Vacuum brake booster

In prior art vacuum brake boosters the partition wall is supported by a control hub. The boosting force is transmitted through the hub to a force transmitting serving to actuate the master cylinder. According to the present invention the partition wall includes a central sleeve of a stepped configuration which serves to transmit the boosting force without loading the hub. As a result the hub can be produced from a thermoplastic material of just sufficient strength to meet the reduced force transmitting requirements and which is easier to mold.

BACKGROUND OF THE INVENTION 
The present invention relates to vacuum brake booster for motor vehicles 
comprising a low pressure housing which is divided into a vacuum chamber 
and a working chamber by a movable rigid partition. The partition is 
connected to an axially movable control hub containing therein a valve 
arrangement with the control hub acting via a reaction device on a force 
transmitting element. 
From French Pat. No. 2,118,925 a vacuum brake booster has become known in 
which the movable partition is fixed on the control hub. The control hub 
in the vacuum chamber has a shoulder around the circumference thereof and 
the partition positively abuts the shoulder in the direction of power 
delivery. By a circlip which positively engages the control hub, the 
partition and a rubber diaphragm adjoining the partition is held at the 
shoulder of the control hub opposite the direction of power delivery. The 
depth of engagement of the circlip into the control hub is such that it 
engages a groove of the valve piston of the valve arrangement and, thus, 
limits the axial motion of that valve piston. 
The force transmitting element is coaxial of the control hub and sealingly 
guided out of the vacuum chamber and is supported by the front surface of 
the control hub by means of a rubber-like reaction disc. The reaction disc 
lies opposite to the valve piston with a small clearance and, thus, can 
receive the vacuum forces transmitted by the partition onto the control 
hub and also that part of the foot or pedal forces which are produced by 
the piston bar mechanically connected to the valve piston. 
In an arrangement of this type the control hub must be constructed in such 
a way that it can transmit the vacuum forces produced onto the force 
transmitting element without being damaged. For this purpose, on one hand, 
an expensive resistant material having a high resistance to pressure must 
be used for the production of the control hub, and, on the other hand, the 
element of the control hub having the shoulder in the vacuum chamber must 
be made from such a strong material that the air channels arranged in the 
control hub do not create any critical weak areas regarding stress. 
From German Pat. No. DE-AS 2,365,903 another vacuum brake booster is known 
in which the partition itself is constructed as the reaction device which 
adds up the vacuum forces and the foot forces and transmits them to the 
force transmitting element. 
The force transmitting element is a stepped piston bar whose smaller 
diameter portion is guided in a bore of the valve piston of the valve 
arrangement. 
The valve piston carries a circlip which comes into engagement with the end 
of the control hub facing the vacuum chamber and which axially secures the 
piston opposite the power direction. The partition is radially segmented, 
is supported by the valve piston and acts on the force transmitting 
element via a supporting plate which is slightly arched. The supporting 
plate abuts the step of the smaller portion of the force transmitting 
element in a form-locking manner and is kept in a defined position 
relative to the control hub and to the partition by spacers projecting 
through the partition. The supporting plate abuts the partition only by 
means of its outer circumference. By the rubber diaphragm which is 
buttoned onto a shoulder of the control hub and is fixed in the wall of 
the low pressure housing, the partition of the working chamber and vacuum 
chamber is effected. With operation of a pedal mechanically connected to 
the valve piston, the vacuum forces acting on the partition will act on 
the force transmitting element via the supporting plate. As a result the 
partition will conically deform and move the valve piston against the 
force of the pedal axially opposite to the power direction in order to 
transmit a reaction force relative to the output force onto the pedal. 
In this brake booster the control hub is relieved to a large extent of the 
operating forces, since the partition is constructed as the reaction 
device. However, construction of the brake booster is complex due to the 
many components necessary. 
SUMMARY OF THE INVENTION 
Therefore, an object of the present invention is to provide a vacuum brake 
booster which has a simple construction, keeps the control hub free of the 
forces, which due to the produced pressure difference, act on the 
partition, allows a simple and, consequently, an advantageous assembly 
regarding costs and can transmit the operating force onto the force 
transmitting element without loading the control hub. 
A feature of the present invention is the provision of a vacuum brake 
booster comprising: a housing having a longitudinal axis; a movable rigid 
partition wall disposed in the housing coaxial of the axis to divide the 
housing into a working chamber and a vacuum chamber; a control hub 
containing a valve arrangement therein disposed in the housing coaxial of 
the axis, the control hub being axially movable in the housing and acts 
via a reaction device on a force transmitting element; and a sleeve 
disposed in the housing coaxial of the axis connected to the control hub 
and the partition wall and supported by the force transmitting element. 
Thus, it is possible to produce the control hub of an inexpensive material 
which must meet only very negligible requirements. Since the sleeve is 
made of a compression-proof material capable of loading, the use of 
expensive material of high quality is reduced to a minimum. 
Since the sleeve is made of a metal, stops can be attached by cold 
deformation so that all elements which are interlocking can be axially 
secured. When the control hub is made of a synthetic material which can be 
sprayed, e.g. thermoplast, the production of the control hub can employ 
cheaper materials and a shorter production period can be achieved. 
Apart from that, when the control hub is produced of a thermoplastic 
material all undercuts and locking profiles can be integrally molded and 
into these undercuts supporting discs can be pressed to increase stability 
and precision of form. Furthermore, a saving of weight can be achieved 
through an appropriate choice of material. 
If the sleeve and the partition form one constructional unit, it would be 
possible to stamp the sleeve out of the middle of the partition. The 
control hub could then be attached by gluing or shrinkage. 
When the control hub of a vacuum brake booster contains a valve arrangement 
including a poppet valve and valve piston to be operated by a piston rod, 
the valve piston can be conveniently sealed in the sleeve. All radial 
forces produced by the piston rod are directly transmitted onto the sleeve 
and do not act on the control hub. 
When the sleeve is of a stepped construction the valve piston slides in the 
portion thereof having a smaller diameter and the force transmitting 
element is guided in the portion thereof having a larger diameter. The end 
of the force transmitting element adjacent the sleeve bears against the 
reaction unit disposed in the larger diameter portion of the sleeve which 
in turn is supported by the step formed between the two portions of the 
sleeve. This construction enables the piston rod and the valve piston to 
be aligned exactly coaxially with the force transmitting element. 
The valve piston is axially secured by a stop disposed in the portion of 
the sleeve having the smaller diameter. At the same time it is possible to 
dimension precisely the idle path of the vacuum brake booster by means of 
a precise, axial arrangement of the stop, which is made only during the 
assembly and does not require any additional elements. Owing to the fact 
that the sealing diaphragm is axially arranged between the partition and 
the control hub which are positively clamped together by the inserted 
sleeve, fastening of the diaphragm is easy and the diaphragm also acts as 
a seal between control hub and partition. 
For air circulation, the connection between the vacuum chamber and valve 
arrangement in the control hub is achieved by a bore disposed in the 
surface of the partition adjoining the control hub. 
The assembly of the arrangement is simplified by constructing the radial 
opening for air circulation in a plane axially arranged at the valve 
piston so that an implement can be run in for placing the stops. One 
assembly step as well as additional elements can be saved. 
By constructing hub and sleeve in such a way that a projection of the 
control hub positively engages a groove of the sleeve or a projection of 
the sleeve positively engages a groove of the control hub, a firm seat for 
the sleeve is guaranteed without any additional means. By gluing the 
sleeve into the control hub, a fixed coherence is also guaranteed. 
Furthermore, by modern welding processes, such as ultrasonic welding, the 
sleeve can be tightly fixed in the control hub against atmospheric 
pressure.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, the low pressure housing of the vacuum brake booster 
includes a pot 1 and a cover 2. 
A partition 4 axially movable together with the control hub 3 divides the 
low pressure housing into a working chamber 5 and a vacuum chamber 6. A 
rubber diaphragm 7, the outer border 8 of which is sealingly clamped 
between pot 1 and cover 2 of the low pressure housing and the internal 
border 9 of which is sealingly clamped in control hub 3, abuts partition 4 
on the side of partition 4 facing working chamber 5 and, thus, seals 
vacuum chamber 6 from working chamber 5. Partition 4 is positively held at 
the front end of control hub 3 facing away from the direction of force 33 
by the flange 10 of a sleeve 11 screwed into control hub 3. Thereby 
partition 4 firmly presses internal border 9 of rubber diaphragm against 
control hub 3. Internal border 9 lies in a circular groove 12 arranged on 
the front end of control hub 3 opening toward the power delivery direction 
33. Inserted sleeve 11 is a stepped sleeve whose smaller diameter portion 
14 projects into the interior of control hub 3. The force transmitting 
element 16 is guided in the larger diameter portion 13 of sleeve 11. 
Element 16 is supported by step 15 between portions 13 and 14 of sleeve 11 
via a reaction disc 17. Force transmitting element 16 extends through the 
bottom of pot 1 to the outside and is sealed thereto by a seal 18 
frictionally held in a stepped portion in the bottom of pot 1. On the 
other side of the housing, control hub 3 projects through cover 2 and is 
axially opened to the atmosphere. By means of a seal 19 locked in cover 2, 
working chamber 5 is sealed against the atmosphere. 
The interior of control hub 3 is hollow and essentially has only one step 
20 between the smaller and larger diameter portions of the air chamber 21 
which is open to the atmosphere. Step 20 serves as a sealing edge for the 
valve arrangement inserted into air chamber 21 which is present on the 
right and on the left of step 20. On the left of step 20 the valve piston 
22 is arranged in section 14 of sleeve 11 having projections 23 which 
limit the axial movement of valve piston 22. Valve piston 22 is moveable 
and is clamped onto the wall ball head of a stepped piston rod 24 without 
any axial clearance. Piston rod 24 projects through air chamber 21 and is 
connected to an operating pedal (not shown). A disc valve 25 is inserted 
into air chamber 21 to the right of step 20 and piston rod 24 projects 
through valve 25 with clearance. 
By a step 26, disc 27, having a cross-section in the form of a z and having 
an axial aperture, is kept in its position in control hub 3 and is 
sealingly pressed against the interior wall of air chamber 21. Within disc 
valve 25, a sleeve 28 is inserted which is pressed against valve disc 32 
stiffened by an insert 31 by a spring 29 supporting a first step 30 of 
piston rod 24. Piston rod 24 has a force applied thereto opposite to the 
direction of force 33 by another spring 46 acting between another step 34 
of piston rod 24 and disc 27 so that valve piston 22 mechanically 
connected to piston rod 24 comes into sealed engagement with valve disc 32 
by means of its bead-shaped outer border 35 and, thus, separates air 
chamber 21 from the interior of the booster including a chamber 36. This 
separated chamber 36 is to the left of step 20 and via two openings 37 
disposed in control hub 3 is connected to working chamber 5. Chamber 36 is 
also connected to the right of step 20 to vacuum chamber 6 via two 
channels 38 in control hub 3 disposed radially outside and above step 20, 
two openings 39 in rubber diaphragm 7 and two bores 40 in partition 4. 
The rest position of the vacuum brake booster shown in FIG. 1 is determined 
by the spring 41 which shifts partition 4 opposite to the power delivery 
direction until the knub 42 of rubber diaphragm 7 abuts cover 2 of the low 
pressure housing. In the rest position of the pedal the air supply to 
working chamber 5 is blocked by valve piston 22 coming into engagement 
with valve disc 32. Consequently, there is a vacuum in working chamber 5, 
since it is, as shown in the drawing, connected to vacuum chamber 6 via 
openings 37, chamber 36, step 20, channels 38 and bores 39 and 40. Vacuum 
chamber 6 is connected to a continuously working vacuum source (not shown) 
via the connection 43. 
In this embodiment control hub 3 is made of a duroplastic or metal or the 
like, so that sleeve 11 may be equipped with a thread and can be screwed 
into control hub 3. During the assembly of the vacuum brake booster the 
valve arrangement is slipped on piston rod 24 and then valve piston 22 is 
clamped on the ball head of piston rod 24. Control hub 3 is prepared to 
the extent that sleeve 11 is screwed into control hub 3 and sealed thereto 
by seal 44 inserted into control hub 3 and rubber diaphragm 7 is clamped 
between partition 4 and control hub 3. Now the prepared piston rod 24 is 
inserted into control hub 3 such that the valve head lies axially moveable 
in portion 14 of sleeve 11. Piston rod 24 is shifted into portion 14 of 
sleeve 11 until valve piston 22 butts against projections 23 in order to 
rivet a disc 45 having a diameter corresponding to the diameter of portion 
14 of sleeve 11 at the free end of valve piston 22. Thereby the axial 
movement of piston rod 24 in control hub 3 is limited. In the rest 
position of the vacuum brake booster shown in the drawing, the distance Y 
between disc 45 and reaction disc 17 represents the idle path of the 
booster. 
When the brake pedal is operated and hence piston rod 24 and valve piston 
22 are shifted to the left when viewing FIG. 1, the disc valve 25 will 
follow this movement until valve disc 32 abuts on step 20. Then channels 
38 are separated from chamber 36 and vacuum chamber 6 and working chamber 
5 are no longer connected. The vacuum brake booster is in the position of 
readiness for operation. 
When the brake pedal is further pressed down part of the force is 
transferred directly onto force transmitting element 16 via reaction disc 
17. At the same time the bead-shaped outer border 35 of valve piston 22 
will lift off of valve disc 32 and will connect chamber 36 to air chamber 
21 and, consequently, to the atmosphere. Air enters working chamber 5 and 
the developing pressure difference between working chamber 5 and vacuum 
chamber 6 tries to shift partition 4 in the direction of force 33. 
However, all forces occurring are being directly transmitted from 
partition 4 via sleeve 11 onto force transmitting element 16. Control hub 
3 remains completely free from these actuating forces. 
Consequently, lower requirements, for instance, strength requirements, are 
possible in fabricating control hub 3. This results in advantages 
regarding manufacture as well as costs. Furthermore, a saving of weight 
can be achieved which meets the requirements of modern brake boosters. 
An embodiment of this type is shown in FIG. 2. Since the construction only 
slightly differs from the one in FIG. 1 identical parts are provided with 
identical reference characters and changed parts are provided with the 
reference characters employed in FIG. 1 marked with a apostrophe. The 
essential difference is to be found in the construction of sleeve 11', in 
the fastening of sleeve 11' in control hub 3' and in the execution and 
limitation of the axial play of valve piston 22'. 
The control hub 3' e.g. is moulded of thermoplast, wherein openings 37' to 
working chamber 5 were intentionally chosen large. Additional undercuts 
26' are provided and a projection 48 is provided in the part receiving 
sleeve 11'. The advantages of this embodiment can best be explained by 
describing the steps of the assembly. 
The valve arrangement is, as already described, slipped onto piston rod 24 
and valve piston 22' is clamped thereon. During the following assembly 
step sleeve 11' is pressed into control hub 3'. Since control hub 3' is 
made from thermoplast capable of sealing, sealing ring 44 shown in FIG. 1 
can be eliminated. There are several possibilities concerning the secure 
fastening of sleeve 11'. As shown in FIG. 2 groove 47 and projection 48 
provides a positive engagement between sleeve 11' and control hub 3'. 
Alternatively, sleeve 11' and control hub 3' can be glued together. 
Additionally, it would be possible to provide this connection by shrinking 
control hub 3' onto sleeve 11' or to connect both of these construction 
units by means of modern welding processes, e.g. ultrasonic welding. 
Now piston rod 24 with the valve arrangement already preassembled is 
inserted into control hub 3', wherein valve piston 22' is placed in 
portion 14 of sleeve 11' and the z-shaped, rigid disc 27' is pressed into 
an undercut 26' for form stabilization of control hub 3' and holding of 
the valve arrangement. Further, form-stabilizing discs 49 can be pressed 
into other undercuts 26'. 
Now the assembler can insert an implement through openings 37' and 
following the adjustment to the necessary or allowed idle path y to 
produce stop 23', which engages the groove of valve piston 22' and limits 
the axial shifting path of valve piston 22' in sleeve 11'. 
Compared to the embodiment in FIG. 1 one assembly step can be saved and 
sealing ring 44 as well as disc 45 fastened on valve piston 22 are 
eliminated. It is obvious that the mechanical elements which would be 
necessary for producing a stop prohibiting an extraction of valve piston 
22 out of its guide in sleeve 11 (locking elements, clamp rings, discs) 
can be omitted and one assembly step (riveting of the disc) is saved. 
This invention also comprises brake boosters which are operated by 
compressed air or pressure fluid as well as brake boosters for actuation 
of other aggregates, e.g. clutches. 
While I have described above the principles of my invention in connection 
with specific apparatus it is to be clearly understood that this 
description is made only by way of example and not as a limitation to the 
scope of my invention as set forth in the objects thereof and in the 
accompanying claims.