Measurement of Friction Forces between Piston and Bushing of an Axial Piston Displacement Unit

Abstract

In industrial applications mineral oil based fluids are used for power transmission and lubrication, enriched with additives for additional functions. These fluids are not fast biologically degradable and often used additives are even environmentally toxic. In case of leakage and disposal into the environment these fluids cause bad damage or high costs for damage prevention. Within the Collaborative Research Centre 442 „Environmentally Friendly Tribosystems by Suitable Coatings and Fluids with Respect to the Machine Tool” at RWTH Aachen the aim is to replace mineral oil based fluids for power transmission and lubrication by fast biologically degradable fluids, which are based on native esters. To compensate the loss of functions in consequence for avoiding the usage of toxic additives in each tribological system, one of two sliding partners of a tribological system gets a PVD-coating. The subproject “Tribological Systems in Hydrostatic Displacement Units” at IFAS strives to modify a hydrostatic axial piston machine in a way, that it can be run with fast biologically degradable fluids without any disadvantages compared with today commonly used units performing with mineral oil based fluids. The axial piston pump is chosen, because its tribological systems include different geometries, loads and moving behaviour. The most critical point in this machine is the tribological system piston-bushing. The contact zone between both bodies is characterised by areas with high pressure, especially in case of complete evacuation of lubrication fluid from the gap and direct contact of the metallic bodies. To ensure the lifelong performance of coated pistons this load has to be reduced. The strategy for this reduction is realised by changing the contour of piston and bushing. In this paper it will be demonstrated, that the friction behaviour of piston and bushing, which is an indicator for the contact pressure, can be changed by the manipulation of the piston geometry. For the friction measurement a new test facility was designed and built up. A detailed introduction to the design and performance of this facility is given in this paper.

Keywords:

• hydrostatic displacement unit
• fast biologically degradable fluid
• biofluids
• PVD
• coating
• piston
• friction
• piezosensors