The crankshaft bearing is the key component of a rotate vector (RV) reducer. However, owing to the harsh working load and restricted available space, the bearing often suffers from fatigue failure. Therefore, this study proposes a novel optimization method for RV reducer crankshaft bearings. A nonlinear constraint optimization model for the design of the bearing considering the crowned roller profile is formulated and is solved by using a crow search algorithm. The goal of the optimization is to maximize the fatigue life of the bearing. The design variables corresponding to the bearing geometry and crowned roller profile are considered. The load working conditions of the bearing and structure of the RV reducer are analyzed. Various constraints, including geometry, lubrication, strength of the bearing, and structure of the RV reducer, are established. Through the optimization design, the optimum crowned roller profile suitable for the working load of the bearing is obtained, and the stress concentration between the roller and raceway is eliminated. Taking the crankshaft bearing of RV-20E and RV-110E type reducers as examples, the bearings were optimized by the proposed method. After optimization, the bearing life of the RV-20E type reducer is increased by 196%, and the bearing life of the RV-110E type reducer is increased by 168%.

Previous lubrication and wear analysis for the planetary roller screw mechanism (PRSM) has mainly concentrated on the isothermal conditions without considering temperature rise. However, the PRSMs applied in aviation and aerospace always operate in intensely varying thermal conditions with intricate thermal convection and thermal radiation environments. For this purpose, a modified Archard wear model is established by incorporating the entraining speed of threads and the load distribution, considering axial thermal errors. The variation of bulk temperature is characterized by the thermal network model (TNM). The modified Weierstrass–Mandelbrot (W-M) function is used to evaluate the lubrication conditions of the machined surfaces. Meanwhile, the contact flash temperature, wear depth, and precision loss rate are investigated in dry contact and lubricated phrase for mixed thermal elastohydrodynamic lubrication (TEHL) point contact. The results reveal that the temperature rise has a more pronounced influence on the precision maintenance capability compared to the uneven load sharing. The wear results are also compared to previous experimental simulation obtained using a rolling-sliding tribometer that utilize a rolling wheel on a rotating disk with similar patterns. It indicates that the wear life span of PRSMs can be improved by reasonable machining trajectory, thermal management, and lubrication parameters.

A systematic process of matrices-based wear profile modelling and a new optimization approach for planetary roller screw mechanism (PRSM) was proposed in this paper to improve the anti-wear performance with varied thread geometries. The wear evolution characteristics of thread surfaces for convex-concave contact and convex-straight contact were captured by incorporating updated axial clearances originated from accumulated thread wear, which provides a new perspective to predict the wear process of PRSM. The results show that the wear depth in convex-concave contact is 22% shallower than convex-straight contact with 10⁵ numbers of cycles, which suggests that thread wear can be limited with plausible geometrical parameters. Further analysis showed that as the surface roughness increases, the true equivalent pressure, the plastic strain and maximum wear depth becomes larger. In addition, the effect of the surface textures on the tribological properties of threads is also discussed to improve the applicability of the proposed methodology.

Obtaining the load distribution and radial deformations of planetary roller screw mechanism (PRSM) for the load carrying capacity design under complex loading conditions (multi-directional loads) is beneficial to decelerate fatigue and wear of threads. However, the combined effects of axial load, radial load and turning torque (overturning moment) were ignored in previous studies due to the complicated structure of PRSM. This paper presents a multi-roller beam-bar-spring (MR-BBS) model that incorporate radial load, turning torque and thread modification. The proposed model extended the theoretic method for calculating bending behavior of screw and rollers, which can more precisely describe the locus of the threads and roller axis. The load distribution of threads and load sharing among rollers of a PRSM under different external loads and cycling process are analyzed. The results show that the load distribution is more sensitive to turning torque of screw but relatively insensitive to the radial load. Moreover, the load distribution moderately changes with the variation of modification factor. Based on thread modification, the variable lead PRSM is proposed so that the uniformity of load sharing with complex load can be optimized.