Research On The Characteristics Of Thermohydrodynamic Lubrication Of High Speed Small Diameter Liquid Suspension Spindle And The Effect Of Surface Microstructures

Liquid suspension spindles are widely applied in the precision high speed machines due to their high rotation accuracy, high stiffness, high damped vibration, and long maintaining life. For the structural integration of spindle function and motor function, it reduces complex intermediate transmission system and thus develops liquid suspension spindle into a new type of spindle unit featuring merits such as wider speed range, low vibration noise, quick start-up and accurate stop, which can greatly enhance the productivity and improve the machining accuracy and surface quality in parts processing. Meanwhile, although the small diameter and high speed liquid suspension spindle becomes the core function component of the high-efficiency high-precision grinding equipment for the concave cams in modern automobile engines, the small diameter and high speed liquid suspension spindle with both high efficiency and high accuracy are in short at home and abroad. To better meet the need of ultrahigh speed and precision machining, it is urgently need to control bearing temperature rise of due to high-speed and structural integration and improve the spindle stiffness restricted by construction dimension and oil film temperature rise in high speed.This thesis puts forward a method to regulate and control the thermodynamic property of small diameter and high speed liquid suspension spindles by surface microstructure technology, builds a thermo-hydrodynamic lubrication theoretical analysis and design method of liquid suspension spindles based on computational fluid dynamics, explores the influence of bearing working parameters, structure parameters, and geometric and distribution parameters of microstructure to the flow field characteristics of oil film and basic properties of thermo-hydrodynamic lubrication and relevant matching rules in optimization, develops small diameter and high speed liquid suspension motorized spindles with both high bearing stiffness and low temperature rise and carries out comprehensive performance test. The main contents and key conclusions of this thesis include:(1) To overcome the weakness of the Reynolds equation-based thermo-hydrodynamic lubrication model of liquid suspension bearing and relevant computational method, a high-speed liquid suspension bearing three-dimensional thermo-hydrodynamic lubrication model and high-efficiency numerical computational method based upon CFD and finite volume method, has been established, and the comparative study on the application of this method and Reynolds Assumptions based one-dimensional and two-dimensional lubrication theory in the classic hydrodynamic lubrication problems has been conducted as well. According to the results, the method, with high accuracy, can be utilized as basic tool in the theoretical research of liquid suspension bearing thermo-hydrodynamic lubrication. Three-dimensional oil film CFD computational grids in accordance with the geometric scale feature of hole inlet hybrid pressure radial bearing, annular recessed hydrostatic thrust bearing and relevant surface microstructure spindles have been established, making advantages of unstructured grid, block division method, and grid size function; selection type and computational parameter optimizing method to liquid suspension bearing thermo-hydrodynamic lubrication boundary factors have been thoroughly discussed; the problem of easy distortion of three-dimensional oil film griding and poor coordination has been solved; and the comprehensive optimization of computational efficiency and accuracy has been ensured as well.(2) The influences of working parameters and structure parameters on the flow field and essential thermohydrodynamic characteristics of high speed hybrid journal bearing have been studied. The inlet diameter and the circumferential and radial distribution number have notable impact to matching ratio of dynamic and static pressure effect of high speed hybrid bearings have been pointed out that a bigger diameter can help reduce film temperature rise and even temperature fields, when the inlet circumferential distribution number is above10, its increase has limited influence on temperature control. When considering the load stiffness, film temperature rise, and power consumption, supply pressure of5-6MPa will be appropriate at30000r/min. Choosing a larger oil-sealing factor and a smaller inlet diameter can improve the bearing comprehensive performance, and a smaller L/D ratio can ensure low temperature rise when the load stiffness is met. It is possible to increase the bearing load capacity and stiffness under the same level of film temperature rise by increasing simultaneously L/D ratio, oil-sealing factor, and number of inlet rows. On this basis, the engineering calculation formulas based on Reynolds model for multihole-entry hybrid journal bearings have been derived, which indicates that, with small inlet parameters, this analytical computing can replace three-dimensional CFD computing as the engineering optimization design method.(3) The influences of working parameters and recess structure parameters on the flow field and essential thermo-hydrodynamic characteristics of high speed hydrostatic thrust bearing have been studied. The notable hydrostatic effect of high speed oil can complicate the flow field of oil film have been pointed out that the assumption of uniformed oil film pressure in recess no longer exists, and that the influence of static pressure effect has opposite impact on pressure fields of both inner and external sealing land. Meanwhile, the matching rule of bearing working parameters and structure parameters have been provided to ensure the characteristics of high load capacity and low temperature rise is put forward. When the spindle is at the speed of30000r/min and the supply pressure exceeds4MPa, the increasing of the supply pressure has limited effect on temperature rise control. Choosing a moderate inlet diameter and a shallower recess can ensure the characteristics of low power consumption, low temperature rise and high stiffness; the analytical theoretical model of high speed hydrostatic thrust bearing considering fluid inertia effect has been established, testing the feasibility of its replacement of three-dimensional CFD computing, and the optimal formula of the recess depth of hydrostatic thrust bearing based on the mathematical model of oil film pressure with centrifugal inertia only considered and the power consumption model based on the assumption of lumped parameter have been obtained under the restriction of the lowest oil film temperature rise, indicating the influence of recess depth on the bearing performance can’t be neglected on the condition of high speed, and the recess depth of hydrostatic thrust bearing is inadvisable to exceed0.35mm.(4) The analytical theoretical model of hydrodynamic lubricated sufficiently for periodic microstructure unit has been established, and the steplike hydrodynamic effect and fluid inertia effect are the principle mechanics for the hydrodynamic lubrication building is discovered; the analytical theoretical model of the parallel sliding bearing with partial microstructures has been established, and the influences of geometrical parameter and distribution parameter of partial microstructures on the load capacity and friction performance have been exposed; choosing the combination of smaller microstructure depth ratio and larger length ratio or the combination of smaller microstructure depth ratio and moderate length ratio the based on the optimal partial microstructures distribution length ratio are benefit for obtaining high load capacity and low friction coefficient. At last, the numerical thermo-hydrodynamic lubrication model of the periodical microstructure unit has been established based on CFD method; the influences of geometrical parameter and distribution parameter of partial microstructures on the load capacity and friction performance have been exposed; and the conclusion is reached that the speed and pressure inlet boundary condition, microstructure depth and diameter have major impact on the steplike hydrodynamic effect and hydrostatic effect.(5) The geometrical parameters and distributional parameters of the microstructure on the thermo-hydrodynamic performance of high speed liquid suspension bearings have been exposed, with the conclusion that the similar step hydrodynamic effect of the microstructure compensate the decline of hydrostatic effect induced by microstructure, as the final flow field is determined by the above two types of effects; the patterns of flow fields are determined by the matching of the microstructure depth and bearing macrostructure parameters, and the optimal microstructure depth exists for optimal bearing comprehensive performance; microstructure hydrodynamic effect can compensate the decreasing of the load capacity and stiffness resulted from the fluid inertia; overlarge area ratio leads to the remarkable decrease of load capacity and stiffness and the decline of suppression of temperature rise; whereas under small area ratio leads to high level of load capacity stiffness and oil film temperature rise, and the optimal microstructure area ratio exists for optimal bearing temperature rise; on condition that the load capacity and stiffness are satisfied, proper microstructure depth and diameter can notably reduce the temperature rise notably, and the bearing performance can be promoted by further optimizing the microstructure geometrical parameters in addition to the optimizing of the bearing macrostructure parameters; for a given microstructure geometrical parameters and bearing working parameters, the high stiffness and low temperature rise characteristics can be ensured by choosing smaller bearing clearance and larger circumferential distribution number; the radial distribution number seldom affects the flow rate and pump power consumption, and the larger radial distribution number should be chosen as large as possible.(6) The influences of working parameters on the rotation accuracy and static and dynamic stiffness of the liquid suspension spindle has been carried out via the established small diameter high-speed liquid suspension spindle test rig and vibration and dynamic signal collection and analyzing system. According to the results, the experimental principle and scheme for the comprehensive performance of small diameter high-speed liquid suspension spindle are correct and efficient; the technical specification of the manufacturing and assembling processing of key components and parts in bearing-motor-sealing-cooling system is scientific and rational; the theoretical design and analysis of the loading stiffness and critical structure parameters of small diameter high-speed liquid suspension spindle bearing system is highly efficient and reliable; and this experiment provides thus experimental instructions for the theoretical analysis, engineering design and comprehensive performance evaluation for the small diameter high-speed liquid suspension spindle system.The thesis provides theoretical analysis method and engineering design instruction for the thermohydrodynamic performance of small diameter high-speed liquid suspension motorized spindle bearing system, explores a new way of using surface microstructure effect to achieve the small diameter high-speed liquid suspension motorized spindle with high stiffness and low temperature rise, unfolds the new picture of tribological pair interface science for classical liquid sliding bearings, and is expected to promote the small diameter high-speed liquid suspension motorized spindle technology, thereby providing strong technical support for the development of concave grinding equipments.