Method for producing track ring member, method for producing rolling bearing, method for producing hub unit bearing, and method for producing vehicle

A centerless grinding processing in which a sliding contact surface (10) is subjected to grinding by pressing a grindstone against the sliding contact surface (10) of an inner ring (3) while rotating the inner ring (3) relative to the grindstone in a prescribed direction, and then, in the centerless grinding process, a finishing step in which grinding lines in a direction intersecting grinding lines formed on the sliding contact surface (10) are formed on the sliding contact surface (10) and/or processing for improving the surface roughness of the sliding contact surface (10) is performed is performed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No. PCT/JP2019/046946, filed Dec. 2, 2019, claiming priority to Japanese Patent Application No. 2019-015850, filed Jan. 31, 2019, the contents of all of which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a method for producing an inner ring and an outer ring constituting a rolling bearing and a track ring member such as a hub constituting a hub unit bearing.

BACKGROUND ART

Rolling bearings such as ball bearings, roller bearings, and tapered roller bearings are incorporated in rotation support portions of various mechanical devices. Rolling bearings include an outer ring having an outer ring track on an inner circumferential surface, an inner ring having an inner ring track on an outer circumferential surface, and a plurality of rolling elements rotatably disposed between the outer ring track and the inner ring track. Furthermore, when the rolling bearing includes a sealing device, the grease sealed in an internal space having the rolling elements installed therein is prevented from leaking to the outside and foreign matter such as rainwater, mud, and dust is prevented from entering the internal space. The sealing device has at least one seal lip having a distal end portion in contact with a sliding contact surface formed on the inner ring in a slidable manner over the entire circumference.

In order to keep the sliding torque of a distal end portion of the seal lip with respect to the sliding contact surface low during relative rotation between the outer ring and the inner ring, a portion of the inner ring including at least the sliding contact surface is subjected to grinding using a grindstone. As a method for grinding a portion of the inner ring including at least the sliding contact surface, for example, the centerless grinding method described in Japanese Unexamined Patent Application, First Publication No. 2001-138186 is known. That is to say, in a state in which the inner ring is supported from below by means of a work rest, when a rotary grindstone is pressed against an outer circumferential surface of the inner ring from a side substantially opposite to that of the adjustment wheel in a radial direction while the inner ring rotates by pressing a rotating adjustment wheel against an outer circumferential surface of the inner ring, the outer circumferential surface of the inner ring is subjected to grinding.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

When the outer circumferential surface of the inner ring is subjected to grinding using a grindstone, as the grindstone rotates relative to the inner ring, abrasive grains in the grindstone come into contact with the outer circumferential surface of the inner ring. As the grindstone rotates relative to the inner ring, the abrasive grains move in a circumferential direction while being pressed against the outer circumferential surface of the inner ring. Thus, a metal present on the outer circumferential surface of the inner ring is removed using the abrasive grains. After that, as the grindstone rotates relative to the inner ring, the abrasive grains are separated from the outer circumferential surface of the inner ring. Therefore, when the inner ring is subjected to grinding by pressing a rotary grindstone having a certain degree of hardness against the outer circumferential surface of the inner ring while the inner ring is rotating relative to the grindstone in a prescribed direction, deformation such as ridges and burrs occurs on both sides of a portion through which the abrasive grains have passed. A tendency (directivity) exists in such deformation in a circumferential direction. For this reason, assuming that a portion in which deformation exists is a sliding contact surface with which the distal end portion of the seal lip is in contact in a slidable manner, a difference in sliding torque of the distal end portion of the seal lip with respect to the sliding contact surface between a first relative rotation direction and a second relative rotation direction of the outer ring and the inner ring, that is, a difference in rotational torque (a difference in rotational torque between forward rotation and reverse rotation) of the inner ring with respect to the outer ring is likely to be a significant difference. To be specific, assuming that the outer ring is fixed, the sliding torque of the distal end portion of the seal lip with respect to the sliding contact surface is likely to be different (a difference occurs in sliding torque) between a case in which the inner ring is rotated in a prescribed direction (first relative rotation) and a case in which the inner ring is rotated in a direction opposite to the prescribed direction (second relative rotation). One reason for this is thought to be that the sliding torque of the distal end portion of the seal lip with respect to the sliding contact surface is different depending on whether the sliding direction of the distal end portion of the seal lip with respect to the sliding contact surface is the same as the moving direction of the grindstone with respect to the inner ring when the inner ring is subjected to grinding by means of the grindstone.

According to the present invention, for example, an object of the present invention is to realize a method for producing a track ring member having excellent rotational characteristics such as being able to minimize a difference in sliding torque of a distal end portion of a seal lip with respect to a sliding contact surface due to a relative rotation direction of a pair of track ring members.

Solution to Problem

An aspect of a method for producing a track ring member of the present invention is to produce a track ring member having a sliding contact surface with which a distal end portion of a seal lip is in contact in a slidable manner over the entire circumference, including: performing a centerless grinding process in which the sliding contact surface is subjected to grinding by pressing a grindstone against the sliding contact surface while rotating the track ring member relative to the grindstone in a prescribed direction, and then performing a finishing step in which a large number of grinding lines in irregular directions are formed on the sliding contact surface and/or processing for ameliorating a surface roughness of the sliding contact surface is performed.

The finishing step can be performed by subjecting the sliding contact surface to polishing processing using an abrasive grain-containing brush, a non-woven fabric abrasive, or a polishing tape (a wrapping film), polishing processing using an elastic grindstone, or super-finishing processing.

An aspect of a method for producing a rolling bearing of the present invention has, as a target, a rolling bearing which includes: a first track ring member; a second track ring member having a surface having a sliding contact surface over the entire circumference and disposed coaxially with the first track ring member; a plurality of rolling elements rotatably disposed between the first track ring member and the second track ring member; and a sealing device having at least one seal lip having a distal end portion in contact with the sliding contact surface in a slidable manner.

In an aspect of the method for producing a rolling bearing of the present invention, the second track ring member is produced using the method for producing a track ring member of the present invention.

One of the first track ring member and the second track ring member can be set as an outer ring, and the other of the first track ring member and the second track ring member can be set as an inner ring coaxially disposed with the outer ring on an inner diameter side of the outer ring. It is preferable that the first track ring member be set as the outer ring and the second track ring member be set as the inner ring. Here, the first track ring member may be set as the inner ring and the second track ring member may be set as the outer ring.

An aspect of a method for producing a hub unit bearing of the present invention has, as a target, a hub unit bearing which includes: an outer diameter side track ring member having a double-row outer ring track on an inner circumferential surface; an inner diameter side track ring member having a surface having a sliding contact surface over the entire circumference and having a double-row inner ring track on an outer circumferential surface; a plurality of rolling elements rotatably disposed between the double-row outer ring track and the double-row inner ring track; and a sealing device having at least one seal lip having a distal end portion in contact with the sliding contact surface in a slidable manner, in which one of the outer diameter side track ring member and the inner diameter side track ring member is a fixed side track ring member supported and fixed to a suspension device, and the other of the outer diameter side track ring member and the inner diameter side track ring member is a rotation side track ring member which rotates together with a wheel and a braking rotating body.

In an aspect of the method for producing a hub unit bearing of the present invention, the inner diameter side track ring member is produced using the method for producing a track ring member of the present invention.

An aspect of a method for producing a vehicle of the present invention has, as a target, a vehicle including a hub unit bearing and the hub unit bearing is produced using the method for producing a hub unit bearing of the present invention.

An aspect of the present invention is a method for producing a rolling bearing using centerless grinding, in which the rolling bearing has an inner ring, an outer ring, a plurality of rolling elements, a seal lip, and a sliding contact surface which is provided on the inner ring or the outer ring and with which a distal end of the seal lip is in contact in a slidable manner, the method including: subjecting the sliding contact surface to centerless grinding while pressing a grindstone against a portion of the inner ring or the outer ring including the sliding contact surface; and subjecting the sliding contact surface to a process for eliminating directivity of surface roughness generated on the sliding contact surface using centerless grinding.

An aspect of the present invention is a method for producing a hub unit bearing using centerless grinding in which the hub unit bearing has an outer ring, a hub, a plurality of rolling elements, a seal lip, and a sliding contact surface which is provided on the outer ring or the hub and with which a distal end of the seal lip is in contact in a slidable manner, the method including: subjecting the sliding contact surface to centerless grinding while pressing a grindstone against a portion of the outer ring or the hub including the sliding contact surface; and subjecting the sliding contact surface to a process for eliminating directivity of surface roughness generated on the sliding contact surface using centerless grinding.

Advantageous Effects of Invention

According to an aspect of the present invention, for example, it is possible to realize a method for producing a track ring member having excellent rotational characteristics such as being able to minimize a difference in sliding torque of a distal end portion of a seal lip with respect to a sliding contact surface due to a relative rotation direction of a pair of track ring members.

DESCRIPTION OF EMBODIMENTS

First Example of Embodiment

A first example of an embodiment of the present invention will be described with reference toFIGS.1to4.FIG.1shows a rolling bearing1which is a target of this example. The rolling bearing1includes an outer ring2which is a first track ring member, an inner ring3which is a second track ring member, a plurality of rolling elements4, and a pair of sealing devices (seal rings)5.

The outer ring2is made of a hard iron-based alloy such as bearing steel or carbon-immersed steel, has an outer ring track6having a circular arc-shaped cross section over the entire circumference on an inner circumferential surface of a central portion in an axial direction, and has locking concave grooves7recessed outward in a radial direction in inner circumferential surfaces on both side portions in the axial direction over the entire circumference.

The inner ring3is made of a hard iron-based alloy such as bearing steel or carbon-immersed steel and is disposed coaxially with the outer ring2on an inner diameter side of the outer ring2. The inner ring3has an inner ring track8having a circular arc-shaped cross section over the entire circumference on an outer circumferential surface of a central portion in the axial direction and has seal grooves9recessed inward in the radial direction in outer circumferential surfaces on both side portions in the axial direction over the entire circumference. Furthermore, the inner ring3has a circular ring-shaped sliding contact surface10on side surfaces of inner surfaces of the seal grooves9directed in directions opposite to each other (outward in the axial direction) over the entire circumference. In this example, the sliding contact surface10has a large number of grinding lines formed in irregular directions and/or an arithmetic average roughness Ra of the sliding contact surface10is 0.1 μm or less.

With regard to the rolling bearing1, an inside thereof in the axial direction is referred to as a central side of the rolling bearing1in a width direction and an outside thereof in the axial direction is referred to as an outside (both sides) of the rolling bearing1in the width direction.

Each of the rolling elements4is made of an iron-based alloy such as bearing steel or ceramics and is rotatably disposed between the outer ring track6and the inner ring track8in the state of being held by means of a retainer11. In this example, balls are used as the rolling elements4.

Each of the sealing devices5closes opening portions on both sides of an internal space12having the rolling elements4disposed therein in the axial direction to prevent the grease sealed in the internal space12from leaking to the outside and to prevent foreign matters such as rainwater, mud, and dust from entering the internal space12. Each of the sealing devices5includes an annular core metal13and an elastic material14made of an elastomer such as rubber reinforced using the core metal13. Each of the sealing devices5can be produced by disposing the core metal13in a cavity of a mold and then mould-forming a material constituting the elastic material14into the core metal13. As the material constituting the elastic material14, for example, nitrile rubber, acrylic rubber, silicon rubber, fluororubber, ethylene propylene-based rubber, hydrogenated nitrile rubber, and the like can be used.

The core metal13has a substantially L-shaped cross-sectional shape by bending and forming a metal sheet such as a mild steel sheet and is formed in an annular shape as a whole. That is to say, the core metal13includes a cylindrical portion15and an annular portion16bent at a right angle from an axially outer end portion of the cylindrical portion15toward an inner side in the radial direction.

The elastic material14includes an elastic locking portion17existing at a radially outer end portion, a thin-walled circular ring covering portion18which covers an axial outer surface of the annular portion16over the entire circumference, and a seal portion19existing at a radially inner end portion.

The elastic locking portion17has a width dimension slightly larger than a width dimension (an axial dimension) of each of the locking concave grooves7in a free state before the elastic locking portion17is locked to the locking concave groove7of the outer ring2and covers an outer circumferential surface and a distal end surface (an axially inner end surface) of the cylindrical portion15of the core metal13.

The seal portion19includes a seal lip20, a grease lip21, and a dust lip22.

The seal lip20is formed to further protrude radially inward and axially inward than a radially inner end portion of the annular portion16of the core metal13and has a distal end portion in contact with the sliding contact surface10of each of the seal grooves9in a slidable manner over the entire circumference.

The grease lip21has a substantially triangular cross-sectional shape and is formed to further protrude inward in the axial direction from a portion of the seal portion19located at an outer side in the radial direction than the seal lip20. When a distal end portion of the grease lip21is brought close to and to face a connection portion of the outer circumferential surface of the inner ring3between the inner ring track8and the seal groove9, the grease lip21constitutes a labyrinth seal between the portion and a distal end portion of the grease lip21.

The dust lip22has a substantially rectangular cross-sectional shape and is formed to extend inward in the radial direction from the radially inner end portion of the circular ring covering portion18. When a distal end portion of the dust lip22is brought close to and to face a portion of the inner ring3existing at a further outer side in the axial direction than the seal groove9, the dust lip22constitutes a labyrinth seal between the portion and the distal end portion of the dust lip22.

Each of the sealing devices5supports the elastic locking portion17with respect to the outer ring2by disposing (locking) the elastic locking portion17inside the locking concave groove7while elastically compressing the elastic locking portion17in the axial direction and the radial direction and brings the distal end portion of the seal lip20into contact with the sliding contact surface10of the seal groove9in a slidable manner over the entire circumference with a tightening allowance.

When the inner ring3constituting the rolling bearing1is produced, first, an exterior form of the inner ring3is formed by subjecting a material made of a metal to plastic processing such as forging processing or cutting processing.

The centerless grinding process which will be described later can be performed, for example, using a grinding machine24as shown inFIG.2. The grinding machine24includes a grindstone23, an adjustment wheel25, a work rest26, and a grinding fluid nozzle27. In the centerless grinding process, the outer circumferential surface of the inner ring3is subjected to grinding processing by pressing the grindstone23against a portion of the outer circumferential surface of the inner ring3including the sliding contact surface10while rotating the inner ring3in one direction.

The grindstone23is a rotary grindstone which has a disk shape, has a generatrix shape along a generatrix shape of the outer circumferential surface of the inner ring3, and rotates about a central axis (not shown). Examples of the grindstone23include grindstones formed by bonding A (alumina)-based abrasive grains with a glass-based bond, having a bonding particle size of #60 to #400, a degree of bonding of G to O, a degree of concentration of 4 to 12, and a porosity of 20% to 50%.

The adjustment wheel25can be rotationally driven about the central axis inclined at a prescribed angle with respect to a central axis of the grindstone23. The illustrated grinding machine24further includes a cleaning device28for removing foreign matters such as grinding debris adhering to an outer circumferential surface of the adjustment wheel25during grinding processing.

The work rest26is disposed below a portion between the grindstone23and the adjustment wheel25and supports the inner ring3from below.

The grinding fluid nozzle27is disposed above a portion between the grindstone23and the adjustment wheel25so that a grinding fluid discharge port is directed downward.

When the centerless grinding process is performed using the grinding machine24, first, the inner ring3which is a work piece is placed between an upper surface of the work rest26and the outer circumferential surface of the adjustment wheel25. Subsequently, a grinding fluid is discharged toward the outer circumferential surface of the inner ring3through the grinding fluid nozzle27. Moreover, when the grindstone23rotating about the central axis of the grindstone23itself is pressed against the portion of the outer circumferential surface of the inner ring3including the sliding contact surface10while rotating the inner ring3in one direction (counterclockwise in the illustrated example) by rotatably driving the adjustment wheel25, the portion is subjected to grinding processing. In this case, when a circumferential speed of the inner ring3and a circumferential speed of the grindstone23are made different from each other, the inner ring3is rotated relative to the grindstone23in a prescribed direction (one direction).

In this way, in the centerless grinding process, the outer circumferential surface of the inner ring3is subjected to grinding processing by pressing a hard grindstone23against the outer circumferential surface of the inner ring3while rotating the inner ring3relative to the grindstone23in a prescribed direction. For this reason, deformation such as ridges and burrs is likely to occur on both sides of a portion of the sliding contact surface10of the inner ring3obtained using centerless grinding process through which abrasive grains of the grindstone23has passed.

In a subsequent finishing step, the sliding contact surface is subjected to a process for eliminating the directivity of the surface roughness generated on the sliding contact surface10using centerless grinding. For example, in the finishing step, a large number of grinding lines in irregular directions are formed on the sliding contact surface10and/or process for improving the surface roughness of the sliding contact surface10is performed on the sliding contact surface10. The directionality of the surface roughness is based on a relative moving direction of the sliding contact surface10with respect to the grindstone23in the centerless grinding. When the surface roughness of the sliding contact surface10is improved, specifically, an arithmetic average roughness Ra of the sliding contact surface10is set to 0.1 μm or less. When a large number of grinding lines in irregular directions are formed on the sliding contact surface10, it is also possible to improve the surface roughness of the sliding contact surface10.

The finishing step can be performed, for example, as shown inFIG.3, using an elastic grindstone29. The elastic grindstone29has a disk shape and has a generatrix shape along the generatrix shape of the seal groove9. Furthermore, the elastic grindstone29has more pores and is softer (has appropriate elasticity) than the grindstone23used in the centerless grinding process. Specific examples of the elastic grindstone29include elastic grindstones formed by bonding A (alumina)-based abrasive grains with a resin-based bond, having a bonding particle size of #600 to #1200, a degree of bonding of F to T, a degree of concentration of 6 to 14, and a porosity of 40% to 80%, and softer than the grindstone23used in the centerless grinding process.

When the finishing step is performed using the elastic grindstone29, the inner ring3is rotated about the central axis of the inner ring3. Moreover, when a surface of a radially outer portion of the elastic grindstone29rotated about a central axis of the elastic grindstone29itself in the radial direction is pressed against each of the seal grooves9of the inner ring3and an inner surface of each of the seal grooves9including the sliding contact surface10is subjected to grinding processing, the surface roughness of the sliding contact surface10is improved. That is to say, the unevenness due to the ridges and the burrs formed in the centerless grinding process is reduced.

Alternatively, the finishing step can be performed using an abrasive grain-containing brush30as shown inFIG.4, examples of the abrasive grain-containing brush30include abrasive grain-containing brushes made of a wire material having appropriate elasticity obtained by mixing nylon fibers such as nylon 6 with A (alumina)-based or GC (green silicon carbide)-based abrasive grains having a particle size of #80 to #600.

When the finishing step is performed using the abrasive grain-containing brush30, the inner ring3is rotated about the central axis of the inner ring3while the abrasive grain-containing brush30is pressed against each of the seal grooves9of the inner ring3. Thus, a large number of grinding lines in irregular directions are formed on the sliding contact surface10by subjecting the inner surface of each of the seal grooves9including the sliding contact surface10to polishing processing.

The finishing step is not limited to the method in which the elastic grindstone29or the abrasive grain-containing brush30is utilized, the finishing step is not particularly limited as long as a large number of grinding lines in irregular directions are formed on the sliding contact surface10, and/or the finishing step can improve the surface roughness of the sliding contact surface10. To be specific, for example, when the sliding contact surface10is polished by means of a non-woven fabric abrasive obtained by adhering A (alumina)-based or GC (green silicon carbide)-based abrasive grains having a particle size of #80 to #600 to a nylon non-woven fabric, it is possible to form a large number of grinding lines in irregular directions on the sliding contact surface10. Alternatively, the sliding contact surface10is polished by means of a polishing tape (a wrapping film) having a surface coated with abrasive grains or the sliding contact surface10is subjected to a super-finishing processing in which a reciprocating grindstone is pressed, the surface roughness of the sliding contact surface10may be improved.

Even when the finishing step is performed by means of any method, the pair of seal grooves9can be processed at the same time or the pair of seal grooves9can be processed separately (in order).

Also, before, after, or at the same time as the finishing step, the inner ring3is completed by subjecting the inner ring track8to a super-finishing processing for improving the surface roughness of the inner ring track8or, if necessary, performing a heat treatment such as quenching at an appropriate timing. Moreover, the rolling bearing1is obtained by combining the inner ring3with the outer ring2, the rolling elements4, the pair of sealing devices5, and the retainer11. For example, conventionally known methods can be applied to the method for producing the outer ring2, the rolling elements4, the pair of sealing devices5, and the retainer11, and the method for assembling the rolling bearing1.

Since the finishing step is performed after the centerless grinding process is performed in this example, a large number of grinding lines in irregular directions can be formed on the sliding contact surface10and/or the surface roughness of the sliding contact surface10can be improved. Therefore, according to this example, a difference in sliding torque of the distal end portion of the seal lip20with respect to the sliding contact surface10between the relative rotation of the inner ring3with respect to the outer ring2in a first direction and the relative rotation thereof in a second direction opposite to the first direction can be kept small as compared with a structure as it is subjected to the centerless grinding processing.

That is to say, when a large number of grinding lines in irregular directions are formed on the sliding contact surface10in the finishing step, an uneven shape of the sliding contact surface10becomes complicated or the distal end portion of the seal lip20does not easily enter a concave portion. The directivity of the surface roughness generated on the sliding contact surface10using centerless grinding is eliminated. As a result, a difference in sliding torque of the distal end portion of the seal lip20with respect to the sliding contact surface10due the relative rotation direction of the inner ring3with respect to the outer ring2can be reduced.

On the other hand, when the surface roughness of the sliding contact surface10is improved in the finishing step, that is, when the unevenness of the sliding contact surface10is reduced, the relative rotation of the inner ring3with respect to the outer ring2is less affected by deformation due to ridges and burrs formed in the centerless grinding process on the distal end portion of the seal lip20. The directivity of the surface roughness generated on the sliding contact surface10using centerless grinding is eliminated. As a result, a difference in sliding torque of the distal end portion of the seal lip20with respect to the sliding contact surface10due to the relative rotation direction of the inner ring3with respect to the outer ring2can be reduced. The rolling bearing in which the directivity of the surface roughness generated on the sliding contact surface10is eliminated has high rotational characteristics and is preferably applied to rotating components used under both forward and reverse rotation conditions.

Although the method for producing a rolling bearing in this example has, as a target, a rolling bearing (ball bearing)1in which balls are used as the rolling elements4, the method for producing a rolling bearing of the present invention can also have, as a target, a needle bearing, a cylindrical roller bearing, and a tapered roller bearing. Furthermore, the present invention is not limited to a single-row rolling bearing, can also have, as a target, a multi-row rolling bearing including multiple rows, and can also have, as a target, a structure in which the distal end portion of the seal lip constituting the sealing device is in contact with the sliding contact surface included in the outer ring in a slidable manner. Furthermore, the present invention is not limited to a radial rolling bearing and can also have, as a target, a thrust rolling bearing as long as the thrust rolling bearing includes a sealing device.

Second Example of Embodiment

A second example of the embodiment of the present invention will be described with reference toFIGS.5to9. This example has, as a target, a hub unit bearing which rotatably supports a wheel and a braking rotating body with respect to a suspension device for a vehicle. As shown inFIG.5, a hub unit bearing31rotatably supports a hub33which is an inner diameter side track ring member and a rotation side track ring member inside an outer ring32which is an outer diameter side track ring member and a fixed side track ring member via a plurality of rolling elements34aand34b.

The outer ring32is made of a hard metal such as medium carbon steel and includes a double-row outer ring tracks35aand35band a stationary flange36. The double-row outer ring tracks35aand35bare formed on the inner circumferential surface of the axially intermediate portion of the outer ring32over the entire circumference. The stationary flange36is formed to protrude outward in a radial direction of the axial intermediate portion of the outer ring32and has support holes37which are screw holes at a plurality of places of the radially intermediate portion in a circumferential direction. The outer ring32is supported and fixed to the knuckle38by screwing bolts40configured to pass through through holes39formed in a knuckle38constituting the suspension device of the vehicle into the support holes37of the stationary flange36from the inside in the axial direction and further tightening the bolts40.

The inside of the hub unit bearing31in the axial direction is referred to as the right side ofFIGS.5and6which is a center side of a vehicle body in a state in which the hub unit bearing31is assembled to an automobile. On the other hand, the outside thereof in the axial direction is referred to as the left side ofFIGS.5and6which is an outer side of the vehicle body in a state in which the hub unit bearing31is assembled to an automobile.

Thu hub33is disposed coaxially with the outer ring32on the inner diameter side of the outer ring32and includes a double-row inner ring tracks41aand41band a rotary flange42. The double-row inner ring tracks41aand41bare formed on a portion of the outer circumferential surface of the hub33facing the double-row outer ring tracks35aand35bover the entire circumference. The rotary flange42is formed to protrude outward in the axial direction at a portion of the hub33located further outward in the axial direction than an axially outer end portion of the outer ring32and has attachment holes43configured to pass through the radially intermediate portion in the axial direction at a plurality of places of the radially intermediate portion in the circumferential direction. In this example, in order to join and fix a braking rotating body44such as a disc and a drum to the rotary flange42, serration portions formed on portions near a base end of a stud45are press-fitted into the attachment holes43and an intermediate portion of the stud45is press-fitted into through holes46formed in the braking rotating body. Furthermore, in order to fix a wheel47constituting the wheel to the rotary flange42, a nut49is screwed into a male threaded portion formed on a distal end portion of the stud45and further tightened in a state in which the male threaded portion has passed through a through hole48formed in the wheel47. Since the hub unit bearing31in this example is for a drive wheel, an engagement hole61configured to engage with a drive shaft (not shown) exists at a central portion of the hub33.

In this example, the hub33is constituted by joining and fixing a hub main body50made of a hard metal such as medium carbon steel and having an inner ring track41aon the outer side in the axial direction and an inner ring51made of a hard metal such as bearing steel and having an inner ring track41bon an inner side in the axial direction to each other. To be specific, when an axially inner end surface of the inner ring51is pressed using a staking portion53obtained by elastically deforming an axially inner end portion of a cylindrical portion52existing at an axial inner end portion of the hub main body50outward in the radial direction in a state in which the inner ring51is externally fitted to an axially inner portion of the hub main body50, the hub main body50and the inner ring51are joined and fixed. The hub33in this example has a sliding contact surface54over the entire circumference within a range from the radially inner end portion of the axially inner surface of the rotary flange42to a portion of the outer circumferential surface existing further outward in the axial direction than the inner ring track41aon the outer side in the axial direction. The sliding contact surface54has a large number of grinding lines formed in irregular directions and/or has an arithmetic average roughness Ra of 0.1 μm or less.

The rolling elements34aand34bare each made of a hard metal such as bearing steel or ceramics and a plurality of rolling elements34aand34bare rotatably disposed between the double-row outer ring tracks35aand35band the double-row inner ring tracks41aand41bwhile being held using retainers55aand55b. With such a constitution, the hub33is rotatably supported on the inner diameter side of the outer ring32. In this example, balls are used as the rolling elements34aand34b.

Also, in the hub unit bearing31in this example, an axially inner opening portion of an internal space56having the rolling elements34aand34bdisposed therein is closed by means of a combination seal ring57and an axially outer opening portion of the internal space56is closed by means of a sealing device58. Thus, the grease sealed in the internal space56is prevented from leaking to the outside and foreign matters such as rainwater, mud, and dust are prevented from entering the internal space56.

The combination seal ring57is obtained by bring distal end portions of a plurality of seal lips constituting a seal ring60which is internally fitted and fixed to the axially inner end portion of the outer ring32into contact with a slinger59which is externally fitted and fixed to the axially inner end portion of the hub33in a slidable manner.

The sealing device58includes an annular core metal62and an elastic material63made of an elastomer or the like such as rubber reinforced using the core metal62.

The core metal62has a substantially L-shaped cross sectional shape formed by bending and forming a metal sheet such as mild steel sheet and is constituted to have an annular shape as a whole. That is to say, the core metal62includes a fitting cylinder portion64which has a cylindrical shape and is internally fitted and fixed to the axially outer end portion of the outer ring32and a bent portion65which is bent inward in the radial direction from an axially outer end portion of the fitting cylinder portion64.

The elastic material63includes a base portion66and a plurality of (three in the illustrated example) seal lips67. The base portion66is fixed to the axially outer surface and the radially inner end portion of the bent portion65of the core metal62through vulcanization adhesion to cover the axially outer surface and the radially inner end portion. Each of the seal lips67extends from the base portion66toward the sliding contact surface54of the hub33and has a distal end portion in contact with the sliding contact surface54in a slidable manner over the entire circumference.

In this example, when the hub main body50constituting the hub33of the hub unit bearing31is produced, first, an exterior form of the hub main body50is formed by subjecting a material made of a metal to forging processing, cutting processing, or the like.

In the subsequent centerless grinding process, as shown inFIG.7, the outer circumferential surface of the hub main body50is subjected to grinding processing by pressing a grindstone68against a portion of an outer circumferential surface of the hub main body50including the sliding contact surface54while the hub main body50is rotated relative to a grindstone68in a prescribed direction.

The grindstone68in this example is a so-called general type rotary grindstone having a generatrix shape along a generatrix shape of the outer circumferential surface of the hub main body50in a range from the sliding contact surface54to the axially inner end portion of the cylindrical portion52. The grindstone68is, for example, formed by bonding A (alumina)-based abrasive grains with a glass-based bond and can include grindstones having a bonding particle size of #60 to #400, a degree of bonding of G to O, a degree of concentration of 4 to 12, and a porosity of 20% to 50%.

When the centerless grinding process is performed using the grindstone68, first, a magnetic chuck69is joined to the axially outer surface of the rotary flange42through magnetic attraction. When the magnetic chuck69is rotated, the hub main body50is rotated. Furthermore, the hub main body50is positioned in the radial direction by supporting the outer circumferential surface of the intermediate portion of the hub main body50in the axial direction using a pair of shoes70(only one is shown inFIG.6). Moreover, when an outer circumferential surface of the grindstone68is pressed against a portion of the outer circumferential surface of the hub main body50including the sliding contact surface54while the grindstone68which is a general type rotary grindstone is rotated about a central axis O68of the grindstone68itself, the portion is subjected to grinding processing. A generatrix shape of the grindstone68is appropriately adjusted using a general type rotary dresser71to have a shape along the generatrix shape of the hub main body50which has been subjected to the centerless grinding process. Furthermore, deformation such as ridges and burrs is likely to occur on both sides of a portion of the sliding contact surface54of the hub main body50obtained using centerless grinding process through which the abrasive grains in the grindstone68has passed.

In the subsequent finishing step, the sliding contact surface54is subjected to a process for eliminating the directivity of the surface roughness generated on the sliding contact surface54using centerless grinding. For example, a large number of grinding lines in irregular directions are formed on the sliding contact surface54and/or a process for improving the surface roughness of the sliding contact surface54is performed.

For example, as shown inFIG.8, in the finishing step, the sliding contact surface54of the hub main body50can be subjected to the super-finishing processing. When the super-finishing processing is performed, the hub main body50is rotated by pressing the hub main body50in the axial direction using a pressurizing roll72, pressing the axially outer end portion of the hub main body50against a backing plate73, and rotating the backing plate73. Furthermore, a super-finishing grindstone76is pressed against the sliding contact surface54by displacing an arm74in the axial direction using a feed mechanism77while the super-finishing grindstone76supported on a distal end portion of the arm74is swaged (oscillated) by swaging the arm74of a super-finishing board81about a swing shaft75disposed at a twisted position with respect to a central axis of the hub main body50. In this way, the surface roughness of the sliding contact surface54is improved by performing the super-finishing processing on the sliding contact surface54of the hub main body50. That is to say, the unevenness due to the deformation such as the ridges and the burrs formed in the centerless grinding process is reduced.

Specific examples of the super-finishing grindstone76include super-finishing grindstones in which A (alumina)-based or GC (green silicon carbide)-based abrasive grains are bonded with a glass-based bond and which have a bonding particle size of #1000 to #8000 and Rockwell hardness H scale (RH) of −120 to 40.

Alternatively, as shown inFIG.9, the finishing step can also be performed using a polishing tape (a wrapping film)78having a surface coated with abrasive grains. The polishing tape78is formed by substantially uniformly applying abrasive grains such as white alumina, green carborundum, and diamond having a bonding particle size of #1000 to #8000 onto a surface of a polyester film having a thickness of 25 μm to 75 μm using an adhesive in a thickness range of 5 μm to 10 μm.

When the finishing step is performed using the polishing tape78, the hub main body50is rotated about the central axis O50of the hub main body50. Moreover, when the polishing tape78is fed from the outer side in the axial direction toward the inner side in the axial direction along the sliding contact surface54using a plurality of (only two rolls are shown inFIG.9) rolls80while the polishing tape78is pressed against the sliding contact surface54using the head79, the sliding contact surface54is subjected to tape polishing processing. This improves the surface roughness of the sliding contact surface54. That is to say, the unevenness due to the ridges and the burrs formed in the centerless grinding process is reduced.

Here, the finishing step is not limited to super-finishing processing and tape polishing processing. Other processing methods in which a large number of grinding lines in irregular directions can be formed on the sliding contact surface54and/or the surface roughness of the sliding contact surface54can be improved can be adopted. To be specific, for example, the finishing step can be performed by grinding or polishing the sliding contact surface54using an elastic grindstone having appropriate elasticity, an abrasive grain-containing brush, or a non-woven fabric abrasive.

Also, before, after, or at the same time as the finishing step, the hub main body50is completed by subjecting the inner ring track41aon the outer side in the axial direction to super-finishing processing and heat treatment such as quenching at an appropriate timing as necessary. Moreover, the hub unit bearing31is obtained by combining the hub main body50with the outer ring32, the rolling elements34aand34b, the retainers55aand55b, the inner ring51, the combination seal ring57, and the sealing device58. For example, a conventionally known method can be applied to a method for producing the outer ring32, the rolling elements34aand34b, the retainers55aand55b, the inner ring51, the combination seal ring57, and the sealing device58and a method for assembling the hub unit bearing31.

Since the centerless grinding process is performed and then the finishing step is performed in this example, a large number of grinding lines in irregular directions can be formed on the sliding contact surface54and/or the surface roughness of the sliding contact surface54can be improved. Therefore, according to this example, the difference in sliding torque of the distal end portion of the seal lip67with respect to the sliding contact surface54between the relative rotation of the hub33with respect to the outer ring32in the first direction and the relative rotation thereof in the second direction opposite to the first direction can be kept small. The composition and the action effect of other parts are the same as those of the first example of the embodiment.

Although the method for producing a hub unit bearing in this example has, as a target, the hub unit bearing31using balls as the rolling elements34aand34b, the method for producing a hub unit bearing of the present invention can also have, as a target, the hub unit bearing using the tapered roller as a rolling element. Furthermore, the method for producing a hub unit bearing of the present invention is not limited to the hub unit bearing for a drive wheel having engagement holes for engaging the drive shaft in the central portion of the hub and can also have, as a target, the hub unit bearing for a driven wheel whose hub is solid. Furthermore, the method for producing a hub unit bearing of the present invention is not limited to the inner ring rotation type hub unit bearing and also has, as a target, the outer ring rotation type hub unit bearing in which the outer diameter side track ring member is used as the rotation side track ring member and the inner diameter side track ring member is used as the fixed side track ring member as long as the hub unit bearing includes a sealing device.

FIG.10is a partial schematic view of a vehicle200including a hub unit bearing (a bearing unit)151. The present invention can be applied to both of the hub unit bearing for a drive wheel and the hub unit bearing for driven wheel. InFIG.10, the hub unit bearing151is for a drive wheel and includes an outer ring152, a hub153, and a plurality of rolling elements156. The outer ring152is fixed to a knuckle201for a suspension device using bolts or the like. A wheel (and a braking rotating body)202is fixed to a flange (a rotary flange)153A provided on the hub153using bolts or the like. Furthermore, the vehicle200can have a support structure as in the above description to the hub unit bearing151for a driven wheel.

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