If you know the rated load ( required torque ) for the set you are using, you can select the optimum motor the S-T characteristics ( relationship between speed and torque ).
However, if there are large variations in the load or speed, it may not be sufficient to just calculate the average load. In such cases, it is necessary to confirm that the motor characteristics can provide the required torque and speed for the expected operating range. The typical torque/speed characteristics for each motor size are shown each shape for your reference when selecting a motor.
DC Brush motors