Research On Intelligent Pre-tightening Technology Of Ball Screw Nut Based On Giant Magnetostriction And Its Application

When there is an axial clearance in the ball screw or when there is an elastic deformation between the ball and race under the role of axial load,the ball screw's positioning or transmission accuracy will be influenced and its stiffness will be reduced.In order to eliminate the axial clearance and improve the axial stiffness,a pre-tightening structure is added between screw and nut when the ball screw is bearing load,meanwhile an initial load is applied between the two nuts to produce a certain axial pressure(or pre-tightening force),which causes an axial displacement between the two nuts,so as to achieve the goals of reducing the axial clearance and improving the stiffness.A tiny pre-tightening force can't reduce the clearance and improve the stiffness,while a great pre-tightening force will make the no-load frictional torque increase sharply,the rolling wear exacerbate and the ball screw service life reduce.Therefore the pre-tightening force must be controlled within a suitable range.The conventional ball screw pre-tightening methods can't realize the timely adjustment of the pre-tightening force,so the pre-tightening mechanism of functional materials is studied to provide a greater pre-tightening force and to achieve its accurate and automatic adjustment.This has become a key scientific and technological problem for the positioning or transmission of a precision ball screw.Focusing on the application of Giant Magnetostrictive Material(GMM)to ball screw nut pre-tightening,the axial stiffness and pre-tightening force of ball screw are analyzed in this thesis,the pre-tightening models are established,the multi-physical field parameters are detected,the giant magnetostrictive structure model is constructed and the pre-tightening force and static stiffness experiments are done.(1)The axial stiffness of ball screw depends on the screw characteristics,the preload and the operating conditions.The friction force between the ball and the raceway is ignored since it's far smaller than the raceway pressure.By analyzing the relationship between the axial force and the raceway pressure as well as the relationship between the normal deformation at the raceway contact point and the screw-nut axial displacement,the stiffness of the double-nut ball screw for positioning pre-tightening is analyzed theoretically.(2)Three ball screw pre-tightening structures based on giant magnetostriction are compared with each other in terms the structural features,energy distribution and electromagnetic field analysis.The pre-tightening structure made of a cylindrical GMM with hinge-levers is finally determined to be studied,its magnetic circuit structure is calculated and simulated,its magnetic field distribution is analyzed,and the multi-body static analysis is done on the pre-tightening mechanical structure so as to ensure the allowable stress,strain and deformation of the structure.This structure realizes the transmission of the GMM's mechanical strain to the ball screw nut pre-tightening force,which can be easily adjusted by adjusting the excitation current of the GMM.(3)The cylindrical GMM in the magnetostrictive structure is placed in a cavity of finite volume,when the detection coil is placed in this cavity to obtain the GMM's magnetic parameters(B,H),the induced magnetic field of the coil will have mutual inductance with the original driving magnetic field of GMM.In order to minimize the influence of mutual inductance and obtain the relatively accurate GMM magnetic parameters,it is proposed on the basis of the GMM B-H curve of single coil that the Helmholtz coil principle be used to design a multi-coil way to detect GMM magnetic parameters,and the detection process is simulated and analyzed.The results show that the method of reversing the two Helmholtz coils and placing the detection coil in between can effectively reduce the mutual inductance between the detection coils and the drive coils,therefore it is suitable for GMM magnetic flux detection in finite cavity space.(4)Take the cylindrical GMM as the object of study.The giant magnetostrictive structure composed of cylindrical GMM is optimized to determine its parameters,mainly including the GMM's geometrical shape,drive mode,drive coil structure and preload stress.In order to improve the uniformity and linearity of the magnetic field in the GMM,the optimization method of the driving mode is a key factor to be considered.The analysis results show that when a constant bias magnetic field is applied on the cylindrical GMM in an alternating manner,the yoke structure is optimized and the uniformity and linearity of GMM are improved.Under the quasi-dynamic condition,the characteristics of the giant magnetostrictive structure output force are analyzed and its input-output features are verified by the no-load displacement-force output experiment.(5)The pre-tightening force adjustment experiment and the ball screw static stiffness experiment are carried out on the designed giant magnetostrictive structure.Results show that in a certain range of driving current,the giant magnetostrictive structure has a certain linear working area,where the giant magnetostrictive structure's piezoelectric equation is approximately considered to be linear,making it easy to achieve timely adjustment of pre-tightening force.Based on the giant magnetostrictive ball screw pre-tightening structure,the axial contact stiffness of the ball screw can be effectively improved by the adjustment of the pre-tightening force,which is consistent with the theoretical analysis of the ball screw contact stiffness.In this paper,a ball screw pre-tightening system with hinge-levers based on giant magnetostriction is established,a relatively accurate GMM magnetic field detection method based on Helholtz coil principles and an optimization method of GMM driving method are proposed.This study realizes the goal of converting the mechanical strain of GMM into the pre-tightening force of ball screw nut so as to timely adjust the pre-tightening force of ball screw nut,laying the theoretical and experimental foundation for the application of GMM to ball screw pre-tightening.