WO2015043630A1

WO2015043630A1 - Rolling bearing with axial pre-stressing member

Info

Publication number: WO2015043630A1
Application number: PCT/EP2013/069970
Authority: WO
Inventors: Laurent Varnoux
Original assignee: Aktiebolaget Skf
Priority date: 2013-09-25
Filing date: 2013-09-25
Publication date: 2015-04-02

Abstract

The rolling bearing comprises an inner ring (16), an outer ring (14), at least one row of rolling elements (18) disposed radially between said rings and at least an axially elastic member (26) fixed to one of the outer and inner rings (14, 16) radially on the side opposite to the rolling elements (18) with respect to said ring. Said elastic member is mounted in axial contact with a lateral surface of said ring delimiting axially said ring.

Description

Rolling bearing with axial pre-stressing member

The present invention relates to rolling bearings, in particular rolling bearings having an inner ring and an outer ring with one or more rows of rolling elements, for example balls .

In particular, the present invention relates to rolling bearings used in industrial electric motors . An electric motor generally comprises a casing inside which is mounted a stator and a rotor, and two rolling bearings located at each end of the rotor' s shaft.

In such application, known ISO deep groove ball bearings are usually used. These bearings comprise inner and outer rings of the massive or solid type. A "so lid ring" is to be understood as a ring obtained by machining with removal o f material (by turning, grinding) from steel tube stock, bar stock, rough forgings and/or rolled blanks .

Document EP-B l - 1 037 05 1 disclo ses an instrumented rolling bearing mounted into the casing o f the electric motor and stacked conical washers axially interposed between said rolling bearing and the casing. Such a device uses a p lurality of pre-stressing washers that are independent of the rolling bearing. Accordingly, the motor manufacturer has to manage, handle and assemble a large number o f components.

One aim o f the present invention is to overcome these drawbacks.

It is a particular object of the present invention to provide a rolling bearing that is economical and easy to install in a machine for which it is intended, for example in an electric motor.

In one embodiment, the rolling bearing comprises an inner ring, an outer ring and at least one row of rolling elements disposed radially between the rings. The rolling bearing further comprises at least an axially elastic member fixed to one o f the outer and inner rings radially on the side opposite to the rolling elements with respect to said ring, said elastic member being mounted in axial contact with a lateral surface of said ring delimiting axially said ring. The axial retention of the elastic member on the inner ring or on the outer ring o f the rolling bearing makes it possible to reduce the number of components to be procured and to be fitted. Manufacturing errors resulting from forgetting the elastic member are thus avoided, for example for a motor manufacturer when the rolling bearing is intended to an electric motor.

Preferably, the elastic member is provided with a bearing surface axially spaced apart from the lateral surface of said ring and adapted to be in contact with an element outside said rolling bearing, and with an opposite abutment surface in axial contact with said lateral surface and axially located outside from a radial space existing between the inner and outer rings .

In one embodiment, the elastic member is fixed into a groove formed on said ring . Alternatively, the elastic member may be fixed to said ring by permanent interference fit between said elastic member and said ring.

In one embodiment, the elastic member is fixed on an outer surface o f the outer ring. Alternatively, the elastic member may be fixed into a bore of the inner ring.

Advantageously, the elastic member may comprise at least one axial retaining portion for fixing said member to said ring. The elastic member may comprise a plurality o f axial retaining portions regularly spaced apart in the circumferential direction.

Preferably, the elastic member is made in one part. The elastic member may be a washer.

In one embodiment, the elastic member comprises radial portions mounted in axial contact with the lateral surface of said ring and elastic portions connected to said radial portions and axially spaced apart from said lateral surface. Preferably, the elastic portions are located axially on the side opposite to the retaining portion(s) with respect to the radial portions. The elastic portions may be regularly spaced apart in the circumferential direction. The elastic portions may comprise corrugations. In another embodiment, the elastic portions may comprise tongues . Alternatively, the elastic member may be a conical washer.

In one embodiment, at least one o f the outer and inner rings is of the massive type. Alternatively or in combination, at least one o f the outer and inner rings comprises at least two separate parts and an annular housing inside which said separate parts are arranged, each o f the two separate parts delimiting with the housing a closed space.

The ro lling bearing may further comprise at least one seal fixed to one o f the inner and outer rings and arranged radially between said rings, the elastic member being located radially on the side opposite to said seal with respect to said ring.

The present invention and its advantages will be better understood by studying the detailed description o f the specific embodiments given by way o f non-limiting examp les and illustrated by the appended drawings on which:

Figure 1 is an axial section o f a ro lling bearing according to a first example of the invention,

Figure 2 is an axial section o f a ro lling bearing according to a second examp le o f the invention, and

Figure 3 is a half axial section o f a rolling bearing according to a third example of the invention.

As illustrated on Figure 1 , a rolling bearing 10, with an axis 12 , comprises an outer ring 14, an inner ring 16, a plurality o f ro lling elements 1 8 , which in this case are balls, interposed between the rings, and a cage 20 for maintaining said rolling elements circumferentially spaced apart. The ro lling bearing 10 further comprises on each side an annular seal 22, 24 to close the radial space that exists between the rings 14 and 16, and an axially elastic washer 26 mounted on the outer ring radially on the side opposite to the rolling elements 1 8 and the seals 22 , 24 with respect to said ring. As will be described later, the elastic washer 26 is adapted to exert an axial preload.

The outer and inner rings 14, 16 are concentric and o f the solid type . One of the rings rotates while the other is fixed or also rotates . The outer ring 14 comprises a bore 14a of cylindrical shape from which a toroidal circular raceway 14b for the rolling elements 1 8 is formed, said raceway being directed radially inwards. Two annular recesses or grooves are also formed radially towards the outside from the bore and arranged laterally to the raceway 14b. The grooves are symmetrical relative to a radial plane passing through the centre o f the rolling bearing 10. The outer ring 14 also comprises an outer cylindrical surface 14c and two opposite radial lateral surfaces 14d, 14e which axially delimit the bore 14a and said outer surface. The outer ring 14 further comprises an annular groove 14f formed on the outer surface 14c of said ring and directed radially outwards. The groove 14f is located axially near to the lateral surface 14d. In the disclo sed example, the groove 14f has in cross-section a rectangular shape. Alternatively, the groove may have a different profile in cross- section for example a circular one, or have a frustoconical form.

The inner ring 16 comprises a cylindrical bore 16a and an outer cylindrical surface 16b from which a toroidal circular raceway 16c for the rolling elements 1 8 is formed, said raceway being directed radially outwards. The inner ring 16 further comprises two opposite radial lateral surfaces 16d, 16e which axially delimit the bore 1 6a and the outer surface 1 6b of said ring. The lateral surfaces 16d, 16e are respectively coplanar with the lateral frontal surfaces 14d, 14e of the outer ring.

Each seal 22, 24 is arranged radially between the outer and inner rings 14 and 1 6, fixed to said outer ring and arranged laterally to the rolling elements 1 8. Each seal 22, 24 is entirely housed into the radial space delimiting between the outer and inner rings 14, 1 6. Each seal 22, 24 is axially offset towards the inside with regard to associated lateral surface 14d, 14e of the outer ring.

In this example, each seal 22, 24 is mounted into one of the grooves of the outer ring, is made in one part and has a form o f a shield which is advantageously made o f metal, for examp le by cutting and stamping a metal sheet. The seals 22, 24 are identical to one another and symmetrical relative to the transverse radial plane passing through the centre of the rolling bearing 10.

The elastic washer 26 is made in one part and may be advantageously manufactured in an economic way by cutting, stamping and fo lding a metal sheet. Alternatively, the washer 26 may be made from synthetic material like po lyamide.

In this example, the elastic washer 26 is a corrugated washer. The washer 26 bears axially against the lateral surface 14d of the outer ring. The washer 26 has an outer diameter equal or smaller than the outer diameter of the outer ring 14. The washer 26 comprises a plurality o f radial portions 26a coming into axial contact with the lateral surface 14d of the outer ring and a plurality o f corrugated portions 26b connected to said radial portions. Each corrugated portion 26b is connected to two successive radial portions 26a. The corrugated portions 26b are regularly spaced apart in the circumferential direction. The washer 26 comprises here four corrugated portions 26b . The corrugated portions 26b extend axially outwards, i. e. axially on the side opposite to the outer and inner rings 14, 16. The corrugated portions 26b are axially spaced apart from the lateral surface 14d o f the outer ring 14 and more generally from said ring. The radial portions 26a and the corrugated portions 26b of the washer are located outside from the radial space that exists between the outer and inner rings 14, 16. Each radial portion 26a delimits a radial abutment surface 28 of the washer against the lateral surface 14d o f the outer ring and each corrugated portion 26b delimits a bearing surface 30 lo cated axially on the opposite side to said abutment surface 28 and axially spaced apart from said lateral surface 14d.

The washer 26 further comprises a plurality o f lugs 26 c proj ecting axially from the edge of large-diameter of the radial portions 26a. Each lug 26c extends axially the associated radial portion 26a in the direction o f the outer ring 14 and radially surrounds the outer surface 14c of said ring. Each lug 26c comprises at its free end a retaining hook 26d extending radially inwards into the groove 14f o f the outer ring 14 to secure the washer 26 to said ring. The hooks 26d are regularly spaced apart in the circumferential direction. The lugs 26c and the hooks 26d are lo cated axially on the side opposite to the corrugated portions 26b with regard to the radial portions 26a. The lugs 26c and the hooks 26d are located radially on the side opposite to the rolling elements 1 8 and the seals 22, 24 with regard to the outer ring 14. The lugs 26c and the hooks 26d are axially lo cated between the lateral surfaces 14d, 14e of the outer ring and radially located on the outside of said ring. The hooks 26d are identical to one another and are adapted to engage within the groove 14f o f the outer ring to secure the elastic washer 26 on said ring. The elastic washer 26 is directly secured on the outer ring 14. The hooks 26d form retaining or anchorage means cooperating with the outer ring 14 to fix or attach the washer 26 to said ring. The retaining means o f the washer are radially located on the side opposite to the rolling elements 1 8 with respect to outer ring 14. The mounting of the washer 26 on the outer ring 14 can be easily achieved for example with a simple axial pushing movement without angular indexation.

When such a rolling bearing 10 is mounted in a motor, the bore of the inner ring 16 is fitted tightly on the shaft supporting the rotor and the outer surface of the outer ring 14 is fitted with a sliding fit in the motor casing. During the mounting of the rolling bearing 10 into the casing, the bearing surface 30 of washer axially comes into contact with a radial shoulder of said casing. An axial compression o f the elastic washer 26 between the outer ring 14 and the shoulder of the housing is achieved. In return, by reaction from the shoulder of the housing, the axial washer 26 exerts a permanent axial force on the outer ring 14. The axial pre-stress exerted by the axial washer 26 is transmitted from the outer ring 14 to the inner ring 16 by way o f the rolling elements 1 8.

The use of the elastic washer 26 allows the rolling bearing 10 to be axially preloaded while accommodating different expansions o f the rotor' s shaft, generally made of steel, and of casing, generally based on aluminium, said rolling bearing being able to slide axially with respect to the casing. The ro lling bearing is compact and may be transported and handled with complete safety without any risk o f loss of the elastic washer. Besides, the motor manufacturer has one less component to procure, manage and fit. The roller bearing may be fitted in the electric motor, particularly easily without requiring any special precaution.

The second example illustrated on Figure 2, in which identical parts are given identical references, mainly differs from the embodiment of Figure 1 in that the elastic washer 26 is secured to the inner ring 16. The inner ring 16 comprises an annular groove 1 6f formed on its bore and directed radially inwards. The groove 16f is lo cated axially near to the lateral surface 16d of the inner ring. In this example, the groove 16a has in cross-section a rectangular shape . Alternatively, the groove may have a different profile in cross-section, for example a circular one. In another variant, the groove may also have a frustoconical form. In this case, the groove extends obliquely outwards from the lateral surface 16d delimiting axially said groove.

The washer 26 bears axially against the lateral surface 16d o f the inner ring. The washer 26 has an inner diameter equal or greater than the diameter of the bore 1 6a of the inner ring. The abutment surface 28 of each radial portion 26a of the washer comes axially into contact with said lateral surface 16d. The washer 26 comprises a plurality o f lugs 26e projecting axially from the bore of the radial portions 26a. Each lug 26e extends axially inwards the associated radial portion 26a in the direction o f the inner ring 16 into the bore 16a o f said ring. Each lug 26e comprises at its free end a retaining hook 26f extending radially outwards into the groove 16f o f the inner ring. The hooks 26f are regularly spaced apart in the circumferential direction. The lugs 26e and the hooks 26f are located axially on the side opposite to the corrugated portions 26b with regard to the radial portions 26a. The lugs 26e and the hooks 26f are lo cated radially on the side opposite to the rolling elements 1 8 and the seals 22, 24 with regard to the inner ring 1 6. The lugs 26e and the hooks 26f are axially lo cated between the lateral surfaces 16d, 16e of the inner ring and radially located into its the bore 16a. The hooks 26f are identical to one another and are adapted to engage within the groove 16 f of the inner ring to secure the elastic washer 26 on said ring. The elastic washer 26 is directly secured on the inner ring 16. The hooks 26f form retaining or anchorage means cooperating with the inner ring 16 to fix or attach the washer 26 to said ring. The retaining means o f the washer are radially lo cated on the side opposite to the rolling elements 1 8 with respect to the inner ring 16.

When such a rolling bearing 10 as disclosed in this second example is mounted in a motor, the elastic washer 26 secured to the inner ring 16 axially comes into contact with a radial shoulder provided on the rotor' s shaft.

In the previous examples, the elastic washer is fixed into an annular groove formed on the inner or outer ring. Alternatively, it may also be possible to fix the elastic washer by providing a plurality o f recesses on the inner or outer ring which are spaced apart in the circumferential direction to cooperate with the hooks of washer. However, with such a design, it is necessary to foresee an angular indexation o f the washer relative to the inner and outer rings . In another variant, it could also be possible to press-fit the elastic washer into the annular groove or into the recesses formed on the associated ring to secure said washer. Otherwise, in the illustrated examples, the rolling bearing comprises two seals to close the radial space existing between the inner and outer rings. Alternatively, the ro lling bearing may be deprived of such seals.

In the third example illustrated on Figure 3 , in which identical parts are given identical references, the outer ring 14 of the rolling bearing comprises two separate parts or half-rings 40a, 40b and an annular housing 41 surrounding said separate parts . The half-rings 40a, 40b are identical and symmetric with respect to the radial plane of symmetry of the bearing passing through the centre of the rolling elements 1 8 in order to reduce manufacturing costs . These two outer half-rings 40a, 40b may advantageously be manufactured by cutting and stamping a metal sheet. Since the two half-rings 40a, 40b are identical in this examp le, only one of them, having the reference "a" will be described here, it being understood that the identical elements of the other half-ring 40b bear the reference "b" in the Figure 3.

The half-ring 40a of the outer ring comprises an outer cylindrical axial portion 42a, an annular radial portion 44a, a toroidal portion 46a and an inner cylindrical axial portion 48 a. The radial portion 44a is connected to the outer axial portion 42a and to the toroidal portion 46a. The outer axial portion 46a radially surrounds the inner axial portion 48a. The toroidal portion 46a defines part of a raceway 50a for the ro lling elements 1 8. The toroidal portion 46a extends axially towards the outside o f the rolling bearing with the inner axial portion 48a. The two outer half-rings 40a, 40b are arranged with the radial faces o f the radial portions 44a, 44b in axial contact with one another, substantially in the radial symmetry plane of the rolling bearing and the ro lling elements 1 8. Alternatively, it might be possible to foresee an axial clearance between the radial portions 44a, 44b o f the half-rings 40a, 40b in order to obtain an internal prelo ad for the rolling bearing.

The housing 41 o f the outer ring, which is advantageously made o f a stamped metal sheet, comprises two distinct annular parts 52, 54 surrounding the two outer half-rings 40a, 40b so as to hold them firmly together in the axial direction. The parts 52, 54 o f the housing 5 may advantageously be produced in an economical way from a single metal sheet by cutting and pressing. Each distinct part 52, 54 has an L-shaped structure.

The first part 52 comprises an inner axial cylindrical portion 56 for retaining radially said outer half-rings 40a, 40b . The inner axial portion 56 radially surrounds the outer half-rings 40a, 40b and is in radial contact with the outer surface of the outer axial portions 42a, 42b o f said rings . The first part 52 further comprises a radial flange 58 extending radially from the inner axial portion 56 towards the immediate vicinity o f the outer surface 1 6b of the inner ring so as to leave a radial clearance therebetween. The second part 54 of the housing 4 1 comprises an outer axial cylindrical portion 60 surrounding the inner axial portion 56 o f the first part 52 and mounted in radial contact. The second part 54 further comprises a radial flange 62 extending radially from the outer axial portion 60 towards the immediate vicinity o f the outer surface 1 6b of the inner ring so as to leave a radial clearance therebetween.

The half-rings 40a, 40b are centred in the inner axial portion 56 of the first part 52 of the housing by radial contact between the outer axial portions 42a, 42b and the bore of the said inner axial portion 56. The outer radial faces which form the annular outer edges of the outer axial portions 42a, 42b are respectively in contact with the radial flanges 58 , 62 o f the housing, thus axially clamping the two half-rings 2a, 2b together. The outer radial faces which form the outer edges of the inner axial portions 48 a, 48b are also in contact with the radial flanges 58 , 62. As an alternative, an axial clearance may be provided between the outer edges of the outer axial portions 42a, 42b or inner axial portions 48a, 48b and the radial flanges 58 , 62of the housing.

Each of the half-rings 40a, 40b delimits, with the housing 41 , an annular closed space 64a, 64b . More specifically, the clo sed space 64a is delimited by the outer axial portion 42a, the radial portion 44a, the toroidal portion 46a, and the inner axial portion 48a, and, adj acent to these portions, the radial flange 58 of the first part 52 of the housing. In the disclosed embodiment, the closed spaces 64a, 64b are empty, i. e. no lubricant or other means is contained in these spaces. Alternatively, lubricant, such as grease or oil, may be foreseen into the clo sed spaces 64a, 64b . In this case, each half-ring 40a, 40b is provided with passage means for the lubricant which can comprise a plurality through-ho les made in the thickness of the inner axial portions 48a, 48b o f the said half-ring.

The outer axial portion 60 of the second part 54 of the housing is fixed to the inner axial portion 56 of the first part 52 by any appropriate means, for example by welding, brazing or gluing. The free end o f the outer axial portion 60 remains axially distant from the lateral flange 58 of the first part 52 and only covers a part of the outer surface o f the inner axial portion 56. The outer axial portion 60 leaves the outer surface of the inner axial portion 56 partly uncovered. Said uncovered outer surface of the inner axial portion 56 o f the first part and the outer surface of the outer axial portion 60 o f the second part delimit together the outer surface of the outer ring 14.

The elastic washer 26 bears axially against the outer lateral surface of the radial flange 58 of the outer ring. The washer 26 has an outer diameter equal or smaller than the outer diameter of the outer ring 14. Said outer lateral surface and the outer lateral surface o f the radial flange 62 axially delimit the outer ring 14. The abutment surface 28 o f each radial portion 26a of the washer comes axially into contact with the outer lateral surface of the radial flange 58 of the outer ring and the bearing surface 30 of said radial portion is axially spaced apart from said outer lateral surface.

With regard to the elastic washer as disclo sed in the first example, the elastic washer 26 has here an increased radial dimension and the edge of large-diameter of each radial portion 26 radially protrudes outside with regard to the outer surface of the inner axial portion 56 of the outer ring.

The washer 26 further comprises a plurality of bended lugs 26h proj ecting obliquely inwards from the edge of large-diameter of the radial portions 26a. Each lug 26h extends obliquely inwards the associated radial portion 26a in the direction of the outer ring 14 and radially surrounds said ring. Each lug 26h extends in the direction o f the outer surface of the inner axial portion 56 o f the outer ring which is not covered by the outer axial portion 60. The free end of each bended lug 26h radially presses against the outer surface o f the inner axial portion 56 of the first part 52 of the housing in order to have by elasticity a permanent interference fit between the elastic washer 26 and the outer ring 14. This surely prevents an axial displacement of the elastic washer 26 relative to the outer ring 14. The bended lugs 26h forms an axial retention means o f the elastic washer 26 on the outer ring 14. The bended lugs 26h form external clipping portions which interfere diametrically with the outer surface of the outer ring 14 and which are lo cated radially on the side opposite to the rolling elements 1 8 with respect to the outer ring 14. The lugs 26h are identical to one another and lo cated axially on the side opposite to the corrugated portions 26b with regard to the radial portions 26a. The lugs 26h are located radially on the side opposite to the rolling elements 1 8 and the seals 22, 24 with regard to the outer ring 14. The lugs 26h are axially located between the outer lateral surfaces of the first and second parts 52, 54 of the outer ring 14 and radially lo cated on the outside of said ring.

In this example, the retaining of the elastic washer 26 on the outer ring 14 is obtained by the radial elasticity o f the lugs 26h. Alternatively, an annular groove may be made on the outer surface of the inner axial portion 56 of the outer ring, and the lugs of the washer 26 may be press-fitted into said groove.

In the third example as shown on Figure 3 , the outer ring 14 comprises a housing having two distinct parts 52, 54 for retaining the two half-rings . Alternatively, the housing may be made into one part. In this illustrated examp le, the outer ring 14 comprises two half-rings 40a, 40b and the housing 41 , and the inner ring 1 is of the solid type . Alternatively, it might be possible to have the outer ring so lid while the inner ring comprises two half-rings produced in a similar way and a housing made in one part or two parts and inside which are mounted said half-rings. In this case, the elastic washer may be secured to the bore of the inner ring delimited by said housing. In another embodiment, it could also be possible to have a rolling bearing in which both the inner and outer rings each comprise two half-rings enclo sed in a housing as previously described.

In the illustrated examples, a corrugated washer is used for axially pre-stressing or preloading the rolling bearing. Alternatively, it is also possible to provide another type of elastic member, such as conical washer of the Belleville type or a washer provided with a plurality o f elastic tongues, having retaining means cooperating with the associated ring.

Claims

1 . Ro lling bearing comprising an inner ring ( 16), an outer ring ( 14) and at least one row of rolling elements ( 1 8) disposed radially between said rings, characterized in that it further comprises at least an axially elastic member (26) fixed to one of the outer and inner rings ( 14, 16) radially on the side opposite to the rolling elements ( 1 8) with respect to said ring, said elastic member being mounted in axial contact with a lateral surface of said ring delimiting axially said ring.

2. Ro lling bearing according to claim 1 , wherein the elastic member (26) is provided with a bearing surface (30) axially spaced apart from the lateral surface o f said ring and adapted to be in contact with an element outside said rolling bearing, and with an opposite abutment surface (28) in axial contact with said lateral surface and axially lo cated outside from a radial space existing between the inner and outer rings.

3. Ro lling bearing according to claim 1 or 2, wherein the elastic member (26) is fixed into a groove formed on said ring.

4. Ro lling bearing according to claim 1 or 2, wherein the elastic member (26) is fixed to said ring by permanent interference fit between said elastic member and said ring .

5. Ro lling bearing according to any o f the preceding claims, wherein the elastic member (26) is fixed on an outer surface ( 14c) o f the outer ring.

6. Ro lling bearing according to any o f the preceding claims 1 to 4, wherein the elastic member (26) is fixed into a bore ( 16a) of the inner ring.

7. Ro lling bearing according to any o f the preceding claims, wherein the elastic member (26) comprises at least one axial retaining portion (26d; 26f; 26h) for fixing said member to said ring.

8. Ro lling bearing according to any o f the preceding claims, wherein the elastic member (26) is made in one part.

9. Ro lling bearing according to any o f the preceding claims, wherein the elastic member (26) is a washer.

10. Ro lling bearing according to any o f the preceding claims, wherein the elastic member (26) comprises radial portions (26a) mounted in axial contact with the lateral surface of said ring and elastic portions (26b) connected to said radial portions and axially spaced apart from said lateral surface.

1 1 . Ro lling bearing according to claim 10 dependent to claim 7, wherein the elastic portions (26b) are lo cated axially on the side opposite to the retaining portion(s) (26d; 26f; 26h) with respect to the radial portions (26a).

12. Ro lling bearing according to claim 10 or 1 1 , wherein the elastic portions (26b) are regularly spaced apart in the circumferential direction.

13. Ro lling bearing according to any o f the preceding claims, wherein at least one o f the outer and inner rings ( 14 , 16) is of the massive type.

14. Ro lling bearing according to any o f the preceding claims, wherein at least one o f the outer and inner rings ( 14, 16) comprises at least two separate parts (40a, 40b) and an annular housing (41 ) inside which said separate parts are arranged, each o f the two separate parts delimiting with the housing a clo sed space.

15. Ro lling bearing according to any o f the preceding claims, further comprising at least one seal (22) fixed to one o f the inner and outer rings and arranged radially between said rings, the elastic member (26) being located radially on the side opposite to said seal with respect to said ring.