Transient Evolution of the Piston–Cylinder Oil Film and Thermo–Fluid–Solid Coupling Response in an Axial Piston Pump Under Complex Operating Conditions

Existing piston–cylinder lubrication studies often simplify the pressure boundary as a constant load or a single field, making it difficult to capture pump-level pressure excitation, local oil-film response, and non-concentric posture under variable loading. This paper establishes a thermo–fluid–solid coupling framework integrating an AMESim full-pump model, a Fluent transient oil-film model, and a Transient Structural model; UDF transfer of periodic pressure, dynamic meshes, and a calibrated Roelands law were used to analyze parallel-offset and center-tilted postures. As the load pressure increased from 10 to 30 MPa, the maximum discharge–half-cycle temperature rose from 28.39 to 36.95 °C, and the average positive leakage during the third-cycle high-pressure stage increased from 0.0201 to 0.1026 L/min; increasing speed from 1000 to 3000 r/min reduced cycle-averaged leakage by 8.93%. At 500 r/min and 30 MPa, the parallel-offset case reached 46.34 °C, 41 kPa, and 0.0990 L/min in maximum temperature, maximum shear stress, and average leakage, whereas the center-tilted case produced a peak resultant force of 3537.12 N, a cylinder inner-wall high-stress band of 76.96 MPa, and a maximum piston deformation and equivalent stress of 4.31 μm and 83.16 MPa. These results clarify the distinct lubrication behavior and potential uneven-wear risk associated with the two representative non-concentric postures, and provide a basis for clearance design and posture-sensitive condition assessment of axial piston pumps.