Piston arrangement of a hydraulic actuating device on motor vehicles

A piston arrangement for master and/or slave cylinders of a hydraulic actuating device for clutch and/or brake systems on motor vehicles is disclosed. An insert is inserted into a recess of the piston up to a first stop, the insert having a ball seat and an expandable insertion sleeve which is undercut with respect to the ball seat and protrudes with its expandable region. A ram is pushed into the sleeve with its ball end until it snaps into the ball seat. As the ram is further pressed axially, the insert slides over the first stop, is firmly pressed into a narrow hole section of the recess and strikes a second stop at the inner end of the recess. The insertion sleeve is then tightly enclosed by the piston wall and can no longer expand even in the event of very high pull-out forces, i.e. cannot release the ball end of the ram.

BACKGROUND OF THE INVENTION

The invention relates to a piston arrangement for master and/or slave cylinders of a hydraulic actuating device for clutch and/or brake systems on motor vehicles.

In a known piston arrangement of this type (DE 100 62 988 A1), assembly is carried out in two separate steps. Firstly, the snap connection is brought about between the ram and the insert, referred to in said document as the piston link head. The unit preassembled in this way is then inserted into the recess and screwed into the latter by means of a threaded engagement. For this purpose, the recess is provided with an internal thread and the insert is provided with a corresponding external thread. On account of the threaded engagement, this known piston arrangement is expensive to manufacture, and special measures moreover have to be taken in order to prevent loosening of the screw connection. In terms of the requirements in mass production, assembly in two steps is complicated, wherein the handling of the preassembled unit consisting of the ram and the insert is made more difficult on account of the ball-and-socket connection of these parts.

OBJECT OF THE INVENTION

It is an object of the invention to simplify the manufacture and assembly of such piston arrangement and to fix the insert in the recess of the piston so that it is secured against high pull-out forces introduced via the ram, even without threaded engagement.

SUMMARY OF THE INVENTION

According to the invention, there is provided a piston arrangement for one of a master and slave cylinder of a hydraulic actuating device for one of a clutch and brake system on a motor vehicle, comprisinga piston having an outer end with a recess,an insert suitable for fixing in the recess, said insert having a ball seat and an insertion sleeve which is undercut with respect to the ball seat, said insertion sleeve having slits and thus being expandable and, when it is fully inserted in the recess, being tightly enclosed by a piston wall which delimits the recess, anda ram which transmits actuating travels and forces, said ram having a ball end which is shaped so as to be complementary to the ball seat, which ball end can be coupled to the insert in order to mount the ram in an articulated manner via a snap connection brought about by the expandable insertion sleeve;wherein the insert is provided with a peripheral chamfer or the like at its inner end, said peripheral chamfer being assigned, within the recess of the piston, a first stop for delimiting a first insertion travel of the insert in the recess, wherein, for bringing about the snap connection, the insert bearing against the first stop protrudes outwards with its expandable insertion sleeve substantially out of the recess of the piston,an axial pressure force that is to be exerted on the ram in order to bring about the snap connection being smaller than an axial pressure force that is to be exerted on the ram in order for the insert to slide over the first stop in the recess, anda second insertion travel of the insert which starts with the sliding-over being delimited by a second stop at the inner end of the recess.

In the piston arrangement designed according to the invention, firstly the insert is introduced into the recess of the piston until the peripheral chamfer of the insert strikes the first travel-delimiting stop. At the end of this first insertion travel, the insertion sleeve of the insert, which can be expanded to produce the snap connection, protrudes out of the recess of the piston.

The ram which is then pushed with its end ball first into the insertion sleeve in the axial direction of the piston drives the end ball with the axial force required to overcome the spring forces of the insertion sleeve until it meets the ball seat in the insert, as a result of which the snap connection is brought about. During this, the insert remains against its first stop in the recess of the piston because the axial pressure force required to produce the snap connection is smaller than the axial pressure force which has to be exerted on the ram in order to slide over the first stop, that is to say is smaller than the retaining force at the first stop.

When the axial pressure force on the ram is increased, the insert slides over the first stop in the recess of the piston, with partial elastic and also plastic deformation of the regions of the insert which bear against the first stop. The second insertion travel of the insert, which starts at this point, is delimited by the second stop at the inner end of the recess. Once the insert bears against the second stop, the expandable insertion sleeve of the insert is located within the recess and is tightly enclosed by the piston wall delimiting the recess. In this position, the end ball of the ram is located without play in the ball seat and cannot be pulled out of the insert and the insert cannot be pulled out of the piston, even if very high pull-out forces are applied.

The design of the piston arrangement according to the invention permits very simple assembly, in which the ram is driven with its end ball in a single axial movement into the insert inserted in the piston, wherein firstly the snap connection is brought about and immediately thereafter the insert is pushed into its final position in the recess. This does not lead to any deformation of the ball-and-socket joint which would give rise to play, since the transmission of pressure forces to the insert takes place via the form-fitting full bearing between end ball and ball seat.

Continuing the concept of the invention, the recess is designed as an elongate blind hole with a certain profiling which also produces the first and second stop for the insert. Accordingly, the insert is designed with an inner cylindrical section and an outer cylindrical section of larger diameter, in a manner adapted to the profiling of the elongate blind hole, in order to ensure the abovementioned simple assembly and the ability to withstand even very high pull-out forces.

Preferably, grooves are provided on the circumferential surface of the inner section of the insert in a uniformly distributed manner, which grooves reduce the bearing surface contact with that cylindrical hole section of the elongate blind hole which is slid over when the retaining force of the first stop is overcome by partial elastic/plastic deformation of the relevant region, as a result of which the necessary axial press-in force is reduced at the same time. Some of these grooves may have a greater axial length than the others, in order to evacuate air from the inner closed region of the elongate blind hole in the piston as the insert is pushed in.

The deformability of the inner section of the insert during the press-in operation may advantageously be increased by virtue of a concentric cut-out in the end face of its inner section, and thus the axial force required to press it in can be reduced. By virtue of ribs on the outer circumference of the insertion sleeve of the insert, the bearing surface portion of the insertion sleeve as it is pressed into the outer, first hole section of the elongate blind hole can also advantageously be reduced, and this also reduces the necessary axial press-in force.

A design of the ball end of the ram according to which the ball end is shaped so as to be free of any ridges in its surface regions which transmit pressure forces and tensile forces, allows the completely ridge-free moulding of an ideal ball joint, so that a play-free fit in the ball seat of the insert is made possible.

Advantageously, the materials for the three components of the piston arrangement are selected from aluminum, aluminum alloy and plastic. By using aluminum or an aluminum alloy for the piston, the latter can be designed to be relatively very thin-walled in the region of the insertion sleeve of the insert, without any strength considerations and without any fear of deformations of the piston wall when the insert is pressed in or in the event of axial tensile stresses via the ram. As a result, the insert, preferably molded from plastic as the ram, can also be given a larger diameter in this region, and this in turn permits an advantageously large ball seat and ball diameter.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The piston arrangement, the parts of the master or slave cylinder connected thereto and its individual parts are shown in the drawings on an enlarged scale compared to the actual dimensions for illustrative purposes, whereinFIGS. 2 to 9are in turn shown on an enlarged scale compared toFIG. 1.

Of the master or slave cylinder, only the part of the housing1which accommodates the piston arrangement is shown broken open inFIG. 1, since the invention does not extend to the design of the master or slave cylinder. The same applies in respect of the arrangement of seals between the housing1and the guide sleeve3which encloses the piston2and the seal arrangements to be fitted at the inner end of the piston2, none of said seals or seal arrangements being shown inFIG. 1. The piston2is a so-called plunger piston which has a cylindrical outer circumferential surface which is guided displaceably in a sealed manner in the guide sleeve3and can be moved in the pushing and pulling direction by means of the ram4.

The piston2has a recess5in its outer end, said recess being designed as an elongate blind hole which is concentric with the central axis of the piston2and has a number of cylindrical hole sections of different diameter, as can best be seen fromFIG. 2and will be described in more detail below. An insert6suitable for fixing in the recess5has a ball seat7and an insertion sleeve8which is undercut with respect to the ball seat7. The insertion sleeve8has slits and is thus expandable, in a manner to be described in more detail below. When it is fully inserted in the recess5, the insertion sleeve8is tightly enclosed by the piston wall which delimits the recess5, as can be seen inFIGS. 1 and 9.

The ram4which transmits the actuating travels and forces is provided with a ball end9which is shaped so as to be complementary to the ball seat7in the insert6, which ball end9can be coupled to the insert6in order to mount the ram4in an articulated manner via a snap connection brought about by the expandable insertion sleeve8. The achieved coupling is shown inFIGS. 1,8and9.

As can be seen fromFIG. 2, the recess5designed as an elongate blind hole has essentially four hole sections, of which the outer, first hole section10has the largest diameter d1which is adapted to the external diameter of the insertion sleeve8of the insert6. The first hole section10is followed by a comparatively smaller diameter d2′ of the second hole section11. This is followed by the third hole section12which has an only slightly smaller diameter d2with a transitional chamfer13which forms a first stop for the insert6. The third hole section12ends with a sharp edge15produced by an annular groove14, so that, seen from the outside in, an undercut is formed behind the third hole section12. The annular groove14is followed by the fourth hole section16which adjoins the annular groove14with a conical transition face17and which has a diameter d3that is larger than the diameters d2′ and d2of the second and third hole sections11and12, respectively. The recess5ends, after the fourth hole section16, at a face18perpendicular to the central axis of the piston2, which face18forms a second stop for the insert6.

In order to describe the insert6, reference will now be made toFIGS. 3 to 5. The insert6is provided at its inner end with a peripheral chamfer19which may also be designed as an edge rounding. This peripheral chamfer19strikes the transitional chamfer13which forms the first travel-delimiting stop when the insert6is introduced into the recess5. The insert6has two essentially cylindrical sections20and21, of which the outer section20has the larger diameter and forms the insertion sleeve8. The latter is expandable by virtue of a number of axis-parallel slits22, in the illustrated example by virtue of six slits22, in its wall which are distributed uniformly over the circumference, said slits forming resilient tongues23, in the example of embodiment six tongues23. The inner section21of the insert6has the smaller diameter which forms a press fit with the diameter d2of the third hole section12of the recess5of the piston2.

The inner section21of the insert6has in its circumferential surface a number of axis-parallel grooves25which continue up to its end face24, some of which grooves25′ have a greater axial length and serve to evacuate air from the recess5as the insert6is pressed into the latter. For the rest, however, the grooves25,25′ reduce the bearing surface portion of the inner section21relative to the third hole section12and therefore reduce the necessary press-in forces. Comparable effects are achieved on the one hand in that the insert6has a concentric cut-out26in the end face24of its inner section21, which facilitates deformations of the inner end of the inner section21during the press-in operation, and on the other hand in that the insertion sleeve8of the insert6has a number of axis-parallel small ribs27distributed over its outer circumference, which ribs reduce the bearing surface of the outer section20of the insert6relative to the wall surface of the first hole section10.

The ball end9of the ram4is molded so as to be free of any ridges in its surface regions which transmit pressure forces and tensile forces, as a result of which the ball-and-socket joint formed of the ball seat7and the ball end9can be manufactured with narrow tolerances and practically without any play, since no deformable and thus play-forming ridge is present in the area of contact of the convex and concave ball surfaces.

Suitable material pairings are obtained when the piston2is made of aluminum or an aluminum alloy, whereas the insert6and the ram4with its ball end9are molded from thermoplastic plastics. By way of example, glass-fiber-reinforced polyamides are suitable for this purpose.

The order of the operating steps for assembling the described piston arrangement is shown inFIGS. 6 to 9. Firstly, the insert6is pushed with its inner section21into the second hole section11and at the same time with its outer section20into the first hole section10, until the peripheral chamfer19strikes the transitional chamfer13which forms the first travel-delimiting stop. In this position of the parts, the insert6protrudes with its expandable insertion sleeve8substantially out of the recess5of the piston2, as shown inFIG. 6. The piston2and the insert6are thus ready to produce the snap connection.

The piston2with the insert6can now be pushed into the guide sleeve3, as shown inFIG. 7. The ram4is introduced by its ball end9through an opening28in the end wall of the guide sleeve3until it reaches the insertion sleeve8, wherein it strikes a peripheral chamfer29of the insertion sleeve8, said peripheral chamfer29being shaped approximately like a truncated cone and allowing expansion of the insertion sleeve8. As the insertion operation continues, the resilient tongues23now move laterally outwards, until they snap behind the ball end9as a result of the undercut in front of the ball seat7, wherein the ball end9passes in a form-fitting manner into the ball seat7. This position is shown inFIG. 8.

The axial pressure force to be exerted on the ram4in order to bring about the snap connection is smaller than the axial pressure force to be exerted on the ram4in order to slide over the first stop. The insert6therefore remains at the first travel-delimiting stop in the position shown inFIGS. 7 and 8while the snap connection is produced, that is to say the peripheral chamfer19of the insert6is pressed against the transitional chamfer13between the second and third hole sections11and12.

If the axial pressure force on the ram4is now increased, the insert6thus slides over the first stop with partial elastic and plastic deformation of the front end of its inner section21, since the retaining force of the first stop is overcome. A second insertion travel of the insert6is thus started, said second insertion travel being delimited and therefore ended by the end face24striking the face18of the recess5which forms the second stop. This position of the parts can be seen fromFIG. 9and alsoFIG. 1. In practice, the insertion of the ram4by its ball end9is not carried out in two insertion movements which are separated from one another over time, but rather takes place in a single, coherent press-in operation.

Behind the sharp edge15, the operation of pressing in and sliding over the first stop leads to a partial elastic back-deformation of the insert6, so that the latter is held in the recess5in the position shown inFIGS. 1 and 9even against very high pull-out forces introduced via the ram4. Since the insertion sleeve8is also completely enclosed by the solid wall of the piston2, very high pull-out forces moreover cannot pull the ball end9out of the ball seat7since the resilient tongues23cannot move out to the side.

There is thus proposed a piston arrangement for master and/or slave cylinders of a hydraulic actuating device for clutch and/or brake systems on motor vehicles, which piston arrangement is characterized by simplified manufacture and assembly of its components and also withstands high pull-out forces introduced via the ram. A suitable insert is inserted into a recess of the piston up to a first stop, said recess being designed as an elongate blind hole. The insert has a ball seat and an expandable insertion sleeve which is undercut with respect to the ball seat and initially protrudes outwards with its expandable region. The ram is pushed into the expandable insertion sleeve in the axial direction with its ball end which is shaped so as to be complementary to the ball seat, until the ball end snaps into the ball seat. As the ram continues to be pressed in the axial direction, the insert slides over the first stop, is firmly pressed into a narrow hole section of the recess and finally strikes a travel-delimiting second stop at the inner end of the recess. In this position, the insertion sleeve is tightly enclosed by the piston wall and can no longer expand even in the event of very high pull-out forces, that is to say cannot release the ball end of the ram.