Grease-mixed lubrication of double circular-arc spiral bevel gears for nutation drive considering shaft misalignment

Double circular-arc spiral bevel gears for nutation drive are characterized by their compact structure, small size, large transmission ratio, and exceptional load-bearing capacity. The gear is suitable for industrial robotic joint reducers. Grease lubrication is widely adopted to avoid the need for bulky oil reservoirs due to the spatial constraints in robotic joint reducers. This paper establishes for the first time a grease-mixed elastohydrodynamic lubrication model of double circular-arc spiral bevel gears for nutation drive based on the Ostwald grease lubrication rheological model. The model's accuracy is experimentally validated. The effects of shaft misalignment, rheological index, and rotational speed on the lubrication characteristics of the gear are analyzed. The results show that an appropriate positive shaft misalignment can increase film thickness, suppress the peaks of pressure and temperature rise, and mitigate edge contact effects. Moreover, a decrease in the rheological index m weakens the shear-thinning effects of the grease, thereby reducing the film thickness and increasing the risk of asperity squeezing. Boundary lubrication may occur at m  = 0.75. A decline in rotational speed n shifts the lubrication regime from full-film to mixed lubrication. At n  = 10 rpm, the necking phenomena of the film thickness disappear, resulting in oil film rupture and a transition of the gear into mixed lubrication.