Why is shaft torque less than total armature torque in a DC motor?

Question

Why is shaft torque less than total armature torque in a DC motor?

Answer 1

because the circumference of the shaft is less than the circumference of the armature.

Answer 2

Because of the resistance involved.. cant remember the formula

Answer 3

Some of the torque that turns the armature is used to overcome bearing friction, the friction between the brushes and the comutator, and windage. Windage is the air drag resistance to rotation and the torque required to move any air that is intentionally moved though the motor to keep it from getting too hot. Those components of the armature torque represent losses and reduce the torque available as shaft torque transmitted to the load.