Bearing device and method of manufacturing the bearing device

A bearing device is provided with a bearing ring and a shaft body that is made of carbon steel including pearlite having a cementite layer structure, and has an end portion caulked onto the bearing ring, and in this device, at least in the end portion of the shaft body, the layer gap of the cementite in the pearlite is made greater.

FIELD OF THE INVENTION

The present invention relates to a rolling bearing device that is preferably applied to, for example, an axle and the like of a vehicle, and also relates to a manufacturing method thereof.

BACKGROUND OF THE INVENTION

The rolling bearing device for an axle is generally provided with an outer ring member that is attached to the vehicle body side and an inner ring member that is rotatably supported on the outer ring member so as to freely rotate around the bearing axis through two rows of rolling elements. This inner ring member is constituted by a hub shaft to which a wheel is attached and an inner ring bearing element that is fitted to the end portion of this hub shaft. The end portion of the hub shaft is deformed radially-outward to be caulked onto the end face of the inner ring bearing element so that the hub shaft is rotatably integrated with the inner ring bearing element. The hub shaft of this type is generally made of a steel material such as carbon steel. Such a steel material has a mixed structure of pearlite and pro-eutectoid ferrite.

When the end portion of the hub shaft is caulked onto the end face of the inner ring bearing element, minute cracks tend to occur on the caulked portion.

The present invention relates to the bearing device made of such a steel material having the mixed structure, and its object is to provide a bearing device that is free from minute cracks even when subjected to the caulking process, and a manufacturing method of such a bearing device.

DISCLOSURE OF THE INVENTION

A bearing device of the present invention includes a bearing ring and a shaft body that is made of carbon steel including pearlite having a cementite layer structure and has an end portion caulked onto the bearing ring, and in this device, at least in the end portion of the shaft body, the layer gap of the cementite in the pearlite is made greater.

Preferably, at least in the end portion of the shaft body, the average value of the layer gaps of the cementite is set in a range from not less than 0.15 μm to not more than 0.4 μm. The inventors have found that the generation of minute cracks is caused by the existence of cementite in the pearlite, and based upon this fact, have set the average value of the layer gaps of cementite to the above-mentioned range. With this arrangement, even when the end portion of the shaft body is caulked onto the bearing ring, the rate of occurrence of the minute cracks on its end portion is reduced to zero. Thus, it becomes possible to improve the supply rate of the bearing device as a product.

The average value of the layer gap of the cementite is preferably set in a range from not less than 0.15 μm to not more than 0.35 μm, with the end portion of the shaft body having a hardness of not less than 15 HRC. In this structure, the shaft body preferably has a sufficient hardness as a bearing device.

The bearing device of the present invention can be applied to axles of various vehicles such as automobiles, train vehicles and aircrafts. The bearing ring includes not only inner rings and outer rings of bearing devices, but also a hub wheel which is used in a bearing device having a constant velocity joint, with the outer ring end portion of the constant velocity joint being caulked onto the end face of the hub wheel. The shaft body includes a hub shaft of a vehicle-use rolling bearing or a shaft formed on an outer ring of a constant velocity joint that is allowed to rotate integrally with the hub wheel. With respect to the steel material to be used for the shaft body, carbon steel having a carbon content specified to 0.37% to 0.65% is used.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to drawings, the following description will discuss a rolling bearing device in accordance with a preferred embodiment of the present invention in detail. A rolling bearing device applied to a bearing for a vehicle axle, more specifically, a rolling bearing device on the driven wheel side will be explained.FIG. 1is a cross-sectional view illustrating the entire structure of the rolling bearing device,FIG. 2is an enlarged cross-sectional view of a caulked portion,FIGS. 3(a) to3(f) are schematic cross-sectional views illustrating the composition of the caulked portion, andFIG. 4is an enlarged view illustrating the composition of pearlite. InFIG. 1, the right side in the axial direction shows a vehicle inner side, and the left side in the axial direction shows a vehicle outer side.

With respect to bearing rings that are placed radially inside and outside of a double row angular contact ball bearing with vertex of contact angles outside of bearing, a rolling bearing device1, shown in the figures, is provided with two inner ring members2and3, each having a row of inner ring raceway in the axial direction, and one outer ring member4having two rows of outer ring raceways in the axial direction.

The rolling bearing device1also has groups of balls5and6that are interposed between the respective raceways of the two inner ring members2,3and the outer ring member4in two rows in the axial direction, snap cages7and8that respectively hold the groups of balls5and6of the respective rows, and a seal member9attached to the end. The one inner ring member2is made by a hub shaft that functions as a shaft body to which a brake disc, a wheel and the like are attached. The other inner ring member3is formed by an inner ring itself that is fitted to the outer periphery of this hub shaft2. Hereinafter, the one inner ring member2and the other ring member3are referred to as a hub shaft2and an inner ring3, respectively.

The hub shaft2and the inner ring3form a bearing ring placed on the inside in the radial direction of the bearing. The outer ring member4forms a bearing ring placed on the outside in the radial direction of the bearing. In this case, when the end portion of the hub shaft2is caulked onto the end face of the inner ring3, the hub shaft2and the inner ring3may be referred to as the shaft member and the bearing ring, respectively.

The hub shaft2has a radially-outward hub flange11, to which a brake disk and the like, not shown, are attached through hub bolts12. The outer ring member4has a radially-outward mounting flange13, and is non-rotatably supported on the vehicle side not shown through the use of this mounting flange13.

A caulking-use concave portion14is formed into a cylindrical shape in the end portion on the vehicle inner side of the hub shaft2. This cylindrical end portion on the vehicle inner side is deformed radially-outward to be caulked onto the outer end face of the inner ring3. The end portion of the hub shaft2, caulked in this manner, is referred to as a caulked portion15. This caulked portion15allows the hub shaft2and the inner ring3to rotate integrally with each other so that a predetermined preload is applied to the inner ring3.

As described above, the hub shaft2is compatibly used as one of the inner ring members of the bearing so that the rolling bearing device1constitutes a bearing device in which the hub shaft2and this bearing are integrally formed.

The following description will discuss features of the present embodiment in detail. The hub shaft2is made of carbon steel. With respect to the carbon steel, machine-structure-use carbon steel materials having a carbon content of 0.37% to 0.65% are selected, and among these, those classified as S55C based upon JIS standard are preferably selected. This S55C has a carbon content of 0.52% to 0.58%.

In the case of the carbon steel mentioned above, the metal structure has a mixed structure of pearlite16and pro-eutectoid ferrite. Further, as shown inFIG. 4, the pearlite16contains cementite18formed into layers. In the embodiment of the present invention, the average value of the layer gaps of this cementite18, that is, lamella gaps, is set in a range from not less than 0.15 μm to not more than 0.35 μm. The following description will discuss the reason why the average value of the layer gaps of the cementite18is set to the range mentioned above.

FIGS. 3(a) to3(f) schematically show cross-sections corresponding to vehicle inner side portions of the hub shafts2. These hub shafts2were manufactured through lot productions of a plurality of sets under a plurality of kinds of conditions. Each ofFIGS. 3(a) to3(f) show a state in which the hub shaft2, cut in the radial direction, was boiled in an alkali solution of picric acid soda so that it was subjected to an etching process so as to make its metal structure observable. When the etching process is applied to the cross-section of the hub shaft2, a black etched portion A (indicated by a cross-hatched portion inFIG. 3)and a white etched portion B (indicated by a portion having no cross-hatching) appear. With respect to each of these hub shafts2, the presence of crack initiation on the caulked portion upon a caulking process was observed. As a result, the hub shafts2having minute cracks occurred on the caulked portion15and those having no cracks were observed. Here, the plurality of kinds of conditions refer to differences in the period of time required for the hub shafts2after a forging process to be cooled to a predetermined temperature (for example, room temperature). In other words, supposing that the annealing time of the hub shaft2shown inFIG. 3(a) is 1, the annealing time thereof shown inFIG. 3(b) is given as 1.2, the annealing time thereof shown inFIG. 3(c) is given as 1.3, and the annealing time thereof shown inFIG. 3(d) is given as 1.5. The annealing time thereof shown inFIGS. 3(e) and3(f) is given as 0.5.

The respective hub shafts2shown inFIGS. 3(a) to3(d) had no cracks generated on the caulked portion15. Those hub shafts2, shown inFIGS. 3(e) and3(f), had minute cracks generated on the caulked portion15. InFIGS. 3(e) and3(f), a black portion (indicated by a cross-hatched portion A) was observed at the end portion of each of the hub shafts2. This is caused by the fact that much pearlite16having cementite18with a small average lamella gap exists at the end portion of the hub shaft2.

Here, the following description will discuss a difference in the metal structures between the black etched portion A and the white etched portion B that are shown in a cross-sectional view of the hub shaft2inFIG. 3.

When the black etched portion A was observed under a microscope, much pearlite16having cementite18with the average lamella gap of less than 0.15 μm existed. Moreover, when the white etched portion B was observed under a microscope, much pearlite16having cementite18with the average lamella gap in a range of 0.15 μm to 0.4 μm existed.

It is considered that such a difference is caused by the length of time required for the hub shaft2after the forging process to be cooled to a predetermined temperature. In other words, when the time required for the hub shaft2after the forging process to be cooled to a predetermined temperature (for example, room temperature) is short, pearlite16having a narrow lamella gap, that is, black etched portion A, tends to occur, while, when the length of time required for the hub shaft2after the forging process to be cooled to a predetermined temperature is set longer, much pearlite16having a wider lamella gap, that is, white etched portion B, tends to occur. Moreover, since the portion corresponding to the caulked portion15is located at a tip portion, this portion is easily cooled even in a short time; thus, black etched portion A tends to occur.

The results of these detailed examinations show that the layer gap of the cementite18located as layers in the pearlite16is closely related to the generation of cracks in the caulked portion15; and it is found that, in particular, in the case when the layer gap of the cementite18in the pearlite16is narrow, there is a higher possibility of crack initiation in the caulked portion15.

FIG. 4is a schematic drawing showing a case in which the pearlite16was observed by a microscope with a higher magnification. This drawing shows that the pearlite16has a lamella structure in which ferrite19and cementite18are alternately located as layers.

In accordance with the results of examinations relating to the critical value of the crack initiation, as shown inFIG. 4, it is found that, for example, supposing that there are 10 gaps, a1, a2, a3, . . . , a10in the respective layers of the cementite18, cracks occur in the caulked portion15when the average value (a1+a2+a3+ . . . +a10)/10 is smaller than 0.15 μm. This average value is taken from 10 layer gaps of a1, a2, a3, . . . , a10on one straight line that intersects11layers of cementite18.

Based upon the results of the examinations, in order to prevent cracks from occurring in the caulked portion15, the lower limit value of the average value of the layer gaps of the cementite18is set to 0.15 μm.

The following Table 1 shows the results of the examinations. In Table 1, the layer gap refers to an actual layer gap of the cementite18layers, the rate of crack initiation refers to the rate of crack initiation when the end portion of the hub shaft2is caulked, and the hardness refers to Rockwell C hardness (HRC) of the end portion of the hub shaft2.

Table 1 shows that, when the inter-layer gap (lamella gap) of the cementite18in the pearlite16is 0.4 μm, the hardness of the hub shaft2becomes 12 HRC. Here, the hardness required for the hub shaft2of a rolling bearing device1of this type is preferably not less than 15 HRC. For this reason, the upper limit value of the average in the layer gap of the cementite18needs to be set to 0.35 μm.

From the reasons described above, the average value of the layer gaps of the cementite18is set in a range of not less than 0.15 μm to not more than 0.35 μm.

As described above, the average value of the layer gaps of the cementite18forming the pearlite16is preferably set in the range of not less than 0.15 μm to not more than 0.35 μm; thus, it becomes possible to prevent cracks from occurring when the end portion of the hub shaft2is caulked on the inner ring3. Moreover, it is possible to provide a sufficient hardness for use as the hub shaft2of the rolling bearing device1. Thus, it becomes possible to eliminate the necessity of disposing defective products, and consequently to improve the supply rate of the products.

The present invention is not limited to the embodiments, and various applications and modifications may be proposed.

(1) The following description will discuss a method in which the lamella gap of pearlite in the end portion of the hub shaft2is made greater than that in the other portions of the hub shaft2, as in the case of the present invention.

To form the hub shaft2, a base material is first molded through a hot forging process into a shape close to the final shape, and required portions such as a raceway surface are then subjected to a turning process to be formed into the final shape. The hub shaft2is heated to approximately 1200° C. when subjected to the hot-forging process, and cooled down after the completion of the forging process. See a cooling curve b inFIG. 5. During this cooling process, by cooling down only the end portion of the hub shaft2more slowly than the other portions of the hub shaft2, the lamella gap of pearlite in the end portion of the hub shaft2is made greater than that of the other portions of the hub shaft2. As a specific example, the following description will discuss a method in which the cooling rate of the shaft end portion is made slower by high-frequency heating only the shaft end portion of the hub shaft2during the cooling process. As shown inFIG. 5, at the time when, after the start of the cooling process, the temperature has dropped to 750° C., only the shaft end portion is high-frequency heated to make the cooling rate of the shaft end portion slower (cooling curve a) so that it becomes possible to make the lamella gap of the pearlite of the shaft end portion greater than that of the other portions of the hub shaft2. Here, the temperature to which the shaft end portion is heated is adjusted in a range of about 750° C. to about 800° C. depending on the materials of the hub shaft2.

Here, as an another methods for making the cooling rate of the shaft end portion slower, a method in which only the shaft end portion is covered with a thermal insulating material during the cooling process is proposed. The method for making the cooling rate of the shaft end portion slower in the present invention is not limited to the high-frequency heating process and thermal insulating material, and another methods may be used. In the embodiment, the lamella gap of the pearlite is widened by making the cooling rate of only the shaft end portion of the hub shaft2slower; however, the present invention is not limited to this method, and the cooling rate of the entire hub shaft2may be adjusted to widen the lamella gap of the pearlite in the entire hub shaft2. As methods for widening the lamella gap of the pearlite in the entire hub shaft2, the followings are proposed. The entire hub shaft2is temporarily heated by using high frequency waves and the like during the cooling process. In the cooling process, a heating furnace in which the cooling rate can be adjusted is used so as to gradually cool the material. The entire hub shaft2is covered with a heat insulating material and cooled down gradually.

(2) In the present invention, as shown inFIG. 6, both of the inner ring members of the rolling bearing device1may be formed by the inner rings3and3athemselves. In this case, since the end portion of the hub shaft2is caulked onto the end face of the inner ring3, the shaft body is formed by the hub shaft2, and the bearing ring is formed by the inner ring3. InFIG. 6, those members that are the same as those ofFIG. 1are indicated by the same reference numerals. In the present embodiment, one portion of the roller bearing is compatibly used as the hub shaft2so that the hub shaft and the roller bearing are integrated.

In this case also, as shown inFIG. 4, the average value of the layer gaps of the cementite18constituting the pearlite16in the caulked portion15is set in a range from not less than 0.15 μm to not more than 0.35 μm. Thus, it becomes possible to prevent cracks from occurring in the caulked portion15when the end portion of the hub shaft2is caulked onto the inner ring3. Moreover, it is possible to provide a sufficient hardness for use as a hub shaft2of a rolling bearing device1. Thus, it becomes possible to eliminate the necessity of disposing defective products, and consequently to improve the supply rate of the products.

(3) As shown inFIG. 7, the present invention is also applied to a rolling bearing device1on the driving ring side. In this rolling bearing device1, inner ring members are rotatably supported through balls5and6on an outer ring member4that is non-rotatably supported on the vehicle side. One of the inner ring members is a bowl-shaped outer ring member20of a constant velocity joint, and the other inner ring member is a hub shaft (in this case, a hub wheel)2. The end portion of the shaft body21that is integrally formed in the bowl-shaped outer ring member20of the constant velocity joint is caulked onto the end face of the hub shaft2. Those portions inFIG. 7that are the same as those ofFIG. 1are indicated by the same reference numerals.

In this case also, as shown inFIG. 4, the average value of the layer gaps of the cementite18constituting the pearlite16in the caulked portion15is set in a range from not less than 0.15 μm to not more than 0.35 μm. Thus, it becomes possible to prevent cracks from occurring in the caulked portion15when the end portion of the hub shaft2is caulked onto the inner ring3. Moreover, it is possible to provide a sufficient hardness for use as a hub shaft2of a rolling bearing device1. Thus, it becomes possible to eliminate the necessity of disposing defective products, and consequently to improve the supply rate of the products.

(4) The present invention is also applied to a rolling bearing except for the double row angular contact ball bearing with vertex of contact angles outside of bearing. With respect to the rolling bearing, except for the double row bearing, the present invention may be applied to, for example, a single row rolling bearing.
(5) The present invention includes a case in which the shaft body of a rolling bearing device is formed as an cylinder member placed on the outside the outer ring member in the radial direction, with the end portion of the cylinder member being caulked onto the end portion of the outer ring member that serves as the bearing ring. With respect to the cylinder member, for example, a housing or the like is proposed.
(6) In the above-mentioned embodiment, upon calculating the average value of the layer gaps of the cementite18, the average value of 10 gaps in the cementite18layers is obtained, and the average value (a1+a2+a3+ . . . +a10)/10 is set in a range from not less than 0.15 μm to not more than 0.35 μm. However, the present invention is not limited to this setting. In other words, the average value of n-number of gaps in the cementite18layers may be obtained, and the average value (a1+a2+a3+ . . . +an−1+an)/n may be set in a range from not less than 0.15 μm to not more than 0.4 μm, or in a range from not less than 0.15 μm to not more than 0.35 μm. In this case also, it becomes possible to prevent cracks from occurring in the caulked portion15when the end portion of the hub shaft2is caulked onto the inner ring3. Moreover, it is possible to provide a sufficient hardness for use as a hub shaft2of a rolling bearing device1. Thus, it becomes possible to eliminate the necessity of disposing defective products, and consequently to improve the supply rate of the products.

As described above, in accordance with the present invention, when the end portion of the shaft body is caulked onto a bearing ring, it is possible to prevent cracks from occurring in the caulked portion, and consequently to improve the supply rate of the products.

INDUSTRIAL APPLICABILITY

The bearing device of the present invention is suitably applied to axles of various vehicles such as automobiles, train vehicles and aircrafts.