Planetary Roller Screw Mechanism Research Articles

Load distribution of planetary roller screw mechanism and its improvement approach

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.

Static Analysis of an Inverted Planetary Roller Screw Mechanism

The paper examines the static behavior of the inverted planetary roller screw (PRS) through numerical and experimental studies. The numerical analysis of the inverted PRS is first presented to capture the global and local deformations in different configurations. Using a three-dimensional finite element (3D FE) method, a sectorial model of the mechanism is built involving an entire roller. The model describes the static behavior of the system under a heavy load and shows the state of the contacts and the in-depth stress zones. The current work also investigates the axial stiffness (AS) and the load distribution (LD) under both compressive and tensile loadings. It is shown that the LDs are not the same at each contact interface of the roller and that they depend on the configuration of the system. Also, the nut is less stressed than the screw shaft because of their contact curvatures. In parallel, complementary experiments are carried out to measure the axial deflection of the screw shaft and the rollers in five cases with different numbers of rollers. In each situation, the mechanism is under the same equivalent axial and static load. The tests reveal that rollers do not have the same behavior, the difference certainly being due to manufacturing and positioning errors that directly affect the number of effective contacts in the device. This stresses the fact that the external load is unequally shared over rollers and contacting threads. By introducing the notion of an equivalent roller, the results are used to validate the previous numerical model of an inverted PRS. As they provide a better understanding of the inverted PRS, these investigations are useful to improve the existing analytical models of the device.

Studies on Meshing Mechanism of Helical Surfaces in Planetary Roller Screw Mechanism

Studies on Meshing Mechanism of Helical Surfaces in Planetary Roller Screw Mechanism

Wear Behavior of Martensitic Stainless Steel in Rolling-Sliding Contact for Planetary Roller Screw Mechanism: Study of the WC/C Solution

Wear Behavior of Martensitic Stainless Steel in Rolling-Sliding Contact for Planetary Roller Screw Mechanism: Study of the WC/C Solution

Theoretical Study of the Load Distribution on the Threads for Roller Screw Mechanisms of a Friction Type

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.

Static Load Distribution and Axial Stiffness in a Planetary Roller Screw Mechanism

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.

Dynamics of the Planetary Roller Screw Mechanism

This paper develops the dynamic equations of motion for the planetary roller screw mechanism (PRSM) accounting for the screw, rollers, and nut bodies. First, the linear and angular velocities and accelerations of the components are derived. Then, their angular momentums are presented. Next, the slip velocities at the contacts are derived in order to determine the direction of the forces of friction. The equations of motion are derived through the use of Lagrange's Method with viscous friction. The steady-state angular velocities and screw/roller slip velocities are also derived. An example demonstrates the magnitude of the slip velocity of the PRSM as a function of both the screw lead and the screw and nut contact angles. By allowing full dynamic simulation, the developed analysis can be used for much improved PRSM system design.

Experimental simulation of rolling–sliding contact for application to planetary roller screw mechanism

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.

Thermo-Mechanical Model and Thermal Analysis of Hollow Cylinder Planetary Roller Screw Mechanism

Planetary roller screw mechanism (PRSM) is widely used for rapid and precise motion translation from rotary into linear motion due to its high stiffness and high position accuracy. However, a high speed PRSM drive system naturally generates significant amount of frictional heat at the contact interfaces, which causes thermal deformation and thermal error and reduces motion accuracy. Preload is usually applied to remove the axial backlash of the PRSM for achieving high accuracy and great stiffness. However, more frictional heat is produced by such preload. On the other hand, larger numbers of angular contact bearings are needed to support the heavy axial load. The friction heat generated in support bearings has also to be investigated. In order to estimate the thermal distribution and thermal error of the hollow cylinder PRSM, a thermo-mechanical model based on finite element method (FEM) is developed, where heat generation from the two main sources of the PRSM, the parameters calculation of heat transfer coefficient and other thermal boundary conditions were studied. The presented model is proven capable of investigating temperature distribution, thermal error, and cooling performances of coolants of the PRSM system.

Kinematics of Planetary Roller Screw Mechanism considering Helical Directions of Screw and Roller Threads

Based on the differential principle of thread transmission, an analytical model considering helical directions between screw and roller threads in planetary roller screw mechanism (PRSM) is presented in this work. The model is critical for the design of PRSM with a smaller lead and a bigger pitch to realize a higher transmission accuracy. The kinematic principle of planetary transmission is employed to analyze the PRSM with different screw thread and roller thread directions. In order to investigate the differences with different screw thread and roller thread directions, the numerical model is developed by using the software Adams to validate the analytical solutions calculated by the presented model. The results indicate, when the helical direction of screw thread is identical with the direction of roller thread, that the lead of PRSM is unaffected regardless of whether sliding between screw and rollers occurs or not. Only when the direction of screw thread is reverse to the direction of roller thread, the design of PRSM with a smaller lead can be realized under a bigger pitch. The presented models and numerical simulation method can be used to research the transmission accuracy of PRSM.

A Frictional Heat Model of Planetary Roller Screw Mechanism Considering Load Distribution

Planetary roller screw (PRS), with higher thrust, higher load capacity, and higher speed, is the best choice of the transmission component of the servo system. However, spinning sliding of rollers and support bearings can cause frictional moments and frictional heat, which is an undesirable phenomenon. Besides, frictional heat will further result in high temperature that causes deterioration of lubrication and eventually lead to destruction of the mechanism. Therefore, it is important to predict frictional moments which result in frictional heat. In order to predict the magnitude of frictional heat of PRS mechanism and study the influence of structural parameters of thread and operating conditions on frictional heat, first, a frictional moment model of bearings is built, and frictional moments models of PRS considering the elastic hysteresis of material, the spinning sliding of the rollers, the viscosity of lubricating oil and the differential sliding of thread raceways are established in this paper, respectively. Second, heat generation models of bearing and PRS are presented, respectively. Finally, relationships between frictional heat in terms of operating conditions of PRS, contact angle, and helix angle of roller thread are investigated. The achievements of this project will provide theoretical basis for the design of PRS with lower frictional moments and higher transmission efficiency.

Расчет радиального смещения осей гайки и роликов и положения точки контакта резьбы гайки и ролика в планетарном ролико-винтовом механизме, выполненном по «перевернутой» схеме

Расчет радиального смещения осей гайки и роликов и положения точки контакта резьбы гайки и ролика в планетарном ролико-винтовом механизме, выполненном по «перевернутой» схеме

Планетарный ролико-винтовой механизм преобразования вращательного движения в поступательное, выполненный по "перевернутой" схеме

Планетарный ролико-винтовой механизм преобразования вращательного движения в поступательное, выполненный по "перевернутой" схеме

Kinematics of Roller Migration in the Planetary Roller Screw Mechanism

This paper develops a kinematic model to predict the axial migration of the rollers relative to the nut in the planetary roller screw mechanism (PRSM). This axial migration is an undesirable phenomenon that can cause binding and eventually lead to the destruction of the mechanism. It is shown that this migration is due to slip at the nut–roller interface, which is caused by a pitch mismatch between the spur-ring gear and the effective nut– roller helical gear pairs. This pitch circle mismatch can be due to manufacturing errors, deformations of the mechanism due to loading, and uncertainty in the radii of contact between the components. This paper derives the angle through which slip occurs and the subsequent axial migration of the roller. It is shown that this roller migration does not affect the overall lead of the PRSM. In addition, the general orbital mechanics, in-plane slip velocity at the nut–roller interface, and the axial slip velocities at the nut–roller and the screw–roller interfaces are also derived. Finally, an example problem is developed using a range of pitch mismatch values for the given roller screw dimensions, and the axial migration and slip velocities are determined.

Kinematics and Efficiency Analysis of the Planetary Roller Screw Mechanism

This paper analyzes the kinematics and the efficiency of the planetary roller screw mechanism (RSM) to provide a fundamental basis to support its various applications. The mechanical structure and practical advantages are presented in comparison with the conventional ball screw mechanism (BSM). Kinematic analysis involves derivation of the angular and axial motions, as well as the development of the slip pattern between the contacting components. Results show that for any motion of the RSM slip always occurs. Kinematic analysis including elastic deformation is also presented. The load carrying capacity and efficiency of the RSM are derived based on geometric and equilibrium conditions, and the results are compared with the BSM.