Abstract: Planetary roller screw mechanism (PRSM) transfers power and movement by the meshing of helical surfaces of a few rollers contacting with the screw and nut. The helical surface equations of roller, screw and nut thread surfaces are established based on the multi-start thread structural characteristics. According to the continuous tangency conditions of meshing surfaces, a method to calculate the meshing point positions and axial clearances of the PRSM is presented by utilizing the vectors described the translation of the screw, roller and nut thread surfaces along their axis. Then, the meshing equations and the axial clearances at the screw/roller and nut/roller interfaces are derived. By comparing the results obtained by existed calculation method with those by the presented meshing equations, the feasibility of meshing equations is validated. The meshing point positions, axial clearances of PRSM and axial gap distributions of two meshing surfaces are analyzed in the case that the screw, roller and nut’s flank angles are identical. The results show that the meshing point at screw/roller interface will incline to the top of the screw and roller’s thread; the meshing point at nut/roller interface will lie on the tangent point of pitch circles of the two meshing components; in order to eliminate the axial clearance during the design, the thread thickness of the screw, roller and nut should be different.

Key words: axial clearance, helical surface, meshing mechanism, planetary roller screw mechanism