Research on the Mechanism of Friction Fluctuation in Piston-Cylinder Kinematic Pair under Fluid Lubrication

:The piston-cylinder kinematic pair is a critical mechanical joint widely used in internal combustion engines, hydraulic presses, and dampers. This study investigates friction fluctuations and their mechanism in the kinematic pair. A counterintuitive phenomenon was observed: under oil lubrication, the friction coefficient fluctuates between 0.1–0.9, significantly exceeding the 0.15–0.22 range under dry friction conditions. To explain this, we propose a mechanistic model where surface asperities induce lateral coupled motion during axial piston movement; lateral fluid damping forces resist this motion, converting into additional fluctuating normal force on the piston, thereby causing friction fluctuations. Using the cylinder surface roughness, piston axial velocity, and fluid-simulated lateral damping forces for calculation, it was numerically confirmed that the fluctuation magnitude predicted by the model matches the experimental data. Four corollaries derive from this model: lower lubricant viscosity reduces fluctuations; reduced axial velocity decreases fluctuations; higher nominal normal force diminishes equivalent nominal friction coefficient fluctuations; reduced surface roughness decreases fluctuations. Experimental verification supported all corollaries. Based on this mechanism, piston geometry was optimized to minimize lateral damping forces under lubrication, achieving stable-friction piston-cylinder kinematic pairs that reached the friction coefficient fluctuation level of dry friction (0.15–0.22) in the silicone oil environment. This work provides a theoretical foundation for designing friction-stable mechanical systems. Of course, it must be mentioned that the proposed mechanistic model has not been directly verified by experiments, but only indirectly validated.