Consider the belt drives, the driven pulley: diameter d1, angular velocity w1 and the follow pulley: diameter d2, angular velocity w2. Assuming that the driven pulley rotate with a constant angular velocity and the belt does not slip on the pulleys, we have w1/w2=d2/d1, so w2 is constant( because w1 is constant). But I think that the follow pulley will have a torque which is caused by the tensions in the tight and loose sides of the belt. Because of this torque, the follow pulley cant rotate with a constant angular velocity. This is opposite to the above statement ( w2 is constant). Please explain this misunderstanding for me, thank you.