A tensioner maintains the correct amount of tension on the belt at all times throughout its duty cycle. It also helps protect other components such as the alternator and water pump from undue stress and premature failure. In addition, a tensioner is a relatively inexpensive part to replace.

A crankshaft pulley, also called a harmonic balance wheel or crankshaft sheave, is a grooved, wheel-shaped device that connects directly to the vehicle's crankshaft. The crankshaft essentially converts the linear, or straight, movement of pistons into a rotational motion.