THE ANNALS OF UNIVERSITY “DUNĂREA DE JOS“ OF GALATI FASCICLE VIII, 2009 (XV), ISSN 1221-4590 TRIBOLOGY 119 Paper presented at the International Conference on Diagnosis and Prediction in Mechanical Engineering Systems (DIPRE’09) 22 - 23 October 2009, Galati, Romania STEEL ON SILICON NITRIDE HYBRID TRIBOLOGICAL SYSTEMS LUBRICATED BY KEROSENE. TESTS ON A FOUR-BALL MACHINE Viorel PALEU Technical University “Gh. Asachi” of Iasi, ROMANIA vpaleu@mec.tuiasi.ro ABSTRACT The kerosene can be used not only as a fuel for jet engines, but also as lubricant of the various mechanisms of modern jet aircrafts and turbine helicopters [1]. In order to determine the wear resistance of hybrid tribological systems lubricated by JET-A1 kerosene, accelerated tests of 1 minute were carried out on a four-ball machine at a constant speed of 1,500 rpm and variable load (from 500 N to 1,000 N). Steel and ceramic were selected as materials for the balls. The scuffing limit was detected at a load corresponding to about 4 GPa mean Hertz pressure and a sliding speed of 0.44 m/s on the concentrated contacts between the upper rolling bearing steel ball and the three lower silicon nitride balls. The diameters of the wear spots were measured on the lower ceramic balls and the structure of the worn surfaces was inspected by optical microscopy. Compared to previous results obtained for hybrid tribological systems lubricated by mineral oil [2], the kerosene film strength was found high enough to recommend it as a lubricant in mechanisms with hybrid concentrated contacts (e.g., hybrid ball bearings). KEYWORDS: Four-ball machine, kerosene, silicon nitride, scuffing. 1. INTRODUCTION When new materials are implemented and new lubricants are used for rolling bearings, tests on both are welcome. Tribological performances of new lubricants are usually determined by tests on tribometers. The most used tribometer is the four-ball machine (also known as the Shell machine). In the standard test, the three lower balls are immersed in lubricant and an upper ball joins them in a pyramid- like form. The lower balls are stationary, while the upper ball rotates at constant or variable speed at a given external load and temperature. The principle of the main device of the four-ball tester is given in figure 1. Four ball machines are usually used to test the different properties of lubricants, i.e., the effect of additives in lubricant on its anti-wear properties [2], as well as material performances: rolling contact fatigue tests (RCF) and failure modes [3, 4, 5]. Fig. 1. Four-ball testing device.