Uneven Load Distribution Between Mating Windings of Roll and Screw with Nut of Planetary Roller Drive

Planetary roller drives (PRD) with axial backlash have the highest load capacity amongst the prospective roller drives. PRD consists of a screw and a nut, between which thread rolls are installed in separators, force calculation of which is sufficient to assess the PRD force loading. For several reasons, the load is unevenly distributed between the roll turns operating with the screw and nut turns. These include rollers size variation by the average diameter of their thread, PRD parts' thread manufacturing errors, primarily thread pitch errors, and other reasons. The article discusses one of them – a special feature of thread roll loading. If we neglect the circumferential forces due to their smallness, then the roll's turns mating with the nut along one of the roll's generatrices and the roll turns mating with the screw along the opposite generatrix will be loaded with axial and radial forces. Since the PRD parts thread turn flank angle is symmetrical and equal to 90°, then the radial force is equal to the axial one on each turn. If we sum the axial forces on the mating roll turns along one of the generatrices, then from the equation of equilibrium one gets that the sum vector value is equal to the sum vector value along the other roll generatrix. Here the sum vectors will have the opposite direction, and the arm between them equals to the average roll thread diameter. Multiplying the sum vector modulus by that arm, one gets the roll overturning moment that does not depend on the distribution of the axial forces along the roll axis. This moment is the cause of uneven load distribution between the roll turns mating with the nut and the screw. The overturning moment can be balanced on the two forming roll generatrices only by radial forces, distribution of which along the roll axis will be uneven. The PRD parts thread turns load distribution irregularity factor is introduced as the ratio of the maximum force acting on the most loaded roll turn to a nominal force. It is determined in the example that the value of this factor is equal to 1.454 or 45.4%, i.e. the influence of the PRD roll loading characteristics on the drive threaded parts turns loading irregularity is strong.