Weight vs Motor


I am calling for help on how to calculate max weight a motor can hold. indeed I cannot find such an info in any datasheet and as I do not own the motors yet I cannot have it by try and error.

while my project is not the same, but a similar situation is:

  • car window lefter
  • segways
  • wheelchairs
  • etc.

your help is pretty much appreciated.



The usual parameters given for motors are stall torque and free-running speed. If you do not understand these yet, you could take a look at the motors section of the 3pi User’s Guide, which explains a little about them. After that, if you have a more specific question, maybe someone can help you out.


thanks Paul… but I still see that I cannot answer my question even after reading the mentioned info

Okay, well, if you can make your question more specific, maybe someone will be able to help you. Describing exactly what you want to do would probably help.


I want to design a vehicle to transport load from one place to another. load will be in the range of 20 to 40 kg. the vehicle will be having 4 wheels driven by 2 motors, each controlling the two wheels in the same side. this leads to a load of 10 to 20 kg on each motor.

I know how to choose a motor based on specs like torque, rpm, current, voltage which are the normal parameters found in any motor datasheet. now comes the case on how to choose a motor based on its load weight. in other words, by knowing the weight, how to translate this to one or more of the above mentioned ones.

example might help. a wheelchair is designed to move around with weights on it: chair weight + the person.
another one, a segway (www.segway.com) is another device that is sold with specs among them is the total weight.

I hope I made myself clear with this.


Okay. There are two issues: first, will weight pushing perpendicular to the shaft damage the motor? Second, will the robot will be strong enough to accelerate quickly or climb slopes?

As for the first issue, a 20kg weight is probably way too much to put directly on the shaft of any of the motors we sell - you really need something like a ball bearing to support the weight, or you will be grinding up metal and deforming gears. You have to figure out some kind bearings anyway, since you have four wheels powered by two motors, right?

The second issue requires you to consider the stall torque of the motor and do a computation like the one in the 3pi documentation. For example, our strongest motor has an 18 kg-cm torque at 12V. That means that, if your wheels have a radius of 6cm, the maximum force from each motor will be about 3 kg. With a maximum force of about 10% of the weight of the robot, it could stall on slopes of about 10% - or if it encounters even a little bit of friction.