Liquid leak, stiff rotation in 50:1 micro-metal gear motor

I bought a Pololu 50:1 Micro-metal gear motor three days back for my line following robot.
Today while testing it, the motor began to heat up, so I stopped to check it.
I noticed some funny looking, transparent liquid underneath the gears of the motor.
In addition to this, this motor’s shaft was not turning as easily as the other motor’s shaft.
What liquid could this be? Is there any problem with the motor? Why has it become stiff?

I had another motor just like this, which also showed the same symptoms, before stopping altogether one day later.

Any help would be much appreciated. (I have a competition coming up)

Thanks. :slight_smile:


I am sorry to hear about that. From what you are describing, it sounds like your gearmotors could be nearing their end-of-life. Gearmotors will wear over time, but this might also be an indication that the gearmotors could be underpowered for your application. Can you describe in more detail how you are using the gearmotors?


The gearmotors are for a line following robot. The weight of this robot is approx. 330 grams.

There are many ways to make and use a line following robot, so that itself does not tell me much about how you were using the motors. How many hours total do you think you were running the gearmotors that are no longer working? How long do you think you have run the still-working gearmotors? What voltage are you supplying to them? Also, we have several micro metal gearmotors with that gear ratio; which one are you using? It might be helpful if you can post pictures or a video of your robot, too.


I am not sure if the question about liquid leak was directly answered:

But the liquid is the grease that was used to lubricate the gears. When the motor heated up, the grease became less viscous and more liquid, allowing it to break free an drip. Think of cold honey that becomes thin when you heat it up a little.

What cannot be clear from your description is whether this was a failure of the gearbox, or the motor. The motor could overheat from excess current applied by the robot, and in turn overheat the gearbox, and permanently damaging the gearbox or motor just through this course of action. More likely the gearbox was dying through normal use and reached a state of runaway thermal breakdown. That is, the gearbox started overheating a bit, which messed up the lubrication, which further encouraged the whole thing to heat up, till death. Not really sure what happened in your case.

The motors with gearbox are not really advised to be spun by hand from the output shaft, because you’re backdriving the motor in that case. Definitively don’t do it with the motor connected to anything or with excess torque on the output shaft.

My line follower uses a 12v Li-ion 3 cell battery pack.
The motor driver, however, supplies only 5 to 6 volts to the motors.
Both, working and non-working, gearmotors were on the same robot, and therefore run at the same time for about 1 hour (with 1 minute gaps in between) almost every day.
This has been going on for about 2 months now. (From the time I bought my first pair of gear motors)

Motor specifications: Pololu 50:1 Micro-Metal Gearmotor HPCB

And i had noticed it warming up ever so slightly these last few days, towards the end of the day. (After about 15 - 20 runs as per the time mentioned above)

Tomek’s explanation of the lubricant seems pretty accurate; we have also observed that as the gearmotors near the end of their life (many cycles of the lubricant being heated and cooled), it looks like the lubricant tends to remain more like a liquid and have a more difficult time returning to its more viscous state as it cools.

It is difficult for me to tell from your description the total run time of your motors (e.g. how frequent were these one minute breaks?), but it sounds like it might be around 50 hours, which would be generally in line with the life times we have observed in our own tests. Does 50 hours sound like a reasonable estimate to you? Have you tried taking apart the non-working motor to see what the brushes look like?

Also, how are you limiting the voltage on the motors to 5 or 6 V? Are you just capping the maximum duty cycle that your motor driver uses?

By the way, could you explain a little bit more about what you are doing? Most of us here have built line followers before, but we have not come close to running our robots 15-20 times a day for two months straight. It sounds like you might be doing something very interesting and unique!