Abstract:
A brake actuating unit for motor vehicles includes a brake master cylinder, a booster stage which is connected in front of the brake master cylinder, and a drive unit for driving the booster stage. In order to make comfortable braking operations possible, there is provision for the transmission both of the actuating force and of the boosting force to the primary piston of the brake master cylinder to take place by a force transmission module which has at least one elastic element and is arranged in the primary piston.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the U.S. national phase application of PCT International Application No. PCT/EP2008/064663, filed Oct. 29, 2008, which claims priority to German Patent Application No. 10 2007 057 041.6, filed Nov. 27, 2007 and German Patent Application No. 10 2008 038 320.1, filed Aug. 19, 2008, the contents of such applications being incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a brake actuating unit, comprising a brake master cylinder which can be actuated by means of a brake pedal and to which wheel brakes of a motor vehicle are connected, a booster stage which is connected in front of the brake master cylinder, and a drive unit for driving the booster stage, which drive unit is formed by an electric motor, a piston rod for transmitting an actuating force being provided between the brake pedal and a primary piston of the brake master cylinder, and the booster stage being arranged in a booster housing and having a first step-down gear mechanism which is formed by a ball screw drive, the threaded nut of which is driven by the electric motor via a second step-down gear mechanism and the threaded spindle of which transmits the boosting force which is generated by the booster stage to the primary piston. 
       BACKGROUND OF THE INVENTION 
       [0003]    A brake actuating unit of this type is known, for example, from DE 199 39 950 A1, which is incorporated by reference. The brake master cylinder and the booster stage with its electromechanical drive form a pressure generator which generates a brake pressure which corresponds to a sum of an actuating force which is produced by the vehicle driver and a booster force which is generated by the booster stage. The special features of the previously known brake actuating unit are, inter alia, an input sensor for detecting the actuating force which is produced by a driver and a control unit which actuates the electromechanical drive of the booster stage as a function of signals which are supplied by the input sensor. 
         [0004]    In the previously known brake actuating unit, the type of the transmission of the forces between a push rod which is actuated by the driver and an output element of the booster stage is considered to be disadvantageous, which transmission takes place by direct mechanical contact of the said components. A considerable impairment in the driving comfort results, in particular in the case of relatively heavy braking operations, from the mentioned mechanical stop of the piston rod and the threaded spindle on the primary piston of the brake master cylinder. 
       SUMMARY OF THE INVENTION 
       [0005]    It is therefore an object of the present invention to propose a brake actuating unit of the generic type mentioned in the introduction, by way of which a high degree of comfort can be achieved, in particular in the case of great actuating forces and associated braking operations. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Further features and advantages of the invention result from the following description of two preferred exemplary embodiments in conjunction with the appended drawing. Those parts of the two design variants which correspond to one another or are identical are denoted by the same reference numerals. In the drawing: 
           [0007]      FIG. 1  shows a longitudinal section of a first embodiment of the subject matter of the invention, 
           [0008]      FIG. 2  shows a simplified longitudinal section of a second embodiment of the subject matter of the invention in an initial position, and 
           [0009]      FIG. 3  shows the embodiment according to  FIG. 2  in a partial braking position. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0010]    The brake actuating unit which is shown in  FIG. 1  in an axial section comprises substantially a brake master cylinder  1 , preferably a tandem master cylinder, a booster stage  2  which is connected in action terms in front of the brake master cylinder  1 , and a drive unit  3  which serves to drive the booster stage  2  and is formed by an electric motor  23 . A brake pedal (not shown) is provided for actuating the brake actuating unit according to aspects of the invention, to which brake pedal a piston rod  4  is coupled which is in force-transmitting connection with the first piston or primary piston  11  of the brake master cylinder  1 . A second or secondary piston of the brake master cylinder is provided with the reference numeral  26 , the two master cylinder pistons  11 ,  26  delimiting pressure spaces which are not denoted in greater detail and to which the wheel brakes of a motor vehicle are connected. The booster stage  2  is driven by means of a first step-down gear mechanism  6  which is configured as a ball screw drive, the threaded nut  16  of which is driven by the electric motor  23  with a second step-down gear mechanism  7  connected in between. The torque which is generated by the electric motor  23  is transmitted to the threaded nut  16  by means of a drive belt which bears the reference numeral  18  and which together with two pulley wheels  20 ,  21  forms a second step-down gear mechanism  7 . Here, the first pulley wheel  20  is seated on an output shaft  24  of the electric motor  23 , whereas the pulley wheel  21  of greater diameter is arranged at the right-hand end (in the drawing) of the previously mentioned threaded nut  16 . Here, the first step-down gear mechanism  6  is preferably arranged in a booster housing  5 , in which a radial bearing  19  is provided, in which the previously mentioned threaded nut  16  is mounted. The output element of the booster stage  2  is formed by the threaded spindle  15  of the ball screw drive  6 . 
         [0011]    Moreover, as can be gathered from  FIG. 1 , the two forces which act during the actuation of the brake actuating unit according to aspects of the invention, namely an actuating or input force which acts on the brake pedal and therefore on the piston rod  4  and a boosting force which is applied by the threaded spindle  15 , are transmitted to the primary piston  11  by means of a force transmission module  9  which is received by an axial extension  38  of the primary piston  11 , which axial extension  38  extends into the interior of the booster housing  5 . The force transmission module  9  comprises substantially a cylindrical pressure sleeve  10 , an elastic element  8  which is mounted in the latter, and a two-part guide ring  12 ,  13 , in which the front end of the piston rod  4  is guided radially. For this purpose, the piston rod  4  has a radial guide shoulder or collar  17  in its region which lies within the guide ring  12 . The elastic element  8  which is configured, for example, as an elastomeric disc serves to add the actuating force which is transmitted by the piston rod  4  and the boosting force which is transmitted by the threaded spindle  15 , a force-transmitting connection existing between the end of the threaded spindle  15  and the guide ring  12 ,  13 . It is particularly advantageous here if that end of the pressure sleeve  10  which is supported on the primary piston  11  is designed in the shape of a spherical cap. 
         [0012]    In order to ensure satisfactory force transmission from the threaded spindle  15  to the guide ring  12 ,  13 , the threaded spindle  15  is provided with a cup-shaped or bell-shaped anti-rotation sleeve  14 , the edge of which of greater diameter forms a plurality of sliding blocks  27  which are guided displaceably in radial grooves  28  of a sliding guide sleeve  22  which is arranged coaxially with respect to the anti-rotation sleeve  14 . The previously mentioned components, that is to say the pulley wheel  21  of the second step-down gear mechanism, the ball screw drive  6 , the anti-rotation sleeve  14 , the sliding guide sleeve  22  and the radial bearing  19 , are previously configured here in such a way that they form a prefabricated assembly which can be handled with a single hand. Here, the sliding guide sleeve  22  carries a supporting ring  29 , on which a compression or restoring spring  25  is supported which is arranged coaxially with respect to the abovementioned assembly and serves to return the threaded spindle  15  into its initial or rest position. As a result of these measures, the pressure which is regulated in the brake master cylinder  1  during the actuation is dissipated completely. 
         [0013]    Finally, it can be gathered from the illustration according to  FIG. 1  that the piston rod  4  is guided or mounted radially at its end which faces the brake pedal (not shown) in a guide housing  30  which is attached to the previously mentioned booster housing  5 . The guide housing  30  is of multiple-piece configuration and has substantially an immovable cylindrical guide  31  and a movable guide sleeve  32  which is guided displaceably in the guide  31 . A cylindrical guide part  33  which is guided in an outer guide sleeve  34  is connected to the guide sleeve  32 . A bearing part  35  which is arranged in the guide sleeve  32  is connected fixedly to the latter, in which bearing part  35  the piston rod  4  is mounted by means of a spherical head  36 . A piston-rod restoring spring  37  which reaches radially around the guide  31  and the guide sleeve  32  and moves the piston rod  4  into its initial position is supported on the guide part  33 . 
         [0014]    A plurality of sensors which are not shown in  FIG. 1  are required for satisfactory operation of the above-described brake actuating unit according to aspects of the invention. These sensors and their method of operation are explained in the following description of the second embodiment of the present invention in conjunction with  FIGS. 2 and 3 . 
         [0015]    A brake pedal  52 , to which the piston rod  4  is connected by means of a force transmission rod  39 , serves to actuate the second embodiment shown in  FIGS. 2 and 3  of the subject matter of the invention. The actuating path “a” (see  FIG. 3 ) of the piston rod  4  is detected indirectly by a rotary angle sensor  40 , by way of which the (actuating) rotational movement of the brake pedal  38  is sensed. The travel of the threaded spindle  15  is determined by way of a second sensor or displacement sensor  41  which can be arranged in the booster housing  5 , the initial position (shown in  FIG. 1 ) of the threaded spindle  15  being monitored by an electric contact arrangement  42  which is configured as a break contact. The position of the rotor (not shown) of the electric motor  23  is detected by a rotor position sensor which is denoted in  FIGS. 2 and 3  by the reference numeral  43 . In order to monitor the hydraulic pressure which is regulated in the brake master cylinder  1  during the actuation of the brake pedal  52 , a pressure sensor  44  is provided which is connected to a first brake circuit  45  which is indicated merely diagrammatically and is connected to the first or primary pressure space of the brake master cylinder  1 , which first or primary pressure space is mentioned in conjunction with  FIG. 1 . The output signals of the said sensors are fed via corresponding signal lines to an electronic control and regulating unit  46  which generates actuating signals for the electric motor  23  according to an algorithm which is stored in the said control and regulating unit  46 . 
         [0016]    Furthermore, it can be gathered from  FIGS. 2 and 3  that the boosting force which is applied by the threaded spindle  15  is transmitted to a force transmission sleeve  47  which is guided displaceably in the booster housing  5 . A transmitter chamber  50  which is filled with a suitable force transmission medium  49  is formed in the interior of the force transmission sleeve  47 . A pasty pressure-resistant compound which is not prone to gap extrusion can be used as the force transmission medium  49 . As an alternative, a plurality of balls of small diameter can be used. The transmitter chamber  50  is delimited firstly by a head flange of a push rod  51  and secondly by the end face of a reaction piston  48  which is guided in the force transmission sleeve  47 . Whereas the push rod  51  is supported on the primary piston  11  of the brake master cylinder  1 , the previously mentioned piston rod  4  is supported on the reaction piston  48 , as a result of which the actuating force which is introduced at the brake pedal  38  is transmitted to the force transmission medium  49 . The reaction piston  48  which, as a result of its construction, transmits a reaction force which results from the boosting force to the piston rod  4  and therefore to the brake pedal  52  during the actuation makes a relative travel “c” of the push rod  51  possible with respect to the force transmission sleeve  47 , which relative travel “c” is defined by the width of a radial groove  53  which is provided for this purpose in the force transmission sleeve  47 . The reference sign “b” denotes the travel which is covered by the threaded spindle  15  during the actuation ( FIG. 3 ).