Effect of running-in on the low-pressure tribological performance of valve plate pair in axial piston pumps

Axial piston pumps are widely used in tunnel boring machines with compact structures and high efficiency. The efficiency and lifetime heavily depend on the tribological performance of the pump's valve plate pair. The running-in process is crucial to the steady-state tribological performance of the valve plate pair and directly influences its friction coefficient and wear rate, etc. Axial piston pumps always run-in with different pressure and speed gradients, starting from low-pressure. However, the initial running-in conditions significantly affect the running-in process of the valve plate pair and their effects on tribological performance are unclear. In this paper, ring-on-block testing is conducted. Effects of the speed (500 r/min and 1500 r/min) and pressure (2 MPa, 5 MPa, and 7 MPa) at different initial roughness (Ra = 0.4 μm and 0.05 μm) on the running-in are investigated. The friction coefficient, running-in time, wear rate, and worn surface are compared. Results show that a higher speed and initial roughness can reduce the running-in time, while the larger pressure will increase the friction coefficient and its fluctuation amplitude. The wear mechanism during the running-in process is mainly abrasive wear, while the plastic deformation wear appears with low initial roughness.