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Servo vs geared motor. Help!

Hi everyone!

I am designing a system that can open/close a valve with 150RPM. It requires a torque of ~3kg.cm. I am choosing between doing it with servo or geared motor.

My main question that I am trying to figure out: why servos have significantly more torque compared to standard geared motors even though they use similar motors and gears.

Here is an example of the servo (motor size 20mm diameter and ~25mm length) that provides 17 kg.cm stall torque and ~71RPM: https://www.pololu.com/product/1057

Here is an example of the geared motor (motor size 20mm diameter and 25mm length) that provides 9.5 kg.cm stall torque and 75rpm: https://www.pololu.com/product/3707

Basically, same same motor provides 2x times more torque in a servo? I am trying to understand how can this be possible? I have even seen similar size servos that that can provide 40 kg.cm stall torque. I would appreciate if somebody could clarify this discrepancy.

Another question I had, why are servos so much louder compared to geared motors?


Brushed D.C. motor torque depends on the permanent magnet field strength in the motor, as well as the winding current.

Given that the stall currents and rotational speeds are similar for the two motors, it is probably safe to assume that the higher torque motor has stronger and more expensive magnets.

Thanks for the tip. I’ll try to make a comparison between same size motors and the torque/rpm and see how much it differs between manufacturers.