Design And Static-dynamic Characteristics Analysis Of Gravity Compensator For Ultra-precision Machine Tool Vertical Axis

Vertical axis for ultra-precision machine tool can expand the machine's processing methods and enhance the processing capacity of machine tool,so that it has been one kind of important component of the multi-axis linkage ultra-precision machine tool.But the gravity compensator of the existing ultra-precision machine tool vertical axis has some flaws currently,such as the friction in the process of movement affecting the positioning accuracy and processing stability of the vertical axis for ultra-precision machine seriously.In order to solve the above problems,this dissertation designed the gravity compensator with aerostatic lubricated low friction cylinder as the core component.The static performance of the aerostatic radial bearing and the static-dynamic characteristics of the low friction gravity compensator were analyzed.The specific research contents are as follows.Based on the design criteria of low friction and the theory of aerostatic lubrication,as well as the typical structures of low friction cylinder,the air float mechanism and unloading mechanism of this new type low friction gravity compensator designed in this dissertation could work independently,because their gas source were supplied respectively.Therefore,this kind of gravity compensator could work smoothly compared with other gravity compensator,especially under the low load.In this dissertation,a mathematical model of the static characteristics of the aerostatic radial bearing was established through the general expression of the Reynolds equation.After that,the selection of the size parameters of the aerostatic bearing was completed.And then complete the pneumatic circuit design to support the gravity compensator,so that the gravity compensator can be in the state of gas lubrication to achieve the goal of balancing the load.Based on the overall structural design of the gravity compensator,the static performance of the aerostatic radial bearing was analyzed by Fluent which is one kind of computational fluid dynamics simulation software widely used around the world.The influence of the gas pressure distribution on the bearing affecting the capacity and static stiffness of the bearing was studied.The effects of eccentricity,bearing radial clearance and gas supply pressure on the static performance of aerostatic radial bearing were analyzed,and the optimum value of each parameter were determined utilizing the results of simulation.According to the structural size of the gravity compensator,the finite element model was established.After that,the structural static analysis of the gravity compensator under different position of the piston rod was carried out to analyze the weak point of the gravity compensator.And its sensitivity of bending moment is studied as well.The dynamic model of the gravity compensator was established,and the modal characteristics of the gravity compensator under different positions of the piston rod were analyzed.The influence of the position change of the piston rod on the modal characteristics of the gravity compensator was studied.In the end,the value of friction force and the change law of the friction force under different gas pressure were discussed,through the performance test of the gas lubricated low friction cylinder for the gravity compensator.