Usually outer surface of pulley is finished with little convexity, which is known as Crowning of pulley.

Creep in belt drive : When the belt passes from slack side to tight side, a certain portion of the belt extends and it contracts again when the belt passes from tight sight to slack side. Due to these changes in length, there is a relative motion between the belt and the pulley surfaces.

A clutch is a mechanical device which engages and disengages power transmission especially from driving shaft to driven shaft. ... In these devices, one shaft is typically attached to an engine or other power unit (the driving member) while the other shaft (the driven member) provides output power for work.