Motor Torque Questions and Motor Controller Compatibility


I am building a robot meant to move rice from one end of a ramp to another. It is my first project ever, so I am unsure if I am doing this completely correct. It is approximated at 20-25 pounds, uses a 12V DC battery supply (AA batteries) and must have the following requirements;

torque range: 2.9865 ~ 3.7331 lb-ft (573.408 ~ 716.755 oz-in)
rpm range: 54.7393 ~ 65.4809 RPM
power range: 0.0311 ~ 0.0465 hP (23.2114 ~ 34.7077 W)

for two motors, the requirements are:
torque range: 1.49325 ~ 1.86655 lb-ft
power range: 0.01555 ~ 0.02325 hP (11.6057 ~ 17.35385 W)

These requirements are for the whole robot, so if I use two motors, I’m assuming the requirements can be cut in half between the two motors; similarly for four motors.

I was thinking of using this motor (if I were using two):
Can anyone suggest a better one to fit the requirements?

I am a little unsure on what parameters to be looking for in a motor controller (I’m guessing power and current? anything else?) and I was wondering which of these may work:

What do you guys think? Will these work? Am I even doing this right? Thanks for any suggestions.



Since you already emailed and called about this, I will keep this response short, but for anybody else who cares, two of the 131:1 Metal Gearmotor 37Dx57L mm are not strong enough for these requirements. Unfortunately, we do not have any stronger options.

For choosing a motor controller, yes, you should start by making sure it can handle the voltage and current required for you motors. You should also decide whether you want a motor driver or a motor controller (you linked to one of each kind); we list the basic differences between the two here.


Hi Emily,

Thank you, and to Kevin, for the responses. I was wondering though, what if I were to do a 2:1 gear ratio. Would the motors then be able to give enough torque? I assume I can further cut the torque requirements in half for each motor, correct?

It is a little hard to figure out the motor capabilities because it gives stall torque (rather than continuous torque) and doesn’t give no-load speed.

But this motor controller would be sufficient?

It may be possible to further reduce the weight and slow down the steady state velocity too, if needed.
Thanks for your time.


Yes, adding another 2:1 gear ratio onto that gearbox would double the motor’s available torque. However, you probably will still lack the torque you need (even if it worked, your robot would be slow). Generally, 20-30% of the stall torque is a good estimate for how much you can load a motor. Ultimately, it is up to you to decide how slow you can tolerate and what kind of external gearbox you can find, but we still do not think this motor is appropriate for your stated application.

The “free-run speed” listed in our specifications is the “no-load speed”. You can get a rough approximation of the torque at different speeds by assuming the torque varies linearly between zero torque at the free-run speed and the stall torque at zero speed.


So I should be looking for a motor such that 20-30% of the stall torque meets my required torque? That would mean I need an extremely high stall torque. Wow.

So if I’m looking at about 60 RPM, then I would be getting a stall torque of about .75*250 = 187.5? And THEN I would consider 20-30% of this value? Thanks.



You can’t talk about a stall torque at 60 RPM: stall torque means the torque when the speed is zero. The point is, if the stall torque is 250, you should figure around 50 for a practical maximum continuous load. Separately, at that kind of load, you’ll get about 80% of the no-load speed, which would be close to your 60 RPM. Another way of viewing this rule of thumb is that your motor won’t last very long if your load slows it down by more than 20-30% from its no-load speed.

- Jan