Influence of a Number of Screw and Nut Thread Starts of Planetary Roller-Screw Mechanisms on Their Main Parameters

Abstract

One of the most important requirements imposed to the modern mechanisms is economic feasibility. Therefore, considered as advanced are mechanical transducers of rotary motion to translational one, where rolling friction is mainly realized. They include planetary roller-screw mechanisms (PRSM).

PRSM has a large variety of features. The design feature of PRSM is multiple starts of screw and nut thread. Rollers, as a rule, are made single-threaded. Number of screw thread starts which equals to a number of nut thread starts, has an effect on almost all performance characteristics of PRSM.

This article covers comprehensively enough the influence of screw thread starts quantity on: kinematical parameters of PRSM, transfer function, mechanism radial dimensions, efficiency, power values, required characteristics of electric motor. As a result of investigations the graphs of dimensionless parameters vs. number of screw thread starts have been plotted, which are demonstrative and common.

Being high enough the PRSM efficiency can vary within 20…25% and more. It depends on a variety of mechanism part parameters; primarily on geometrical ones, as well as on a number of screw thread starts. Previously the methods of PRSM design calculation consisted in determination of the main geometric parameters of mechanism parts, then in determination of mechanism efficiency. At that, it was not always possible to design the economically feasible PRSM structure.

Introduction of a dimensionless value – a relation of PRSM part thread pitch to average screw thread diameter – contributed to successive plotting of the assemblage of curves for relation of efficiency to the indicated ratio and a number of screw thread starts. By taking this assemblage of curves as a basis, the method of economically feasible PRSM structures calculation and design was proposed.  The essence of the developed method lies in that selection or definition of a number of screw thread starts helps to determine the relation of PRSM parts thread pitch to average diameter of screw thread which provides the maximum possible efficiency. Given further are check calculations confirming the operation capability of the selected PRSM design.