the other in the reverse. We went through each motor with a visual check, if the wires are exposed to short, it is bad. If it locks up when turned by hand, it is bad. Then we plug it in to the cortex and run it in both directions. If it is noticeably weaker in one direction, it is bad. If it does not run, it is bad.

Next we want to do some dyno based testing on those that survived the first phase to determine which are good and which are only marginally acceptable.

How do you plan to do your dyno testing? There was a thread from last year that described a dyno and I had a student build it. I haven’t really spent enough time on how to use the result to determine if a motor is good or bad.

We would probably do a few brand new motors to establish what we expect to see, and maybe a few known weak motors to see what that sort of degradation looks like, then just compare the data. The dyno will let you do an approximate torque-speed plot from stall to free, and we would then compare that as a percent of what was expected.

Sorry, maybe my question wasn’t clear. What sort of hardware are you going to use for the dyno testing? Commercial or built with vex parts? and if built, do you have any pictures?


We will probably use the VEX built one you referenced earlier, likely with only marginal mechanical changes. A quick mount for the motors, a weight to turn, and an encoder to read position data.

One thing we do is certainly replace gears fairly often, but we also measure the time to trip the PTC and sort the motors by that. Then we have a spreadsheet for teams to pick from. We use a constant current supply and an oscilloscope, but you can also do it roughly by holding the wheel stopped with full power output and time how long it takes to go “limp”. Just make sure you only do a few of them (5-10) then recharge the battery.

I replace them before big comps like worlds ect. Other than that I replace them when they don’t work anymore.

I replace my motors at the beginning of the year, replace all high stress motors for the states rebuild and then replace every motor about a week before worlds. We’re pretty lucky that our school basically gives us an unlimited budget but in middle school, I just used old motors for early season robots.