Theoretical And Experimental Research On The Meshing And Motion Properties Of Planetary Roller Screw Mechanism

With the rapid development of intelligentization and digitization of equipments and power-by-wire technology,the electro-mechanical actuator is turning towards larger power,higher level of integration,more precision and higher reliability.Planetary roller screw mechanism(PRSM)is a mechanical device for converting rotary motion into linear motion.Due to its advantages of heavier load carrying capacity,higher stiffness,longer lifetime and higher reliability compared with ball screw mechanism,the PRSM becomes a critical component in the electro-mechanical actuator and has been more and more widely used.However,there have been few fundamental researches published to support its engineering applications.In this dissertation,based on the meshing theory of helix and the property of multiple thread meshing points,different types of contact and multi-bodies of the PRSM,the theoretical and experimental research on the meshing and motion properties of the PRSM is conducted.The output of the research can provide a theoretical basis for analyzing the load carrying capacity,stiffness,lubrication,wear and lifetime of the PRSM.Thus,it is of great significance for the design of a high performance PRSM and can promote the application of the PRSM in industry fields,such as aeronautics,astronautics,marine,petrifaction and intelligent manufacturing.In this dissertation,according to the thread surfaces of the screw,roller and nut,the analytical meshing model of the PRSM is proposed.Then,a more comprehensive analytical meshing model of the PRSM is developed,which considers the thread profile errors,dividing errors of multiple threads and the deviation of parts.The kinematic analysis of the PRSM with run-out errors,position errors and dividing errors of multiple threads is performed.A nonlinear six degrees of freedoms dynamic model of the PRSM is developed.A test rig which is used to measure the transmission performance of the PRSM is designed and built.And the feasibility of the proposed kinematic and dynamic models is tested by experiments.The main research works and achievements in this dissertation are as follows:1.According to the equations of thread surfaces and assembly of the screw,roller and nut,the meshing model which is used to compute the contact positions and axial clearances in the PRSM is proposed.Then,the influence of the structural parameters on the meshing properties of the PSRM is studied.The results show that the contact points at the screw-roller interface will move away from the line connecting the centers of the screw and roller,when the pitch of thread is increased or the flank angle of thread is decreased.The thread thicknesses of the screw,roller and nut are different,when the axial clearance of the PRSM is zero.When the radius of the roller profile is increased,the contact points between the screw and roller will move torwards the top of the roller thread or the bottom of the screw thread.2.The meshing equations of the PRSM are derived,which can be used to calculate the contact position and clearance of an arbitrary pair of thread teeth along any directions.The influence of the direction of clearance vector,thread profile errors,dividing errors of multiple threads and deviations of parts on the meshing properties of the PRSM is analyzed.The results show that the influence of flank angle error on the contact position of the PRSM will become larger,when the difference between the flank angles of the threads at the screw-roller or nut-roller interface is enlarged.There is a linear relationship between the pitch diameter errors of threads and the change of the axial clearance in the PRSM.Due to the influence of the dividing errors of multiple threads in the screw and nut,the states of axial clearanes of different rollers are different and those of different thread teeth in the same roller are still different.The deviation of the roller will influence the contact positions and clearances at the screw-roller and nut-roller interfaces.3.A kinematic model of the PRSM with run-out errors,position errors and dividing errors of multiple threads is proposed,which considers the change of motion transfer paths,the fluctuation of the transmission ratio of the spur-ring gear pair and the floating of the roller in the PRSM.A method to analyze the clamping stagnation of the rollers in the PRSM is developed and the influence of above mentioned errors on the motion properties of the PRSM is studied.The results show that the influence of errors on the roller floating regions should be considered in order to avoid clamping stagnation of the PRSM.When the magnitude of the nut run-out error is equal to that of the nut installation error,the influence of the run-out error and installation error of the nut on the transmission error of the PRSM can be counteracted if the value of the nut installation angle is reasonable.The dividing errors of multiple threads have effect on the roller floating regions and the number of the thread teeth on the roller that contact with the screw or nut.4.A nonlinear six degrees of freedoms dynamic model of the PRSM is proposed by using Newton's Second Law.The influence of the friction coefficients,load conditions and structural parameters on the dynamic properties of the PRSM is studied.The results show that the ratio between the rotational velocities of the carrier and screw,the contact force at the ring gear-roller interface and the contact force at the carrier-roller interface are increased,when the friction coefficients of threads are increased.At the same time,the efficiency of the PRSM is decreased.The external load acting on the nut has effect on the meshing properties of the spur-ring gear pair.The steady-state efficiency and the ratio between the rotational velocities of the carrier and screw of the PRSM under light load are less than those of the PRSM under heavy load.If the pitch of thread is increased or the flank anlge of thread is decreased,the efficiency of the PRSM is increased.When the flank angle of thread is less than 45?,it will have little influence on the efficiency of the PRSM.5.A test rig which is used to measure the transmission performance of the PRSM is designed and built.The no-load transmission error,efficiency and rotational velocity of the carrier of a PRSM specimen are measured.The effectiveness of the proposed kinematic and dynamic models of the PRSM is validated by comparing the results obtained from the proposed models and experiments.The results show that the no-load transmission error of the PRSM is cyclic with a period corresponding to the rotation period of the screw.The magnitude of the no-load transmission error is much larger than that of the screw lead error.When the directions of the external load and the velocity of the nut are opposite,the efficiency of the PRSM and the ratio between the rotational velocities of the carrier and screw are larger than those when the directions of the external load and the velocity of the nut are the same.