EXPERIMENTAL AND THEORETICAL INVESTIGATIO N ON TEMPERATURE AND MICRO-SCUFFING IN EHL CONTACTS

This paper describes experimental research conducted on a high-speed t win-disk machine. The twin-disk machine, originally designed to simula te the operation of gears, is used at high speed and low load to simul ate the operation of high-speed rolling bearings at the inner ring - r olling element contact. Operating conditions, ie, rolling and sliding speeds, contact pressure, lubricant, temperature, disk material combin ation and surface finish are typical of aerospace practice. The lubric ant used in these tests is a tetra ester of five centistoke viscosity at 100 degrees C, qualified for use in gas turbine engine lubrication systems under the MIL-L-23699 specifications. In the experimental proc edure, the scuffing limit is reached by increasing progressively the s liding speed, other operating conditions such as the normal load and t he rolling speed being kept constant. The evidence of disk scuffing is a sudden increase of the friction force, which stops automatically th e machine at an earlier stage of the damage process. Moreover, a model to estimate the surface and subsurface temperature distribution in th e contacting bodies is presented and used to complement our experiment al results. Scuffing damages at sliding speeds of up to 40m.s(-1) have been obtained. A total of 32 tests have been carried out. Damages obs erved on the surface of the disks after tests at very high sliding spe eds, appear to be similar in nature to those of the inner raceway of r olling bearings subject to skidding. They consist of transfer of compo nent surface material in microscopic patches from a location on one co ntacting surface to a location on the other contacting surface. The sc uffing limits of two material combinations (M50-M50 and M50-16NCD13), for two mean rolling speeds (25 and 50 m/s), three maximum Hertz press ures (0.5, 0.8 and 1 Gpa) and three oil feed temperatures (40, 80 and 120 degrees C) are presented and discussed. Finally, from this experim ental and theoretical investigation, it is proposed to call 'micro-scu ffing damage' the surface distress observed as a consequence of skiddi ng.