Engineers typically use belts to transmit rotary motion between shafts. Tension on the belts creates friction between belt and pulley. This allows the belt to grip the pulley, and rotate without slipping. Belts are either flat or V-shaped. Flat belts run on pulleys and are more suitable for transmitting power between shafts that are far apart. V-belts run in a grooved pulley, or sheave, and are good for connecting shafts that are closer together.
Because a V-belt interfaces with a sheave on its flanks, it has more surface area contacting the pulley than flat belts do, and subsequently achieves greater friction with less tension, and in a more compact space. The wedging action of a V-belt increases the force of the belt against the sheave groove and helps prevent slip. Consequently, tension is key for transmitting power and torque, and ultimately dictates the efficiency and life of the drive.
Slippage decreases tension, drive efficiency, and life, and is the biggest problem with V-belts due to normal stretching during operation. As a result, tension must be frequently checked and reset throughout a belt's operating life.