Patent Publication Number: US-2010126167-A1

Title: Electromechanical  brake  booster

Description:
PRIOR ART 
     The invention relates to an electromechanical brake booster having the characteristics of the preamble to claim  1 . 
     In motor vehicles at this time, underpressure brake boosters are usual, which utilize an underpressure in an intake tube of an internal combustion engine of the motor vehicle to generate an auxiliary force that boosts a muscle power exerted by a vehicle driver for actuating a brake system of the motor vehicle. The underpressure brake boosters are typically flanged to a master cylinder; that is, they are disposed between a brake pedal and the master cylinder, and they introduce their auxiliary force between the brake pedal and a piston of the master cylinder. When the term brake pedal is used hereinafter, it is to be understood also to encompass a hand brake lever or other actuating element for a hydraulic vehicle brake system. 
     In modern internal combustion engines, the problem sometimes occurs that they fail to furnish an adequate underpressure for an underpressure brake booster, or that connecting an underpressure brake booster is unwanted because of its influence on the intake performance, or in other words on the delivery of the combustion air. For that reason, electromechanical brake boosters have been proposed that use an electric motor to generate an auxiliary force for actuating a master cylinder of a hydraulic vehicle brake system. In principle, external force actuation is also possible; that is, the master cylinder is actuated solely with the force of the electromechanical brake booster and not partly with muscle power as well. The force generated by the brake booster and exerted on the piston of the master cylinder is in this case called an external force rather than an auxiliary force. This too is intended to be within the scope of the invention. Normally, external force braking is preferred in which a vehicle driver must exert some of the actuation force by muscle power, which gives him feedback about the intensity of the brake actuation. 
     German Published Patent Disclosure DE 103 27 553 A1 discloses an electromechanical brake booster for a master cylinder of a hydraulic vehicle brake system. The known brake booster has an electric motor, which is embodied as a hollow shaft motor and is disposed coaxially around a piston rod that connects a brake pedal with a piston of a master cylinder. A spindle drive is disposed in the hollow shaft motor and its nut is driven by a rotor of the electric motor, while its spindle is embodied as a hollow rod that likewise concentrically surrounds the piston rod. The spindle cooperates with a stop of the piston rod, by way of which stop it displaces the piston rod in the direction of an actuation of the master cylinder. If the known brake booster should fail, then the master cylinder can be actuated by muscle power using the brake pedal, without brake force boosting. In an accident, with the known brake booster the brake pedal can be pulled away in the direction of brake actuation in order to reduce the risk of injury to a vehicle driver. The accident can be detected with an acceleration sensor, which is present for instance for tripping vehicle air bags as well. 
     A further electromechanical brake booster is disclosed in U.S. Pat. No. 6,634,724 B2. This brake booster has a conventional electric motor, which via a step-down gear and a rack and pinion gear acts on the piston of a master cylinder or on a piston rod connecting the piston to a brake pedal. The electric motor is angularly offset by 90°, or more precisely is disposed at a tangent to the piston rod. 
     A further electromechanical brake booster is disclosed in German Published Patent Disclosure DE 101 13 346 A1. In this brake booster, torque from an electric motor is introduced into a pedal shaft of a brake pedal via a worm gear. 
     Explanation and Advantages of the Invention 
     The electromechanical brake booster of the invention having the characteristics of claim  1  has an electric motor and a rotation-to-translation conversion gear that converts a rotary driving motion of the electric motor into a linear motion for actuating a master cylinder of a hydraulic vehicle brake system. A spindle drive or a rack and pinion gear can for instance be instance be used as the rotation-to-translation conversion gear. A rotatable or pivotable cam that is pivoted by the electric motor and presses directly or indirectly against a piston of the master cylinder can also be considered for the rotation-to-translation conversion gear. Furthermore, a lever gear, for instance in the form of a crank drive, can be used as the rotation-to-translation conversion gear. The electric motor drives the crank to a pivoting motion, which is converted via a connecting rod into a translational motion for actuating the master cylinder. 
     The brake booster of the invention furthermore has a mechanical gear with a variable gear ratio. A variable gear ratio is for instance also possible with a rack and pinion gear (claim  2 ). For the sake of changing the gear ratio, a pitch of a toothing of the rack can vary over a length of the rack. Also by changing a spacing a gear wheel from the rack of a rack and pinion gear, or by using a non-circular gear wheel with a variable diameter, a change in the gear ratio is possible. A variable gear ratio is also possible with a coupling mechanism (claim  3 ), for instance a toggle mechanism. Coupling mechanisms are also called lever gears or kinematic chains. A gear with a variable gear ratio enables a major travel boost at the onset of actuation of the master cylinder, or in other words makes fast brake actuation and a major boost in force or torque possible at the end of the actuation, at high hydraulic pressure and with strong braking and actuation force. 
     As noted, by means of the variable gear ratio, a major force or torque boost is possible with a strong brake and actuation force. Correspondingly, a torque that the electric motor of the brake booster must exert to generate a predetermined maximum auxiliary force decreases. The invention thus makes a lower-power and hence smaller and lighter-weight electric motor with lower current consumption possible. 
     If a brake actuation by muscle power is effected via the gear with the variable gear ratio of the brake booster of the invention, then this additionally has the advantage that both the muscle power and the external force of the brake booster are boosted; with a strong actuation force, a major boost of the muscle power is attained via the gear with the variable gear ratio of the brake booster. The muscle power required to generate a strong braking force is reduced. Reducing the muscle power for actuating a brake with strong braking force is an advantage particularly if the brake booster fails, since for actuating the brake the brake booster has to be moved (driven), which requires some of the muscle power. 
     The dependent claims have advantageous features and refinements of the invention defined by claim  1  as their subject. 
     Claim  8  contemplates a worm gear for a brake booster, because it has a major step-down in rpm and a major step-up in torque. The worm gear is preferably provided as a first or only gear stage and is driven directly by the electric motor of the brake booster. Besides its high step-down or step-up ratio, a worm gear has the advantage that it reduces the rpm sharply, thus reducing the noise that is generated particularly as a result of high rotary speeds. A refinement of the invention in accordance with claim  9  contemplates a plastic wheel of the worm gear, which also serves to reduce noise. 
     According to claim  13 , the brake booster of the invention has a free wheel mechanism, which can also be called a directionally shifted clutch. The free wheel mechanism transmits the motion of the electric motor in the actuation direction to the master cylinder. If a motion of the brake pedal is faster than the motion of the electric motor or of the brake booster, or if the electric motor does not move at all because of some defect, then the free wheel mechanism allows an actuation of the master cylinder with the brake pedal. The free wheel mechanism of the brake booster of the invention is used in its function as an overrunning clutch. Both form-locking free wheel mechanisms, such as ratchet free wheel mechanisms, and force-locking clamping free wheel mechanisms can be used. Besides rotating, or in other words or rotationally acting, free wheel mechanisms, linear free wheel mechanisms are also possible, which transmit a linear motion of the brake booster directly or indirectly to a piston of the master cylinder and allow a relative motion in the opposite direction, that is, the free wheel mechanism direction, so that once again, a motion of the piston of the master cylinder in the direction of actuating the hydraulic vehicle brake system is possible without motion of the brake booster, or with a slower motion of the brake booster. A linear free wheel mechanism may for instance have a ratchet that cooperates with a rack that has sawtooth toothing. Besides form-locking free wheel mechanisms, in the case of linear free wheel mechanisms force-locking free wheel mechanisms with clamping bodies are also possible. 
     Preferably, the free wheel mechanism of the brake booster of the invention is disposed such that upon an actuation of the master cylinder by muscle power if the brake booster is defective, as few parts as possible of the brake booster are also jointly moved, so that a motion resistance of the brake booster is low. Hence the free wheel mechanism is preferably disposed as close as possible to a final member of the brake booster, possibly even between the last member of the brake booster and a piston rod of the master cylinder; with the disposition, it is the action, that is, the introduction of the force of the brake booster, that is meant in particular, and not so much the locational disposition. However, a free wheel mechanism that is disposed directly on the electric motor of the brake booster is also possible within the scope of the invention. 
     The advantage of this feature of the invention is that an actuation of the master cylinder is possible if the brake booster fails, and only a few, or in any case not all, of the parts of the brake booster have to be jointly moved. There is low motion resistance of the non-moved brake booster to an actuation of the master cylinder. A further advantage of the invention is a comparatively simple, economical design and the possibility of using a conventional electric motor. In comparison with a hollow rotor motor, the lower moment of mass inertia is another advantage. In an accident, a retraction, that is, a motion of the brake pedal in the direction of a brake actuation, is possible with the brake booster of the invention, for reducing the risk of injury. 
     Claim  14  contemplates a shiftable free wheel mechanism, which in the engaged state acts a free wheel mechanism and in the disengaged state disconnects the electric motor of the brake booster, and preferably also its gear or parts of the gear, mechanically from the master cylinder. By disengaging the free wheel mechanism, a release of the master cylinder without a motion of the brake booster is possible; at the least, not all the parts of the brake booster have to be jointly moved. The force required for releasing the master cylinder is reduced as a result. 
     Further characteristics of the invention will become apparent from the ensuing description of embodiments of the invention in conjunction with the claims and the drawings. Individual characteristics can each be implemented on their own or in groups in embodiments of the invention. For instance, the free wheel mechanism does not necessarily require the mechanical gear with the variable gear ratio. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is described below in terms of embodiments shown in the drawings. The drawings show four embodiments of electromechanical brake boosters according to the invention. The drawings must be understood to be schematic, simplified illustrations for the sake of comprehension and explanation of the invention. 
     
    
    
     EMBODIMENTS OF THE INVENTION 
       FIG. 1  schematically shows a master cylinder  1  of a hydraulic vehicle brake system, not otherwise shown, of a motor vehicle. A rod piston  2  and a floating piston  3  are received in the master cylinder  1 . The rod piston  2  is moved mechanically with a brake pedal  4 , via a tappet rod  5  and a piston rod  6 . The tappet rod  5  connects the piston rod  6  in articulated fashion to the brake pedal  4 . The floating piston  3  is subjected to and moved by a hydraulic pressure that the rod piston  2  generates upon its displacement into the master cylinder  1 . If there is a leak, the floating piston  3  is moved by contact with the rod piston  2 . This is known per se and requires no further explanation. The piston rod  6  is rigidly connected to the rod piston  2 , and the piston rod  6  may be in one piece with the rod piston  2 . 
     An electromechanical brake booster  7  in accordance with the invention is disposed between the brake pedal  4  and the master cylinder  1 . The brake booster  7  has an electric motor  8  with a step-down gear flanged to it and a rack and pinion gear  9  with a rack  10  and a gear wheel  11  that meshes with the rack  10 . The rack and pinion gear  9  fowls a rotation-to-translation conversion gear, which converts a rotary driving motion of the electric motor  8 , or of the gear flanged to it, into a translational motion for displacing the rod piston  2 . For introducing a force of the brake booster  7 , the rack  10  can be connected in articulated fashion or rigidly to the rod piston  2  or its piston rod  6 . In the embodiment shown of the invention, the piston rod  6  has the toothing of the rack  10 ; thus the piston rod  6  also forms the rack  10  of the rack and pinion gear  9 . 
     Between the step-down gear of the electric motor  8  and the gear wheel  11  of the rack and pinion gear  9  is a free wheel mechanism  12 , which transmits a rotary motion from the gear to the gear wheel  11  in one direction of rotation and allows a rotation of the gear wheel  11  relative to an output shaft of the gear in the reverse direction of rotation. The free wheel mechanism  12  is also called a directionally shifted clutch, and in the brake booster  7  it is used in its function as an overrunning clutch. The blocking direction, in which the free wheel mechanism  12  transmits a rotary motion from the gear of the electric motor  8  to the gear wheel  11  of the rack and pinion gear  9 , is the direction with which the rod piston  2  is displaced into the master cylinder  1 , or in other words in which the master cylinder  1  is actuated. 
     For actuating the master cylinder  1 , the brake pedal  4  is depressed by a vehicle driver using muscle power and in this way, the rod piston  2  is displaced into the master cylinder  1  via the coupling rod  5 , the rack  10 , and the piston rod  6 . Upon the actuation of the master cylinder  1 , the electric motor  8  of the brake booster  7  of the invention is supplied with current, and via its gear, the free wheel mechanism  2  and the gear wheel  11 , it drives the rack  10 . This means that the electric motor  8  or the brake booster  7  exerts a force in the actuation direction on the rod piston  2  of the master cylinder  1 . The force exerted by the brake booster  7  onto the rod piston  2  is called the auxiliary force. It acts on the rod piston  2  in addition to the actuation by muscle power by means of the brake pedal  4 . The auxiliary force of the brake booster  7  and the muscle power exerted by means of the brake pedal  4  add up to an actuation force that acts on the rod piston  2 . Conversely, this means that the muscle power required for generating a certain actuation force is reduced by the auxiliary force exerted by the brake booster  7 . Controlling or regulating the auxiliary force of the brake booster  7  is effected for instance as a function of a displacement travel of the rod piston  2 , which is measured for instance with a travel sensor  13  on the piston rod  6  or the rack  10 , by means of a force sensor  14  and/or a pressure sensor  15 , which measures the hydraulic pressure in the master cylinder  1 . The control or regulation is effected as a linear or nonlinear function of the aforementioned measured variables, by means of an electronic controller or regulator, not shown. 
     By means of the free wheel mechanism  12 , actuation of the master cylinder  1  with the brake pedal by muscle power is possible without boosting by the brake booster  7 , for instance if the brake booster  7  is defective. Upon an actuation of the master cylinder  1  with the brake pedal  4  without the action of the brake booster  7 , the rack  10  jointly moves the gear wheel  11  that meshes with it, and the free wheel mechanism  12  disconnects between the gear wheel  11  and the step-down gear flanged to the electric motor  8 . As a result, the motion resistance of the brake booster  7  is negligibly slight. 
     The free wheel mechanism  12  is shiftable; in the engaged state, it has the described function of an overrunning clutch. In the disengaged state, in both directions of rotation, the free wheel mechanism  12  disconnects the gear wheel  11  from the gear of the electric motor  8 . In this way, easy restoration of the rod piston  2  in the event of failure of the brake booster  7  is possible. In an accident, which can be ascertained with an acceleration sensor known per se and not shown, it is possible by supplying current to the electric motor  8  to move the brake pedal  4  in the actuation direction via the rack  10  and the coupling rod  5 , in order to reduce the risk of injury to a vehicle driver. 
     The rack and pinion gear  9  is a mechanical gear. The toothing of the rack  10  has a pitch which varies over the length of the rack  10 . The pitch of the toothing of the rack  10  is greater at an end of the rack  10  that is near the master cylinder  1  and the rod piston  2 , and with increasing distance from the rod piston  2  and the master cylinder  1  it decreases. As a result, the rack and pinion gear  9  has a variable gear ratio. At the onset of a displacement of the rack  10  and of the rod piston  2 , a travel boost of the rack and pinion gear  9  is great, and as a result, a fast motion of the rod piston  2  is attained. With increasing displacement, the travel boost of the rack and pinion gear  9  decreases, and to the same extent a force boost of the rack and pinion gear  9  increases. As a result, the auxiliary force exerted by the brake booster  7  becomes greater, at a constant torque of the electric motor  8 , with increasing displacement of the rod piston  2 , or in other words with increasing hydraulic pressure in the master cylinder  1  and an increasing actuation force. 
     In the embodiment of the invention shown in  FIG. 1 , the rack  10  is rigidly connected to the piston rod  6  and the rod piston  2 . However, this is not compulsory for the invention. The rigid connection of the rack  10  and the rod piston  2  makes it possible to guide the rack  10  displaceably with a single guide  16 . In the embodiment shown, the guide  16  is disposed in the region of the piston rod  6 , which is connected rigidly to the rack  10  and in particular is in one piece with it. The guide  16  of the rack  10  is disposed offset from the gear wheel  11  of the rack and pinion gear  9  by an offset d in the direction of the master cylinder  1 . The offset d is selected such that an imaginary line of application  17  of the rack and pinion gear  9  intersects the guide  16  on a side diametrically opposite the toothing of the rack  10 . The line of application  17  is an imaginary straight line through a point of contact or a line of contact on the flanks of the teeth of the gear wheel  11  and of the rack  10 , which touch one another upon actuation of the master cylinder  1 . The line of application  17  is a normal to the tooth flanks at the point of contact or line of contact. The Line of application  17  indicates the direction in which the meshing tooth at that time of the gear wheel  11  exerts a force on the corresponding tooth of the rack  10 . Thus the line of application  17  indicates the direction of the force, exerted by the gear wheel  11  on the rack  10 , upon actuation of the master cylinder  1 . Because of the offset d by which the guide  16  of the rack  10  is intersected by the line of application  17  on a side diametrically opposite the toothing of the rack  10 , the guide  16  braces the rack  10  without torque. A single guide  16  is sufficient for the rack  10 , and a torque or a transverse force on the rod piston  2 , which would cause increased wear, is avoided. Moreover, the offset d of the guide  16 , which because of the offset d braces the rack  10  without torque, makes it possible to embody the guide  16  as shown with a ring bearing as a slide bearing for the rack  10  or the piston rod  6 . A slide bearing, especially in the structural form of a ring bearing, is inexpensive and makes easy assembly possible. An additional advantage is that a ring bearing as a guide  16  braces the rack  10  against a transverse force that occurs if the rack and pinion gear  9  has a helical toothing. 
     In the ensuing description of  FIGS. 2 through 4 , the same reference numerals are used for components that match  FIG. 1 . In agreement with  FIG. 1 , the electromechanical brake booster  7  of the invention in  FIG. 2  has a rack and pinion gear  9 , with a rack  10  with which a gear wheel  11  meshes. The rack and pinion gear  9  has a helical toothing. The rack  10  is rigidly connected to the piston rod  6  of the rod piston  2 . The brake pedal  4  is also present, which is connected in articulated fashion to the rack  10  and the piston rod  6  via the coupling rod  5 . The free wheel mechanism  12  and the electric motor  8  that drives the rack and pinion gear  9  via the free wheel mechanism  12  are also present. In a distinction from  FIG. 1 , the electric motor  8  drives the free wheel mechanism  12  via an angular gear  18 . In the exemplary embodiment, the angular gear  18  is a worm gear, with a worm  19  that is disposed on the shaft  20  of the electric motor  8  in a manner fixed against relative rotation and meshes with a worm wheel  21 . The worm wheel  21  is disposed on an input shaft of the free wheel mechanism  12  in a manner fixed against relative rotation and drives the free wheel mechanism. The win in wheel  21  is of plastic, for the sake of quiet gear operation. As an angular gear  18 , the worm gear has a high step-down ratio, making an additional step-down gear unnecessary. However, the invention is not limited to a worm gear as the angular gear  18 . The rack and pinion gear  9  also forms an angular gear, and both gears  9 ,  18  have a deflection of 90° each. Overall, the electric motor  8  is deflected relative to the rack  10  and thus to the master cylinder  1  by 180° by means of the two angular gears  9 ,  18 ; that is, the electric motor  8  is disposed parallel to the master cylinder  1  and beside it. This makes a compact embodiment of the brake booster  7  of the invention with the master cylinder possible. 
     For the description of  FIGS. 2-4 , the description of  FIG. 1  is additionally referred to, to avoid repetition. 
     In  FIG. 3 , the electromechanical brake booster  7  of the invention acts on the rod piston  2  of the master cylinder  1  via a coupling mechanism  22 . In the embodiment selected, the coupling mechanism  22 , which can also be conceived of as a kinematic chain, is a toggle mechanism, or in other words a flat lever mechanism. The coupling mechanism  22  has a piston rod  23 , one end of which is connected in articulated fashion to the rod piston  2  of the master cylinder  1 . The other end of the piston rod  23  is connected in articulated fashion to a support lever  24 , which is braced in articulated fashion on an abutment  27  that is stationary relative to the master cylinder  1 . The joint connecting the piston rod  23  to the support lever  24  will hereinafter be called a knee joint. It is engaged on the one hand by the brake pedal  4 , although with a sliding fit, to allow a longitudinal motion of the knee joint  26  on the brake pedal  4 . The knee joint  26  is also engaged in articulated fashion by the rack  10  of the electromechanical brake booster  7 . In principle, the rack  10  may also engage the knee joint  26  of the coupling mechanism  22  with pressure, instead of with tension as shown. 
     The brake booster  7  of  FIG. 3  is constructed like the brake booster  7  of  FIG. 1 , so that the description above of it may be referred to. The brake booster  7  of  FIG. 3  has an electric motor  8  with a step-down gear flanged to it, and the electric motor, via a shiftable free wheel mechanism  12 , drives a gear wheel  11  that meshes with the rack  10 . The gear wheel  11  and the rack  10  form a rack and pinion gear  9 . Only the introduction of the auxiliary force generated by the brake booster  7  into the actuation of the rod piston  2  of the master cylinder  1  is done differently in  FIG. 3  from  FIG. 1 , namely in articulated fashion, via the coupling mechanism  22  embodied as a toggle mechanism, instead of rigidly as in  FIG. 1  with the rack  10  of the rack and pinion gear  9  onto the piston rod  6  of the rod piston  2 . Also in  FIG. 3 , the piston rod  23  is connected in articulated fashion with the rod piston  2 , while in  FIG. 1 , both the piston rod  6  and the rack  10  are connected rigidly to the rod piston  2 . 
     In  FIG. 3 , the guide  16  of the rack  10  has a roller bearing, and it is disposed without offset relative to the gear wheel  11  of the rack and pinion gear  9 . The guide  16  makes pivoting of the rack  10  possible. 
     For actuating the master cylinder  1  in  FIG. 3 , the brake pedal  4  is depressed, which displaceably engages the knee joint  26 . Depressing the brake pedal  4  puts the coupling mechanism  22 , embodied as a toggle mechanism, out of the angled position shown into a straighter position; that is, a knee angle α between the piston rod  23  and the support lever  24  increases. As a result, the coupling mechanism  22  pushes the rod piston  2  into the master cylinder  1 , which is thus actuated. 
     The actuation is reinforced by the brake booster  7 , which in the manner already described in conjunction with  FIG. 1  exerts an auxiliary force that it introduces via the rack  10  at the knee joint  26 . The auxiliary force acts in the direction of a greater straightening of the coupling mechanism  22  embodied as a toggle mechanism, or in other words in the direction of an actuation of the master cylinder  1 . Controlling or regulating the auxiliary force of the brake booster is effected as described in conjunction with  FIG. 1 . 
     The coupling mechanism  22  embodied as a toggle mechanism is a mechanical gear with a variable gear ratio: The small knee angle α, at the onset of an actuation of the master cylinder  1 , between the piston rod  23  and the support lever  24  causes a long displacement travel of the rod piston  2 , given a predetermined travel of the knee joint  26 . At the onset of actuation of the master cylinder  1 , the rod piston  2  is thus moved quickly. With increasing straightening of the coupling mechanism  22  embodied as a toggle mechanism, or in other words with the knee angle α increasing, the travel boosting becomes less and force boosting becomes greater. With a straightened toggle mechanism, or in other words upon approach to a knee angle α of 180° between the piston rod  23  and the support lever  24 , the force exerted by the toggle mechanism on the rod piston  2  tends toward infinity. 
     The boosting of the coupling mechanism  22  is determined not only by its geometry and position but also by the location of the abutment  25 . In the embodiment shown in  FIG. 3 , a joint of the abutment  25  is located on an imaginary cylinder axis  27  of the master cylinder  1 . As indicated by lines with dashes and double dots and by reference numeral  28 , the abutment  25  of the coupling mechanism  22  can also be offset from the cylinder axis  27  of the master cylinder by an angle β. In addition to having an influence on the boosting of the coupling mechanism  22 , the angular offset β of the abutment  25 ,  28  also has an influence on a transverse force exerted by the coupling mechanism  22  on the rod piston  2 . The transverse force is created because the piston rod  23  engages the rod piston  2  not axially but rather at an attack angle γ to the cylinder axis  27 . As a result of the angular offset β of the abutment  25 , the attack angle γ of the piston rod  23  varies; the attack angle γ can decrease, causing the transverse on the rod piston  2  to decrease. In this connection, it should be taken into account that upon actuation of the master cylinder  1 , the coupling mechanism  22  embodied as a toggle mechanism is increasingly straightened, and thus the attack angle γ by which the piston rod  23  meets the rod piston  2  is reduced. This reduces an increase in the transverse force, exerted by the piston rod  23  on the rod piston  2 , while the actuation force is increasing. 
     The boosting of the coupling mechanism  22  can also be varied by draining brake fluid from the master cylinder  1 . As a result, the rod piston  2  and the floating piston  3  are displaced into the master cylinder  1 ; that is, at a certain hydraulic pressure in the master cylinder  1 , the pistons  2 ,  3  are displaced farther into the master cylinder  1  than without the reduction of the brake fluid volume in the master cylinder  1  by draining off brake fluid. As a result, the coupling mechanism  22  is more markedly straightened, and its force boosting is greater. In a vehicle brake system that has traction control (ABS, TCS, VDC, ESP), draining off brake fluid from the master cylinder  1  is effected by opening valves of the vehicle brake system, not shown. The brake fluid flows out of the master cylinder  1  into hydraulic reservoirs. With hydraulic pumps, which vehicle brake systems of this kind have, it is conversely also possible to pump brake fluid into the master cylinder  1  and thereby to lessen the straightening of the coupling mechanism  22 . 
     This possibility of a hydraulic variation of the gear ratio of the mechanical coupling mechanism  22  by draining brake fluid from the master cylinder  1  is equally possible in the embodiment of the invention shown in  FIG. 1 . By draining brake fluid from the master cylinder  1 , the pistons  2 ,  3  there as well are displaced farther into the master cylinder  1 . The gear wheel  11  of the rack and pinion gear  9  as a result meshes at a different point of the rack  10 , whose pitch varies over its length. As a result, the gear wheel  11  meshes at a point of the rack  10  at which the pitch of its toothing is less and the force boost is therefore greater. 
     In comparison to  FIG. 3 , the embodiment of the invention shown in  FIG. 4  has a “half toggle mechanism” as its coupling mechanism  22 . In comparison to  FIG. 3 , the support lever  24  and its abutment  25  are absent. In  FIG. 4  as in  FIG. 3 , the piston rod  23  is connected in articulated fashion to the rod piston  2 . The other end of the piston rod  23  is connected in articulated fashion to the rack  10  of the brake booster  7 . The joint connecting the piston rod  23  to the rack  10 , as in  FIG. 3 , is also called a knee joint  26  in  FIG. 4 . In  FIG. 4  as in  FIG. 3 , the brake pedal  4  engages the knee joint  26  displaceably. 
     In the embodiment of  FIG. 4 , the rack  10  is disposed transversely to the master cylinder  1  and is guided displaceably with two guides  16 . The guides  16  are offset in both directions longitudinally to the rack  10  relative to the gear wheel  11  of the rack and pinion gear  9 . The guides  16  brace the rack  10  in the axial direction relative to the master cylinder  1 . By a displacement of the rack  10  with the brake pedal  4  and/or with the brake booster  7 , the piston rod  23  is pivoted in the direction of the cylinder axis  27  of the master cylinder  1  and in the process displaces the rod piston  2 . The coupling mechanism  22  of  FIG. 4 , which is a mechanical gear, has a variable gear ratio, of the kind that has been described in conjunction with the coupling mechanism  22  of  FIG. 3 . In this respect, see the discussion of  FIG. 3 . 
     As in  FIG. 2 , in  FIG. 4  the electric motor  8  of the brake booster  7  drives the free wheel mechanism  12  via a worm gear, that is, an angular gear  18 , and via the free wheel mechanism, it drives the gear wheel  11  of the rack and pinion gear  9 . The angular gear  18  embodied as a worm gear has a worm  19  and a worm wheel  21 , which for reasons of noise and expense made from plastic. As a result of the deflection by 90° each with the rack and pinion gear  9  and the worm gear  18 , the electric motor  8  is disposed parallel to and beside the master cylinder  1 . This makes a compact, space-saving disposition of the electromechanical brake booster  7  on the master cylinder  1  possible.