Toothed belt drives for driving camshafts or balancer shafts have been used successfully in mass production internal combustion engines for 40 years. In older designs, the toothed belt is preloaded via either an assembly with an eccentric bearing arrangement in the toothed belt drive (e.g. the water pump) or so-called fixed tension pulleys (eccentric tension pulleys etc).

Optimum setting of the belt force is not possible with such systems since they cannot compensate for fluctuations in belt force due to temperature or wear or for dynamic effects (belt oscillations, influences due to the valve train, etc). The compensation of such fluctuations and effects by automatic belt tensioning systems is absolutely essential in modern toothed belt drives, since this is the only way that the system life values of up to 240 000 km and beyond (corresponding to the life of the engine) as required by the automotive industry can be achieved.

When an automatic belt tensioning system is used, the scatter of preload force at initial assembly can be reduced and the preload force can be held practically constant over the operating temperature range of the engine. Automatic belt tensioning systems have been used on toothed belt drives in internal combustion engines since the beginning of the 1990s and have largely displaced the fixed systems from the market for the reasons described above.

The above conditions have resulted in the following principal requirements for automatic tensioning systems:

• simple setting of the specified belt force at initial assembly and service (compensation of belt, diameter and positional tolerances)
• maintenance of a defined belt force as consistently as possible under all operating conditions throughout the required system life (compensation of thermal expansion, belt stretch and wear, taking account of crankshaft and camshaft dynamics)
• ensuring an optimum noise level while reducing belt oscillations
• preventing tooth jump