The high speed of machine tool spindles is an important trend in the development of machine tool technology in recent years. Rolling bearings are important mechanical supports that provide high-speed rotary motion and are an important foundation for military and civilian industries. As mechanical products continue to develop in the direction of high precision, high speed and high automation, the working speed of various equipments continues to increase, so that the corresponding dn value (the product of bearing diameter and rotational speed) has reached 3 million, and attempts to improve the structure of the rolling bearing by simply And the lubrication conditions to increase the dn value are far from meeting this requirement. The way to solve this problem is to reduce the mass of the ball from the structural parameters, such as small ball bearings (the ball diameter is smaller than the general bearing), and the rotation speed can be increased by about 40% while keeping other conditions basically unchanged. This kind of bearing has already started production in China; another method is to change the material of the bearing (such as ceramic ball bearing). Because of the ceramic material, the 3 teams are used as balls, and its density is only 40% of the steel, and the ceramics with small diameter are used. After the balls, the quality of the balls can be further reduced, thereby further increasing the rotational speed, which is particularly suitable for ultra-high speed machine spindles.
Ceramic ball bearings come in many forms. Commonly used on machine tools are hybrid ceramic ball bearings. The inner and outer rings are still made of bearing steel, but the material used for the balls is engineering ceramics, referred to as ceramic ball bearings. Compared with traditional bearing steel, engineering ceramics have many excellent physical and mechanical properties: wear resistance, high temperature resistance, corrosion resistance, non-magnetic, low density (about 40%) and electrical insulation, low thermal expansion coefficient (25%), The large modulus of elasticity (150%), etc., can play an important role in rolling bearings and can greatly improve the fatigue life of bearings.
This paper focuses on the analysis of several important performances of ceramic ball bearings applied to the spindle of high-speed machine tools, and introduces the results of tests with hot-pressed silicon nitride.
1 The rated static load of ceramic ball bearings The most important basis for designing and selecting bearings is their rated static load and dynamic load rating. The former is the maximum static load that the bearing can withstand, while the latter relates to the rolling fatigue life of the bearing. The rated static load of the rolling bearing defined in the international standard LSO76 refers to the static load when the total plastic deformation of the contact between the rolling element and the raceway at the maximum contact stress in the bearing is one ten thousandth of the diameter of the rolling element. When the static load is too large, the plastic deformation in the steel bearing will affect its normal use, so the above definition is more suitable for the steel bearing, but the plastic deformation of the Si3N4 ceramic material under the static load is almost negligible compared with the steel. Therefore, it is obviously not suitable for the ceramic ball bearing to use the above method to define its rated static load. Therefore, it is stipulated in the SO-TC4 standard that the rated static load of a general ball bearing is suitable for a static load ceramic ball bearing when the calculated contact stress at the center of the contact between the rolling element and the ferrule raceway of the bearing is 4200 MPa, and Steel bearings still apply.
Based on the above two definitions, we analyze the ability of steel bearings and ceramic ball bearings to withstand static loads.
It is a test method for measuring the ball crushing load prescribed in Japanese Industrial Standard JISB1501, and is a comparison of the crushing load values â€‹â€‹of the 3 ceramic balls and the steel balls obtained by this method. It can be seen from the above that when the crushing load is simply compared, the minimum crushing load of the S3N4 ball is about 1/21/3 of the steel ball. However, according to the above two standards, the static load limit of the ball used in the bearing is much lower than the crushing load value. The static load limit of steel ball bearings is controlled by the amount of plastic deformation. When the maximum contact stress between the steel ball and the ferrule raceway exceeds 4200 MPa, the largest rolling element in the bearing is in contact with the ferrule raceway. The total plastic deformation will exceed one ten thousandth of the diameter of the sphere, so the maximum contact stress in the ball bearing cannot exceed this value. Since the Si3N4 ceramic is hardly plastically deformed when loaded, the static load limit of the ceramic ball bearing is controlled by the maximum contact stress between the ball and the raceway in the bearing. According to the crushing load value of S3N4 ceramic ball, the maximum contact stress at the time of failure can be calculated as the maximum contact stress inside the ball bearing specified in ISO-TC4 standard. The engineering ceramic is essentially a brittle material, when the load is greater than a certain load. A brittle fracture occurs at a limit value. However, due to the development of the material industry, the strength and toughness of ceramic materials have been greatly improved. At present, the S3N4 ceramic ball shaft generally does not undergo brittle failure when it is operated under normal working load, but has the same failure mode and fatigue peeling as the steel bearing.
The thrust load type ball bearing fatigue testing machine adopted by Japan Koyo Seiko and Toshiba is shown in Table 1, and the test conditions are shown in Table 1. Under such test conditions, the experimental conditional speed in Table 1 produced between the Si3N4 test piece and the steel ball is 1200r/min. Lubrication No. 60 spindle oil ball number 3 load 4000N ball diameter 9. Out, Si ceramic than high carbon chromium Bearing steel has a long rolling fatigue life. Small, so the better the contact state. Since the density of SigN ceramics is only for deep groove or angular contact ball bearings, the fatigue life is related to the maximum contact stress between the ball and the ferrule raceway. This contact stress comes from the normal working load on the one hand, and the S-heart force from the rolling elements on the other hand. When the bearing is running at high speed, the additional load of 10 caused by the centrifugal force of the f S0 rolling element is considerable. Since the density of Si3N4W ceramics is only about 40% of that of steel, the number of stress cycles / ä¸¨% under high speed conditions using ceramic ball bearings can greatly reduce the contact fatigue stress between the ball and the ferrule raceway, thereby prolonging the rolling fatigue life of silicon nitride. The service life of the bearing.
3 ceramic ball bearings in high-speed conditions of the rolling ratio angular contact ball bearings in high-speed operation, the ball and the inner and outer ring connection B.) outer ring raceway control assumes that the ball in the outer ring raceway contact occurs pure rolling And the inner ring raceway f contact has both rolling and spin sliding, and the ratio of the spin angular velocity to the rolling angular velocity is the rolling ratio. The rolling ratio can be used to evaluate the rolling contact state of the shaft 1 bearing. The smaller the value of /, X16mmr/min), the ceramic ball bearing of the bearing ball in the bearing and the steel ball shaft have 40% of the steel, under the same high speed condition. During operation, the centrifugal force of the same size ceramic ball is only 40% of the steel ball, so the difference between the contact angle of the ball and the inner and outer rings in the ceramic ball bearing is smaller than that in the steel ball bearing, so the rotation ratio is also Smaller than steel ball bearings (as shown).
By comparing the calculation results of the rolling ratio of ceramic ball and steel ball angular contact ball bearing, it can be concluded that the ball rolling ratio of the steel ball bearing is about 50% higher than that of the ceramic ball bearing. This shows that the ceramic ball bearing is in high speed operation. The friction and temperature rise are smaller than steel bearings, especially in the harsh lubrication conditions, the advantages of ceramic ball bearings are more prominent.
The corrosion resistance, insulation and non-magnetic Si3N4 ceramics of ceramic ball bearings are much better than steel, which makes ceramic ball bearings suitable for use in special environments such as seawater. However, the J corrosion resistance of SigN4 ceramics is also significantly different due to the difference in the quality and ratio of the raw material powder, the sintering additive, and the sintering conditions. |In the A, B, C curve table!
The weight reduction rate of three different SigN4 materials in 90 Â° C, g 30% hydrochloric acid is shown. Therefore, for ceramic ball bearings requiring corrosion resistance, attention must be paid to the corrosion-resistant manufacturing methods and additives of silicon nitride silicon nitride.
Another advantage of ceramic ball bearings is that they can be used where complete electrical insulation is required. The resistivity of steel ranges from 108 to 104.m, while that of ceramics is above 1C6. It can be seen that the insulating properties of ceramic materials are much better than steel.
In a strong magnetic environment, when a steel bearing is used, the fine powder that has been worn out from the bearing itself is adsorbed between the rolling element and the raceway surface, which is the main cause of early peeling damage and increased noise of the bearing, since the ceramic ball bearing is completely Non-magnetic and with normal load carrying capacity, it can be used in Applications where a completely non-magnetic bearing is required.
5 Conclusions Due to the use of Si3N4 ceramic materials, ceramic ball bearings can withstand greater static loads than steel bearings and have a longer rolling fatigue life. Moreover, since the ceramic ball bearing has a smaller rolling ratio than the steel bearing, the friction and temperature rise of the ceramic ball bearing at high speed are smaller than those of the steel bearing. From the perspective of the material itself, silicon nitride ceramics have excellent mechanical and physical properties such as corrosion resistance, insulation, and non-magnetic properties. Therefore, rolling bearings made of these non-metallic materials as rolling elements are also particularly suitable for use in certain steel systems. Work in special environments where the bearings cannot be adapted.
The spin/rolling ratio of spin rolling and frictional heat generation (down to page 46) With the improvement of the flexural strength and fracture toughness of ceramic materials, ceramic materials are increasingly improved by the above excellent properties. More attention has also made it play an important role in the rolling bearing material. The finite element flow line ara after the 5 meter is taken as an example of the working process of the cone valve. When the valve core is in the position of 0.2 mm up, the high pressure from the hydraulic pump The oil enters the P port of the valve, the input liquid pressure at the valve inlet P is 3.3 MPa, and the output hydraulic pressure at the outlet O is ft=1.5 MPa. The liquid particle adheres to the fixed wall surface, satisfies the no-slip condition, and is at rest. The state, there are: 4 results analysis software analysis of the above situation 3. Through the calculation and post-processing of the finite element program, the distribution law of liquid pressure and liquid flow rate in the valve cavity is obtained. The fluid flow function cloud diagram when the valve opening amount is 0.2 mm is an equivalent diagram of the overall fluid flow function with the valve opening amount being 0.
The distribution law of the liquid pressure along the surface of the cone valve is obtained by finite element calculation, and then the integral force of the X and y axes can be obtained by integrating the entire surface of the cone valve. For the axial force analysis of the cone valve, the integral diagram is shown. It can be seen that a conical surface is taken on the surface of the cone valve. Because of its symmetry, the force value in the x-axis is zero, and on the conical surface, the small annular area dS the y-axis is fluid force acting 1542N. from the above results of FEM data are reliable, the absolute error is 1, and the relative error of 0.065%. cONCLUSIONS whichever valve chamber 5 is closed during operation of the valve cone For the fluid, the mathematical model of the fluid continuum inside the cone valve was established by the finite element method. The distribution of the flow function of the liquid in the cone valve cavity was obtained. The important parameter curve of the fluid pressure and velocity distribution in the valve was further calculated.
The fluid pressure distribution law of the fluid to the cone surface of the poppet valve core was calculated by the finite element software. Then, the total force of the fluid to the y-direction of the poppet valve spool was obtained.
The above conclusions provide a theoretical basis for further research on the structure of hydraulic cone valves and the design of preload springs and return springs, and have a wide range of practical significance.
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