Abstract:
A vacuum brake booster comprising a vacuum chamber ( 16 ) and a working chamber ( 18 ), separated by a movable wall ( 14 ) coupled to a housing ( 22 ). A control valve ( 20 ) includes a first valve seat ( 40 ) formed on a displacement valve member ( 38 ) and cooperating with a first valve seat member ( 42 ), to control a supply of atmospheric pressure to the working chamber. A rear side of the valve member is constantly subjected to pressure in the working chamber. When the valve member is displaced more than a predetermined distance in the actuating direction relative to the housing, a front side of the valve member is subjected to pressure in the vacuum chamber, such that a differential pressure acts on the valve member, to hold the valve member in a displaced position until pressure equalization between the first and rear sides of the valve member is effected.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/EP99/08203 filed Oct. 28, 1999, which claims priority to German Patent Application No. 19850478.0 filed Nov. 2, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to vacuum brake boosters with a vacuum chamber and a working chamber, which is separated in a pressure-tight manner from the latter by a moveable wall, and a control valve, which comprises a housing coupled in a working manner to the moveable wall and a first valve seat, which is disposed in the housing and which, interacting with a first valve sealing member, can control the supply of at least atmospheric pressure to the working chamber in accordance with the displacement of an input member, which is coupled to the first valve seat, of the brake booster in order to obtain a pressure difference at the moveable wall. Vacuum brake boosters of this kind have been known for a long time and are used in a vast number of cases to boost the actuating forces of a hydraulic vehicle brake system and thereby maintain them at a low level which is acceptable to the driver of a vehicle. 
     So-called brake assistants are also known. This term is usually understood to mean a system which can make increased braking power available to a driver with substantially the same actuating force in an emergency braking situation. Systems of this kind were developed because tests revealed that, although when emergency braking most vehicle users depress the brake pedal quickly, they do not do so forcefully enough to achieve the maximum possible braking power. The stopping distance of the vehicle is therefore longer than necessary. Systems of this kind which are already in production use a brake booster which can be electromagnetically actuated together with a device which can determine the actuating speed of the brake pedal. If this device detects an actuating speed lying above a predetermined threshold value, it is assumed that an emergency braking situation exists and the brake booster is fully driven by means of the electromagnetic actuating device, i.e. it provides its maximum boost power. A brake booster with an electromagnetically actuated brake assistant of this kind is known from DE 44 05 092 C1. 
     However brake boosters with an electromagnetic actuation facility are too expensive for motor vehicles of the lower and middle price category. Solutions which achieve a brake assistant function at less expense are therefore required. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide a vacuum brake booster of the above-mentioned type with a brake assistant function without having to resort to a control valve which can be electromagnetically actuated. Unintentional initiation of the brake assistant function is at the same time to be prevented as far as possible. 
     Valve member which is coupled to the input member in the actuating direction of the brake booster. According to the invention the displaceable valve member is always subjected to the pressure prevailing in the working chamber at its rear side, which faces the input member. If, however, the displaceable valve member is displaced relative to the control valve housing in the actuating direction by more than a prefixed distance, then the displaceable valve member is subjected over at least a part of its front side, which lies opposite the rear side, to the pressure prevailing in the vacuum chamber, and the pressure difference then acting on the displaceable valve member holds the valve member in the position which is reached until pressure equalisation between the front side and the rear side of the displaceable valve member takes place. 
     This means that, according to the invention, the pressure difference prevailing at the moveable wall at the time is used to exert a force acting on the displaceable valve member in the actuating direction in order to hold the first valve seat formed at the displaceable valve member completely open in certain situations in which the actuating force exerted by the driver is not sufficient for this, so that the brake booster builds up the greatest possible pressure difference between its vacuum chamber and its working chamber, i.e. provides its maximum boost power. 
     After exceeding the above-mentioned, prefixed relative displacement, the displaceable valve member of the vacuum brake booster according to the invention is therefore virtually “sucked” into a position in which the first valve seat is completely open. However this necessary relative displacement is only achieved when the actuating speed of the input member exceeds a defined value. In the vacuum brake booster according to the invention the brake assistant function is therefore activated solely by the skilful utilisation of pressure differences present within the brake booster. No magnet is required to initiate the brake assistant function. 
     In the vacuum brake booster according to the invention the brake assistant function is disengaged by means of a reduction in the pressure difference acting on the displaceable valve member. The reduction in this pressure difference is initiated by a return movement of the input member which exceeds a certain measure. 
     In preferred embodiments of the vacuum brake booster according to the invention the displaceable valve member is resiliently biased opposite to the actuating direction of the brake booster. This resilient biasing advantageously ensures that the displaceable valve member is coupled to the input member in the actuating direction of the brake booster and on the other hand enables the displaceable valve member to be uncoupled from the input member when the brake assistant function is activated. In embodiments of this kind the force acting on the displaceable valve member on account of the pressure difference must be greater than the opposing spring force acting on the valve member in order to initiate the brake assistant function. This requirement can easily be taken into account by appropriately dimensioning the surfaces of the valve member which are subjected to the pressure difference. 
     A second valve seat is preferably formed at the displaceable valve member in the vacuum brake booster according to the invention, which seat co-operates with a second valve sealing member which in turn co-operates with a third valve seat which establishes a connection between the working chamber and the vacuum chamber in the open state. After the valve member has exceeded the prefixed displacement relative to the control valve housing, the second valve seat is closed and the third valve seat open, so that the pressure prevailing in the vacuum chamber can now act on the displaceable valve member. 
     In particularly preferred embodiments the second valve seat is formed at the front side of the displaceable valve member. An annular cavity is defined between the second valve seat and the third valve seat, both of which are preferably annular, the end boundary of which cavity is formed on one side by the displaceable valve member. The third valve seat is in particular disposed concentrically with and radially outside of the second valve seat. The annular cavity is connected to the vacuum chamber when the second valve seat is closed and the third valve seat open, while it is connected to the working chamber when the second valve seat is open and the third valve seat closed. The surface of the displaceable valve member which is located radially between the second valve seat and the third valve seat can thus be subjected either to the pressure in the vacuum chamber or the pressure in the working chamber. 
     According to one embodiment of the vacuum brake booster according to the invention, in order to be able to reduce a pressure difference acting on the displaceable valve member, the latter comprises a duct which connects the front side of the valve member to its rear side. This duct, which is normally closed, may be opened by displacing the input member or a component coupled to the latter, this displacement taking place relative to the displaceable valve member opposite to the actuating direction of the brake booster and exceeding a prefixed measure. The prefixed measure ensures that the brake assistant function is not unintentionally disconnected too soon. 
     According to a preferred configuration of the brake booster according to the invention, the duct connecting the front side to the rear side of the displaceable valve member can be closed by means of a ring seal comprising two axially spaced, circulating sealing lips. The ring seal is accommodated in a component which is guided on or in the displaceable valve member and can be displaced relative to the valve member. The axial spacing of the two circulating sealing lips in this case substantially determines the extent of the relative displacement between the component and the valve member which is necessary to open the duct and initiate pressure equalisation. 
     In preferred embodiments of the brake booster according to the invention the second valve sealing member is resiliently biased opposite to the actuating direction of the brake booster and can be axially displaced against this spring biasing. The extent of the axial displaceability of the second valve sealing member in this case represents the switching threshold which must be overridden in order to initiate the brake assistant function. In embodiments of this kind the force resulting from the pressure difference at the displaceable valve member must be greater than the sum of the spring forces which act in the opposite direction and which bias the second valve sealing member or displaceable valve member, respectively, in order to initiate the brake assistant function. 
     The displaceable valve member is preferably substantially sleeve-shaped in order to achieve a space-saving construction. The first valve seat is in this case formed at the end of the valve member which faces the input member, while the second valve seat is located at the opposite end of the valve member. A sleeve-shaped, displaceable valve member of this kind can be integrated into conventional control valve constructions without noticeably affecting the diameter or overall length thereof. 
     The input member is preferably also resiliently biased opposite to the actuating direction in all embodiments of the vacuum brake booster according to the invention. When the brake is released this resilient biasing returns the input member to the initial position. This resilient biasing of the input member is used to advantage in constructional terms during its return movement to the initial position to move back the above-mentioned component in which the ring seal is accommodated relative to the displaceable valve member in order thus to open the duct provided in the valve member and disconnect the brake assistant function. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal section through the region concerned of a vacuum brake booster according to the invention in a rest position, 
     FIG. 2 is a view according to FIG. 1 in an actuating position, in which a brake assistant function is activated, 
     FIG. 3 is a view according to FIG. 2 with reduced actuating force, and 
     FIG. 4 is a view according to FIG.  1  and FIG. 2 just before the brake assistant function is disconnected. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a vacuum brake booster  10  with a housing  12 , the interior space of which is divided by a moveable wall  14  into a vacuum chamber  16  and a working chamber  18 . 
     The vacuum chamber  16  is constantly connected to a vacuum source, for example to the intake tract of an internal combustion engine or to a vacuum pump, when the brake booster  10  is in operation. A control valve  20  with a housing  22  serves to establish a connection either between the vacuum chamber  16  and the working chamber  18 , in order to evacuate the working chamber  18 , or a connection between the evacuated working chamber  18  and the ambient atmosphere, i.e. the ambient pressure. The moveable wall  14  is coupled in a working manner to the control valve housing  22 . 
     The brake booster  10  is actuated by means of a rod-shaped input member  24 , which is resiliently biased into its initial position, projects along an axis A into the control valve housing  22  and is fastened by way of one, spherical end in a transmission piston  26 . 
     A locking bar  28  is rigidly connected to the transmission piston  26 , extends at a right angle to the axis A away from the transmission piston  26  and through a duct  30  formed in the control valve housing  22 . In the position which is shown in FIG. 1 the locking bar  28  lies against a stop  32  of the brake booster housing  12  which defines the rest position of the control valve  20 , i.e. the position of all the components of the control valve  20  relative to one another which they take up when the brake booster  10  is not actuated (LTF [lost travel free] position). The side walls of the duct  30  limit the mobility of the locking bar  28  along the axis A, i.e. the maximum travel of the locking bar  28  along the axis A is determined by the axial spacing of the side walls of the duct  30 . 
     The end of the transmission piston  26  lying opposite the spherical end of the input member  24  lies against a force delivery ram  34 , which transmits an actuating force introduced via the input member  24  into the brake booster  10  to a master cylinder, which is connected downstream of the brake booster, of a hydraulic vehicle brake system, only a part of an input piston  36  of which cylinder being reproduced in the figures. 
     The transmission piston  26  and a part of the force delivery ram  34  are surrounded by a displaceable valve member  38 , which is disposed concentrically with them and is substantially sleeve-shaped. A first annular valve seat  40  of the control valve  20  is formed at the free end of the valve member  38 , which is adjacent to the input member  24 . The first valve seat  40  co-operates with a first valve sealing member  42 , which is resiliently biased against it and is likewise annular, and can control the connection between the ambient atmosphere and the working chamber  18  of the brake booster  10 . 
     A further annular valve seat  44  of the control valve  20  is formed inside the control valve housing  22  radially outside of the first valve seat  40  and concentrically with the latter, which valve seat  44  is called fourth valve seat in the following and likewise co-operates with the first valve sealing member  42 . This fourth valve seat  44  can control the connection between the vacuum chamber  16  and the working chamber  18  of the brake booster  10  in order to evacuate the working chamber  18  again after the brake booster has been actuated. 
     A second annular valve seat  50  is formed at the free end of a sleeve  46 , which has a smaller diameter than the valve member  38  and is connected in one piece to the valve member  38  via an annular flange  48 , at the end of the displaceable valve member  38  which is remote from the input member  24 , which valve seat  50  co-operates with a second valve sealing member  52 , which is resiliently biased against it and is likewise annular. 
     A third valve seat  53 , again annular, is formed at the control valve housing  22  radially outside of the second valve seat  50 , which valve seat  53  is disposed concentrically with the latter and likewise co-operates with the second valve sealing member  52 . The function of both the second valve seat  50  and of the third valve seat  53  shall be explained in detail in the following. 
     The second valve sealing member  52  is guided in an axially displaceable manner by means of a cylindrical prolongation  54 , which extends from the member  52  in the direction of the input piston  36 , on a correspondingly constructed section  56  of an auxiliary housing  58  and is sealed off from the section  56  by a seal  60 . The auxiliary housing  58 , which is inserted in the inner end region of the control valve housing  22  and firmly connected to the latter, comprises a hollow cylindrical prolongation  62 , which projects into the control valve housing  22  and has a smaller outside diameter than the section  56 , which is disposed concentrically with it and is likewise of a hollow cylindrical form. The end of the force delivery ram  34  which faces the input piston  36  is accommodated in the hollow cylindrical prolongation  62 . An annular valve seat  50 ′ is formed at the free end of the hollow cylindrical prolongation  62 , which seat is of the same diameter as the second valve seat  50 , with which it forms a functional unit in this embodiment. The valve seat  50 ′ co-operates with the second valve sealing member  52  on the side of the latter which faces away from the second valve seat  50 . 
     A compression spring  64  is disposed between the hollow cylindrical prolongation  62  and the section  56 , this spring being supported on one side at the bottom of the auxiliary housing  58  and on the other, opposite side at the second valve sealing member  52 . The compression spring  64  biases the second valve sealing member  52  opposite to the actuating direction of the brake booster  10  and in the direction of the second valve seat  50  and the third valve seat  53 . 
     A further compression spring  66 , which concentrically surrounds the force delivery ram  34  and is supported by way of one end at the auxiliary housing  58 , presses with its other end against a circulating collar  67 , which is directed radially inwards, of the sleeve  46  of the displaceable valve member  38 , so that the valve member  38  is biased opposite to the actuating direction of the brake booster  10  and is normally held in contact with an annular collar  68  formed on the force delivery ram  34 . 
     As represented in FIG. 1, the locking bar  28  projects through a recess  70  in the displaceable valve member  38 . The locking bar  28  is located in the recess  70  with a clearance along the axis A which is smaller than the maximum possible travel of the locking bar  28  in the duct  30 . 
     The function of the represented brake booster  10  is now explained in detail on the basis of FIGS. 1 to  4 . Actuation of the brake booster  10  displaces the input member  24  into the brake booster  10  or into its control valve  20 , i.e. to the left in the figures. This displacement of the input member  24  is transmitted to the transmission piston  26  and from the latter to the force delivery ram  34 . The force delivery ram  34  in turn transmits this displacement to the displaceable valve member  38  by means of the annular collar  68  which is formed on it, so that the first valve seat  40  is lifted off the first valve sealing member  42 , whereby ambient air can flow through a duct  72  surrounding the input member  24 , past the open first valve seat  40  and further through the duct  30  into the working chamber  18  of the brake booster  10 . A pressure difference is consequently produced at the moveable wall  14 , and the resulting force is transmitted from the moveable wall  14  to the control valve housing  22 , which delivers the force via the input piston  36  to the above-mentioned master cylinder. 
     Because the displaceable valve member  38  is coupled as described to the input member  24  in the actuating direction of the brake booster, the first valve seat  40  of the control valve  20  is opened to a greater or lesser degree in dependence upon the displacement of the input member  24  relative to the control valve housing  22 . A corresponding assisting force of the brake booster  10  results from the pressure difference acting at the moveable wall  14  at the time. 
     Where conventional service braking, which here is called normal braking, is concerned, the input member  24  and therefore also the displaceable valve member  38  are only displaced over a relatively short distance in the actuating direction. As a result, although the second valve seat  50  is applied to the second valve sealing member  52  during normal braking of this kind, the valve seat  50 ′ co-operating with it is not. There is no appreciable axial displacement of the second valve sealing member  52 , so that the third valve seat  53  remains closed. When normal braking takes place the pressure prevailing in the working chamber  18  is therefore the same as that both at the rear side and at the front side of the annular flange  48  of the valve member  38  via an opening  74  (see FIG. 2) provided in the transmission piston  26 , also via a duct  76  (see FIG. 2) formed at the annular collar  68  and via the valve seat  50 ′, which is then open, and pressure equalising openings  78 , which are provided in the second valve sealing member  52 . Therefore no pressure difference is operative at the displaceable valve member  38  and, in particular, at its annular flange  48  during normal braking. 
     If an actuating force applied to the input member  26  during normal braking is not increased, the first valve sealing member  42  again comes into contact with the first valve seat  40  on account of the displacement of the control valve housing  22 , which is brought about by the pressure difference present at the moveable wall  14 , so that the air supply into the working chamber  18  is interrupted (position of equilibrium, both valve seats  40  and  44  closed). 
     If, however, the input member  24  is actuated rapidly and so that it travels relatively far, as is typical when panic braking (emergency braking), the displaceable valve member  38  is displaced relative to the control valve housing  22  in the actuating direction to an extent such that both the axial spacing between the second valve seat  50  and the second valve sealing member  52  and the axial spacing between the valve seat  50 ′ and the second valve sealing member  52  are covered virtually instantaneously. Considering the process in greater detail, the second valve seat  50  is firstly applied in sealing fashion to the second valve sealing member  52  and then displaces the latter against the force of the compression spring  64  in the actuating direction, so that the valve seat  50 ′ also comes into sealing contact with the second valve sealing member  52 . 
     The displacement of the second valve sealing member  52  opens the third valve seat  53 , so that the vacuum in the vacuum chamber  16  can enter an annular cavity B defined between the closed valve seats  50 ,  50 ′ and the third valve seat  53 . After the valve seat  53  has opened, the pressure in the annular cavity B reaches that of the vacuum chamber relatively quickly and then acts on the front side of the annular flange  48  of the valve member  38 . 
     However the pressure prevailing in the working chamber  18  acts as before on the rear side of the annular flange  48 . The pressure difference therefore applied to the annular flange  48  results in a force which acts on the displaceable valve member  38  in the actuating direction and which, through appropriately dimensioning the surface of the annular flange  48 , is greater than the opposing force of the two compression springs  64  and  66 . This ensures that the displaceable valve member  38  is held in the position reproduced in FIG. 2, i.e. the first valve seat  40  remains open (brake assistant function, see FIG.  2 ), even if the input member  24  is displaced slightly opposite to the actuating direction in the further course of the emergency braking action due to the high reactive forces then occurring (see FIG.  3 ). Because of the vacuum in the annular cavity B in this operating state, the displaceable valve member  38  remains “stuck by suction”, as it were, to the second valve sealing member  52 , even when the annular collar  68  comes away from the collar  67  of the sleeve  46  (see FIG.  3 ). 
     Pressure equalisation relative to the annular flange  48  must take place in order to disconnect the brake assistant function. A radial bore  80  is provided in the displaceable valve member  38  for this purpose, which bore is, an the one hand, constantly connected to the annular cavity B and can, on the other hand, be connected to the rear side of the annular flange  48 . A ring seal  82 , which is accommodated in the transmission piston  26  and comprises two axially spaced, circulating sealing lips  84 , controls the connection to the interior space of the displaceable valve member  38  and thus to the rear side of the annular flange  48 . As long as the radial bore  80  is located axially between the two sealing lips  84 , the connection between the radial bore  80  and the interior space of the displaceable valve member  38  is interrupted. Only after the transmission piston  26  has been moved back relative to the valve member  30  to an extent such that the sealing lip  84  on the left-hand side in the figures has moved the radial bore  80  over opposite to the actuating direction of the brake booster is there a fluid connection between the interior space of the displaceable valve member  38  and the annular cavity B, so that the above-mentioned pressure equalisation can take place. 
     Once the pressure equalisation has been carried out, the force in the actuating direction resulting from the pressure difference at the annular flange  48  ceases, so that the compression spring  64  moves the second valve sealing member  52  into contact with the third valve seat  53 , and the compression spring  66  presses the displaceable valve member  38  into contact with the annular collar  68  of the force delivery ram  34 . The brake assistant function is thus disconnected and the valve seats  50  and  50 ′ open again. 
     If the input member  24  moves back very quickly, the brake assistant function can also be disconnected in a second way. For a rapid return movement of the input member  24  causes the locking bar  28  to strike the edge of the recess  70  in the valve member  38  which is on the right-hand side in FIG. 3 (see FIG.  4 ), so that it is not just the force of the two compression springs  64  and  66  which acts on the displaceable valve member  38 , but also the force of a compression spring  86  which biases the input member  24  opposite to the actuating direction. The combined force of these three compression springs  64 ,  66  and  68  is greater than the force resulting from the pressure difference at the annular flange  48  and acting in the actuating direction, so that the displaceable valve member  38  is forced away from the second valve sealing member  52  and the brake assistant function is disconnected, even if the pressure equalisation at the annular flange  48  has not yet taken place or not yet been completed. 
     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.