Rolling Friction of Single Balls in a Flat-Ball-Flat-Contact as a Function of Surface Roughness

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ORIGINAL PAPER

Rolling Friction of Single Balls in a Flat-Ball-Flat-Contact as a Function of Surface Roughness B. Guddei • S. I.-U. Ahmed

Received: 16 December 2012 / Accepted: 7 March 2013 / Published online: 20 March 2013 Ó Springer Science+Business Media New York 2013

Abstract This paper reports on the dependence of rolling resistance on surface roughness of single balls in a flat-ballflat configuration at sub-Newton loads using an specially designed rolling resistance tribometer. The tribometer setup features an ultra-stable drive and a two-dimensional force transducer with a high ratio between load and friction force sensitivity. Using this setup, the rolling resistance was examined for different materials as well as a selfmated sapphire contact. Results indicate a very strong dependence of rolling resistance on surface roughness for the tested loads. Rolling resistance was found to decrease monotonically with decreasing roughness. A comparison was also attempted between rolling resistance and sliding friction. Sliding friction measurements show that decreased roughness does not necessarily decrease the coefficient of friction. In fact, a friction minimum is reached at a certain roughness. A further decrease of roughness increases the friction. This indicates that sliding friction measurements do not seem to be transferable to rolling resistance in every case, especially when the average surface roughness is in the range of a few nanometres. Keywords Rolling resistance  Flat-ball-flat-contact  Surface roughness

B. Guddei (&)  S. I.-U. Ahmed Faculty of Mathematics and Natural Sciences, Institute of Physics, Technical Physics I, Ilmenau University of Technology, PF 100565, 98684 Ilmenau, Germany e-mail: [email protected] S. I.-U. Ahmed Hochschule Braunschweig/Wolfenbu¨ttel, Faculty of Mechanical Engineering, Institute of Construction and Applied Mechanical Engineering, Ostfalia University of Applied Sciences, Salzdahlumer Strasse 46/48, 38302 Wolfenbu¨ttel, Germany

1 Introduction The rolling contact is well-established in technological applications, the ball bearing being a classic example of such a contact. Rolling contacts are characterized by a low resistance to motion. The energy losses in such contacts during operation originate from several factors, among them viscoelastic [1] and/or plastic material properties [2], slip in the contact zone [3] and surface roughness [2, 4], to name a few. This study focuses on the dependence of the rolling resistance on surface roughness and surface irregularities. The first attempt to measure the dependence of roughness on rolling resistance was performed by Reynolds [5]. In that study, the angle at which an iron cylinder starts to roll over plates of different material and surface texture was investigated. Later, Stribeck [6] found that a smooth surface finish is advantageous for ball bearings. Bikerman improved Reynolds’ setup and measured the initial rolling resistance of bearing steel balls depending on the root mean square roughness of flat samples,