Analysis of the Run-in Behavior of Axial Piston Pumps

Axial piston pumps are universal displacement machines that are used in a vast variety of applications. Their high pressure resistance and ease of operation make them very popular, especially in mobile applications. Each pump is designed to last several thousand operating hours, but some fail before their expected time. Aspects that play a role in premature failure, are the load cycle, the application, and also the manufacturing tolerances of the parts that define the sealing interface. These tolerances are already very tight for these machines, nevertheless every pump will behave differently in their first operating hours - the run-in. This paper gives a small glimpse into this phenomenon. The run-in is the period where the pump parts experience higher than usual wear, which means that the sealing lands will conform to a geometry that will support the load. It is known that the run-in varies even within one pump model. However, there are many unknowns, for example the ideal run-in operating conditions or the dependency on external conditions such as oil, temperature or loads. Another unknown is whether the geometry after the run-in will always be the same, for the same run-in procedure, or does it vary depending the tolerances of the parts. To give some answers to these questions an innovative test rig was built, to measure the temperatures and fluid film thickness of the slipper/swash plate and cylinder block/valve plate interface of an axial piston pump. Other information such as particles, pressures and vibration were recorded, to give a unique insight into the inner workings of a pump. In addition, a very sophisticated simulation tool, called Caspar FSTI, was used to model the tribological interfaces, place the sensors in critical regions and predict run-in patterns. Previous publications of the authors have shown first trends of wear, within this paper the run-in of the pump will be analyzed more in detail.