Choosing DC motor: max needed torque vs nominal torque

Question

I have just sized the DC motors I want to use (corresponding to my robot and its intended applications - my figures include a 50% uncertainty factor to account for friction in reducers and other losses). Now I need to actually choose the exact motors I want to buy from the manufacturer (I am targeting maxon motors as I am not an expert and want no problem). I have a few down to earth questions about linking the mechanical needs to the electrical characteristics, among them:

Question #2:

As far as I understand, the nominal torque corresponds to the maximum torque the motor can sustain continuously. So I guess, as a rule of thumb, I should find a motor with a nominal torque = my max needed torque (after reduction), or around. Right?

Answer 1

The other answer is not quite right. Nominal (rated) torque is the torque at which the motor can run continuously without overheating. This is not the same as the torque at maximum efficiency. A well designed motor will have its rated torque slightly higher than the torque at max. efficiency. Maximum torque of a DC motor is the stall torque.

Answer 2

No. The nominal (or rated) torque is a torque that motor delivers at nominal speed. The motor is the most efficient at that point. When you take a look at any DC motor characteristics, you will see that the more torque you get, the smaller is the speed (and bigger is the current). Maximum torque is when the motor is stopped (stalled).

You shouldn't stall the motor for too long (more than couple of seconds), because it can burn, as it receives maximum current then. However, as long as you don't stall the motor, it is completely safe to operate with loads bigger than nominal torque.