Shangjun Ma

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.

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.

A New Study on the Parameter Relationships of Planetary Roller Screws

As a more powerful transmission device, planetary roller screws (PRSs) recently have received more attention, compared to conventional ball screws. However, due to the complicated and unclear relationships among the PRS components’ parameters, it is difficult to design high-quality PRSs. To facilitate the PRS design, a new study on the parameter relationships of PRS is conducted in this work. New models of the axial stiffness and the frictional moment of PRS are developed, and the relationships of the axial stiffness and the frictional moment in terms of contact angle, helical angle, and tooth number of the roller thread are investigated. This study could contribute to the research of PRS to improve its transmission performance, especially to increase its positioning accuracy.

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.

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.

Thread Modification Method for Load Balance on Planetary Roller Screw Mechanism

Based on the relationship between the load distribution and the load deformation of the thread of the planetary roller screw mechanism (PRSM), a modification method of the roller thread is proposed. The nut side and the screw side of the roller thread are modified by different amount of modification, and the modified thread load distribution is calculated and analyzed. The results show that the effects of roller thread modification on the load distribution are significant, and the sensitivity of the load capacity of the nut and screw to the modification is different. Through the study of the influence of roller thread modification on thread load distribution, the optimal value of the roller thread modification on the nut side and the screw side is obtained under the PRSM parameters given in this paper, which provides a theoretical guidance for the future PRSM load balancing design.

An improved thermal estimation model of the inverted planetary roller screw mechanism

The inverted planetary roller screw mechanism has recently become competitive in the electro-mechanical actuation system due to its high load-carrying capacity and small assembly size. However, a significant amount of heat at the frictional contact interfaces and power loss inside the electrical machine can be naturally generated in a compact and high-load inverted planetary roller screw mechanism system. The conductive heat leads to the temperature rise of inverted planetary roller screw mechanism components that subsequently results in thermal drift and error as well as the actuation accuracy degradation. An analytical approach is applied to calculate the friction torque of the contact pairs and support bearings in the inverted planetary roller screw mechanism system. As the thermal load, heat generation is derived from the friction in nut-roller-screw section and bearings. Then, the heat generation and convection boundary conditions are formulated to facilitate thermal behavior analysis. Finally, using the finite element method, steady-state and transient thermal-mechanical coupling analyses are performed to estimate the temperature distribution and thermal expansion of the inverted planetary roller screw mechanism components. Computational results reveal that operating conditions of rotation speed and external load have significant influence on the thermal characteristics of the inverted planetary roller screw mechanism. This study can serve as a foundation for modeling temperature field and analyzing coupled thermal-mechanical response of inverted planetary roller screw mechanism in electro-mechanical actuation system, which can be useful in determining thermal error compensation.

A review of electromechanical actuators for More/All Electric aircraft systems

Conventional hydraulic actuators in aircraft systems are high maintenance and more vulnerable to high temperatures and pressures. This usually leads to high operating costs and low efficiency. With the rapid development of More/All Electric technology, power-by-wire actuators are being broadly employed to improve the maintainability, reliability, and manoeuvrability of future aircraft. This paper reviews the published application and development of the airborne linear electromechanical actuator. First, the general configuration, merits, and limitations of the gear-drive electromechanical actuator and the direct-drive electromechanical actuator are analysed. Second, the development state of the electromechanical actuator testing systems is elaborated in three aspects, namely the performance testing based on room temperature, testing in a thermal vacuum environment, and iron bird. Common problems and tendencies of the testing systems are summarized. Key technologies and research challenges are revealed in terms of fault-tolerant motor, high-thrust mechanical transmission, multidisciplinary modelling, thermal management, and thermal analysis. Finally, the trend for future electromechanical actuators in More/All Electric Aircraft applications is summarized, and future research on the airborne linear electromechanical actuators is discussed.