Variable Stiffness Tensioner for a Belt Driven Transmission System

Abstract

A new tensioner mechanism is designed for a belt driven transmission system with variable gear ratios. The preliminary design of such transmission system includes a morphing driver pulley, a driven pulley with constant radius, and a tensioner. With the aim of changing the gear ratio, the radius of the morphing pulley (driver) varies gradually through a stepwise shifting procedure. Hence, the geometry of the belt and pulley system, including the tensioner pulley position, changes noticeably while the shifts occur. A regular tensioner, commonly used in the belt and pulley systems, includes a linear spring, which is attached to an idler pulley. This type of tensioner is designed and installed in the transmission systems with respect to the geometry and loading condition in order to guarantee a certain value of static pretension in the belt. The longitudinal vibration of the belt and pulley system is also controlled by setting proper value of tensioner spring stiffness and other dynamic parameters. The regular tensioner is commonly modeled as a linear spring. These tensioners are mainly used for the belt and pulley systems with constant gear ratios. In the case of variable drive system, while the radius of the driver pulley changes, a large displacement in the tensioner pulley position is expected. This large displacement results in large variation into the belt static tension, and might cause unpredicted fatigue failure in the belt or pulleys. Lower values of static tension can cause belt dynamics instability in a belt driven system. [Wickert 1990] A new tensioner is proposed for the morphing pulley transmission system in order to control the static tension and the belt longitudinal vibrations for different gear ratios. The tensioner includes a linear spring that is connected to the tensioner pulley via a convertor mechanism. The convertor system consists of a traveling pin and guide rail mechanism. The traveling pin is connected to the tensioner pulley while the rail is attached to the linear spring. Using this mechanism, the tensioner system behaves like a variable spring element. By defining a proper curve, it is possible to maintain the static tension within the desired range and control the longitudinal vibration of the belt and pulley system. The design parameters of the tensioner system are investigated through a linear optimization process in order to minimize the longitudinal vibration of the belt as well as the pretension.