Why Slack Side Is On Upper Side Of Flat Belt Drive

The slack side is the side where tension on a belt is low compared to the other side. In a flat belt drive, driver pulley pulls the belt through the bottom side and deliver it on the opposite side (upper side), so the top side is always loose. One of the reasons for doing so is to increase the angle of contact.


Consider if slack is present at the lower side as shown fig, it decreases the angle of contact and hence contacting area or grip between belt and pulley. It increases the chance of slipping and reduces power transmission capacity and efficiency.

If slack is on the upper side of the pulley, the hanging belt increases the angle of contact and hence the grip between rope and pulley. It is lowering the chance of slippage and improve the efficiency.

Read:
Flat belt drive - Types, advantage and disadvantage
Causes of belt failure and power loss