Ceramic bearing ball

Abstract: A ceramic bearing ball capable of preventing irregular vibrations of, for example, a hard disk drive and having a high accuracy is characterized by a sphericity of not more than 0.08 µm, a surface roughness of not more than 0.012 µm and a maximum surface pore size of 5 µm.

Description: This invention relates to bearing balls of the type which find use in, for example, computer hard disk drives, semiconductor wafer transfer devices and other devices which require low vibration operation. Bearing devices which make use of balls or rollers arranged between outer and inner races are widely used for the smooth running of rotary components in equipment of the most diverse sizes from heavy industrial equipment to electronic devices. Traditionally, such bearing balls and rollers have been manufactured from steel but, in recent years, ceramic materials, such as, for example, silicon nitride have been used also. US 5,642,947 describes a method to prolong the life of a steel ball bearing by improving its ability to form oil films. Steel and super-steel have heretofore been the material of choice for bearing ball in, for example, hard disk drives for computers. However, with the improvements in computer performance, higher rotational frequencies have been demanded, and, accordingly, it has become necessary to make use of ceramic materials (ball manufacturing material) for the manufacture of bearing balls. One type of hard disk drive of the nature incorporating ceramic bearing balls according to the preamble of claim 1 is described and illustrated in U.S. Patent No. 5,485,331 to Dunfield et al., issued January 16, 1996, however there is no disclosure in this document of ceramic balls having all of the features of claim 1 hereinbelow. Since the density of ceramic materials is generally lower than that of steel, the weight and hence the frictional resistance of ceramic bearing balls is correspondingly lower. Also, since the thermal expansion coefficient of ceramic materials is lower than that of steel, the dimensional accuracy can be improved. A ceramic material also has the advantage of requiring less lubricant. However, merely selecting a ceramic material for a bearing ball is not necessarily sufficient in itself. For example, in a computer hard disk drive, irregular bearing vibrations during the storage (writing) and reading of data constitutes a potentially fatal performance drawback. Balls for such a bearing therefore demand a greater performance accuracy than that of the bearing balls used in a machine tool, for example. The present invention seeks to provide a ceramic bearing ball which is capable of preventing irregular vibration of, for example, a hard disk drive incorporating it. There is provided in accordance with the invention a ceramic bearing ball comprising a sphericity of not more than 0.08 µm, and a surface roughness of not more than 0.012 µm, characterized by a maximum surface pore size of 5 µm. A ceramic bearing ball according to the present invention having the above characteristics has an excellent surface condition and high dimensional accuracy. Accordingly, when such a ceramic bearing ball is used in a bearing in, for example, a computer hard disk drive, irregular bearing vibration rarely occurs, so that the writing and reading of data can be achieved accurately and reliably. The invention is described below in greater detail by way of example only with reference to the accompanying drawings, in which Fig. 1 is an explanatory view showing maximum pore sizes; and Fig. 2 is an explanatory view showing laboratory equipment for determining vibration levels. The sphericity, surface roughness and maximum pore size can be characterized as follows. 1. Sphericity: The specifications concerning steel balls for ball bearings according to JIS B1501 are used for the ceramic bearing balls according to the present invention. Sphericity is therefore determined by measuring two or three equatorial dimensions, at mutual right angles, using an instrument which measures maximum radial distances between the respective circumscribed circles and the spherical surface. 2. Surface roughness: The specifications concerning steel balls for ball bearings according to JIS B1501 are used also for the ceramic bearing balls according to the present invention. Surface roughness is therefore determined as the center line average roughness from measuring an equatorial surface of a ceramic bearing ball with a measuring instrument of the type stipulated in JIS B 0651 (tracer type surface roughness measuring instrument). 3. Maximum pore size: The maximum pore size is the largest pore diameter as shown in Fig. 1 of the drawings of pores exposed at the surface of the ceramic bearing ball, and the largest diameter of the largest pore when there are a plurality of such pores. The measurement of the maximum pore size can be made by using, for example, the image of the surface of the ceramic bearing ball taken from, for example, a metallurgical microscope or SEM. Preferably, the ceramic bearing ball of the present invention has a maximum internal pore size of not more than 5µm. It is considered that, when a ceramic bearing ball has pores, those pores exist not only on the surface thereof but also in an inner portion thereof. Accordingly, when the shape and size of the ceramic bearing ball are adjusted or regulated by polishing the ball, the pores in the interior portion thereof are necessarily exposed to the surface. Preferably, silicon nitride is used as the main component of the ceramic bearing ball, endowing the ball with advantageous properties such as hardness, weight and friction loss. Other ceramic materials, for example alumina, SiC, zirconia, composite materials of alumina zirconia and cermet can also be employed. An example of a ceramic bearing ball according to the present invention will now be described with reference to the drawings. The ceramic bearing ball in this embodiment has as its main component silicon nitride, and the ceramic ball comprises, for example, 94 wt.% silicon nitride, 3 wt.% aluminum oxide and 3 wt.% yttrium oxide. This ceramic bearing ball is a regular sphere of, for example, 2 mm diameter, which has a sphericity of not more than 0.08 µm, for example 0.02 µm, a surface roughness of not more than 0.012 µm, for example 0.001 µm, and a maximum surface pore size of not more than 5 µm, for example 2 µm. The raw material for the ceramic bearing ball is prepared as a slurry obtained by compounding 1-10 parts by weight of sintering aid powder (for example, a mixture of aluminum oxide and yttrium oxide) with 100 parts by weight of silicon nitride powder, and subjecting the resultant mixture to wet mixing (or wet mixing-pulverization) in a ball mill or an attritor using, for example, pure water as a solvent. This slurry is then supplied to a raw powder preparation apparatus of the type shown in, for example, Japanese Patent Application No. 290798/1998, to prepare raw powder. The raw powder thus prepared is molded (granulated) to a spherical shape by a known rolling/granulation apparatus. The spherical molded bodies obtained by the granulation process are sintered by regular gas pressure sintering or hot hydrostatic pressure sintering to obtain spherical sintered bodies of silicon nitride. In an alternative method, the slurry described above is formed into raw powder by spray drying, and the raw powder is molded into spherical bodies using a known metal press. The spherical molded bodies are subjected to regular gas pressure sintering or hot hydrostatic pressure sintering to form spherical sintered bodies of silicon nitride. The size of pores in the sintered bodies is regulated by controlling the type and the quantity of the sintering aid, sintering temperature and pressure and the average particle size of the raw material. For example, when the quantity of the sintering aid and the sintering temperature and pressure are increased with the average particle size of the raw material kept small, the densification of the raw material is greatly promoted during sintering, so that the pores are generally small. Accordingly, the sintering aid and sintering conditions are regulated so that a maximum pore size in the interior portions (which change into surface pores after the completion of polishing) of the sintered bodies is not more than 5 µm. The diameter, sphericity and surface roughness of the sintered bodies thus obtained are regulated by subjecting the sintered bodies to polishing, so that ceramic bearing balls of silicon nitride of not more than 4 mm diameter are formed. The ceramic bearing balls thus manufactured have a sphericity of not more than 0.08 µm, a surface roughness of not more than 0.012 µm and a maximum surface pore size of not more than 5 µm. When these ceramic bearing balls are used in a bearing in, for example, a hard disk drive of a computer, irregular vibrations rarely occurs, so that the writing and reading of data can be carried out accurately and reliably. Examples Details of experiments carried out so as to ascertain the advantages of the present invention will now be described. Balls of sintered bodies containing silicon nitride as a main component were manufactured by hot hydrostatic pressing. The sintering aid(s) and calcination conditions were varied so that the sizes of the internal pores of the balls varied within the range of the invention. These sintered balls were then polished so that they had the sphericity and surface roughness shown in Table 1 below, to obtain the ceramic bearing balls of Samples Nos. 1-10 falling within the scope of the present invention. Sintered balls were manufactured as comparative examples in the same manner by varying the sintering aid(s) and calcination conditions so that they had various pore diameters, and these balls were then polished so that they had the sphericity and surface roughness shown in Table 1 below, to form ceramic bearing balls of Samples Nos. 11-16 outside the scope of the present invention. The ceramic bearing balls were cut and maximum pore sizes in the cut surfaces thereof were measured. The results are also shown in Table 1. When the surfaces of the balls are polished, the pores in the inner portions thereof are exposed to the outside and the maximum sizes of internal pores and those of surface pores were regarded as being equal. These ceramic bearing balls 1 were arranged between outer and inner races 2, 3 as shown in Fig. 2, to form ball bearings, and the inner race 3 of each bearing was rotated with the outer race 2 fixed, to conduct acoustic (vibration) tests. The intensity of sound occurring when the inner race was rotated at 5000-10000 rpm was picked up at the outer race 2 by a sensor (microphone) 4 to examine whether the output level exceeded a predetermined permissible range (i.e. 30 dB), whereby it was determined whether each ceramic bearing ball 1 had suitable surface characteristics and accuracy. The results are also shown in Table 1. In Table 1, the samples having a sound intensity not greater than 30 dB are marked with O, (satisfactory) and the samples having a sound intensity greater than 30 dB with X (fail). Sphericity [µm] Surface roughness [µm] Maximum pore size [µm] Results of vibration (acoustic) experiments Examples of the present invention 1 0.08 0.008 2 ○ 2 0.06 0.006 3 ○ 3 0.04 0.005 2 ○ 4 0.04 0.004 2 ○ 5 0.03 0.005 5 ○ 6 0.03 0.002 3 ○ 7 0.02 0.001 2 ○ 8 0.08 0.012 5 ○ 9 0.03 0.002 1 ○ 10 0.03 0.002 0 ○ Comparative examples 11 0.25 0.025 10 X 12 0.13 0.020 3 X 13 0.25 0.025 2 X 14 0.10 0.005 3 X 15 0.08 0.014 5 X 16 0.08 0.008 7 X As is clear from Table 1, it has been ascertained that ball bearings using the ceramic bearing balls (Samples Nos. 1-10) within the scope of the present invention cause little vibration and are suitable to be used in, for example, a hard disk drive of a computer. They may be used also in a hard disk drive carriage arm, for example. On the other hand, the ball bearings using balls of comparative examples (Samples Nos. 11-16) do not satisfy the conditions for the present invention, so that they cause much vibration and are not suitable.

Claims: A ceramic bearing ball (1) comprising a sphericity of not more than 0.08 µm and a surface roughness of not more than 0.012 µm characterized by a maximum surface pore size of 5 µm. A ceramic bearing ball (1) according to Claim 1, having a maximum internal pore size of 5 µm. A ceramic bearing ball (1) according to Claim 1 or Claim 2, characterized by a sphericity of about 0.02 µm, a surface roughness of about 0.001 µm and maximum surface pore size of about 2 µm. A ceramic bearing ball (1) according to any one of Claims 1 to 3, comprising silicon nitride as the main component thereof. A disk drive motor bearing having inner (3) and outer (2) bearing races and, accommodated between the races a plurality of ceramic bearing balls (1) according to any one of claims 1 to 4.

Inventors: NIWA TOMONORI, NIWA, TOMONORI
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG COMPANY LIMITED
Application year: 2000
Main classification: F16C33
IPC codes: F16C33