Hub unit for supporting a wheel shaft

A caulked portion is formed by plastic deformation of a cylindrical portion which is formed at the inner end of a shaft member and is reduced in thickness toward the leading end in a diametrical outward direction. The force required to form the caulked portion is reduced, and the caulked portion is less susceptible to damage. Further, the force of the caulked portion for supporting the inner ring is ensured.

BACKGROUND OF THE INVENTION 
The present invention relates to a hub unit for supporting a wheel shaft, 
which is utilized for supporting a wheel of a vehicle rotatively relative 
to a suspension system. 
Wheels of a vehicle are supported on the suspension system by means of hub 
units for supporting wheels. FIG. 8 shows one example of a hub unit for 
supporting a wheel described in U.S. Pat. No. 5,490,732. A hub unit 1 for 
supporting a wheel comprises a shaft member 2, a pair of inner rings 3a, 
3b, an outer ring 4, and a plurality of rolling elements 5, 5. A flange (a 
first flange) 6 for supporting a wheel is formed along the outer 
peripheral surface of outer-edge portion of the shaft member 2 (here the 
expression "out or outside" designates a direction toward the outside of 
the vehicle in a widthwise direction when the hub unit is mounted on the 
vehicle, that is, a leftward direction in FIG. 8, and in contrast, an 
expression "in or inside" designates a direction toward the center of the 
vehicle in the widthwise direction, that is, a rightward direction in FIG. 
8.). A stepped portion 7 is formed along the base of the flange 6 in the 
vicinity of the axial center of the axial member 2. 
The pair of inner rings 3a, 3b are provided and fitted around the outer 
surface of the axial member 2 which ranges from the middle portion to the 
inner end of the axial member. The outer side surface of the inner ring 3a 
abuts a vertical surface of the stepped portion 7, and the outer side 
surface of the inner ring 3b abuts the inner side surface of the inner 
ring 3a. A cylindrical portion 8 is formed in the inner end surface of the 
shaft member 2. A caulked portion 9 is formed by bending, in a diametrical 
outward direction, the edge of the cylindrical portion 8 which protrudes 
toward the inside beyond the inner end surface of the inner ring 3b. The 
pair of inner rings 3a, 3b are sandwiched between the vertical surface of 
the stepped portion 7 and the caulked portion 9. 
The plurality of rolling elements 5, 5 are provided between a pair of outer 
(first and second) raceways 10, 10 formed along the inner peripheral 
surface of the outer ring 4 and a pair of inner (first and second) 
raceways 11, 11 formed respectively along the outer peripheral surfaces of 
the inner rings 3a, 3b. Although balls are used as the rolling elements 5, 
5 in an illustrated example, tapered rollers may also be used as the 
rolling elements for a heavy hub unit for supporting a vehicle wheel. 
Alternatively, the (first) inner raceway formed adjacent to the flange 6 
may be directly formed along the outer peripheral surface of the shaft 
member 2, and the outer inner ring 3a may be omitted in some cases. In 
such a case, the stepped portion 7 is formed in an inner position relative 
to the inner ring 3a shown in FIG. 8. 
The foregoing hub unit 1 is locked to the vehicle by securing the outer 
wheel 4 to the suspension system through use of an outwardly extending 
flanged mount (a second flange) 12 formed along the outer peripheral 
surface of the outer wheel 4, and by fastening the wheel to the flange 6. 
As a result, the wheel can be rotatively secured to the suspension system. 
In the case of the conventional construction shown in FIG. 8, the caulked 
portion 9 is susceptible to damage such as cracks when being formed in 
order to fixedly fit the inner rings 3a, 3b on the shaft member 2. 
Further, at the time of formation of the caulked portion 9, force is 
exerted, in a diametrical outward direction, on the inner peripheral 
surface of the inner ring 3b adjoining the caulked portion 9. In short, in 
the case of the conventional construction, for the purpose of forming the 
caulked portion 9, the cylindrical portion 8 is formed in the inner end 
face of the shaft member 2 in such a way that the inner and outer 
peripheral surfaces of the cylindrical portion 8 become concentric with 
each other in the axial direction of the shaft member. Since the outer and 
inner peripheral surfaces of the shaft member are formed into mere 
cylindrical surfaces which are concentric with each other, the cylindrical 
portion 8 is formed with uniform thickness over its entire length. Such a 
construction requires great force to produce the caulked portion 9 by 
caulking the protruding edge of the cylindrical portion 8, rendering the 
caulking operation laborious. Further, the caulking operation entails 
exertion of great tensile force on the leading edge of the caulked portion 
9, rendering the caulked portion susceptible to damage. 
Since great force is exerted on the leading edge of the caulked portion 
when the caulked portion is formed by caulking, the force exerted on the 
internal peripheral surface of the inner ring 3b from the caulked portion 
is also increased correspondingly. Although not to a greater extent, the 
diameter of the inner ring 3b is slightly changed. If there is an increase 
in the amount of change in the diameter of the inner ring, there arises a 
risk of the inner ring 3b being subjected to damage such as cracks, and a 
risk of a change in the diameter of the inner raceways 11, 11 formed along 
the outer peripheral surface of the inner ring 3b or of deterioration of 
geometrical accuracy (e.g., roundness or the degree of accuracy of a 
cross-sectional profile) of the inner ring. This in turn makes it 
laborious to maintain at an optimum value the preload imparted to the 
rolling elements 5, 5 interposed between the inner raceways 11, 11 formed 
along the outer peripheral surface of the inner ring 3b and the outer 
raceways 10, 10 facing the inner raceways 11, 11, posing the risk of 
difficulty of ensuring the durability of the hub unit 1. 
SUMMARY OF THE INVENTION 
The hub unit according to the present invention has been conceived in view 
of the foregoing problems, and an object of the present invention is to 
prevent a caulked portion from being subjected to damage such as cracks 
when an inner ring is fixed, as well as to prevent a change in the inner 
diameter of the inner ring or the diameter of inner raceways formed along 
the outer periphery of the inner ring, which would otherwise caused by a 
caulking operation to such an extent as to pose practical problem. 
Similar to the conventional hub unit mentioned previously, a hub unit for 
supporting a wheel of a vehicle according to the present invention also 
includes: a shaft member having a first flange which is formed at one end 
of the shaft member on the outer peripheral surface of the same; a first 
inner raceway which is formed directly on the outer peripheral surface of 
the intermediate portion of the shaft member or indirectly on the same by 
way of an inner raceway separated from the shaft member; a stepped portion 
which is formed in the vicinity of the other end of the shaft member and 
which has an outer diameter smaller than that of the area of the shaft 
member where the first inner raceway is formed; an inner ring which has a 
second inner raceway formed on the outer peripheral surface thereof and 
which is fitted around the stepped portion; an outer ring which has a 
first outer raceway formed on the internal peripheral surface thereof so 
as to face the first inner raceway, a second outer raceway formed on the 
same so as to face the second inner raceway, and a second flange formed on 
the outer peripheral surface thereof; and a plurality of rolling elements 
which are interposed between the first inner raceway and the first outer 
raceway and between the second inner raceway and the second outer raceway. 
The hub unit of the present invention further comprises a caulked portion 
formed by caulking and spreading in a diametrical outward direction a 
portion of a cylindrical portion which is formed on the other end of the 
shaft member and which extends beyond at least the inner ring fitted 
around the stepped portion. The caulked portion presses the inner ring 
fitted around the stepped portion toward the end face of stepped portion, 
thereby fixedly fastening the inner ring fitted around the stepped portion 
to the shaft member. 
Particularly, the hub unit according to the present invention is 
characterized by the feature that the cylindrical portion has a smaller 
thickness toward the leading end of the cylindrical portion before it is 
caulked and spread in the diametrical outward direction, and the thickness 
of the caulked portion, which is formed by caulking and spreading in the 
diametrical outward direction the cylindrical portion and which presses 
the end face of the inner ring fitted around the stepped portion, is 
gradually reduced toward the leading end of the caulked portion in 
comparison with the thickness of the base of the cylindrical portion. 
More preferably, the hub unit according to the present invention has one or 
both of the following requirements 1 and 2. 
1 The opening formed on the other end of the ring has a chamfer which has a 
circular-arc cross section and which connects the inner end flat surface 
of the inner ring to the inner peripheral and cylindrical surface of the 
inner ring. The outer peripheral edge of the caulked portion is placed at 
an inner position in a diametrically inward direction relative to the 
point of intersection between the outer peripheral edge of the chamfer and 
the inner peripheral edge of the inner end surface of the inner ring. 
2 A first axial position is set at a point which is spaced away from the 
inner end surface of the inner ring toward the center axis of the shaft 
member by a distance which is 1.26 times the width to be caulked equal to 
half of the difference between the outer diameter of the caulked portion 
and the inner diameter of the inner ring. Further, the inner most end of 
the inner peripheral surface used for forming the caulked portion is taken 
as a second axial position. A third axial position is set at a point on 
the side surface close to the inner end surface of the inner ring among 
the surfaces of the inner raceway formed along the outer peripheral 
surface of the inner ring. In a state in which the caulked portion is 
formed, the second axial position is placed at a position, in the axial 
direction of the shaft member, between the first axial position and the 
third axial position. 
The operation of the hub unit for supporting a vehicle wheel having the 
foregoing structure according to the present invention is the same as that 
of the conventional hub unit; that is, the hub unit rotatively supports a 
wheel with respect to a suspension system. 
Particularly, in the case of the hub unit according to the present 
invention, since the thickness of the cylindrical portion used for forming 
the caulked portion is gradually reduced toward the leading edge, 
excessively great force is not required to form the caulked portion. 
Accordingly, the caulked portion is protected from damage such as cracks, 
which would otherwise be caused when the caulked portion is machined. 
Further, it is possible to prevent force, which would otherwise change the 
diameter of the inner ring fitted to the shaft member by the caulked 
portion such an extent as to affect the preload exerted on the inner ring 
or the durability of the inner ring such as the rolling fatigue life of 
the inner ring. 
In a case where the hub unit satisfies the requirement 1, the outer 
peripheral edge of the caulked portion can be effectively protected from 
imperfections such as cracks, burrs, or underfill. 
Further, in a case where the hub unit satisfies the requirement 2, no 
clearance arises between the outer peripheral surface of the caulked 
portion and the inner peripheral surface of the inner ring, and the 
supporting strength of the inner ring imparted by the caulked portion is 
ensured. Further, the deformation of the inner raceway is also prevented.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1 through 7 show one example in which the present invention is 
carried out. The features of the present invention lie in a structure for 
securing an inner ring 3 relative to a shaft member 2. In contrast with 
the conventional structure shown in FIG. 8, the structure according to the 
present embodiment comprises one inner ring 3 and a pair of inner raceways 
11, 11 an outer one of which is directly formed along the outer peripheral 
surface of the shaft member 2. Accordingly, the stepped portion 7 is 
formed along an inner end portion of the shaft member 2. Since the hub 
unit according to the embodiment is the same in structure and operation as 
the conventional hub unit shown in FIG. 8, and hence overlapping 
explanations thereof will be omitted or simplified. An explanation will be 
primarily given of the characteristic portion of the present invention. 
FIG. 6 shows the wall thickness of a cylindrical portion 8a which is formed 
in the inner end of the shaft member 2 in order to constitute a caulked 
portion 9a used for fixing the inner ring 3. Before being caulked in a 
diametrical outward direction, the cylindrical portion 8a is tapered so as 
to have a smaller thickness toward the leading end. To this end, in the 
case of the illustrated shaft member, a tapered hole 13 is formed in the 
inner end surface of the shaft member 2 so as to have a smaller inner 
diameter toward the inner most end of the hole. 
To caulk the leading edge of the cylindrical portion 8a having such a 
structure in order to fix the inner ring 3 around the outer periphery of 
the inner end portion of the shaft member 2, the shaft member 2 is 
fastened so as not to axially move off. In addition, as shown in FIG. 7, 
while the outer peripheral surface of the inner ring 3 is fixedly held by 
a pressure piece 14 so as to prevent the shaft member 2 having the inner 
ring 3 fitted thereon from developing deflections in the direction of the 
diameter of the shaft member, a press mold 15 is forcibly pressed against 
the leading end of the cylindrical portion 8a in a manner as shown in the 
drawing. A truncated conical protuberance 16 which is removably inserted 
into the inside of the cylindrical portion 18a is formed at the center of 
the end face (or the lowermost end face shown in FIG. 7) of the press mold 
15. A recess 17 having a circular-arc cross section is formed so as to 
surround the truncated conical protuberance 16. 
The shape and size of the cylindrical portion 8a, and the cross-sectional 
profile, outer diameter, and depth of the recess 17 are determined in such 
a way that the caulked portion 9a having the following predetermined shape 
and size is formed by plastic deformation of the cylindrical portion 8a 
while force is imparted to metal (or steel) constituting the cylindrical 
portion 8a in a direction in which the metal is compressed. An inner end 
face 18 in the form of a plane surface at right angles to the center axis 
of the inner ring 3, is formed around an inner-side opening of the inner 
ring 3 which is used for fixing the inner ring to the inner end portion of 
the shaft member 2 (i.e., the right end portion of the shaft member shown 
in FIG. 1 and the upper end portion shown in each of FIGS. 2, 4, 5, 6, and 
7) by the caulked portion 9a. The inner peripheral edge of the inner end 
surface 18 is continually connected to an inner peripheral and cylindrical 
surface 19 of the inner ring 3 by way of a chamfer 20 having a 
circular-arc cross section. 
The caulked portion 9a used for fixing the inner ring 3, whose inner end is 
formed into the foregoing shape, to the stepped portion 7 of the shaft 
member 2, is formed by caulking and extending the caulked portion 9a in a 
diametrical outward direction. The thickness of the cylindrical portion is 
gradually reduced toward the leading end in comparison with the thickness 
a.sub.0 (shown in FIG. 2) of the base of the caulked portion. More 
specifically, as shown in FIG. 4, taking the thickness of the base of the 
caulked portion 9a as a.sub.0, the thickness of the caulked portion 9a 
changes in the order of a.sub.0, a.sub.1, a.sub.2, . . . a.sub.n toward 
the leading end of the caulked portion. The cross-sectional profile of 
each of the protuberance 16 and the recess 17 is determined in such a way 
that the caulked portion 9a is formed so as to maintain the relationship 
of wall thickness represented by a.sub.0 &gt;a.sub.1 &gt;a.sub.2 &gt; . . . 
&gt;a.sub.n and that the thickness a.sub.n of the leading end of the caulked 
portion 9a does not become zero (a.sub.n &gt;0). 
The reason why the tapered hole 13 for forming the cylindrical portion 8a 
is formed in the inner end face of the shaft member 2 in such a way that 
the inner diameter of the tapered hole becomes gradually smaller toward 
the innermost end of the hole, is for forming the caulked portion 9a 
mentioned previously. In short, given that the volume V.sub.9a of the 
caulked portion 9a is formed so as to have a constant value by caulking 
and extending the cylindrical portion 8a in the diametrically outward 
direction, the relationship between the height H.sub.8a of the cylindrical 
portion 8a (see FIG. 6) and the angle .theta..sub.21 (FIG. 6) relative to 
the center axis of the cylindrical portion 8a is expressed by a curved 
line .alpha. shown in FIG. 3. The height H.sub.8a of the cylindrical 
portion Ba ensures formation of the caulked portion 9a and designates an 
axial dimension of the tapered hole 13 with the exception of a conical 
portion 22 formed at the innermost end of the tapered hole 13. 
The volume V.sub.9a of the caulked portion 9a strongly affects the strength 
of the caulked portion used for fixing the inner ring 3 on the shaft 
member 2. More specifically, the greater the volume V.sub.9a is increased, 
the greater the fixing strength of the caulked portion is increased. On 
the other hand, the caulked portion 9a becomes difficult to form, and the 
weight of a hub unit for supporting a wheel is also increased. For these 
reasons, it is necessary to maintain the volume V.sub.9a of the caulked 
portion 9a to such an extent as to be able to ensure required strength to 
prevent a defect in the caulked portion 9a. Turning again back to the 
curved line .alpha. from the foregoing viewpoint, as the height H.sub.8a 
and the angle .theta..sub.21 become smaller; that is, the cylindrical 
portion 8a becomes shorter in the axial direction and the inner peripheral 
surface 21 becomes close to a cylindrical surface (i.e., within the range 
of curved line indicated by 1' in FIG. 3), the outer peripheral edge of 
the caulked portion 9a departs from the surface of the inner ring 3, 
thereby causing a so-called underfilled portion. The underfilled portion 
is not desirable, because the underfilled portion causes a reduction in 
the strength of the caulked portion 9a used for fixing the inner ring 3. 
In contrast, as the height H.sub.8a and the angle .theta..sub.21 become 
larger (i.e., within the range of curved line indicated by 2' in FIG. 3); 
that is, as the cylindrical portion 8a becomes longer in the axial 
direction and the inner peripheral surface 21 of the cylindrical portion 
8a is changed from a cylindrical surface to a conical surface, thin burrs 
which are not useful for fitting the inner ring 3 on the stepped portion 7 
arise in the outer peripheral edge of the caulked portion 9a or there 
readily occur cracks which cause a reduction in the strength of the area 
of the caulked portion that presses the inner ring 3 against the stepped 
portion 7. Accordingly, in order to ensure strength sufficient to fix the 
inner ring 3 on the shaft member 2 with a given volume V.sub.9a, it is 
desirable to maintain the height H.sub.8a and the angle .theta..sub.21 
within a given range between lines 3' and 4' shown in FIG. 3. According to 
the research performed by the inventors, it is conceivable that so long as 
the angle .theta..sub.21 is set to about 20 degrees, strength most 
sufficient to fix the inner ring 3 on the shaft member 2 can be secured 
with a given volume V.sub.9a. 
The overall outer peripheral edge of the caulked portion 9a is set at an 
inner position in the diametrical direction relative to the inner 
peripheral edge of the inner end surface 18. In other words, as show in 
FIG. 2, the overall outer peripheral edge of the caulked portion 9a is 
placed at an inner position in the diametrical direction relative to a 
point of junction I between the outer peripheral edge of the chamfer 20 
and the inner peripheral edge of the inner end surface 18. The reason why 
the overall outer peripheral edge of the caulked portion 9a is placed in 
an inner position in the diametrical direction relative to the point of 
junction I, is also for preventing burrs or cracks from arising in the 
outer peripheral edge of the caulked portion 9a. 
The position of the innermost end of the inner peripheral surface 21 of the 
cylindrical portion 8a used for forming the caulked portion 9a is defined 
by the relationship between the width W.sub.9a of the caulked portion 9a 
(see FIG. 5) and the inner end of the inner raceway 11 (i.e., the second 
inner raceway) formed along the outer peripheral surface of the inner ring 
3. In this connection, an explanation will be given by reference to FIG. 
5. First, a first axial position A is set at point which is spaced away 
from the inner end surface 18 of the inner ring toward the center in the 
axial direction of the shaft member by a distance which is 1.26 times 
(1.26 W.sub.9a =L.sub.9a) the width W.sub.9a {=(D.sub.9a -R.sub.3)/2} 
equal to half of the difference between the outer diameter D.sub.9a of the 
caulked portion 9a and the inner diameter R.sub.3 of the inner ring 3. 
Further, the inner most end of the inner peripheral surface 21 is taken as 
a second axial position C. A third axial position B is set at a point on 
the side surface close to the inner end surface 18 of the inner ring 3 
among the surfaces of the inner raceway 11 formed along the outer 
peripheral surface of the inner ring 3. In such a case, in a state in 
which the caulked portion 9a is formed, the height H.sub.8a of the 
cylindrical portion 8a is determined in such a way that the second axial 
position C comes to a position in the vicinity of the first axial position 
A in the axial direction of the shaft member 2 within the range from the 
first axial position A to the third axial position B. 
The reason why the second axial position C is determined in consideration 
of the relationship between the first and third axial positions A and B, 
is also for causing the caulked portion 9a to exert the effect of fixing 
the inner ring 3 to the maximum, and for preventing the inner raceway 11 
to be deformed. If the second axial position C is set at a position closer 
to the leading edge of the cylindrical portion Ba relative to the first 
axial position A, a gap is apt to arise between a part of the outer 
peripheral surface of the caulked portion 9a formed by caulking the 
cylindrical portion 8a and the chamfer 20 formed along the inner opening 
of the inner ring 3. If such a gap arises, the strength of the caulked 
portion 9a used for fixing the inner ring 3 is reduced. In contrast, if 
the second axial position C is set at a position closer to the inner 
raceway 11 relative to the third axial position B, force is exerted, in a 
diametrical outward direction, on the area of the inner raceway 11 where 
the inner ring 3 is formed when the caulked portion 9a is formed, thereby 
rendering the inner raceway 11 susceptible to a dimensional change. For 
these reasons, it is desirable to determine the second axial position C in 
consideration of the relationship between the first and third axial 
positions A and B. 
In order to form the caulked portion 9a having a geometry such as that 
mentioned previously by plastic deformation of the cylindrical portion 8a 
of such dimension and shape as those mentioned previously, the tilt angle 
.theta..sub.21 of the inner peripheral surface 21 of the cylindrical 
portion 8a is preferably set to about 20 degrees. The recess 17 of the 
press mold 15 is formed so as to have a combined curved surface whose 
curvature radius becomes smaller in the direction of outer diameter. With 
this combined curved surface, when the caulked portion 9a is formed by 
plastic deformation of the leading end of the cylindrical portion 8a, the 
cross section of the caulked portion 9a becomes gradually smaller in 
thickness from the base to the leading edge of the caulked portion. 
Particularly, the caulked portion 9a becomes abruptly smaller in thickness 
at the leading edge of the caulked portion. The outer diameter R.sub.17 of 
the recess 17 (see FIGS. 2 and 4) is set so as to become equal to or 
slightly smaller than the outside diameter R.sub.9a of the caulked portion 
9a of the caulked portion 9a to be formed (i.e., R.sub.17 
.ltoreq.D.sub.9a) The depth D.sub.17 of the recess 17 (see FIGS. 2 and 4) 
is determined in such a way as to ensure a clearance 23 between the 
leading end surface of the press mold 15 and the inner end surface 18 of 
the inner ring 3 while the caulked portion 9a is formed and while the 
leading end of the cylindrical portion 8a is held along the inner portion 
of the inner peripheral surface of the inner ring 3 and along the inner 
end surface 18 of the same. 
So long as the press mold 15 having the protuberance 16 and the recess 17 
which have the shapes and dimensions such as those mentioned previously is 
pressed against the leading end of the cylindrical portion 8a, the leading 
end of the cylindrical portion 8a is caulked and extended in a diametrical 
outward direction, enabling formation of the caulked portion 9a. The inner 
ring 3 is sandwiched between the thus-formed caulked portion 9a and a 
vertical surface 24 of the stepped portion 7 formed in the vicinity of the 
inner end portion of the shaft member 2, thereby enabling the inner ring 3 
to be fixedly fitted to the shaft member 2. In the case of an illustrated 
embodiment, compressive force is exerted in a diametrical inward direction 
on the outer peripheral surface of the caulked portion 9a from the inner 
surface of the recess 17 in the final stage where the caulked portion 9a 
is formed by plastic deformation of the inner end surface of the 
cylindrical portion 8a. Accordingly, damage such as cracks can be 
effectively prevented from arising in the outer peripheral edge of the 
caulked portion 9a. Further, the chamfer 20 having a circular-arc cross 
section is formed along the periphery of the inner opening of the inner 
ring 3 with which the outer peripheral surface of the base of the caulked 
portion 9a comes into contact. Accordingly, the curvature radius of the 
base of the caulked portion 9a is prevented from being reduced, and hence 
the base of the caulked portion is prevented from being subjected to 
excessive stress. 
As mentioned previously, in the hub unit for supporting a wheel according 
to the present invention, the cylindrical portion 8a used for forming the 
caulked portion 9a is formed so as to become smaller in thickness toward 
the leading end of the cylindrical portion. By virtue of such a 
configuration, it is not required to apply excessively strong force to 
form the caulked portion 9a by plastic deformation of the leading end of 
the cylindrical portion Ba through use of the press mold 15 having the 
foregoing structure. Accordingly, the caulked portion 9a is prevented from 
being subjected to damage such as cracks when being formed, or the inner 
ring 3 to be fixed by the caulked portion 9a is prevented from being 
subjected to force which would otherwise change the diameter of the inner 
ring 3 to such an extent as to affect the preload exerted on the inner 
ring or the durability of the inner ring such as rolling-fatigue life. 
Particularly, in the case of the illustrated embodiment, compressive 
stress is exerted on the leading end of the caulked portion 9a, and the 
curvature radius of the base of the caulked portion 9a is increased. 
Therefore, the caulked portion 9a is effectively protected from damage. 
It is desirable to set the line of action of the load exerted on the inner 
ring 3 from the plurality of rolling elements 5, 5 (i.e., chain line 
.beta. which is shown in FIG. 1 and indicates a contact angle of the 
rolling element 5) so as to pass through a mating surface between the 
inner peripheral surface of the inner ring 3 and the leading end of the 
shaft member 2 and so as not to pass through the caulked portion 9a. The 
reason for this is that the caulked portion 9a is protected from damage by 
preventing the load from directly acting on the caulked portion 9a as 
deforming force in the diametrical inward direction. 
Next, taking the cross-sectional area of the portion of the inner ring 3 
closer toward the outside relative to the inner raceway 11 (i.e., the area 
indicated by line X--X shown in 
FIG. 2) as S.sub.3 and the cross-sectional area of the shaft member 2 in 
the corresponding portion as S.sub.2, the relationship between S.sub.2 and 
S.sub.3 is set to S.sub.3 &lt;S.sub.2 and more preferably to S.sub.3 
&lt;0.94S.sub.2. The reason why the cross-sectional area of each of these 
areas is limited, is for ensuring the holding strength of the inner ring 3 
relative to the shaft member 2. In other words, force to axially press the 
inner ring 3 and to prevent rotation of the inner ring 3 (i.e., axial 
force) while the inner ring 3 is held between the caulked portion 9a and 
the vertical surface 24, is determined by the difference between the 
amount of axial deflection of the shaft member 2 and the amount of axial 
deflection of the inner ring 3. In short, during the course of a caulking 
operation, the amount of elastic deflection of the inner ring 3 is greater 
than that of the shaft member 2. After completion of the caulking 
operation, the inner ring 3 and the shaft member 2 elastically return to 
their original states, imparting axial force to the inner ring 3. Since 
the inner ring 3 is formed from substantially the same material as that of 
the shaft member 2, they have substantially the same elastic modulus. As 
mentioned previously, so long as the relationship between S.sub.2 and 
S.sub.3 is set to S.sub.3 &lt;S.sub.2 the amount of elastic deformation of 
the inner ring 3 is greater than that of the shaft member 2 during the 
caulking operation. Accordingly, so long as the cross-sectional areas of 
corresponding portions of the inner ring and the shaft member are limited 
in a manner as mentioned previously, sufficient compressive load is 
continually imparted to the inner ring 3, thereby effectively preventing a 
so-called creeping phenomenon in which the inner ring 3 rotates around the 
shaft member 2. 
EXAMPLE 
An explanation will be given of one example of dimensions of respective 
elements suitable for implementing the structure such as that shown in 
FIGS. 1 and 2. The shaft member 2 and the inner ring 3 are formed of 
structural carbon steel (S53C) including 0.4 to 0.6 wt % carbon, and the 
inner raceway 11 and other required portions are subjected to induction 
hardening treatment. First, the inner diameter R.sub.3 of the inner ring 3 
to be fitted around the shaft member 3 (see FIG. 5) is set to 26.0 mm. 
Further, axial distance L.sub.C from the axial leading edge of the caulked 
portion 9a to the second axial position C set at a point of the innermost 
end of the inner peripheral surface 21 of the cylindrical portion 8a (see 
FIG. 6) used for forming the caulked portion 9a is set to 7.5 mm. Further, 
distance L.sub.11 from the inner end surface 18 of the inner ring 3 to the 
third axial position B, which is set at an end of the inner raceway 11, 
which is formed along the outer periphery of the inner ring 3, at the side 
of the inner end surface 18, is set to 9.36 mm. Further, the inner 
diameter r.sub.21 of the inner peripheral surface 21 at the second axial 
direction C is set to 11.4 mm. Axial distance L.sub.BC between the second 
and third axial positions B and C is set to 3.86 mm. In such a case, axial 
distance L.sub.AC (not shown) between the first axial position A and the 
second axial position C is set to 0.5 mm. Still further, the thickness a 
of the caulked portion 9a at the first axial position A is set to 7.3 mm. 
Furthermore, the distance from the outer peripheral edge of the caulked 
portion 9a to the point of junction I between the inner end surface 18 and 
the chamfer 20 is set to 0.96 mm. 
The caulked portion 9a mentioned previously is formed by elastic 
deformation of the cylindrical portion 8a formed at the inner end of the 
shaft member 2 by oscillating pressing operations such as those shown in 
FIG. 7. In the oscillating pressing operation, a oscillating press machine 
called a rocking press having a capacity of about 100 tons for example is 
used to machine the cylindrical portion for about five seconds through 
oscillating pressing while the angle of oscillation .theta..sub.15 is set 
to about two degrees. 
A hub unit for supporting a vehicle wheel according to the present 
invention has the foregoing structure and operates in a manner as 
mentioned previously, and hence a caulked portion is protected from damage 
such as cracks or burrs which cause an excessive thickness of the caulked 
portion. Further, the caulked portion prevents the diameter of an inner 
ring to be fitted around a shaft member from being changed to such an 
extent as to pose practical problems. Moreover, the risk of the inner ring 
or the caulked portion being damaged or subjected to imperfections by 
fitting of the inner ring around the shaft member is reduced, and the 
preload exerted on the inner ring can be maintained at an optimum value.