Effect of fluid contaminant on the behaviour of a hydrostatic thrust bearing

Abstract

This thesis presents an experimental study of the friction and wear behaviour of hydrostatic thrust bearings due to the solid contaminating particles of ACFTD in the hydraulic oil. The thrust bearing apparatus represents the basic geometry of the valve plate of an axial piston pump. The test rig simulates the operating conditions of a valve plate as the most critical pump component under the contaminated conditions. A preliminary study of the effects of contaminants was undertaken using a Four-ball extreme pressure machine, Ferranti-Shirley viscometer, Redwood viscometer, Ferranti viscometer, and surface tension apparatus. The viscosity and friction was found to show an increase due to contaminating particles when measured by the Ferranti-Shirley viscometer. The experiments using the hydrostatic thrust bearing apparatus have shown an increase in frictional torque and wear due to the increasing amount of gravimetric concentration of ACFTD (contaminant) in hydraulic oil. The increase in wear particles has been supported by the particle counting techniques. A mechanism of contaminated wear has been proposed to represent the stages of wear due to the increasing levels of contaminant concentration. The mechanical polishing theory of surface interaction explains that at lower concentrations, the surfaces are finely polished, lapped and ground. When the onset of wear takes place, it goes on increasing due to the abrasive action of particles. This produces a new surface every time a contaminating particle interacts with the bearing surface. And, when the tufftride layer of iron-nitride has been removed, severe wear takes place as shown by heat spotting and the surface fatigue cracks due to the stress concentration over the hills of the rough surfaces, leading to slip in the grain boundaries of the underlying substrate. The investigation demonstrates the relative importance of some design and operating parameters for hydraulic systems as a whole and valve plate of an axial piston pump in particular. The conclusions arrived at indicate what needs to be done in future.