Land friction effects in the meshing of timing belts

Abstract

It is observed experimentally that local peaks occur in the tangential component of the force on a pulley pitch when a timing belt seats at entry to a driving pulley and unseats at exit from a driven pulley. In this paper it is shown that these peaks are caused by forces acting between the belt and pulley lands. It is further shown experimentally, in tests on nominally 9.525 mm pitch belts running on 19-tooth pulleys, that the direction of sliding of a belt land over a pulley land can differ in seating conditions from the direction in fully meshed contact expected from the pitch difference between the pulley and belt. A theory to predict the direction of sliding is developed that combines the constraints of synchronous motion with velocity differentials between the belt cord and land surface that depend on the belt radius. The theory almost agrees with experiments. It needs measurements of belt cord extension stiffness, belt land radial compliance and bending stiffness and belt land shear compliance. Data on all these quantities have been obtained and are presented: the belt land properties are very anisotropic.