Selection of Graphite Materials Used for High-speed Spindle Bearings

Abstract: The electric spindle of machine tool can obtain higher speed to realize high speed and high efficiency machining when using the low viscosity lubrication medium，and because of its advantages of small expansion coefficient，good heat conduction performance and short-time dry friction resistance，graphite bearing is especially suitable for using at the condition of low viscosity lubrication medium． In order to select a suitable material of graphite bearing to meet the need of high speed machining，through the analysis of the spindle working condition，the demand parameters for the material of graphite bearing were obtained． The graphite material performance testing system consisting of material mechanics experiment，thermal physics experiment and tribology experiment was designed． Four kinds of common graphite materials were selected，and their mechanical properties，thermal physical properties and friction and wear properties were tested and evaluated． The reasons for differences in performance were explained from the microstructure． The research results show that the graphite materials with moderate degree of graphitization and after impregnation process are more suitable for using as the material of high-speed spindle bearing.
Precision machining technology is an important development direction of modern mechanical engineering. It is not only the foundation and key of advanced manufacturing technology, but also an important symbol of a country’s manufacturing level. With the rapid development of national economy and national defense modernization, manufacturing industry requires not only precision but also high-speed and high-efficiency, which requires CNC machine tools to achieve high-speed cutting, cutting process is more than ten times higher than conventional cutting speed., in order to improve processing efficiency and precision. The realization of high-speed cutting depends on the performance of machine tool spindle. The bearing with low viscosity medium lubrication can be used in machine tool spindle system with low viscosity medium lubrication, and the spindle can keep dynamic stability in large load and speed range. At the same time, due to the graphite self-lubrication performance, when the spindle instantaneous overload or lubrication interruption occurs dry friction, the journal will not be abrasive. However, it is difficult to select suitable graphite material for bearing because there are many kinds of graphite materials and the properties of different types of graphite materials are very different. Therefore, it is a very important basic research work to select a suitable graphite material for bearing to meet the needs of high-speed machining. Graphite material selection involves many subjects such as materials science, thermophysics and tribology, which need to be considered comprehensively. Some researches have explored this topic. The strength, wear resistance and friction coefficient of the graphite bearing material of the shield pump were studied by Xu Hongyuan et al. The elastic modulus of the ordinary graphite and the wetting resin graphite material was measured, and the tensile and compressive strength limits of the material were determined. The wear rate of graphite with different graphitization degree in corrosive environment was studied by JIA. The basic tribological data obtained from the study provide a reference for the selection of sealing and bearing impregnated graphite materials and processes. The preparation and performance testing of graphite bearings for nuclear main pump were studied by Zhang Fan and others. The graphite materials and bearing preparation and testing datasets were established. The graphite materials suitable for the operation of the main pump motor were selected.
However, the above research is not deep enough, for the selection of bearing graphite material should be combined with bearing operating conditions – bearing structure – graphite material for comprehensive consideration. Based on the actual requirement of high speed machine tool, the author analyzes the operating condition of spindle, obtains the performance requirements of bearing graphite material, compares and evaluates the performance of the alternative graphite material by experiment according to the requirement parameters, and explains the reason of the performance difference from the microscopic point of view.
performance requirements of graphite materials for main bearings
Commonly used low viscosity lubricating media generally have air, liquid oxygen, kerosene and so on. Liquid oxygen and kerosene are commonly used in aerospace engines and turbopumps, and water and air are commonly used in motorized spindles, micro gas turbines, submarines, etc. The object of this paper is the graphite bearing of high-speed precision machine tool spindle lubricated by low viscosity medium. The main objects of such high-speed precision machine tools are optical lenses, plane mirrors, precision mechanical parts and so on.

The structure of static pressure spindle
The hydrodynamic and hydrostatic spindles use hydrodynamic bearings to support the rotor. The common hydrodynamic spindles contain two hydrodynamic journal bearings and two hydrodynamic thrust Journal bearings. Bearing materials of dynamic and static spindle are usually Babbitt, stainless steel, tin bronze and self-lubricating material.
The main shaft bearings studied are radial thrust combined bearings. Bearing with metal matrix inlaid graphite tile structure, see Fig. (a). Radial and thrust tile are shallow cavity with small hole structure, see Fig. (b). The inner diameter of the radial graphite tile is 48 mm, the width is 100 mm, and the thickness is 5 mm; the outer diameter of the thrust graphite tile is 100 mm, the inner diameter is 50 mm, and the thickness is 5 mm. In the grinding process, the spindle is mainly subjected to radial, tangential and axial forces in 3 directions. Generally speaking, the radial grinding force is the most important load on the spindle, which has the greatest influence on the operation of the spindle. Therefore, the radial graphite tile bears more force than the thrust graphite tile.
According to the working conditions, the graphite material of spindle bearing should meet the following performances: (1) it has better mechanical properties, the ability to resist the invasion of surface foreign bodies, the ability to withstand certain bending moments and the ability to resist deformation and failure under compressive stress; (2) it should have similar thermophysical properties to metal matrix. Linear expansion coefficient and large thermal conductivity; (3) good tribological properties, such as stable friction coefficient and small wear.
According to the analysis of the working condition of graphite bearing, the choice of graphite material needs three experiments, namely mechanical property experiment, thermophysical property experiment and tribological property experiment.
In order to select suitable graphite bearing materials, four typical graphite materials were selected for experiments, and they were numbered 1 #, 2 #, 3 #, 4 #. The selection of alternative materials is based on the degree of graphitization and whether there is any impregnation treatment. The degree of graphitization is a measure of the degree to which carbon material rearranges from amorphous carbon through its structure and its crystals approach perfection of graphite. Impregnation is a process to fill the surface pores of graphite materials by metal or non-metal.
The four typical graphite samples are all made of coke by firing. The graphitization degree of 1# sample is 0 without graphitization but impregnated by phenolic resin, and there is no hexagonal crystal of carbon, so it can be said that it is a pseudo-graphite; 2# sample is semi graphitization treatment but not impregnated at 1500 or so; 3# sample is both semi graphitization treatment and semi-impregnated. Graphitization is also treated by phenolic resin impregnation; 4 # sample is treated by full graphitization above 3000 ~C and impregnated by phenolic resin. 4 samples were made for 3 groups of samples. Each experiment was repeated 3 times, and the average value of 3 experiments was obtained. It should be pointed out that because of the anisotropy of carbon-graphite materials, the properties of carbon-graphite materials in the pressing direction are superior to those in other directions.
The results of mechanical properties, thermophysical properties and tribological tests of four alternative graphite materials are summarized.
(1) the mechanical properties of 1# graphite are good, but the thermal physical properties and tribological properties are poor.
(2) the mechanical properties, thermal physical properties and Tribological Properties of 2# graphite are poor.
(3) the mechanical properties, thermophysical properties and Tribological Properties of 3# graphite are the best among four materials.
(4) 4# graphite has poor mechanical properties, better thermal and physical properties and tribological properties.
Considering comprehensively, all properties of the 3# graphite material treated by semi-graphitization and impregnation are ideal, which meets the material selection requirements of the main shaft graphite bearing.
Conclusion
(1) For spindle bearings lubricated with low viscosity medium, the choice of graphite material needs to be considered comprehensively in many subjects, and the mechanical properties, thermophysical properties and Tribological Properties of the material are tested.
(2) The impregnation process can improve the graphite properties. The phenolic resin impregnated graphite is more suitable for high-speed spindle graphite bearings than the unimpregnated graphite.
(3) The properties of graphite materials are greatly affected by the degree of graphitization. The graphite treated by semi-graphitization is more suitable for high-speed spindle than the graphite treated by non-graphitization or full graphitization.
(4) The research results in this paper are also suitable to guide the selection of friction pairs for high-speed rotating machinery using graphite materials such as mechanical seals.
Author: WANG Jianlei, YAN Yudong, ZHANG Jian, CUI Yahui, JIA Qian