Load-responsive braking pressure control device especially for vehicle brake systems

A load-responsive brake pressure control device for vehicle brake systems includes a control valve actuatable by a stepped piston. The stepped piston has one surface of a larger axially effective area which is acted upon in the valve closing direction by the pressure prevailing downstream of the valve, and another surface of a smaller axially effective area which is acted upon in the valve closing direction by the pressure prevailing upstream of the valve. A load-responsive spring acts in the opening direction of the valve on a movable component of a cylinder-and-piston unit, especially on a flange of a cylinder component of such a unit, to urge the same into its initial position in which it abuts a stop surface of a support. The cylinder and piston components of the unit together define a chamber which communicates with a flow path having a stationary portion accommodating a sealing element bounding a passage of a reduced cross section. The stepped piston carries a plunger which penetrates into the passage after a predetermined amount of movement of the stepped piston in the closing direction, so as to entrap the fluid present in the chamber of the unit. Thereafter, further movement of the stepped piston in the closing direction is hindered by the pressure of the entrapped fluid, until this pressure rises to a level at which the movable component of the unit yields in opposition to the spring force. Then, the valve will close and the pressure downstream thereof will be maintained at a level determined by the ratio of the axially effective areas of the aforementioned surfaces of the stepped piston.

BACKGROUND OF THE INVENTION 
The present invention relates to load-responsive braking pressure control 
devices in general, and more particularly to a device of this type which 
is especially suited for use in vehicle brake systems. 
In conventional devices of this type, a control valve is actuatable by a 
stepped piston which is slidable in a housing. The control valve is urged 
in its closing direction by the outlet pressure which acts on a larger 
effective surface of the stepped piston and in its opening direction by 
the inlet pressure which acts on a step of the stepped piston which has a 
relatively smaller effective surface. A load-responsive spring, in 
particular a suspension spring supporting the vehicle body on the axle, 
acts on the stepped piston in the opening direction through the 
intermediary of a transmitting device. 
In a load-responsive braking pressure control device of this type which is 
disclosed in the U.S. Pat. No. 4,150,855, a suspension spring designed as 
a compression spring bears against the vehicle body, on the one hand, and 
against a spring seat element, on the other hand. The spring seat element 
surrounds the housing for the stepped piston. The spring seat element acts 
on several radially extending levers. The outer ends of these levers are 
seated on a vehicle unsprung mass, and their inner ends engage a cap. The 
levers compress a preloading spring through a pin, and the spring loads 
the stepped piston. The provision of such levers is necessary in this 
construction to ensure that only part of the vehicle spring force is 
transmitted to the stepped piston. The transmitting device resulting 
therefrom is complicated from the structural and manufacturing points of 
view. Difficulties are involved in designing the lever ratios such that 
only a small amount of the force of the suspension spring is transmitted 
to the stepped piston; this has to be taken into account when choosing the 
size of the stepped piston. Moreover, the stepped piston is under constant 
preload. The stepped piston is thus required to be dimensioned in 
conformity with such continuous load. The stepped piston moves only upon 
attainment of the changeover pressure defined by the preload. To ensure 
timely closing of the control valve, the piston stroke until the instant 
of closing is permitted to be but short. This results in tolerance 
problems during the manufacture and assembly and in difficulties during 
the operation. 
SUMMARY OF THE INVENTION 
Accordingly, it is a general object of the present invention to avoid the 
disadvantages of the prior art. 
More particularly, it is an object of the present invention to provide a 
load-responsive pressure control device which is particularly suited for 
use in vehicle braking systems and which does not possess the 
disadvantages of the conventional devices of this type. 
Still another object of the invention is to so construct the device of the 
type here under consideration that the total extent of the stroke of the 
stepped piston can be relatively large, and yet the control valve closes 
quite rapidly after the effective pressure has risen to the change-over 
point. 
It is yet another object of the present invention to so design the device 
of the above type that the stepped piston need not be dimensioned for 
withstanding constant loading originating at the load-responsive spring. 
A concomitant object of the present invention is to develop a 
load-responsive pressure control device which is simple in construction, 
inexpensive to manufacture, and reliable in operation nevertheless. 
In pursuance of these objects and others which will become apparent 
hereafter, one feature of the present invention resides in a 
load-responsive pressure control device, especially for use in a vehicle 
braking system between an upstream and a downstream section of a conduit 
for controling the pressure in the downstream section in dependence on the 
vehicle load; this device comprising a support having a stop surface; a 
control valve interposed between the upstream and downstream sections and 
displaceable between its open and closed positions; means for actuating 
the control valve, including means for defining an enclosed space having 
an axis, a stepped piston sealingly accommodated in the space for axial 
movement and having oppositely facing first and second surfaces 
respectively having a larger and a smaller axially effective area, means 
for subjecting the first and second surfaces to the pressures prevailing 
respectively in the downstream and upstream sections, and means for 
displacing the control valve between its open and closed positions in 
dependence on the movement of the stepped piston; and means for applying a 
load-dependent force to the actuating means, including a 
cylinder-and-piston unit including a cylinder component and a piston 
component together bounding a chamber, one of these components being 
stationary and the other movable relative to the support, resilient 
load-responsive means for urging the movable component into abutment with 
the stop surface, means for bounding a flow path into the chamber, 
including at least a path portion stationary relative to the support, 
means for forming a passage of a restricted cross-sectional area in the 
portion path, and a plunger extending into the path portion and connected 
to the stepped piston for joint movement therewith from a position in 
which it permits to a position in which it interrupts flow through the 
passage, so that further movement of the plunger beyond the latter 
position takes place against the opposition of the urging means as 
reflected in the pressure prevailing in the chamber. Advantageously, the 
forming means includes a seal mounted on the stationary component and 
extending into the path. 
In this construction, the stepped piston is not loaded in its inactive 
position by the load-responsive urging means as the entire spring force is 
transmitted to the support via the stop surface. During a braking 
operation, the stepped piston moves, without having to overcome high 
forces, until it assumes a position in which it penetrates the 
circumferential seal. It remains in this position until the forces acting 
on the stepped piston in the closing direction have reached an amount 
large enough to displace the piston as a plunger into a control pressure 
chamber which communicates with the chamber bounded by the cylinder and 
piston components. This occurs when the plunger generates such a pressure 
in the control pressure chamber that this pressure, multiplied by the 
effective piston surface of the piston-cylinder arrangement, exerts a 
force on the movable component which exceeds the preloading force of the 
load-responsive urging means, such as a spring, especially a vehicle 
suspension spring. Thereafter, only a small additional stroke of the 
stepped piston will suffice to close the control valve. This mode of 
operation applies for any amount of preload of the load-responsive spring. 
It will be particularly advantageous to have the piston component fixed to 
a vehicle component, which thus constitutes the support, and to have the 
stop surface surround the piston component. In the inactive position, the 
end face of the cylinder can then directly abut the stop surface. 
It is further advantageous for the cylinder component to have at its end 
face an outer flange which serves as a seat for the load-responsive 
spring. This results in a space-saving design, inasmuch as the spring 
surrounds the cylinder component over its entire length. Furthermore the 
outer flange may be used to increase the magnitude of the area of abutment 
of the cylinder component with the stop surface. 
It is furthermore advantageous, when the housing for the stepped piston is 
constituted by the piston component of the cylinder-and-piston unit or, in 
other words, when the space accommodating the stepped piston is formed in 
the piston component. This accomplishes a compact construction of the 
device and the shortest possible connection between the space 
accommodating the pressure control valve and the cylinder chamber. 
A control chamber is advantageously provided on the side of the 
circumferential seal which is remote from the cylinder chamber. This 
control chamber is in permanent communication with the inlet side of the 
pressure control device. This avoids the otherwise existing need for the 
provision of an additional supply device for filling the cylinder chamber. 
In this arrangement, the stepped piston may advantageously contain a 
channel interconnecting the control chamber and the inlet side. A channel 
of this kind can be made simultaneously with machining the stepped piston, 
without necessitating much additional effort. 
Advantageously, the stepped piston includes a third surface subjected to 
atmospheric pressure and situated between the plunger and the second 
surface which is acted upon by the inlet pressure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawing in detail, it may be seen that a suspension 
spring 1 extends between an unsprung mass 2 of a vehicle and a sprung mass 
3. For example, the unsprung mass 2 is a spring seat connected to the 
vehicle axle, while the sprung mass 3 is the vehicle body, or the chassis 
or a similar support. 
The pressure control device of the invention includes a cylinder-and-piston 
unit 4 which incorporates a piston component 5 which is fixed to the 
vehicle unsprung mass 3 and a cylinder component 6 including an end wall 7 
and an outer flange 8 at an end face 9 remote from the end wall 7. The 
flange 8 serves as a spring seat to support the suspension spring 1. In 
the inactive position, the end face 9 cooperates with a stop surface 10 
which is provided on the sprung mass or support 3 and surrounds the piston 
component 5. A cylinder chamber 11 is disposed between the piston 
component 5 and the cylinder component 6 and is sealed at the periphery of 
the piston component 5 with respect to the exterior of the 
cylinder-and-piston unit 4 by a seal 12. 
The piston component 5 of the piston-cylinder unit 4 bounds an enclosed 
space which accommodates a stepped piston 13 carrying a valve seat 14 for 
a closure member 16 of a control valve 17. The closure member 16 is urged 
towards its closed position by a closure spring 15. The control valve 17 
is interposed between an inlet side 18 of the illustrated braking pressure 
control device which is connectible to a conventional master cylinder by 
an upstream conduit section, and the outlet side 19 which is connectible 
to a wheel cylinder via a downstream conduit section. The stepped piston 
13 is subjected to the action of a weak return spring 20 and has an end 
face 40 which is exposed to an outlet pressure p.sub.2 prevailing at the 
outlet side 19, a stepped surface 22 separated from the end face 40 by a 
seal 21 and exposed to an inlet pressure p.sub.1 prevailing at the inlet 
side 18, a stepped surface 24 separated from the surface 22 by a seal 23 
and connected via a channel 25 to atmosphere, and a plunger 27 separated 
from the surface 24 by a seal 26. The interior of the plunger 27 is 
provided with a radial channel 28 close to the inlet side 18 and an axial 
channel 29 commencing thereat. The axial channel 29 extends to a valve 
chamber 30, on the one hand, and via another radial channel 31 to a 
control chamber 32, on the other hand. Adjacent to the control chamber 32 
is a circumferential seal 33 which sealingly receives the plunger 27 upon 
axial displacement of the stepped piston 13. Neighboring thereto is a 
control pressure chamber 34. The closure member 16 of the valve 17 
includes a pin 36 which extends through an axial bore 35 of the stepped 
piston 13 and which normally bears against an end face 37 delimiting an 
enclosed space 38 receiving the stepped piston 13 in the piston component 
5. 
It will be appreciated that the large piston component 5 and the stepped 
piston 13 may each be of a multipart construction to enable assembly of 
the individual parts. 
The illustrated braking pressure control device operates as follows: the 
suspension spring 1 is compressed by the load of the vehicle and therefore 
urges the cylinder component 6 to exert a load-responsive force on the 
stop surface 10. When a braking action is started and the inlet pressure 
p.sub.1 rises by actuation of the master cylinder, the stepped piston 13 
moves downwardly as considered in the drawing since the end face 40 has a 
larger axially effective area than the stepped surface 22 and an end face 
39 of the plunger 27 combined and since the inlet pressure p.sub.1 acts on 
all of these surfaces. The stepped piston 13 moves until the plunger 27, 
after conducting a partial motion, penetrates into a passage bounded by 
the circumferential seal 33. Thereafter, further motion of the stepped 
piston 13 is hindered by the fluid entrapped in the control pressure 
chamber 34 and in the cylinder chamber 11. The control valve 17 has 
already performed a part of its closing stroke before reaching this 
position. The stepped piston 13 will remain in this position until the 
force acting on it as a result of the continuing increase in the inlet 
pressure p.sub.1 becomes so large that the plunger 27 develops such a 
pressure in the fluid entrapped in the chambers 11 and 34 that the force 
resulting from the action of this pressure on the axially effective 
internal surface of of the end wall 7 of the cylinder component 6 exceeds 
the force of the spring 1. Then the cylinder component 6 will yield and 
the stepped piston 13 will be able to continue its downward movement. The 
fluid displaced by the plunger 27 shifts the cylinder component 6 
downwardly in opposition to the force of the suspension spring 1. The 
change-over point is thereby attained, and the control valve 17 closes 
shortly after that. If the inlet pressure p.sub.1 continues to rise, the 
outlet pressure p.sub.2 has a value reduced relative thereto, the amount 
of the reduction depending upon the ratio of the stepped surface 22 which 
is acted on by the pressure of the inlet side 18 to the end face 40 which 
is subjected to the pressure of the outlet side 19. 
The housing accommodating the stepped piston 13 can also be arranged 
outside the cylinder component 5, rather than being constituted by the 
latter as illustrated in the drawing. In this case, the pressure control 
chamber 34 will communicate with the cylinder chamber 11 via a connecting 
conduit. 
A braking pressure control device of this kind will also be suited for use 
in an arrangement in which the load is applied by load-responsive springs 
other than a suspension spring which supports the vehicle body on the 
axle. 
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.