Stiff motors

Over the years, a number of our motors have gone dead by becoming “stiff” – they still turn, but very slowly even without a load, and when we open up them up and change the internal gears (smearing around the extra lubricant), it doesn’t seem to help. When you try to turn them externally (forcing an inserted axle to turn), there’s a lot of resistance, but they do turn a bit. Does anyone know the cause and/or solution? We now have 4 motors with this condition that would be nice to rejuvenate.

I actually enjoy watching motors die by reaming out – change an internal gear or 2, and they’re good as new.

Maybe the brushes are bad or the copper coils around the armature have burned up, creating a difference in the resistance when the coil passes over the magnet. Or maybe even the armature became magnetizes and resists moving forward. That’s all I can think of. Anyone have any other ideas?

I asked this question a while ago and this is the response I recieved

**Yes, the glorious life has come to an end. Our minimum repair is over twice the cost of a Motor, so just replace it.

There is a current limit internal to the Motor to help protect it. High torque loads put maximum current draw on the Motor causing excessive heat and cause the current limit to trip. As the Motor cools down the current limit is reset and thus allows the Motor to work again. My guess is some of the internal winding of the Motor have melted together over time.**

So basically the windings of your motor melted together causing your problem.


Brushes getting dirty is a common problem in brushed DC motors.

Melting windings is much more rare in most electric motors. They would have to get very hot ! I’m shocked to see this is the problem. Makes me want to open up a few stiff motors and confirm !

“Melting” windings would certainly cause motor failure. Even cooking the insulation off the windings (which seems more likely than melting the metal) would cause shorts and, thereby, motor failure. However, I don’t see how either of those would cause an increase in resistance to turning the motor by hand.

In fairness, Manic Mechanic didn’t say if the misbehaving motors are harder to turn by hand than motors that are well behaved. If they are, then I’m inclined to look for mechanical causes, such as dirt or other contamination in the bearing surfaces (for example, between a gear and an axle around which it is designed to rotate).

@Manic Mechanic,
I suggest checking if the motor itself (that is, *not *through the gear train), when manually rotated, is abnormally stiff. If it is, then look inside the motor. (That, I concede, may not be trivial.;)) OTOH, if the motor itself turns freely, see if it will run freely when not connected to the gear train. If so, the problem’s probably in the gear train, not the motor.

A gear train problem could be contamination or, conversely, wear-induced misalignment. The former you probably can fix; the latter (unless it’s due to wear on replaceable parts) is probably not worth the effort or cost of repair.

Good Luck,

Yes, the stiff motors are much harder to turn by hand than “normal” ones. I test by sticking a single axle into the motor with a large gear on the end so I have something to grip with my hand; there’s no load or other mechanical consideration(s). The normal motors turn rather easily by hand; the stiff ones can barely be turned when extreme force is used.

When I looked inside the motor, I didn’t see anything amiss – no broken internal gears, no residue or crud coming between the gears, and they articulate perfectly. On the other hand, I didn’t know what to look for. So next time I’ll check for melted winding.

I wouldn’t be surprised if these motors are ready to give up the ghost. They are probably 3-4 years old and were driven especially hard during the last tournament. What amazes me is that all our 4-year-old batteries are still going strong.

Manic Mechanic,
Please allow me to clarify one point: By “gear train”, I was referring to the gear train inside the Vex motor module.