Study of Separation Characteristics of Micro-groove Rotary Seal Considering Different Cavitation Boundary Conditions

Micro-grooves have superior hydrodynamic characteristics; therefore, the operating state of the micro-groove rotary seals (MGRSs) used in wet clutches of vehicles change from mixed lubrication to full-film hydrodynamic lubrication with increased rotating speed. These transition characteristics of the lubrication state of the MGRS are called “separation characteristics.” However, simultaneously the increase in rotating speed also leads to the cavitation effect. In contrast to the convergent wedge-shaped gap, “negative effects” exist in the diverging gap that lead to the reduction in the hydrodynamic pressure on the sealing end face. When the pressure is less than the cavitation pressure, the oil film undergoes cavitation and then ruptures. Thus, a large cavitation region reduces the bearing capacity of the oil film. Cavitation effects significantly influence the separation characteristics of the MGRS. Here we report on a theoretical model that has been developed through coupling asperity contact, fluid lubrication with micro-grooves and axial motion dynamic characteristics of the seal. Separation characteristics of the MGRS under Reynolds and Jakobsson–Floberg–Olsson (JFO) cavitation boundary conditions are systematically investigated in this study. The results show that pressure distribution and fluid bearing force under the Reynolds cavitation boundary condition are larger than those under the JFO condition, while friction torques are almost the same under both conditions. The cavitation region is observed in experiments and found to become larger with increases in speed. The JFO cavitation boundary condition predicts larger separation speed of the MGRS than does the Reynolds cavitation boundary, and the separation speed predicted by the JFO cavitation boundary condition is significantly closer to the experimental result than that predicted by the Reynolds condition. For the best separation characteristics of the MGRS, selected micro-groove parameters are as follows: N_g = 35–40, β = 17–23° and h_g = 15–25 μm.