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What Are the Different Types of Car Pulleys?

By Jeremy Laukkonen
Updated May 23, 2024
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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.

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Discussion Comments
By Feryll — On Jul 18, 2014

@Laotionne - I can't disagree with @Drentel when he says that you should start being concerned about your belts at 30,000 miles, but I don't think most people do that. Most people trade in their cars before they ever have to replace a belt. In general, cars are good for 50,000 to 80,000 miles before a belt breaks.

Usually, the pulley system belts will start squeaking when they are near the end, and you can see the cracks in them too. I had a car that would make a loud squealing noise when I started it. It was an older car that I bought used, and I don't think the belts had ever been changed. Anyway, when you start hearing a loud squeal that's a sure sound that the belts need some attention.

By Drentel — On Jul 17, 2014

@Laotionne - The only way to definitely know that a belt needs to be replaced is to open the hood and examine all of the belts closely. Most people take the belts on the pulley systems for granted. This can lead to you being stranded beside the road.

If you're looking for a specific timeline for changing belts, you should find your owner's manual. However, in general, you should start looking closely at your belts after 30,000 miles. They will probably last longer, but this is a good time to start thinking about replacing them, or at least you should have a mechanic look at them at this point.

By Laotionne — On Jul 17, 2014

Until a few weeks ago I had never had a belt on my car pulley systems replaced. I took the car in for maintenance and one of the belts was shredding and the mechanic told me that I was lucky that it had not broken while I was driving. In general, how often should I replace the belts on my car?

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