# Radial load and number of bearings of 50:1 Metal Gearmotor 3

What is the radial load a 50:1 Metal Gearmotor 37Dx54L mm (item 1104) can handle? Also, how many bearings support the shaft? I need two motors to drive a 40-50 lb robot. The motors will be attached straight to the drive wheels and will support the entire weight of the robot (I will have a third wheel to help steering and support some of the weight). Is this motor appropriate for this application and if not could you suggest a motor that is?

Hello.

The output shaft of that gearmotor is supported by a single bushing (it’s the collar you can see around it where it emerges from the gearbox), and I do not think the shafts would be able to support that kind of weight. Furthermore, even if you find a way to relieve the radial load on the motor output shaft, I suspect your robot is going to be too heavy to be driven by just two of those motors. Have you carefully considered how much torque you will need for this project? Please keep in mind that you should generally limit continuous operation of DC motors to around 25% of their stall torques or less.

- Grant

The robot will have 6 inch diameter wheels and will go with a speed of 3 ft/s. Do you have motors that you could recommend I use?

There are a number of tutorials and tools on the web that will allow you to estimate the motor power and torque required for your robot. If you want it to work well, you really need to have an understanding of this material.

Here is a start, and google can help with others: societyofrobots.com/mechanics_dynamics.shtml

I have done all the math on this. I calculated that for desired acceleration of 0.5 ft/s^2 and 65% efficient motors I need torque of 30 oz-in and 115 RPM. I need help purchasing motors from this website that will be suitable for my application. Is this forum the wrong place to ask?

You might have mentioned that you have done the required calculations. It appears that the motor you chose is capable of supplying the required torque at the required RPM (if the acceleration you specified is for a perfectly level surface), but as Grant pointed out, the motor shaft and bearings can’t support the robot.

To use that motor, you would need to couple the shaft to a ball bearing axle assembly that will carry the actual weight of the robot. People often use chain drives, available for example from servocity.com/html/sprockets___chain.html

The acceleration you chose (0.5 ft/s^2 or .15 m/s^2) is quite small. If you want this robot to climb any sort of slope, you will need much more torque.

I made a rough estimate of the climbing ability. The full stall torque from that motor (170 oz-in or 1.2 N m) will allow a 20 kg robot with wheels of radius 3" (7.6 cm) to climb a slope of at most 5 degrees, assuming 100% mechanical efficiency. Having a second motor helps of course, but you don’t want to rely on 50% of the stall torque.

I will definitely take your advice and pick more powerful motors. However, I still run into the issue of not knowing how much radial load the motor can take because this spec is not listed. I have very little experience with motors and don’t have a grasp of what would work for my application. If you have any suggestions on specific motors on the website that would work I would greatly appreciate your input.

Thank You,
Kalina

I don’t work for Pololu but I think that is the largest motor they carry. Most motors like that are not designed for significant radial loads. For a 20 kg direct drive robot, you need to look for dual ball bearing motors designed for the purpose.

However the chosen motor should work fine if your robot is always on smooth level surfaces, and you use a belt or chain linkage with separate wheel axles as I described.

Jim,

Thank you so much for your input, it has been very helpful! I found a motor with dual ball bearing and max torque of 680 oz-in.

Kalina

If you could post a link to the motor, others might find it helpful.

It’s from another seller, I don’t think that would be appropriate.

Hello, Kalina.

It is definitely okay to post to our forum about any parts you are using in your electronics projects that are not from us. If you have any feedback about your experience using something from somewhere else, we would love to hear that, too.

-Jon