Fan Belt? Alternator Belt? Serpentine Belt? Similarities and Differences

Look up "fan belt" in any modern automotive dictionary, and you'll typically find a definition that reads, "An endless belt used to transmit power from a crankshaft-driven pulley to a pulley driving the fan, alternator, or other engine accessory. It is usually V-shaped in cross section with the point of the V fitting into a groove in the pulley".

Even though cooling fans on most modern vehicles are driven by electric motors instead of actual belts, the term fan belt is still incorrectly used as a blanket expression to describe any single accessory belt that may be used to drive a water pump, alternator, power steering pump, or other items. Why?

Fan Belt
Fan Belt Scheme
The term "fan belt" is still incorrectly used as a blanket expression to describe any single accessory belt (shown in this photo) that may be used to drive a water pump, alternator, power steering pump, or other items.

In order to pull cooling air through a vehicle's radiator, a cooling fan needs to be located at the front of the vehicle just behind the radiator. Traditional rear-wheel-drive vehicles with longitudinally-mounted engines had crankshaft pulleys right there at the front of the car, and they were convenient sources of power for cooling fans. Since the fan belt tended to be the biggest and most noticeable one on the vehicle, the expression became universal for all other belts that looked similar – no matter what function they performed. Many original engine layouts also used that fan belt to spin water pump and alternator pulleys, and you may hear some people call this belt a "water pump belt" or "alternator belt" as well as a fan belt.

However, as vehicles with front-wheel-drive and sideways-mounted engines grew in popularity, the front of the engine was no longer at the front of the vehicle where the cooling fan was.

Ford Mustang Fan Belt
Shown in this picture of a 1966 Ford Mustang engine bay is an actual "fan belt" used to drive a cooling fan. Until recent decades, this was the traditional setup on vehicles with longitudinally-mounted engines.
Cooling Fan With Electric Motor
As vehicles with front-wheel-drive and sideways-mounted engines grew in popularity, the front of the engine was no longer at the front of the vehicle where the cooling fan was. Belts could no longer be used, and fans with electric motors were perfected. Because of their greater energy efficiency, they've made belt-driven fans obsolete in virtually all automotive applications today.

Engineers perfected electric motors, temperature sensors, and related computers to get the job done, and belt-driven fans were no longer necessary. Because cooling fans don't always need to be running at steady cruising speeds, electric motors which can shut off proved more energy efficient than direct drive belts which are always connected. Since electric cooling fans are used even in modern vehicles with longitudinally-mounted engines, the term fan belt has become virtually obsolete. More fitting terms to use when searching for a replacement single belt of this kind would be accessory drive belt or V-belt.

Because drive belts are subject to constant rotation, load, and heat, they are constructed in a similar fashion as tires. An outer layer of rubber surrounds internal steel and cord reinforcements to minimize stretching and maximize strength when grip is needed under load.

Accessory "V" Belts

Inner Construction of a Typical Reinforced Rubber Accessory Belt
The inner construction of a typical reinforced rubber accessory belt. Because of a shape that tapers downward at the bottom, these are known as "V belts".
Engine With Three Separate Accessory Belts
This photo shows an engine equipped with three separate accessory belts. One is for the cooling fan and generator, another drives the A/C compressor, and the third drives the power steering pump. Note the slotted adjuster brackets.

Automotive accessory drive belts mounted visibly outside the engine are also known as "V-belts" because their trapezoidal shape tapers from wide to narrow as it forms a V point near the bottom. Tension on individual accessory belts is adjusted manually by tightening an adjuster to change the position of an idler pulley wheel, or by loosening and tightening slotted mounting brackets that move accessories such as an alternator by fractions of an inch.

As vehicle manufacturers added more accessories, their initial solution was to add more drive belts to power these accessories. So cars went from one belt driving the fan, water pump, and alternator, to 2 or 3 separate belts, now also driving the power steering pump, A/C compressor, and emissions air pump.

The "V" belt is one of the most effective belt solutions because of its strength, relative quietness, power transfer, and durability. The belt's ability to provide a small degree of slippage when needed makes it a good match for accessories that shut on and off by the use of a clutch, such as air conditioner compressors and older style radiator cooling fans.

Multi-Grooved Or Ribbed "V" Belts

Multi-Groove, or Ribbed, V-belt
This illustration shows a multi-groove, or ribbed, V-belt. The belt is flatter, thinner, more flexible, and builds up less heat.
Serpentine Accessory Belt
A serpentine accessory belt is simply a longer, endless belt constructed in the same fashion as a single accessory belt. As this picture illustrates, a single serpentine belt will typically power all externally mounted accessories. Idler pulleys help with proper routing of the belt. Unlike individual belts, tension on a serpentine belt is provided by a tensioner pulley which is under constant spring pressure.

A variation on the above theme is provided by the so-called multi-grooved or ribbed belt. In practice, this is still a V-belt, but with smaller, multiple "V" shapes placed side-by-side. The underside surface of the belt features endless parallel ridges designed to slot into mating grooves on the pulley to prevent sideways slippage. While this version of the V-belt is wider, it is also thinner, more flexible, and runs with less heat buildup. As load pressures increase on this type of V-belt, the rubber grooves wedge themselves further into the pulley and friction provides the necessary grip.

Serpentine Belts

Several factors have come together to change the way motor vehicle engineers have designed accessory drive belts. There has been an increased demand on accessories which derive their power from drive belts. At the same time, as vehicles have gotten smaller, engine compartments have become more crowded. Packaging of mechanical components into constricted spaces has become an important part of overall vehicle design.

The above has led engineers to abandon the traditional V-belt in favor of the serpentine belt. A serpentine belt is simply a longer version of a grooved V-belt. It may or may not have grooves on both sides. Instead of being specific to a single accessory, a serpentine belt is designed to wind around multiple pulleys – powering all of the vehicle's pulley-driven accessories. Depending on design, both sides of the belt may provide friction. Freewheeling "idler pulleys" are positioned in various locations in order to reroute the belt in the direction of the next accessory.

Unlike individual belts, tension on serpentine belts is provided by a tensioner pulley which is under constant spring pressure. A serpentine belt tensioner never needs adjustment, and will release its tension when force is placed against it using proper hand tools if the belt needs to be removed or replaced.

Timing Belts

Spring Loaded Serpentine Belt Tensioner
Visible in this photo is a spring-loaded serpentine belt tensioner. This type of tensioner never needs adjustment and will release its tension when force is placed against it using proper hand tools if the belt needs to be removed or replaced.

Timing belts should not be confused with accessory drive belts. Unlike drive belts that run accessories mounted on the outside of the engine, timing belts are encased inside the engine. They feature flatter construction with molded teeth, designed to mate with the teeth on the crankshaft and camshaft drive gears. A spring-loaded pulley provides constant tension to ensure that this belt remains slip-proof so it maintains a fixed relationship among the gears for camshaft timing. Because of their necessity in engine operation, all timing belts have a fixed replacement interval (determined by the manufacturer.) Auxiliary drive belts are checked for wear and replaced when necessary.

Purchasing Replacement Accessory Belts

Whether it is sometimes referred to as a fan belt, alternator belt, or water pump belt, it is most properly called an accessory drive belt, V belt, or serpentine belt. Each vehicle has its own belt configuration, depending on its engine and optional accessories. When purchasing replacement belts, it is important to know your vehicle's year, make, and model, engine size, and in some cases, whether it is equipped with certain accessories such as air conditioning. Knowing these few facts ahead of time is key to ensuring that you purchase the proper belt!


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