HYDRAULIC DEVICE

Abstract

Hydraulic device such as a motor or a pump comprising a first assembly of parts (1C) including a housing part, and a second assembly of parts (1D) fixed to said first assembly.

Description

The present invention relates to a hydraulic device such as a motor or a pump comprising a first assembly of parts comprising a housing part wherein a cylinder block is arranged, and a second assembly of parts fixed to said first assembly, said assemblies having respectively a first and a second contact face, the two contact faces being stressed towards each other and placed in mutual contact by means exerting axial stress.

The means exerting axial stress are here means exerting stress tending to bring the two contact faces towards each other, along an axis perpendicular to these faces.

In hydraulic motors or pumps, the housing generally comprises several parts which are machined individually and which are assembled together. This assembly must allow the passage of torque. For example, the housing of a hydraulic motor has a cam part whereof the internal periphery is corrugated to cooperate with the pistons of the cylinder block and a part known as a distribution cover, arranged around the internal distributor which distributes fluid to the cylinders of the cylinder block. These two parts must be fixed together so as to be perfectly well fastened together.

In the case of a fixed housing, it is generally the distribution cover part which is fixed to a fixed part such as the chassis of a vehicle, and the cam part must be perfectly well fixed to this cover part without being capable of turning, since it is the rotation of the cylinder block relative to the cam which conditions the functioning of the motor. In this case, the torque which is transmitted between the distribution cover and the cam part is the torque resisting the motor torque.

Also, some hydraulic motors are provided with braking systems which are arranged in a housing part called a brake cover. This part is fixed to another housing part, for example the distribution cover and it must be perfectly well connected thereto to transmit braking torque.

It can eventuate especially that instead of being uniform, braking occurs with considerable jolts. During these jolts, considerable torque must be transmitted from one part to the other of the motor. Consequently, the quality of fastening between the different parts of the motor is particularly important in motors equipped with braking means.

In the case of a motor with rotating housing, the motor torque or the braking torque must also be transmitted between the different housing parts assembled together.

In this way, in numerous devices such as hydraulic motors or pumps, considerable torque must be transmitted between a first assembly and a second assembly of parts (each assembly can comprise just a single part). This torque is generally transmitted mainly via housing parts forming part respectively of the first and of the second assembly.

First, to assemble two assemblies of a hydraulic motor, a contact face is usually provided on each of the assemblies. Means (screws, for example) exerting axial stress perpendicularly to the contact faces push the two contact faces towards each other and place them in mutual contact.

Also, rotation relative to the other rotation of the two assemblies is generally blocked by means for fastening the two assemblies in fixed rotational position, generally screws.

In such a motor, transmission of torque occurs as follows: First of all, in addition to their role of axial stressing and locking in rotation, screws serve as shearing element, that is, they transmit some of the torque between the two assemblies. Also, in addition to screws and when the torque to be transmitted is substantial, extra shearing elements, such as balls, pins, etc., can also be used if necessary.

However, when considerable torque must be transmitted between the assemblies, the number and diameters of the screws and/or of the other shearing elements used must be considerable. This poses problems especially of bulk and complexity of assembly in a certain number of applications.

To avoid having to use too many screws or other shearing elements or of excessive diameter, it is known to glue the contact faces on each other. However, this process is delicate to carry out industrially, and also makes assembly/disassembly operations highly problematical.

A first aim of the invention is to overcome the above disadvantages using a device of the type presented in the introduction but comprising improved fastening means, for transmitting considerable torque between two parts of a hydraulic device without the need to multiply the screws or shearing elements connecting these two parts. These means must especially enable sealing to be maintained relative to leaks of hydraulic fluid, at the level of the junction between the two assemblies of parts.

This aim is attained due to the fact that in the device at least one of said first and second contact faces has a rough friction surface so it can allow transmission of high torque between the two assemblies.

It appeared indeed that because of this rough friction surface, arranged on one of the contact faces, the contact faces are capable of transmitting considerable torque between the two assemblies, and perhaps even more significant than with earlier known embodiments. In this way in a device according to the invention, the need to use shearing elements for transmission of torque is reduced, or even eliminated.

The invention particularly specifies the assembly of contact faces which are both metallic, the metals enabling transmission of high stress.

The invention can be implemented especially in a hydraulic device comprising a rotor and a stator, a hydraulic device of this type generally requiring the transmission of high torque, in particular if the device comprises braking means.

The invention can also be implemented in particular in devices wherein the second assembly of parts also comprises a housing part, the means exerting axial stress serving to secure the two housing parts to each other.

The rough friction surface can be arranged on all or part of the contact face.

In an embodiment, the first assembly also comprises a fastening plate; the first and second contact faces are formed respectively on walls opposite said fastening plate and said second assembly; also, a third and a fourth contact face are formed respectively on walls opposite said housing part and said fastening plate.

Advantageously, this embodiment is fairly easy to execute. In fact, the specific operations, specifically especially those operations for preparing the rough friction surface, can relate to the fastening plate only.

Since the fastening plate is (generally) smaller and less complex than the other parts making up the first or the second assembly (such as the housing part), it remains relatively easy to undertake the previously mentioned specific operations on the fastening plate.

The fastening plate can be made by common processes: the plate can be made by being cut from sheet metal, followed by peening and surface treatment increasing the surface hardness of the plate. So in this case, neither the housing part nor the second assembly needs any specific additional operation. Simple machining allowing them to receive the fastening plate may be necessary however.

In an optional variant of the previous embodiment (with fastening plate), one of said third and fourth contact face has a rough friction surface for increasing the value of maximal torque transmissible between the housing part and the fastening plate. The two rough friction surfaces are arranged on the two opposite sides of the fastening plate, and the principle of securing in rotation according to the invention is implemented on each side of the fastening plate.

According to the invention, the friction surface must be roughened such that the device is capable of transmitting high torque between the two assemblies.

With this aim, in an embodiment said friction surface or one of said friction surfaces has a roughness index Ra greater than 12 μm and preferably greater than 18 μm, or a roughness index Rz greater than 80 μm (roughness parameters Ra and Rz being defined by the standard ISO 4287). In this way, the friction surface has considerable roughness, adapted for transmission of considerable torque between the two assemblies of parts.

To achieve such a roughness index, said friction surface or at least one of said friction surfaces can be the object of surface treatment for increasing roughness, peening for example. Such treatment (or grit blasting) not only cleans the contact face of the part, but also ensures that the relevant contact face has adequate roughness. Other processes, for example grooving, are feasible for increasing the roughness of this contact face.

Preferably, the surface treatment for increasing roughness is surface treatment without material input (in the sense where materials used are not intended to be incorporated into the friction surface). In this way treatments comprising input of grains of material are to be avoided, as such grains would be likely to flake off from the friction surface and could damage the hydraulic device if they were to be disseminated in the latter.

Preferably, the friction surface occupies the major part of the contact face on which it is arranged, for example at least 80%, or even 95% of this surface.

In an embodiment, said or at least one of said friction surfaces has a Vickers hardness greater than 450 Hv (Vickers hardness being measured as per standard EN ISO 6507-1).

In fact, a particularly large coefficient of adherence between the friction surface and the contact face which faces the latter can be attained when the reliefs formed on the friction surface penetrate the contact face opposite. Substantial torque can be transmitted from one assembly to the other. To fully achieve this aim, the reliefs must be retained at least partially when the two faces are stressed towards each other at the time of assembling together the two assemblies. The Vickers hardness of the friction surfaces must therefore preferably be high. More generally, the invention can also be executed with a friction face whereof the Vickers hardness is greater than 400 Hv, or even 370 Hv.

To achieve such Vickers hardness, said friction surface or at least one of said friction surfaces can follow surface treatment for increasing surface hardness, nitriding for example.

In an embodiment, at least one contact face placed opposite a friction surface has a surface hardness of less than that of said friction surface. Preferably, the surface hardness HV_contact of the relevant contact face is less by at least 20%, and if possible by at least 30%, than that HV_friction of the friction surface opposite it. In other terms, HV_contact is preferably less than HV_friction * 0.8 (or even HV_friction * 0.7).

Due to this differential of surface hardness, during assembling of the two assemblies the reliefs formed on the friction surface can penetrate relatively easily into the contact face opposite, the surface hardness of the latter being less. The result is that the reliefs of the friction surface are really engaged with the contact face opposite, which allows transmission of considerable torque. Also, this torque can be transmitted without the need for the pressing or traction force between the two parts being particularly high.

It is therefore understood that the invention is particularly adapted to securing housing parts made of metals of relatively low hardness, for example cast iron, molten steel or even forged steel.

Also, when a fastening plate is used, the fact that the fastening plate is a part separate from the parts which it secures allows relatively free choice of the material of the plate and surface treatments applied to the latter. It is possible to select particularly hard material for the plate; it is also relatively easy to apply treatments to the plate, whether for roughening the plate or for boosting its surface hardness. By comparison, it would generally be more complicated to apply such treatments to an entire housing part, these housing parts comprising zones machined with precision, which are relatively fragile and have to be preserved.

In conclusion, the invention offers an effective solution in terms of torque transmission, and is particularly simple to implement and inexpensive on an industrial scale.

A second aim of the invention is to propose a process for producing a hydraulic device of the type presented in the introduction, allowing a link for transmission of considerable torque between these two assemblies of parts to be made between the two assemblies of parts making up the device.

This aim is attained because of the fact that the process comprises the following steps:

a) a first and a second assembly of parts are provided, such as:

• • the two assemblies are capable of being fixed to each other to constitute a hydraulic device such as a motor or a pump,
  • the two assemblies present respectively a first and a second contact face,
  • the first assembly comprises a housing part wherein a cylinder block is arranged;b) a rough friction surface is formed on one of said contact faces for transmission of high torque between the two assemblies; andd) the two assemblies are assembled so as to place the two contact faces in mutual contact by means of means exerting axial stress.

It can be noted in this process that step b) for formation of the rough friction surface can be made while step a) for providing the different assemblies of parts of the device is not yet complete.

The rough friction surface can be formed by applying to the relevant contact face one of the surface treatments for increasing roughness presented previously.

To further improve the efficacy of the process, in an implementation mode, the production process also comprises, after step b), the following step c):

c) surface treatment for increasing the surface hardness of the friction surface is carried out, such that the contact face placed opposite the friction surface has a surface hardness less than that of the friction surface.

The invention will be easily understood and its advantages will emerge more clearly from the following detailed description of embodiments illustrated by way of non-limiting examples. The description refers to the attached diagrams, in which:

FIG. 1 is an axial sectional view of a hydraulic motor according to the invention;

FIG. 2 is an exploded perspective view of part of the motor of FIG. 1, in a first embodiment;

FIG. 3 is an exploded perspective view of part of a motor similar to that of FIG. 1 but comprising a fastening plate, and constituting a second embodiment of the invention;

FIG. 3A is a section of a detail extracted from FIG. 3, locally showing the form (in section) of the fastening plate;

FIG. 4 is an axial section of part of the motor of FIG. 3;

FIG. 5 is a section of a detail extracted from FIG. 4, showing the arrangement of the fastening plate;

FIG. 6 is an exploded local axial section of the part of the motor illustrated in FIG. 3, showing the arrangement of the fastening plate;

FIG. 7 is an exploded perspective view of part of a motor similar to the motor of FIG. 3, showing the fastening plate, in a third embodiment;

FIG. 8 is a section of a detail of the motor of FIG. 7, showing the arrangement of the fastening plate; and

FIGS. 9A and 9B are axial sections of a detail of a motor according to the invention in a fourth and a fifth embodiment, showing two other possible arrangements for the fastening plate.

In the figures, different embodiments of the invention are presented in relation to different variants of a motor 100. In these different variants, identical or similar elements retain the same reference numeral.

The hydraulic motor 100 of FIG. 1 is a motor with radial pistons. It has a housing in four parts 1A, 1B, 1C and 1D. The part 1B has an undulating internal periphery and forms the cam against which the pistons of the cylinder block 2 react. The cylinder block 2 illustrated in this case comprises two rows of offset cylinders and whereof the respective cylinders 2′ are offset angularly. Pistons 2″ are arranged and slide in these cylinders. The motor shaft 3 is arranged non-rotatably with respect to the cylinder block 2 by splines and extends in the part 1A of the housing, which bears the bearings 4 of the motor. The latter comprises an internal fluid distributor 5 whereof the distribution conduits 6 are connected alternatively to the cylinder conduits 7 of the cylinder block 2. The distributor extends inside the part 1C of the housing, called a distribution cover. A brake shaft 8 also extends inside the distributor, which, as for the shaft 3, is fixed in rotation with the cylinder block 2. The end of this shaft opposite the cylinder block 2 extends in the part 1D of the housing, called a brake cover. This end and this brake part support braking members constituted in this case by discs 9 interposed between each other. A brake piston 10 is stressed by a spring 11 to push the discs 9 into braking contact and can be controlled in reverse by pressurising fluid in a braking chamber 12.

The motor 100 is of the type with rotating shaft, since its housing is fixed, the housing part 1A comprising fastening elements not illustrated with for example the chassis of the vehicle.

Such a motor needs transmission of substantial torque, mainly in two operating modes: motor mode, and braking mode.

In motor mode, torque is transmitted by the turning cylinder block 2 to the shaft 3 which, is designed to drive an external element via flanges 3′. For the motor to operate, the housing part 1B or cam must remain in a perfectly fixed rotational position around axis A vis-à-vis the part 1A of the housing which is that fixed to a stationary element. Torque resisting the motor torque must therefore be transmitted between parts 1A and 1B of the housing.

Inversely during braking, braking torque must be transmitted between the part 1D of the housing and the parts 1A, 1B and 1C.

In this way, the different housing parts 1A, 1B, 1C and 1D must be perfectly fastened together and especially enable transmission of considerable torque.

With this aim, to secure the parts 1A, 1B and is in rotation the motor 100 comprises first of all contact faces for attaching these parts in pairs to the parts 1A, 1B and 1C. These faces are flat and perpendicular to the axis of rotation A of the motor; these are faces 1A′ and 1B′ between parts 1A and 1B, 1B″ and 1C″ between parts 1B and 1C, 1C′ and 1D′ between parts 1C and 1D.

In addition, fastening screws 14 ensure that the parts 1A, 1B and 1C are joined together, as is known.

Inversely, securing the parts 1C and 1D is carried out as per the invention. These parts 1C and 1D are representative of a first and second assembly in terms of the invention, between which considerable torque must be transmitted.

To fix the parts 1C and 1D, the motor 100 first comprises screws 15. These pull towards each other parts 1C and 1D and consequently, the contact faces 1C′ and 1D′, which constitute the first and second contact faces in terms of the invention. The screws 15 are means exerting axial stress along axis A of the motor 100, and press the contact faces 1C′ and 1D′ against each other.

The screws 15 are capable of transmitting a certain torque between the parts 1C and 1D.

However, in the motor presented here, because of the diameter and/or of the limited number of screws 15 the latter do not enable transmission of sufficient torque between the parts 1C and 1D.

Also, to increase the value of the torque being transmitted between the parts 1C and 1D, the contact face 1C′ has been the object of a particular arrangement. It has undergone peening followed by surface-hardening treatment. As a result of these treatments the face 1C′ has become rough and is a friction surface capable of transmitting considerable torque between the parts 1C and 1D.

A motor part illustrating a second embodiment of the invention is illustrated in FIG. 3.

In this embodiment the first assembly in terms of the invention comprises not only the part 1C, but also an additional part, specifically a fastening plate 22. The latter serves to transmit torque between the parts 1C and 1D. This plate 22 is a washer arranged about an axis A of rotation of the device (motor 100). Here washer designates a substantially flat part, pierced by a hole wherein another part enters.

(It is noted that the fastening plate can take the form of a washer also in the embodiments wherein the fastening plate comprises a rough surface on one side only). Each of the two sides opposite the plate 22 has a rough friction surface (24,25). The plate 22 is interposed between the contact faces 1C′ and 1D′ such that the friction surfaces 24 and 25 are respectively in contact with the contact faces 1C′ and 1D′. The surfaces 24 and 1D′, at the level of which the first assembly (part 1C and plate 22) and the second assembly (part 1D) are in contact constitute the first and second contact faces in terms of the invention. The surfaces 1C′ and 25 constitute the third and fourth contact faces in terms of the invention.

In the embodiment of FIG. 2, the friction surfaces 24 and 25 are aligned axially on either side of the fastening plate 22. In fact, the friction surfaces 24 and 25 extend exactly to the right of each other, in the axial direction, on either side of the plate 22.

The technical effect of the plate 22 can be better understood by way of FIG. 2A which shows the form of the friction surfaces 24,25.

On said friction surface or on at least one of said friction surfaces (24,25) the projecting parts are pointed; they form peaks or points 26 (viewed in a section perpendicular to the plane of the plate). When the parts 1C, 22 and 1D are assembled and placed under axial stress by the screws 15, the points 26 are pressed into the opposite contact surface (in this case, surfaces 1C′ and 1D′); they prevent any rotation, any relative sliding between the parts 1C, 22 and 1D.

Also, in the embodiment illustrated, the parts 1C and 1D present a surface hardness less than that of the plate 22. Also, when the motor is assembled, the plate is clamped by means of the screws 15 between the contact faces 1C′ and 1D′. A specific pressing system can optionally be used to impose particularly strong pressure on the fastening plate, if necessary. Under the effect of this pressure, the points 26 penetrate therefore (superficially) the parts 1C and 1D. Also after installation, the thickness really occupied by the plate 22 between the contact faces is substantially less than its initial thickness and is around the minimal thickness of the plate measured at the base of the reliefs of the friction surfaces (thickness e, FIG. 2A). In practice, for a washer having a thickness of around 1 mm, a loss in thickness of the order of 0.1 to 0.3 mm during assembly can be confirmed.

When mounted in such a manner, the plate 22 is closely connected to the parts 1C and 1D and enables transmission of considerable torque to each other of these parts. Also, the invention can advantageously be implemented on novel devices during maintenance operation on devices, by retrofitting.

For lodging the fastening plate 22, it can be provided that the second and/or the third contact face (1C′, 1D′) has an uptake recess 28 for the plate provided to receive the fastening plate 22. (It is noted that such an uptake recess for the plate can be provided in an embodiment wherein the fastening plate has a rough surface on one side only).

Different configurations are possible.

In the second and third embodiments (FIGS. 4 to 8), the recess 28 is provided on the face 1D′, the face 1C′ remaining smooth. This is also the case in the variant presented in FIG. 9A. In this way, only one (1D′) of the second and the third contact faces comprises an uptake recess 28 for the plate, the other contact face 1C′ being provided a recess.

Inversely, in the variant presented in FIG. 9B, uptake recesses 30,31 of the plate are made symmetrically on the two contact faces 1C′ and 1D′. In this configuration, a sealing joint is positioned symmetrically between the two faces 1C′ and 1D′.

If the plate 22 gives a clear gain in terms of torque transmission, generally specific arrangements must also be made to ensure sealing of the connection between the parts 1C and 1D. It can in fact be necessary to prevent any fluid leak from the inside to the outside of the motor 100 between the parts 1C and 1D.

To ensure this sealing, the device 100 comprises sealing means 40 to prevent radial fluid passage between two contact faces (1C′,25; 24,1D′) in mutual contact.

These means can take various forms. In the example presented in FIGS. 3, 4, 5 and 6, the sealing means 40 comprise two O-rings 42,43 arranged on either side of the washer 22 and coaxial with the latter. These joints extend continuously over the entire periphery of the motor to the right of the contact faces, and prevent any fluid going from the interior to the exterior of the motor, between the faces 1C′ and 1D′. They ensure continuous contact between the face 1C′ and the plate 22 on the one hand, and between the plate 22 and the face 1D′ on the other hand.

Also, the plate 22 has sealing surfaces facing the joints 42, 43, that is, surfaces capable of creating sealing when the joints 42,43 are pressed on them. In this case, these are smooth surfaces provided on the inner periphery of the plate 22, on each side of the latter.

In another embodiment, illustrated by FIGS. 7 and 8, the sealing means 40 comprise two joints 44,45 fixed respectively on both sides of the washer 22, overmoulded, for example. This embodiment simplifies installation of the plate 22 to the extent where the two joints are solid with the latter.

The sealing joint or sealing joints provided in the sealing means 40 can be arranged in different configurations.

In an embodiment, the sealing means (40) comprise at least one sealing joint (42,43,46), arranged so as to create sealing between both a flat surface and also a groove 50 formed in the contact face 1D′ and arranged to receive said sealing joint 46 and allow deformation of the latter.

For example, in FIG. 9A, the joint 46 is arranged to create sealing on the face 1C′. The face 1D′ comprises a groove 50 provided to receive the joint 46. In this configuration, the plate 22 contributes to holding the joint 46 and the groove 50 is relatively shallow.

In the above expression, the term <<flat surface>> signifies in particular that said flat surface against which the joint seals has no relief(s) likely to make contact with the joint so as to promote sealing. In this way advantageously creating sealing on this surface (said flat surface) requires no specific treatment, such as machining.

In an embodiment, the sealing means 40 comprise a joint 46,47 interposed between the second contact face 1D′ and the third contact face 1C′ and placed directly in contact with each of these faces.

In this case, this joint can be placed radially inside or outside the washer. There is accordingly dissociation between the sealing function, ensured by the joint, and the torque transmission function, ensured especially by the plate 22.

An exemplary embodiment is given in FIGS. 9A and 9B. In these figures, the sealing between the parts 1C and 1D of the housing is ensured conventionally by means of an O-ring arranged between the surfaces 1C′ and 1D′. The joint in this case is placed radially inside the plate 22, and is relatively independent of the latter. Advantageously, the edge of the washer 22 is used to hold the joint (46, 47).

The invention has been presented in the foregoing as a motor with radial pistons. Naturally, it can be implemented in numerous other types of hydraulic devices, especially pumps with axial pistons.

Various tests were conducted to verify the efficacy of the invention. It was able to be confirmed that maximal torque able to be transmitted between two assemblies of parts of a hydraulic motor can be tripled (and go for example from 9000 Nm to 27500 Nm) when a securing washer such as that presented in FIG. 3 is inserted in between two assemblies of parts to be secured.

Claims

1. A hydraulic device such as a motor or a pump comprising a first assembly of parts comprising a housing part wherein is arranged a cylinder block, and a second assembly fixed to said first assembly, said assemblies having respectively a first and a second contact face, the two contact faces being pushed towards each other and placed in mutual contact by means exerting axial stress,wherein at least one of said first and second contact faces has a rough friction surface for transmission of high torque between the two assemblies.

2. The device as claimed in claim 1, wherein said first assembly also comprises a fastening plate; said first and second contact faces are formed respectively on walls opposite said fastening plate and said second assembly;also, a third and a fourth contact face are formed respectively on walls opposite said housing part and said fastening plate; andone of said third and fourth contact faces has a rough friction surface for transmission of high torque between said housing part and said fastening plate.

3. The device as claimed in claim 2, wherein said friction surfaces are aligned axially on either side of the fastening plate.

4. The device as claimed in claim 2, wherein the second and/or the third contact face has an uptake recess for the plate provided to receive the fastening plate.

5. The device as claimed in claim 4, wherein one only of the second and the third contact faces comprises an uptake recess for the plate, the other contact face being provided a recess.

6. The device as claimed in claim 2, wherein the plate is a washer arranged about an axis of rotation of the device.

7. The device as claimed in claim 1, wherein said friction surface or one of said friction surfaces has a roughness index Ra greater than 12 μm or a roughness index Rz greater than 80 μm.

8. The device as claimed in claim 1, wherein the projecting parts on said friction surface or on at least one of said friction surfaces are pointed.

9. The device as claimed in claim 1, wherein said friction surface or at least one of said friction surfaces has a Vickers hardness greater than 450 HV.

10. The device as claimed in claim 1, wherein at least one contact face placed opposite a friction surface has a surface hardness less than that of said friction surface.

11. The device as claimed in claim 1, wherein the device comprises sealing means to prevent radial fluid passage between two contact faces in mutual contact.

12. The device as claimed in claim 11, wherein the sealing means comprise at least one sealing joint, arranged so as to create sealing between both a plane surface and also a groove formed in a contact face and arranged so as to receive said sealing joint and allow deformation of the latter.

13. The device as claimed in claim 11, wherein the sealing means comprise a joint interposed between the second and the third contact face and placed directly in contact with each of these faces.

14. A production process of a hydraulic device comprising the following steps: a) a first and a second assembly of parts are provided, such that: the two assemblies are capable of being fixed to each other to constitute a hydraulic device such as a motor or a pump,the two assemblies present respectively a first and a second contact face,the first assembly comprises a housing part wherein is arranged a cylinder block;d) the two assemblies are assembled so as to place the two contact faces in mutual contact by means of means exerting axial stress,the process wherein it also comprises a following step b):b) a rough friction surface is formed on one of said contact faces to enable transmission of high torque between the two assemblies.

15. The production process as claimed in claim 14, also comprising after step b) the following step c): c) surface treatment increasing the surface hardness of the friction surface is carried out such that the contact face placed opposite said friction surface has a surface hardness of less than that of the friction surface.