The two main categories of car pulleys are those that spin freely or provide power to an accessory or other device. Each car also has a crank pulley, which is typically the only one that is directly driven by the rotation of the engine. Accessory pulleys, which are present on components such as alternators, air conditioning (AC) compressors, and power steering pumps, are driven by crank pulleys via belts. Idler and tensioner pulleys spin freely because they are used to properly route the belts or keep them tight. Some car pulleys, such as those present on AC compressors, have clutch mechanisms that allow them to either spin freely or engage and activate the devices.
Most car pulleys are driven by a crank pulley, which is typically bolted or pressed on to the crankshaft. The crankshaft spins when the engine is in operation, which in turn causes the crank pulley to rotate. This motion is then transferred to all of the other car pulleys by either v-belts or serpentine belts. In some cases, the camshaft also has a power takeoff. Camshafts are typically connected to crankshafts by sprocket-driven chains or belts, so accessories that are powered by a cam pulley are indirectly driven by the crankshaft as well.
The most common car pulleys are used to power accessory devices. When an accessory pulley is rotated by the motion of a drive belt, it causes a device to activate. The pulley on a power steering pump causes fluid to pressurize and circulate through the system, and one on an alternator results in the formation of a magnetic field which can be converted into usable electricity. Most of these pulleys activate the corresponding accessories whenever the engine is running, though there are a few exceptions. AC compressors commonly have clutch mechanisms built in, which can allow a pulley to spin freely when the air conditioning is not turned on.
Car pulleys that do not provide power or operate accessories are typically referred to as tensioners and idlers. An idler pulley can take the place of a missing accessory, such as an AC compressor, though they are often included in the design of serpentine belt systems to create a complex belt path. These pulleys consist of a simple bearing and cylindrical pulley mechanism, and spin freely when turned. Tensioners operate in the same way, but they have the additional function of keeping belts tight. A loose belt cannot properly turn accessory pulleys, so tensioners use screw mechanisms and spring-loaded arms to apply pressure to the system.