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Using Gearmotors for robotic arm

I am trying to design a Raspberry Pi / Arduino arm with reasonably precise control with arms made of lightweight acrylic rod. Rather than using stepper motors, I was thinking of using high power metal gearmotors instead. I want to count quadrature encoder ticks instead of relaying on the stepper motor steps.


Bolting a Gearmotor Bracket (A) to one acrylic link.
Attaching the gearmotor (B) to the bracket (A)
Attaching the shaft of the gearmotor (B) to a mounting hub ©
Bolting the mounting hub © to a second acrylic joint.


A) Pololu 25D mm Metal Gearmotor Bracket Pair

B) 172:1 Metal Gearmotor 25Dx71L mm HP 6V with 48 CPR Encoder

C) Pololu Universal Aluminum Mounting Hub for 4mm Shaft, #4-40 Holes (2-Pack)

L1) and L2) Acrylic arm link assembly

I have two questions:

Is this kind of motor appropriate to use? If I put torque load on the motor and provide power to keep it in a fixed position, will it suffer damage being “stalled” like that for extended periods of time (assuming I keep the power below the specified limits).

Do I need to worry about bending stresses on the motor shaft? I was thinking of placing a thrust bearing (D) between the gearmotor and the mounting hub to handle some of the stresses (see below). Is that necessary?



It sounds like you are essentially trying to use the 25D gearmotor with encoder to do closed-loop position control, similar to the function of a servo. I cannot say if the particular 172:1 25D gearmotor you linked to is appropriate for your application based on the information you provided since that mostly depends on the load and speed requirements of your system. As noted on the gearmotor’s product page, the recommended upper limit for instantaneous torque is 15 kg-cm and we generally recommend operating them at 25% or less of the stall current.

As far as loads on the motor shaft, adding a bearing to support the shaft like you described could help decouple those forces from the gearbox and motor assembly, which is especially important at larger loads.