Pre-Sliding Stiffness Collapse in Hydraulic Cylinder Start-Up: Temperature–Stroke Coupled Effects and Underlying Mechanism

Accurate characterization of hydraulic cylinder start-up characteristics is critical for precision electro-hydraulic servo systems. Conventional methods rely solely on maximum static friction force, a force-domain metric that fails to capture pre-sliding micro-displacement and thus cannot reliably indicate interfacial contact state or positioning accuracy. This paper investigates the coupled effects of temperature and stroke on pre-sliding stiffness through high-precision force-displacement joint measurement. A test bench integrating 50 kHz high-frequency pressure sensors, a 25 nm resolution laser displacement sensor, and a precision temperature control system is constructed, with a 12th-order zero-phase Butterworth filter employed to preserve phase fidelity of transient dynamic signals. Under 11 MPa constant pressure, measurements reveal that when oil temperature rises from 50°C to 60°C, the pre-sliding stiffness drops by over 75% across all three stroke lengths, while maximum static friction force varies by less than 15%. The stiffness collapse is dominated by thermal softening and partial desorption of the boundary lubrication film triggered by frictional heat accumulation, with PTFE material softening as a secondary factor. This framework provides a reliable measurement basis for non-destructive online monitoring of hydraulic seal states and adaptive gain scheduling in precision servo control.