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?