Rolling bearing and machine tool

Abstract

To provide a rolling bearing capable of achieving reduction of NRRO.SOLUTION: An outer peripheral surface 14a of a cage 14 is brought into contact with an outer ring 12. A point at which pocket center axes P of respective pockets 14c intersect is defined as a pocket pitch center Op. On a projection plane obtained by axially parallel projection of the cage 14, a center of gravity Og of the cage 14, an outer diameter center O of the cage, and the pocket pitch center Op are provided at mutually different positions. An angle θ formed between a first line segment L1, which has the outer diameter center O as a starting point and the center of gravity Og as an end point, and a second line segment L2, which has the outer diameter center O of the cage 14 as the starting point and the pocket pitch center Op as the end point, is set to be greater than 135° and less than or equal to 180°.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a rolling bearing and a machine tool. [Background technology]

[0002] With rolling bearings, the issue is how to reduce non-repeatable run-out (NRRO). NRRO refers to the maximum difference in the amount of displacement in the same phase in a Lissajous figure, which is a plot of shaft displacement as a trajectory, as shown in Figure 9. In recent machine tools, improvements in machining accuracy and improvements in machining efficiency through faster spindle speeds are required, and there is a strong demand for reducing NRRO, which has a significant impact on the quality of the machined surface.

[0003] NRRO is caused by factors such as shape errors in the raceway surfaces of the inner and outer rings and the surfaces of the rolling elements. For example, when used with the inner ring rotating, if the number of peaks in the waviness of the inner ring raceway surface is two (for example, if the inner ring raceway surface is elliptical), runout will occur with a period twice the inner ring rotation frequency. Known existing technology aimed at reducing NRRO is a method of controlling the difference in diameter between the balls and the roundness of the raceway surfaces of the inner and outer rings, as described in Patent Document 1 listed below. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-353692 Summary of the Invention [Problem to be solved by the invention]

[0005] As mentioned above, runout caused by geometric errors in the raceway or rolling elements does not change significantly even when the rotation speed changes. On the other hand, as the rotation speed increases, the cage rotation period runout component increases rapidly, and it has become clear that this runout component accounts for most of the NRRO under high-speed rotation conditions (e.g., 5000 rpm or higher). Therefore, suppressing cage rotation period runout under high-speed rotation is an important issue in reducing NRRO. The technology described in Patent Document 1 is effective in reducing NRRO in the low- to medium-speed range, but is not sufficiently effective against cage rotation period runout under high-speed rotation.

[0006] Therefore, an object of the present invention is to provide a rolling bearing that can achieve a reduction in NRRO, particularly a reduction in NRRO under high speed rotation. [Means for solving the problem]

[0007] During rotation of the rolling bearing, centrifugal force acts on the rotating cage. During rotation, the cage rotates while displacing in a certain radial direction due to mass imbalance caused by manufacturing errors. In this case, the center of gravity of the cage is offset from the center of the bearing, so the cage revolves around the center of the bearing while pointing its center toward the outer diameter. As shown in FIG. 10 , when the cage 140 moves radially (in the direction of the arrow) due to centrifugal force, the rolling elements are pushed against the inner surface 142 (called the pocket surface) of the pocket 141 of the cage 140, causing the rolling elements 130 to be unevenly distributed in one radial direction (upward in FIG. 10 ). This results in an area where the rolling elements 130 are densely distributed (the upper area in FIG. 10 ) and an area where they are sparsely distributed (the lower area in FIG. 10 ). At high speeds, where centrifugal force is particularly large, the rolling elements tend to move more easily, making the uneven distribution of the rolling elements more pronounced. Incidentally, the dashed lines in FIG. 10 represent the rolling elements 130 at evenly spaced positions.

[0008] In this way, the rolling elements 130 are unevenly distributed in the circumferential direction, which causes the rigidity between the inner and outer rings to vary in the circumferential direction. This is thought to be a factor in generating a runout component in the cage with a rotation period of the cage during high-speed rotation.

[0009] Based on the above findings, the inventors of the present application came up with the idea of ​​controlling the direction in which the retainer displaces by intentionally creating a mass imbalance in the retainer, and then configuring the retainer so that when the retainer displaces in that direction, the pocket pitch center and the bearing center are as close as possible to each other.

[0010] In order to realize the above idea, the present invention provides an outer ring guide type rolling bearing comprising an inner ring having an inner raceway surface on its outer peripheral surface, an outer ring having an outer raceway surface on its inner peripheral surface, a plurality of rolling elements arranged between the inner raceway surface and the outer raceway surface, and a cage that holds each of the rolling elements, wherein a plurality of spaces (pockets) that accommodate the rolling elements are formed in the cage and the outer peripheral surface of the cage is in contact with the outer ring, wherein the point where the pocket central axes of each of the pockets intersect is defined as the pocket pitch center, and on a projection plane obtained by projecting the cage parallel to the axial direction, the center of gravity of the cage, the center of outer diameter of the cage, and the pocket pitch center are all located at different positions, and the angle formed by a first line segment that starts at the outer diameter center of the cage and ends at the center of gravity of the cage, and a second line segment that starts at the outer diameter center of the cage and ends at the pocket pitch center is greater than 135° and not greater than 180°.

[0011] The present invention also provides an inner ring guide type rolling bearing comprising an inner ring having an inner raceway surface on its outer peripheral surface, an outer ring having an outer raceway surface on its inner peripheral surface, a plurality of rolling elements arranged between the inner raceway surface and the outer raceway surface, and a retainer that holds the rolling elements, wherein a plurality of spaces (pockets) that accommodate the rolling elements are formed in the retainer, and the inner peripheral surface of the retainer is in contact with the inner ring, wherein the point where the pocket central axes of the pockets intersect is defined as the pocket pitch center, and on a projection plane obtained by projecting the retainer parallel to the axial direction, the center of gravity of the retainer, the inner diameter center of the retainer, and the pocket pitch center are all located at different positions, and the angle formed by a first line segment that starts at the inner diameter center of the retainer and ends at the center of gravity of the retainer, and a second line segment that starts at the inner diameter center of the retainer and ends at the pocket pitch center is greater than 135° and not greater than 180°.

[0012] In outer ring guide type rolling bearings, the cage rotates during bearing rotation. Due to the influence of centrifugal force, it displaces in the direction of the offset of its center of gravity relative to the center of the outer diameter, contacting the guide surface of the outer ring. As the bearing rotates, the cage revolves around the center of the inner circumferential surface of the outer ring while displacing in the direction of the offset of its center of gravity. When the cage displaces in the direction of the offset of its center of gravity, the pocket pitch center approaches the center of the inner circumferential surface of the outer ring. By moving the pocket pitch center closer to the center of the inner circumferential surface of the outer ring, the rolling elements are evenly and circumferentially spaced or nearly so, thereby suppressing circumferential variation in rigidity between the inner and outer rings. This suppresses runout components of the cage rotation period during high-speed rotation, enabling the bearing's NRRO to be reduced. The same effect can be achieved with inner ring guide type rolling bearings.

[0013] It is more preferable that the angle between the first line segment and the second line segment be 160° or greater. This allows the pocket pitch center to be closer to the outer diameter center O when the cage is displaced by centrifugal force, thereby improving the effect of suppressing NRRO.

[0014] The length of the second line segment is preferably 1 / 4 to 3 / 4 of the guide clearance. If the length of the second line segment is outside this range, the effect of bringing the pocket pitch center closer to the outer diameter center when the cage is displaced by centrifugal force is reduced, and the effect of suppressing NRRO is reduced.

[0015] A machine tool in which the spindle is supported by the above-described rolling bearing can improve the quality of the machined surface. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a rolling bearing with reduced NRRO. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is an axial cross-sectional view of a rolling bearing according to an embodiment of the present invention. [Figure 2] FIG. [Figure 3] FIG. 4 is an enlarged plan view showing a pocket of the cage. [Figure 4] FIG. 3 is a radial cross-sectional view of the cage projected onto a projection plane. [Figure 5] FIG. 2 is an enlarged view of a partial area projected onto a projection surface. [Figure 6] FIG. 2 is a radial cross-sectional view of the cage during revolution projected onto a projection plane. [Figure 7] FIG. 2 is a radial cross-sectional view of the cage during revolution projected onto a projection plane. [Figure 8] FIG. 2 is an enlarged view of a partial area projected onto a projection surface. [Figure 9] FIG. 10 is a diagram showing a Lissajous figure in which the displacement of an axis is plotted as a locus. [Figure 10] FIG. 1 is a radial cross-sectional view of a cage and rolling elements in a conventional rolling bearing. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0019] 1 is a cross-sectional view showing an angular contact ball bearing 1, which is a type of rolling bearing. This angular contact ball bearing 1 comprises an inner ring 11 having an inner raceway surface 11a on its outer peripheral surface, an outer ring 12 having an outer raceway surface 12a on its inner peripheral surface, a plurality of rolling elements (balls) 13 interposed so as to be able to roll between the inner raceway surface 11a and the outer raceway surface 12a, and an annular cage 14 that holds the rolling elements 13.

[0020] An inner ring shoulder surface 11b with a larger diameter than the bottom of the inner ring raceway surface 11a is formed in an area of ​​the outer peripheral surface of the inner ring 11 on one axial side of the inner raceway surface 11a, and an outer ring shoulder surface 12b with a smaller diameter than the bottom of the outer raceway surface 12a is formed in an area of ​​the inner peripheral surface of the outer ring 12 on the other axial side of the outer raceway surface 12a. When the bearing 1 is of an outer ring guide type, the outer peripheral surface 14a of the cage 14 comes into contact with the outer ring shoulder surface 12b, which serves as a guide surface, as the bearing 1 rotates. In this case, the difference A (see FIG. 6) between the diameter of the outer peripheral surface 14a of the cage 14 and the diameter of the outer ring shoulder surface 12b is called the guide clearance.

[0021] When the bearing 1 is of the inner ring guide type, the inner peripheral surface 14b of the cage 14 comes into contact with the inner ring shoulder surface 11b, which serves as the guide surface, as the bearing 1 rotates. In this case, the difference A between the diameter of the inner peripheral surface 14b of the cage 14 and the diameter of the inner ring shoulder surface 11b serves as the guide gap. The present invention is applicable to both inner ring guide types and outer ring guide types. The following explanation illustrates the case where the bearing 1 is of the outer ring guide type.

[0022] As shown in FIG. 2, the cage 14 is formed with a plurality of spaces (pockets 14c) for accommodating the rolling elements 13, spaced at equal intervals around the cage. In this embodiment, the pockets 14c penetrate the cage 14 in the radial direction and are closed around the entire circumference without any openings. As shown in FIGS. 3 and 4, this embodiment illustrates a case in which the inner surface 14c1 (referred to as the pocket surface) forming the pocket 14c is formed as a cylindrical surface. As shown in FIG. 4, the entire pocket surface 14c1 may be formed as a cylindrical surface. Alternatively, a region including at least the rolling element PCD (denoted by the symbol "PCD" in FIG. 1) may be formed as a cylindrical surface, and either or both of the region on the outer diameter side and the region on the inner diameter side may be formed as a conical surface. In this case, the conical surface on the inner diameter side is tapered toward the inner diameter, and the conical surface on the outer diameter side is tapered toward the outer diameter. As shown in FIG. 3, a minute gap called a pocket gap α is provided between the pocket surface 14c1 and the surface of the rolling element 13.

[0023] In this embodiment, the line connecting the pocket centers in circumferential cross sections of the pocket 14c at multiple radial locations (particularly multiple radial locations in the region including the rolling elements PCD) is referred to as the pocket center axis P (see FIG. 4). For example, in the case of a ball bearing, the line connecting the centers of the circular pocket surfaces 14c1 that appear when each circumferential cross section is developed on a plane is the pocket center axis P. In the case of a roller bearing, the line connecting the centers of the quadrangular pocket surfaces (the points where the diagonals of the quadrangle intersect) that appear when each circumferential cross section is developed on a plane is the pocket center axis P. The pocket center axis P is formed linearly at least in the region including the rolling elements PCD. Note that FIG. 4 illustrates an example in which the pocket center axis P is linear throughout the entire radial direction of the pocket 14c. The pocket center axes P of each pocket 14c are designed to intersect at a single point. The intersection of the pocket center axes P is referred to as the pocket pitch center Op.

[0024] When the pockets 14c of the cage 14 are formed by drilling, the annular cage blank is rotated by equal angles around the pocket pitch center Op. While the cage blank is stopped from rotating, a tool such as a drill is fed along the pocket center axis P to form each pocket 14c. In conventional rolling bearings, the pocket pitch center Op is aligned with the outer diameter center O of the cage 14, so the feed direction of the tool is aligned with the radial direction of the cage 14. However, in the cage 14 of this embodiment, the pocket pitch center Op is set at a position radially offset from the outer diameter center O of the cage 14, as described below, so the feed direction of the tool (pocket center axis P) is slightly inclined with respect to the radial direction of the cage 14.

[0025] The pockets 14c can be formed by other known forming methods, such as molding molten resin or molten metal, instead of drilling. In this case, each pocket surface 14c1 is formed so that the pocket pitch center Op is offset from the outer diameter center O of the cage in the radial direction.

[0026] The characteristic features of the present invention will be described in detail below.

[0027] The present invention is characterized by specifying the positional relationship between the center of gravity Og of the cage 14, the outer diameter center O of the cage 14, and the pocket pitch center Op of the cage 14. The outer diameter center O of the cage 14 is the center of the outer surface 14a of the cage 14, and serves as the rotation center of the cage 14 when the outer surface 14a of the cage 14 is finish-machined. Conventionally, the pocket pitch center Op overlaps with the outer diameter center O, and therefore exists on the bearing center axis X when the cage is in the neutral position. In the present invention, as already mentioned, the cage 14 is designed so that the pocket pitch center Op is located at a position radially offset from the outer diameter center O (the bearing center axis X when the cage is in the neutral position).

[0028] The center of gravity Og of the cage 14 may be located at a position offset in the axial direction from the center of the width direction of the cage 14. Furthermore, due to processing errors or the like, the pocket pitch center Op of the cage 14 may be located at a position offset in the axial direction from the center of the width direction of the cage 14. In order to identify the positions where the above three points should be located while excluding the influence of such axial position offsets, in this embodiment, a projection plane is imagined by projecting the cage 14 parallel in the axial direction, including the above three points, and the positional conditions that the center of gravity Og, outer diameter center O, and pocket pitch center Op should satisfy on this projection plane are determined.

[0029] Specifically, in the present invention, as shown in Figures 4 and 5, the center of gravity Og, outer diameter center O, and pocket pitch center Op of the retainer 14 are located at positions on the aforementioned projection plane that satisfy both of the following conditions (1) and (2):

[0030] (1) The center of gravity Og of the cage 14, the outer diameter center O of the cage 14, and the pocket pitch center Op are located at different positions. (2) The angle θ formed by the first line segment L1, which starts at the outer diameter center O of the retainer 14 and ends at the center of gravity Og of the retainer 14, and the second line segment L2, which starts at the outer diameter center O of the retainer 14 and ends at the pocket pitch center Op, is greater than 135° and less than or equal to 180°.

[0031] [Regarding condition (1)] In the present invention, the center of gravity Og of the cage 14 is located at a position offset in the radial direction from the outer diameter center O of the cage 14. This offset can be achieved, for example, by creating a mass imbalance in one direction on the axial projection of the cage 14 by forming a circumferentially extending notch 14d in a partial region of the inner diameter end of the cage 14, as shown in FIG. 2. For example, when the notch 14d is formed at the position shown in FIG. 2, the center of gravity is shifted upward in FIG. 2 compared to when the notch 14d is not formed. The shape and position of the notch are not limited to the illustrated example. The center of gravity Og of the cage 14 can also be radially offset from the outer diameter center O by other methods, such as by varying the radial thickness of the cage 14 in the circumferential direction.

[0032] The pocket pitch center Op of the cage 14 is located at a position offset in the radial direction from the outer diameter center O of the cage 14. As a result, on the projection plane described above, the pocket center axes P of some or all of the pockets 14c are slightly inclined with respect to the normal direction of the rolling elements PCD (see FIG. 4).

[0033] The actual position of the outer diameter center O of the cage 14 can be calculated by measuring the two-dimensional contour of the outer peripheral surface 14a of the cage 14 using a roundness measuring device such as a Talyrond. The position of the center of gravity Og of the cage 14 can be geometrically calculated from the overall shape of the cage 14. Alternatively, the cage itself can be fixed to a rotating shaft (preferably a rigid shaft where the effects of gravity can be ignored) and the outer peripheral surface 14a in the case of an outer ring guide system or the inner peripheral surface 14b in the case of an inner ring guide system is constrained while the cage is rotated, and the load (measured from the magnitude and direction of centrifugal force) applied to the rotating shaft can be calculated. The position of the pocket pitch center Op of the cage 14 can be determined by using a roundness measuring device to measure the two-dimensional contour at multiple locations on the pocket inner surface 14c1 of the cage 14, connecting each measured pocket center with a straight line to determine the pocket center axis P, and performing this operation for multiple pockets 14c to find the intersection of the pocket center axes P.

[0034] [Regarding condition (2)] The angle θ formed by a first line segment L1, which starts at the outer diameter center O of the cage 14 and ends at the center of gravity Og of the cage 14, and a second line segment L2, which starts at the outer diameter center O of the cage 14 and ends at the pocket pitch center Op, is ideally set to 180°, as shown in Figure 5. Furthermore, the length D2 of the second line segment L2 is ideally set to coincide with half the guide gap A (D2 = A / 2). Below, the operation of the present invention will be explained assuming that the three points are located at the above ideal positions.

[0035] As the bearing 1 rotates, the cage 14 rotates due to centrifugal force. As shown in FIG. 6, the cage 14 is displaced in the offset direction of the center of gravity Og relative to the outer diameter center O and comes into contact with the outer ring shoulder surface 12b, which is the guide surface of the outer ring 12. As shown in FIG. 7, as the bearing 1 rotates, the cage 14 revolves around the center Or of the outer ring guide surface 12b (hereinafter referred to as the outer ring guide surface center Or) while displacing in the offset direction of the center of gravity Og relative to the outer diameter center O. When the outer peripheral surface 14a of the cage 14 comes into contact with the outer ring shoulder surface 12b due to the displacement of the cage 14, the pocket pitch center Op coincides with the outer ring guide surface center Or. Since the pocket pitch center Op and the outer ring guide surface center Or coincide in this way, the rolling elements 13 are evenly and uniformly distributed in the circumferential direction, thereby suppressing circumferential variation in rigidity between the inner and outer rings. This suppresses the occurrence of runout components due to the cage rotation period during high-speed rotation, thereby reducing the NRRO of the bearing 1.

[0036] However, in a mass-produced cage 14, it is extremely difficult to set the center of gravity Og, outer diameter center O, and pocket pitch center Op to the above-mentioned ideal positions due to problems such as manufacturing errors. In consideration of this, the positions of the center of gravity Og, outer diameter center O, and pocket pitch center Op are set with a certain degree of tolerance as follows.

[0037] (1) The angle θ between the first and second line segments As shown in Fig. 8, the angle θ formed by the first line segment L1 and the second line segment L2 is set to θ ≥ 135°, preferably θ ≥ 160°. The angle θ is set to θ ≤ 180°. If the angle θ is less than 135°, the effect of bringing the pocket pitch center Op closer to the outer ring guideway center Or when the cage 14 is displaced by centrifugal force will be reduced, resulting in an insufficient effect in suppressing NRRO.

[0038] (2) Length of the second line segment The length D2 of the second line segment L2 is set to be equal to or greater than 1 / 4 and equal to or less than 3 / 4 of the guide gap A. If the length D2 of the second line segment L2 is outside the above range, the effect of bringing the pocket pitch center Op closer to the outer diameter center O when the cage 14 is displaced due to centrifugal force is weakened, and the effect of suppressing NRRO becomes insufficient.

[0039] In the above explanation, an outer ring guide type rolling bearing has been given as an example, but the present invention can also be applied to an inner ring guide type rolling bearing in which the inner peripheral surface 14b of the cage 14 contacts the inner ring 11. In the case of this inner ring guide type rolling bearing, the positions of the center of gravity of the cage 14, the bore center of the cage 14 (the center of the inner peripheral surface of the cage 14), and the pocket pitch center of the cage 14 are determined. Specifically, as with the outer ring guide type, the center of gravity, bore center, and pocket pitch center of the cage 14 are each located at different positions on a projection plane obtained by projecting the cage 14 parallel to the axial direction, and the angle formed by a first line segment starting at the bore center of the cage 14 and ending at the center of gravity of the cage and a second line segment starting at the bore center of the cage 14 and ending at the pocket pitch center is set to be 135° (preferably 160°) or more and 180° or less. Also, similar to the outer ring guide type, the length D2 of the second line segment L2 is set to be equal to or greater than 1 / 4 and equal to or less than 3 / 4 of the guide gap A.

[0040] Furthermore, in the above explanation, an angular contact ball bearing 1 has been used as an example of a rolling bearing, but the present invention is not limited to angular contact ball bearings and can also be applied to cages used in other bearings, such as deep groove ball bearings, cylindrical roller bearings, and tapered roller bearings. For example, in a cage for a cylindrical roller bearing, the pocket surface 14c1 is a square cylindrical surface. In this case, the pocket center axis P is determined from the position of the pocket center of each pocket 14c, and the intersection of the pocket center axes P of each pocket 14c is treated as the pocket pitch center Op.

[0041] The rolling bearing 1 described above is suitable for supporting the main shaft of a machine tool, but the application of the bearing 1 is not limited to machine tools, and it can be widely used in automobiles or industrial equipment. [Explanation of symbols]

[0042] 1. Rolling bearings (angular contact ball bearings) 11 Inner Circle 11a Inner raceway surface 11b Inner ring shoulder surface 12 outer ring 12a Outer raceway surface 12b Outer ring shoulder surface 13 Rolling element (ball) 14 Cage 14a Outer surface 14b Inner surface D1 Length of the first line segment D2 Length of the second line segment L1 First line segment L2 Second line segment Og Center of gravity of cage O Cage outer diameter center Op cage pocket pitch center Or outer ring guideway center P Pocket center axis

Claims

1. An outer ring guide type rolling bearing comprising: an inner ring having an inner raceway surface on its outer peripheral surface; an outer ring having an outer raceway surface on its inner peripheral surface; a plurality of rolling elements arranged between the inner raceway surface and the outer raceway surface; and a cage that holds each of the rolling elements, wherein a plurality of pockets that house the rolling elements are formed in the cage, and the outer peripheral surface of the cage comes into contact with the outer ring, The point where the pocket central axes of the pockets intersect is defined as the pocket pitch center, On a projection plane obtained by projecting the cage parallel to the axial direction, the center of gravity of the cage, the outer diameter center of the cage, and the pocket pitch center are located at different positions, and an angle formed by a first line segment starting from the outer diameter center of the cage and ending at the center of gravity of the cage and a second line segment starting from the outer diameter center of the cage and ending at the pocket pitch center is greater than 135° and is equal to or less than 180°; A rolling bearing characterized by:

2. An inner ring guide type rolling bearing comprising an inner ring having an inner raceway surface on its outer peripheral surface, an outer ring having an outer raceway surface on its inner peripheral surface, a plurality of rolling elements arranged between the inner raceway surface and the outer raceway surface, and a cage that holds each of the rolling elements, wherein a plurality of pockets that house the rolling elements are formed in the cage, and the inner peripheral surface of the cage comes into contact with the inner ring, The point where the pocket central axes of the pockets intersect is defined as the pocket pitch center, On a projection plane obtained by projecting the cage parallel to the axial direction, the center of gravity of the cage, the center of the inner diameter of the cage, and the pocket pitch center are located at different positions, and an angle formed by a first line segment starting from the center of the inner diameter of the cage and ending at the center of gravity of the cage and a second line segment starting from the center of the inner diameter of the cage and ending at the pocket pitch center is greater than 135° and is equal to or less than 180°; A rolling bearing characterized by:

3. 3. The rolling bearing according to claim 1, wherein the angle between the first line segment and the second line segment is 160° or greater.

4. 3. The rolling bearing according to claim 1, wherein the length of the second line segment is not less than 1 / 4 and not more than 3 / 4 of the guide clearance.

5. A machine tool having a main spindle supported by the rolling bearing according to claim 1 or 2.

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