The first modern automotive accessory drive belt to operate without a tensioning device is making its debut on the 5.3-L V8 in the 2008 Hummer H3. The ribbed belt, which wraps around the air-conditioning compressor and the crankshaft pulley, has a low modulus of elasticity that makes it sufficiently “stretchy” to be installed without permanent deformation. The belt is also able to maintain required tension for as long as a conventional belt with a tensioner, while transferring the power required to operate the compressor.
Eliminating the mechanical tensioner saves weight, space, and cost. Although the belt is of the same basic construction as a conventional one, its tensile cord is made from a polyamide material that is three times more elastic than the polyester cord normally used, explained Paul McVicar, Accessory Drive Engineer at GM Powertrain.
In addition, McVicar said, the rubber layers around the cord layer were formulated to provide the superior durability and adhesion to the cord needed to accommodate the greater amount of stretch.
The belt was developed with several suppliers during a two-year period, he said. The GM testing, he added, included operation on test rigs that simulate failure modes, plus hot, dry city traffic road-testing run out to 150,000 mi (240,000 km).
Low-modulus belts have been tried since 1998, McVicar said, but durability was an issue, and tensioners always were used. It would be possible to wrap the new belt around three pulleys in some applications, he added. However, the accessory-drive tolerances are greater with a third pulley, he pointed out, and increasing belt stretch and initial loading to compensate could affect the life of the pulley and shaft bearings.
The new belt is installed with a special tool that lifts and guides the belt onto the pulley. Although another special tool could have been developed to unseat and remove a belt for service operations, GM decided that its recommended procedure would be to cut the belt and replace it.
Other H3 V8 front-end accessories, such as alternator, water pump, and power steering pump, use a conventional serpentine belt with a spring-loaded automatic tensioner.