Abstract:
The invention relates to a master brake cylinder arrangement with actuation detection for a motor vehicle braking system, having at least one piston arrangement with a piston, which is movably guided in a cylinder bore in a cylinder housing, said piston with the cylinder bore defining a pressure chamber which is fluidically coupled to a hydraulic brake circuit. During a forward stroke, the piston can be moved from a start position into a possible actuation piston along a movement axis, which coincides essentially with a longitudinal axis of the cylinder bore, and during a return stroke, said piston is moved from an actuation position into a start position. The piston is associated with a coupling element which is coupled to a position detecting rod for common movement therewith, said position detecting rod being moved essentially parallel to the first cylinder bore, the position of the position detecting rod can be detected by means of a position detecting sensor placed on the cylinder housing. According to the invention, in order to save space, the coupling element can be locked relative to the piston for a common lift movement therewith and after exceeding a predetermined path of the lift movement, the piston can be moved further relative to the coupling element.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage of International Application No. PCT/EP2013/073847 filed Nov. 14, 2013 and which claimed priority to German Patent Application No. 10 2012 022 519.9 filed Nov. 16, 2012, the disclosures of which are incorporated herein by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a master brake cylinder arrangement with actuation detection for a motor vehicle braking system with at least one piston arrangement with a piston which is dislocatably guided in a cylinder bore in a cylinder housing, wherein the piston confines a pressure chamber with the cylinder bore which is fluidically coupled with a hydraulic brake circuit, and can be dislocated during a forward stroke from an initial position to a possible actuated position along a dislocation axis which substantially coincides with a longitudinal axis of the cylinder bore, and is dislocated from an actuated position to the initial position during a back stroke, wherein a coupling element is assigned to the piston which is coupled for a common dislocation to a position detection rod which is dislocatably guided substantially in parallel to the first cylinder bore, wherein the position of the position detection rod is detectable with a position detection sensor attached to the cylinder housing. The present invention further relates to a motor vehicle braking system with such a master brake cylinder arrangement. 
     A master brake cylinder arrangement of the kind described at the outset is already known from the prior art according to the document DE 10 2008 020 934 A1. In this arrangement it is provided that the position detection rod is fixedly coupled to the piston and moves with the piston over the entire stroke movement thereof. This has the result that a corresponding space-requiring arrangement must be provided on the master brake cylinder housing, which arrangement has a sufficiently long position detection rod with sufficient freedom of movement corresponding to the stroke movement of the piston. The cylinder housing must be configured with a correspondingly large volume. However, this requirement is contrary to the increasingly important requirement for space- and weight-saving components in modern motor-vehicle construction. 
     Similar arrangements are found in the documents DE 37 23 842 A1, DE 37 23 916 A1 and DE 38 42 225 A1. 
     BRIEF SUMMARY OF THE INVENTION 
     A feature of the present invention is to provide a master brake cylinder arrangement of the kind described at the outset which differs from the prior art by having a compact and cost-effective design together with reliable functioning. 
     This feature is achieved by a master brake cylinder arrangement in which it is provided that the coupling element is lockable relative to the piston for a common stroke movement therewith and after over-travelling of a predetermined distance of the stroke movement, the piston is further dislocatable relative to the coupling element. 
     In a departure from the prior art, the present invention provides that the coupling element, which is coupled or couplable for common dislocation to the position detection rod, does not move jointly over the entire stroke movement of the piston, but only over a partial region of this stroke movement. For this partial region, the coupling element is coupled to the piston by locking for common dislocation. If, however, the predetermined distance of the stroke movement is reached and over-travelled, the coupling element is so to speak uncoupled from the piston. The piston then moves further without taking along the coupling element. Accordingly, the position detection rod is not moved further either. As a result, the movement range for the latter can be dimensioned significantly smaller. At first sight, this may appear disadvantageous. However, it has been realised that, both for the generation of a brake light signal and for the control of traction control systems, it is not necessary to detect the entire stroke movement of the piston. Rather, it is sufficient to monitor only a partial region of the stroke movement of the piston, namely in particular the stroke movement of the piston at the beginning between an initial position, in which the piston is unactuated, and a predetermined piston travel, at which a substantial braking effect has already occurred. The invention makes use of this realization, whereby space and weight can be saved. 
     A development of the invention provides that the coupling element is assigned a first abutment surface of the piston, of the cylinder housing or of a component coupled thereto, against which surface the coupling element in the initial position abuts or is supported. As a result, it can be ensured that the coupling element with an unactuated brake is in a defined initial position, so that the unactuated state of the brake is also reliably detectable via the position detection rod. 
     Furthermore, according to the invention it can be provided the coupling element is assigned a second abutment surface, against which the coupling element abuts on over-travelling of the predetermined distance of the stroke movement. As a result, it is ensured that the coupling element after over-travelling of the predetermined distance of the stroke movement is not moved further, but remains, relative to the cylinder housing, in a predetermined position until a back stroke movement of the piston. 
     Preferably, in this connection it can be provided that the first or/and the second abutment surface is part of an integrally formed section of the cylinder housing. Alternatively to this it is possible that the first or/and the second abutment surface is provided on an additional element formed in the cylinder housing, wherein the additional element is configured in particular in the form of a disc surrounding the piston in the circumferential direction or of an annular sealing component. The use of an additional element allows resort to be had to conventional designs of the cylinder housing and these to be modified only slightly, if possible at all. Moreover, the production and the assembly are thereby facilitated. 
     An embodiment of the invention provides that the coupling element is a body at least partially encompassing the piston in the circumferential direction. In this case, it is possible according to the invention that the coupling element has an abutment ring which is provided with an abutment surface, via which it engages on the piston. This abutment ring can be closed or slotted. The abutment surface can, for example, be formed by the inner circumferential surface of a tubular section which is integrally connected to the coupling element. Furthermore, an embodiment variant of the invention provides that the coupling element abuts against the piston under radial bias and is coupled to the piston by frictional engagement. In order to ensure that within a predetermined distance of the stroke movement of the piston a common movement between coupling element and piston takes place, additional measures for guaranteeing the frictional engagement over the service life of the master brake cylinder arrangement are taken. In this case, it can in particular be provided that the abutment surface is provided with a special friction lining. 
     Additionally or alternatively to a frictional engagement, it is further possible that the coupling element is coupled via at least one spring element for common movement with the piston. Thus, for example an arrangement can be employed, with which the coupling element is biased via the spring element in such a way that the coupling element executes a common movement with the piston until it strikes a stop and then is impeded from a further movement together with the piston. In this case, it can be provided that owing to the further movement of the piston the spring element is compressed, as explained in detail below. Alternatively, it is possible that until the over-travelling of the predetermined distance x the spring element relaxes and then has a constant length. In this connection, it is possible in an embodiment variant of the invention that the coupling element is biased via the spring element into an initial position relative to the piston and in this case in particular is of pot-like configuration. Furthermore, in this connection it can be provided that the coupling element has a stop ring, with which it can be brought, owing to the stroke movement of the piston, into abutment against the cylinder housing on over-travelling of the predetermined distance of the stroke movement, wherein the coupling element on advancing stroke movement of the piston, in particular under the action of the spring element, remains stationary relative to the cylinder housing. 
     An alternative embodiment of the invention provides that the spring element is supported, at one end, on the coupling element and, at the other end, on a holding bush fixed to the piston or a closure element fixed to the cylinder housing. In this case, the holding bush can be integrally formed on the piston. Preferably, however, it is provided that the holding bush is formed as a separate component and fixed to the piston, for instance by latching or via additional fixing means. This too facilitates the production and the assembly, because in this way the piston can, for example, firstly be preassembled with the coupling element and the holding bush and also the spring element arranged therebetween to form an assembly and subsequently be inserted into the master brake cylinder housing. In the case of a supporting of the spring element on a closure element fixed to the cylinder housing, the coupling element can, according to a further embodiment of the invention, be supported on a diameter step or shoulder of the piston. As a result, the coupling element can, solely via the spring element and the abutment surfaces on piston and closure element, be completely fixed in its position over its entire movement distance. Regarding the position detection device, a development of the invention provides that the guide shaft is configured with a diameter step, wherein the position detection rod has a detection section or is coupled thereto, on which a sensor element is provided. Furthermore, in this connection it is possible that the sensor element has a magnetic body and that the position detection sensor is a Hall sensor. 
     The invention further relates to a motor vehicle braking system with a master brake cylinder arrangement of the kind described above. 
     Other advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an axis-containing sectional view of a first embodiment of the master brake cylinder arrangement according to the invention in an initial position; 
         FIG. 2  shows a partial sectional view of the embodiment according to  FIG. 1  in an actuated position; 
         FIG. 3  shows a partial sectional view corresponding to  FIG. 2  of a second embodiment of the invention in the initial position; 
         FIG. 4  shows a partial sectional view corresponding to  FIG. 3  of the second embodiment of the invention in an actuated position; 
         FIG. 5  shows a partial sectional view corresponding to  FIG. 3  of a third embodiment of the invention in the initial position; 
         FIG. 6  shows a partial sectional view corresponding to  FIG. 5  of the third embodiment of the invention in the initial position; 
         FIG. 7  shows a partial sectional view corresponding to  FIG. 3  of a fourth embodiment of the invention in the initial position; 
         FIG. 8  shows a three-dimensional representation of an embodiment of the coupling element; 
         FIG. 9  shows a partial sectional view corresponding to  FIG. 2  of a fifth embodiment of the invention in the initial position; 
         FIG. 10  shows a partial sectional view corresponding to  FIG. 9  of the fifth embodiment of the invention in an actuated position; 
         FIG. 11  shows a partial sectional view corresponding to  FIG. 2  of a sixth embodiment of the invention in the initial position; 
         FIG. 12  shows a partial sectional view corresponding to  FIG. 11  of the sixth embodiment of the invention in an actuated position; and 
         FIG. 13  shows a partial sectional view corresponding to  FIG. 2  of a seventh embodiment of the invention in the initial position. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In  FIG. 1 , a master brake cylinder arrangement according to the invention is shown and generally designated by  10  in a sectional view containing the longitudinal axis A of the cylinder bore. This arrangement comprises a cylinder housing  12 , in which a cylinder bore  14  is provided. A piston arrangement with a first piston  16  and a second piston  18  is accommodated in the cylinder bore  14 . The second piston  18  is supported on the base of the cylinder bore  14  via a return spring  20 . The return spring  20  is biased via a biasing device  22  with a pin  24  and a bush  26 . The first piston  16  is supported on the second piston  18  via a further return spring  28 , which is provided via a further biasing device  30  with a pin  32  and a bush  34 . The two pistons  16  and  18  are guided in a sealed manner in the cylinder bore  14  using seals  36 ,  38 ,  40 ,  42 , so that they enclose with the cylinder bore  14  pressure chambers  44 ,  46 . These pressure chambers  44 ,  46  are hydraulically coupled with a hydraulic circuit (not shown) of a motor vehicle braking system. Furthermore, there can be seen connections  48 ,  50  for connecting a fluid container (not shown) in a known manner. 
     The seal  36  is accommodated in a separate closure element  52  which is received in the cylinder housing  12  by a caulking or a latching connection  54 . The piston  16  is also guided in this element. 
     On the outer circumferential surface  56  of the piston  16  is mounted a coupling element  58 , which is shown in  FIG. 6  in a three-dimensional single-part representation. This element has a slotted, annular body with a tubular section  60  running in the direction of the longitudinal axis A and an annular section  62  running in the radial direction. Provided on both sides of the slot  64  are spreading openings  66  for facilitating the mounting with a spreading tool. The inner circumferential surface  68  of the tubular section  60  is formed as a friction surface, optionally with a friction lining. 
     The coupling element  58  is seated under radial bias on the outer circumferential surface  56  of the first piston  16 . In the initial position shown in  FIG. 1 , it abuts in a planar manner against the surface of the closure element  52  facing the interior of the cylinder housing  12 . The coupling element  58  acts, with a place on its annular section  62 , on a position detection rod  70  which is guided in a guide shaft  72  inside the cylinder housing  12 . At its end facing away from the coupling element  58 , the position detection rod  70  is coupled to a sensor element  76 , displaceably accommodated in a bore  74 , with a magnetic body  78 . The sensor element  76  is biased via a compression spring  80  into its position shown in  FIG. 1 . The bore  74  is closed via a spherical closure body  82 . Furthermore, it can be seen that on the cylinder housing  12  there is provided a position detection sensor  84  which delivers a position signal to the vehicle electronics according to the deflection of the sensor element  76 . 
     If now from the initial position according to  FIG. 1  the master brake cylinder arrangement  10  is actuated via a brake pedal (not shown), a force F is exerted on the first piston  16  and the latter is dislocated along the longitudinal axis A into the cylinder housing  12  to generate a brake pressure. On account of the frictional engagement between the first piston  16  and the coupling element  58 , the coupling element  58  moves with the first piston  16 . Owing to the movement of the coupling element  58 , the position detection rod  70  is also dislocated and with the latter the sensor element  76 , for position detection. 
     However, as soon as the first piston  16  has been dislocated by a predetermined distance x, the coupling element  58  abuts against an abutment projection  86  of the master brake cylinder housing  12 . This state is shown in  FIG. 2 . A further movement of the coupling element  58  with the first piston  16  is no longer possible because of this mutual abutment. However, if the piston  16  is further dislocated as a result of the force F, this piston, with its outer circumferential surface  56 , so to speak slips through the tubular section  60 , bearing on this surface under bias, of the coupling element  58 . Accordingly, any further dislocation of the piston  16  is no longer detected via the position detection rod  70  and the position detection sensor  84 . 
     If the brake pedal is subsequently released, the first and the second piston  16 ,  18  move back due to the action of the return springs  20 ,  28 . In the process, the coupling element  58  is carried along with the piston on account of the frictional engagement between the inner circumferential surface  68  of the tubular section  60  and the outer circumferential surface  56  of the first piston  16 , until the coupling element  58  abuts against the closure element  52  again. The coupling element  58  cannot follow a further return movement of the piston  16  because of this abutment situation, so that the piston  16  once again slips through the coupling element  58  by overcoming the frictional engagement. Finally, the initial position according to  FIG. 1  is reached again, so that upon a renewed actuation of the brake the same conditions and relative positions regarding the position detection arise as described above. It follows from this that the intended switching point for a brake light signal is always reproducible, because the coupling element  58  always returns to its originally provided initial position. 
     The invention is distinguished in that the position detection no longer extends over the complete piston stroke but only over a partial region, designated above as distance x. The position detection in this partial region is sufficient on the one hand to generate the brake light signal and on the other hand to carry out appropriate control measures on traction control systems or the like. A detection of the complete piston stroke is not necessary, as has been realised by the inventors. As a result, the master brake cylinder arrangement according to the invention can be of considerably more compact and also lighter design than conventional devices, in which the complete piston stroke is detected during its stroke movement. 
       FIGS. 3 and 4  show a second embodiment of the invention, wherein, to avoid repetitions and to facilitate the description, for components which have the same effect or are of the same kind, the same reference symbols are used as in the description of the first embodiment. Once again, the initial position and an actuated position are shown. 
     An essential difference of this embodiment from the first embodiment consists in that the coupling element  58  no longer abuts against an abutment formed integrally with the cylinder housing  12 , but against a disc-shaped component  90  which is inserted into an accommodating groove  92  in the cylinder housing  12 . This component also has a guide bore  94  for guiding the position detection rod  70 . A further difference of this embodiment from the first embodiment consists in that the piston is provided with a central valve  96  which is formed with a valve tappet  98  in a manner known per se. The valve tappet  98  can be opened or closed against the action of a compression spring  100  depending on the position of a locking bar  102 . In the initial position, the valve  96  is open, so that there is a fluid connection to the fluid container. As soon as the first piston  16  is deflected by a specific distance, the locking bar  102  lifts off from the disc  90  and the central valve  96  changes into its closed state. 
     The from functioning regarding the position detection is the same as described above with reference to the first embodiment. 
     In  FIGS. 5 and 6 , a modification of the embodiment according to  FIGS. 3 and 4  is shown. Once again, an initial position and an actuated position are illustrated. An essential difference from the embodiment according to  FIGS. 3 and 4  is the provision of a diameter step  142  on the piston  16 . The encircling end surface defined by the diameter step  142  and facing away from the base of the cylinder bore  14  serves as an abutment surface for the coupling element  58 . A compression spring  140  is arranged between the coupling element  58  and the closure element  52  fixed to the housing. In the initial position shown in  FIG. 5 , the compression spring  140  is maximally compressed and biases the coupling element  58  against the diameter step  142 . Analogously to the embodiment according to  FIGS. 3 and 4 , the coupling element after over-travelling of the predetermined distance abuts against a disc-shaped component  90 , with the compression spring  140  not yet being completely relaxed. In this case, the compression spring  140  abuts against the coupling element  58  over the entire movement distance thereof and finally biases the latter also against the disc  90 . The embodiment affords the advantage that the position of the coupling element  58  is always uniquely defined solely by a cooperation of the compression spring  140  and the abutment surfaces formed from diameter step  142  and disc  90 . A radial bias of the coupling element  58  against the piston  16  can therefore be of less strong design or be completely omitted. This simplifies the assembly and reduces friction losses. 
       FIG. 7  shows a further modification of the embodiment according to  FIGS. 3 and 4 . The only difference of this embodiment consists in that the disc  90  is not accommodated in an accommodating groove, but that the closure element  52  is provided with an axial extension  104  which fixes the disc  90  in the cylinder housing  12 . 
       FIGS. 9 and 10  show a fifth embodiment of the invention, wherein, to avoid repetitions and to facilitate the description, for components which have the same effect or are of the same kind, the same reference symbols are used as in the above description of the preceding embodiment. Once again, the initial position ( FIG. 9 ) and an actuated position ( FIG. 10 ) are shown. 
     In this fifth embodiment, the position detection rod  70  projects from the cylinder housing  12  in the axial direction A. The coupling element  58  in this embodiment is of pot-like and stepped configuration. It surrounds, on one side, the free right-hand end of the first piston  16  with its annular section  62  and the tubular section  68 . Furthermore, it surrounds with a tubular extension  106  a plunger-like force input member  108 , this tubular extension  106  being connected to the tubular section  68  via a bottom  110 . The force input member  108  is of T-shaped configuration and accommodates in an accommodating section  114  a rubber body  112 . Arranged between the accommodating section  114  and the bottom  110  of the coupling element  58  is a compression spring  116  which presses the coupling element  58  into abutment with an end surface  118  at the free end of the first piston  16 . It should be noted that the force input member  108  is accommodated in a manner known per se in an accommodating opening  120  of the first piston  16 . 
     Upon an actuation of the brake, the force input member  108  is subjected to the force F. Subsequently, the first piston  16  moves into the cylinder housing  12 . The coupling element  58  follows this movement until the predetermined distance x has been travelled. During this movement, the current position of the piston  16  is detected via the position detection rod  70  in the manner described above. When the predetermined distance x has been covered and over-travelled, the coupling element  58  abuts with its annular section  62  against an end surface  122  of the cylinder housing  12  and cannot move further with the piston  16 . If the piston is dislocated further into the cylinder housing  12 , this piston slips inside the tubular section  68  through the coupling element  58 . This can take place under friction. Alternatively, however, a sufficient play can also be provided between coupling element  58  and piston  16  in order to keep the friction too low as possible. In any case, on reaching the predetermined distance x and further piston movement into the cylinder housing  12 , a relative movement between the coupling element  58  and the piston  16  occurs, the spring  116  being compressed in accordance with this relative movement. This state is shown by way of example in  FIG. 10  for a particular actuated position. 
     If the brake is released to reduce the braking effect, so that it returns to its initial position again due to the return forces, the coupling element  58  finally assumes the position according to  FIG. 9  relative to the piston  16  again due to the action of the compression spring  116 . 
     The embodiment according to  FIGS. 9 and 10  has the particular advantage that, apart from the guidance of the position detection rod  70  in the cylinder housing  12 , no interventions have to be made on the cylinder housing. The actuation of the position detection rod for position detection takes place solely by measures which are taken outside the cylinder housing. This results in a particularly cost-effective and simple design. 
     Finally,  FIGS. 11 and 12  show a sixth embodiment of the invention, which is similar to the fifth embodiment of the invention according to  FIGS. 9 and 10 . In this embodiment, once again the coupling element  58  is accommodated on the outer circumferential surface  56  of the first piston  16 , optionally with frictional engagement or with play, i.e. largely without friction. Furthermore, a latching groove  130  is provided in the receiving opening  120 . This groove serves to fix a tubular section  132  with latching projections  134  of a holding bush  136  relative to the first piston  16 . The holding bush  136  has a disc-shaped annular section  138  which projects in the radial direction with respect to the longitudinal axis A. Provided between the annular section  138  of the holding bush  136  and the annular section  62  of the coupling element  58  is a compression spring  116  which is dimensioned with such a strength that it is not or not substantially compressed as a result of an actuation of the positioning rod  70 . 
     If now once again the force input member  108  is subjected to the force F, the first piston  16  is dislocated in a known manner into the cylinder housing  12 . The coupling element  58  follows this movement until the predetermined distance x has been travelled. During this movement, the current position of the piston  16  is detected via the position detection rod  70  in the manner described above. When the predetermined distance x has been covered and over-travelled, the coupling element  58  abuts with its annular section  62  against an end surface  122  of the cylinder housing  12  and cannot move further with the piston  16 . If the piston is dislocated further into the cylinder housing  12 , this piston slips inside the tubular section  68  through the coupling element  58 . In any case, on reaching the predetermined distance x and further piston movement into the cylinder housing  12 , a relative movement between the coupling element  58  and the piston  16  occurs, the spring  116  being compressed in accordance with this relative movement. This state is shown by way of example in  FIG. 12  for a particular actuated position. 
     If the brake is released, so that it returns to its initial position again due to the return forces, the coupling element  58  finally assumes the position according to  FIG. 11  relative to the piston  16  again due to the action of the compression spring  116 . 
     The embodiment according to  FIGS. 11 and 12  is also distinguished by a comparatively simple design. Measures have only to be undertaken on the first piston  16  in order to be able to utilise the above-described advantages of the position detection performed only over a partial region of the stroke movement. 
     Furthermore, in  FIG. 13  there is shown a seventh embodiment, which is based on the fifth and sixth embodiment according to  FIGS. 9 and 10, and 11 and 12 . In the broadest sense, in this embodiment there is provided a combination of a pot-like coupling element  58  analogously to  FIGS. 9 and 10  with a holding bush  136  engaging in the accommodating opening  120  of the piston  16  according to  FIGS. 10 and 11 . Arranged between these components is once again a compression spring  116 . In  FIG. 13 , the master brake cylinder arrangement  10  is generally in a position where the braking is not active and the piston  16  is positioned in its initial position. 
     As can be seen from  FIG. 13 , the coupling element  58  is again configured with an annular section  62 , a tubular section  68 , a tubular extension  106  and a bottom  110 . The holding bush  136 , which is preferably made of plastic, once again has a tubular section  132  with latching projections  134  which engage in a latching groove  130  inside the accommodating opening  120 . Likewise, an annular section  138  is provided on the right-hand end in  FIG. 13 , and serves as an abutment surface for the compression spring  116 . The fixing of the compression spring  116  to the holding bush  136  takes place, in what is shown, by means of a diameter step  140 , onto which the compression spring  116  is pushed with elastic widening. On the coupling element  58 , the compression spring  116  abuts against the outer wall of the bottom  110  and is likewise fixed thereto by means of a diameter step  142 . 
     Compared with the sixth embodiment, the tubular section  132  of the holding bush  136  is configured with a markedly larger axial length, so that the annular section  138  is arranged axially spaced from the piston  16 . Analogously to the previous examples, an actuating member (not shown) can be fastened in a known manner in the accommodating opening  120  of the piston  16  and extend along the longitudinal axis A through the holding bush  136 . 
     In the embodiment shown, the coupling element  58  sectionally surrounds the tubular section  132  of the holding bush  136  with its tubular extension  106  and bears directly thereon. Thus, the coupling element  58  is dislocatably guided axially along the tubular section  132 . In this case, a frictional engagement can also be provided between these elements or alternatively frictionless play. 
     The functioning of the arrangement from  FIG. 13  corresponds essentially to that of the fifth and sixth embodiments described above. Accordingly, on exertion of an actuating force F, firstly the coupling element  58  is commonly dislocated with the piston  16  over a distance x and in the process also displaces a position detection rod  70 . After travelling the distance x, the annular section  62  of the coupling element  58  comes into abutment with an end surface  112  of the brake cylinder housing  12 , so that the piston  16  is further dislocated relative to the stationary coupling element  58 . In the process, the holding bush  136  moves through the tubular extension  106  under a compression of the compression spring  116 . On the return movement of the piston  16  to the initial position shown in  FIG. 13 , this piston once again comes into abutment with the bottom  110  of the coupling element  58 , so that the latter is moved back to its initial position under additional action of the return force of the compression spring  116 . 
     In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.