Angular contact ball bearing

Abstract

An angular ball bearing comprising: an inner ring having an inner ring track groove having a cross-sectionally arc shape in the outer peripheral surface; an outer ring having an outer ring track groove having a cross-sectionally arc shape in the inner peripheral surface; and a plurality of balls provided so as to be able to roll between the inner ring track groove and the outer ring track groove. A groove curvature radius ratio Ri of the inner ring track groove to the ball diameter is 54-58%, a groove curvature radius ratio Ro of the outer ring track groove to the ball diameter is 51-58%, and Ri-Ro≥0 points is satisfied. Further, at least the inner ring track groove has a maximum contact pressure of 4.7-6.0 GPa when the sum of permanent deformation amounts of the ball and the inner ring track groove at the center of a contact portion between the ball and the inner ring track groove is 1 / 10000 of the ball diameter. Accordingly, the present invention provides an angular ball bearing capable of reducing a heat generation amount and suppressing damage due to an external impact load under a static state.

Description

[Technical field]

[0001] The present invention relates to an angular contact ball bearing, and more particularly to a ball bearing used for various machine tool spindles, motors, etc. [Background technology]

[0002] In recent years, the speed of the spindles of machine tools has been increasing to improve processing efficiency and productivity, and the rotation speed of angular ball bearings used in machine tool spindles is also increasing. In general, when an angular ball bearing rotates at high speed, a large slip occurs at the contact point between the balls and the raceway due to spinning motion or gyroscopic motion, and the contact pressure between the balls and the raceway decreases due to the influence of centrifugal force acting on the inner ring and balls, resulting in high heat generation. When the amount of heat generated increases, the viscosity of the oil decreases, causing the oil film to break at the rolling contact between the balls and the raceway, causing problems such as bearing seizure and large thermal displacement of the spindle, which deteriorates processing accuracy.

[0003] Known prior art techniques for reducing the amount of heat generated in angular contact ball bearings include, for example, an outer ring groove curvature radius ratio of 50.5 to 53% and an inner ring groove curvature radius ratio of 52.5 to 60% (see Patent Document 1), and an outer and inner ring groove curvature radius ratios of 54 to 57% (see Patent Document 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2000 / 37813 [Patent Document 2] Japanese Patent Application Publication No. 2005-240881 Summary of the Invention [Problem to be solved by the invention]

[0005] In Patent Documents 1 and 2, the ratio of the groove curvature radii of the outer and inner rings is set large to reduce heat generation, but the contact pressure of the rolling elements and the raceway surface tends to be high. When the bearing is viewed from the axial side, the contact area between the rolling elements and the raceway surface of the inner ring and the contact area between the rolling elements and the raceway surface of the outer ring are contact between the outer periphery of the rolling elements arc and the outer periphery of the inner ring raceway surface arc, and between the outer periphery of the rolling elements arc and the inner periphery of the outer ring raceway surface arc, respectively, so the contact pressure of the raceway surface of the inner ring tends to be particularly high. For this reason, when an external impact load is applied to the angular contact ball bearing when stationary, indentations are likely to occur on the inner ring raceway surface. The external impact load here does not refer to the processing load applied during normal cutting, but to the collision load caused by the sudden interference of the spindle (including parts up to the blade) with the workpiece, fixtures, or parts that make up the machine tool in the processing chamber, the unclamping load when replacing the blade, the unexpected collision during the spindle assembly process, and the load caused by the vibration and impact that the bearing receives when transporting the bearing alone, which is a large load that is at least one order of magnitude larger than the processing load during operation. Therefore, if the bearing is operated with indentations on the raceway surface caused by the external impact load, vibrations will be generated, reducing the quality of the machined surface of the workpiece, and there is a concern that peeling will occur starting from the indentations on the raceway surface.

[0006] The present invention has been made to solve these problems, and its object is to provide an angular contact ball bearing that can reduce the amount of heat generated and suppress damage caused by external impact loads when stationary. [Means for solving the problem]

[0007] In order to solve the above problems, the present invention provides an angular contact ball bearing as described below. (1) an inner ring having an inner ring raceway groove having an arc-shaped cross section on an outer peripheral surface; an outer ring having an outer ring raceway groove having an arc-shaped cross section on its inner circumferential surface; a plurality of balls rollably disposed between the inner ring raceway groove and the outer ring raceway groove, A ratio (Ri) of the groove radius of curvature of the inner ring raceway groove to the ball diameter is 54 to 58%, a ratio (Ro) of the groove radius of curvature of the outer ring raceway groove to the ball diameter is 51 to 58%, and Ri-Ro≧0 points are satisfied, At least the inner ring raceway groove has a maximum surface pressure of 4.7 to 6.0 GPa when the sum of the permanent deformation of the ball and the inner ring raceway groove at the center of contact between the ball and the inner ring raceway groove is 1 / 10,000 of the ball diameter. (2) The angular contact ball bearing according to (1), wherein Ri-Ro is equal to or greater than 1 point. (3) The angular contact ball bearing according to (1) or (2), wherein at least the inner ring raceway groove is formed with a surface hardened layer by machining. (4) The angular contact ball bearing according to (1) or (2), wherein, of the inner ring raceway groove and the outer ring raceway groove, only the inner ring raceway groove has a surface hardened layer formed by machining. (5) The angular contact ball bearing according to any one of (1) to (4) above, wherein the material of the balls is ceramic. (6) An angular contact ball bearing according to any one of (1) to (5) above, wherein the ball diameter / section height ratio is 0.39 to 0.65. (7) The angular contact ball bearing according to (6) above, wherein the ball diameter / section height ratio is 0.55 to 0.65. (8) The angular contact ball bearing according to any one of (1) to (7) above, which is used in a machine tool spindle having a dmn of 800,000 or more and is preloaded. (9) At least one of the inner ring and the outer ring is made of steel containing 0.2 to 1.2 mass% C, 0.7 to 1.5 mass% Si, 0.5 to 1.5 mass% Mo, 0.5 to 2.0 mass% Cr, the balance being Fe and unavoidable impurity elements, and The surface carbon concentration is 0.8 to 1.3 mass% and the surface nitrogen concentration is 0.2 to 0.8 mass%. The angular contact ball bearing according to any one of (1) to (8) above. Effect of the Invention

[0008] The angular contact ball bearing of the present invention can reduce heat generation and suppress damage caused by external impact loads when stationary. In particular, the angular contact ball bearing of the present invention is useful as an angular ball bearing for a machine tool main shaft used at high speeds of dmn 800,000 or more. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a partially enlarged cross-sectional view of an angular contact ball bearing, which is an example of the ball bearing of the present invention. [Diagram 2] FIG. 2 is a schematic diagram for explaining spin slip. [Diagram 3] FIG. 3 is a schematic diagram for explaining spin slip, showing an enlarged view of the inner ring raceway groove of the inner ring. [Figure 4] FIG. 4 is a schematic diagram for explaining spin slip, where (a) is a diagram showing the direction in which centrifugal force acts, (b) is a diagram showing the amount of spin when the inner ring groove curvature radius ratio is large, and (c) is a diagram showing the amount of spin when the inner ring groove curvature radius ratio is small. [Diagram 5] FIG. 5 is a graph showing the relationship between the inner ring groove curvature radius ratio (Ri) and the total amount of spin heat generated on the inner ring side and outer ring side under analysis condition 1, calculated. [Figure 6] FIG. 6 is a graph showing the relationship between the outer ring groove curvature radius ratio (Ro) and the total amount of spin heat generated on the inner ring side and the outer ring side under analysis condition 1, calculated. [Figure 7] FIG. 7 is a graph showing the relationship between Ri-Ro and the ratio of the inner ring surface pressure to the outer ring surface pressure calculated under analysis condition 1. [Figure 8] FIG. 8 is a graph showing the relationship between the inner ring groove curvature radius ratio (Ri) and the total amount of spin heat generated on the inner ring side and outer ring side under analysis condition 2, calculated. [Figure 9] FIG. 9 is a graph showing the relationship between the outer ring groove curvature radius ratio (Ro) and the total amount of spin heat generated on the inner ring side and the outer ring side under analysis condition 2, calculated. [Figure 10]FIG. 10 is a graph showing the relationship between the inner ring groove curvature radius ratio (Ri) and the total amount of spin heat generated on the inner ring side and outer ring side under analysis condition 3, calculated. [Figure 11] FIG. 11 is a graph showing the relationship between the outer ring groove curvature radius ratio (Ro) and the total amount of spin heat generated on the inner ring side and the outer ring side under analysis condition 3, calculated. [Figure 12] FIG. 12 is a graph showing the relationship between the inner ring groove curvature radius ratio (Ri) and the total amount of spin heat generated on the inner ring side and outer ring side under analysis condition 4, calculated. [Figure 13] FIG. 13 is a graph showing the relationship between the outer ring groove curvature radius ratio (Ro) and the total amount of spin heat generated on the inner ring side and the outer ring side under analysis condition 4, calculated. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, an angular contact ball bearing according to an embodiment of the present invention will be described in detail with reference to the drawings. In this specification, the use of "to" indicating a numerical range means that the numerical values ​​before and after it are included as the lower limit and upper limit.

[0011] FIG. 1 shows an angular ball bearing used in a spindle of a machine tool as an example of the angular ball bearing of the present invention. The angular ball bearing 1 includes an inner ring 2 having an inner ring raceway groove 2a with an arc-shaped cross section on its outer peripheral surface, an outer ring 3 having an outer ring raceway groove 3a with an arc-shaped cross section on its inner peripheral surface, a plurality of balls 4 provided to be rollable between the inner ring raceway groove 2a and the outer ring raceway groove 3a, and a cage 5 for holding the plurality of balls 4. A counterbore 3b is formed on one axial side of the inner peripheral surface of the outer ring 3, and the balls 4 are arranged between the inner ring raceway groove 2a and the outer ring raceway groove 3a with contact angles αi and αo. The contact angles αi and αo are defined as the angles formed by a plane P perpendicular to the bearing center axis X and a line of action connecting the contact points of the balls 4 with the inner ring 2 and the outer ring 3, respectively, and the center of the balls 4.

[0012] The balls 4 used have a ball diameter / cross-sectional height ratio, ie, diameter of ball 4 / {(outer diameter of outer ring 3-inner diameter of inner ring 2) / 2} of 0.39 to 0.65, preferably 0.55 to 0.65.

[0013] Moreover, the inner ring raceway groove 2a and the outer ring raceway groove 3a have a groove curvature radius ratio of the inner ring raceway groove 2a to the ball diameter (Ri = inner ring raceway groove radius of curvature / ball diameter) of 54 to 58%, and a groove curvature radius ratio of the outer ring raceway groove 3a to the ball diameter (Ro = outer ring raceway groove radius of curvature / ball diameter) of 51 to 58%, and are set to Ri-Ro ≧ 0 point, preferably Ri-Ro ≧ 1 point. Hereinafter, the groove curvature radius ratio Ri of the inner ring raceway groove 2a to the ball diameter will also be referred to as the inner ring groove curvature radius ratio Ri, and the groove curvature radius ratio Ro of the outer ring raceway groove 3a to the ball diameter will also be referred to as the outer ring groove curvature radius ratio Ro.

[0014] Furthermore, in this embodiment, the inner ring raceway groove 2a is formed with a surface-hardened layer 10 by roller burnishing, which is a machining process, and the inner ring raceway groove 2a has a maximum surface pressure of 4.7 to 6.0 GPa when the sum of the permanent deformation amounts of the balls 4 and the inner ring raceway groove 2a at the center of the contact area between the balls 4 and the inner ring raceway groove 2a becomes 1 / 10,000 of the ball diameter. On the other hand, the outer ring raceway groove 3a is not subjected to roller burnishing, and no surface-hardened layer is formed thereon.

[0015] The roller burnishing process is carried out after heat treatment and finish processing of the inner ring raceway groove 2a of the inner ring 2 formed by cutting. Furthermore, precision processing may be carried out after the roller burnishing process, if necessary.

[0016] Below, we will explain the groove curvature radius ratios Ri, Ro of the above-mentioned raceway grooves 2a, 3a, the maximum surface pressure when the sum of the permanent deformation amounts of the balls 4 and the inner ring raceway groove 2a in the inner ring raceway groove 2a becomes 1 / 10,000 of the ball diameter, and the critical significance of the ball diameter.

[0017] [The ratio of the inner raceway groove curvature radius to the ball diameter (Ri) is 54 to 58%, and the ratio of the outer raceway groove curvature radius to the ball diameter (Ro) is 51 to 58%] First, in an angular contact ball bearing 1 for high-speed rotation applications used in machine tool spindles, as shown in Figure 2, if we assume that balls 4 simply roll in the outer ring raceway groove 3a of the outer ring 3, at the contact area (contact ellipse) between the inner ring raceway groove 2a of the inner ring 2 and the surface of the ball 4, the relative circumferential speed (symbol C in the same figure) between the circumferential speed on the surface of ball 4 due to rotation (shown by symbol A in the same figure, proportional to the vertical distance from the rotation axis AX of ball 4 to the outer peripheral surface arc of ball 4) and the circumferential speed on the inner ring raceway groove 2a of the inner ring 2 due to revolution (shown by symbol B in the same figure, proportional to the vertical distance from the rotation axis of the inner ring 2 to the inner ring raceway groove 2a) appears as spin slip. As shown by D1 and D2 in Fig. 3, the larger the contact angle αi, the larger the circumferential speed on the inner ring raceway 2a of the inner ring 2 due to revolution, and the larger the semi-major axis of the contact ellipse formed by the contact surface between the ball 4 and the inner ring raceway 2a, the larger the difference in circumferential speed between both ends of the contact ellipse (shown as Δd1 and Δd2 in the figure, where Δd1>Δd2) becomes, and thus the relative circumferential speed C also becomes larger. Therefore, in order to suppress spin slip, it is effective to suppress the circumferential speed on the inner ring raceway 2a of the inner ring 2 due to revolution at the contact portion between the ball 4 and the inner ring raceway 2a, and to reduce the semi-major axis of the contact ellipse formed by the contact surface between the ball 4 and the inner ring raceway 2a. In Fig. 2, the symbol AX represents the rotation axis of the ball 4 of the outer ring control.

[0018] As shown in FIG. 4(a), in the angular contact ball bearing 1 during operation, the contact angle αo of the outer ring raceway groove 3a of the outer ring 3 becomes smaller and the contact angle αi of the inner ring raceway groove 2a of the inner ring 2 becomes larger due to the balance of the forces between the centrifugal force F acting on the ball 4 and the preload from the inner ring 2 or the outer ring 3. In the inner ring 2, when the contact angle αi between the inner ring raceway groove 2a and the ball 4 becomes larger, the amount of spin slip increases and the amount of heat generated also increases. For this reason, by increasing Ri in the inner ring 2, the contact angle αi is less likely to increase during high-speed rotation, and the contact ellipse length can be reduced, so that heat generation due to spin slip can be suppressed. That is, as shown in FIG. 4(b), when Ri is increased, the change in the contact angle due to the centrifugal force becomes smaller and the amount of spin slip also becomes smaller. On the other hand, as shown in FIG. 4(c), when Ri is decreased, the change in the contact angle due to the centrifugal force F becomes larger and the amount of spin slip also becomes larger. For this reason, it is considered preferable to increase Ri in order to suppress the amount of spin slip.

[0019] On the other hand, with the outer ring 3, as the outer ring groove curvature radius ratio Ro increases, the semi-major axis of the contact ellipse becomes smaller, which has the effect of suppressing heat generation. However, this does not act in the direction of decreasing the contact angle αo, and therefore, in terms of reducing the amount of heat generated by spin slip, this is less effective than increasing the inner ring groove curvature radius ratio Ri.

[0020] Here, using an angular contact ball bearing under the following analysis condition 1, the inner ring groove curvature radius ratio Ri and the outer ring groove curvature radius ratio Ro were changed to calculate the total spin heat generation amount on the inner ring side and the outer ring side. The calculation results of the total spin heat generation amount (W) corresponding to each Ri and Ro are shown in Table 1.

[0021] (Analysis condition 1) Bearing inner diameter: 70mm Bearing outer diameter: 110mm Bearing width: 20mm Initial contact angle: 18° Ball diameter / section height ratio: 0.595 Rotation speed: 20,000 min -1 Preload: 1,000N

[0022] [Table 1]

[0023] FIG. 5 is a graph showing the relationship between the inner ring groove curvature radius ratio Ri and the total spin heat generation amount, and FIG. 6 is a graph showing the relationship between the outer ring groove curvature radius ratio Ro and the total spin heat generation amount. First, from the graph of FIG. 5, it can be seen that the heat generation amount decreases by increasing the inner ring groove curvature radius ratio Ri regardless of the outer ring groove curvature radius ratio Ro, and the heat generation amount becomes extremely large when the inner ring groove curvature radius ratio Ri is less than 54%. However, if the inner ring groove curvature radius ratio Ri is made too large, the surface pressure between the inner ring raceway groove 2a and the ball 4 when a load is applied increases, and indentations tend to occur more easily. In particular, if the inner ring groove curvature radius ratio Ri is made larger than 58%, the indentation resistance decreases compared to the conventional product even if the indentation resistance is improved by surface hardening. In addition, in order to increase the degree of surface hardening, it is necessary to make the processing conditions stricter, but this reduces productivity, so there are restrictions on the processing conditions. Therefore, the inner ring groove curvature radius ratio Ri is set to 54 to 58%. On the other hand, from the graph of FIG. 6, when the outer ring groove curvature radius ratio Ro is less than 51%, the amount of heat generated is extremely large, reaching a minimum value at around 52%. When the outer ring groove curvature radius ratio Ro is 52% or more, the increase in the amount of heat generated with an increase in Ro is relatively gradual, and considering the variation in the quality of Ro during manufacturing, the variation in the amount of heat generated due to the variation in the quality of Ri during manufacturing can be kept small by aiming for a range slightly larger than the minimum value of 52%. This is because when the outer ring groove curvature radius ratio Ro is 58%, the value is roughly equivalent to 51%. From the viewpoint of the effect of reducing the amount of heat generated by spin, the outer ring groove curvature radius ratio Ro is set to 51 to 58%, which includes the minimum value of the amount of heat generated.

[0024] [The maximum surface pressure when the sum of the permanent deformation of the ball and the inner ring raceway at the center of the contact area between the ball and the inner ring raceway is 1 / 10,000 of the ball diameter is 4.7 to 6.0 GPa on the inner ring raceway surface] As described above, by setting the inner ring groove curvature radius ratio Ri to 54-58% and the outer ring groove curvature radius ratio Ro to 51-58%, the amount of heat generated by spin during rotation can be reduced, but it is considered that the contact surface pressure increases when an external impact load is applied while stationary, increasing the possibility of indentations occurring. For this reason, by forming a surface-hardened layer 10 with surface residual stress imparted to at least the inner ring raceway groove 2a of the inner ring 2, it is possible to further prevent indentations from occurring.

[0025] To form the surface-hardened layer 10, roller burnishing is performed on the raceway groove. In this roller burnishing, a ceramic or carbide ball (indenter) held hydraulically is pressed against the inner ring raceway groove 2a and moved along the axial cross section of the inner ring raceway groove 2a while being in rolling contact with the inner ring raceway groove 2a. The surface is hardened by this roller burnishing, and processing conditions such as the indenter diameter and pressure of the burnishing tool are selected so that the maximum surface pressure is 4.7 to 6.0 GPa when the sum of the permanent deformation amounts of the balls 4 and the inner ring raceway groove 2a at the center of the contact area between the balls 4 and the inner ring raceway groove 2a is 1 / 10,000 of the ball diameter. In addition, the limit for preventing smooth rotation of the bearing is set at 1 / 10,000 of the ball diameter for the sum of the permanent deformation of the ball 4 and the permanent deformation of the raceway at the center of contact between the ball 4 and the raceway.

[0026] In the case of a ball bearing that does not have residual compressive stress applied to the surface of the raceway, the maximum surface pressure when the sum of the permanent deformation of the ball and raceway groove at the center of the contact area between the ball and raceway groove is 1 / 10,000 of the ball diameter is 4.2 GPa (in accordance with JIS B1519), so the application of residual compressive stress has the effect of making it less likely for indentations to occur when the ball is subjected to an external impact load when stationary.

[0027] According to the applicant's research, it has been found that bearings returned from the market due to collision damage are subjected to a surface pressure of approximately 4 GPa or more. If a residual compressive stress is applied to the surface layer of the raceway surface, and the maximum surface pressure is set to 4.7 GPa when the sum of the permanent deformation of the balls 4 and the inner ring raceway groove 2a is 1 / 10,000 of the ball diameter, products that previously suffered from a surface pressure of approximately 4 GPa or more but less than 4.7 GPa and were deemed defective will no longer be recognized as defective. This will eliminate the need to replace the bearings. In addition, the maximum surface pressure of the inner ring raceway 2a when the sum of the permanent deformation of the ball 4 and the inner ring raceway 2a becomes 1 / 10,000 of the ball diameter is set to 4.7 to 6.0 GPa, taking into consideration processing conditions that do not reduce productivity. Furthermore, the surface hardened layer is not limited to the inner ring raceway groove 2 a of the inner ring 2 , but may also be applied to the outer ring raceway groove 3 a of the outer ring 3 . The technique of forming a hard coating having a thickness of 0.05 to 8 μm on the raceway surface shown in Patent Document 1 is to coat the raceway ring after machining with a hardened layer by chemical processing to improve wear resistance and reduce the friction coefficient. On the other hand, the surface-treated layer 10 of the present embodiment is to harden the surface by machining and improve impression resistance.

[0028] [Ri-Ro≧0 points] The inner ring groove curvature radius ratio Ri and the outer ring groove curvature radius ratio Ro are set within the above-mentioned ranges, but by making Ri equal to or larger than the outer ring groove curvature radius ratio Ro, the increase in surface pressure when a load is applied is kept lower in the outer ring 3 than in the inner ring 2. Meanwhile, since the inner ring raceway groove 2a has a surface-hardened layer 10, if it is possible to prevent the outer ring 3 from being indented before the inner ring 2 from being damaged when an external impact load is applied while the bearing is stationary, then the maximum surface pressure at which the sum of the permanent deformation amounts of the balls 4 and the inner ring raceway groove 2a becomes 1 / 10,000 of the ball diameter can be set within the above-mentioned range in the inner ring raceway groove 2a, thereby sufficiently improving the indentation resistance.

[0029] Fig. 7 is a graph showing the relationship between Ri-Ro and the ratio of the inner ring surface pressure to the outer ring surface pressure calculated for the bearing specifications under analysis condition 1, and shows the relationship between the outer ring surface pressure and the inner ring surface pressure in the difference between Ri and Ro when a load is applied to the bearing. In other words, in the region where the inner ring surface pressure / outer ring surface pressure>1, the inner ring surface pressure is high, in the region where the inner ring surface pressure / outer ring surface pressure<1, the outer ring surface pressure is high, and when the inner ring surface pressure / outer ring surface pressure=1, the inner ring surface pressure and the outer ring surface pressure are equal. For example, if we consider a typical bearing in which the inner ring raceway groove 2a and the outer ring raceway groove 3a are not surface hardened, the maximum surface pressure defined in JIS B1519 at which the sum of the permanent deformation of the ball and raceway groove at the centre of contact between the ball and raceway groove is 1 / 10,000 of the ball diameter is 4.2 GPa for both the inner ring raceway surface and the outer ring raceway surface, which means that in the region where the inner ring surface pressure / outer ring surface pressure>1, the indentation will occur on the inner ring raceway surface first, in the region where the inner ring surface pressure / outer ring surface pressure<1, the indentation will occur on the outer ring raceway surface first, and in the region where the inner ring surface pressure / outer ring surface pressure=1, the indentation will occur on both the inner ring raceway surface and the outer ring raceway surface simultaneously.

[0030] When the inner ring raceway 2a of the inner ring 2 is surface-hardened to form a surface-hardened layer 10, the maximum surface pressure at which the sum of the permanent deformation of the balls 4 and the inner ring raceway 2a is 1 / 10,000 of the ball diameter is 4.7 to 6.0 GPa, and the outer ring raceway of the outer ring is not surface-hardened, the ratio of the inner ring surface pressure to the outer ring surface pressure at which indentations occur on the inner ring raceway surface before the outer ring raceway surface is in the range of 1.120≦inner ring surface pressure / outer ring surface pressure≦1.429 shown in Fig. 7. The lower limit value 1.120 and the upper limit value 1.429 of the ratio of the inner ring surface pressure to the outer ring surface pressure at which indentations occur on the inner ring raceway surface before the outer ring raceway surface are the ratio of the surface pressure between the inner ring raceway surface that has been surface-hardened and the outer ring raceway surface that has not been surface-hardened, and are calculated as 4.7÷4.2=1.120 and 6.0÷4.2=1.429, respectively.

[0031] 7, in a bearing in which the inner ring raceway groove has been surface-hardened, when Ri-Ro≧0, the inner ring surface pressure / outer ring surface pressure≧1.120, so that the outer ring does not develop indentations first, and the effect of surface hardening on the inner ring can be fully obtained. It can also be seen that even without surface hardening the outer ring raceway groove 3a of the outer ring 3, it is possible to obtain indentation resistance equal to or greater than that of an inner ring 2 in which the inner ring raceway groove 2a has been surface-hardened. Therefore, in this embodiment, surface hardening of the outer ring raceway groove 3a is not necessary, which provides manufacturing advantages. From the results of FIG. 7, when Ri is in the range of 54-58% and Ro is in the range of 51-58%, the range of the inner ring surface pressure / outer ring surface pressure at the point Ri-Ro=0 is 1.120≦inner ring surface pressure / outer ring surface pressure≦1.124, and at the point Ri-Ro=1, it is 1.151≦inner ring surface pressure / outer ring surface pressure≦1.191. That is, since the value of the inner ring surface pressure / outer ring surface pressure at the point Ri-Ro=1 is larger and the range is wider than that at the point Ri-Ro=0, the inner ring surface pressure tends to be higher than the outer ring surface pressure, and the condition is such that the inner ring surface pressure is more likely to be formed first and the outer ring surface pressure is less likely to be formed first, which makes it easier to obtain the effect of the surface strengthening treatment. Therefore, it is preferable to set the point Ri-Ro≧1.

[0032] [Relationship with ball diameter / section height ratio] Under the above analysis condition 1, it was confirmed that the total spin heat generation can be reduced by using a relatively large ball diameter (large ball) and setting the inner ring groove curvature radius ratio Ri to 54-58% and the outer ring groove curvature radius ratio Ro to 51-58% when the ball diameter / section height ratio is 0.595. Below, under analysis condition 2, it was confirmed whether the total spin heat generation can be reduced according to the above Ri and Ro regulations when a smaller ball diameter (small ball) is used and the ball diameter / section height ratio is 0.437. Table 2 shows the calculation results of the total spin heat generation (W) corresponding to each Ri and Ro.

[0033] (Analysis condition 2) Bearing inner diameter: 70mm Bearing outer diameter: 110mm Bearing width: 20mm Contact angle: 18° Ball diameter / section height ratio: 0.437 Rotation speed: 20,000 min -1 Preload: 1,000N

[0034] [Table 2]

[0035] Fig. 8 is a graph showing the relationship between the inner ring groove curvature radius ratio Ri on the horizontal axis and the total amount of spin heat generated, and Fig. 9 is a graph showing the relationship between the outer ring groove curvature radius ratio Ro on the horizontal axis and the total amount of spin heat generated on the inner ring side and the outer ring side. In this case, as in analysis condition 1, an effect of reducing the total amount of spin heat generated on the inner ring side and the outer ring side is recognized when the inner ring groove curvature radius ratio Ri is in the range of 54 to 58% and the outer ring groove curvature radius ratio Ro is in the range of 51 to 58%.

[0036] Next, analysis condition 3, which has a ball diameter / section height ratio of 0.572 and analysis condition 4, which has a ball diameter / section height ratio of 0.635, was used, and confirmed whether the total spin heat generation could be reduced according to the above Ri and Ro regulations, although the bearing size is different from analysis condition 1. Table 3 shows the calculation results of the total spin heat generation (W) corresponding to each Ri and Ro under analysis condition 3, and Table 4 shows the calculation results of the total spin heat generation (W) corresponding to each Ri and Ro under analysis condition 4.

[0037] (Analysis condition 3) Bearing inner diameter: 30mm Bearing outer diameter: 55mm Bearing width: 13mm Contact angle: 18° Ball diameter / section height ratio: 0.572 Rotational speed: 43,000 min -1 Preload: 440N

[0038] [Table 3]

[0039] (Analysis condition 4) Bearing inner diameter: 110mm Bearing outer diameter: 170mm Bearing width: 28mm Contact angle: 18° Ball diameter / section height ratio: 0.635 Rotation speed: 13,000 min -1 Preload: 2,200N

[0040] [Table 4]

[0041] FIG. 10 is a graph showing the relationship between the inner ring groove curvature radius ratio Ri on the horizontal axis and the total spin heat generation under analysis condition 3, and FIG. 11 is a graph showing the relationship between the outer ring groove curvature radius ratio Ro on the horizontal axis and the total spin heat generation under analysis condition 3. FIG. 12 is a graph showing the relationship between the inner ring groove curvature radius ratio Ri on the horizontal axis and the total spin heat generation under analysis condition 4, and FIG. 13 is a graph showing the relationship between the outer ring groove curvature radius ratio Ro on the horizontal axis and the total spin heat generation under analysis condition 4. In the case of analysis conditions 3 and 4, too, an effect in reducing the total amount of spin heat generated on the inner and outer ring sides was observed when the inner ring groove curvature radius ratio Ri was in the range of 54 to 58% and the outer ring groove curvature radius ratio Ro was in the range of 51 to 58%.

[0042] Therefore, it can be seen that by specifying the inner ring groove curvature radius ratio Ri and the outer ring groove curvature radius ratio Ro, the total spin heat generation can be reduced and the effect of reducing indentation resistance does not change even if the bearing size changes.

[0043] In addition, the smaller the ball diameter / section height ratio, the more advantageous it is for reducing heat generation, but if it is too small, the centrifugal and thermal expansion of the inner ring will cause the effective radial clearance to become too small during high-speed operation, which will lead to seizure. For this reason, the ball diameter / section height ratio must be 0.39 or more. In addition, the larger the ball diameter / section height ratio, the more advantageous it is for indentation resistance, but if this value is greater than 0.65, the thickness of the raceway will become too thin, which is undesirable in terms of manufacturing, as it will cause large deformation during heat treatment and processing. For this reason, the ball diameter / section height ratio is preferably 0.39 to 0.65, and when indentation resistance is important, it is more preferable to use large balls with a ball diameter / section height ratio of 0.55 to 0.65.

[0044] Furthermore, the inner ring 2 and the outer ring 3 are usually made of bearing steel such as SUJ2 (high carbon chromium bearing steel). Bearing steel such as SUJ2 is used at relatively low temperatures because at high temperatures its hardness drops significantly, shortening its lifespan. Therefore, when faster rotation is required, the contact pressure at the contact surfaces where the balls 4 come into contact with the inner ring 2 and the outer ring 3 and the sliding of the balls 4 increase, generating heat and causing localized high temperatures. For this reason, it is desirable for the inner ring 2 and the outer ring 3 to be made of a material with excellent heat resistance and wear resistance.

[0045] Therefore, materials in which secondary hardening precipitation-type eutectic carbides are formed, such as high-speed steel, semi-high-speed steel, and martensitic stainless steel, are suitable, such as SKD, SKH, M50, and SUS440C. In addition, general bearing steel (SUJ2) tempered at a high temperature of 240°C to 330°C may be used and hard-coated. In this case, the hardness of the base material itself decreases, but the hardness of the raceway surface can be increased by the hard coating, so that the same performance as when the above-mentioned metallic materials are used can be obtained.

[0046] Also, materials that improve the tempering resistance and stabilize the dimensions by the constituent elements (materials similar to high carbon chromium steels) are suitable, and an example is SHX material. In this case, at least one of the inner ring 2 and the outer ring 3 is made of a steel material containing 0.2-1.2 mass% C, 0.7-1.5 mass% Si, 0.5-1.5 mass% Mo, 0.5-2.0 mass% Cr, the balance being Fe and unavoidable impurity elements, and by performing carbonitriding treatment and then quenching and tempering treatment, the surface carbon concentration is set to 0.8-1.3 mass% and the surface nitrogen concentration is set to 0.2-0.8 mass%. Here, the critical meaning of the effective range of each of the above constituent elements will be explained.

[0047] (1)Si;0.7~1.5% by mass Silicon is an element that is effective in improving temper softening resistance, improving high-temperature strength, and retarding the decomposition of retained austenite, which is effective in preventing indentation-initiated spalling in high-temperature environments. If the Si content is less than 0.7 mass%, the high-temperature strength is insufficient and indentation-initiated spalling occurs, so the lower limit is set at 0.7 mass%. On the other hand, if the Si content exceeds 1.5 mass%, the mechanical strength decreases and carburization is inhibited, so the upper limit is set at 1.5 mass%.

[0048] (2)Mo; 0.5~1.5% by mass Like Si, Mo is an element that is effective in temper softening resistance and has the effect of improving high-temperature strength. Mo also acts as a carbide-forming element that forms minute carbides on the carbonitrided surface. If the Mo content falls below 0.5 mass%, the high-temperature strength is insufficient and the carbides precipitated on the surface are insufficient, so the lower limit is set to 0.5 mass%. On the other hand, if the Mo content exceeds 1.5 mass%, macro carbides are formed at the material stage, which leads to the carbide falling off and reduces the rolling fatigue life of the bearing, so the upper limit is set to 1.5 mass%.

[0049] (3)Cr; 0.5~2.0% by mass Cr is an additive element that has the same action and effect as Mo. If the Cr content is below 0.5 mass%, the high-temperature strength is insufficient and the amount of carbides precipitated on the surface becomes insufficient, so the lower limit is set to 0.5 mass%. On the other hand, if the Cr content exceeds 2.0 mass%, giant carbides are formed at the material stage, which leads to the carbide falling off and shortens the rolling fatigue life of the bearing, so the upper limit is set to 2.0 mass%.

[0050] (4)C; 0.2~1.2% by mass As mentioned above, if the amount of retained austenite becomes too large, the retained austenite decomposes and changes in shape over time, which impairs the dimensional stability of the bearing. On the other hand, the presence of retained austenite on the surface of the raceway is effective in preventing indentation-initiated spalling. Therefore, it is preferable to limit the amount of retained austenite in the entire bearing while allowing the retained austenite to exist on the surface, and for this purpose, it is necessary to suppress the amount of retained austenite in the core of the bearing. From this perspective, it is preferable to make the average amount of retained austenite in the steel, including the surface and core, 5% by volume or less, and for this purpose, it is necessary to make the carbon concentration on which the retained austenite depends 1.2% by mass or less, so the upper limit is set to 1.2% by mass. On the other hand, if the carbon concentration is below 0.2% by mass, it takes a long time to obtain the desired carburization depth by carbonitriding, which leads to an overall increase in costs, so the lower limit is set to 0.2% by mass.

[0051] (5) Surface carbon concentration; 0.8~1.3% by mass Adding carbon to the surface by carbonitriding can strengthen the martensite structure that forms the matrix through solid solution, and can form a large amount of retained austenite that is effective in preventing indentation-initiated spalling in the extreme surface layer. If the surface carbon concentration falls below 0.8 mass%, the surface hardness is insufficient, resulting in reduced rolling fatigue life and wear resistance, so the lower limit is set to 0.8 mass%. On the other hand, if the surface carbon concentration exceeds 1.3 mass%, macrocarbides precipitate during carbonitriding, reducing the rolling fatigue life, so the upper limit is set to 1.3 mass%.

[0052] (6) Surface N concentration; 0.2~0.8% by mass Adding nitrogen to the surface by carbonitriding improves tempering resistance, increases high-temperature strength, improves wear resistance, and allows a large amount of retained austenite to exist in the extreme surface layer, which is effective in preventing indentation-initiated spalling. If the surface nitrogen concentration falls below 0.2 mass%, high-temperature strength and wear resistance decrease, so the lower limit is set to 0.2 mass%. On the other hand, if the surface nitrogen concentration exceeds 0.8 mass%, grinding finish during bearing manufacturing becomes difficult, and the productivity of bearings decreases due to the difficulty of grinding, so the upper limit is set to 0.8 mass%.

[0053] (7) Other component elements The balance is Fe and unavoidable impurities, but it is preferable to add a small amount of Ti as another component element. When Ti is added, fine titanium carbide (TiC) and carbonitride (Ti(C+N)) are precipitated and dispersed in the matrix, improving wear resistance and seizure resistance. In this case, the Ti content is preferably 0.1 to 0.3 mass%. If the Ti content is less than 0.1 mass%, the carbide precipitation effect cannot be obtained, so the lower limit is set to 0.1 mass%. On the other hand, if the Ti content exceeds 0.3 mass%, huge precipitates are likely to be formed, which may become defects and may adversely reduce the rolling fatigue life, so the upper limit is set to 0.3 mass%. Incidentally, if the size of the titanium precipitates (TiC, Ti(C+N)) is 0.1 μm or less, it contributes to improving wear resistance and seizure resistance.

[0054] It is preferable to avoid, as much as possible, the inclusion of unavoidable impurity elements such as S, P, H, and O. In particular, if the oxygen (O) content exceeds 12 ppm, oxide-based inclusions are likely to form, which can become defects and reduce the rolling fatigue life, so it is preferable to keep the oxygen content below 12 ppm.

[0055] Furthermore, the balls 4 may be made of steel, which has excellent heat resistance and wear resistance, but may also be made of ceramics such as Si3N4 (silicon nitride), SiC (silicon carbide) or Al2O3 (aluminum oxide). Since ceramic balls 4 have a higher Young's modulus than steel balls, the surface pressure with the raceway groove is high and indentations are likely to occur, so an angular ball bearing with improved indentation resistance as in this embodiment works more effectively.

[0056] As described above, the angular contact ball bearing of this embodiment is configured such that the groove curvature radius ratio (Ri) of the inner ring raceway groove 2a to the ball diameter is 54-58%, the groove curvature radius ratio (Ro) of the outer ring raceway groove 3a to the ball diameter is 51-58%, and Ri-Ro≧0 point, and at least the inner ring raceway groove 2a is configured such that the maximum surface pressure when the sum of the permanent deformation amount of the ball 4 and the inner ring raceway groove 2a at the center of the contact portion between the ball 4 and the inner ring raceway groove 2a is 1 / 10,000 of the ball diameter is 4.7-6.0 GPa. This suppresses heat generation and has excellent indentation resistance, making it suitable for use in applications where it is used at high speed rotation and where excessive loads are applied while stationary, and is particularly useful as an angular contact ball bearing used in a machine tool main spindle with a dmn of 800,000 or more and preloaded. Furthermore, with the above-described configuration, it is only necessary to form a surface hardened layer by machining only on the inner ring raceway groove 2a out of the inner ring raceway groove 2a and the outer ring raceway groove 3a, and this offers advantages in terms of manufacturing.

[0057] The present invention is not limited to the above-described embodiment, and various modifications and improvements are possible. For example, the lubrication system of the angular contact ball bearing of the present invention may be oil-air lubrication or grease lubrication.

[0058] This application is based on a Japanese patent application (Patent Application No. 2020-162504) filed on September 28, 2020, the contents of which are incorporated by reference into this application. [Explanation of symbols]

[0059] 1 Angular contact ball bearing 2. Inner Circle 2a Inner ring raceway groove 3 Outer ring 3a Outer ring raceway 4 balls 5 Cage 10 Surface hardening layer

Claims

1. an inner ring having an inner ring raceway groove having an arc-shaped cross section on an outer peripheral surface; an outer ring having an outer ring raceway groove having an arc-shaped cross section on its inner circumferential surface; a plurality of balls rollably disposed between the inner ring raceway groove and the outer ring raceway groove, A ratio (Ri) of the groove radius of curvature of the inner ring raceway groove to the ball diameter is 54 to 58%, a ratio (Ro) of the groove radius of curvature of the outer ring raceway groove to the ball diameter is 51 to 58%, and Ri-Ro≧0 points are satisfied; At least the inner ring raceway groove has a maximum surface pressure of 4.7 to 6.0 GPa when the sum of the permanent deformation of the ball and the inner ring raceway groove at the center of contact between the ball and the inner ring raceway groove is 1 / 10,000 of the ball diameter.

2. 2. The angular contact ball bearing according to claim 1, wherein Ri-Ro≧1 point.

3. 3. The angular contact ball bearing according to claim 1, wherein at least said inner ring raceway groove is provided with a surface hardened layer by machining.

4. 3. The angular contact ball bearing according to claim 1, wherein a surface hardened layer is formed by machining only on the inner ring raceway groove out of the inner ring raceway groove and the outer ring raceway groove.

5. 5. The angular contact ball bearing according to claim 1, wherein the material of the balls is ceramic.

6. 6. The angular contact ball bearing according to claim 1, wherein the ball diameter / section height ratio is 0.39 to 0.

65.

7. 7. The angular contact ball bearing according to claim 6, wherein the ball diameter / section height ratio is 0.55 to 0.

65.

8. 8. The angular ball bearing according to claim 1, which is used in a machine tool spindle having a dmn of 800,000 or more and is preloaded.

9. at least one of the inner ring and the outer ring is made of steel containing 0.2 to 1.2 mass% C, 0.7 to 1.5 mass% Si, 0.5 to 1.5 mass% Mo, 0.5 to 2.0 mass% Cr, the balance being Fe and unavoidable impurity elements, and The surface carbon concentration is 0.8 to 1.3 mass% and the surface nitrogen concentration is 0.2 to 0.8 mass%. The angular contact ball bearing according to any one of claims 1 to 8.

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