Ball bearing

A cage of a ball bearing includes an annular portion located on a first axial side with respect to balls, the first axial side being one side in an axial direction of the ball bearing, and cage bars extending from the annular portion toward a second axial side, the second axial side being the other side in the axial direction of the ball bearing. A space located on a second axial side of the annular portion and defined between any two of the cage bars adjacent to each other in a circumferential direction serves as a pocket that accommodates a corresponding one of the balls. The pocket has a contact surface that is brought into point contact with the ball at an intersection point between a rotational center axis of the ball and a surface of the ball when the cage is displaced toward the second axial side.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2017-019428 filed on Feb. 6, 2017 including the specification, drawings and abstract, is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a ball bearing.

2. Description of the Related Art

A ball bearing is a low torque bearing (with low rotational resistance), and ball bearings with even lower torque are increasingly required in recent years. The ball bearing includes an inner ring, an outer ring, a plurality of balls and an annular cage. A rotational performance of the ball bearing is secured by grease (lubricant) provided in an annular space formed between the inner ring and the outer ring. The cage functions as a separator that separates the balls disposed between the inner ring and the outer ring. The cage is made of steel or resin. A steel cage has a pair of annular members. The balls are sandwiched between the annular members from both sides and the annular members are coupled using rivets, for example (see Japanese Patent Application Publication No. 2014-70669 (JP 2014-70669 A), for example).

As illustrated inFIG. 7, a resin cage has an annular portion91located on a first axial side with respect to balls93and a plurality of cage bars92extending from the annular portion91toward a second axial side. A space located on the second axial side of the annular portion91and defined between two cage bars92adjacent to each other in the circumferential direction serves as a pocket94that accommodates the ball93.

The conventional ball bearing adopts a structure in which a cage90is guided by the balls93and that is referred to as a rolling element guide. In other words, the cage90is positioned by the balls93. The same structure is adopted in the steel cage. In the cage90illustrated inFIG. 7, each of the pockets94has a pocket surface95shaped along a virtual spherical surface with a diameter slightly larger than the diameter of the balls93in order to achieve the rolling element guide structure.

The balls93need to be held by the cage90to adopt the rolling element guide structure. Thus, as described above, the pocket surface95has a shape along a virtual spherical surface with a diameter slightly larger than the diameter of the balls93. Therefore, the pocket surface95facing the ball93is wider compared to the surface of the ball93. A fine clearance is formed between the entire area of the pocket surface95and the surface of the ball93. The entire area of the pocket surface95can be brought into contact with the ball93and thus positions the cage90.

When the ball bearing having such rolling element guide structure rotates, grease (lubricant) is sheared in the fine clearance. When grease is sheared in a wide range between the pocket surface95and the ball93, the resistance (shear resistance) caused by the shearing increases. When the ball bearing rotates, the balls93rotate (revolve) around the inner ring along with the cage90while rotating (turning) about their rotational center axes at high speed. This causes a significant difference in a rotational speed (sliding speed) between the balls93and the cage90, resulting in significantly high shearing speed of grease. As described above, in the conventional ball bearings, shearing of grease generates rotational resistance (running torque). Further, shearing reduces the service life of grease.

SUMMARY OF THE INVENTION

One object of the invention is to suppress shearing of lubricant provided in an annular space formed between an inner ring and an outer ring, thereby reducing rotational resistance and achieving longer service life of the lubricant.

An aspect of the present invention provides a ball bearing that includes an inner ring, an outer ring, a plurality of balls, and an annular cage. The inner ring has an inner raceway groove on an outer peripheral surface of the inner ring. The outer ring has an outer raceway groove on an inner peripheral surface of the outer ring. The balls are interposed between the inner raceway groove and the outer raceway groove. The cage holds the balls. The cage includes an annular portion located on a first axial side with respect to the balls, the first axial side being one side in an axial direction of the ball bearing and a plurality of cage bars extending from the annular portion toward a second axial side, the second axial side being the other side in the axial direction of the ball bearing. A space located on a second axial side of the annular portion and defined between any two of the cage bars adjacent to each other in a circumferential direction serves as a pocket that accommodates the corresponding one of the balls. The pocket has a contact surface that is brought into point contact with the ball at an intersection point between a rotational center axis of the ball and a surface of the ball when the cage is displaced toward the second axial side.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, example embodiments of the invention will be described with reference to the accompanying drawings.FIG. 1is a sectional view of a ball bearing according to an embodiment of the present invention. A ball bearing1includes an inner ring2, an outer ring3, a plurality of balls4, and an annular cage5. The outer ring3is disposed radially outward of the inner ring2. The balls4are interposed between the inner ring2and the outer ring3. The annular cage5holds the balls4. In the ball bearing1, grease is provided as lubricant in an annular space formed between the inner ring2and the outer ring3. In the present embodiment, an axial direction includes a direction parallel to a center line CL of the ball bearing1(hereinafter referred to as “bearing center line CL”), and a radial direction corresponds to a direction perpendicular to the bearing center line CL.

The ball bearing1illustrated inFIG. 1further includes a sealing device6on each end in the axial direction. The sealing device6prevents grease provided inside of the bearing (the annular space) where the balls4and the cage5are disposed from leaking outside. The sealing device6also prevents entry of foreign matter into the bearing.

The inner ring2is an annular member, and an inner raceway groove21is formed in an outer peripheral surface thereof along which the balls4roll. In a longitudinal sectional view illustrated inFIG. 1, the inner raceway groove21has a concaved arc shape with a radius slightly larger than the radius of the balls4. The inner ring2has a first shoulder22on a first axial side of the inner raceway groove21and a second shoulder23on a second axial side of the inner raceway groove21.

The outer ring3is an annular member, and an outer raceway groove31is formed in an inner peripheral surface thereof along which the balls4roll. In a longitudinal sectional view illustrated inFIG. 1, the outer raceway groove31has a concaved arc shape with a radius slightly larger than the radius of the balls4. The outer ring3has a first shoulder32on the first axial side of the outer raceway groove31and a second shoulder33on the second axial side of the outer raceway groove31. A groove39is formed in the inner peripheral surface of the outer ring3at each end in the axial direction. The sealing device6is attached to the groove39. The ball bearing1in the present embodiment is a deep groove ball bearing.

The balls4are interposed between the inner raceway groove21and the outer raceway groove31. Rotation of the ball bearing1(inner ring2) brings the balls4into rolling contact with the inner raceway groove21and the outer raceway groove31. That is, the balls4rotate (revolve) around the bearing center line CL along the inner raceway groove21and the outer raceway groove31while rotating (turning) around their respective rotational center axes CO. In the present embodiment, the rotational center axis CO of each ball4is a linear axis that is parallel to the bearing center line CL and that passes through a center C1of the ball4(hereinafter referred to as “ball center C1”). At the intersection point P of the rotational center axis CO of the ball4and the surface7of the ball4, the peripheral velocity caused by the rotation of the ball4is zero. The ball4is brought into contact with the inner raceway groove21at the deepest point (area S3) thereof and with the outer raceway groove31at the deepest point (area S1) thereof. Each ball4is a steel member made of, for example, bearing steel. The inner ring2and the outer ring3are steel members made of, for example, bearing steel or steel for machine structural use.

FIG. 2is a perspective view illustrating a part of the cage5. As illustrated inFIGS. 1 and 2, the cage5has an annular portion11located on the first axial side with respect to the balls4and a plurality of cage bars13extending from the annular portion11toward the second axial side. The cage5is a so-called snap cage. The cage5in the present embodiment has guide portions14. A space located on the second axial side of the annular portion11and defined between two cage bars13adjacent to each other in the circumferential direction serves as a pocket15that accommodates the ball4. The pockets15are open on the second axial side. The pockets15are arranged in a circumferential direction. The cage5can hold the balls4at intervals in the circumferential direction.

FIG. 3is a sectional view of the inner ring2, the outer ring3, and the cage5. InFIG. 3, the sealing device6is removed. The annular portion11is located between the shoulder22of the inner ring2(seeFIG. 1) and the shoulder32of the outer ring3. The cage bar13extends linearly from a radially outer side portion11bof the annular portion11toward the second axial side. The guide portion14extends from a radially inner side portion11aof the annular portion11toward the second axial side. Although not illustrated, the guide portion14may be configured so as to separate from a portion (base portion) of the cage bar13on the first axial side and project radially inward. The guide portion14is disposed radially inward of the cage bar13and a cutout16is formed between the guide portion14and the cage bar13. The cutout16is formed by cutting out a part of the cage bar13in a recessed shape viewed in section. All cage bars13have the same shape and all guide portions14have the same shape. The cage5is made of resin (synthetic resin) and formed by injection molding. The annular portion11, the cage bars13, and the guide portions14are integrally formed, and the cage5is a single piece.

FIG. 4is a view from an outer peripheral side of the ball4and the cage5.FIG. 4partially shows a section taken along a plane that passes through the ball center C1and that is perpendicular to the virtual line extending in the radial direction of the ball bearing1. The pocket15has a pocket surface50that faces the surface (outer peripheral surface)7of the ball4. As illustrated inFIG. 4, when the center of the pocket15and the ball center C1coincide with each other (hereinafter referred to as “neutral state”), a clearance is formed between the entire pocket surface50and the ball4, which are not in contact with each other. Thus, the cage5can be displaced, with respect to the ball4, from the neutral state toward the first axial side and the second axial side, and also (slightly) in a first circumferential direction and a second circumferential direction. The center of the pocket15is a point on a circumference having the same diameter as that of the pitch circle of the ball4. The cage5having such pocket15with the center described above can be displaced by the same amount with respect to the ball4toward the first axial side and the second axial side, and can be displaced by the same amount with respect to the ball4in the first circumferential direction and the second circumferential direction. As illustrated inFIG. 3, the cage5is placed coaxially with the inner ring2and the outer ring3around the bearing center line CL in the neutral state.

As illustrated inFIG. 3, the guide portion14of the cage5can be brought into contact with the inner raceway groove21. The guide portion14brought into contact with a part of the inner raceway groove21restricts displacement of the cage5in the radial direction (toward the inside and outside in the radial direction) and further restricts displacement of the cage5toward the first axial side. A part of the annular portion11(contact surface55described below) brought into contact with the ball4restricts displacement of the cage5toward the second axial side.

As illustrated inFIG. 1, each sealing device6has a core metal6aand a rubber sealing portion6bfixed to the core metal6a. The outer peripheral portion (radially outside portion) of the sealing portion6bis fitted to the groove39of the outer ring3to attach the sealing device6to the outer ring3. A lip formed in the inner circumference (radially inside portion) of the sealing portion6bis in contact with the outer peripheral surface of the shoulder22(23) of the inner ring2. The sealing device6may have a different structure. For example, although not illustrated, the sealing device6may be configured only by an annular member and may constitute a labyrinth seal between the inner ring2and the annular member.

As illustrated inFIG. 4, the pocket surface50of the pocket15of the cage5has a side surface51, arc surfaces52, and flat surfaces53. The side surface51is located on the second axial side of the annular portion11. The arc surfaces52extend from the side surface51in the opposite directions along the circumferential direction. The flat surfaces53each extend from the corresponding one of the arc surfaces52. The arc surfaces52and the flat surfaces53define the surfaces of the cage bar13in the circumferential direction.FIG. 5is a sectional view taken along the line A-A inFIG. 4, and illustrating a section along a plane including the ball center C1and the bearing center line CL (seeFIG. 1). As illustrated inFIG. 5, the side surface51has a contact surface55, a radially outward facing surface56, and a radially inward facing surface57. The contact surface55is located in the center in the radial direction. The radially outward facing surface56(hereinafter referred to as “outer facing surface56”) extends from the contact surface55in the radially outward direction. The radially inward facing surface57(hereinafter referred to as “inner facing surface57”) extends from the contact surface55in the radially inward direction.

The contact surface55is a planar rectangular surface and is brought into point contact with the ball4when the cage5is displaced toward the second axial side from the neutral state. The contact surface55is brought into contact with the ball4at the intersection point P between the rotational center axis CO of the ball4and the surface7of the ball4. Thus, the contact surface55brought into contact with the ball4restricts displacement of the cage5toward the second axial side. In the neutral state, a fine clearance e1is formed between the intersection point P and the contact surface55.

The outer facing surface56extends from an upper end of the contact surface55in the radially outer direction. The outer facing surface56extends along a sphere with a radius larger than the radius of the ball4and faces the surface7of the ball4. The inner facing surface57extends from a lower end of the contact surface55in the radially inner direction. The inner facing surface57extends along a sphere with a radius larger than the radius of the ball4and faces the surface7of the ball4. As described above, the contact surface55is provided as the center of the pocket15in the rear side surface (the side surface51). Further, the pocket15has the outer facing surface56disposed on a radially outward side of the contact surface55and facing the ball4and the inner facing surface57disposed on a radially inward side of the contact surface55and facing the ball4.

In the neutral state, a clearance e2is formed between the outer facing surface56and the surface7of the ball4, and a clearance e3is formed between the inner facing surface57and the surface7of the ball4. Both clearances e2and e3are larger than the clearance e1formed between the surface7(intersection point P) of the ball4and the contact surface55. The clearances e2and e3do not have to be constant. However, the minimum dimensions of the clearances e2and e3are larger than the clearance e1. Dimensions of the clearances e2and e3are preferably set to a value two times or more and four times or less of the clearance e1. The clearances e2and e3have wide dimensions such that grease is less likely to be sheared.

The outer facing surface56and the inner facing surface57project toward the second axial side with respect to the contact surface55and are close to the ball4. However, the outer facing surface56and the inner facing surface57are not in contact with the ball4in the normal rotating state. That is, even if the cage5is displaced toward the second axial side from the neutral state, neither the outer facing surface56nor the inner facing surface57is brought into contact with the ball4. The cage5can be displaced in the radial direction only by a small amount since the guide portion14restricts displacement of the cage5. In the normal rotating state, even if the cage5is displaced toward the second axial side assisted by a component in the radial direction from the neutral state, neither the outer facing surface56nor the inner facing surface57is brought into contact with the ball4.

As illustrated inFIG. 4, the pocket15is open on the second axial side and has a pair of the flat surfaces53in the opened area on the second axial side. The flat surfaces53are provided facing each other in the circumferential direction with an interval slightly larger than the diameter of the ball4. The flat surfaces53have planar shapes and are arranged in parallel to each other. When the cage5and the ball4relatively approach each other in the circumferential direction from the neutral state, a part of the pocket surface50and the surface7of the ball4are brought into contact with each other. The ball4is brought into contact with a point on the flat surface53(contact point) of the pocket surface50. The arc surfaces52are interposed between the side surface51and the flat surfaces53and are not brought into contact with the ball4. Of all the clearances formed between the pocket15and the ball4in the neutral state, the clearance e1formed between the contact surface55and the ball4is the smallest.

Referring toFIG. 3, the guide portion14of the cage5is further described. The guide portion14has a projection17that projects radially inward toward the inner raceway groove21. A diameter (inner diameter) of the inner peripheral end of the projection17is smaller than the diameter (outer diameter) of the shoulder22of the inner ring2. Thus, the guide portion14is elastically deformed when the cage5is attached to the balls4interposed between the inner ring2and the outer ring3.

The guide portion14is provided to position the cage5. When the inner ring2and the cage5are coaxially arranged (in the neutral state), the projections17of the guide portions14each face the inner raceway groove21with a clearance. When the cage5is displaced in the radial direction, some of the projections17are brought into contact with the inner raceway groove21in the radial direction. This enables the guide portions14to position the cage5in the radial direction. Furthermore, when the cage5is displaced toward the first axial side from the neutral state, the guide portions14are brought into contact with the inner raceway groove21in the axial direction, thereby enabling the guide portions14to position the cage5in the axial direction. Thus, the guide portions14brought into contact with the inner raceway groove21restrict the displacement of the cage5toward the first axial side.

In the present embodiment, each guide portion14is brought into contact with the inner raceway groove21in the non-contact area S2other than the area S3(hereinafter referred to as “contact area S3”) where the ball4is brought into contact with the inner raceway groove21. Further, the guide portion14is brought into contact with the inner raceway groove21at one point on the non-contact area S2. The guide portion14and the inner ring2are brought into line contact or point contact with each other. The non-contact area S2is provided at a position closer to the shoulder22with respect to the contact area S3. The ball4is in contact with the contact area S3when an axial load is not applied to the entire bearing. The ball4is not in contact with the non-contact area S2when the axial load is not applied to the entire bearing. When the axial load is not applied to the entire bearing, the center of the outer raceway groove31in the axial direction, the center of the inner raceway groove21in the axial direction, and the centers of the balls4are aligned in a plane perpendicular to the bearing center line CL.

As described above, in the ball bearing1in the present embodiment, the pocket15(seeFIG. 5) of the cage5has the contact surface55. When the cage5is displaced toward the second axial side, the contact surface55is brought into point contact with the ball4at the intersection point P between the rotational center axis CO of the ball4and the surface7of the ball4. That is, the pocket surface50of the pocket15includes the planar-shaped contact surface55that is brought into point contact with the ball4. The intersection point P between the rotational center axis CO of the ball4and the surface7of the ball4is the contact point with the cage5. In the ball bearing1, when the cage5holding the ball4between the inner ring2and the outer ring3is displaced toward the second axial side, the ball4and the cage5are brought into point contact with each other at a position where the peripheral velocity therebetween becomes zero, thereby positioning the cage5. Thus, the cage5is positioned at an area where the shearing speed of grease between the cage5and the ball4is small. This can suppress the shearing of grease between the ball4and the cage5. Reduction in rotational resistance of the ball bearing1and longer service life of grease can thus be achieved. In the conventional ball bearing (seeFIG. 7), grease is sheared in a wide range between the pocket surfaces95and the balls93, and the balls93rotate (turn) relative to the pocket surfaces95at high rotational speed. This causes the grease to be sheared with a significant speed difference (significant shearing speed). Thus, with the conventional ball bearing (seeFIG. 7), there is a possibility that increased rotational resistance shears the grease significantly, thereby reducing the service life of the grease.

As illustrated inFIG. 3, rotation of the ball bearing1(inner ring2) causes the balls4to rotate while revolving along the inner raceway groove21and the outer raceway groove31. Therefore, the rotational speed difference between the cage5and the inner ring2(outer ring3) is smaller than the rotational speed difference between the cage5and the balls4. Since the rotational speed differences correspond to the shearing speed, grease is less likely to be sheared with smaller rotational speed difference. Thus, in the present embodiment as described above, when the cage5is displaced toward the first axial side, the cage5is brought into contact with the inner ring2(inner raceway groove21) at the guide portions14instead of with the balls4, thereby positioning the cage5. That is, in order to position the cage5that is displaced toward the first axial side, the cage5and the inner ring2are brought into contact with each other, since the rotational speed difference is small therebetween. In the present embodiment, grease is sheared due to the speed difference between the rotational speed of the inner ring2and the rotational speed of the cage5. This rotational speed difference is by far smaller (than the rotational speed difference between the rotational speed of the balls4and the rotational speed of the cage5), which suppresses shearing of grease. The guide portion14and the inner ring2are brought into line contact or point contact with each other. The small contact range can further suppress shearing of grease. This contributes to reducing rotational resistance and achieving longer service life of grease.

As described above, of all the clearances formed between the pocket15and the ball4in the neutral state (seeFIG. 4), the clearance e1formed between the contact surface55and the ball4is the smallest. In the neutral state, the smallest dimension between the pocket surface50and the ball4is the dimension between the contact surface55and the ball4. Thus, of the clearances formed between the pocket15and the ball4, grease is less likely to be sheared in the area other than the clearance between the contact surface55and the ball4. This further contributes to reducing rotational resistance and achieving longer service life of grease.

Even if the cage5in the present embodiment is excessively deformed for some reason, the deformation causes a part of the cage5to be brought into contact (touched down) with the ball4or the inner ring2(or the outer ring3) (at the contact surface55and at other portions). This configuration allows obstruction of the excessive deformation. Excessive deformation of the cage5may break the cage5. In order to obstruct such deformation, in the present embodiment (seeFIG. 5), the pocket15has the inner facing surface57facing the ball4as described above. In the neutral state, the large clearance e3is formed between the inner facing surface57and the ball4. The clearance e3is larger than the clearance e1formed between the ball4and the contact surface55in the neutral state. When the cage5is about to be excessively deformed, this configuration obstructs the deformation by bringing the inner facing surface57and the ball4into contact with each other, thereby preventing the cage5from breaking. During normal rotation (when excessive deformation is not generated), the large clearance e3formed between the ball4and the inner facing surface57prevents grease from being sheared, thereby making grease less likely to be sheared. When the cage5is about to be excessively deformed, the outer facing surface56can be brought into contact with the ball4depending on the deformation mode. This allows obstruction of excessive deformation.

In the present embodiment, the guide portion14is brought into contact with a part of the inner ring2(inner raceway groove21). Thus, the entire cage5is easily disposed at a position closer to the inner ring2, thereby enabling reduction in resistance generated by grease sheared by the cage5. That is, when the ball bearing1rotates, grease in the annular space between the inner ring2and the outer ring3is likely to be gathered at a position closer to the outer ring3by a centrifugal force. Therefore, providing the cage5at a position closer to the inner ring2makes the cage5less likely to shear grease. This enables reduction in resistance caused by shearing of grease.

In the above-described embodiment (seeFIG. 3for example), the guide portion14of the cage5is brought into contact with the inner ring2(inner raceway groove21). However, as illustrated inFIG. 6, the guide portion14of the cage5may be brought into contact with the outer ring3(outer raceway groove31). In this case, the guide portion14may be provided at a position closer to the outer ring3with respect to the cage bar13. The guide portion14is brought into contact with the outer raceway groove31in the non-contact area S2other than the area S1where the ball4is brought into contact with the outer raceway groove31. This configuration is preferably used to position the cage5.

As described above according to the embodiments, the cage5has a guide portion14. When the cage5is displaced toward the first axial side from the neutral state, the guide portion14is brought into contact with the inner ring2(inner raceway groove21) (as illustrated inFIG. 3) or with the outer ring3(outer raceway groove31) (as illustrated inFIG. 6), thereby positioning the cage5. As illustrated inFIG. 6, even in the case where the guide portion14is brought into contact with the outer ring3(outer raceway groove31), the pocket of the cage5has a contact surface55that is brought into point contact with the ball4when the cage5is displaced toward the second axial side. The contact surface55is brought into point contact with the ball4at the intersection point P between the rotational center axis CO of the ball4and the surface7of the ball4, thereby positioning the cage5. Thus, the cage5is positioned in an area with a smaller shearing speed of grease between the cage5and the ball4. As a result, shearing of grease between the ball4and the cage5can be supressed, thereby achieving reduction in rotational resistance of the ball bearing1and longer service life of grease.

The embodiments described above are to be considered as illustrative and not restrictive in all respects. That is, the ball bearing according to the invention is not limited to the configurations or structures illustrated in the drawings but may have any other configuration or structure within the scope of the invention. In the above-described embodiment, the case has been described where the ball bearing is a deep groove ball bearing. However, the ball bearing may be an angular contact ball bearing. In the case of the angular contact ball bearing, a rotational center axis of each ball is inclined. A cage may be configured such that a contact surface of each pocket surface of the cage is brought into point contact with the ball at an intersection point between the inclined rotational center axis and a surface of the ball.

According to the invention, shearing of lubricant between the ball and the cage can be supressed, thereby achieving reduction in rotational resistance of the ball bearing and longer service life of the lubricant.