Schaeffler North America

Due to the modified boundary conditions, the use of a conventional tensioning system in such systems is only possible in exceptional cases and represents a compromise in any respect. The characteristics of the tensioning system can in this case only be matched to one operating condition, while normally startup (shown in blue) represents the more critical operating condition. The increased preload for startup then lies in generator operation (shown in red). The force level is thus at a higher level than is usual today. This effect leads to a restriction on the expected life of all belt drive components.

In order to circumvent this, it is necessary to develop suitable tensioning systems, where the requirements described below must be fulfilled:

• ensuring a suitably matched and almost constant belt preload both in startup and during generator operation
• ensuring sufficient belt preload for reliable engine startup over the whole temperature range
• ensuring long life of all components through the lowest possible load
• reduced belt slip and running noise
• reduction in dynamic peak forces in the belt drive
• compensation of belt stretch and wear during service life

In belt drives for starter generator applications, a clear allocation of the slack strand is no longer possible since the generator is the driving element during startup and the driven element during generator operation. The direction of the torque acting on the generator pulley is reversed as a function of the operating condition. The major challenge in the direction of tensioning systems suitable for belt-driven starter generator applications is therefore in tensioning the slack strand in any situation independent of the operating condition. The ideal situation is achieved as soon as it is possible to additionally increase the preload during startup while reducing the preload during generator operation. Furthermore, the possibility of boost and recuperative operation must also be ensured in high performance belt-driven starter generator applications.

A very promising approach is the use of the generator itself as a tensioning system.