Heat treatment method for the outer ring of a hub unit bearing

Abstract

The present invention provides a heat treatment method for the outer ring of a hub unit bearing that suppresses fluctuations in the circumferential heat capacity of the outer ring of the hub unit bearing, thereby enabling a consistent thickness of the heat-treated hardened layer in the circumferential direction. [Solution] The invention provides a hardening jig 40 having a contact portion 42 made of a fine wire 43 that has a higher thermal conductivity and flexibility than steel, and the contact portion 42 of the hardening jig 40 is placed in contact with the portion of the outer circumferential surface 8a of the outer ring 8 excluding the stationary flange 12 to suppress circumferential fluctuations in the heat capacity of the outer ring 8, and a heat treatment step in which the outer ring raceway 13b is subjected to high-frequency induction hardening treatment to harden it.

Description

Technical Field

[0001] The present invention relates to a heat treatment method for an outer ring of a hub unit bearing.

Background Art

[0002] The thickness of the hardened layer by high-frequency quenching (HFQ) becomes thicker when the heat capacity of the processed part is small (for example, thin), and becomes thinner when the heat capacity is large (for example, thick). When forming a hardened layer by high-frequency quenching the outer ring raceway surface of a hub unit bearing, the hardened layer of the part far from the stationary flange and with a small heat capacity becomes thicker, and the hardened layer of the part near the stationary flange with a large heat capacity becomes thinner. Therefore, on the outer ring raceway surface, the depth of the hardened layer varies in the circumferential direction.

[0003] In the hardened layer, since the steel undergoes martensite transformation, the volume increases by 0.1 to 0.2% compared to the non-hardened layer. Therefore, the variation in the circumferential depth of the hardened layer may lead to an increase in the major axis of the part with a thick hardened layer, resulting in a decrease in the roundness of the outer ring raceway surface. When grinding the outer ring raceway surface in such a state, more of the thin part of the hardened layer is ground, further increasing the variation in the circumferential depth of the hardened layer. Along with the decrease in the roundness of the outer ring raceway surface, there is also concern about an adverse effect on the life of the outer ring.

[0004] In Patent Document 1, as shown in FIG. 5, the diameter D1 of the outer peripheral surface 8a of the outer ring 8 excluding the stationary flange 12 is set to be between the diameter D2 of the thread groove bottom 16 of the knuckle mounting screw hole 15 and the diameter D3 of the thread crest 17 (D2 ≤ D1 ≤ D3), substantially eliminating the variation in circumferential heat capacity and preventing the circumferential variation in the hardened layer thickness. A hub unit bearing is disclosed.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

[0006] However, since the pitch circle diameter (PCD) of the knuckle mounting screw hole 15 is set in relation to the knuckle, the above relationship cannot always be maintained, causing the heat capacity of the outer ring to fluctuate in the circumferential direction, and as a result, it may be difficult to form a uniform thickness of the hardened layer in the circumferential direction.

[0007] The present invention has been made in view of the aforementioned problems, and its objective is to provide a heat treatment method for the outer ring of a hub unit bearing that can suppress fluctuations in the circumferential heat capacity of the outer ring of the hub unit bearing and make the thickness of the heat-treated hardened layer constant in the circumferential direction. [Means for solving the problem]

[0008] Therefore, the above objective of the present invention is achieved by the configuration described below [1] relating to a heat treatment method for the outer ring of a hub unit bearing. [1] An outer ring having a cylindrical outer ring body having an outer ring raceway on its inner circumferential surface, and a stationary flange projecting radially outward from the outer circumferential surface of the outer ring body, with at least a portion of its axial direction overlapping radially with the outer ring raceway, A hub having an inner ring track on its outer surface, A plurality of rolling elements are arranged to roll freely between the outer ring raceway and the inner ring raceway, A heat treatment method for the outer ring of a hub unit bearing comprising, The quenching jig has a contact portion made of a fibrous material that has higher thermal conductivity and flexibility than steel, A preparatory step to suppress circumferential fluctuations in the heat capacity of the outer ring, which involves positioning the contact portion of the quenching jig in contact with the portion of the outer circumferential surface of the outer ring body excluding the stationary flange, A heat treatment step in which the outer ring raceway of the outer ring body is subjected to high-frequency induction hardening treatment to harden it, A method for heat-treating the outer ring of a hub unit bearing equipped with the following features. [Effects of the Invention]

[0009] According to the heat treatment method for the outer ring of a hub unit bearing of the present invention, fluctuations in the heat capacity in the circumferential direction of the outer ring of the hub unit bearing can be suppressed, and the thickness of the heat-treated hardened layer can be made to be constant in the circumferential direction. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 is a cross-sectional view of a hub unit bearing according to the first embodiment of the present invention. [Figure 2] Figure 2 is a plan view of the outer ring of a hub unit bearing, showing the heat treatment method for the outer ring. [Figure 3] Figure 3 is a plan view of the outer ring showing the experimental heat treatment method. [Figure 4] Figure 4 is a plan view of the outer ring showing a heat treatment method for the outer ring of a hub unit bearing according to a second embodiment of the present invention. [Figure 5] Figure 5 is an enlarged view of the main part of the outer ring of a conventional hub unit bearing. [Modes for carrying out the invention]

[0011] As shown in Figure 3, the inventors of the present invention considered covering the outer circumferential surface 8a of the outer ring 8 with a heat capacity adjustment jig 50, thereby making the outer diameter substantially circular, eliminating fluctuations in circumferential heat capacity, and preventing circumferential fluctuations in the thickness of the hardened layer formed on the heat-treated outer ring raceway 13b. However, because the outer circumferential surface 8a of the outer ring 8 is a hot-forged surface, its surface roughness and shape accuracy are poor, and it was not possible to achieve a level of close contact between the outer circumferential surface 8a and the heat capacity adjustment jig 50 that could be considered an integrated unit in terms of heat conduction.

[0012] Therefore, after diligently researching methods to suppress fluctuations in the circumferential heat capacity of the outer ring of a hub unit bearing, we have invented the heat treatment method for the outer ring of a hub unit bearing described below.

[0013] Hereinafter, each embodiment of the heat treatment method for the outer ring of a hub unit bearing according to the present invention will be described in detail with reference to the drawings.

[0014] (First Embodiment) FIG. 1 is a cross-sectional view of a hub unit bearing according to a first embodiment of the present invention, and FIG. 2 is a plan view of an outer ring showing a heat treatment method of the outer ring of the hub unit bearing.

[0015] As shown in FIG. 1, the hub unit bearing 1 of the present embodiment includes an outer ring 8, a hub 9, and a plurality of balls 10 as rolling elements. Regarding the hub unit bearing, the axially inner side (inboard side) refers to the center side in the vehicle width direction (the right side in FIG. 1) in the assembled state to the vehicle, and the axially outer side (outboard side) refers to the outer side in the vehicle width direction (the left side in FIG. 1) in the assembled state to the vehicle.

[0016] The outer ring 8 is made of medium carbon steel and includes a cylindrical outer ring body 11 and a stationary flange 12 for fixing the outer ring 8 to the knuckle of the suspension device. The outer ring body 11 has a double row of outer ring raceways 13a, 13b on the inner peripheral surface. The cross-sectional shapes of the outer ring raceways 13a, 13b are arcuate.

[0017] A knuckle pilot portion 31 having an outer diameter smaller than that of a portion adjacent axially outward is provided on the outer peripheral surface of the axially inner end portion of the outer ring body 11. The stationary flange 12 is constituted by large flange portions 14 provided so as to project radially outward from a plurality of locations spaced apart in the circumferential direction of the outer peripheral surface of the outer ring body 11. In the example of FIG. 2, four large flange portions 14 are provided.

[0018] In the embodiment shown in the figure, there are no elements constituting the stationary flange 12 between the adjacent large flange portions 14 in the circumferential direction. That is, the outer peripheral surface 8a of the outer ring 8 deviated in the circumferential direction from the large flange portion 14 is the outer peripheral surface of the outer ring body 11. A screw hole 15 penetrating axially is formed in each of the large flange portions 14. The outer ring 8 is fixed to the knuckle by a knuckle bolt screwed into the screw hole 15 with the knuckle pilot portion 31 fitted into a support hole of a knuckle (not shown) constituting the suspension device.

[0019] The stationary flange 12 (large flange portion 14) has its axially outer portion superimposed radially on the axially inner outer ring raceway 13b. In other words, the stationary flange 12 (large flange portion 14) is provided so that its axially outer portion protrudes radially outward from the portion of the outer circumferential surface of the outer ring body 11 that superimposes radially on the axially inner outer ring raceway 13b.

[0020] In the illustrated example, the portion of the outer circumferential surface 8a of the outer ring 8 that overlaps radially with the outer ring raceway 13b on the axial side is a tapered surface that is inclined in a direction in which the outer diameter increases as it moves inward in the axial direction. This tapered surface is the draft angle of hot forging, and the portion that does not overlap circumferentially with the stationary flange 12 (large flange portion 14) can be machined to create a cylindrical surface in which the outer diameter does not change in the axial direction.

[0021] A heat-treated hardened layer 32 is formed on the surface of the outer ring raceways 13a and 13b, as shown in Figure 1 with an oblique grid. For this purpose, when manufacturing the outer ring 8, the material made of medium carbon steel is hot-forged and shaped, then roughly machined, and then the outer ring raceways 13a and 13b are subjected to high-frequency heat treatment to form the heat-treated hardened layer 32.

[0022] The hub 9 comprises double-row inner ring raceways 18a and 18b, and a rotating flange 19 for supporting the wheel and the braking rotating body. The double-row inner ring raceways 18a and 18b are provided on the outer circumferential surface of the hub 9, on the portion facing the double-row outer ring raceways 13a and 13b. The cross-sectional shape of the inner ring raceways 18a and 18b is arc-shaped. The rotating flange 19 is provided on the axially outer portion of the hub 9, which is located axially outward from the axially outer end face of the outer ring 8 in the axial direction, so as to project radially outward.

[0023] The hub 9 is constructed by combining a hub body 20 made of a hard metal such as medium carbon steel and an inner ring 21 made of a hard metal such as bearing steel. Specifically, the hub body 20 and the inner ring 21 are combined by fitting and fixing the inner ring 21 onto a small-diameter stepped portion 22 provided on the axially inner side of the hub body 20. In addition, the axially inner end surface of the inner ring 21 can also be pressed down by a crimped portion formed by plastically deforming the axially inner end of the hub body radially outward.

[0024] Furthermore, the axially outer inner ring raceway 18a is provided on the outer circumferential surface of the axially intermediate portion of the hub body 20, and the axially inner inner ring raceway 18b is provided on the outer circumferential surface of the inner ring 21. The rotating flange 19 is provided on the axially outer portion of the hub body 20. The hub unit bearing 1 shown in the figure is for a drive wheel, and a spline hole 23 for spline engagement of the drive shaft is provided in the radial center of the hub 9 (hub body 20). Note that the present invention can also be applied to a hub unit bearing for a driven wheel with a solid hub body.

[0025] The balls 10 are made of hard metal such as bearing steel or ceramic, and are arranged to roll freely in multiples in each row between the double-row outer ring raceways 13a, 13b and the double-row inner ring raceways 18a, 18b. Preload is applied to the balls 10 in both rows, along with a back-to-back contact angle. Therefore, in the axially inner outer ring raceway 13b, the axially outer side of the groove bottom becomes the load side 27 of the rolling element load, and the axially inner side becomes the unloaded side 28 of the rolling element load. Note that the rolling elements can also be tapered rollers.

[0026] Furthermore, the cylindrical space 24, which is located between the inner surface of the outer ring 8 and the outer surface of the hub 9 and contains multiple balls 10, has its axial outer end opening sealed by a seal ring 25 and its axial inner end opening sealed by a combination seal ring 26.

[0027] Next, with reference to Figure 2, the heat treatment method for the outer ring of the hub unit bearing will be described. The hardening jig 40 of this embodiment comprises a plurality of base portions 41 arranged circumferentially spaced apart from each other on the radially outer side of the outer ring body 11, and contact portions 42 fixed to each base portion 41.

[0028] The base portion 41 is positioned between adjacent stationary flanges 12 in the circumferential direction. In this embodiment, the pair of base portions 41, 41 positioned at the upper and lower positions in Figure 2, where the distance between the stationary flanges 12 is short, are formed in a substantially rectangular shape when viewed from the axial direction. The pair of base portions 41, 41 positioned at the left and right positions in Figure 2, where the distance between the stationary flanges 12 is long, are formed in a substantially arc shape when viewed from the axial direction. Furthermore, the base portion 41 is formed of, for example, steel, having a thickness approximately the same as the axial thickness of the stationary flange 12.

[0029] Furthermore, the contact portion 42 is formed of numerous fine wires 43, which are fibrous members made of a material that has a higher thermal conductivity and flexibility than steel, such as an aluminum alloy, a copper alloy, or carbon fiber. The contact portion 42 is fixed to the inner circumferential surface of the base portion 41 by being implanted in a brush-like manner.

[0030] The contact portion 42 is adjusted so that its thermal conductivity is substantially the same as that of the steel material by adjusting the thermal conductivity of the material of the fine wire 43, the thickness of the fine wire 43, and the implantation density on the base portion 41. The fine wire 43 may be formed in a cotton-like or sponge-like form.

[0031] In the hub unit bearing 1, the outer ring 8 has a stationary flange 12 that is radially superimposed on the outer ring raceway 13b on the axially inner side. As a result, the heat capacity near the outer ring raceway 13b varies in the circumferential direction. Specifically, the heat capacity near the outer ring raceway 13b corresponding to the stationary flange 12 is large, while the heat capacity near the outer ring raceway 13b corresponding to the portion without the stationary flange 12 is small.

[0032] Therefore, if high-frequency induction hardening is performed on the outer ring raceway 13b using a conventional method, the hardened layer will be thin in the portion overlapping the stationary flange 12 and thick in the portion not overlapping the stationary flange 12, resulting in a circumferential variation in the hardened layer thickness. However, for the outer ring raceway 13a on the axial side where the stationary flange 12 is not provided, it is not necessary to consider the circumferential variation in heat capacity.

[0033] Thus, the heat treatment of the outer ring 8 of the hub unit bearing 1, whose heat capacity fluctuates circumferentially due to the stationary flange 12, can be performed using a hardening jig 40 to suppress circumferential fluctuations in the hardened layer thickness. Specifically, as shown in Figure 2, the contact portion 42 of the hardening jig 40 is placed in contact with the portion of the outer circumferential surface 8a of the outer ring 8 that excludes the stationary flange 12. In other words, by bringing the contact portion 42 of the hardening jig 40 into contact with the portion of the outer circumferential surface 8a of the outer ring 8 between adjacent stationary flanges 12 in the circumferential direction, the heat capacity of that portion is increased.

[0034] Since the contact portion 42 is flexible, it adheres closely to the outer circumferential surface 8a of the outer ring 8. Furthermore, the thermal conductivity of the contact portion 42 is adjusted to be the same as that of the steel material (stationary flange 12). As a result, the circumferential fluctuation of the heat capacity near the outer ring raceway 13b is effectively suppressed.

[0035] In this way, the contact portion 42 of the hardening jig 40 is brought into close contact with the outer circumferential surface 8a of the outer ring 8, excluding the stationary flange 12, to suppress circumferential fluctuations in the heat capacity of the outer ring 8 (preparation step). Then, the outer ring raceway 13b of the outer ring 8 is subjected to high-frequency induction hardening treatment to harden it (heat treatment step). This prevents circumferential fluctuations in the thickness of the hardened layer, making it possible to achieve a constant thickness in the circumferential direction and improve the roundness of the outer ring raceway 13b.

[0036] (Second Embodiment) Figure 4 is a plan view of the outer ring showing a heat treatment method for the outer ring of a hub unit bearing according to a second embodiment of the present invention.

[0037] As shown in Figure 4, in this embodiment, the quenching jig 40 has each contact portion 42 formed as a bag 45. The bag 45 is formed by weaving fine wires 43, which are flexible fibrous members, into a bag shape. Inside the bag 45, a filling material consisting of a fluid such as air or coolant is placed directly or in a sealable container to adjust the thermal conductivity. As a result, the thermal conductivity of the contact portion 42 (bag 45) is adjusted to be the same as the thermal conductivity of the steel material.

[0038] Then, as shown in Figure 4, the contact portion 42 (bag 45) of the hardening jig 40 is placed in contact with the portion of the outer circumferential surface 8a of the outer ring 8 that excludes the stationary flange 12. In other words, by placing the contact portion 42 (bag 45) of the hardening jig 40 in contact with the portion of the outer circumferential surface 8a of the outer ring 8 between adjacent stationary flanges 12 in the circumferential direction, the heat capacity of that portion is increased. Since the contact portion 42 is flexible, it adheres closely to the outer circumferential surface 8a of the outer ring 8, and furthermore, the thermal conductivity of the contact portion 42 is adjusted to be the same as the thermal conductivity of the steel material (stationary flange 12), so that fluctuations in the circumferential direction of the heat capacity near the outer ring raceway 13b are suppressed.

[0039] The contact operation between the contact portion 42 (bag 45) and the outer circumferential surface 8a of the outer ring 8 may be performed by filling the bag 45 with a filling material to adjust the thermal conductivity, inflating it, and making it adhere tightly to the outer circumferential surface 8a, or by filling the bag 45 with a filling material and inflating it, then placing the bag 45 between the outer circumferential surface 8a of the outer ring 8 and a movable plate (not shown) that can move in the vertical or horizontal direction, and moving the movable plate toward the center of the outer ring 8 to make the bag 45 adhere tightly to the outer circumferential surface 8a of the outer ring 8.

[0040] In this way, the bag 45 is pressed tightly against the outer surface 8a of the outer ring 8 to suppress circumferential fluctuations in the heat capacity of the outer ring 8 (preparation step), and then the outer ring raceway 13b of the outer ring 8 is subjected to high-frequency induction hardening treatment to harden it (heat treatment step). This prevents circumferential fluctuations in the thickness of the hardened layer, making it possible to achieve a constant thickness in the circumferential direction and to form a well-rounded outer ring raceway 13b.

[0041] Furthermore, the present invention is not limited to the embodiments described above, and can be modified, improved, etc., as appropriate. In the above description, an outer ring of a hub unit bearing having a plurality of stationary flanges spaced apart in the circumferential direction on its outer circumference was described, but it is not limited to the outer ring of a hub unit bearing, and can be applied to any shape of hardened member whose heat capacity varies in the circumferential direction.

[0042] As described above, the following matters are disclosed in this specification: (1) An outer ring having a cylindrical outer ring body having an outer ring raceway on its inner circumferential surface, and a stationary flange that protrudes radially outward from the outer circumferential surface of the outer ring body, with at least a portion of its axial direction overlapping radially with the outer ring raceway, A hub having an inner ring track on its outer surface, A plurality of rolling elements are arranged to roll freely between the outer ring raceway and the inner ring raceway, A heat treatment method for the outer ring of a hub unit bearing comprising, The quenching jig has a contact portion made of a fibrous material that has higher thermal conductivity and flexibility than steel, A preparatory step to suppress circumferential fluctuations in the heat capacity of the outer ring, which involves positioning the contact portion of the quenching jig in contact with the portion of the outer circumferential surface of the outer ring body excluding the stationary flange, A heat treatment step in which the outer ring raceway of the outer ring body is subjected to high-frequency induction hardening treatment to harden it, A method for heat-treating the outer ring of a hub unit bearing equipped with the following features. This configuration prevents circumferential variation in the hardened layer thickness, allowing the hardened layer to maintain a constant thickness in the circumferential direction, and enabling the formation of a well-rounded outer ring raceway.

[0043] (2) The contact portion of the quenching jig is adjusted so that the thermal conductivity of the contact portion is the same as the thermal conductivity of the steel material, based on the thermal conductivity, thickness, and arrangement density of the fibrous member. (1) A method for heat treatment of the outer ring of the hub unit bearing described above. With this configuration, the thermal conductivity of the contact area is adjusted to be the same as that of the steel material, thereby suppressing circumferential fluctuations in the heat capacity of the outer ring.

[0044] (3) The contact portion of the quenching jig is a bag formed from the fibrous member, and the thermal conductivity of the contact portion is adjusted to be the same as the thermal conductivity of the steel material by filling the inside of the bag with the material. (1) A method for heat treatment of the outer ring of the hub unit bearing described above. With this configuration, the thermal conductivity of the contact area is adjusted by the material filled inside the bag formed by the fibrous member to be the same as the thermal conductivity of the steel material, thereby suppressing circumferential fluctuations in the heat capacity of the outer ring. [Explanation of Symbols]

[0045] 1 Hub unit bearing 8 Outer ring 9 Hubs 10 Balls (rolling elements) 11 Outer ring body 12 Stationary flange 13a, 13b Outer ring track 18a, 18b Inner ring track 40 Hardening jigs 42 Contact area 43 Thin line 45 bags

Claims

1. An outer ring having a cylindrical outer ring body having an outer ring raceway on its inner circumferential surface, and a stationary flange that protrudes radially outward from the outer circumferential surface of the outer ring body, with at least a portion of its axial direction overlapping radially with the outer ring raceway, A hub having an inner ring track on its outer surface, A plurality of rolling elements are arranged to roll freely between the outer ring raceway and the inner ring raceway, A heat treatment method for the outer ring of a hub unit bearing comprising, The quenching jig has a contact portion made of a fibrous material that has higher thermal conductivity and flexibility than steel, A preparatory step to suppress circumferential fluctuations in the heat capacity of the outer ring, which involves positioning the contact portion of the quenching jig in contact with the portion of the outer circumferential surface of the outer ring body excluding the stationary flange, A heat treatment step in which the outer ring raceway of the outer ring body is subjected to high-frequency induction hardening treatment to harden it, A method for heat-treating the outer ring of a hub unit bearing equipped with the following features.

2. The contact portion of the quenching jig is adjusted so that the thermal conductivity of the contact portion is the same as the thermal conductivity of the steel material, based on the thermal conductivity, thickness, and arrangement density of the fibrous member. A method for heat-treating the outer ring of a hub unit bearing according to claim 1.

3. The contact portion of the quenching jig is a bag formed from the fibrous member, and the thermal conductivity of the contact portion is adjusted to be the same as that of the steel material by filling the inside of the bag with a material. A method for heat-treating the outer ring of a hub unit bearing according to claim 1.

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