EP3784918A1

EP3784918A1 - Piston-cylinder arrangement for a hydraulic coupling system

Info

Publication number: EP3784918A1
Application number: EP19720358.1A
Authority: EP
Inventors: Martin Böckler
Original assignee: Schaeffler Technologies AG and Co KG
Current assignee: Schaeffler Technologies AG and Co KG
Priority date: 2018-04-24
Filing date: 2019-04-03
Publication date: 2021-03-03
Also published as: DE112019002120A5; WO2019206361A1; DE102018109811A1; CN112041581A

Abstract

The present invention relates to a piston-cylinder arrangement for a hydraulic coupling system, having a cylinder (12) and a piston (14) that is positioned in the cylinder (12) so as to be axially displaceable, wherein between the piston (14) and the cylinder (12) there is provided at least one seal (16, 18) for sealing an interspace (20) between the piston (14) and the cylinder (12), wherein the at least one seal (16, 18) is secured to either one of the piston (14) and the cylinder (12) and bears movably against the corresponding counter-element - either the cylinder (12) or the piston (14) – at a bearing face (22), wherein the bearing face (22) of the piston (14) or of the cylinder (12) is made from a material having a thermoplastic polytetrafluoroethylene.

Description

Piston-cylinder arrangement for a hydraulic Kupplunqssvstem

The present invention relates to a piston-cylinder arrangement for a hydraulic clutch system. The present invention further relates to a hydraulic clutch system having such a piston-cylinder arrangement.

In hydraulic clutch systems, it is known that these have a clutch master cylinder which is actuated by a piston rod and thus promotes hydraulic fluid from a reservoir to a clutch slave cylinder or vice versa from the clutch slave cylinder into the reservoir.

Depending on the materials used, it may be in the operation of corresponding piston-cylinder assemblies to noise, which can reduce the comfort of operating such piston-cylinder assemblies or hereby equipped clutch systems.

For example, DE 102 05 177 A1 describes a hydraulic system with a master cylinder and a slave cylinder and a line filled with pressure medium between them, in particular for a clutch release device in a motor vehicle. The master cylinder and the slave cylinder each comprise an axially displaceable piston in the respective cylinder, wherein the master cylinder builds pressure by means of an actuating device and thus acts on the slave cylinder via a line filled with pressure medium, at least one of the pistons being made of duroplastic material by means of a shaping process is made.

DE 10 2014 222 992 A1 describes a piston arrangement, comprising a piston, which can be guided movably in a cylinder space, and a metallic piston rod for transmitting a movement to the piston or for receiving a movement from the piston, the piston rod having a contact surface for contacting a piston Contact surface of the piston has. It is provided that at least one of the contact surface of the piston and the contact surface of the piston rod provided with a noise-inhibiting or with a wear-resistant coating is. The coating may comprise a plastic, such as thermoplastic materials, such as polyethylene or polypropylene, or a thermosetting plastic, such as an epoxy resin, or else amorphous carbon.

However, the solutions known from the prior art can still offer further potential for improvement, in particular with regard to a simple and cost-effective design of the piston-cylinder arrangement with simultaneous low-noise operation.

It is therefore the object of the present invention to at least partially overcome the disadvantages known from the prior art. In particular, it is the object of the present invention to provide a solution by means of which a piston-cylinder arrangement can be formed in a cost-effective and / or simple manner, which is characterized by a low-noise operation.

The object is achieved according to the invention by a piston-and-cylinder arrangement having the features of claim 1. According to the invention, the object of the present invention is also achieved by a hydraulic coupling system having the features of claim 10. Preferred embodiments of the invention These features are described in the subclaims, in the description or the figures, wherein further features described or shown in the subclaims or in the description or the figures can be an object of the invention individually or in any combination, if the opposite does not result from the context.

It is proposed a piston-cylinder arrangement for a hydraulic clutch system, comprising a cylinder and an axially movably positioned piston in the cylinder, wherein between the piston and the cylinder at least one seal for sealing a gap between the piston and the cylinder is provided, wherein the at least one seal is fixed to one of the piston and the cylinder and movably against the corresponding counter element selected from cylinder and piston on a contact surface, wherein the abutment surface of the piston or the cylinder is formed of a material which comprises a thermoplastic polytetrafluoroethylene (PTFE). A piston-cylinder arrangement described above makes it possible, in particular, to provide a solution by means of which, in particular in a cost-effective and simple manner, a piston-cylinder arrangement can be formed, which is characterized by low-noise operation.

The piston-cylinder arrangement is used in particular for use in a hydraulic clutch system and in particular in a hydraulic clutch system of a motor vehicle. In principle, the piston-cylinder arrangement can be any unit, in particular of a hydraulic clutch system, in which a piston is movably positioned in a cylinder. Correspondingly, the piston-cylinder arrangement comprises a cylinder and an axially movably positioned piston in the cylinder. In particular, the piston-cylinder assembly serves to convey hydraulic fluid so as to actuate a clutch.

In order to enable the desired functionality of the hydraulic system, it is provided that between the piston and the cylinder, a seal for sealing a gap between the piston and the cylinder is provided. As a result, a desired delivery of the hydraulic fluid can be ensured by axially displacing the piston in the cylinder in a manner known per se. The seal is either fixed to the cylinder or the cylinder inner wall and slides in an axial movement of the piston on the corresponding piston or the piston outer wall along or the seal is fixed to the piston or the piston outer wall and slides at an axial movement of the piston on the corresponding cylinder or the cylinder inner wall along. In order to make this possible, it is provided that the seal is fixed to one of the piston and the cylinder and is movably abutting against the corresponding counter element selected from cylinder and piston on a contact surface.

In particular, in such embodiments may possibly noise occur, which can reduce the comfort in the operation of such a piston-cylinder assembly. In order to prevent or at least significantly reduce this in a piston-cylinder arrangement described here, it is provided that the contact surface of the piston or of the cylinder is formed from a material which comprises a thermoplastic polytetrafluoroethylene (PTFE). In other words, the surface on which the seal slides during a movement of the piston, ie the corresponding surface of the piston or the piston outer wall or of the cylinder or the inner wall of the cylinder, is formed from a material which comprises a thermoplastic polytetrafluoroethylene ,

In particular, this embodiment can be compared to the solutions of the prior art, significant advantages can be achieved.

Nowadays and also within the scope of the present invention, seals made of elastomers are used with particular preference, wherein seals formed from ethylene-propylene-diene rubber (EPDM), in particular in the hydraulic coupling systems, can enable very good sealing effects and longevity. Furthermore, nowadays usually pistons of a thermoset, such as from the designated by the trade name Vyncolit X680 thermoset used. The purpose of the thermosetting resin is currently to ensure that the relative movement between seals, such as primary and secondary seals, and pistons for the non-limiting example of standing seals, ie seals affixed to the cylinder, does not produce any disturbing noises. In other words, no operation sounds should be audible using a thermoset.

In this regard, it could be found that the speed-dependent friction caused by the relative movement in the contact point between the sealing material and a contact surface is the cause of the friction-excited vibrations between the friction partners. The resulting vibrations then lead to audible noise.

However, in a piston-cylinder arrangement as described here, such noises can be prevented precisely by the provision of a thermoplastic polytetrafluoroethylene as the material of the contact surface. Because by the provision of a Such contact surface can be a particularly smooth running of the seal, which is molded in particular of EPDM, are made possible, so that strained vibrations and thus achieved noise can be significantly reduced. Thus, for example, with EPDM as the friction partner, the same or a similar low noise development can be achieved as when using the thermoset, but without having to share its disadvantages, as described below.

In addition, a contact surface of a material comprising polytetrafluoroethylene, which allow for the application of thermal and mechanical stability and sufficient media compatibility allow. This allows a particularly high long-term stability, which can make the reliability of such systems high and can reduce service. Furthermore, if necessary, further properties which are necessary for the application can be made possible.

In addition to these mentioned advantages of a contact surface made of a material which comprises thermoplastic polytetrafluoroethylene, in the case of the piston-cylinder arrangement described here, there are also production-related or processing-technical advantages.

Because in the case of the piston / cylinder arrangement described here, provision can be made for the provision of a piston which is formed from a thermosetting material, the disadvantages which are associated with the production or processing of thermosetting materials can also be overcome. For example, thermoset materials have disadvantages in comparatively long cycle times or in a necessary elaborate reworking.

Compared to the solutions, which are based on the use of a piston made of a thermosetting material, piston-cylinder arrangements described here have the advantage that the production can be simplified by dispensing with such complex processes. By using a thermoplastic material simple and straightforward manufacturing steps can be used, which require no post-processing and thus can form the corresponding component in a few steps. For example, the injection molding of thermoplastic materials or the application of coating called thermoplastic materials that are technically mature and easy to implement and implement.

With respect to thermoplastic polytetrafluoroethylene can be understood in a conventional manner, in particular, one which is moldable under the influence of temperature and then hardens again on cooling and thereby has a high stability in the hard state. In other words, a thermoplastic polytetrafluoroethylene is melt processable.

For example, the material sold by ElringKlinger under the name "Moldflon" can be used as such a thermoplastic polytetrafluoroethylene. Such a material or a material of this kind may be chemically modified, for example, by incorporating an additive known as a modifier in its polymer chain and thereby being able to have improved properties compared with standard PTFE. For example, as mentioned above, thermoplastic polytetrafluoroethylene, unlike conventional or otherwise modified PTFE, may be melt-processable.

Thus, there are significant processing advantages over conventional PTFE, since due to the very high molecular weight, the melt viscosity of this is so high that it was previously not thermoplastic process. So far, PTFE components have been manufactured using special pressing and sintering techniques, with the components subsequently achieving their geometry through machining, for example.

In contrast, thermoplastic PTFE offers economic advantages, for example through the elimination of post-processing steps, which are particularly noticeable in large-scale production. For example, thermoplastic PTFE, for example the material "Moldflon", enables component geometries that were previously difficult or impossible to realize by machining. This property makes it possible to implement thermoplastic polytetrafluoroethylene in a material which forms the contact surface, for example by means of injection molding. Finally, it may be possible for thermoplastic polytetrafluoroethylene to exhibit excellent sliding friction behavior with good wear resistance even at high temperatures, and these parameters may possibly even be better than in the case of conventional polytetrafluoroethylene. Furthermore, thermoplastic polytetrafluoroethylene has an extremely low coefficient of friction. Since static friction and sliding friction are the same for PTFE, the transition from standstill to motion is without jerking, so that there is no stick-slip effect or stick-slip effect. This further shows the advantageous suitability of thermoplastic polytetrafluoroethylene for a contact surface of a seal in the piston-cylinder arrangement described here.

In principle, however, the present invention is not limited to the use of "Moldflon" as a material, but any thermoplastic polytetrafluoroethylene can be used. Basically, in these materials in a manner known to those skilled in the polymer structure is adapted so as to reduce the melt viscosity. The structure of these particularly partially crystalline materials is composed, for example, of in particular lamellar crystallites and amorphous zones lying between them. Thermoplastic processability thus permits processing such as by injection molding and extrusion, but also by deep drawing, blow molding or melt spinning, without being limited thereto. This processing has significant advantages over, for example, processing of thermoset materials as described above.

Further advantages of Moldflon or other thermoplastic polytetrafluoroethylene can be seen, for example, in that it is possible to work with a melting temperature of approximately 320 ° C. and a continuous operating temperature of approximately 260 ° C. without problems in the region of, for example, a clutch master cylinder. Due to the very good temperature resistance, it can even be used in the slave cylinder, such as a slave cylinder designed as a CSC (Concentric Slave Cylinder), ie in the clutch gap, where usually much higher temperatures prevail than in the area of the master cylinder.

In addition, compared to other thermoplastics, only a very low water absorption of <0.01% by weight can be observed. which offers the advantage that the material has a high dimensional stability.

Finally, through the use of thermoplastic materials, it is possible to encase or inject smallest components, which eliminates the need for additional expensive assembly processes or post-processing steps.

Furthermore, polytetrafluoroethylene exhibits good resistance to almost all chemicals used in the respective application. The use of brake fluids, which can be used in the present case as an example hydraulic fluid, is therefore harmless with respect to the media compatibility of the mate- rials of the contact surface.

It can preferably be provided that the at least one seal is fixed to the cylinder and the contact surface is part of the piston. This embodiment may also be referred to as a so-called standing seal. In particular, in this embodiment, the present invention may be advantageous. Especially with a vertical seal or with several upright seals, the piston is often molded from duroplastic materials to allow a suitable friction behavior to the seals. Thus, in particular in this embodiment, the thermosetting piston can be replaced, which can allow the advantages described above.

For example, in this embodiment, but not limited thereto, the piston-cylinder assembly may be a clutch master cylinder or a clutch slave cylinder of a hydraulic clutch system.

It can further be provided that a seal is provided or that a plurality of seals is provided, such as in a clutch master cylinder. For example, a seal may be a so-called primary seal, which may be a pressure build-up in the direction of a clutch slave cylinder connected to the clutch master cylinder, and a second seal may be a so-called secondary seal, which keeps the hydraulic fluid in the wake region depressurized by a seal Atmosphere. It may also be preferred that the contact surface is part of a layer which is arranged on a base body, in particular of the piston or of the cylinder. In particular, when the contact surface is part of a layer which is arranged on a main body of the piston, it can be provided that the layer partially encloses the basic body, for example completely. The layer can, for example, completely encase the base body or at least be provided on the circumference at which the piston or the contact surface comes into contact with the seal.

In this embodiment, it may be possible that the core or the base body and the layer are formed of different materials, which allows an individual adaptability of the materials to the desired properties.

For example, it can be provided that the base body is formed from a thermoplastic plastic.

Thus, in particular in this embodiment, it may be possible, for example, for a piston formed from a thermosetting material known from the prior art to be replaced by such a piston which is completely formed from a non-thermosetting material. For example, it can be provided that the main body of the piston or the cylinder comprises or consists of a thermoplastic standard material or a technical thermoplastic. Examples of such thermoplastic materials from which the body may be formed include, for example, polyamide (PA) or polyphthalamide (PPA) or a mixture thereof. In this case, the material of the base body but also of the layer having the contact surface can be fiber-reinforced, for example by means of carbon fibers or glass fibers.

As a result, the basic body can also be formed by the abovementioned production steps, which can significantly simplify the production of the piston or of the cylinder. In particular, if the base body is formed from a thermoplastic material, for example, the base body is easier to handle compared to thermoset materials. As a result, the base body and thus, for example, the piston or the cylinder can be cheaper to manufacture, if the base body is made of a thermoplastic material and dispenses with duroplastic materials, for example, where the cost-effectiveness may be due in particular to processability. The use of a thermoplastic such as PA or PPA or a combination of different thermoplastic materials is therefore particularly suitable, whereby the initial cost of this material or of these materials can be similar or even better than that of the thermoset and the production costs can be lowered against the piston made of duroplastic.

The material of the base body, for example a thermoplastic material as described above, can provide, for example, the necessary mechanical stability of the component, wherein the layer with the contact surface should also reduce or prevent noise as described above. As a result of the configuration of the contact surface described above, the noises can be bypassed so that the advantages of a thermoplastic material without its disadvantages can be achieved.

It may preferably be provided that the layer is formed as a sleeve. Thus, the body of the sleeve is at least partially, for example, completely give. In particular, in this embodiment, the piston-cylinder arrangement may be advantageous, since low production costs are combined with a high stability and longevity in a particularly effective manner.

In this embodiment, the layer or the sleeve, which has the contact surface, be prefabricated, which may allow a particularly adaptable connection to a selectable body. In this case, the sleeve may be fixed to the base body in a basically selectable form. Particularly preferably, it can be provided that the sleeve is fixed by means of positive locking on the base body. For example, the positive connection can be made possible by means of a snap connection or clip connection. Such compounds can allow for permanent stability and also be easy to train. Finally, such compounds may optionally be soluble. As a result, even if the contact surface is damaged or worn, it can be renewed in a simple manner, as a result of which the main body can continue to be used.

It may further be provided that the layer is formed as a coating applied to a base body of the piston or the cylinder. In this embodiment, a particularly easy application of the layer can be made possible. Because coating operations can be carried out in many ways and are easy and inexpensive to carry out.

For example, if the base body is formed from a thermoplastic material, the layer or the coating can be produced on the base body by an injection molding process. In other words, the base body can be encapsulated by the material of the layer or the coating. In this regard, in particular a two-component injection molding (2K) can be used.

The layer, such as the coating, may for example consist of thermoplastic polyethylene terephthalate or, for example, in addition to the thermoplastic polymer terephthalate, reinforcing fibers, such as glass fibers or carbon fibers as reinforcing fibers or other components.

By way of example, the layer or the coating can also be the material distributed with the product name XYLAN 1290 / F1230 LAMP BLACK from Whitford. This is a resin-bonded dry lubricant coating with a high proportion of PTFE. This material allows a low friction to the wear ratio. The application is possible, for example, in the spray process, if appropriate after cleaning or degreasing the surfaces of the base body which have, for example, PA and / or PPA.

In principle, therefore, in addition to a coating of pure or fiber-reinforced PTFE, a coating can be used which, in addition to polytetrafluoroethylene, has a synthetic resin. The abovementioned coatings, as described above, allow a particularly quiet operation due to a very low stick-slip effect. Furthermore, such coatings have a high temperature resistance, which allows their use in the master cylinder and the slave cylinder without problems. In addition, they have a high chemical resistance, which allows the unconsidered contact with brake fluid and other media, such as mineral oil, lubricating greases or others.

It may further be provided that the piston or the cylinder is made entirely of a material which comprises, for example consists of, thermoplastic polytetrafluoroethylene. In this case, reference is made in particular to the piston or cylinder, which has the contact surface. In this embodiment, not only the material of the contact surface is thus formed from a material which comprises thermoplastic polytetrafluoroethylene, but preferably the entire piston or the entire cylinder is constructed from this material. In this embodiment, a particularly simple producibility can be made possible since the piston or the cylinder does not have to be formed from a base body with a layer of the material of the contact surface arranged thereon. Rather, the entire piston or the entire component can be formed in one production step, such as in an injection molding process. This can thus combine the advantages described above with a particularly advantageous manufacturability.

It can also be provided that the thermoplastic polytetrafluoroethylene, whether as a material of the contact surface or possibly also of the entire piston, is fiber-reinforced. For example, the thermoplastic polytetrafluoroethylene may be reinforced by glass fibers or by carbon fibers. In this case, it can also be provided that the material from which the contact surface or also the entire piston is formed consists of the reinforcing fibers and the thermoplastic polytetrafluoroethylene or that further substances are incorporated. In this embodiment, it may be possible to maintain or at least not significantly reduce the advantageous properties as described above, while at the same time increasing mechanical stability and long-term stability. With regard to further technical features and advantages of the piston-cylinder arrangement, reference is made to the further description of the hydraulic coupling system, the figures and the description of the figures.

Also described is a hydraulic clutch system, in particular for a motor vehicle, wherein the clutch system has a piston-cylinder arrangement, as described in detail above.

Such a coupling system may in particular comprise a clutch master cylinder, which is actuated by a piston rod and thus promotes hydraulic fluid from a reservoir to a clutch slave cylinder, such as in a clutch bell, or vice versa from the clutch slave cylinder into the reservoir for engagement or disengagement of the clutch. Between the hydraulic functional elements, in particular clutch master cylinder, clutch slave cylinder or clutch bell and reservoir hydraulic lines are in particular designed as pressure lines. In particular, and in no way limiting, the piston-cylinder arrangement can be a clutch master cylinder of the clutch system.

By means of such a configuration, as described in detail above, a smooth and, in particular, low-noise running of the piston in the cylinder can be made possible in a simple and cost-effective manner.

With regard to further advantages and technical features of the hydraulic clutch system, reference is made to the description of the piston-cylinder arrangement, the figures and the description of the figures.

In the following, a particularly preferred embodiment of the invention will be explained with reference to the accompanying drawings, wherein it is explicitly pointed out that the object according to the invention is not limited to the exemplary embodiment described. Show it:

Fig. 1 is a schematic sectional view through a piston-cylinder arrangement; FIG. 2 shows a sectional view through a piston for a piston-cylinder arrangement from FIG. 1; FIG.

Fig. 3 test results showing the effect of the invention; and

Fig. 4 further test results showing the effect of the invention.

Figure 1 shows a piston-cylinder assembly 10, which may be part of a hydraulic clutch system in particular. For example, the piston-cylinder assembly 10 may be a clutch master cylinder.

The piston-cylinder arrangement 10 comprises a cylinder 12 and a piston 14 axially displaceably positioned in the cylinder 12. Between the piston 14 and the cylinder 12, in the embodiment according to FIG. 1, two are made of ethylene-propylene-diene rubber molded seals 16, 18 are provided. The seals 16, 18 serve to seal off a gap 20 between the piston 14 and the cylinder 12.

The embodiment of Figure 1 is one which is referred to as a design with standing seals. Correspondingly, the seals 16, 18 are fixed to the cylinder 12 and abut on a contact surface 22 of the piston 14, or slide along it. The contact surface 22 is formed from a material which comprises thermoplastic polytetrafluoroethylene.

In the figure 1, a pedal connection 24 is also shown, which is intended to show a connection to a clutch pedal. The pedal connection 24 is connected by a piston rod 26 to the piston 14. Furthermore, a follow-up nozzle 28 is shown, which is connected to a reservoir, and there is a Flochdruckanschluss 30 shown, which can connect the piston-cylinder assembly 10 formed as a clutch master cylinder with a clutch slave cylinder. With respect to the seals 16, 18, the seal 18 is a primary seal and allows a pressure build-up in the direction of a clutch slave cylinder, whereas the seal 16 is a secondary seal and holds the hydraulic fluid pressure in the trailing area.

An embodiment of the piston 14 is shown in FIG. As shown in FIG. 2, it is provided that the contact surface 22 forms part of a layer 34 is, which is arranged on a base body 32 of the piston 14. More specifically, the layer 34 is formed as a sleeve 36. The layer 34 may consist of thermoplastic polytetrafluoroethylene, for example, and the base body 32 may be made of a likewise thermoplastic material, such as polyamide (PA) or polyphthalamide (PPA).

In Figures 3 and 4, the positive effect of the piston-cylinder assembly 10 described here is described.

FIGS. 3 and 4 each show diagrams which show on the X-axis a relative velocity between a piston 14 and a cylinder 12 in [mm / s] and show the dimensionless so-called stick-slap factor on the Y-axis which represents a higher noise level at higher values.

In FIG. 3, curve A represents the friction partners glass-fiber-reinforced polyamide (PA-GF) and ethylene-propylene-diene rubber (EPDM), curve B represents the friction partners glass-fiber-reinforced polyphthalamide (PPA-GF) and ethylene-propylene-diene rubber (EPDM) and curve C represents the friction partners polytetrafluoroethylene (PTFE) and ethylene-propylene-diene rubber (EPDM).

In FIG. 4, curve A represents the friction partners polyamide (PA) and ethylene-propylene-diene rubber (EPDM), curve B represents the friction partners polyphthalamide (PPA) and ethylene-propylene-diene rubber (EPDM), Curve C represents the friction partners polytetrafluoroethylene (PTFE) and ethylene-propylene-diene rubber (EPDM) and curve D represents the friction partners duroplastic and ethylene-propylene-diene rubber (EPDM).

With reference to FIGS. 3 and 4, it was already possible to observe the positive behavior in basic tribological experiments with specimens made of standard PTFE and EPDM specimens. Compared with thermoplastic PA or PPA, pure PTFE with EPDM as a friction partner shows a significant improvement in the vibration behavior of the friction partners, which are piston materials compared to the sealing material EPDM, which leads to significantly lower noise emissions. It can be clearly seen that the stick-slip effect is reduced by pure PTFE, so that there are only too few / no vibrational vibrations, so that no audible / disturbing noises.

The present experiments could be made regardless of whether the thermoplastic polytetrafluoroethylene is present as a coating, solid material or sleeve. Further, the presence of brake fluid as the hydraulic fluid has not caused any changes in the positive characteristics.

This clearly shows that a simple replacement of a thermosetting plastic against a thermoplastic material makes possible improved manufacturability, but increases the noise load. Only by a contact surface 22 comprising polytetrafluoroethylene a replacement of thermosetting plastic with the same noise development and improved production can be made possible.

LIST OF REFERENCES

Piston-cylinder arrangement

cylinder

piston

poetry

gap

contact surface

pedal connection

piston rod

trailing socket

High pressure port

body

layer

shell

Claims

claims

1. Piston-cylinder arrangement for a hydraulic clutch system, comprising a cylinder (12) and a piston (14) axially displaceably positioned in the cylinder (12), wherein between the piston (14) and the cylinder (12 ) at least one seal (16, 18) for sealing a gap (20) between the piston (14) and the cylinder (12) is provided, the at least one seal (16, 18) on one of the piston (14) and is fixed to the cylinder (12) and is movably attached to the corresponding mating element selected from cylinder (12) and piston (14) on a contact surface (22), characterized in that the abutment surface (22) of the piston (14) or the cylinder (12) is formed of a material which comprises a thermoplastic polytetrafluoroethylene.

2. Piston-cylinder assembly according to claim 1, characterized in that the at least one seal (16, 18) is formed of ethylene-propylene-diene rubber.

3. piston-cylinder arrangement according to one of claims 1 or 2, characterized in that the at least one seal (16, 18) on the cylinder (12) is fixed and the contact surface (22) part of the piston (14). is.

4. piston-cylinder arrangement according to one of claims 1 to 3, character- ized in that the contact surface (22) is part of a layer (34) on a nem body (32) of the piston (14) or the Cylinder (12) is arranged.

5. piston-cylinder assembly according to claim 4, characterized in that the base body (32) is formed of a thermoplastic material.

6. piston-cylinder arrangement according to one of claims 4 or 5, character- ized in that the layer (34) is designed as a sleeve (36).

7. piston-cylinder arrangement according to one of claims 4 or 5, character- ized in that the layer (34) as a on a base body (32) of the

Piston (14) or the cylinder (12) applied coating is formed.

8. piston-cylinder arrangement according to one of claims 1 to 7, character- ized in that the piston (14) or the cylinder (12) consists entirely of a material comprising thermoplastic polytetrafluoroethylene.

9. Piston-cylinder arrangement according to one of claims 1 to 8, character- ized in that the thermoplastic polytetrafluoroethylene is fiber-reinforced.

10. Hydraulic clutch system, in particular for a motor vehicle, characterized in that the coupling system comprises a piston-cylinder arrangement (10) according to one of claims 1 to 9.