A driving belt or, more commonly, drive belt is any belt that transfers axial motion from a motor to another device. In automotive applications, a driving belt can take the form of either a V-belt or a serpentine belt and allows the motor to drive accessories such as an alternator, power-steering pump or air-conditioning compressor. Both V- and serpentine driving belts are made of rubber and can be reinforced with a variety of materials, such as steel, polyester and other fibers.
Though they perform the same basic functions, V-belts and serpentine belts differ in design and appearance. When viewed in cross section, a V-belt is wedge-shaped and typically taller than it is wide. Serpentine belts are flat and generally have one smooth surface and one grooved surface. Depending on the width of the serpentine belt, from three to six grooves in a row will typically run the full length of the inner surface. Where V-belts ride in a wedge-shaped pulley, serpentine belts must be matched to pulleys that have an identical number of grooves.
Generally, older vehicles will make use of multiple V-belts, while newer vehicles will make use of a single serpentine driving belt. Certain applications will, however, utilize two or more serpentine belts or pair a small V-belt with a larger serpentine belt. Each v-belt typically requires its own tensioner pulley and drives one or two accessories, while a serpentine belt can be looped around and routed to power all of the accessories present in an engine. With only one belt and one tensioner, this can represent a savings both of space in the design and time when replacement is required. Serpentine belt pulleys must typically be wider than V-belt pulleys, though this reduced clearance is often negated by the fact that one serpentine belt can do the job of multiple V-belts.
Replacing a serpentine driving belt is often a simpler operation than replacing multiple V-belts, though care must be taken to note the path of the belt around the multiple pulleys. It's often possible to route a serpentine belt more than one way, with only one routing being correct. Some serpentine belts are tensioned by an automatic spring tensioner that must be pulled back to release tension on the belt. Others utilize the same type of tensioner as many V-belt systems, in which a nut or bolt has to be backed off to release the tension. Once the tension has been released, the belt may be removed from the pulleys and replaced by a new part.
Regardless of the type of tensioner or type of driving belt, it's important to achieve the correct tension after replacing a belt. If a driving belt is too loose, the accessories won't turn; a belt that is too tight can potentially damage the pulley bearings. If tension specification can be obtained, a device that measures belt deflection can sometimes be used to set the correct tension. It's also possible for belts to stretch or loosen, so checking the tension after the vehicle has been driven for a while can help extend the life of the new belts.