JP2003214451A - Installing structure of rolling bearing - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent the occurrence of a creep between a knuckle 3 and a rolling bearing 2 at high temperature time even if the knuckle 3 for rotatably supporting a shaft body 1 via the rolling bearing 2 is made of light metal having a linear expansion coefficient larger than the bearing in an installing structure of the rolling bearing 2 applied to a vehicular hub unit. <P>SOLUTION: This installing structure of the rolling bearing 2 rotatably supports the shaft body 1 to the knuckle 3, and is characterized in that the knuckle 3 is made of the light metal, and has a cylindrical part 31 capable of inserting the rolling bearing 2, and the rolling bearing 2 is inserted into the cylindrical part 31 of the knuckle 3, and an aligning plate spring 5 is interposed in a fitting surface of an outer peripheral surface of an outer race 23 of the rolling bearing 2 and an inner peripheral surface of the cylindrical part 31. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rolling bearing mounting structure applied to a wheel supporting device for a vehicle such as an automobile.

[0002]

2. Description of the Related Art An example of a conventional vehicle wheel support device is shown in FIG.

In FIG. 8, reference numeral 1 is a shaft body, and has a hub flange 11 for bolting the wheel to the vehicle outer side (right side in FIG. 8).

A rolling bearing 2 is press-fitted onto the outer peripheral surface of the shaft body 1 and fixed by a nut 42 screwed to a threaded portion formed at the vehicle inner side (left side in FIG. 8) end portion of the shaft body 1. ing.

The rolling bearing 2 is a double-row angular contact ball bearing, and has inner rings 21 and 22, an outer ring 23, and inner rings 21 and 21.
2 and two groups of balls 24 and 25 arranged along the race formed on the outer ring 23 and held by a retainer, and seal members 26 and 27 for sealing the annular space between the inner and outer rings from the outside Has been done.

The rolling bearing 2 is fixed to a knuckle 3 provided on the vehicle body. Knuckle 3 is rolling bearing 2
Has a cylindrical portion 31 in which a mounting hole 37 into which the rolling bearing 2 can be inserted is formed, and the rolling bearing 2 is inserted into the mounting hole 37 of the cylindrical portion 31 from the vehicle outer side, and the vehicle inner end 23a of the outer ring 23 is inserted.
Is brought into contact with the annular protrusion 32 of the knuckle 3. Further, the snap ring 4 is fitted into the groove 33 formed on the inner peripheral surface of the tubular portion 31 on the vehicle outer side, the snap ring 4 is brought into contact with the vehicle outer side end portion 23b of the outer ring 23, and the outer ring 23 is fixed to the annular projection. Three
It is sandwiched between 2 and snap ring 4 and fixed to knuckle 3.

In this way, the shaft body 1 has the rolling bearing 2
Is accurately positioned with respect to the knuckle 3 and is rotatably supported.

[0008]

In the conventional vehicle wheel support device for a vehicle, the rolling bearing 2 and the knuckle 3 are provided.
Is composed of an iron-based material.

However, in the wheel supporting device, in order to reduce the weight of the vehicle body, it has been proposed to form the knuckle 3 which is an iron-based material with a light metal such as aluminum.

With this structure, the outer ring 2 made of a ferrous material is used.
Since the linear expansion coefficient of the knuckle 3 is different from that of the light metal knuckle 3, a gap is generated between the outer ring 23 and the knuckle 3 at high temperature. That is, light metal has a larger linear expansion coefficient than iron-based materials, and the axial extension of the tubular portion 31 of the knuckle 3 at high temperature is larger than that of the outer ring 23, so that the annular protrusion 32 of the tubular portion 31 is Since the axial distance between the snap rings 4 is larger than the axial dimension of the outer ring 23, a gap is generated in the axial direction.

Further, since the inner diameter of the cylindrical portion 31 is larger than the outer diameter of the outer ring 23, a radial gap is generated between them.

When the axial and radial gaps are generated between the outer ring 23 and the knuckle 3 as described above, the fixing strength between the outer ring 23 and the knuckle 3 is reduced, and the outer ring 23 is rotated by the rotation of the shaft body 1. So-called creep, which slides in the rotational direction with respect to 3, occurs.

According to the present invention, even if the support member for rotatably supporting the shaft body through the rolling bearing is made of a light metal having a linear expansion coefficient larger than that of the bearing, creep occurs between the support member and the rolling bearing at high temperature. It is an object of the present invention to provide a rolling bearing mounting structure capable of preventing the occurrence of the rolling bearing.

[0014]

According to a first aspect of the present invention, there is provided a rolling bearing mounting structure for rotatably supporting a shaft member with respect to a supporting member, wherein the supporting member has a wire more than an outer ring of the rolling bearing. It is made of a material having a large expansion coefficient and has a tubular portion into which the rolling bearing can be inserted. The rolling bearing is inserted into the tubular portion of the support member, and the outer peripheral surface of the outer ring of the rolling bearing and the tubular portion. A bearing holding member that is expandable and contractable in the radial direction is provided on the fitting surface with the inner peripheral surface of the cylindrical portion, which gives a tightening margin to the fitting surface when the cylindrical portion linearly expands.

According to the rolling bearing mounting structure of claim 1 of the present invention, the support member made of a light metal, which has a larger linear expansion coefficient than the outer ring of the rolling bearing, has a radial extension at a high temperature as compared with that of the outer ring. The size of the rolling bearing increases, and a radial gap is generated between the outer ring of the rolling bearing and the support member. However, even if a gap is generated between the outer ring and the support member, since the bearing holding member that can expand and contract in the radial direction is provided on the fitting surface of the outer ring and the supporting member, the fitting surface of the bearing holding member Can be given a tightening margin. As a result, the fixing strength between the outer ring and the support member does not decrease, and it is possible to prevent the occurrence of so-called creep in which the outer ring slides in the rotational direction with respect to the support member as the shaft rotates.

A peripheral groove may be formed along the inner peripheral surface of the cylindrical portion of the support member, and the bearing holding member may be housed in the peripheral groove.

According to a third aspect of the present invention, there is provided a rolling bearing mounting structure for rotatably supporting the shaft member with respect to the supporting member, wherein the supporting member is made of a material having a linear expansion coefficient larger than that of the outer ring of the rolling bearing. It is configured and has a tubular portion into which the rolling bearing can be inserted, and the rolling bearing is inserted into the tubular portion of the support member. At both axial end portions of the rolling bearing, at the axial end surface of the tubular portion. The fixing member is provided so as to abut and regulate the linear expansion of the tubular portion in the axial direction.

According to the mounting structure of the rolling bearing of claim 3 of the present invention, since both axial ends of the tubular portion are sandwiched by the fixing members provided at both axial end portions of the rolling bearing, the tubular portion of the tubular portion can be heated at high temperature. With the expansion in the axial direction, both axial ends of the tubular portion press the fixing members in the axial direction, and the axial force increases.
As a result, the fixing strength between the outer ring and the support member does not decrease, and it is possible to prevent the occurrence of so-called creep in which the outer ring slides in the rotational direction with respect to the support member as the shaft rotates.

For example, in the rolling bearing mounting structure according to any one of claims 1 to 3, the shaft body is an axle and the support member is a knuckle which is a part of a vehicle frame.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (Embodiment 1) Embodiment 1 of the present invention will be described with reference to FIGS.

FIG. 1 is a partial cross-sectional view of a wheel support device for an inner ring rotating vehicle to which the rolling bearing mounting structure according to this embodiment is applied, FIG. 2 is a perspective view of a centering leaf spring, and FIG. 3 is a rolling bearing. The action explanatory drawing in the attachment structure of is shown.
The same parts as those in the example shown in FIG. 8 are designated by the same reference numerals and the description thereof will be omitted.

Further, the vehicle inner side in the following description of each embodiment means the left side of FIGS. 1 and 3 to 7.
The outer side of the vehicle is the right side of the figure.

In this embodiment, between the knuckle 3 made of a light metal such as aluminum and the outer ring 23 of the rolling bearing 2,
The centering leaf spring 5 serving as a bearing holding member is interposed.

That is, the peripheral groove 34 is formed along the inner peripheral surface of the mounting hole 37 of the tubular portion 31 of the knuckle 3, and the centering leaf spring 5 is fitted in the peripheral groove 34.

As shown in FIG. 2, the centering leaf spring 5 is made of a corrugated steel sheet having elasticity and is formed annularly with a notch 53, and has a mountain portion 51 on the inner peripheral surface. The troughs 52 are alternately arranged and have elasticity capable of expanding and contracting in the radial direction.

Next, the assembly of the vehicle wheel support device will be described.

First, the rolling bearing 2 is assembled. next,
The assembled inner rings 21 and 22 of the rolling bearing 2 are press-fitted into the outer peripheral surface of the shaft body 1 from the vehicle inner side, and the inner rings 21 and 22 are shafted by the caulking portion 12 formed at the vehicle inner side end of the shaft body 1. It is fixed to the body 1, and the shaft body 1 and the rolling bearing 2 are integrated.
Next, the centering leaf spring 5 is housed in the circumferential groove 34 in a radially compressed state. Then, the rolling bearing 2 integrated with the bearing 1 is inserted into the mounting hole 37 of the tubular portion 31 of the knuckle 3 from the vehicle outer side, and the vehicle inner side end 23a of the outer ring 23 contacts the annular projection 32 of the knuckle 3. Contact.

When the rolling bearing 2 is inserted into the mounting hole 37 of the tubular portion 31, the centering leaf spring 5 is interposed between the circumferential groove 34 and the rolling bearing 2 in a radially compressed state. To be dressed.

Further, the snap ring 4 is fitted into the groove 33 formed in the inner peripheral surface of the tubular portion 31 on the vehicle outer side, and the snap ring 4 is brought into contact with the vehicle outer side end portion 23b of the outer ring 23. In this way, the outer ring 23 is sandwiched between the annular projection 32 and the snap ring 4 and fixed to the knuckle 3.

With this structure, as shown in FIG. 3, the knuckle 3 made of light metal has a larger linear expansion coefficient than the outer ring 23 made of an iron-based material. And an axial gap S is generated.

However, even if a gap S occurs between the outer ring 23 and the knuckle 3, the centering leaf spring 5 that expands and contracts in the radial direction on the fitting surface of the outer ring 23 and the knuckle 3 is compressed in the radial direction. The fixing force between the outer ring 23 and the knuckle 3 does not decrease due to the biasing force of the centering leaf spring 5, so that the shaft 1
It is possible to prevent the occurrence of so-called creep, in which the outer ring 23 slides in the rotational direction with respect to the knuckle 3 with the rotation of the.

(Second Embodiment) The second embodiment of the present invention will be described with reference to FIG.

FIG. 4 is a partial cross-sectional view of a vehicle wheel support device to which the rolling bearing mounting structure of this embodiment is applied. The same parts as those of the example shown in FIG. 8 are designated by the same reference numerals. And its description is omitted.

In this embodiment, the knuckle 3 made of light metal is used.
The cylindrical portion 31 of the rolling bearing 2 is fixed to the outer ring 23 of the rolling bearing 2 at both axial ends by fixing members so that the cylindrical portion 31 does not extend in the axial direction as compared with the outer ring 23 of the rolling bearing 2. Is.

That is, a screw thread 23c is formed on the outer circumference of the outer ring 23 on the vehicle inner side, and a screw groove 35 with which the screw thread 23c of the outer ring 23 is screwed is formed on the inner circumference of the tubular portion 31 of the knuckle 3 on the vehicle inner side. Form. The screw thread 23c and the screw groove 35 constitute one fixing member.

When fixing the rolling bearing 2 fitted onto the shaft body 1 to the knuckle 3, first, the snap ring 4 is fitted into the groove 23e formed in the outer peripheral surface of the outer ring 23 on the vehicle outer side. Next, the rolling bearing 2 is inserted into the tubular portion 31 of the knuckle 3 from the vehicle outer side, and the snap ring 4 is inserted into the tubular portion 31.
2 to the position where it abuts against the vehicle outer side end 31a
3c is screwed into the thread groove 35. In addition, snap ring 4
The vehicle outer side end portion 31a serves as the other fixing member.

According to the rolling bearing mounting structure constructed as described above, the tubular portion 31 has one axial end portion which is one fixing member (the screw thread 23c and the screw groove 35) formed on the vehicle inner side. Since it is sandwiched by the other fixing member (snap ring 4) formed on the vehicle outer side, at the time of high temperature, the vehicle outer side end 31a of the tubular portion 31 is expanded by the axial expansion of the tubular portion 31. The axial force that presses in the axial direction increases.

Thus, even if a radial gap is generated between the outer peripheral surface of the outer ring 23 and the inner peripheral surface of the tubular portion 31 due to the tubular portion 31 expanding in the radial direction at a high temperature, the above-mentioned shaft The outer ring 23 (rolling bearing 2) and the tubular portion 31 (knuckle 3) can be coupled by the increase in the axial force in the direction. As a result, the fixing strength between the outer ring 23 and the knuckle 3 does not decrease at high temperatures, and it is possible to prevent the occurrence of so-called creep in which the outer ring 23 slides in the rotational direction with respect to the knuckle 3 as the shaft 1 rotates.

(Third Embodiment) A third embodiment of the present invention will be described with reference to FIG.

FIG. 5 is a partial cross-sectional view of a vehicle wheel support device to which the rolling bearing mounting structure of this embodiment is applied. The same parts as those of the example shown in FIG. 8 are designated by the same reference numerals. And its description is omitted.

This embodiment is a modification of the second embodiment, in which the tubular portion 31 of the knuckle 3 made of light metal at high temperature is
By utilizing the phenomenon that the rolling bearing 2 expands more in the axial direction than the outer ring 23, creep is prevented from occurring between the tubular portion 31 and the outer ring 23.

First, the rolling bearing 2 is press-fitted into the tubular portion 31 of the knuckle 3 from the vehicle outer side. Then, the snap ring 4 serving as one of the fixing members is fitted into the groove 23g formed on the outer peripheral surface of the outer ring 23 on the vehicle inner side, and the snap ring 4 is brought into contact with the vehicle inner side end 31b of the tubular portion 31. Next, the thread groove 2 formed on the outer periphery of the outer ring 23 on the vehicle outer side
An annular nut 6 serving as the other fixing member is screwed into 3f and fastened to the vehicle outer side end 31a of the tubular portion 31. In this way, the shaft body 1 is inserted from the vehicle outer side into the inner ring of the rolling bearing 2 fixed to the knuckle 3 and fixed by the nut 42.

According to the rolling bearing mounting structure thus constructed, the tubular portion 31 has one axially opposite end portion which is one fixing member (snap ring 4) formed on the vehicle inner side.
And the other fixing member (nut 6) formed on the vehicle outer side, the vehicle outer side end portion 31a of the tubular portion 31 is accompanied by the axial expansion of the tubular portion 31 at high temperature.
Indicates the nut 6 and the vehicle inner side end 31b of the tubular portion 31 axially presses the snap ring 4 to increase the axial force.

Thus, even if a radial gap is generated between the outer peripheral surface of the outer ring 23 and the inner peripheral surface of the tubular portion 31 due to the tubular portion 31 expanding in the radial direction at a high temperature, the above-mentioned shaft The outer ring 23 (rolling bearing 2) and the tubular portion 31 (knuckle 3) can be coupled by the increase in the axial force in the direction. As a result, the fixing strength between the outer ring 23 and the knuckle 3 does not decrease at high temperature, and it is possible to prevent the occurrence of so-called creep in which the outer ring 23 slides in the rotational direction with respect to the knuckle 3 as the shaft 1 rotates.

Incidentally, when fixing the rolling bearing 2 to the knuckle 3, the rolling bearing 2 may be press-fitted into the tubular portion 31 of the knuckle 3 from the vehicle inner side.

(Fourth Embodiment) A fourth embodiment of the present invention will be described with reference to FIG.

FIG. 6 is a partial cross-sectional view of a vehicle wheel support device to which the rolling bearing mounting structure of this embodiment is applied. The same parts as those of the example shown in FIG. 8 are designated by the same reference numerals. And its description is omitted.

This embodiment is a modification of the second embodiment, in which the tubular portion 31 of the knuckle 3 made of light metal is
By utilizing the phenomenon that the rolling bearing 2 expands more in the axial direction than the outer ring 23, creep is prevented from occurring between the tubular portion 31 and the outer ring 23.

First, the rolling bearing 2 is press-fitted into the tubular portion 31 of the knuckle 3 from the vehicle outer side. The annular pressing member 7 serving as the other fixing member is provided on the vehicle outer side end portion 23b of the outer ring 23 and the vehicle outer side end portion 31a of the tubular portion 31.
To the screw hole 23 of the outer ring 23 by inserting the bolt 8 therethrough.
Fasten to h. Further, the snap ring 4 serving as one fixing member is fitted in the groove 23g formed on the outer peripheral surface of the outer ring 23 on the vehicle inner side, and the snap ring 4 is brought into contact with the vehicle inner end 31b of the tubular portion 31. . In this way
The shaft body 1 is inserted into the inner ring of the rolling bearing 2 fixed to the knuckle 3 from the vehicle outer side, and fixed by the nut 13.

According to the rolling bearing mounting structure thus constructed, the tubular portion 31 has one axial end portion which is one fixing member (snap ring 4) formed on the vehicle inner side.
And the other fixing member (pressing member 7) formed on the vehicle outer side, the cylindrical portion 31 is held at high temperature.
The vehicle outer side end 31 of the tubular portion 31 due to the axial expansion of the
a presses the pressing member 7, and the vehicle inner end 31b of the tubular portion 31
Pushes the snap rings 4 in the axial direction to increase the axial force.

Thus, even if a radial gap is generated between the outer peripheral surface of the outer ring 23 and the inner peripheral surface of the tubular portion 31 due to the tubular portion 31 expanding in the radial direction at a high temperature, the above-mentioned shaft The outer ring 23 (rolling bearing 2) and the tubular portion 31 (knuckle 3) can be coupled by the increase in the axial force in the direction. As a result, the fixing strength between the outer ring 23 and the knuckle 3 does not decrease at high temperature, and it is possible to prevent the occurrence of so-called creep in which the outer ring 23 slides in the rotational direction with respect to the knuckle 3 as the shaft 1 rotates.

When fixing the rolling bearing 2 to the knuckle 3, the rolling bearing 2 may be press-fitted into the tubular portion 31 of the knuckle 3 from the vehicle inner side.

(Fifth Embodiment) The fifth embodiment of the present invention will be described with reference to FIG.

FIG. 7 is a partial cross-sectional view of a vehicle wheel support device to which the rolling bearing mounting structure according to this embodiment is applied. The same parts as those of the example shown in FIG. And its description is omitted.

In this embodiment, in the third embodiment shown in FIG. 5, between the cylindrical portion 31 of the knuckle 3 and the outer ring 23 of the rolling bearing 2, the centering leaf spring shown in the first embodiment is provided. The bearing holding member such as No. 5 is interposed.

That is, when the rolling bearing 2 is fixed to the knuckle 3, the centering leaf spring 5 is fitted in the circumferential groove 34 of the tubular portion 31, and other fixing work is performed in the embodiment. Same as 3.

According to the rolling bearing mounting structure thus configured, as in the third embodiment, the axial force applied to the nut 6 and the snap ring 4 increases due to the thermal expansion of the tubular portion 31, and creep is generated. Can be prevented. Further, even if this axial force is insufficient, since the centering leaf spring 5 that can expand and contract in the radial direction is interposed in the fitted surface of the outer ring 23 and the knuckle 3 in a compressed state, The occurrence can be reliably prevented.

The configuration in which the centering leaf spring 5 is interposed between the outer ring 23 and the knuckle 3 may be applied to the second and fourth embodiments.

The present invention is not limited to the above-mentioned embodiment, and various applications and modifications can be considered.

(1) The bearing holding member is not limited to the centering leaf spring 5, but may be various radial springs that can expand and contract in the radial direction, or elastic bodies such as rubber that can expand and contract in the radial direction. Good.

Moreover, the bearing holding member is not limited to being provided over the entire circumference of the fitting surface of the outer ring 23 and the knuckle 3, but may be provided intermittently in the circumferential direction. Further, the bearing holding member may be housed and provided in a circumferential groove formed on the outer peripheral surface of the outer ring 23.

(2) The fixing member for fixing both axial ends of the tubular portion 31 to the outer ring 23 of the rolling bearing 2 is also the combination of the screws 23c and 35 and the snap ring 4 and the nut 6 as in the above-described embodiment. Combination of snap ring 4 and pressing member 7
The member is not limited to the combination of the snap ring 4 and the snap ring 4, and may be any member that can regulate the axial expansion of the rolling bearing 2 of the tubular portion 31 with respect to the outer ring 23.

(3) In the above embodiment, the rolling bearing 2 is applied to the vehicle wheel supporting device, and the supporting member corresponds to the knuckle 3. However, the invention is not limited to this. Absent. That is, the present invention can be applied as long as the shaft body is rotatably supported by the support member via the rolling bearing, and the support member is not limited to the knuckle, and may be various cases or housings.

(4) The shape of the rolling bearing 2 is not limited to the above embodiment. For example, in addition to the double-row outward angular ball bearing, various types of oblique contact rolling bearings such as tapered rollers may be used.

[0065]

According to the rolling bearing mounting structure of the present invention, even if the supporting member for rotatably supporting the shaft body through the rolling bearing is made of a light metal having a linear expansion coefficient larger than that of the bearing, at a high temperature. The effect that creep can be prevented from occurring between the support member and the rolling bearing is obtained.

[Brief description of drawings]

FIG. 1 is a partial cross-sectional view of a vehicle wheel support device to which a rolling bearing mounting structure according to a first embodiment of the present invention is applied.

FIG. 2 is a perspective view of an aligning leaf spring according to the first embodiment of the present invention.

FIG. 3 is an operation explanatory view of the rolling bearing mounting structure according to the first embodiment of the present invention.

FIG. 4 is a partial cross-sectional view of a vehicle wheel support device to which a rolling bearing mounting structure according to a second embodiment of the present invention is applied.

FIG. 5 is a partial cross-sectional view of a vehicle wheel support device to which a rolling bearing mounting structure according to a third embodiment of the present invention is applied.

FIG. 6 is a partial cross-sectional view of a vehicle wheel support device to which a rolling bearing mounting structure according to a fourth embodiment of the present invention is applied.

FIG. 7 is a partial cross-sectional view of a vehicle wheel support device to which a rolling bearing mounting structure according to a fifth embodiment of the present invention is applied.

FIG. 8 is a cross-sectional view of a vehicle wheel support device in a conventional example.

[Explanation of symbols]

1 axis 2 Rolling bearing 23 outer ring 23c screw thread (fixing member) 3 knuckles (support members) 31 tube 35 thread groove (fixing member) 4 Snap ring (fixing member) 5 Centering leaf spring (bearing holding member) 6 Nut (fixing member) 7 Pressing member (fixing member)

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 3J017 AA01 AA05 DA01 DB04 DB07                 3J101 AA02 AA32 AA43 AA54 AA62                       BA77 FA35 GA02 GA03

Claims (4)

[Claims] 1. A rolling bearing mounting structure for rotatably supporting a shaft member with respect to a supporting member, wherein the supporting member is made of a material having a linear expansion coefficient larger than that of an outer ring of the rolling bearing, and A rolling bearing has a cylindrical portion into which the rolling bearing can be inserted, and the rolling bearing is inserted into the cylindrical portion of the support member, and the outer peripheral surface of the outer ring of the rolling bearing and the fitting surface of the inner peripheral surface of the cylindrical portion, A rolling bearing mounting structure in which a bearing holding member that expands and contracts in a radial direction and gives a tight margin to the fitting surface at the time of linear expansion of the tubular portion is interposed. 2. The rolling bearing mounting structure according to claim 1, wherein a peripheral groove is formed along the inner peripheral surface of the cylindrical portion of the support member, and the bearing holding member is housed in the peripheral groove. 3. A rolling bearing mounting structure for rotatably supporting a shaft member with respect to a supporting member, wherein the supporting member is made of a material having a linear expansion coefficient larger than that of an outer ring of the rolling bearing, and It has a tubular portion into which a rolling bearing can be inserted, the rolling bearing is inserted into the tubular portion of the support member, and the tubular portion is brought into contact with axial end faces of the tubular portion at both axial end portions of the rolling bearing. Rolling bearing mounting structure with a fixing member that regulates linear expansion in the axial direction. 4. The rolling bearing mounting structure according to claim 1, wherein the shaft body is an axle and the support member is a knuckle which is a part of a frame of the vehicle. Rolling bearing mounting structure.