Patent Description:
When mounting a rolling bearing between a shaft and a housing, the inner ring of the rolling bearing is fitted onto the shaft, and the outer ring is fitted to the inner periphery of the housing. The fit between the inner ring or the outer ring and the corresponding shaft or housing is selected, taking into consideration the loading conditions, assemblability of the device, etc., from among an interference fit, a normal fit and a clearance fit. If a clearance fit is selected for the inner ring or the outer ring, the inner ring or the outer ring which is in a clearance fit may creep, that is, rotate in the circumferential direction relative to the mating member, namely the shaft or the housing.

By way of example, in a bearing assembly including a rolling bearing through which a shaft of, for example, an automotive transmission or an EV (electric vehicle) motor is supported on a housing, for easy mounting of the rolling bearing to the housing, its outer ring is fitted to the housing in a clearance fit, and the clearance fit could result in creeping of the outer ring due to unbalanced load on the shaft under high loads or during high-speed rotation.

To address this problem, the below-identified Patent Document <NUM> proposes a rolling bearing which stably maintains good anti-creep performance. This rolling bearing includes a calcined film on the radially outer surface of the outer ring, which is a fitting surface with the housing, or on the radially inner surface of the inner ring, which is a fitting surface with the shaft. This calcined film contains an organic binder, solid lubricant powder such as molybdenum disulfide powder, and a friction/wear adjustor such as antimony oxide powder.

In a bearing supporting an EV motor, discharge may occur between the rolling elements and the raceway surfaces due to leakage current from the motor. The discharge could cause electrolytic corrosion damage to the inner ring, the outer ring or the rolling elements.

To address this problem, the below-identified Patent Document <NUM> proposes a structure capable of preventing electrolytic corrosion of a rolling bearing. This rolling bearing includes ceramic films impregnated with a synthetic resin having insulating properties and formed on the radially inner surface and the end surfaces of the inner ring, and on the radially outer surface and the end surfaces of the outer ring. Bearing devices according to the preamble of claim <NUM> are disclosed in <CIT> and <CIT>.

However, in the rolling bearing of Patent Document <NUM>, although containing solid lubricant powder, and a friction/wear adjustor, if this bearing is used for supporting the shaft of an EV motor, there is a concern that leakage current from the motor passes through the bearing, causing discharge between the rolling elements and the raceway surfaces, resulting in electrolytic corrosion damage.

On the other hand, in the rolling bearing of Patent Document <NUM>, while ceramic coatings are used in order to provide electrolytic corrosion resistance, if the outer ring or the inner ring creeps, because its fitting surface with the shaft or the housing lacks lubricity, friction and wear to such an extent as to affect the shaft supporting function could occur at the fitting surfaces of the bearing ring that has creeped and the corresponding housing or shaft.

In view of the above-described background, the object of the present invention is to provide a rolling bearing which achieves, in an environment where leakage current is transmitted to the inner ring or the outer ring, and the inner ring or the outer ring is fitted in a clearance fit to the corresponding shaft or housing, both the creep resistance and the electrolytic corrosion resistance.

In order to achieve this object, the present invention provides a rolling bearing comprising an inner ring having a radially inner surface, an outer ring having a radially outer surface, and rolling elements disposed between the inner ring and the outer ring, wherein at least one of the radially inner surface of the inner ring and the radially outer surface of the outer ring is covered with a coating layer, characterized in that, the coating layer comprises a coating layer composite composed of a plurality of layers, wherein a surface layer of the plurality of layers of the coating layer composite is composed of an anti-creep film having lubricity, and wherein at least one of the plurality of layers of the coating layer composite excluding the surface layer is composed of an insulating film having insulation properties.

In this arrangement, the anti-creep film, which is a surface layer of the plurality of layers of the coating layer composite that covers the radially inner surface of the inner ring or the radially outer surface of the outer ring, is a fitting surface with the shaft or the housing. Thus, the anti-creep film exhibits lubricity during creeping, thereby preventing friction and wear at the fitting surface of the radially inner surface of the inner ring or the radially outer surface of the outer ring, and the fitting surface of the corresponding shaft or housing. Also, since at least one of the plurality of layers of the coating layer composite excluding the surface layer is composed of an insulating film having insulating properties, the insulating layer breaks the circuit of the flow of leakage current, preventing discharge between the inner ring or the outer ring and the rolling elements, and thereby preventing electrolytic corrosion damage to the inner ring, outer ring and rolling elements. In other words, the rolling elements of the above-described structure provides both of the above-described two benefits.

According to the invention, the insulating film is a calcined film containing at least one of a ceramic material, an epoxy-based resin and a polyamide-based resin.

The anti-creep film is, for example, a sintered film containing a resin binder and solid lubricant powder.

Preferably, the insulating film of the coating layer composite includes a side covering portion covering one of two side surfaces defining the width of the outer ring, or one of two side surfaces defining the width of the inner ring.

Preferably, the anti-creep film of the coating layer composite includes an extended covering portion covering one of a portion of the inner ring other than the radially inner surface thereof, and a portion of the outer ring other than the radially outer surface thereof, the extended covering portion being in contact with the shaft or the housing.

By employing the above configuration, the present invention provides a rolling bearing which, in a use condition of the rolling bearing in which leakage current reaches the inner ring or the outer ring, and in which the inner ring or the outer ring is in a clearance fit with, respectively, the shaft or one of the housing and the flange bracket, the rolling bearing exhibits both creep resistance that prevents friction and wear at the fitting surfaces, and resistance to electrolytic corrosion that prevents electrolytic corrosion of the bearing races and the rolling elements.

Referring to the drawings, the first embodiment of the present invention is described.

As shown in <FIG> and <FIG>, the rolling bearing <NUM> (either a first or a second rolling bearing <NUM>) according to the first embodiment is disposed between a shaft <NUM> and a housing <NUM> surrounding the shaft <NUM>.

In the following description, in an ideal state in which the rotation center axis, in design, of the rolling bearing <NUM> coincides with the rotation center axis of the shaft <NUM>, the direction along the rotation center axis is referred to as the "axial direction", "axially" or "axial"; the direction along the circumference of a circle having a center at the rotation center axis is referred to as the "circumferential direction", "circumferential" or "circumferentially"; and a direction perpendicular to the rotation center axis is referred to as a "radial direction", "radially", or "radial".

The shaft <NUM> rotates relative to the housing <NUM>. The shaft <NUM> is, for example, the rotary shaft of a motor of an electric vehicle (EV). The shaft <NUM> has cylindrical fitting surfaces 1a extending in the circumferential direction.

The housing <NUM> is stationary relative to the shaft <NUM>, and supports the shaft <NUM> through the rolling bearing <NUM>. The housing <NUM> is, for example, a partition wall forming a portion of a case of the above-described motor. The housing <NUM> has a cylindrical fitting surface 2a extending in the circumferential direction. This fitting surface 2a is concentric with the corresponding fitting surface 1a of the shaft <NUM>.

The rolling bearing <NUM> supports the shaft <NUM> for rotation relative to the housing <NUM>, and bears, for example, radial loads that act between the shaft <NUM> and the housing <NUM>.

The EV motor shown in <FIG> includes the housing <NUM>, which defines the outer perimeter of the motor; the shaft <NUM>, which is inserted in the housing <NUM>; and a rotor <NUM> mounted around the shaft <NUM> for rotation in unison with the shaft <NUM>. Outside the rotor <NUM>, a stator <NUM> is fixed to the inner side of the housing <NUM>. A coil is mounted in the stator <NUM> and electricity is supplied to the coil via a lead wire <NUM>. A flange bracket <NUM> is mounted to an opening of the housing <NUM>. The shaft <NUM> extends through a center hole of the flange bracket <NUM>. The flange bracket <NUM> has a cylindrical fitting surface 3a extending in the circumferential direction. This fitting surface 3a is also concentric with the corresponding fitting surface 1a of the shaft <NUM>. The first rolling bearing <NUM> is disposed between the fitting surface 2a of the housing <NUM> on the bottom side (i.e., right side in <FIG>) of the housing <NUM> and the corresponding fitting surface 1a of the shaft <NUM>. The second rolling bearing <NUM> is disposed between the fitting surface 3a of the flange bracket <NUM> and the corresponding fitting surface 1a of the shaft <NUM>.

The rolling bearings <NUM> each include an inner ring <NUM> mounted on the shaft <NUM>, an outer ring <NUM> mounted to the housing <NUM> or the flange bracket <NUM>, a plurality of rolling elements <NUM> disposed between the inner ring <NUM> and the outer ring <NUM>, and a retainer <NUM> maintaining the circumferential distances between the rolling elements <NUM>. The rolling bearings <NUM> shown are deep groove ball bearings.

The inner ring <NUM> is an annular bearing part having, on the outer periphery thereof, a raceway surface 11a extending in the circumferential direction, and on the inner periphery thereof, a radially inner surface 11b extending in the circumferential direction. The radially inner surface 11b is a cylindrical surface concentric with the fitting surface 1a of the shaft <NUM>. The radially inner surface 11b of the inner ring <NUM> is fitted to the fitting surface 1a of the shaft <NUM>.

The fit between the radially inner surface 11b of the inner ring <NUM> and the fitting surface 1a of the shaft <NUM> is an interference fit. Due to the interference fit, the inner ring <NUM> is fixed to the shaft <NUM> for rotation in unison with the shaft <NUM>.

The outer ring <NUM> is an annular bearing part having, on the inner periphery thereof, a raceway surface 12a extending in the circumferential direction, and on the outer periphery thereof, a radially outer surface 12b extending in the circumferential direction. The radially outer surface 12b is a cylindrical surface concentric with the radially inner surface 11b of the inner ring <NUM>.

The outer ring <NUM> is in a clearance fit with the housing <NUM>.

The inner ring <NUM> and the outer ring <NUM> are both composed of steel such as SUJ2, SCM420, SCr420, SCR420 or SUS440. The inner ring <NUM> and the outer ring <NUM> are subjected to an appropriate treatment such as a quench and temper treatment, a carburizing treatment, or a carbonitriding treatment.

As shown in <FIG>, in the first embodiment, the radially outer surface 12b of the outer ring <NUM> is covered with a coating layer <NUM>. The coating layer <NUM> is a coating layer composite <NUM> comprising a plurality of layers. Of the plurality of layers of the coating layer composite <NUM>, the surface layer at the radially outermost position is composed of an anti-creep film <NUM> having lubricity. The radially outer surface of this anti-creep film <NUM> serves as a fitting surface which is in a clearance fit with the fitting surface 2a of the housing <NUM> or the fitting surface 3a of the flange bracket <NUM>.

The anti-creep film <NUM> may be, for example, a calcined film containing a resin binder and a solid lubricant. The resin binder is composed of a base material and a hardening agent and hardened by reacting the hardening agent. By solidifying the outside of the solid lubricant with the resin binder, the anti-creep film exhibits excellent adhesion properties and wear resistance. The anti-creep film also reduces wear of the housing <NUM> or the flange bracket <NUM>. As the base material, due to its durability, a polyamideimide resin is preferable. As the hardening agent, for easy hardening, an epoxy resin and a reactive compound to be reacted with the epoxy resin are preferably used in combination.

The type of the epoxy resin used is not particularly limited provided it can be used as a hardening agent, such epoxy resins including bisphenol A epoxy resin, cresol novolac epoxy resin, biphenyl epoxy resin, brominated epoxy resin, and alicyclic epoxy resin. Examples of the reactive compound include aliphatic polyamine, polyaminoamide, polymercaptans, aromatic polyamine, acid anhydrides, and dicyandiamide. Besides these components, a hardening accelerator may be added for increased reactivity. Examples of the hardening accelerator include tertiary amines, tertiary amine salts, imidazole, phosphine, phosphonium salts and sulphonium salts.

In order for the solid lubricant to exhibit lubricity, it is preferably composed of a material softer than the material forming the housing <NUM>, specifically a material having a hardness of Hv <NUM> to <NUM>. Examples of the solid lubricant material includes molybdenum disulfide powder, graphite powder, tungsten disulfide, and polytetrafluoroethylene. Among them, preferably, molybdenum disulfide may be used alone, or a mixture of molybdenum disulfide and another or other materials may be used.

The anti-creep film <NUM> may further contain a friction/wear adjustor. The friction/wear adjustor is a material that improves the wear resistance of the calcined film, and is preferably selected from materials softer than the material forming the housing <NUM>. Examples of such materials include antimony oxide, talc, mica, potassium titanate, tin, copper, zinc and nickel. Among them, antimony oxide is particularly preferable.

In order to form the anti-creep film <NUM> as the calcined film, a coating liquid is prepared by, for example, adding solid lubricant powder, friction/wear adjuster powder, and other components to a solvent into which the resin binder is dissolved. The coating liquid thus obtained is applied to the surface of the below-described insulating film or another intermediate film, and is heated to evaporate the solvent, thereby forming the anti-creep film <NUM>.

At least one of the layers forming the coating layer composite <NUM> other than the anti-creep film <NUM> comprises an insulating film <NUM> having insulating properties. The insulating film <NUM> is formed on the outer periphery of the radially outer surface 12b, and the anti-creep film <NUM> is formed outside of the insulating film <NUM>.

The insulating film <NUM> is composed of a material having insulating properties. The insulating film <NUM> provides insulation between the radially outer surface 12b of the outer ring <NUM> and the fitting surface 2a, 3a of the housing <NUM> or the flange bracket <NUM> to prevent leak current from the fitting surface 2a, 3a from reaching the radially outer surface 12b.

Examples of the material of the insulating film <NUM> include a ceramic material, an epoxy resin, and a polyamideimide resin. If a ceramic material, an epoxy resin, or a polyamideimide resin is used, the calcined film may be formed by heating the coated material. If an epoxy resin or a polyamideimide resin is used, calcining may be performed together with a hardening agent.

In order to prevent discharge between the rolling elements <NUM> and the raceway surfaces 11a and 12a by breaking the circuit of the flow of leakage current between the outer ring <NUM> and the inner ring <NUM>, it is necessary to cover at least one of the radially outer surface 12b of the outer ring <NUM> and the radially inner surface 11b of the inner ring <NUM> with the coating layer composite containing the insulating film <NUM>. By preventing discharge between the rolling elements <NUM> and the raceway surfaces 11a and 12a, electrolytic corrosion of the rolling elements <NUM> and the raceway surfaces 11a and 12a is prevented.

The insulating film <NUM> shown includes a center covering portion 32a covering the radially outer surface 12b of the outer ring <NUM>, a side covering portion 32b covering a side surface 12c of the outer ring <NUM>. The side surface 12c of the outer ring <NUM> is one of two side surfaces defining the width of the outer ring <NUM>. The side surface 12c axially opposes the side surface of the housing <NUM> or the flange bracket <NUM>.

The center covering portion 32a and the side covering portions 32b form an integral film. The side covering portion 32b provides insulation between the housing <NUM> or the flange bracket <NUM> and the side surface 12c of the outer ring <NUM>, and by preventing discharge therebetween, the electrolytic corrosion of the housing <NUM> or the flange bracket <NUM> and the side surface 12c is prevented.

In the above-described rolling bearing <NUM> according to the first embodiment, among the plurality of layers constituting the coating layer composite <NUM> covering the radially outer surface 12b of the outer ring, the anti-creep film <NUM> as the surface layer serves as the fitting surface with the housing <NUM> or the flange bracket <NUM>, exhibiting lubricity during creeping, thereby preventing friction and wear of the radially outer surface 12b of the outer ring <NUM> and the fitting surface 2a, 3a of the corresponding one of the housing <NUM> and the flange bracket <NUM>. Further, because at least one of the plurality of layers constituting the coating layer composite <NUM> other than the surface layer or the anti-creep film <NUM> is composed of an insulating film <NUM> having insulating properties, the insulating film <NUM> breaks the circuit of the flow of leakage current, preventing discharge between the inner ring <NUM> or the outer ring <NUM> and the rolling elements <NUM>, and thereby preventing electrolytic corrosion damage to the inner ring <NUM>, the outer ring <NUM> and the rolling elements <NUM>. Thus, in a use condition of the rolling bearing <NUM> in which leakage current reaches the inner ring <NUM> or the outer ring <NUM>, and in which the inner ring <NUM> or the outer ring <NUM> is in a clearance fit, respectively, with the shaft <NUM> or one of the housing <NUM> and the flange bracket <NUM>, the rolling bearing <NUM> exhibits both creep resistance and resistance to electrolytic corrosion.

Further, since the insulating film <NUM> of the coating layer composite <NUM> includes the side covering portion 32b covering the side surface 12c of the outer ring <NUM>, the insulating film <NUM> prevents discharge between the side surface 12c of the outer ring <NUM> and one of the housing <NUM> and the flange bracket <NUM> that opposes the side surface 12c, thereby preventing electrolytic corrosion of e.g., the side surface 12c.

The coating layer composite <NUM> is not limited to the two-layer structure as shown. For example, a film having a different function may be disposed between the radially outer surface 12b and the anti-creep film <NUM>, and also, an additional insulating film and an additional anti-creep film may be laminated between the insulating film <NUM> and the anti-creep film <NUM>. Since, in an environment where alternating current flows, the capacitance is involved in the electrolytic corrosion resistance performance, the films need to have a certain amount of capacitance. Since the necessary capacitance is determined by the structure of the parts, the materials and thicknesses of the films, etc., the necessary capacitance is properly determined according to the use environment.

The second embodiment is described with reference to <FIG>. The description is limited to what differs from the first embodiment.

In the rolling bearing of the second embodiment, the radially inner surface 11b of the inner ring <NUM> is covered with a coating layer composite <NUM> comprising a plurality of layers. The coating layer composite <NUM> includes an insulating film <NUM> having a center covering portion 41a covering the radially inner surface 11b of the inner ring <NUM>, and a side covering portion 41b covering a side surface 11c of the inner ring <NUM>. The side surface 11c is one of two side surfaces defining the width of the inner ring <NUM>. The side surface 11c axially opposes the side surface of the housing <NUM> or the flange bracket <NUM>.

The coating layer composite <NUM> further includes an anti-creep film <NUM>, as a surface layer of the coating layer composite <NUM>, which forms a fitting surface fitted to the fitting surface 1a of the shaft <NUM>. In this embodiment, the anti-creep film <NUM> exhibits anti-creep properties against the shaft <NUM>.

The insulating film <NUM> and the anti-creep film <NUM> may have the same structures and may be formed by the same methods, as the insulating film and the anti-creep film of the first embodiment.

The coating layer composite <NUM> further includes an intermediate layer <NUM> between the insulating film <NUM> and the anti-creep film <NUM>. The anti-creep film <NUM> is formed, as the surface layer, on the front side of the intermediate layer <NUM>.

The structure of the intermediate layer <NUM> is not particularly limited provided it does not interfere with the performances of the insulating film <NUM> and the anti-creep film <NUM>. For example, it may have the capability of improving insulating properties, have the capability of strongly fixing the anti-creep film <NUM> in position, or may have other properties. For example, where the adhesion between the insulating film <NUM> and the anti-creep film <NUM> tends to be insufficient if they are directly laminated together, an intermediate layer <NUM> that shows good adhesion to both of them may be used. If the insulating film <NUM> and the anti-creep film <NUM> alone are not capable of achieving the required capacitance, an intermediate layer <NUM> may be selected which enables, in combination of the three, to achieve the required capacitance. While not shown, the intermediate layer <NUM> may be formed not only between the insulating film <NUM> and the anti-creep film <NUM> but also between the insulating film <NUM> and the inner ring <NUM>. The intermediate layer <NUM> may have, instead of the single-layer structure, a multi-layer structure.

According to the second embodiment, the insulating film <NUM> breaks the circuit of the flow of leakage current, thereby preventing electrolytic corrosion damage to the inner ring <NUM>, the outer ring <NUM>, and the rolling elements <NUM> by preventing discharge between the inner ring <NUM> or the outer ring and the rolling elements <NUM>, and also preventing the electrolytic corrosion of, for example, the side surface 11c by preventing discharge between the side surface 11c of the inner ring <NUM> and one of the housing <NUM> and the flange bracket <NUM> that opposes the side surface 11c of the inner ring <NUM>.

The third embodiment is described with reference to <FIG>.

The third embodiment differs from the second embodiment in that the anti-creep film <NUM> includes an extended covering portion 42b covering a portion of the inner ring <NUM> other than its radially inner surface 11b, and in contact with the housing <NUM>. The center covering portion 43a of the intermediate layer <NUM> is superposed on the center covering portion 41a of the insulating film <NUM>, while the center covering portion 42a of the anti-creep film <NUM> is superposed on the center covering portion 43a of the intermediate layer <NUM>. The side covering portion 43b of the intermediate layer <NUM> is superposed on the side covering portion 41b of the insulating film <NUM>, while the extended covering portion 42b of the anti-creep film <NUM> is superposed on the side covering portion 43b of the intermediate layer <NUM>.

When the inner ring <NUM> creeps, the extended covering portion 42b of the anti-creep film <NUM> slides on the side surface of the housing <NUM> in the circumferential direction. Thus, the extended covering portion 42b prevents friction and wear between the side surface 11c of the inner ring <NUM> and the housing <NUM>.

Thus, according to the third embodiment, because the anti-creep film <NUM> of the coating layer composite <NUM> includes an extended covering portion 42b covering a portion of the inner ring <NUM> other than its radially inner surface 11b (i.e., side surface 11c), and in contact with the housing <NUM>, the extended covering portion 42b prevents wear and friction between the above inner ring portion (side surface 11c) and the housing <NUM>.

While, in the embodiment, because the inner ring <NUM> is axially supported by the housing <NUM>, the side surface 11c of the inner ring <NUM> is protected against the housing <NUM> by the extended covering portion 42b, if the inner ring <NUM> includes on its outer peripheral side a portion to be supported by the housing <NUM> (such as a shoulder or a chamfer), the extended covering portion may be superposed on this outer peripheral portion.

In an arrangement of the first embodiment, in which the coating layer composite is formed on the outer ring <NUM>, the coating layer composite may include a portion corresponding to the extended covering portion, thereby preventing friction and wear between the side surface of the outer ring and the shaft.

In the second and third embodiments, the outer ring of the first embodiment may be used, or the outer ring may be free of the coating layer composite.

Claim 1:
A rolling bearing comprising:
an inner ring (<NUM>) having a radially inner surface (11b), an outer ring (<NUM>) having a radially outer surface (12b), and rolling elements (<NUM>) disposed between the inner ring (<NUM>) and the outer ring (<NUM>),
wherein at least one of the radially inner surface (11b) of the inner ring (<NUM>) and the radially outer surface (12b) of the outer ring (<NUM>) is covered with a coating layer (<NUM>, <NUM>),
wherein the coating layer (<NUM>, <NUM>) comprises a coating layer composite (<NUM>, <NUM>) composed of a plurality of layers,
wherein a surface layer of the plurality of layers is composed of an anti-creep film (<NUM>, <NUM>) having lubricity, and
wherein at least one of the plurality of layers excluding the surface layer is composed of an insulating film (<NUM>, <NUM>) having electrical insulation properties,
characterized in that
the insulating film (<NUM>, <NUM>) is a calcined film containing at least one of a ceramic material, an epoxy-based resin and a polyamideimide-based resin.