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Performance analysis of ceramic ball bearings for high-speed machine tool spindles

by:JNSN     2021-10-22
The high speed of machine tool spindle is an important trend in the development of machine tool technology in recent years. Rolling bearings are important mechanical supports that can provide high-speed rotary motion, and are also important basic parts for military and civilian industries. Due to the continuous development of mechanical products in the direction of high precision, high speed and high automation, the working speed of various equipment continues to increase, so that the corresponding dn value (product of bearing diameter and speed) has reached 3 million, and trying to improve the structure of rolling bearings simply And the lubrication conditions to increase the dn value is far from being able to meet this requirement. One way to solve this problem is to consider the structural parameters to reduce the quality of the ball, such as small ball bearings (the diameter of the ball is smaller than that of ordinary bearings), while keeping other conditions basically unchanged, the speed can be increased by about 40% , This kind of bearing has been produced in our country; another way is to change the material of the bearing (such as ceramic ball bearing). Because the ceramic material is used as the ball, its density is only 40% of that of steel, and small diameter ceramics are used. After the ball is rolled, the quality of the ball can be further reduced, thereby further increasing the speed, which is especially suitable for ultra-high-speed machine tool spindles.

  Ceramic ball bearings come in many forms. Hybrid ceramic ball bearings are commonly used on machine tools. Its 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 elastic modulus is large (150%), etc., so it can play an important role in rolling bearings, and can greatly improve the fatigue life of the bearing.

   This article focuses on the analysis of several important performances of ceramic ball bearings used in high-speed machine tool spindles, and introduces the results of experiments with hot-pressed silicon nitride.

  1 Static load rating of ceramic ball bearings When designing and selecting a bearing, the most important basis is its static load rating and dynamic load rating. The former is the maximum static load that the bearing can withstand, and the latter involves the rolling fatigue life of the bearing. The static load rating of a rolling bearing defined in the international standard LSO76 refers to the static load when the total plastic deformation of the rolling element and the raceway at the point where the contact stress is the largest 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 appropriate for the steel bearing, but the plastic deformation of the Si3N4 ceramic material under the static load is so small that it is almost negligible compared to the steel. Therefore, it is obviously inappropriate to use the above method to define the static load rating of ceramic ball bearings. Therefore, in the SO-TC4 standard, the static load rating of a general ball bearing refers to the static load ceramic ball bearing when the calculated contact stress of the center of the contact between the rolling element and the raceway of the ring reaches 4200MPa. Steel bearings are still suitable.

   On the basis of the above two definitions, let's analyze and compare the static load capacity of steel bearings and ceramic ball bearings.

   is the test method for measuring the crushing load of a sphere specified in the Japanese Industrial Standard JISB1501. It is the comparison of the crushing load value of the Zhu 3 ceramic ball and the steel ball obtained by this method. It can be seen from this that when simply comparing the crushing load, the minimum crushing load of the S3N4 ball is about 1/21/3 of that of the steel ball. However, according to the above two standards, the static load limit of the ball used in the bearing is far lower than its crushing load value. The static load limit of the steel ball bearing is controlled by the amount of plastic deformation. When the maximum contact stress between the steel ball and the raceway of the ring exceeds 4200MPa, the bearing where the most loaded rolling element is in contact with the raceway of the ring The total plastic deformation will exceed one ten thousandth of the diameter of the sphere, so the maximum contact stress in the steel ball bearing cannot exceed this value. Since Si3N4 ceramics hardly undergo plastic deformation when loaded, the static load limit of ceramic ball bearings is controlled by the maximum contact stress between the ball and the raceway in the bearing. According to the crushing load value of the S3N4 ceramic ball, it can be calculated that the maximum contact stress at failure is about the maximum contact stress inside the ball bearing specified in the ISO-TC4 standard. Engineering ceramics are essentially brittle materials. When the load is greater than a certain 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. The current S3N4 ceramic ball bearings generally do not undergo brittle failure when operating under normal working loads, but have the same failure mode as steel bearings—fatigue spalling.

  The thrust load type ball bearing fatigue testing machine used by Koyo Seiko and Toshiba is shown in Table 1. Table 1 shows the test conditions. Under such test conditions, the experimental conditions in Table 1 produced between the Si3N4 test piece and the steel ball, the rotation speed is 1200r/min, the number of lubricating No. 60 spindle oil balls, the load is 4000N, the ball diameter is 9. Bearing steel has a long rolling fatigue life. Smaller, the better the contact state. Since the density of SigN ceramics is only for deep groove ball or angular contact ball bearings, its fatigue life is related to the maximum contact stress between the ball and the raceway of the ring.

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