The accessory drive in modern internal combustion engines is based in almost all cases on multiribbed belts.

The main demands placed on accessory drives and automatic tensioning systems are as follows:

• Automatic setting of belt force at initial assembly and service (tolerance compensation of all drive components)
• Almost constant belt force over the whole life of the belt drive (compensation of belt stretch and wear)
• Almost constant belt force across the whole engine temperature range (compensation of thermal expansation in all components affecting the drive)
• Reduction of dynamic belt force peaks
• Minimisation of slip, noise and belt wear
• Increased life of the whole belt drive system
• Optimum reliability of the whole belt drive system

When matched ideally to their operating conditions, modern accessory drive systems are maintenance-free and can achieve a service life of up to 240 000 km assuming optimum system development.