Rolling bearings

Abstract

By sufficiently reducing bearing vibration, the occurrence of cracks and damage can be prevented, thereby improving the durability of the rolling bearing. [Solution] The rolling bearing 10 comprises a pair of raceway rings (outer ring 1, inner ring 3) having a pair of opposing raceway surfaces (outer ring raceway surface 1a, inner ring raceway surface 3a), and a plurality of rolling elements 5 that are held to roll freely between the pair of raceway surfaces (outer ring raceway surface 1a, inner ring raceway surface 3a). The rolling elements 5 are made of alumina-zirconia ceramics. When the wobble component of the rolling elements 5 is separated into low band, medium band, and high band bands, the webbing values ​​are LB(npc), MB(npc), and HB(npc), respectively, the webbing value LB is 100npc or less, the webbing value MB is 100npc or less, and the webbing value HB is 250npc or less.

Description

Technical Field

[0001] The present invention relates to a rolling bearing that is mounted on a rotating shaft such as a motor or a generator, and is particularly suitable for mounting on a part where current may flow.

Background Art

[0002] Conventionally, in equipment having a rotating body, a metal bearing is mounted on the rotating shaft in order to reduce the frictional resistance generated by the rotation. For example, in a motor that rotates a rotating body (so-called rotor) by electromagnetic force or a generator that generates electricity by rotating a rotating body (so-called turbine) by hydraulic power or the like, a bearing is mounted on the rotating shaft of the rotating body.

[0003] In these motors, generators, etc., there is a problem that current leaks into the bearing and corrosion (electro-erosion) caused by the flow of current tends to progress. Therefore, bearings using rolling elements made of ceramics are used not only for the purpose of improving electrolytic corrosion resistance and corrosion resistance, but also for high-speed rotating devices and for the purpose of improving wear resistance and sound resistance. As a ceramic material used for bearings, silicon nitride, which has excellent rolling fatigue characteristics, can be mentioned. In bearing members made of a rigid member or silicon nitride, damage and cracks do not progress. However, when alumina is used as the material of the rolling element made of ceramics, since alumina is a material with low toughness, damage and cracks progress.

[0004] Therefore, in recent years, bearings using a ceramic material in which alumina (Al2O3) is compounded with zirconia (ZrO2) to improve strength have also been proposed. For example, Patent Document 1 proposes a rolling bearing in which at least the rolling surfaces of the inner ring, the outer ring, and the rolling elements are formed of a composite ceramic material, and the composite ceramic includes a main component composed of alumina and zirconia. Patent Document 1 describes that by specifying the contents of alumina and zirconia in the main component, a composite ceramic material having extremely excellent corrosion resistance, excellent surface workability and mechanical strength, and little strength reduction due to the crystal transformation of zirconia in water can be obtained.

[0005] Furthermore, Patent Document 2 discloses a rolling bearing in which at least one of the outer ring, inner ring, and rolling elements is made of a ceramic material mainly composed of Al2O3 and ZrO2. Patent Document 2 states that durability and mechanical strength can be improved by specifying the size of the aggregated ZrO2 particles in the ceramic material. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 11-153142 [Patent Document 2] Japanese Patent Publication No. 2000-319064 [Overview of the project] [Problems that the invention aims to solve]

[0007] In the rolling bearing described in Patent Document 1, the maximum surface roughness Rmax and average surface roughness Ra of the rolling surface are specified. However, simply specifying the roughness increases the vibration of the rolling bearing and does not adequately suppress the occurrence of damage. Furthermore, the lubrication state may deteriorate, causing wear on the rolling elements and inner and outer rings, thus reducing durability. Moreover, as described in Patent Document 2, even when the aggregate particle size of the ceramic material ZrO2 is strictly controlled, it is not possible to adequately suppress cracks and damage to the bearing, making it difficult to obtain the desired durability.

[0008] This invention has been made in view of the above problems, and aims to provide a rolling bearing that can prevent cracks and damage caused by vibrations of the bearing, thereby improving durability. [Means for solving the problem]

[0009] The rolling bearing according to the present invention has the configuration shown in [1] below.

[0010] [1] A rolling bearing comprising a pair of raceway rings having a pair of opposing raceway surfaces, and a plurality of rolling elements held to roll freely between the pair of raceway surfaces, The rolling element is made of alumina-zirconia ceramics. When the wobble component of the rolling element is separated into low-band, medium-band, and high-band frequencies, the resulting webiness values ​​are denoted as LB(npc), MB(npc), and HB(npc), respectively. A rolling bearing characterized in that the webiness value LB is 100 npc or less, the webiness value MB is 100 npc or less, and the webiness value HB is 250 npc or less.

[0011] A preferred embodiment of the rolling bearing according to the present invention is configured as shown in [2] to [4] below.

[0012] [2] The rolling bearing according to [1], characterized in that the arithmetic mean roughness Ra of the rolling elements is 0.01 μm or less.

[0013] [3] The rolling bearing according to [1] or [2], characterized in that the rolling elements contain 70% to 95% by volume of alumina and a total of 5% to 30% by volume of zirconia and its stabilizer, wherein the content of the stabilizer is 2.0 mol% to 5.0 mol% relative to the content of the zirconia.

[0014] [4] The rolling bearing according to [3], characterized in that the stabilizer contains yttria. [Effects of the Invention]

[0015] According to the present invention, by sufficiently reducing bearing vibration, it is possible to prevent the occurrence of cracks and damage, thereby providing a rolling bearing that can improve durability. [Brief explanation of the drawing]

[0016] [Figure 1] Figure 1 is a cross-sectional view showing an example of a rolling bearing according to an embodiment of the present invention. [Figure 2] Figure 2 is a micrograph showing the rolling element surfaces of the invention example and the comparative example. **Embodiments for Carrying out the Invention**

[0017] As a result of intensive research by the inventor of the present application, it has been found that by controlling the waviness value for each undulation component of the rolling elements, the vibration of the rolling bearing can be sufficiently reduced and the occurrence of cracks and damage can be prevented.

[0018] Hereinafter, embodiments of the present invention will be specifically described. Note that the present invention is not limited to the embodiments described below.

[0019] **(Rolling Bearing)** Figure 1 is a cross-sectional view showing an example of a rolling bearing according to an embodiment of the present invention. As shown in Figure 1, the rolling bearing 10 has a steel outer ring 1 having an outer ring raceway surface 1a on its inner peripheral surface, and a steel inner ring 3 having an inner ring raceway surface 3a on its outer peripheral surface. By combining this outer ring 1 and inner ring 3, a raceway ring having a pair of opposing raceway surfaces is formed. Further, a plurality of rolling elements 5 are arranged between the pair of raceway rings (between the outer ring raceway surface 1a and the inner ring raceway surface 3a). These respective rolling elements 5 are held in a state of being equally spaced in the circumferential direction and are held by a cage 7 so as to be freely rotatable. Furthermore, seal members 9 are attached to both axial sides of the outer ring 1 and the inner ring 3, and the lubricating oil enclosed between the outer ring 1 and the inner ring 3 is retained by these seal members 9.

[0020] **(Rolling Element)** Hereinafter, the rolling elements of the rolling bearing according to the present embodiment will be described in detail.

[0021] (Component: Alumina-Zirconia-based Ceramics) The rolling element 5 is made of alumina-zirconia ceramics. Unlike steel, ceramics are materials that hardly undergo plastic deformation. When steel rolling elements are used, any localized abnormal roughness (protrusions) disappear due to plastic deformation caused by "break-in" that occurs during the initial use of the bearing. However, in the case of ceramic rolling elements, since they do not undergo plastic deformation, there is a risk of minute damage or cracks occurring.

[0022] Incidentally, when alumina and zirconia are combined, the grain size of the alumina crystals is refined and crack propagation is bypassed, thereby preventing the detachment of alumina crystal grains and improving bending strength. Furthermore, in ceramic rolling elements made by combining alumina and zirconia, delamination resistance can also be improved. However, zirconia is a material with multiple crystal structures, and since the crystal structure differs depending on the temperature, and the volume also differs depending on the crystal structure, it undergoes volume expansion when cooled from the sintering temperature during manufacturing, and cannot maintain its shape. Therefore, when zirconia is used in ceramics, stabilized zirconia with added stabilizers is used to stabilize the tetragonal and cubic crystals even at room temperature, suppressing large volume changes.

[0023] However, even when using stabilized zirconia, temperature and stress can cause the zirconia to transform into its inherently stable monoclinic state at room temperature. When this phase transformation, or volume change, occurs in the zirconia grains, the surrounding crystal grains cannot absorb the expansion, leading to crack initiation and easy crack propagation, resulting in the detachment and delamination of alumina crystal grains.

[0024] Therefore, the inventors of this invention focused on webiness, which represents the undulation of the surface of the rolling elements, in order to suppress the generation of vibration in rolling bearings. The webiness values ​​defined in this embodiment will be described in detail below.

[0025] (LB: less than 100npc, MB: more than 100npc, HB: less than 250npc) In the AFBMA standard, bearing vibrations are evaluated by dividing them into three bands: low band (50Hz to 300Hz), medium band (300Hz to 1800Hz), and high band (1800Hz to 10000Hz). These vibrations can be separated into vibration components from the raceway surface and vibration components from the rolling elements.

[0026] In other words, the surface of the rolling element has a shape error from a geometrically perfect sphere, and this shape error consists of a periodic undulation component and random irregularities. In this embodiment, we focus on the webbing, which is the periodic undulation on the surface of the rolling element.

[0027] Furthermore, the three vibration bands in the bearing can be associated with the vibration bands of the rolling element's wobble component, and can be separated into low band (2-5 (lobes / c)), medium band (5-30 (lobes / c)), and high band (30-166 (lobes / c)) (Kenji Goto, Hiroshi Mizumoto, "Journal of the Japan Society for Precision Engineering", Vol. 63, No. 1, 1997, p. 96). For this reason, for each band of the rolling element's wobble component, the total height of the peaks per circumference of the rolling element is defined as the webiness value (unit: npc: nm per circumference), and the webiness value for each band is defined accordingly. Bearing vibration is generally affected by the condition of the raceway surface of the raceway ring, but in this embodiment, since the above webiness value is strictly defined, bearing vibration can be sufficiently reduced regardless of the condition of the raceway surface.

[0028] In the above classification of frequency bands, the webiness value at 5 (lobes / c) shall be included in both the low-band and medium-band webiness values. Similarly, the webiness value at 30 (lobes / c) shall be included in both the medium-band and high-band webiness values.

[0029] In this embodiment, it is important to specify that all values ​​of the rolling element's webiness values ​​LB, MB, and HB are below a predetermined value. If the rolling element's webiness value LB exceeds 100 npc, the webiness value MB exceeds 100 npc, or the webiness value HB exceeds 250 npc, the vibration of the rolling bearing cannot be suppressed, and excellent durability cannot be obtained. Therefore, the rolling element's webiness value LB should be 100 npc or less, MB 100 npc or less, and HB 250 npc or less. If any one of the webiness values ​​LB, MB, or HB falls outside the above range, vibration may occur, so it is important that all of them are satisfied. Furthermore, it is preferable that the rolling element's webiness value LB be 90 npc or less, MB 90 npc or less, and HB 230 npc or less, and it is more preferable that the rolling element's webiness value LB be 80 npc or less, MB 80 npc or less, and HB 200 npc or less.

[0030] On the other hand, smaller webiness values ​​LB, MB, and HB for the rolling elements indicate less waviness and improved durability; therefore, in this embodiment, there is no particular lower limit to the webiness values. However, in order to make the webiness values ​​LB, MB, and HB extremely small, special techniques and equipment for polishing the rolling elements are required. Therefore, within a feasible range, the webiness values ​​LB can be 5 npc or more, MB 10 npc or more, and HB 40 npc or more. Furthermore, in order to suppress increases in manufacturing costs, it is more preferable that the webiness values ​​LB 10 npc or more, MB 15 npc or more, and HB 70 npc or more, and even more preferable that LB 20 npc or more, MB 20 npc or more, and HB 100 npc or more.

[0031] As described above, in this embodiment, since the webiness value for each waviness component of the rolling element is defined, the rotational motion of the rolling bearing can be made smoother and the vertical force acting on the rolling element can be reduced. This suppresses localized heat generation and stress increase of the rolling element, and suppresses deterioration of the surface properties of the rolling element. As a result, it is possible to suppress the phase transformation of zirconia itself and the occurrence of delamination, which is a typical type of bearing damage.

[0032] The webiness value can be measured using a commercially available webiness meter; for example, the Ball Wavemeter BWM-102 (manufactured by Sugawara Laboratory Co., Ltd.) can be used. The rotation speed during measurement is approximately 300 min⁻¹. -1 ~2000 min -1 You can set any rotation speed within the above range, and it is believed that the rotation speed will not affect the webbing value within that range.

[0033] Furthermore, one method for controlling the webiness values ​​LB, MB, and HB in each band to the above range is to thoroughly polish the surface of the rolling elements and the raceway surface of the raceway rings. Generally, the roughness of rolling elements can be improved more easily than that of the raceway surface, and polishing makes it easier to reduce the webiness values ​​LB, MB, and HB. Specific methods for reducing the webiness values ​​LB, MB, and HB include adjusting the type (hardness) of the grinding wheel, the size of the abrasive grains, the processing speed, and the processing pressure, and increasing the processing time.

[0034] (Arithmetic mean roughness Ra: 0.01 μm or less) Normally, an oil film is formed between the rolling elements of a rolling bearing and the raceway surface of the raceway ring by a lubricant, and the two do not come into direct contact. However, since the thickness of the oil film is usually several hundred nanometers, if the surface roughness of the rolling elements is large, contact can occur beyond the oil film, making the rolling elements more susceptible to damage. In this embodiment, the webiness value of the rolling elements is specified, but by controlling the surface roughness of the rolling elements, it is possible to suppress contact between the rolling elements and the raceway surface beyond the oil film. That is, if the arithmetic mean roughness Ra of the rolling elements is 0.01 μm or less, direct contact between the rolling elements and the raceway surface becomes less likely, and the occurrence of damage to the rolling elements can be further reduced. Therefore, it is preferable that the arithmetic mean roughness Ra of the rolling elements be 0.01 μm or less, more preferably 0.007 μm or less, and even more preferably 0.005 μm or less. The arithmetic mean roughness Ra of the rolling element can be measured, for example, by using a contact measurement method in accordance with JIS B 0601:2013, or by using a non-contact measurement method such as a laser microscope or optical interferometer in accordance with ISO 25178.

[0035] The following provides a more detailed explanation of the content of the components that make up the rolling elements.

[0036] (Alumina: 70% to 95% by volume, Stabilized zirconia: 5% to 30% by volume) In this embodiment, since rolling elements made of alumina-zirconia ceramics are used, it is particularly important to control the webiness value of the rolling elements in order to suppress localized heat generation and stress increase of the rolling elements and to suppress phase transformation of zirconia. Furthermore, in order to suppress the phase transformation of zirconia itself and to suppress the occurrence of delamination, which is a typical type of bearing damage, it is preferable to use stabilized zirconia as the ceramic component. Stabilized zirconia refers to zirconia that has been stabilized by adding a stabilizing agent such as yttria. The stabilized zirconia may be fully stabilized zirconia or partially stabilized zirconia. Examples of stabilizing agents include yttria (Y2O3), calcia (CaO), and magnesia (MgO), but it is preferable that the stabilizer contains yttria.

[0037] In this embodiment, the rolling elements contain alumina and stabilized zirconia, but their content is not particularly limited. However, increasing the alumina content can reduce the manufacturing cost of the rolling elements. Also, the higher the proportion of stabilized zirconia, the more likely the rolling elements are to degrade at low temperatures. Therefore, the alumina content is preferably 70% by volume or more, and more preferably 80% by volume or more. The stabilized zirconia content is preferably 30% by volume or less, and more preferably 20% by volume or less.

[0038] If the alumina content is 95% by volume or less, sufficient strength can be imparted to the rolling element. Therefore, the alumina content is preferably 95% by volume or less, and more preferably 90% by volume or less. Furthermore, the stabilized zirconia content is preferably 5% by volume or more, and more preferably 10% by volume or more. Note that the stabilized zirconia content refers to the total content of zirconia and its stabilizer.

[0039] As described above, when a stabilizer is added to zirconia and dissolved in a solid solution, the tetragonal crystal structure becomes metastable even at room temperature. This suppresses the transformation to a monoclinic crystal structure accompanied by volume expansion that occurs during the cooling process of sintering, thereby suppressing the occurrence of crack defects originating from the manufacturing process and improving strength. In this embodiment, the amount of stabilizer used to obtain stabilized zirconia is not particularly limited, but if the stabilizer content relative to the zirconia content is 2.0 mol% or more, the tetragonal crystal structure can be stabilized at room temperature, and the effect of improving strength can be sufficiently obtained. Therefore, the stabilizer content relative to the zirconia content is preferably 2.0 mol% or more, and more preferably 2.5 mol% or more.

[0040] Examples of stabilizers include yttria, calcia, and magnesia. Only one of these stabilizers may be used, or a combination of multiple stabilizers may be used. It is preferable that the stabilizer includes yttria. Specifically, the molar ratio of yttria to the total amount of stabilizer is preferably 80 mol% or more, more preferably 90 mol% or more, and even more preferably 95 mol% or more. It is particularly preferable to use only yttria as the stabilizer.

[0041] On the other hand, if the stabilizer content relative to the zirconia content is 5.0 mol% or less, the added stabilizer can be uniformly dissolved in the zirconia, preventing a decrease in strength due to the presence of undissolved stabilizer. Furthermore, the increase in cost due to an increase in the stabilizer content can be suppressed. Therefore, the stabilizer content relative to the zirconia content is preferably 5.0 mol% or less, more preferably 4.0 mol% or less, and even more preferably 3.5 mol% or less.

[0042] <Orbit ring> Next, the raceway rings of the rolling bearing according to this embodiment will be described.

[0043] (Materials for the raceway) The material of the raceway used in this embodiment is not particularly limited, but for example, in addition to steel raceways, resin raceways or ceramic raceways can be used. Examples of steel raceways include high-carbon chromium bearing steel (SUJ2) and martensitic stainless steel (SUS440C). Examples of resin raceways include fluororesin, polyether ether ketone (PEEK) resin, and polyacetal (POM) resin. [Examples]

[0044] The following describes examples and comparative examples of rolling bearings according to this embodiment.

[0045] [Manufacturing of test rolling elements and raceways] First, alumina powder, zirconia powder, and yttria powder were mixed, and the mixture was molded into a spherical shape to obtain a molded product. The volume ratio of alumina to zirconia was adjusted so that alumina was 85 vol%, and the total amount of zirconia and yttria as a stabilizer was 15 vol%, with the yttria content being between 2.5 mol% and 3.5 mol% relative to the zirconia content. Next, the molded product was degreased and sintered, and then treated by hot isostatic pressing (HIP) to produce a primary sphere. Subsequently, the primary sphere was ground and polished to produce a test rolling element with a predetermined spherical shape.

[0046] Furthermore, by heat-treating a ring-shaped base body and performing grinding and grooving on the base body, raceway rings (inner and outer rings) made of SUJ2 were fabricated.

[0047] [Measurement of surface properties of rolling elements and raceway wheels] The surface properties of the rolling elements and the raceway surfaces of the raceway rings were measured. For the surface properties of the rolling elements, the measurement was performed by referring to "Rolling bearings - Steel balls" as described in JIS B 1501:2009 and applying it to ceramic rolling elements. The raceway surfaces were measured in accordance with JIS B 0601. A brief explanation of each item and measurement method follows below.

[0048] <Surface properties of rolling elements> (Variable diameter) Diameter variation is the difference between the maximum and minimum diameters of a single rolling element. The rolling element was placed between a plane and a measurement point perpendicular to it, and its diameter was measured while randomly changing its direction. The difference between the maximum and minimum values ​​was then calculated.

[0049] (Sphericity) Using a V-block, rolling elements were placed between measuring probes perpendicular to it, and the direction of the rolling elements was randomly changed. The sphericity was defined as the maximum range of motion of the measuring probes when the diameter was measured, divided by 2.

[0050] (mutual difference) The rolling elements were placed between a plane and a measuring probe perpendicular to it, and their diameters were measured while they were rotated randomly. The average of the maximum and minimum values ​​of a single rolling element was then taken as its average diameter, and the difference between the average diameter of the largest rolling element and the average diameter of the smallest rolling element in the lot was calculated.

[0051] (Surface roughness (arithmetic mean roughness Ra)) The arithmetic mean roughness Ra of the rolling element was measured using a laser microscope, a non-contact measuring instrument.

[0052] (Webiness value) Using a vibiness meter (the same specification as the ball vibiness meter BWM-102 manufactured by Sugawara Laboratory Co., Ltd.), the vibiness value of the rolling element was measured. Using the vibiness meter, first, the rolling element was rotated at a constant rotational speed, and the undulation on the surface of the rolling element was detected with a velocity-type sensor. Then, the obtained output was amplified by an amplifier, segmented by the number of peaks of the unevenness per circumference of the rolling element, and the total value of the peak heights in each band was calculated. The rotational speed was specified between about 300 min -1 ~2000 min -1 AFBMA standard, the vibration of the bearing is evaluated by dividing it into three bands: low order (low band: 50 Hz to 300 Hz), medium order (medium band: 300 Hz to 1800 Hz), and high order (high band: 1800 Hz to 10000 Hz). In this example, these bearing vibration bands were separated into bands of low band (2 to 5 (lobes / c)), medium band (5 to 30 (lobes / c)), and high band (30 to 166 (lobes / c)) corresponding to the undulation components of the rolling elements that affect the bearing vibration, and the vibiness value was measured.

[0053] <Surface texture of raceway surface> (Maximum peak height Rp) Using a laser microscope, which is a non-contact measuring instrument, the maximum peak height Rp of the raceway surface defined in JIS B 0601:2013 was measured. The maximum peak height Rp represents the maximum value of the peak height Zp of the roughness curve in the evaluation length. The arithmetic mean roughness Ra of the raceway surface was measured. When the arithmetic mean roughness Ra was in the range of Ra ≦ 0.02 μm in accordance with JIS B 0633:2001, the reference length was set to 0.08 mm, and when the arithmetic mean roughness Ra was in the range of 0.02 μm < Ra ≦ 0.1 mm, the reference length was set to 0.25 mm. Other measurement conditions were as follows.

[0054] · Filter: Use a Gaussian filter · Cutoff value λc: Set it to a value equal to the reference length. That is, 0.08 mm or 0.25 mm. · Cutoff λs: 0.0025 mm · Evaluation length: 5 times of λc • Roughness curve: A curve to which λc and λs are applied. • Measurement direction: The direction in which the roughness increases was set as the measurement direction. Specifically, for the raceways (inner and outer rings) of thrust bearings, the radial direction was set as the measurement direction.

[0055] [Durability evaluation test] By combining test rolling elements and raceway rings, rolling bearings of model numbers 51305 and 51205 were manufactured.

[0056] <Calculation of computational lifetime Lcal> For each rolling bearing, the calculated lifespan Lcal (hours) was determined based on the following equation (1). Formula (1): Lcal=(Ca / P) 3 However, in equation (1) above, Ca(N) represents the basic dynamic load rating (capacity) of a rolling bearing subjected to an axial load, and P represents the bearing load (N). Specifically, for the rolling bearing with model number 51305, the Lcal at a rotational speed of 2000 rpm was approximately 90 hours, and the Lcal at a rotational speed of 1000 rpm was approximately 180 hours (at 1000 rpm). For the rolling bearing with model number 51205, the Lcal at a rotational speed of 2000 rpm was approximately 100 hours, and the Lcal at a rotational speed of 1000 rpm was approximately 200 hours.

[0057] <Testing methods for durability> Durability tests were conducted on each rolling bearing using a thrust rolling fatigue testing machine. During the durability tests, vibration monitoring was performed continuously, and if the vibration exceeded twice the initial vibration, the durability test was terminated at that point to allow observation of damage to the rolling elements. In addition, if the rotation time exceeded the calculated lifespan before the rotational vibration exceeded twice the initial vibration, the durability test was terminated at that point to allow observation of damage to the rolling elements. The conditions for the durability tests are shown in Table 1 below.

[0058] [Table 1]

[0059] <Observation of damage> Microscopic images were taken of any area of ​​the rolling element after the durability test described above, and the presence, number, and size of defects in the captured field of view were measured. The imaging area was approximately 716 μm × 538 μm.

[0060] <Evaluation Criteria> The photographs were observed, and if no defects were found, or if defects smaller than 5 μm were observed but no defects larger than 5 μm were found, the result was rated as A (Excellent). If at least one defect between 5 μm and 20 μm was present, but no defects larger than 20 μm were found, the result was rated as B (Good). Furthermore, if at least one defect between 20 μm and 50 μm was present, but no defects larger than 50 μm were found, the result was rated as C (No Problem). Finally, if at least one defect larger than 50 μm was present, the result was rated as D (Poor).

[0061] The types of rolling bearings used in the inventive examples and comparative examples, as well as the measurement results of the surface properties of the rolling elements and raceways, are shown in Table 2 below, and the results of the durability tests are shown in Table 3 below. In addition, micrographs of the rolling element surfaces of each inventive example and comparative example are shown in Figure 2.

[0062] [Table 2]

[0063] [Table 3]

[0064] In Invention Examples No. 1 to 4 and Comparative Examples No. 1 and 4, the vibration did not exceed twice the initial vibration even after Lcal time had elapsed, so the test was terminated at Lcal time. In Comparative Examples No. 2 and 3, the vibration exceeded twice the initial vibration before the test time reached Lcal × 0.5 hours, so the test was terminated at Lcal × 0.5 hours or less.

[0065] As shown in Tables 2 and 3 and Figure 2 above, Invention Examples No. 1 to 4 use rolling elements made of alumina-zirconia ceramics, and the webiness values ​​LB, MB, and HB of the rolling elements are all within the range defined in this invention. Therefore, regardless of the surface properties of the raceway, no defects larger than 50 μm were formed in any of them. Thus, Invention Examples No. 1 to 4 were confirmed to satisfy the required durability.

[0066] On the other hand, Comparative Examples No. 1 to 4 used rolling elements made of alumina-zirconia ceramics, but at least one of the webiness values ​​LB, MB, and HB of the rolling elements fell outside the range defined in the present invention. As a result, defects larger than 50 μm were formed in all of them, and the desired durability could not be obtained. In Figure 2, only Comparative Example No. 4 shows a micrograph of the surface of the rolling element, but similar defects were observed in the other Comparative Examples No. 1 to 3 as well.

[0067] For example, when comparing the mutual differences, diameter variations, sphericity, and surface roughness of the rolling elements between Invention Example No. 4 and Comparative Example No. 1, there was almost no difference in the values ​​between the two, and the maximum peak height Rp of the raceway rings was also the same. However, the webiness values ​​MB and HB of Invention Example No. 4 were smaller than those of Comparative Example No. 1 and within the range defined in the present invention, thus satisfying the required durability. From this, it was shown that defining the webiness values ​​of the rolling elements is extremely useful as a condition for obtaining a rolling bearing with good durability. [Explanation of symbols]

[0068] 1 Outer ring 1a Outer ring raceway surface 3. Inner ring 3a Inner ring raceway surface 5 Rolling element 7 Cage 9. Sealing member 10 Rolling bearings

Claims

1. A rolling bearing comprising a pair of raceway rings having a pair of opposing raceway surfaces, and a plurality of rolling elements held to roll freely between the pair of raceway surfaces, The rolling element is made of alumina-zirconia ceramics. When the wobble component of the rolling element is separated into low-band, medium-band, and high-band bandwidths, the resulting webiness values ​​are denoted as LB(npc), MB(npc), and HB(npc), respectively. The webiness value LB is 100 npc or less, the webiness value MB is 100 npc or less, and the webiness value HB is 250 npc or less. A rolling bearing characterized in that the rolling elements contain 70% to 95% by volume of alumina, and a total of 5% to 30% by volume of zirconia and its stabilizer, wherein the content of the stabilizer is 2.0 mol% to 5.0 mol% relative to the content of the zirconia.

2. The rolling bearing according to claim 1, characterized in that the arithmetic mean roughness Ra of the rolling elements is 0.01 μm or less.

3. The rolling bearing according to claim 1 or 2, characterized in that the stabilizer contains yttria.

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