Q: How Do Belt Pulleys Work?

Your car has two main types of pulleys, crank pulleys and accessory pulleys. The crank pulley delivers motion to the other pulleys by means of either a serpentine belt or a v-belt, and they in turn activate the accessories in your vehicle. Idler pulleys may take the place of an accessory. Tensioner pulleys simply work to keep the proper tension on the belts.

Every engine-driven accessory has a drive pulley attached to it. Drive pulleys are responsible for operating each of the engine accessories. These accessories include the crankshaft, water pump, generator, power steering pump, A/C compressor, oil pump, supercharger (if equipped), and secondary air pump (if equipped).

The crankshaft is the main driver. The accessory drive belt is routed so that it rides on every pulley. When the crankshaft turns, the crank pulley moves the drive belt which, in turn, moves all of the other pulleys on the engine. This causes the pulleys to turn the accessories that they are attached to. The faster the crank turns, the faster the pulleys turn.