Geared DC Motor Selection


Disclaimer. This is my first time selecting a motor for a project. I have a DC motor in mind, but I’m not sure if the motor will produce enough torque for its intended purpose. So I need a motor that should be able to rotate horizontal disk of about 6kg at about 800rpm. I am looking at the 37D Metal gearbox motors, particularly item number 4758 with the built in encoder. Will this motor be fine?

Any help would be thoroughly appreciated.



Hello, Lee.

Ultimately, you will need to calculate the required torque for your system, but a 6kg disk is likely too much for our 37D gearmotors. If you are talking about connecting the motor to the center of the disk and having it oriented horizontally, the torque is given by \text{τ}=\text{I}*\text{α} , where I is the moment of inertia and α is the angular acceleration.

A disk is just a thin cylinder, so you can calculate the moment of inertia using this equation for a solid cylinder:


Where M is the mass and R is the radius.

Angular acceleration is the change in angular velocity over the change in time:


You did not give the radius or how quickly you want the system to get up to speed, and ultimately those specifics will matter, but as an example for you to reference with the correct values, let’s say the disk has a 10 cm radius and you want it to take 2 seconds to reach your desired speed of 800 RPM (which is ~83.8 rad/s), then:

\text{I}=\frac{1}{2}*\text{6}*\text{0.1}^{2}=0.03\text{ kg m}^{2}
\text{α}=\frac{83.8}{2}=41.9\text{ rad/s}^{2}

With those givens, the required torque would be:

\text{τ}= 0.03* 41.9 = 1.26\text{ kg m} = 126 \text{ kg cm}

Also, a good rule of thumb for brushed DC motors is to run them at no more than 20-25% of their stall torque, so with those givens, you should be looking for a motor with a stall torque of roughly 500-600 kg cm.

Lastly, you should consider the radial or axial load that the disk would put on the motor shaft. We have not done detailed characterizations of the effects those loads on the performance and lifetimes of our gearmotors yet, but the practical radial and axial load limits for our 37D gearmotors is probably around 1 - 1.5 kgf (per motor). So, even if we had one powerful enough, I would suggest that decoupling your load from the motor, such as through external gearing or belts and pulleys.