JP2008202792A - Rolling bearing unit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a rolling bearing unit comprising a rolling bearing mounted on the outer periphery of a shaft body such as a vehicular hub unit, for preventing the occurrence of leaving crack in an inner ring. <P>SOLUTION: The rolling bearing unit 3 comprises the inner ring 32 having a surface layer which is formed of carbon steel, to the surface of which heat treatment is applied, and in which retained austenite exists, for preventing the occurrence of leaving crack in the inner ring 32 to be broken when the rolling bearing unit 3 is left to be stored in the state that the inner ring 32 is caulked to the shaft body 2 of a hub wheel 1, and a fastened part 4 fastened to the axial outer end face of the inner ring 32 for operating tensile stress on the inner ring 32 in the direction of the outer periphery. The inner ring 32 has an edge 32b on the radial outside. The surface layer is removed from the edge 32b. Since the surface layer which may cause leaving crack is removed from the edge 32b of the inner ring 32, the influences of volume expansion are reduced when the retained austenite is deformed into martensite. This prevents the occurrence of leaving crack at the edge of the inner ring due to an increase of tensile stress. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a rolling bearing unit in which a rolling bearing is mounted on the outer periphery of a shaft body such as a vehicle hub unit.

  In general, a vehicle hub unit has a structure in which a double-row rolling bearing is secured to the outer periphery of a shaft body of a hub wheel.

The shaft body of the hub wheel is provided with a cylindrical portion used for retaining the bearing on the free end side. The cylindrical portion is bent and deformed radially outward using a caulking jig, and is caulked to an axial outer end surface of an inner ring included in the bearing to form a caulking portion. The bearing is prevented from coming off from the hub wheel by this caulking portion. At the same time, the inner ring of the bearing is preloaded from this caulking portion. Carbon steel is used as the material for the inner ring of the bearing. In order to improve the bearing life, the inner ring is subjected to heat treatment to increase the strength of its raceway surface (see, for example, Patent Document 1).
Japanese Patent Application Laid-Open No. 11-129703 (all pages, FIG. 1)

  By the way, the present inventors have advanced various studies on the strength of the raceway surface of the inner ring when carbon steel is used as the material of the inner ring and the inner ring is heat-treated. As a result, the inventors have completed an invention that can stably maintain the strength of the inner ring even when the rolling bearing unit is left unattended.

  That is, the present invention has prevented the occurrence of a phenomenon in which the inner ring is cracked and broken (placement crack) when the rolling bearing unit is left for storage in a state where the inner ring is caulked to the shaft body of the hub wheel. Providing a rolling bearing unit is a problem to be solved.

  The rolling bearing unit according to the present invention includes a shaft body and an inner ring that is attached to the outer peripheral surface of the shaft body, is made of carbon steel, and has a surface layer on which heat treatment is performed and residual austenite exists. And a fastening portion that is fastened to an axially outer end surface of the inner ring and exerts a tensile stress in the outer circumferential direction of the inner ring, and the inner ring has an edge radially outward, and from the edge, the edge The surface layer has been removed.

  According to the rolling bearing unit of the present invention, since the surface layer causing the cracking is removed from the edge of the inner ring, the influence of volume expansion when the residual austenite is transformed into martensite can be reduced. As a result, it is possible to prevent cracks from occurring at the edge of the inner ring due to an increase in tensile stress.

  In the present invention, the surface layer is preferably removed by polishing or turning.

  In the present invention, the amount of retained austenite at the edge is preferably set to 3% or more and 20% or less by the removal.

  According to the present invention, it is possible to effectively prevent the inner ring from being cracked when left for storage or the like.

  Details of embodiments of the present invention will be described with reference to the drawings. 1 and 2 relate to an embodiment of the present invention, FIG. 1 is a sectional view of a rolling bearing unit comprising a hub unit for a drive wheel in one embodiment of the present invention, and FIG. It is sectional drawing of the inner ring | wheel of the rolling bearing unit in embodiment.

  A rolling bearing unit according to an embodiment of the present invention will be described with reference to these drawings. A hub wheel 1 to which a wheel is attached has a shaft body 2 serving as a rotating shaft. The double row rolling bearing 3 is formed of an angular ball bearing, and is fitted into the outer peripheral surface of the shaft body 2 by being press-fitted from the vehicle inner side. The rolling bearing 3 has an inner ring 32. The inner ring 32 is attached to the annular recess 2 a on the outer peripheral surface of the shaft body 2 on the vehicle inner side. The inner ring 32 has an inner ring raceway 32d, while the shaft body 2 has an inner ring raceway 2b on the outer peripheral surface in the middle in the axial direction. The inner ring raceway 2b and the inner ring raceway 32d of the inner ring 32 form a pair of inner ring raceways in the axial direction. The shaft body 2 also has a radially outward flange 2c on the outer peripheral surface on the vehicle outer side. A tire wheel and a brake disk (not shown) are mounted on the side surface of the flange 2c on the vehicle outer side. The rolling bearing 3 also has an outer ring 33. The outer ring 33 is fixed to the vehicle body via a steering knuckle (not shown), and two rows of outer ring raceways 38 and 39 in the axial direction are formed on the inner peripheral surface. The outer ring 33 is disposed concentrically on the radially outer side with respect to the inner ring 32. The rolling bearing 3 also has two axial rows of balls 34 and 35 interposed in the inner ring raceways 32d and 2b and the outer ring raceways 38 and 39, and two axial rows of cages holding the balls 34 and 35 in each row. 36 and 37, and seal rings 7 and 8 for sealing both ends of the rolling bearing 3 in the axial direction.

  The vehicle inner side shaft end portion of the shaft body 2 is bent and deformed radially outward and is caulked to the vehicle inner side outer end surface 32 a of the inner ring 32 to constitute the caulking portion 4. As a fastening portion, the caulking portion 4 applies an appropriate preload to the balls 34 and 35 and prevents the rolling bearing 3 from coming off from the shaft body 2.

  The inner ring 32 will be described with reference to FIG.

  As the material of the inner ring 32, carbon steel such as high carbon chrome steel (Japanese Industrial Standard SUJ2, also called bearing steel) and carbon steel for machine structure (Japanese Industrial Standard S55C) is used.

  In order to enhance the bearing life by increasing the surface strength of the race 32d of the inner ring 32, the inner ring 32 is heat treated in the manufacturing process of the inner ring 32. In this heat treatment, in order to prevent decarburization, heating is performed in a slight carburizing atmosphere, or heating is performed in a carbonitriding atmosphere in which a small amount of NH3 (ammonia gas) is added to the atmospheric gas. .

  When the cracking phenomenon was examined, the following can be considered. The phenomenon of occurrence of cracking will be described with reference to FIGS. The end portion of the shaft body 2 is caulked to the outer end surface 32 a of the end portion in the axial direction of the inner ring 32 to form a caulking portion 4. A circumferential tensile stress acts on the inner ring 32 in the vicinity of the caulking portion 4 from the caulking portion 4. Since the outer end surface 32 a of the inner ring 32 is held by the caulking portion 4, the tensile stress concentrates on the outer diameter side edge 32 b of the inner ring 32. On the surface of the inner ring 32, as shown in FIG. 4, a surface layer L having a larger amount of retained austenite than other portions is formed after the heat treatment.

  Here, the outer end surface 32a of the inner ring 32 and the outer peripheral surface 32c of the shoulder portion thereof constitute an outer diameter side edge 32b of the outer end surface 32a of the inner ring 32. The surface layer L of the outer diameter side edge 32b has a larger amount of retained austenite than the other surface layers L due to heating and carbon diffusion from the outer end surface 32a and the outer diameter surface 32c of the inner ring 32 during heat treatment. After the heat treatment, the retained austenite in the surface layer L is transformed into martensite, and the surface layer L expands in volume.

  This volume expansion due to martensite, combined with the tensile stress acting on the outer diameter side edge 32b of the inner ring 32 described above, causes an increase in tensile stress, and cracks occur on the outer diameter side edge 32b of the inner ring 32. It becomes easy. Thus, the amount of retained austenite at the outer diameter side edge 32b of the inner ring 32 is considered to be related to the occurrence of the cracking phenomenon. Therefore, the present invention proposes a rolling bearing unit that can prevent the cracking phenomenon by reducing the amount of retained austenite at the outer diameter side edge 32b of the inner ring 32 in order to prevent the cracking. Hereinafter, a method for reducing the amount of retained austenite at the outer diameter side edge 32b of the inner ring 32 will be described.

  The first method pays attention to the fact that the amount of retained austenite in the surface layer L decreases from the surface of the inner ring 32 toward the inside, and the surface layer L at the outer diameter side edge 32b of the inner ring 32 is polished or turned to a predetermined value. By removing to the depth, the amount of retained austenite of the outer diameter side edge 32b of the inner ring 32 is reduced. Therefore, by removing the surface layer L until the amount of retained austenite in the range a where tensile stress is concentrated is at least 3% and not more than 20% at the outer diameter side edge 32b, the probability that the inner ring 32 will be cracked is increased. Can be reduced. The specific range a is determined as follows.

The range a to be removed includes the outer diameter of the caulking portion 4 as φA, the outer diameter of the outer peripheral surface 32c of the shoulder portion of the inner ring 32 as φB, the radius from the edge 32b in the vicinity of the caulking portion 4 of the inner ring 32 as r, and the inner ring 32. When the value of the surface layer where the amount of retained austenite generated on the surface of the surface becomes excessive (for example, the amount of retained austenite is 20% or more) is substantially equal to D (mm), the radius r is (φB− φA) / 2 ≧ r ≧ D
The range r of the radius r satisfying the relational expression Here, D is preferably 0.5 [mm] as an example.

  Thus, by removing the surface layer existing within the radius r from the edge 32b of the inner ring 32 that satisfies the relational expression, the amount of retained austenite at the edge 32b is 3% or more and 20% or less, preferably 5%. It is 15% or less, more preferably 5% or more and 10% or less.

  When the amount of retained austenite was 20% or less, no cracking occurred even after days passed. When the amount of retained austenite was 23%, cracks occurred on the 10th day. When the amount of retained austenite was 30%, cracks occurred on the 7th day. The amount of retained austenite decreases with increasing distance from the outer diameter side edge 32b of the inner ring 32.

  According to the rolling bearing unit configured as described above, the retained austenite amount in the range of the radius r satisfying the relational expression from the edge 32b of the inner ring 32 is 3% or more and 20% or less. The influence of volume expansion when transformed into a site can be kept small. As a result, it is possible to prevent a crack from occurring on the edge 32b of the inner ring 32 due to an increase in the tensile stress caused by the caulking portion 4, thereby improving the reliability of the bearing.

  The present invention is not limited to the case where the amount of retained austenite at the edge 32b is 3% or more and 20% or less by polishing or turning the edge 32b of the inner ring 32 as shown in the above embodiment. The amount of retained austenite at the edge 32b may be 3% or more and 20% or less by optimizing the heat treatment conditions in the 32 manufacturing steps.

  Conventional heat treatment conditions are as follows. That is, the inner ring 32 is put into the heat treatment furnace. The internal temperature of the heat treatment furnace at this time is set to 850 ° C to 930 ° C. The heat treatment time for the inner ring 32 is 5 hours. The carbon concentration C · P (Carbon · Potential) in the heat treatment furnace is 1.1 to 1.4%, and the ammonia concentration is 7 to 11 CFH. However, CFH means ft3 / H (cubic foot per hour). Thereafter, the internal temperature of the heat treatment furnace is lowered to 800 to 830 ° C., and the temperature lowered state is maintained for 30 minutes, and then the inner ring 32 is taken out from the heat treatment furnace and is put into oil at 60 ° C. to 100 ° C. and quenched. (Oil cooling). After this oil cooling, the inner ring 32 is tempered again in a heat treatment furnace having an internal temperature of 160 ° C. to 200 ° C. over 2 hours. Thereafter, the inner ring 32 is taken out from the heat treatment furnace and air-cooled.

  In the present invention, the conventional heat treatment conditions for the inner ring 32 are improved. That is, the inner ring 32 is put into the heat treatment furnace. The internal temperature of the heat treatment furnace at this time is set to 850 ° C to 930 ° C. The heat treatment time for the inner ring 32 is 5 hours. The carbon concentration C · P inside the heat treatment furnace is 0.9 to 1.1%, and the ammonia concentration is 4 to 7 CFH. Thereafter, the internal temperature of the heat treatment furnace is lowered to 800 ° C. to 830 ° C., and the temperature lowered state is maintained for 30 minutes, and then the inner ring 32 is taken out from the heat treatment furnace and put into oil at 60 ° C. to 100 ° C. and quenched. (Oil cooling). After this oil cooling, the inner ring 32 is tempered again in a heat treatment furnace having an internal temperature of 160 ° C. to 200 ° C. over 2 hours. Thereafter, the inner ring 32 is taken out from the heat treatment furnace and air-cooled.

  As described above, in the present invention, by improving the carbon concentration and the ammonia concentration among the above heat treatment conditions, the amount of retained austenite at the edge 32b is 3% or more and 20% without polishing or turning the edge 32b of the inner ring 32. % Or less.

  The present invention is not limited to the inner ring 32 having the structure shown in the above embodiment. For example, as shown in FIG. 5, a step 32e having a reduced diameter is provided at a vehicle inner side end of the shoulder portion of the inner ring 32, and this step 32e is used as a space for installing a rotation speed sensor, and the vehicle inner of the step 32e is provided. The side end is the edge 32b. The amount of retained austenite within the range of the radius r satisfying the relational expression at the edge 32b may be 3% or more and 20% or less.

  The present invention is not limited to the caulking portion 4 as shown in the above-described embodiment, and may be a nut that is screwed into the shaft body 2 and fastened to the outer end surface in the axial direction of the inner ring 32. . In short, the shaft body 2 includes a fastening portion that is fastened to the outer end surface in the axial direction of the inner ring 32 and applies a tensile stress in the outer circumferential direction of the inner ring 32.

  The present invention is not limited to a hub unit composed of a combination of a hub wheel and an angular ball bearing, as shown in the above-described embodiment, and a rolling bearing such as an angular ball bearing using a hub wheel or the like as a shaft body. You may apply to the various rolling bearing units provided in the outer periphery of the shaft body.

Sectional drawing of the rolling bearing unit which consists of the hub unit for drive wheels in one embodiment of this invention Sectional drawing of the inner ring | wheel of the rolling bearing unit in one embodiment of this invention Cross section of inner ring of rolling bearing unit Explanatory drawing of the surface layer generated on the surface of the inner ring of the rolling bearing unit Sectional drawing of the inner ring | wheel of the rolling bearing unit in other embodiment of this invention

Explanation of symbols

1 Hub wheel 2 Shaft body 3 Rolling bearing 32 Inner ring

Claims (10)

A shaft,
An inner ring which is attached to the outer peripheral surface of the shaft body and which is made of carbon steel and has a surface layer on which heat treatment is applied and residual austenite is present;
A fastening portion fastened to the outer end surface in the axial direction of the inner ring and exerting a tensile stress in the outer circumferential direction of the inner ring;
Including
The inner ring is a rolling bearing unit having an edge on the radially outer side, and the surface layer is removed from the edge. The rolling bearing unit according to claim 1,
The inner ring is a rolling bearing unit in which the surface layer is removed from the edge by grinding or turning. The rolling bearing unit according to claim 1,
The inner ring is a rolling bearing unit in which the amount of retained austenite at the edge is set to 3% or more and 20% or less by the removal. The rolling bearing unit according to claim 1,
The inner ring has a step at a vehicle inner side end of the shoulder, the step being a space for installing a rotation speed sensor, and the vehicle inner side end of the step being the edge. Rolling bearing unit. A shaft,
An inner ring that is attached to the outer peripheral surface of the shaft body, is made of carbon steel, and is subjected to heat treatment to increase its surface hardness;
A fastening portion fastened to the outer end surface in the axial direction of the inner ring and exerting a tensile stress in the outer circumferential direction of the inner ring;
Including
A rolling bearing unit in which the inner ring has an edge on the radially outer side, and the amount of retained austenite at least at the edge is 3% or more and 20% or less by the heat treatment. In the rolling bearing unit according to claim 5,
There is a step at a vehicle inner side end of the shoulder of the inner ring, and the step is a space for installing a rotation speed sensor, and the vehicle inner side end of the step is a rolling edge. Bearing unit. A shaft,
An inner ring that is attached to the outer peripheral surface of the shaft body, is made of carbon steel, and is subjected to heat treatment to increase its surface hardness;
A ball interposed between the inner ring and an outer ring disposed concentrically on the radially outer side with respect to the inner ring,
By forming a caulking portion that is caulked to the axially outer end surface of the inner ring on the shaft body, a preload is applied to the ball and the inner ring is fixed to the shaft body,
A rolling bearing unit in which the inner ring has an edge on the radially outer side in the vicinity of the caulking portion, and the amount of retained austenite at the edge is set to 3% or more and 20% or less. A shaft,
An inner ring mounted on the outer peripheral surface of the shaft body;
A fastening portion fastened to the axially outer end face of the inner ring;
Including
The inner ring is a rolling bearing unit in which the amount of retained austenite at the radially outer edge is 3% or more and 20% or less. In the rolling bearing unit according to claim 8,
A rolling bearing unit in which at least a portion where the amount of retained austenite at the edge is 20% or more is removed. In the rolling bearing unit according to claim 8,
A rolling bearing unit in which the amount of retained austenite at the edge is 3% or more and 20% or less by heat treatment.

Images