A kinematic model of the planetary roller screw mechanism (PRSM) is proposed, which accounts for the run-out errors of the screw, roller, nut, ring gear, and carrier, and the position errors of the nut and the pinhole in the carrier. The roller floating region, which contains all the possible positions of the roller inside the pinhole, is obtained by analyzing the axial clearances between mating thread surfaces and the radial clearance between the roller and carrier. The proposed model is based on the constraint that the set of roller floating region is not empty. Then, the additional rigid-body movement on the nut is derived and the path of motion transfer from the screw to the nut is obtained. According to the fundamental property of rigid-body kinematics, the axial velocity of the nut is derived and the transmission error of the PRSM is calculated. The proposed model is verified by comparing the calculated transmission error with experimental one. The results show that the transmission error of the PRSM with run-out and position errors is cyclic with a period corresponding to the rotation period of the screw and the magnitude of the transmission error can be much larger than the lead error of the screw. Besides, due to the run-out and position errors, the roller can move radially or transversally inside the pinhole of the carrier when the elements in the PRSM are regarded as rigid bodies.

A considerable portion of space mechanism failures are related to space tribological problems. Cold welding in high vacuum; surface erosion and collision damage caused by various radiations, high temperature oxidation under atomic oxygen (AO) bombardment; and thermal stress caused by temperature alternation all alter the physical, chemical, and friction properties of materials. In particular, the space vibration caused by alternating temperatures and microgravity environments can alter the motion of the contact body, further affecting its friction properties. Improving the friction properties of contact surfaces in the space environment is an important way to extend the service life of spacecraft. Traditional lubricants can no longer meet the lubrication requirements of the space environment. This study describes the characteristics of the space environment and the applications of solid lubricants. The friction properties of MoS₂, a solid lubricant widely used in space, are discussed. The synergistic lubrication of MoS₂ with surface textures or metals is presented. Advances in research on the friction properties of collision sliding contacts in the space environment are reviewed. The combination of MoS₂ and soft metals with surface textures is introduced to reduce the effects of vibration environments on the friction properties of moving parts in space mechanisms. Finally, the challenges and future research interests of MoS₂ films in space tribology are presented.

Nanoscale sliding contacts are the major factors that influence the friction and result in wear in micro/nanoelectromechanical systems. Many experimental studies indicated that some surface textures could help improve the contact characteristics and reduce friction forces. However, the experimental results may be biased, due to the contamination of the sample surface or substantial defects in the materials. Numerical methods, such as continuum mechanics, meet great challenges when they are applied at length of nanoscale, and the time cost of molecular dynamics (MD) simulation can be extremely high. Therefore, multiscale method, which can capture atomistic behaviors in the region underlying micro/nano physical processes by MD simulations and models other regions by continuum mechanics, offers a great promise. Coupling MD simulation and finite element method, the multiscale method is used to investigate two dimensional nanoscale sliding contacts between a rigid cylindrical tip and an elastic substrate with textured surface, in which adhesive effects are considered. Two series of nanoscale surface textures with different asperity shapes, different asperity heights, and different spacings between asperities are designed. For different heights of asperities or different spacings between asperities, average potential energy, normal forces, mean normal forces, friction forces, and mean friction forces are compared to observe how these parameters influence friction characteristics; then, the optimal asperity height or spacing is discovered. Through the average potential energy, normal forces, mean normal forces, friction forces, and mean friction forces comparisons between smooth surface and textured surfaces, a better shape is advised to indicate that asperity shape plays an important role in friction force reduction. The influences of the indentation depth and radius of the rigid cylindrical tip are analyzed to find out the sensitivity of surface textures to these two parameters. Effects of sliding speed on the characteristics of nanoscale sliding contacts are also discussed. The results show that, with proper asperity height and proper spacing between asperities, surface textures can reduce friction forces effectively. Coefficients of friction (COFs) of all the cases are calculated and compared. Some negative COFs caused by significant adhesive effects are discovered, which are different from traditional macroscopic phenomena.

In nanomaterials, the surface or the subsurface structures influence the friction behaviors greatly. In this work, nanoscale friction behaviors between a rigid cylinder tip and a single crystal copper substrate are studied by molecular dynamics simulation. Nanoscale textured surfaces are modeled on the surface of the substrate to represent the surface structures, and the spacings between textures are seen as defects on the surface. Nano-defects are prepared at the subsurface of the substrate. The effects of depth, orientation, width and shape of textured surfaces on the average friction forces are investigated, and the influence of subsurface defects in the substrate is also studied. Compared with the smooth surface, textured surfaces can improve friction behaviors effectively. The textured surfaces with a greater depth or smaller width lead to lower friction forces. The surface with 45° texture orientation produces the lowest average friction force among all the orientations. The influence of the shape is slight, and the v-shape shows a lower average friction force. Besides, the subsurface defects in the substrate make the sliding process unstable and the influence of subsurface defects on friction forces is sensitive to their positions.