The contact and friction between flat belts and pulleys

Rubber coated nylon flat belts running over pulleys in practice display friction coefficients between 0·3 and 0·8. This paper studies the causes of the friction variations. Adhesive friction theory considers the friction force to be the product of the real areas of contact between the sliding surfaces and the shear stress at the contacts: these two quantities have been separately measured by running belts over transparent perspex pulleys and directly observing the contacts. It has been found that variations in contact from one belt to another due to their method of manufacture are as significant in explaining differences in their friction behaviour as are variations in shear stress caused by their different rubber formulations. Real areas of contact were less than one third of the apparent area and varied with load, elastic modulus and roughness of the belt surface in a way broadly understandable in terms of elastic contact mechanics. Shear stress were about 0·5 Nmm⁻², perhaps determined by hydrocarbon films. Some belts showed real areas of contact not directly proportional to load. This led to their friction coefficients being load dependent.