Servo mechanism, especially for intensifying the braking power in a motor vehicle

To provide a power brake having relatively small hysteresis between brake pedal pressure and brake hose pressure and with compact construction, the push rod leading from the brake pedal to the brake master cylinder is divided into an input rod and an output rod. A pressure sensitive element is disposed between the rods and is displacable therewith. The pressure-sensitive element determines the force with which the servo member acts upon the output rod.

BACKGROUND OF THE INVENTION 
This invention pertains to a servo mechanism for intensifying the braking 
power of a motor vehicle. 
Presently so-called servo brake systems are being installed in nearly all 
motor vehicles. These servo brake systems intensify the pressure exerted 
on the brake pedal by the driver to a sufficiently high brake pressure. 
The auxiliary power necessary for this purpose is gained from diverse 
systems. For instance there are hydraulic power brakes for the operation 
of which a pump for the fluid has to be available. In other cases the 
vacuum in the intake manifold of the motor is utilized. However, vacuum 
assisted systems are not easily utilized in vehicles with fuel injection. 
From DE-OS No. 27 58 644 a brake unit is known, in which an electric motor 
serves as an accessory drive. Via a friction clutch and an overunning 
clutch the electric motor drives a pinion serving as a servo member. The 
pinion mates with a push rod partly formed as a rack which leads from the 
brake pedal to the brake master cylinder. The friction clutch may be 
regarded as a pressure-sensitive element. The greater the pressure which 
presses the two clutch halves together, the greater is the torque which 
can be transmitted to the servo member via the clutch, and the larger is 
the counterforce against which the push rod can still be adjusted. If at a 
given value of the frictional connection between the two clutch halves 
this counterforce is exceeded by a certain amount, there occurs a clutch 
slippage and the push rod is not further adjusted. 
The support axes of the servo member, of the clutch disks and the motor 
shaft of the known device extend vertically to the push rod. The 
consequence is that the clutch can only be controlled from the brake pedal 
via a complicated lever power transmission which requires significant 
space. During braking big transverse forces appear in the bearing of the 
pinion and in one clutch half which result in significant 
bearing-friction. This bearing-friction is also noticeable, when by 
increasing the pressure exerted onto the brake pedal, the pinion and the 
one clutch disk are somewhat displaced in axial direction. Because of the 
bearing-frictions which appear in the lever system, an extremely high 
hysteresis between brake line pressure and brake pedal pressure is 
created. 
SUMMARY OF THE INVENTION 
Starting from the prior art mentioned the invention is based on the problem 
of providing an improved servo mechanism in which the brake line pressure 
quickly follows the brake pedal pressure and in a compact design. The 
servo mechanism is not to exceed the size of vacuum intensifiers presently 
used. 
This problem is solved according to the invention by a servo mechanism in 
which the push rod has an input and an output rod co-axially positioned 
towards each other, in which the pressure-sensitive element is arranged 
between input rod and output rod and this is displaceable and in which the 
servo member acts upon the output rod. 
By arranging the pressure-sensitive element between input and output rod 
this element can in a simple manner be acted upon by pressure on the brake 
pedal, because a counterforce acts on the output rod. The pressure on the 
pressure-sensitive element can also be maintained, when output rod and 
input rod are adjusted, because the pressure-sensitive element can be 
displaced with them. In a servo mechanism according to the invention, it 
is not necessary to provide a complicated lever power transmission and 
therefore the space required is considerably reduced. This also 
contributes to a reduction of the hysteresis between brake line pressure 
and brake pedal pressure. Additionally, to increase the pressure on the 
pressure-sensitive element no parts on which high transverse forces are 
acting have to be displaced in axial direction. 
Having the servo member supported co-axially to the output rod is 
especially favorable with regard to its spatial arrangement and with 
respect to a low bearing friction. The support axis of the servo member is 
in line with the direction into which the output rod has be be displaced 
and into which the force has to act on the output rod. Having the driven 
wheel of the accessory drive formed as a hollow cylinder within which the 
output rod extends contributes to a compact design. Because one can reduce 
the rotational speed to a small number of revolutions this driven wheel 
may have a big diameter. Thus, the interpenetrative arrangement of output 
rod and driven wheel does not cause an enlargement of the servo mechanism 
in a direction transverse to the output rod, but in the longitudinal 
direction of the output rod which accordingly makes the mechanism very 
short. 
In accordance with an embodiment of the invention the rotation of the 
rotating driven wheel is transformed into an axial displacement of the 
output rod via a screw joint and a push joint. Such a design is generally 
called a worm gear. Thereby the necessary transformation of motion can be 
effected on a small space. One of the two parts interconnected by a screw 
joint is normally stationary in the axial direction, whereas the other has 
a component of motion in the axial direction. Therefore counterforce to 
the axially directed force for moving the one part has to be absorbed by 
the other part. Further in accordance with the invention, stationary part 
of the two parts interconnected by the screw joint may be directly 
connected with a structural part to be acted upon by the output rod. This 
design has the particular advantage in that the entire servo power is 
directly applied to the structural part acted upon by the output rod. The 
structural part to be acted upon by the output rod can for instance be the 
brake master cylinder of a brake system. 
When the screw joint is arranged between the driven wheel and a further 
part and the push joint is arranged between the housing and the further 
part, the servo mechanism has only a few rotating parts. Under certain 
circumstances only the output of the driven wheel executes a rotary 
movement. Because the input rod and the output rod are only moved in an 
axial direction without undergoing a rotary movement only a small number 
of rotating parts are necessary. The number of bearing between parts being 
fixed in a rotational direction and parts moving in a rotational direction 
is small. 
Further in accordance with the invention, an overruning clutch can 
advantageously be realized in the screw joint. An overruning clutch is 
provided in a servo mechanism according to the invention to permit 
adjustment of input and output rods for the braking of a motor vehicle 
when the motor-driven accessory drive is blocked. If for instance the 
motor vehicle engine is used as an accessory drive, braking should also be 
possible in case the engine is idle. Braking should also be possible if an 
electric motor is used as an accessory drive and its current supply is 
interrupted, because the ignition has been switched off or a defect with 
regard to the electric or mechanical function has appeared. 
In another embodiment of the invention, the screw joint can also be 
realized without a thread and without a roller or slide body. The screw 
motion between the two parts connected by the joint is effected in the 
following manner: if the two parts would execute a pure rotational motion 
relative towards each other the spacing between the two moving joints of 
the connecting element on both parts would vary. However, the connecting 
element does not allow such a change. Under the additional condition that 
the spacing between the moving joints must remain the same the two parts 
can therefore only be rotated towards each other, if they also move 
relative towards each other in axial direction. This results in a screw 
motion in which there is a non-linear correlation between the angle by 
which the two parts connected by the screw joint are twisted against each 
other and the distance covered in axial direction. For this reason the 
amplification of the servo mechanism is not constant. Depending on what is 
required the joint can be developed in such a way that the amplification 
is increased or reduced in accordance with the adjustment of the output 
rod. The most favorable condition might be that the amplification 
increases with increasing adjustment of the output rod. For a brake system 
the pressure required for slight braking is mainly provided via the brake 
pedal. Only if the braking becomes stronger does the power assist of the 
servo mechanism become noticeable, and increasingly so. A conventional 
roller screw joint can be developed in a non-linear manner. 
If the connecting element, be it a rigid rod or a flexible rope, is subject 
to tensile strain, an overrunning clutch can be integrated in the screw 
joint. A flexible connecting element, e.g. a rope, can fold when the two 
parts connected by the screw joint have to execute a purely axial movement 
towards each other. If a rigid rod is used the overrunning clutch is 
achieved in that according to claim 14 the two moving joints of the rod in 
the condition of the biggest possible axial spacing from each other are in 
axial direction at least nearly in alignment and that at least one moving 
joint is developed as a passage for the rod. But if the screw is developed 
in such a way that the rods are subject to tensile strain, an overrunning 
clutch can be achieved according to claim 16 in that a guide groove 
originates from at least one moving joint of each rod, in which guide 
groove the rod is captivated. 
Further in accordance with the invention, the pressure-sensitive element 
may be a non-positive friction clutch co-axially arranged to the push rod. 
This clutch comprises two halves movable against each other. One half is 
articulated to the driven wheel of the servo motor and rotatably mounted. 
The other half is articulated to the housing. Thereby, the second clutch 
half can be acted upon by the input rod. Depending on whether the screw 
joint is positioned between the driven wheel and the first clutch half and 
the push joint is positioned between the second clutch half and the 
housing or vice versa, upon actuation of the clutch by exerting a pressure 
on the input rod the first clutch half is braked by the second clutch half 
or the second clutch half is carried along by the first clutch half. The 
first case has the advantage that no relative rotary movement takes place 
between the second clutch half and the input rod, so that a rolling 
bearing between the second clutch half and the input rod is not needed. 
Other embodiments of the invention utilize a friction clutch. The 
difficulties in articulating the driven wheel to the first clutch half and 
coupling it with the auxiliary motor are overcome by providing the driven 
wheel with two portions positioned on behind the other in axial direction. 
The driven wheel is coupled with the servo motor on one portion and on the 
other portion carries a part of the joint between itself and the first 
clutch half is a screw joint then as far as the thread is concerned, the 
torque predetermined by the non-positive connection of the friction clutch 
and the force to be transmitted in axial direction is inversely 
proportional to the radius and to the tangent of the angle of inclination 
of the thread. If the radius is reduced, the axial force is larger. 
Because the axial force must, however, not exceed a given value the 
smaller radius can be compensated by a bigger angle. Thus, one can provide 
an angle of 45.degree. which has the greatest power transmission 
efficiency, if one disregards the friction. 
Further, in accordance with one aspect of the invention, a small and 
compact design of the servo mechanism may be achieved by locating each 
friction lining of the friction clutch radially outside of the second 
portion of the driven wheel between a flange of the first clutch half and 
a flange of the second clutch half. Then the second clutch half may be 
formed as a bell which at least partly covers the driven wheel and the 
first clutch half. 
If the push joint is located between the driven wheel and the first clutch 
half, both the link joint between the driven wheel and the first clutch 
half and the coupling between the first clutch half and the output rod are 
realized in a simple manner. 
A servo mechanism in which the coupling between the two clutch disks is 
provided with the friction linings outside of the driven wheel can, under 
certain circumstances, involve difficulties with respect to the design, 
because some connection must be effected between the friction linings 
arranged outside of the driven wheel and the output rod extending within 
the driven wheel. It can therefore be more favorable to arrange in 
addition to the output rod also the friction linings and the friction 
disks or the entire clutch in the interior of the driven wheel resulting 
in a symmetrical arrangement as in another embodiment of the invention. 
Upon actuation of the clutch the one clutch half is therefore only exposed 
to small forces. 
In yet another embodiment, the screw joint is located between the second 
clutch half and the housing with the radius of the thread of the screw 
joint as small as possible. The structural part of the second clutch half 
is articulated to the structural part fixed on the housing in at least one 
direction of rotation and is connected with a friction disk of the second 
clutch half in a manner protected against twisting and in axial direction 
adjusts each clutch disk and the output rod. This facilitates the 
production of the piece parts of the second clutch half. With the separate 
structural part connected with a clutch disk of the second clutch half in 
a manner protected against twisting in at least one direction of rotation, 
an overrunning clutch can be inserted between the separate structural part 
and the corresponding clutch disk. This overrunning clutch permits the 
separate structural part to rotate relative to the housing without taking 
along the clutch disks, when the brake is operated without power 
assistance. In order to provide that the separate structural part is 
easily rotatable relative to the housing the one clutch disk of the second 
clutch half directly acts upon the output rod independently of the 
separate structural part. 
Other embodiments of the invention avoid the use of a friction clutch, 
which is subject to wear. These other embodiments are based on the 
principle of controlling an electric motor by means of the pressure 
exerted by the driver's foot. For this purpose the pressure-sensitive 
element is a sensor which serves as a transmitter for an electronic 
circuit controlling an electric motor in such a way that the torque 
created is proportional to the pressure measured by the sensor, and that 
the driven wheel of the motor is at least in one direction of rotation 
continuously and directly operatively connected to the output rod by a 
joint. A screw joint is advantageously positioned between the output of 
the motor and the output rod and a push joint between the output rod and 
the housing. By this arrangement it is provided that the output rod does 
not carry out a rotational motion without any rolling bearings. The 
armature of the motor only rotates so far at a time until the counterforce 
and the feeding power created by the torque are equal. If the pressure on 
the input rod is eased, the motor is controlled via the electronic circuit 
with a lower voltage. When the input rod and the output rod are reset, the 
armature of the motor is rotated in the opposite direction. In order to be 
able to brake quickly and without additional resistance, when the electric 
motor is defective, an overrunning clutch can be built in between the 
motor and the joint between the output of the motor and the output rod. 
Advantageously the rotor of the electric motor may be directly used as an 
output. Such a design is of simple construction and small size.

DETAILED DESCRIPTION 
The terms "push joint" and "screw joint" or variations thereof are used 
herein. It will be understood by those skilled in the art that the term 
"screw joint" refers to a coupling between two elements such that rotary 
motion of one element results in a relative axial displacement of the 
other element. It will also be understood by those skilled in the art that 
the term "push joint" refers to a coupling between two elements such that 
relative movement between the two elements can only occur in a lateral or 
axial direction, however, both elements may jointly exhibit rotary 
movement relative to a third element. In accordance with the invention, a 
screw joint and a push joint are both used such that lateral or axial 
movement of an input rod is combined with rotary movement of a motor 
driven wheel to produce an axial movement of an output rod. 
One example of a push joint utilized in one embodiment of the invention 
shown in FIG. 4 is a one or more radially extending tongue like portion 
carried on one element which engages one or more corresponding laterally 
or axially extending grooves on a second element. 
The servo mechanisms shown in the Figures serving as power brakes have an 
input rod 30 which can be connected with a brake pedal and an output rod 
31 which can act upon the piston of a brake master cylinder. So the 
necessary force for adjusting the output rod 31 does not have to be 
entirely supplied via the brake pedal during braking, an additional force 
created by a motor-driven accessory drive is conducted onto the output rod 
31. A friction clutch 32 is provided for initiating the servo power of the 
power brakes according to FIGS. 1 to 17. The first clutch half of clutch 
32, namely the driving part, is articulated to the driven wheel 34 of the 
motor-driven accessory drive. The second clutch half 35, i.e., the driven 
part, is articulated to the housing 36. The rotational movement of the 
driven wheel 34 is transformed into an axial movement of the output rod 31 
via a screw joint 37 between the driven wheel 34 and the first clutch half 
33 and via a push joint between the second clutch half 35 and the housing. 
It should be noted that instead of the screw joint a push joint can be 
used and vice versa. In the versions shown the motor-driven accessory 
drive is provided by an electric motor 39 whose rotational speed is 
reduced by means of the worm shaft 40 and by the driven wheel 34 developed 
as a worm wheel. 
The housing 36 is cup-shaped with a sidewall or jacket 45 and an end wall 
46. A face plate 47 is provided opposite end walls 46. The face plate 47 
includes a central cavity 48 having a smaller diameter than the jacket 45. 
In the power brake unit of FIG. 1 the driven wheel 34 is rotatably mounted 
in a slide bearing 50. The slide bearing 50 is held on face plate 47. The 
driven wheel is formed as a bushing 51. On the side of the bushing 51 
pointing to the interior of the housing is a flange 52. A hollow cylinder 
53 projects from the outer rim of flange 52. Toothing is formed on the 
outer surface of cylinder 53. A pressing rod 54 extends through the 
bushing 51 of the driven wheel 34. One end portion 55 of the rod 54 is of 
square cross-section. The other end is connected with the input rod 30 via 
an intermediary member 56. Between intermediary member 56 and pressing rod 
54 there are ball bearings. The connection between intermediary 56 and 
input rod 30 is provided by a ball joint. The pressing rod 54 is provided 
with a total of four longitudinal groove 57 in which a total of eight 
balls 58 are projecting. The balls 58 are held in receptacles on the 
inside of the bushing 51. Thus, two balls 58 are each located in one 
longitudinal groove 57. By means of the longitudinal grooves 57 and the 
balls 58 are held in the bushing 51 a push joint is provided between the 
driven wheel 34 and the pressing rod 54. Thus, these parts can move 
relative to each other only in axial direction. 
The clutch disk 66 of the first clutch half 33 includes a hub 65 for 
engaging square portion 55. The length and width of square portion 65 is 
smaller than the diameter of the cylindrical part of the pressing rod 54. 
The clutch disk 66 is surrounded by a hollow-cylindrical portion 67 of the 
clutch disk 68 of the second clutch half 35. There is a predetermined 
spacing between the disks 66 and 68. Several longitudinal grooves are 
provided on the inside of the portion 67 of the clutch disk 68 and on the 
outside of the hub 65 of the clutch disk 66. The grooves are distributed 
over 360.degree.. Each clutch disk 66, 68 is coupled with two clutch 
plates 69 which overlap in the space between the two clutch disks. A 
friction lining 98 is arranged between two clutch plates 69 each and 
between each clutch disk and the neighboring clutch plate. Thus, the first 
clutch half 33, which together with the driven wheel 34 executes a rotary 
movement, is connected to the clutch half 35 in a non-positive manner. 
Therefore, the clutch disk 68 of the second clutch half 35 is rotated, 
which, due to the screw joint between it and the housing 36, is adjusted 
in axial direction as well. Clutch disk 68 serves as a servo member acts 
upon the output rod 31 moving rod 31 in axial direction. When the brake 
pedal is swivelled further, the first clutch half follows the movement of 
the second clutch half. Thus, the strength of the coupling is maintained. 
Finally, the counterforce on the output rod 31 becomes so great that the 
torque necessary to overcome this force can no longer be transmitted by 
the clutch because the frictional connection is to small. Then clutch 
slippage occurs. The driver obviously does not desire a stronger braking 
at the time being. Only a further swivelling of the brake pedal could 
strengthen the frictional connection between the two clutch halves, so 
that greater torque can be transmitted and the output rod 31 can be 
adjusted further. 
If the electric motor of the servo mechanism shown in FIG. 1 should fail, 
braking can be effected with the power of the foot alone. The force acting 
on the brake pedal is transmitted to the output rod 31 via the input rod 
30, the intermediary member 56, the pressing rod 54, the clutch disk 66, 
the clutch plates 69, the friction linings 98 and the clutch disk 68. An 
axial adjustment of the clutch disk 68 is made possible in that the rods 
71 can penetrate the disk 68. Thus, an overrunning clutch in the screw 
joint between the clutch disk 68 and the housing 36 is provided in a 
simple manner. 
When the braking process is terminated the clutch 32 and the pressing rod 
54 are, with the help of pressure spring 73, again brought into the 
position shown. The output rod 31 is readjusted together with the piston 
of the master brake cylinder. Of course, the restoring force in the brake 
system also contributes to resetting of the clutch and of the pressing rod 
54. However, this contribution is relatively small, because the restoring 
forces within the brake system are largely used up for resetting of other 
parts. 
The power brake of FIG. 2 differs from that of FIG. 1 mainly by a reverse 
sequency of driven wheel 34, first clutch half 33 and second clutch half 
35 in such a way that now the clutch disk 66 of the first clutch half 33 
acts upon the output rod 31 and in that the rods 71 of the screw joint 37 
between the clutch disk 68 of the second clutch half 35 and the housing 36 
are subject to compressive stress. As in FIG. 1 the driven wheel 34 has 
two portions 51 and 53 in axial direction being located one behind the 
other, which portions are connected by a flange 52. But now, unlike to the 
example of FIG. 1, the driven wheel is supported on the slide bearing 50 
by the hollow cylinder 53 which on its outside is provided with a toothing 
mating with the worm 40. Unlike FIG. 1 the slide bearing 50 is now located 
on the end wall 46 of the housing 36. Beginning at the flange 52 the 
bushing 51 traverses nearly the entire housing. Bushing 51 is provided 
with a total of four longitudinal slots 80, which are opposite to each 
other and extend to the flange 52. The clutch disk 66 of the first clutch 
half 33 has a total of four spokes 81, which are positioned in the 
longitudinal slots 80 and which interconnect the ring-shaped parts 82 and 
83 inside and outside of the bush 51. Via the ball bearing 70 the clutch 
disk 66 acts upon the output rod 31 by means of the inner ring 82. A push 
joint is formed between the driven wheel 34 and the first clutch half 33 
by the longitudinal slots 80 in the bush 51 and the spokes 81 guided 
therein. 
In addition to the clutch disk 68 the second clutch half 35 has a bell 
portion 84 which is functionally comparable to the pressing rod 54 of FIG. 
1 as it also makes the connection between a clutch disk and the input rod 
30. Between the bell 84 and the input rod 30 is the intermediary member 
56. The bell 84 overlaps the bushing 51 of the driven wheel 34 and, at its 
lower end, widens flange-like to the outside of the clutch disk 68. The 
clutch plates 69 and the friction linings 98 are arranged between the 
clutch disks 66 and 68 as in FIG. 1. 
In contrast to the structure of FIG. 1, the rods 71 with the balls 72 on 
their ends extend between the clutch disk 68 and the front 47 of the 
housing 36. During a braking process the rods 71 press the entire clutch 
and the output rod towards the brake master cylinder. Thereby, the clutch 
disk 66 represents the servo member acting upon the output rod 31. As FIG. 
2 shows, in the lower part of which the servo mechanism is shown in the 
rest condition and in the upper part in the condition of full braking, the 
two moving joints of a pressing rod 71 on the clutch disk 68 and on the 
front 47 of the housing 36 are not in alignment in the condition of full 
braking. This feature of design ensures that the coupling 32 and the 
intermediary member 56 can without difficulties be reset by the pressure 
spring 73, when the brake pedal is released. 
In order to ensure that in the version according to FIG. 2 the braking can 
also be effected, when the electric motor 39 fails, it is provided that at 
least one guide groove originates from a moving joint of each pressing rod 
71, in which guide groove the rod 71 is captured. These guide grooves are 
not shown in FIG. 2. There can be a guide groove on only one moving joint 
of each pressing rod. It is however, possible to provide guide grooves on 
both moving joints. These guide grooves do not impair the servo assistance 
in anyway, because the pressing rods can support on the end of each 
groove. When the servo assistance is blocked, the respective ball 72 can 
be drawn away from the supporting end of a groove. 
Because the force exerted by the connection rods 71 in axial direction is 
greater with a larger angle of inclination towards the clutch disk 68, the 
assistance of the power brake according to FIG. 2 is greatest, when the 
greatest brake effort is required. This is the non-linearity of the screw 
joint 37 of FIG. 2 might be more advantageous than that of the screw joint 
37 of FIG. 1. 
Also the readjusting spring 73 of FIG. 2 is arranged differently that that 
of the example according to FIG. 1. The readjusting spring of FIG. 1 is 
supported with its one end on a stationary part, namely on the front wall 
46 of the housing 36, and with its other end on a rotating part, namely on 
the clutch disk 68, and the spring is thereby twisted. The readjusting 
spring 73 of the embodiment according to FIG. 2 rests against the clutch 
disk 66 of the first clutch half 33. Thus, the two ends of spring 73 in 
the embodiment of FIG. 2 rotate synchronously. 
In comparison to the servo mechanisms of FIGS. 1 and 2, the sequency of 
screw joint 37 and push joint 38 is reversed in the embodiments of FIGS. 3 
and 4, if one regards the driven wheel 34 as the starting point. The screw 
joint 37 is located between the driven wheel 34 and the first clutch half 
33. The push joint 38 is arranged between the second clutch half 35 and 
the housing 36. Because the second clutch half 35 does not carry out a 
rotary movement, it can be coupled without an intermediary member directly 
with the input rod 30. 
As in the embodiment according to FIG. 2, the driven wheel 34 is supported 
in the area of the front face 46 and is arranged in the same manner with 
the portions 51, 52 and 53. At the bushing 51 the driven wheel 34 is 
connected with the first clutch half 33 via the screw joint 37. Because of 
this screw joint a force to the right acts upon the driven wheel 34 when 
the clutch 32 and the output rod 31 are displaced to the left, and the 
driven wheel has to be stably supported in right-hand direction. This 
support is provided by a ball bearing 91, which also acts as a radial 
bearing and as axial bearing for the hollow cylinder 53. The inner ring 92 
of the ball bearing 91 lies with two flange-like outwardly directed eyes 
93 between the heads of the two screws 94 and the front wall 46 of the 
housing through which the screws 94 project to the outside. When the brake 
system is mounted, the brake master cylinder and the housing 36 of the 
servo mechanism are coupled by these screws. The force which seeks to draw 
the driven wheel 34 to the right directly acts upon the screws 94 and 
thereby on the brake master cylinder. The result is the housing 36 is 
relieved of this force. 
The first clutch half 33 has a bell 95 which surrounds the bushing 51 of 
the driven wheel 34. At the open end of the bell 95 not facing the input 
rod 30 there is the flange-like outwardly directed clutch disk 66. Over 
the bell 95 of the first clutch half 33 the bell 84 of the second clutch 
half 35 is positioned. A universal ball joint 96 receives the ball-shaped 
end 97 of the input rod 30. The clutch disk 68 is again formed by an 
outward flange at the open end of the bell 84. In a hollow-cylindrical 
portion 67 of clutch disk 68 clutch plates 69 are inserted between which 
the friction linings 98 are positioned. The package of clutch plates 69 
and friction linings 98 is acted upon by the clutch disks 66 and 68. 
In two places diametrically opposed to each other two pins 99 are fastened 
on the portion 67 of the second clutch half. Pins 99 are pointing in 
radial direction. On each pin 99 the inner ring of a ball bearing 100 is 
firmly seated. The ball bearing 100 is guided in a longitudinal groove 101 
of the housing 46. The longitudinal grooves 101 and the ball bearings 100 
provide the push joint 38 between the second clutch half 35 and the 
housing 36. The push joint 38 functions as a rocker joint. 
The readjusting spring 73 is supported in the two versions of FIGS. 3 and 4 
at its one end on the housing 36 and at its other end on a ring-shaped 
sheet 102 which is riveted to the front side of the hollow-cylindrical 
portion 67 of the second clutch half 35 and which projects inwardly below 
the side of the clutch disk 66 not facing the friction linings. Twisting 
of the spring 73 is avoided because the housing 36 and the second clutch 
half 35 are interconnected by a push joint. When a braking operation is 
terminated the readjusting of the first clutch half is ensured 
simultaneously, because the sheet 102 and the clutch disk 66 overlap. 
The two power brakes of FIGS. 3 and 4 differ primarily in the design of the 
screw joint between the driven wheel 34 and the first clutch half 33. In 
the example of FIG. 3 and the bushing 51 of the driven wheel is provided 
with a quadruple thread on its outside surface. In each thread a ball 111 
is located as a connecting element. Half of each ball projects over the 
outer surface of the bushing 51 and grips in one of four longitudinal 
grooves 112 on the inside of the bell 95 of the first clutch half 33. Each 
pair of the four longitudinal grooves 112 are diametrically opposed to 
each other. 
In the power brake according to FIG. 4, the second clutch half includes a 
stud 113 projecting into the interior of the bushing 51 of the driven 
wheel 34. Stud 113 is connected to the end of the bell 95 and extends 
within the bell 95 towards the clutch disk 66. The outer surface of the 
stud 113 is developed as a quadruple thread 110, having an angle of 
inclination of 45.degree.. The longitudinal grooves 112 are located on the 
inner side of the bushing 51. Again half of each ball is located in a 
thread 110 and the other half in a longitudinal groove 112. Unlike FIG. 3 
the output rod 31 is not acted upon by the end of the bell 95, but by the 
front end of the stud 113. By moving the screw joint into the interior of 
the bushing 51 the radius of the thread 110 is smaller that that of the 
version according to FIG. 3. Thus, the angle of inclination may be 
increased to 45.degree.. This is an angle of inclination with a very good 
power transmission efficiency factor. 
When the brake pedal is actuated the electric motor 39 is switched on via a 
switch on the pedal, so that the driven wheel 34 is put into a rotary 
movement. If the pressure exerted by the brake pedal is very low, the 
first clutch half 33 is taken along by the driven wheel via the screw 
joint 37. Because of the low pressure the friction linings 98, the clutch 
plates 69 and the clutch disks 66 and 68 can move towards each other with 
very low friction. If the pressure initiated by the brake pedal is 
increased, then the two clutch halves 33 and 35 are coupled with each 
other. Thereby, the clutch half 35, which is static in direction of 
rotation, brake the first clutch half 33. This clutch half can no longer 
follow the rotary movement of the driven wheel 34, so that its movement 
receives a component in axial direction. Thus, the tappet stud 113 presses 
the output rod 31 towards the master brake cylinder. Thereby the force 
delivered by the servo mechanism adds to that which is initiated by the 
brake pedal. The readjustment of the various parts from the position shown 
in the upper half of FIG. 3 into the positions they occupy in FIG. 4 and 
in the lower half of FIG. 3 is effected via the conical pressure spring 
73. 
If the electric motor 39 fails completely the entire braking force must be 
generated by the brake pedal. For this reason longitudinal grooves 112 are 
provided in the bushing 51. The longitudinal grooves 112 are pushed over 
the four balls 111, so that it is possible to move the clutch 32 and the 
output rod 31. 
If one exchanges the push joint 38 and the screw joint 37 with each other 
one obtains an alternative to the embodiments of FIGS. 3 and 4. So for 
instance one could develop the guides for the ball bearings 100 as 
threads, so that the screw joint is located between the housing 36 and the 
second clutch half 35. Because of the push joint only axial movement would 
then be possible between the driven wheel 34 and the first clutch half. 
In FIGS. 5 to 9 some piece parts of the power brake assembly of FIG. 4 are 
shown separately. FIGS. 5 and 6 respectively show the tappet 113 in a 
lateral view and in a right-hand end view. The right end is screwed to the 
bell 95. The right end is designed as a pivot with cheeks 114 which can be 
inserted into an appropriately formed opening at the top surface of the 
bell 95, so that the connection between the bell 95 and the tappet 113 is 
protected against twisting. The diameter of the pivot with cheeks 114 is 
smaller than the diameter of bushing 51. Bushing 51 engages the bell 95 
when it is inserted therein. Therefore, the balls 111 are captured between 
bushing 51 and the right-hand end of the tappet 113. An annular groove 115 
is provided in which a locking ring is inserted at the other end. The four 
turns of the thread 110 can be seen in the view according to FIG. 6. 
In FIGS. 7 and 8 which illustrate the driven member 34, the longitudinal 
grooves 112 are to be easily recognized. Grooves 113 have an approximately 
semicircular cross-section. FIG. 7 shows that the bushing 51 of the driven 
wheel 34 is provided with an annular groove 116 at a spacing from the end. 
From this annular groove 116 to the front side 118, the bushing 51 has a 
larger inside diameter than the portion on the other side of the annular 
groove 116. When one inserts the tappet 113 with the four balls 111 in the 
bushing 51, one can press a locking ring into the annular groove 116 
because of the enlarged inside diameter between the tappet 113 and the 
bush 51. Thus the driven wheel 34 and the tappet 113 with the bell 95 are 
captured to each other. 
FIG. 9 shows a left end view of the bell 84 with the clutch disk 68 and the 
hollow-cylindrical portion 67. Clearly shown are the longitudinal grooves 
117 in the hollow-cylindrical portion 67, into which the clutch plates 69, 
which are to be connected with the bell 84 in a manner protected against 
twisting, can be inserted and secured by outwardly directed studs. Thereby 
a protection against twisting and the axial movability is ensured. 
FIGS. 10 and 11 show two embodiments of a power brake servo mechanism 
according to the invention in which the friction linings and the clutch 
disks are located within the hollow driven wheel 34. Driven wheel 34 has a 
first hollow-cylindrical portion 53 at which it is rotatably mounted on a 
rolling bearing 70 and carries a toothing. This first portion 53 is 
followed by a cup-like second portion 125 with an even bigger diameter, 
which portion extends nearly to the front side 47. In the area of the cup 
portion 125, the driven wheel 34 is coupled with the first clutch half 33 
via the push joint 38. This first clutch half consists mainly of a 
ringshaped clutch plate 69 at whose outer pin several pins 99 are fastened 
on which the inner ring of a ball bearing 100 is seated. These ball 
bearings 100 are guided in longitudinal grooves 101 in the interior of the 
cup 125. Thus the push joint 38 between the driven wheel 34 and the first 
clutch half 33 is built up in the same manner as the push joint between 
the second clutch half 35 and the housing 36 in the embodiments of FIGS. 3 
and 4. 
In the versions according to FIGS. 10 and 11 the second clutch half 35 has 
two clutch disks 126 and 127 which each are provided with friction linings 
98 resting against the clutch plate 69. The first clutch disk 126 is made 
in one piece with a hub 128 extending through the ringshaped clutch disk 
127. Four balls 129, are each guided in a recess 130 formed by a groove on 
the outside of the hub 126 and on the inside of the clutch disk 127. With 
this arrangement, the two clutch disks are coupled to each other in a 
manner protected against twisting, but which permits movement in an axial 
direction. 
In the embodiment of FIG. 10 the hub 128 of the clutch disk 126 is 
elongated beyond the side of the clutch disk 126 not facing the clutch 
disk 127. Starting from its end pointing to the front wall 46 of the 
housing 36 the following parts are arranged in the hollow interior of the 
hub 128: a locking ring 131 is located in an annular groove. Behind it 
follows a flange 132 at the rear end of the output rod 31. Thereafter, 
follows an axial needle bearing 133. Bearing 133 is acted upon a spindle 
134. The left end of the spindle 134 is formed as a thread head 135 of an 
overrunning clutch. This overrunning clutch includes rollers 140 which are 
located between the thread head 135 and the hub 128. In an annular groove 
of the hub 128 a further locking ring is inserted behind the thread head 
135. In the space between the spindle 134 and the hub 128, following the 
locking ring 141, there is a bushing 142 having an internal thread 143. 
Thread 143 has four turns as does the thread 144 of the spindle 134. Balls 
145 couple both threads with each other. The balls 145 are secured within 
the bushing 142 by the two rings 146. The bushing 142 projects beyond the 
rear end of the hub 128 and is firmly connected with the housing by 
various extension arms 147 separated by intermediary spaces. Through these 
spaces the intermediary member 56 grips with various fingers 148. 
Intermediary member 56 acts upon the clutch disk 127 via an axial ball 
bearing 149 located between the clutch disk 127 and the extension arms 
147. The intermediary member is captured by the locking ring 150. 
The design of the second clutch half with the clutch disks 126 and 127 and 
the spindle 134 in the embodiment of FIG. 11 corresponds largely to that 
of FIG. 10. Therefore, only the differences are described. Between the 
axial rolling bearing 133 and the spindle 134 a pressure ring 151 is 
inserted in a groove of the hub 128. Thus, clutch disk 126 of the second 
clutch half 35 acts upon the output rod 31 independently of the spindle 
134. The bushing 142 is held in a bulge on the front side 47 of the 
housing 36 and does not have extension arms. Again the connection between 
the input rod 30 and the clutch disk 127 is provided by a rolling bearing 
149 via the intermediary member 56. In this embodiment, the member 56 is 
made in two pieces. It consists of a cap 153, which receives the input rod 
30 in a socket and whose lower rim is provided with recesses, and of a 
finger member 154. Finger member 154 rests upon a rolling bearing 149 with 
a ring and grips through openings 155 in the front wall 47 of the housing 
36 into the recesses at the cap 153 with various fingers separated from 
each other. The two parts 153 and 154 are firmly interconnected by the 
ring 156. 
In the embodiment of FIG. 11 the driven wheel 34 at its end facing the 
front side 47 of the housing 36 is supported via a rolling bearing 157 
both in the axial and in the radial direction. Pressure springs 158 are 
inserted in recesses of the clutch disk 126 and have the tendency to force 
apart the two clutch disks 126 and 127 of the second clutch half 35. 
In FIGS. 12 to 17 parts of the embodiment of FIG. 11 are separately shown. 
In the views of the clutch disk 126 in FIGS. 12 and 14 are shown the 
recesses 165 for the pressure springs 158 and on the outer surface of the 
hub 128 is shown the longitudinal grooves 166 for the balls 129. Grooves 
166 also penetrate the disk 126 as bores 167. In FIGS. 13 and 14 the 
annular groove 168 for receiving the locking ring 131 and the annular 
groove 169 for receiving the pressure ring 151 are shown. FIG. 13 shows a 
groove 170 on the inside of the hub 128 and a groove 171 on its outside. 
The annular groove 171 serves to receive the locking ring 141. The outer 
annular groove 171 receives a locking ring for the rolling bearing 139. 
In view of the clutch disk 127 shown in FIG. 15, the longitudinal grooves 
172 are shown which are opposite the longitudinal grooves 166 on the hub 
128 and together with grooves 166 form the recesses 130 for the balls 129. 
The view of the bushing 142 in FIG. 16 shows clearly the four turns of the 
thread 143. 
FIG. 17 shows a view of the spindle from the direction of the input rod 30 
showing the four turns of the thread 144 and the thread head 135 in the 
outer rim of which conical recesses 173 are brought in. In the bores 174 
pressure springs are inserted which act upon the rolling bearing 140. 
During a braking operation the movements of the apparatus of FIGS. 10 and 
11 are similar to those of FIGS. 1 and 2. As soon as the brake pedal is 
operated the electric motor is switched on, which drives the driven wheel 
34. The driven wheel 34 also starts a rotary movement of the first clutch 
half 33 with the clutch plate 69. When the pressure exerted by the brake 
pedal is great enough the clutch engages, so that the clutch plate 69 
takes along the clutch disks 126 and 127. Via the overrunning clutch the 
spindle 134 is rotated which due to the screw joint between itself and the 
bushing 142 carries out a movement in axial direction. Thereby it also 
moves the clutch disk 126 and the output rod 31 to the left. The clutch 
plate 69, the clutch disk 127, the rolling bearing 149 as well as the 
intermediary member 56 and the input 30 are pushed into the same direction 
by the driver via the brake pedal. If the force exerted on the brake pedal 
is reduced the clutch 32 disengages and the pistons of the brake master 
cylinder can readjust all parts moved, by means of the readjusting spring 
73. Because of the disengaged clutch the clutch disks 126 and 127 can 
assume a relaxed position together with the spindle 134 independently of 
the motional condition of the clutch plate 69. 
If the electric motor blocks for any reason, in the structure of FIG. 10, 
the first initiated via the brake pedal is transmitted via the input rod 
30, the intermediary member 56, the axial ball bearing 149, the clutch 
disk 127, the clutch plate 69, the clutch disk 126, the pressure ring 141, 
the spindle 134 and the axial needle bearing 133 to the output rod 31. 
Because the driven wheel 34 is thereby at rest, the clutch plate 69 can 
carry out no rotary movement nor can clutch disks 126 and 127 due to the 
frictional connection at the clutch. The spindle 134, however, can carry 
out a screw motion due to the overrunning clutch between itself and the 
clutch disk 126. 
In the structure of FIG. 11 the power transmission is carried out in a 
similar manner as in FIG. 10. Because the pressure ring 151 is, however, 
arranged before the spindle 134 and the force is directly transmitted from 
the clutch disk 126 and the hub 128 via the pressure ring 151 to the 
output rod by circumventing the spindle 134. The friction between the 
rotating spindle 134 and the other parts, which are only moved in axial 
direction, becomes smaller. The screw motion of the spindle 134 can 
therefore easily be effected. 
It is possible to design the power brake in such a way that the spindle 134 
is at rest, when the braking is effected without servo assistance. This 
would, however, result in a much larger power brake. If the blocked motor 
becomes suddenly unblocked during a braking operation the spindle 134 
would also hit the parts positioned before the thread head 135 with great 
velocity. 
The two power brakes according to FIGS. 18 and 19 have also a housing 36 
with the jacket 45, the front side 46 and the front side 47. A 
pressure-sensitive element or sensor 180, is located between the input rod 
31 and the output rod 30. The sensor can for instance be a wire strain 
gauge. The sensor 180 is connected to an electronic circuit 182 via a lead 
181. Electronic circuit 182 controls the electric motor 39 in such a way 
that the torque generated at any time is proportional to the force of 
pressure measured by the sensor. The accessory drive unit 39 has a 
rotating output 34 connected to the output rod 30 via a screw joint 37. 
Between the output 34 and the output rod 30 an overrunning clutch 183 can 
be fitted. The rolling-push joint 38 between the output rod 30 and the 
housing 36 has the effect that the output rod can only move in axial 
direction. A rotary movement between the output rod and the housing is not 
possible. 
In the structure of FIG. 18, a conventional electric motor is used on whose 
motor shaft a pinion 184 is seated. This pinion mates with the toothed 
wheel 34. If the brake pedal is now actuated, in accordance with the 
pressure received the sensor transmits a value to the electronic circuit 
182, which supplies the motor 39 with a particular voltage. The pinion and 
with it the toothed wheel begin to rotate. Via the overrunning clutch 183 
the bushing 185 is carried along too. At the inside of bushing 185 there 
is a thread of the screw joint 37. Because it is not possible to twist the 
output rod, the output rod is moved ahead. The counterforce rises until it 
finally reaches a value which is greater than the force which is generated 
by the brake pedal and the motor in the direction of adjusting. The motor 
therefore blocks and the output rod is not adjusted further. If the brake 
pedal is swivelled further, the sensor detects a higher pressure, the 
electronic circuit 182 would supply the motor with a higher voltage so 
that the wheel 34 could be rotated again and the output rod 30 could be 
axially adjusted. After termination of the braking operation the spring 73 
readjusts the output rod 31. The wheel 34, the pinion 184 and the armature 
of the motor 39 must be able to rotate. Thus, the motor may not be 
self-locking. Even in case the motor fails, this means the entire braking 
force has to be provided via the brake pedal. It would be possible to 
adjust the output rod 30 without the overrunning clutch 183. The 
overrunning clutch is of advantage, because when the electric motor fails 
or the brake is operated very rapidly the braking can be effected without 
additional resistance. 
In the structure of FIG. 19, the rotor of the electric motor is directly 
articulated to the output rod 31. Therefore, it represents the output 34. 
The rotor is permanent magnet 186. Diametrically opposed to each other, 
two coils 187 are fixed on the jacket 45 of the housing 36. A core of 
ferro-magnetic material is located in the interior of the coils. The coils 
are wound in such a way that the magnetic north of one coil points into 
the interior of the housing and the magnetic south of the other coild 
points into the interior. When the brake is not operated and the 
readjusting spring has brought the output rod into the position shown, the 
south pole of the permanent magnet 186 is located opposite the coil, which 
when energized generates a magnetic field with an inwardly directed south 
pole. The same is valid for the magnetic north of the permanent magnet 186 
and the second coil. During a braking operation the coils 188 are supplied 
with a particular voltage depending on the force initiated by the brake 
pedal and further dependent on the pressure generated by said force and 
measured by the sensor 180. Therefore a torque acts on the permanent 
magnet 186, so that it rotates. This rotation leads to an axial adjustment 
of the output rod 31 via the screw joint 37 and the push joint 38. 
FIG. 20 is a simplified section of a motor vehicle comprising the engine 
195. This engine drives the power brake 197 via a connecting element 196; 
thus it serves as a motor-driven accessory drive. No auxiliary motor is 
necessary for the power brake. A flexible shaft, a multi-jointed shaft, a 
V-belt or another known technical element could be used as a connecting 
element 196 for transmission of motion.