Research On Oil Film Control Of High Speed Hydrostatic Bearing Of Vertical Lathe

Maximum speed of CNC vertical lathe worktable,which directly determines the efficiency and surface quality of the workpiece,is an important indicator of the lathe performance.Opening and quantitative oil supply hydrostatic bearing are widely used as the axial support of vertical lathe whose diameter is greater than 4000 mm.The disadvantage is that it has poor oil film retainability when the oil temperature is high,the unstable film result in the decrease of the workpiece precision,when the film is too thin the guiderail surface will be worn because of dry friction and then the bearing can’t work long hours at a high speed.In this thesis,oil film control of large vertical lathe will be researched under the condition of high speed and continuous working for a long time,a stable control system with good film thickness retain performance,high oil film stiffness and high precision will be put forward.The structure of hydrostatic bearing and the type of oil supply will be illustrated in the case of rectangular cavity thrust hydrostatic bearing,the influence of oil film thickness on bearing capacity,on film stiffness and the reason why the thickness became unstable will be analyzed.Temperature affects the oil film deeply,aiming at the problem of a thinner oil film caused by high revolving speed and high oil temperature,the solution in which the film thickness can be retained by controlling the oil flow is proposed.Functional relationship between viscosity,deformation and temperature will be established by calculating the thermal deformation of the worktable and oil viscosity-temperature characteristics under different temperature,flow model of the supporting gap will be derived,then the quantitative relationship between temperature and oil film thickness will be given,the influence of temperature on thickness will be proved,the quantitative relationship can also provide quantitative basis for variable flow oil supplying to keep the film thickness under different temperature.The control system to retain the high speed hydrostatic bearing oil film thickness is established.Multi-channel signal acquisition system of the cavity temperature,pressure and film thickness is established by LabVIEW virtual instrument software,then the state of bearing oil film can be observed online,the signal of thickness sensor is processed by designing filter circuit to meet the needs of control system,PLC(programmable logic controller)in the lathe electrical cabinet is used as the processor,vector-variable-frequency speed control system is used as actuator to build a closed-loop system,principles and usages of each link are analyzed.Mathematical model of each link,and the state space model of the whole system are established,adaptive strategy is proposed for the reason that the system modeling is not accurate,complex time-varying and unknown disturbances exist in it.MCS(minimum control comprehensive)adaptive algorithm is designed and the simulation model of oil film control system is established.The oil system which contains temperature time-varying and unknown disturbances is simulated in open-loop,PID control and MCS control state,the results show that,the ideal trajectory of oil film thickness can be almost completely trailed by the real one when using the controller proposed,the controller has strong anti-interference capability,the stability of oil film thickness is ensured and the control accuracy is improved.This research can provide feasible basis for the high speed hydrostatic bearing to work continuously,stably and accurately.