Research On The Mechanism And Law Of Stiffness And Accuracy Of The Hydrostatic Spindle With Controllable Restrictor

Ultra-precision machining is a sophisticated technology which represents the astonishing development of modern manufacturing industry.Ultra-precision machine tools are the hardware foundation for the success of the technology,and they have unique applications in the manufacture of cutting-edge products,such as aerospace structures,optical instruments,modern weapons and so on.Since there are high requirements for rotating accuracy and loading capacity,hydrostatic spindle systems have huge advantage among ultra-precision machine tools for their characteristics "error homogenization effect" and “oil film incompressibility”.As pressure regulating elements of the hydrostatic spindle system,restrictors have large influence on machining accuracy.The controllable restrictor technology can adjust the restrictor parameters by changing the recess pressure,which can improve the spindle system stiffness and rotation precision.However,with conventional hydrostatic spindle theories,the journal position transition from the unbalanced state to the equilibrium state and the significant reduction of the steady-state rotation motion error of the controllable restrictor hydrostatic spindle(CRHS)cannot be quantitatively explained.The physical mechanism of the stiffness and rotating accuracy promotion are still unclear,and it is difficult to provide systematic and in-depth theoretical support for accuracy limitation prediction,which needs to be solved urgently.This paper focuses on the rotation error development process of CRHS journal.A theoretical model of the journal rotation error formation process is established.The model is solved numerically by the finite difference method and the Euler iterative method,which is a quantitative simulation for the process.The journal stiffness and rotating accuracy promotion mechanism,external load influence,and journal shape error effect on the process are revealed.A CRHS test rig is established and the theoretical analysis results is verified by experiments.The main research works are as follows:(1)A pre-loaded and pre-adjustable controllable throttling technology is developed.The load characteristic model of the hydrostatic bearing with controllable restrictor is established based on the principle of flow continuity.The influence of oil supply pressure,spring stiffness and the control oil chamber pressure on the load capacity and bearing stiffness is revealed.The bearing characteristics and experimental results are compared with the fixed restrictor hydrostatic bearing,which verified the theoretical research results.The results show that with appropriate selection of the control oil chamber pressure,the hydrostatic bearing with controllable restrictor can provide optimal stiffness in the whole eccentricity range.Compared to fixed restrictor,the controllable restrictor can increase the oil film stiffness of the same hydrostatic bearing up to 50%.(2)A theoretical model for the CRHS journal rotation error formation process analysis is established.And the model is solved numerically by the finite difference method and the Euler iteration method.The results reveal the rotating accuracy promotion mechanism of the CRHS.This model is based on the working principle of the controllable restrictor,the controllable throttling constraint equation,the fluid dynamic lubrication equation and the spindle dynamic equation.The relationship between oil supply pressure,spring stiffness,control pressure and journal center orbit are quantitatively studied,and the rotation error formation process of the CRHS is compared with the process of a fixed restrictor hydrostatic spindle(FRHS),which revealed the rotation error reduction mechanism of the controllable throttling technology.The results show that the controllable restrictor can reduce the journal rotation error by reducing rotation center variation radius.(3)The influence of external load on the rotation error formation process of the CRHS is studied.Quantitatively analysis of cyclic loads and transient loads on CRHS are performed,and the results are compared with the results of FRHS.The results show that: cyclic loads cause CRHS journal center vibrates cyclically around the rotational center;the rotation radius of the CRHS journal is proportional to the cyclic load amplitude;the transient loads cannot effect the rotational error,but the journal center orbit of the CRHS and the nonlinear oil film force might reflect the loads,and once the transient load disappears,the journal returns to the initial equilibrium position.(4)The influence of the journal profile on the rotation error formation process in CRHS is revealed,and the results are compared with the results of FRHS.The influence of the roundness error and cylindricity error on journal center drift and vibration amplitude is quantitively studied.The influence of the roundness error frequency on the CRHS rotation error formation process is analyzed.The researches find that bearing oil film has homogenization effect on the journal profile error.The results show that the journal roundness error has little effect on the journal fluctuation center,but obviously affects the fluctuation amplitude and the journal center trajectory shape.The homogenization effect of the bearing oil film on the roundness error at even-numbered frequencies is significantly better than the error at odd-numbered frequencies;the journal cylindricity error has little effect on the rotation error,and the journal center orbit is almost circular when the spindle is stable.(5)A CRHS test rig is built.The journal center orbit and the center vibration amplitude are measured,and the presented model and algorithm are verified by the results.The experimental research show that the simulation results and experiment results have similar journal center displacement-time pattern,and in time domain the displacement amplitude changes in sine or cosine form.The influence of oil supply pressure and the spindle speed on the journal center orbit is similar,the theory and experiment difference of the fluctuation amplitude in X-axis is about 23.6%,and the difference in the Y-direction displacement is about 12.8%,so the agreement is acceptable in engineering applications.This research provides an effective theoretical basis for quantitatively explaining the transient process of journal stabilization and radial error reduction.The physical mechanism of controlled restrictor technology in improving bearing oil film stiffness and spindle rotation accuracy is revealed.A new path for developing high stiffness,ultra-precision and intelligent hydrostatic spindles is explored.