Impact of surface roughness on the start-up properties of water-lubricated bearings in energy recovery turbochargers

This paper studies the impact of surface roughness on the start-up properties of water-lubricated bearings (WLB) in energy recovery turbochargers. By combining the Weierstrass–Mandelbrot function, water-film thickness equation, Greenwood–Tripp contact model, and averaged Reynolds equation, a systematic mathematical model was established to study the WLB. This model is utilized to calculate the equilibrium position of WLB with static circumstances and examines the impact of varying surface roughness and roughness patterns on shaft motion and performance characteristics during start-up. The investigation indicated that the asperity contact supports all loads with the bearing-rotor system before starting. Whereas the asperity contact pressure diminishes quickly with increasing rotation velocity, the hydrodynamic pressure rises rapidly. When the contact pressure drops to zero, the shaft entirely detached from the bearing. Moreover, the reduced surface roughness and the transverse roughness pattern can enhance the hydrodynamic force during start-up, resulting in decreased lift-off and speed.