Experimental measuring set-up and procedure for the traction coefficient on roller-on-disc contact A. Gama 1, * ,† , T. Cousseau 2 , B. Graça 2 , J. Castro 3 and J. Seabra 1 1 FEUP, Universidade do Porto, R. Dr. Roberto Frias 400, 4200-465 Porto, Portugal 2 INEGI, Universidade do Porto, Campus FEUP, Rua Dr. Roberto Frias 400, 4200-465 Porto, Portugal 3 ISEP, Instituto Superior de Engenharia do Porto, R. Dr. António Bernardino de Almeida, 431, 4200-465 Porto, Portugal ABSTRACT A pin-on-disc machine was modified to measure traction coefficient and the potential of contact difference under controlled conditions of slide-to-roll ratio, temperature, contact pressure and entrainment speed; oil, grease, grease’s base oil and bleed-oil were tested. This new arrangement proved to be efficient to charac- terise lubricants traction behaviour, film separation and tribofilm generation. Copyright © 2016 John Wiley & Sons, Ltd. Received 19 May 2015; Revised 27 October 2015; Accepted 3 January 2016 KEY WORDS: oil; grease; bleed oil; base oil; EHD; tribofilm INTRODUCTION The supply of lubricating oil or grease to the contact region is very important for long life and maintenance-free operation of several machine elements. 1 Some key factors influencing grease performance on rolling bearings are grease constituents, grease rheology, oil bleed rate, bleed-oil properties, slide-to-roll ratio (SRR), transversal speed and contact geometry. 2,3 The factors related to the grease properties might change substantially along the grease life. In the early stages of grease life (churning and bleeding phase), it was shown that the bleed-oil properties are the dominant factor on film formation. 3 Lubricating oils are formulated products that result from the mixture of base oils and package of additives. Anti-wear and extreme pressure additives form a protective layer on the contacting steel surfaces under severe conditions. Identifying the parameters affecting the additive-derived layer formation and its tribological properties of this layer becomes important as they contribute on friction reduction and wear control in steel–steel contacts. 4 *Correspondence to: A. Gama, FEUP, Universidade do Porto, R. Dr. Roberto Frias 400, 4200-465 Porto, Portugal. † E-mail: algama@fe.up.pt LUBRICATION SCIENCE Lubrication Science (2016) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/ls.1335 Copyright © 2016 John Wiley & Sons, Ltd.