Friction torque in thrust ball bearings lubricated with polymer greases of different thickener content David Gonçalves a,n , Samuel Pinho b , Beatriz Graça a , Armando V. Campos c , Jorge H.O. Seabra b a INEGI, Universidade do Porto, Faculdade de Engenharia, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal b FEUP, Universidade do Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal c ISEP-IPP, Instituto Superior de Engenharia do Instituto Politécnico do Porto, Portugal article info Article history: Received 29 September 2015 Received in revised form 14 December 2015 Accepted 14 December 2015 Available online 21 December 2015 Keywords: Friction torque Lubricating greases Thrust ball bearings abstract In this work a series of experimental tests were performed in thrust ball bearings lubricated with polymer greases. The tested greases were formulated with the same base oil but different thickener content. A multi-purpose lithium thickened grease was also tested as reference. The friction torque was measured at constant temperature and load, while varying the rotational speed. The coefficients of friction under boundary and full film lubrication were numerically calculated through the optimization of a rolling bearing friction torque model to the experimental measurements. The results show that the higher the thickener content, the smaller is the friction torque generated by the lubricating greases, phenomenon which was found to be especially important at low speeds and low specific film thickness. & 2015 Elsevier Ltd. All rights reserved. 1. Introduction About 90% of all the rolling bearings are grease lubricated [1]. Despite this fact, there is very little work on the grease lubrication mechanisms which rule the film thickness formation and friction torque in rolling bearings. Most analytical tools to predict film thickness and friction in grease lubrication only take into account the base oil properties and generally disregard the grease for- mulation, i.e., the type of thickener, its content or its interaction with the base oil, the additive package, etc. Furthermore, the base oil properties are often used to predict film thickness [2–4] and rolling bearing friction torque [5], even though for many grease formulations, the grease shows very different characteristics than the base oil and even the oil bled by the grease during work might have significantly different properties than the original base oil [6]. According to Cann et al. [7,8], the grease lubrication mechan- isms after the churning phase depend mainly on bearing type, operating conditions and grease properties where base oil oxida- tion, thickener degradation, and anti-wear/boundary properties will all play a role. Still, the available rolling bearing torque loss models do not take into account different grease formulations (thickener type and concentration) which may influence the tor- que measurements and it only considers the base oil properties to predict the friction torque of rolling bearings lubricated with grease. Very recently, in single ball-on-disc tests, the thickener influ- ence on the film thickness and friction was addressed by several authors [9–12]. Either by influencing the bleed-oil release, by changing the grease rheology/consistency or by directly con- tributing to the film thickness at low speeds, it has been observed experimentally that the thickener type and content are very important for the lubricant film formation. However, there is still very little work published on how the friction is affected, espe- cially in full bearing tests. Therefore, this work intends to analyse the friction torque behaviour of this recent and still poorly studied type of lubricating greases formulated with polypropylene (PP) thickener and how the different thickener content reflects in the rolling bearing fric- tion torque. The tests were performed in a rolling bearing test rig using thrust ball bearings, running over different rotational speeds at constant load and controlled temperature. 2. Materials and methods 2.1. Tested greases Four greases were tested in this work: M1, M2, M3 and MLi. The greases' main properties are shown in Table 1. Experimental batches of polymer greases were specifically manufactured for this Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/triboint Tribology International http://dx.doi.org/10.1016/j.triboint.2015.12.017 0301-679X/& 2015 Elsevier Ltd. All rights reserved. n Corresponding author. Tel.: þ351 225081742; fax: þ351 225081584. E-mail address: degoncalves@inegi.up.pt (D. Gonçalves). Tribology International 96 (2016) 87–96