Lubricant replenishment in the vicinity of an EHD contact

Abstract: Lubricants are subjected to high pressures in many mechanical devices - notably gears, cams, piston rings and rolling element bearings. The contact pressure between the ball and the race in a ball bearing can be as high as 5 GPa. At such high pressures, it is necessary to be able to measure the rheological behaviour of the lubricant in order to obtain input data for computer simulations of rolling and sliding contacts. Lubricant manufacturers are often unable to provide information about the rheology of the lubricant at high pressures, temperatures, shear stresses and shear rates. In paper A, a new Couette rheometer was developed to measure the rheological properties of fluids at pressures up to 500 MPa. The shearrate range covers 0.5 to 20000 1/s and the instrument can operate up to 1O0°C at maximum pressure. The results from the apparatus show excellent agreement with measurements made with a failing ball viscometer on a Newtonian fluid. Criteria for non-laminar flow and viscous heating have been developed for the apparatus to verify that these effects do not significantly influence the measurements. In grease lubricated machine elements the supply of grease to the contact region is very important for long life and maintenance-free operation. For example, in a bearing application the grease is often applied within the housing but outside the bearing. For good lubrication the grease has to move from the storage space into the bearing in order to separate the moving surfaces. One important part of this grease movement is the transport from near the contact region into the rolling track. This transportation mechanism may depend on different phenomena such as: capillary forces, vibrations, drop formation, oil bleeding etc. The papers B-G in this thesis describe investigations of these various phenomena. Papers B-D deal with the drop formation of oils and greases in the outlet of an elastohydrodynamically lubricated contact and how different surface energies can affect the distribution of lubricant between the surfaces. Paper E describes the elongation of grease filaments formed when two surfaces are separating from each other, as a function of time. Measurements of the lift off (adhesion) force acting between the two solid surfaces as a function of time were also made. In paper F a ball and disc apparatus was used to study the replenishment of grease due to side slip. The inlet distance and the 'butterfly' shaped meniscus that surrounds the contact point was studied as a function of the side slip motion of the ball. The filament formation equations from paper E were extended to include the geometry of the butterfly and the inlet distance. Finally, in paper G, four deep groove ball bearings were tested in a SKF R2F test rig to evaluate how lift-off-speed and breakthrough-speed are affected by ball spin. The results are compared to film thickness calculations. The lubricants used were greases, oil bled from the greases and the base oil used for manufacturing the greases.