Lubrication of a rubber ball rolling with spin

Abstract

Relative spin is present between surfaces at each point of contact when a ball is rolled between two circular contra-rotating plates. This gives rise to tangential frictional tractions transmitted across the contact surfaces which may result in a creep of the ball perpendicular to its direction of rolling. This paper reports a study made of rubber and steel balls rolling between plates of perspex lubricated with various oils and water. In dry contact an unrestrained ball, whether of rubber or steel, was found to creep radially outwards between the contra-rotating plates in a manner anticipated by earlier work, but when surfaces were lubricated with a viscous oil steel balls showed a neutral behaviour and rubber balls actually crept inwards. The inward creep was quite unexpected. Such behaviour suggests a greater traction, due to spin, over the back half of the contact zone arising by tilt between the contact surfaces. Estimates and actual measurements of tilt generated by elastohydrodynamic action were used to predict inward creep which could then be compared directly with experiment. The comparison shows that tilt clearly contributes to inward creep though does not fully explain the observed results.