Abstract:
A vacuum brake booster ( 10 ) is described, comprising a vacuum chamber ( 16 ) and a working chamber ( 18 ) separated from each other by a movable wall ( 14 ). The vacuum brake booster ( 10 ) includes an emergency braking aid comprising a permanent magnet ( 38 ), which is disposed in the control valve housing ( 22 ), and an armature ( 36 A) which cooperates with the permanent magnet ( 38 ) and in the event of emergency braking is drawn into abutment with the permanent magnet ( 38 ), with the result that a control valve ( 20 ) is held open for the supply of atmospheric pressure or above-atmospheric pressure to the working chamber ( 18 ). The permanent magnet ( 38 ) and the armature ( 36 A) form a two-component magnetic module ( 36 A,  38 ), of which a first component is rigidly coupled to an actuating piston ( 28 ) of the brake booster ( 10 ). A coupling device ( 66 ) is provided which is adapted to couple the second component of the magnetic module ( 36 A,  38 ), at least in actuating direction, to the actuating piston ( 28 ) in response to high reaction forces.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/EP01/04188 filed Apr. 11, 2001, which claimed priority to German Patent Application No. 10019424.9 filed Apr. 19, 2000, the disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a vacuum brake booster comprising a vacuum chamber and a working chamber separated from each other by a movable wall, a control valve which comprises a housing coupled workingly to said movable wall and which is capable of controlling the supply of atmospheric pressure or above-atmospheric pressure to the working chamber to achieve a pressure difference at the movable wall, and an emergency braking aid having a permanent magnet, which is disposed in the control valve housing, and an armature, which cooperates with the permanent magnet and in the event of emergency braking is drawn into abutment with the permanent magnet, with the result that the control valve is held open for the supply of atmospheric pressure or above-atmospheric pressure to the working chamber. The permanent magnet and the armature form a two-component magnetic module wherein a first component of the magnetic module is rigidly coupled to an actuating piston in actuating direction and a second component of the magnetic module is adapted to be coupled to the actuating piston at least in the actuating direction of the brake booster. In the context of the present invention, the term “actuating direction” always means the actuating direction of the brake booster. 
     Vacuum brake boosters have been known for quite some time and millions of them are being used to boost the actuating forces of a vehicle hydraulic brake system and therefore keep said forces at a level which is acceptable to the driver of a vehicle. Likewise known are so-called emergency braking aids, which are frequently also referred to as “brake assistants”. These are devices which provide a driver in the event of an emergency braking situation with increased braking power for substantially the same actuating force. 
     Emergency braking aids may be divided into electro-magnetically actuated and mechanically actuated systems. For reasons of cost, the use of a mechanical system is desired for applications in vehicles at the lower end of the price range. 
     A vacuum brake booster having such a mechanical emergency braking aid is known, for example, from WO 00/07862, corresponding U.S. Patent Application Publication 2001/0003947 A1 which is incorporated by reference herein. Said vacuum brake booster has a vacuum chamber and a working chamber separated from each other in a pressure-proof manner by a movable wall. A control valve, which has a housing coupled workingly to the movable wall, comprises an atmospheric valve seat which, to achieve a pressure difference at the movable wall, is capable of controlling the supply of atmospheric pressure to the working chamber in dependence upon the displacement of an input element of the brake booster. The input element is coupled in actuating direction to an actuating piston. 
     For improved boosting of the braking force in emergency braking situations, a mechanical emergency braking aid is disposed in the control valve housing. The emergency braking aid includes a two-component armature comprised of a permanent magnet and an armature. In the actuating direction, the armature cooperating with the permanent magnet is rigidly coupled to the input element via the actuating piston. A coupling device makes it possible to couple the armature to the permanent magnet in such a manner that the axial distance between the armature and the permanent magnet does not change as a braking operation commences. 
     The armature is resiliently preloaded counter to the actuating direction of the brake booster and, in the starting position of the control valve, is held at a first distance from the permanent magnet. In the course of an approach towards the permanent magnet, the armature, when it is less than a predetermined second distance away, which is smaller than the first distance, is pulled by the permanent magnet counter to the resilient preloading force acting upon the armature and with simultaneous cancellation of its, in actuating direction, rigid coupling to the input element into abutment with the permanent magnet. 
     The movement of the armature is transmitted to a valve sleeve, which is rigidly coupled to the armature and on its end facing the input element carries the atmospheric valve seat. When the emergency braking aid is activated, because of the coupling of armature and permanent magnet, the atmospheric valve is held open to the maximum extent. The maximum possible pressure difference therefore builds up, with the result that the maximum possible boosting force of the brake booster is achieved. 
     To deactivate an activated emergency braking aid, the actuating force summoned up by the driver has to be reduced. As a result of the reduction of the actuating force summoned up by the driver, the actuating piston moves counter to actuating direction and a catch rigidly coupled to the actuating piston separates the armature from the permanent magnet. 
     So long as the full-output pressure of the brake booster, i.e. the maximum pressure difference at the movable wall, is not attained, a defined actuating speed excess is needed to move the armature closer than the second distance to the permanent magnet and hence achieve the coupling of armature and permanent magnet. However, once the full-output pressure is attained, a further increase of the actuating force summoned up by the driver is, independently of the actuating speed, always combined with an approach of armature and permanent magnet. Thus, even if the actuating force is increased slowly, e.g. when a vehicle is stopped at traffic lights, after the full-output pressure is attained an inappropriate coupling of armature and permanent magnet may occur. The emergency braking aid is activated even though an emergency braking situation does not exist. 
     To deactivate the inappropriately activated emergency braking aid, the driver—just as in the case of appropriate activation after emergency braking—has to ease off the brake pedal to a relatively large extent before the brake booster drops back to its original performance characteristic and may once more be apportioned in the usual manner by the driver. 
     The driver is not accustomed to the departure from the usual performance characteristic which occurs upon activation of the emergency braking aid in non-emergency braking situations and he therefore perceives it to be a disadvantage. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide a brake booster with a mechanical emergency braking aid, in which activation of the emergency braking aid may be effected only in emergency braking situations. 
     Proceeding from a brake booster of the type described initially, said object is achieved according to the invention in that a coupling device is provided, which may couple the second, as yet uncoupled component of the magnetic module at least in actuating direction of the brake booster to the actuating piston, when increased reaction forces are introduced counter to the actuating direction of the brake booster into the coupling device. 
     In a non-emergency braking situation, i.e. in the case of e.g. a slow increase of the actuating force, both the armature and the permanent magnet are coupled at least in actuating direction to the actuating piston by the coupling device. As a result of the simultaneous coupling of both armature and permanent magnet to the actuating piston the mutual distance of the two components of the magnetic module cannot be reduced any further. Armature and permanent magnet are therefore prevented from moving so close to one another that they are less than the previously mentioned second distance apart and a coupling of armature and permanent magnet occurs. 
     According to the invention, the hydraulic reaction forces of the master brake cylinder acting counter to actuating direction upon the actuating piston are used as a criterion for the existence of an emergency braking situation. Given a comparatively slow increase of the actuating force summoned up by the driver, i.e. in a non-emergency braking situation, the reaction forces of the master brake cylinder are relatively high. In said case, as a response to the high reaction forces, so to say, the coupling device is activated by them and the magnetic module component not yet coupled to the actuating piston is coupled, at least in actuating direction, to the actuating piston. An inappropriate activation of the emergency braking aid is ruled out in this event. 
     In the case of a fast actuation of the brake system which is typical of emergency braking situations, on the other hand, the reaction forces of the master brake cylinder are initially still comparatively low. The coupling device consequently remains deactivated and the emergency braking aid may cut in in the usual manner. 
     According to a preferred embodiment, a mechanical coupling device is provided. The reaction forces of the master brake cylinder advantageously act counter to actuating direction upon the mechanical coupling device so that the second, as yet uncoupled component of the magnetic module is coupled at least in actuating direction to the actuating piston, e.g. when the reaction forces exceed a specific threshold. The coupling device is in said case preferably disposed workingly between the actuating piston and a component of the brake booster which introduces the reaction forces of the master brake cylinder into the brake booster. According to the principle “actio=reactio”, the coupling device is then acted upon not only by the actuating force acting in actuating direction summoned up by the driver but also by the reaction force of the master brake cylinder acting counter to the actuating direction. 
     The coupling of the magnetic module component, which is not yet coupled to the actuating piston, is preferentially effected by means of a clamping joint to the actuating piston. But different constructions of the coupling device are also possible. For example, the coupling device may comprise latch or catch elements to couple the still uncoupled magnetic module component to the actuating piston. 
     In case a clamping connection is provided for coupling the still uncoupled component of the magnetic module component to the actuating piston, the coupling device may comprise a clamping element which is deformable radially to the outside relative to a longitudinal axis of the control valve housing by a force acting in actuating direction, for example. The clamping element is preferably disposed, in actuating direction, downstream of the actuating piston and connected workingly to the latter. At least some of the force needed to deform the clamping element may therefore be summoned up by the actuating piston. 
     According to a preferred embodiment, the deformable regions of the clamping element take the form of fingers, which extend parallel to the longitudinal axis of the control valve housing and all of which concentrically surround said longitudinal axis. The fingers in turn may be connected at their ends facing the vacuum chamber to a common carrier part, which is disposed downstream of the actuating piston. 
     A component disposed e.g. between the actuating piston and the clamping element and provided with sloping surfaces may be used to deform the clamping element. Instead of providing a separate component having sloping surfaces, the sloping surfaces may also be disposed on the actuating piston radially at the outside. 
     The coupling device is actuated through cooperation of the sloping surfaces (e.g. conical surfaces) with the deformable regions of the clamping element e.g. in the form of fingers. For said purpose, a reduction of the distance between the component provided with sloping surfaces and the clamping element is normally required. The sloping surfaces are preferably inclined relative to a longitudinal axis of the control valve housing. 
     A resilient element is preferably disposed between the clamping element and the component provided with sloping surfaces in such a way as to counteract a convergence of both components. The control valve housing may have a step acting counter to the actuating direction as a stop for the component provided with sloping surfaces. A sensing disk, which cooperates with a reaction disk disposed between a reaction piston of the vacuum brake booster and the control valve housing, is preferably disposed in actuating direction downstream of the clamping element. 
     The clamping means may further comprise an extension, which is coupled in actuating direction to the as yet uncoupled component of the magnetic module, cooperates with the clamping element and radially concentrically surrounds the outside of the clamping element at least in sections. In the case of radially outward deformation of the clamping element, the deforming regions of the clamping element come into abutment with the portion of the extension radially surrounding the outside of the clamping element. The coupling of said components to the actuating piston is effected as a result of the clamping element coming into abutment with the extension of the as yet uncoupled component of the magnetic module. The extension is preferably preloaded in actuating direction relative to the component of the magnetic module to be coupled, in order to allow a certain relative motion between said component and the extension. 
     The as yet uncoupled component of the magnetic module is preferably connected by a screw connection in an axially adjustable manner to a holding device. The holding device in turn may be preloaded by means of a resilient element counter to the actuating direction towards a step of the control valve housing. The step of the control valve housing prevents a movement of the component to be coupled of the magnetic module counter to the actuating direction. The resilient element, on the other hand, allows a certain displacement of the component to be coupled of the magnetic module in actuating direction. 
     The holding device for the uncoupled component of the magnetic module is preferably connected rigidly counter to the actuating direction and resiliently in actuating direction to the extension cooperating with the clamping element. To said end, the holding device may, for example, be provided radially at the inside with a groove. The extension may have a collar projecting into the groove; a resilient element disposed between a side surface of the groove facing the vacuum chamber and an opposing face end of the collar advantageously preloads the holding device and the extension towards one another. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a longitudinal section through the control valve of a first embodiment of a vacuum brake booster according to the invention in a standby position; 
     FIG. 2 the control valve of FIG. 1 with the emergency braking aid activated; 
     FIG. 3 the control valve of FIG. 1 in release position after deactivation of the emergency braking aid; 
     FIG. 4 the control valve of FIG. 1 on attaining the full-output pressure in a normal braking position; 
     FIG. 5 the control valve of FIG. 1 after the full-output point is exceeded, with the coupling device activated; 
     FIG. 6 the control valve according to FIG. 1 in a partial braking position, with the coupling device activated. 
     FIG. 7 a longitudinal section through the control valve of a second embodiment of a vacuum brake booster according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a first embodiment of a vacuum brake booster  10  according to the invention having a housing  12  in which a movable wall  14  separates a vacuum chamber  16  in a pressure-proof manner from a working chamber  18 . 
     During operation of the brake booster  10  the vacuum chamber  16  is constantly connected to a vacuum source, e.g. to the intake tract of an internal combustion engine or to a vacuum pump. A control valve  20  having a housing  22  may optionally connect the working chamber  18  to the vacuum chamber  16 , in order to evacuate the working chamber  18 , or the evacuated working chamber  18  to the ambient atmosphere, i.e. the ambient pressure, in order to generate a pressure differential at the movable wall  14 . The movable wall  14  is coupled workingly to the control valve housing  22 . 
     The brake booster  10  is operated by means of a rod-shaped input element  26 , which is preloaded into its starting position by a spring  24 , projects along an axis A into the control valve housing  22  and is fixed by its one, spherically designed end in an actuating piston  28 . 
     The end of the actuating piston  28  opposite the spherically designed end of the input element  26  is in contact via a coupling device  66  with a sensing disk  30  that transmits an actuating force, which is introduced via the input element  26  into the brake booster  10 , via a reaction disk  32  made of elastomeric material, to a reaction piston  34  of a master cylinder, disposed functionally downstream of the brake booster  10  and not shown here, of a vehicle hydraulic brake system. The reaction disk  34  brings together the actuating force summoned up by the driver and the boosting force. 
     The actuating piston  28  penetrates a circular armature  36 A arranged concentrically with it and an annular permanent magnet  38  which is likewise arranged concentrically with the actuating piston  28  and accommodated in a pot-shaped component  40 . 
     The pot-shaped component  40  accommodating the permanent magnet  38  is connected, via a central internal thread, in an axially adjustable manner to a holding device in the form of a hollow-cylindrical retaining ring  42  having a complementary external thread. The retaining ring  42  has a flange-shaped portion of enlarged diameter  42 A, which rests with a part of its surface facing the input element  26  against a step  22 A of the control valve housing  22 . 
     The control valve housing  22  is closed off at a side opposite the input element  26  by a control valve housing insert  44 . An annular resilient element  43  is disposed between a surface—facing the vacuum chamber  16 —of the flange-shaped portion of enlarged diameter  42 A of the retaining ring  42  and an end face—facing the input element  26 —of the control valve housing insert  44 . The resilient element  43 , which is made of an elastomeric material, allows a defined displacement in actuating direction of the retaining ring  42  and/or of the pot-shaped component  40  that is coupled to the retaining ring  42  and accommodates the permanent magnet  38 . 
     The inside diameter of the retaining ring  42  has an annular groove  42 B at its end facing the vacuum chamber  16 . A collar  70 A of an extension  70 , which extends from the retaining ring  42  in the direction of the vacuum chamber  16 , projects into said annular groove  42 B. Between a side surface—facing the vacuum chamber  16 —of the groove  42 B and an end face—facing the input element  26 —of the collar  70 A of the extension  70  a spring element  72  is disposed in such a way that the collar  70 A is preloaded in actuating direction. The extension  70  is therefore connected to the holding device  42  rigidly in actuating direction and resiliently counter to actuating direction. 
     The extension  70  comprises a series of portions  70 B in the form of cylinder envelope segments that extend into a central recess  44 A of the control valve housing insert  44 . In FIG. 1 only a single portion  70 B is illustrated. Said portions  70 B of the extension  70  cooperate with fingers  74 A of a clamping element  74 . FIG. 1 shows two such fingers  74 A, which extend parallel to the housing longitudinal axis A and concentrically surround said axis. The fingers  74 A are connected at their ends facing the vacuum chamber  16  to a common carrier part  74 B. The clamping element  74  therefore has a substantially pot-shaped configuration with the side walls of the pot have recesses in the region between the individual fingers  74 A, however. 
     The sensing disk  30  is disposed in actuating direction downstream of the clamping element  74 . Situated in actuating direction upstream of the clamping element  74  is a conical component  76 , which in actuating direction cooperates, via its conical portion  76 B, with the fingers  74 A and, via its end face facing the vacuum chamber  16 , via a resilient element  78  with the carrier part  74 B. 
     When the conical component  76  penetrates the interior of the pot-shaped clamping element  74 , the fingers  74 A of the clamping element  74  are bent radially outwards by the conical portion  76 B of the conical component  76 , the cross section of which decreases in actuating direction. At the same time, the resilient element  78  disposed in actuating direction upstream of the conical component  76  is correspondingly deformed. 
     The armature  36 A cooperating with the permanent magnet  38  is designed as an integral part of an axially displaceable valve sleeve  36 B guided in the control valve housing. The armature  36 A has a collar facing radially inwards and cooperating with a return spring  58 . The preloaded return spring  58  cooperates with the control valve housing insert  44  firmly connected to the control valve housing  22  and preloads the armature  36 A and the valve sleeve  36 B counter to the actuating direction of the input element  26  towards a shoulder  28 A of the actuating piston  28 . The armature  36 A is consequently coupled rigidly to the actuating piston  28  in actuating direction. 
     A first annular valve seat  54  of the control valve  20  is formed on the free end of the valve sleeve  36 B. The first valve seat  54  cooperates with valve sealing element  56  which is also annular and preloaded towards it by a spring  60  and is adapted to control the connection between the ambient atmosphere and the working chamber  18  of the brake booster  10 . Formed radially outside of and concentrically with the first valve seat  54  and at the inside of the control valve housing  22  is a second annular valve seat  62  of the control valve  20 , which valve seat likewise cooperates with the valve sealing element  56  and is adapted to control the connection between the vacuum chamber  16  and the working chamber  18  of the brake booster  10 . 
     Upon operation of the brake booster, the first valve seat  54  of the control valve  20  in dependence upon the displacement of the input element  26  relative to the control valve housing  22  is opened to a greater or lesser extent, thereby generating a corresponding boosting force of the brake booster  10 , which results from the pressure differential actually effective at the movable wall  14 . 
     When actuation of the input element  26  is effected quickly and with a relatively large stroke, in the manner typical of an emergency braking operation, the armature  36 A moves so close to the permanent magnet  38  that the force of the compression spring  58  is no longer sufficient to keep the armature  36 A away from the permanent magnet  38 . The force exerted by the permanent magnet  38  upon the armature  36 A is then preponderant. The armature then detaches itself from the input element  26  and comes into abutment with the permanent magnet  38 . The valve sleeve  36 B integrally constructed with the armature  36 A also participates in said movement of the armature  36 A so that the valve seat  54 , as a result of the coupling of armature  36 A and permanent magnet  38 , is held open to the maximum extent. The emergency braking aid is therefore activated and the maximum possible boosting force of the brake booster  10  is made available. Said operating position of the brake booster  10  with activated emergency braking aid is shown in FIG.  2 . 
     Since in an emergency braking situation the reaction forces of the master brake cylinder at the start of braking are still relatively low, the actuating force summoned up by the driver and introduced via the actuating piston  28  into the conical component  76  is transmitted without much resistance via the resilient element  78  to the clamping element  74  and from the clamping element  74  to the sensing disk  30 . The reaction forces are consequently insufficient to set up a resistance to the actuating forces summoned up by the driver which would lead to deformation of the clamping element  74  and coupling of the permanent magnet  38  in actuating direction to the actuating piston  28 . The coupling device remains deactivated. 
     In order to release the brake from the operating position shown in FIG.  2  and deactivate the emergency braking aid, the actuating force summoned up by the driver has to be reduced. During a reduction of the actuating force summoned up by the driver, the return stroke of the input element  26  is assisted by the return springs  24  and  58 . Because of the return stroke of the input element  26 , a locking bar  64  rigidly coupled to the actuating piston  28  and extending through a recess  36 C of the valve sleeve  36 B comes into contact with a face end  36 D of the valve sleeve  36 B. Upon a further increase of the return stroke, therefore, the return forces of the return springs  24  and  58  are transmitted via the locking bar  64  to the valve sleeve  36 B and to the armature  36 A rigidly coupled to the valve sleeve  36 B, with the result that the armature  36 A finally detaches from the permanent magnet  38 . The emergency braking aid is deactivated. 
     Said operating position of the brake booster with the brake released upon completion of the return stroke of the input element  26  is shown in FIG.  3 . 
     As a result of the return stroke of the actuating piston  28  the valve sleeve  36 B rigidly coupled to the armature  36 A is displaced by the return spring  58  counter to actuating direction and preloaded towards a step  22 B of the control valve housing  22 . The first valve seat  54  subsequently comes into contact with the valve sealing element  56  and displaces the latter likewise counter to actuating direction. The second valve seat  62  is therefore opened and a connection established between the working chamber  18  and the vacuum chamber  16 . The working chamber  18  is evacuated and the initial state shown in FIG. 1 arises. 
     In the operating position of the brake booster  10  shown in FIG. 3 the conical component  76  has already detached itself from the actuating piston  28  because further displacement of the conical component  76  counter to the actuating direction is prevented by a stop  44 B of the control valve housing insert  44 . 
     When, from the standby position of the brake booster  10  shown in FIG. 1, the actuating force summoned up by the driver is increased slowly, i.e. an emergency braking situation does not exist, progressively higher reaction forces of the master brake cylinder counteract actuation and finally lead to activation of the coupling device. 
     FIG. 4 shows the brake booster  10  of FIG. 1 upon attaining the full-output pressure in a normal braking position with the coupling device activated. The armature  36 A has already moved so close to the permanent magnet  38  that further displacement of the armature  36 A in actuating direction would lead to an undesired coupling of armature  36 A and permanent magnet  38 . In such an actuating position of the brake booster  10 , the coupling device is activated. 
     Because in said state the reaction forces of the master brake cylinder are high, the resilient element  72  disposed between the clamping element  74  and the conical component  76  is deformed upon a further displacement of the actuating piston  28  in actuating direction. The conical component  76  then penetrates even further into the clamping element  74  so that the fingers  74 A of the clamping element  74  are bent radially outwards by the conical portion  76 A of the conical component  76 . The fingers  74 A then become jammed with the inside diameters of the portions  70 B of the extension  70  extending into the recess  44 A of the control valve housing insert  44 . The permanent magnet  38  is consequently coupled via the pot-shaped component  40 , the holding device  42 , the extension  70 , the clamping element  74  and the conical component  76  in actuating direction rigidly to the actuating piston  28 . Because the armature  36 A is also coupled via a stop  28 A of the actuating piston  28  rigidly to the latter, the distance between the armature  36 A and the permanent magnet  38  cannot be reduced further. 
     If, as shown in FIG. 5, the actuating force summoned up by the driver is further increased and the full-output pressure exceeded, the actuating piston  28  does in fact continue to move in actuating direction but the distance between armature  36 A and permanent magnet  38  cannot be reduced further because the permanent magnet  38  is also rigidly coupled to the actuating piston  28 . Instead, the flange-shaped portion of enlarged diameter  42 A of the holding device  42  moves out of contact with the stop  22 A of the control valve housing  22  and the resilient element  43  is deformed. An inappropriate activation of the emergency braking aid is ruled out. 
     When, from the operating position shown in FIG. 5, the brake is released by reducing the actuating force summoned up by the driver, the then relieved resilient element  78  pushes the conical component  76  and the clamping element  74  apart. The clamping action between clamping element  74  and extension  70  is cancelled and the permanent magnet  38  is uncoupled from the actuating piston  28 . 
     In the so-called partial braking position of the brake booster  10  shown in FIG. 6, the opened valve seat  54  is closed by the reaction forces of the master brake cylinder while the actuating force summoned up by the driver remains the same. The position of equilibrium arises. 
     If in the partial braking position the coupling device was already activated, i.e. the fingers  74 A are already clamping the portion  70 B of the extension  70 , given an integral construction of extension  70  and holding device  42 , the reaction forces of the master brake cylinder needed to close the valve seat  54  would be introduced entirely into the control valve housing  22  via the flange-shaped portion of enlarged diameter  42 A of the holding device  42 . In that case, the valve seat  54  could not be closed by the reaction forces of the master brake cylinder and a position of equilibrium would be unattainable. 
     In order nevertheless to enable attainment of the position of equilibrium in the partial braking position, holding device  42  and extension  70  are designed as separate and mutually displaceable components. As already mentioned above, the extension  70  may be displaced counter to the actuating direction relative to the holding device  42  through deformation of the resilient element  43  disposed in the groove  42 B of the holding device  42 . The reaction forces of the master brake cylinder may therefore at least partially retroact upon the actuating piston  28  even when the coupling device is activated. Via the spring  58  the reaction forces are then transmitted from the actuating piston  28  to the valve sleeve  36 B, with the result that the valve seat  54  is closed. 
     FIG. 7 shows a second embodiment of a vacuum brake booster  10  according to the invention, i.e. actually the control valve  20  of this vacuum brake booster  10 . As regards the function, the vacuum brake booster  10  according to the second embodiment is essentially identical with the vacuum brake booster described with reference to FIGS. 1 to  6 . In the vacuum brake booster according to the second embodiment, however, the conical component  76  of FIG. 1 has been omitted. An advantage of this omission is the considerably reduced axial extension of the coupling device  66 . The result of this reduced axial extension is that no assembly space is required within the control valve housing larger than is the case with a control valve housing of a conventional type, i.e. without emergency braking aid. Moreover, the coupling device  66  illustrated in FIG. 7 can be manufactured cheaper due to the fact that the conical component is omitted. 
     In the vacuum brake booster  10  illustrated in FIG. 7 the function of the conical component is assumed by the actuating piston  28 . To this end, the actuating piston  28  is provided at its end facing the sensing disk  30  with sloping surfaces embodied by a conical section  28   a . The actuating piston  28  cooperates with the clamping fingers  74   a  of the clamping element by means of said sloping surfaces  28   a.    
     The clamping element  74  includes a hollow-cylindrical carrier member  74   b  which surrounds a portion of reduced diameter  28   b  of the actuating piston  28  radially at the outside and is guided on the portion of reduced diameter  28  so as to be displaceable in longitudinal direction. In the operating position of the vacuum brake booster  10  illustrated in FIG. 7 the hollow-cylindrical carrier member  74   b  rests with its end against the surface of the sensing disk  30  facing the input element  26 . The clamping fingers  74   a  are arranged in the area of its other end. 
     When the actuating piston  28  is slowly displaced in the actuating direction, the clamping fingers  74   a  are urged radially outwardly by the conical portion  28   a  of the actuating piston  28 , owing to the high reaction forces introduced by the sensing disk  30  into the carrier member  74   b . At the same time, the clamping fingers  74  make a clamping connection with the coupling cylinder  70  surrounding the clamping fingers  74   a  radially at the outside. As a result of this clamping connection, the actuating piston  28  is coupled in the actuating direction via its conical section  28   a , the clamping fingers  74   a  and the coupling cylinder  70  to the permanent magnet  40 . The axial distance between the armature  36   a  and the permanent magnet  40  cannot be reduced any further owing to this coupling of the actuating piston  28  to the permanent magnet  40 . 
     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.