An Investigation of Rotational Response of Accessory Drive Serpentine Belt System

Serpentine belt system has been widely used during past years to drive automotive accessories like power steering, alternator, and A/C compressor from a crankshaft pulley. Instead of using multiple belt drives, the serpentine belt system uses a multi-rib flat belt, which wraps around several idlers and accessory pulleys. This design requires the use of a tensioning device to maintain adequate belt tension for preventing slip. Crankshaft torsional vibrations can lead to excessive rotational vibrations in poorly designed accessory systems. This can lead to undesirable noise and excessive slip, which can hamper the belt and bearing life. The value of these rotational frequencies and system response is of utmost interest to the accessory drive designer. While it is not practical to shift these rotational frequencies out of the operating range of an engine, a belt layout with these frequencies at non-dwell engine speed is highly desirable.

The presence of a tensioner induces non-linearity since the tensioner arm motion changes the adjacent belt span lengths and hence the stiffness. The coulomb friction in the tensioner arm can lead to stick-slip motion of the arm that can induce a higher harmonic response. Thus, standard linear vibration analysis tools cannot be used to predict the response, and one must resort to numerically integrating the equation of motion. A modular integration technique is suggested where an accessory drive system model can be generated by assembling pre-defined blocks which capture the behavior of individual components like tensioner, belt span, and accessory pulley. Implementation of this model into commercially available software is accomplished easily. The system model is excited by a sine sweep signal to determine the response under entire operating range. By predicting the dynamic response of an accessory layout the effect of a design change is determined rapidly, thus reducing the development time.