Synchronous Belt Mechanics and Durability - An Investigation Towards Life Prediction

Abstract: An automotive synchronous belt transmits power and assures accurate timing between the crankshaft and camshafts. The automotive industry is very competitive: there is constant pressure to increase performance and lower costs. For synchronous belts, this means that replacement intervals must be longer at the same time as the loading situation becomes more complex due to engine development. Hence, it has become important to be able to predict belt life in early design stages.

This thesis covers the fundamentals of belt life predictions, namely, synchronous belt mechanics. A quasi-static load distribution model, which includes spring stiffnesses determined by a finite element based method, is presented. Since loads in an automotive synchronous belt drive are not quasi-static, a dynamic load distribution model was developed. These two models have a unique force decomposition that includes a radial degree of freedom for each pitch. It is shown that dynamic models must be used for belt profiles where friction forces take part in the power transmission at the same time as rapid load changes are present. This is due to the friction history dependency.

Load distribution measurements were made under contrasting conditions: quasi-static in a simple two pulley rig and dynamic in a running engine. These measurements were made possible by a strain gauge equipped measurement pulley able to measure four of the force components calculated by the models mentioned above. The models show good agreement with experiments. The pitch difference was shown to be crucial to the load distribution; this finding could be used to optimize a belt-pulley interaction. The finding is also valid under both quasi-static and dynamic conditions. During the experiments, wear-induced geometric changes that led to a redistribution of friction forces were observed.

Descriptions of the three failure modes exhibited by automotive cambelts under normal operation are found in the thesis. For one of these failure modes, land area wear, a method to measure wear is devised. The method uses well-know roughness parameters derived from surface topology measurements. An investigation made on belts from test engines, points to effects from at least two land area wear types. The first is caused by the relative motion between belt and pulley during complete mesh; the second arises from counter-directional friction forces at seating and unseating. It is also shown that there exits an optimum pitch difference (in terms of wear durability); and that this optimum is shifted by a decrease in the extensional belt stiffness.

The results and models presented can be used in the design of improved synchronous belt transmissions.