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Theoretical Study of the Load Distribution on the Threads for Roller Screw Mechanisms of a Friction Type（16年摩擦型滾柱絲杠螺紋載荷分布的理論研究）

t The paper provides theoretical research of load distribution across the thread turns of the planetary roller-screw mechanism (RSM). A characteristic feature of the theoretical approach of the paper is the use of a RSM rod model with two elastic contact layers (the first layer is composed of screw-rollers, the second – of nut-rollers). A threaded mating of each elastic layer is considered as a connection with continuous turns, which is closest to the truth and consistent to classical approaches. A similar approach was applied for the ball-and-screw mechanisms with a member point contact. Thus, differential analytical closed-form equations may be used to solve this task. Based on the initial equation of compatibility of strains, axial offsets of contact layer points and the sum of screw, roller and nut turn deflections are calculated; then, load distribution across the turns may be determined. Load distribution depends both on the RSM geometry (pitch, profile angle, entry number, and thread diameter), material of threaded elements, manufacturing accuracy, and on mating friction forces. With the dependencies of load distribution across the turns of zero-clearance RSMs obtained, we have determined that maximum load is imposed on the outermost turns, which corresponds to classical solutions proposed by N. E. Zhukovski.

閱讀： 2016-07-15
Static Load Distribution and Axial Stiffness in a Planetary Roller Screw Mechanism（16年行星滾柱絲杠的靜載荷分布和軸向剛度）

In this paper, an original approach is proposed to calculate the static load distribution and the axial stiffness of a planetary roller screw (PRS) mechanism. Assuming that the external loading is shared equally over an arbitrary number of rollers, only a sector of the system is represented to save on computing time. The approach consists in using a structure of bars, beams, and nonlinear springs to model the different components of the mechanism and their interactions. This nonlinear model describes the details of the mechanism and captures the shape of the nut as well as the bending deformation of the roller. All materials are assumed to operate in the elastic range. The load distribution and the axial stiffness are determined in three specific configurations of the system for both compressive and tensile loads. Further, the influence of the shape of the nut is studied in the case of the inverted PRS. The results obtained from this approach are also compared to those computed with a three-dimensional finite-element (3D FE) model. Finally, since the calculations appear to be very accurate, a parametric study is conducted to show the impact of the bending of the roller on the load distribution.

閱讀： 2016-01-12
Load distribution of planetary roller screw mechanism and its improvement approach（15年行星滾柱絲杠的載荷分布）

A model of load distribution over threads of planetary roller screw mechanism (PRSM) is developed according to the relationships of deformation compatibility and force equilibrium. In order to make the applied load of PRSM uniformly distributed over threads, an improvement approach is proposed, in which the parameters of thread form of roller and nut are redesigned, and the contact conditions of roller with screw and nut are changed to compensate the axial accumulative deformation of shaft sections of screw and nut. A typical planetary roller screw mechanism is taken as example to analyze the load distribution, and the effects of installation configurations, load conditions and thread form parameters on load distribution are studied. Furthermore, the improvement approach is applied to the PRSM, and it is proved to be beneficial to reach uniform load distribution over threads.

閱讀： 2015-09-01
Kinematics Analysis of the Roller Screw Based on the Accuracy of Meshing Point Calculation（15年滾柱絲杠基于嚙合點精度計算的運動學分析）

This paper investigates the meshing behavior of the roller screw, a mechanical transmission device characterized by threaded rollers that transfer a load between the nut and the screw, by analyzing the meshing characteristics between screw and rollers. This study seeks to establish a more accurate mathematical model for the thread surface by creating a modeling process in which the max radiuses of the threads are calculated more precisely. The contact line distribution and the contact location were also calculated in order to confirm the cross section of the meshing points. In the research presented in this paper, the actual transmission ratio is analyzed and the study results in a new method to calculate the actual transmission ratio. In this study, the helical angle and the vertex angle are proven to be of great significance after a careful analysis of their influence is conducted.

閱讀： 2015-03-25
Experimental simulation of rolling–sliding contact for application to planetary roller screw mechanism（15年滾動滑動接觸的實驗模擬及其應用）

The planetary roller screw mechanism is used in the aeronautics industry for electro-mechanical actuators application. It transforms a rotational movement into a translation movement, and it is designed for heavy loads. The main components are made of martensitic stainless steel, and lubricated with grease. Like most usual rolling mechanisms, smearing and jamming can occur before the theoretical fatigue lifetime, especially in defective lubrication conditions. The actuated load is carried by small contacts between the threads of the screw, the rollers and the nut. The static single contact can be described as an ellipsoid on flat contact; motion consists of rolling with sliding perpendicular to the rolling direction. A calculation method based on elastic theories (Hertz, Carter, Johnson) has been implemented. It calculates the normal and tangential stresses distributions, generated in the micro-slip and stick zones of the contact area, using several input parameters such as material properties, normal force, and creep ratio. A specific apparatus has been developed to support these calculations and to experimentally study the damage of the contacts in this mechanism. It consists of a freely rolling wheel loaded on a rotating disc with a component of sliding that simulates the roller screw contact. The tribometer inputs are the normal load, the speed, the creep ratio, and the lubrication. The wheel rolling speed and the tangential force generated in the direction perpendicular to rolling are measured. The experiments reveal a quick adhesive wear in dry or bad lubricated conditions, while a low friction coefficient remains if the contact is well lubricated. The influence of the input parameters concurs with the theoretical calculation. The evolution of grease lubrication during duty lifetime and the influence of the tribo-chemical films on this lifetime are also studied.

閱讀： 2015-03-19

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