Parking maintaining apparatus for brake booster

A parking maintaining apparatus includes a solenoid-operated open/close valve which opens or closes a constant pressure passage providing a communication between a valve mechanism and a constant pressure chamber of a brake booster. A relief valve is provided which relieves a pressure within the constant pressure passage into the constant pressure chamber when such pressure exceeds a given value. The provision of the relief valve allows the pressure within a variable pressure chamber to be maintained at or below the given value through the valve mechanism under a parking maintaining condition, whereby the pressure within the variable pressure chamber can be rapidly displaced as the parking maintaining condition is to be terminated, thus eliminating a partly braked condition.

FIELD OF THE INVENTION 
The invention relates to a brake booster to be mounted on a vehicle, and 
more particularly, to a parking maintaining apparatus for brake booster 
which enables a braking action to be continued or maintained after the 
release of a depressed brake pedal under a given condition. 
DESCRIPTION OF THE PRIOR ART 
Generally, a brake booster comprises a power piston slidably disposed 
within a shell, a valve mechanism contained within a valve body defined in 
an axial portion of the power piston, a constant pressure chamber defined 
forwardly of the power piston as viewed in the direction of operation 
thereof and connected to a source of negative pressure, a variable 
pressure chamber defined rearwardly of the power piston, as viewed in the 
direction of operation thereof, a plurality of passages including a 
constant pressure passage providing a communication between the valve 
mechanism and the constant pressure chamber, a variable pressure passage 
providing a communication between the valve mechanism and the variable 
pressure chamber and a pressure passage providing a communication between 
the valve mechanism and an atmospheric pressure, and an input shaft for 
operating a valve plunger, which forms part of the valve mechanism, to 
switch the communiction of the respective passages. 
The valve mechanism is constructed so that when a brake pedal is depressed 
to drive the input shaft forward, a communication between the constant 
pressure passage and the variable pressure passage is interrupted while a 
communication is established between the variable pressure passage and the 
pressure passage, thus introducing a pressure fluid into the variable 
pressure chamber. When the brake pedal is now released to cause the input 
shaft to move back, a communication between the variable pressure passage 
and the pressure passage is interrupted while a communication is 
established between the variable pressure passage and the constant 
pressure passage, thus displacing the pressure fluid which has been 
introduced into the variable pressure chamber to the source of negative 
pressure through the constant pressure chamber. 
There has been proposed a parking maintaining apparatus for the brake 
booster of the kind described and including a solenoid-operated open/close 
valve which opens and closes the constant pressure passage (see Japanese 
Patent Application No. 120,834/1986). In this arrangement, when the 
solenoid-operated valve is maintained open, a usual operation of a brake 
booster takes place. By providing an arrangement to close the 
solenoid-operated valve when a vehicle speed sensor detects that a vehicle 
has come to a stop as a result of a braking action, the release of the 
brake pedal which tends to establish a communication between the variable 
pressure chamber and the constant pressure chamber through the variable 
pressure passage and the constant pressure passage cannot allow the 
pressure fluid which has been introduced into the variable pressure 
chamber to be displaced to the source of low pressure through the constant 
pressure chamber since the solenoid-operated valve remains closed. 
Accordingly, the braking action can be maintained even after the brake 
pedal is released, and thus the brake booster can be used as a parking 
maintaining apparatus. 
The parking maintaining condition can be terminated upon starting the 
vehicle. However, if a pressure fluid of unnecessarily high pressure is 
confined within the variable pressure chamber, it takes a certain length 
of time until the pressure fluid can be displaced externally, causing a 
partly braked condition. 
SUMMARY OF THE INVENTION 
In view of the foregoing, the invention provides a parking maintaining 
apparatus for brake booster in which a relief valve is provided to relieve 
the pressure within the constant pressure passage located between the 
valve mechanism and the solenoid-operated valve to the constant pressure 
chamber when such pressure becomes equal to or greater than a preset 
value. 
With the arrangement of the invention, when a pressure fluid of 
unnecessarily high pressure is confined within the variable pressure 
chamber under the parking maintaining condition, the pressure fluid within 
the variable pressure chamber can be relieved to the constant pressure 
chamber through the variable pressure passage, the valve mechanism, the 
constant pressure passage and the relief valve, thus allowing the pressure 
within the variable pressure chamber to be maintained below the preset 
value which is established by the relief valve. In this manner, when the 
parking maintaining condition is to be terminated, the pressure within the 
variable pressure chamber can be rapidly reduced, thus eliminating the 
partly braked condition. 
Since the relief valve is originally designed to relieve the pressure 
within the constant pressure passage into the constant pressure chamber, 
the normal braking action cannot be adversely influenced if the relief 
valve is provided. By depressing a brake pedal to a greater degree, the 
pressure within the variable pressure chamber can be raised above the 
preset value. 
Above and other objects and advantages of the invention will become 
apparent from the following description of an embodiment thereof with 
reference to the attached drawings.

DETAILED DESCRIPTION OF EMBODIMENTS 
Referring to the drawings, several embodiments of the invention will be 
described. Initially referring to FIG. 1, a brake booster includes a shell 
1 which comprises a front shell 1a, and a rear shell 1b. A power piston 2 
is slidably disposed within the shell 1, and a diaphragm 3 is applied to 
the back surface of the power piston 2 so that the combination of the 
power piston 2 and the diaphragm 3 divides the interior of the shell 1 
into a forwardly located, constant pressure chamber 4 and a rearwardly 
located, variable pressure chamber 5. 
An axial portion of the power piston 2 is integrally formed with a valve 
body 6, in which a valve mechanism 7, which switches a fluid path, is 
contained. The power piston 2 and the valve body 6 are normally maintained 
in their inoperative positions, shown, by means of a return spring 8. 
The valve mechanism 7 comprises a first valve seat 10 formed on the valve 
body 6, a second valve seat 12 formed on a valve plunger 11, and a valve 
element 14 adapted to be seated upon either valve seat 10 or 12 from the 
rear side of the power piston 2, or from the right-hand side, as viewed in 
FIG. 1, under the resilience of a spring 13. A region located externally 
of a seal defined between the valve seat 10 and the valve element 14 
communicates with the constant pressure chamber 4 through a constant 
pressure passage 15 and a solenoid-operated open/close valve 16. The 
constant pressure chamber 4 communicates with a source of negative 
pressure such as an intake manifold of an engine, not shown, through a 
piping 17 mounted on the shell 1 which is utilized to introduce a negative 
pressure. 
A region located intermediate a seal defined between the first valve seat 
10 and the valve element 14 and another seal defined between the second 
valve seat 12 and the valve element 14 communicates with the variable 
pressure chamber 5 through a variable pressure passage 18 which is formed 
in the valve body 6. Finally, a region located inside a seal defined 
between the second valve seat 12 and the valve element 14 communicates 
with the atmosphere through a pressure passage 20 in which a filter 19 is 
disposed. It is to be noted that the variable pressure chamber 5 is 
hermetically sealed against the exterior by a seal member 21 through which 
the valve body 6 is slidable. 
The valve plunger 11, which forms the valve mechanism 7, is prevented from 
being withdrawn from the valve body 6 by means of a key member 24, and has 
its one end connected to an input shaft 25 which is in turn mechanically 
coupled to a brake pedal, not shown. The front end face of the valve 
plunger 11 is disposed in opposing relationship with an end face of an 
output shaft 26, with a reaction disc 27 interposed between the opposing 
surfaces. An axial portion of the front shell 1a is formed with a 
cylindrical reentrant extension 1c in which a seal member 28 is disposed, 
which allows an output shaft 26 to extend slidably therethrough to project 
externally of the shell 1 for connection with a piston of a master 
cylinder, not shown. 
The solenoid-operated valve 16 is disposed within the constant pressure 
chamber 4 and is mounted on the valve body 6 at a location outside and 
below the coiled return spring 8 for opening and closing the constant 
pressure passage 15 formed in the valve body 6. 
FIG. 2 shows another embodiment of the invention which differs from the 
arrangement shown in FIG. 1. Referring to FIG. 2, the construction of the 
solenoid-operated open/close valve 16 mentioned above will be described. 
Specifically, the valve 16 comprises a substantially dish-shaped casing 30 
which is mounted on the valve body 6 in a hermetically sealed manner so as 
to cover an opening of the constant pressure passage 15 into the constant 
pressure chamber 4, and a substantially cup-shaped casing 32 integrally 
connected to the casing 30 and containing a ring-shaped solenoid 31 
therein. The dish-shaped casing 30 is formed with a step, on the right end 
face of which is defined a valve seat 33. A plunger 34 is disposed for 
sliding movement through an axial portion of the solenoid 31, and is 
provided with valve means 35 which is adapted to be seated upon the valve 
seat 33. 
The valve means 35 comprises an annular valve element 36 of an increased 
diameter having an outer peripheral portion which is adapted to be seated 
upon the valve seat 33 from the right hand thereof, and a poppet-type 
valve element 37 which slidably extends through an axial portion of the 
valve element 36 and which is a press fit into the plunger 34. The 
poppet-type valve element 37 is adapted to be seated upon a valve seat 38, 
which is formed around the inner periphery of the annular valve element 
36, to close a passage 39 formed in the axial portion of the valve element 
36. 
The plunger 34 and the poppet valve element 37 are normally maintained in 
their inoperative positions shown by a spring 40, and another spring 41 is 
disposed between the annular valve element 36 and the dish-shaped casing 
30 for normally maintaining the valve seat 38 on the valve element 36 in 
abutment against the poppet valve element 37 which assumes its inoperative 
position, thus maintaining it in its inoperative position. Under this 
condition, the annular valve element 36 is spaced from the valve seat 33 
formed on the dish-shaped casing 30, whereby the constant pressure passage 
15 communicates with the constant pressure chamber 4 through a clearance 
between the valve element 36 and the valve seat 33 and a passage 42 formed 
in the casing 30. 
As shown in FIG. 1, harnesses 43 extending from the solenoid-operated valve 
16 are taken out through seal means 44 mounted on the front shell 1a for 
connection with a controller, not shown, which includes a microcomputer. 
In the embodiment shown in FIG. 1, a bypass passage 50 is formed in the 
valve body 6 to establish a communication between the constant pressure 
chamber 4 and the constant pressure passage 15 extending between the valve 
mechanism 7 and the solenoid-operated valve 16. A relief valve 51 is 
disposed in the bypass passage 50 for relieving the pressure from the 
constant pressure passage 15 into the constant pressure chamber 4 whenever 
such pressure exceeds a given value. 
In the described arrangement, the controller mentioned above maintains the 
solenoid-operated valve 16 open, and accordingly, when the brake booster 
is inoperative, a negative pressure which is introduced into the constant 
pressure chamber 4 through the piping 17 is allowed to be introduced into 
the constant pressure passage 15 through the solenoid-operated valve 16, 
namely, through the passage 42 and the clearance between the valve element 
36 and the valve seat 33, and thence into the variable pressure chamber 5 
through the valve mechanism 7 and the variable pressure passage 18. 
Accordingly, no fluid pressure differential is developed across the power 
piston 2 under this condition, and hence the brake booster maintains its 
inoperative condition shown, in a similar manner as with a conventional 
brake booster which is not provided with the solenoidoperated valve 16. 
When a brake pedal is now depressed under this inoperative condition, it 
will be apparent that the brake booster is capable of producing a braking 
force which depends on the degree of depression of the brake pedal, 
generally in a similar manner as in a conventional brake booster. 
When the brake booster mentioned above is to be operated as a parking 
maintaining apparatus, the controller mentioned above energizes the 
solenoid 31 of the solenoid-operated valve 16 when the controller 
mentioned above determines that a given requirement to maintain the 
parking condition is satisfied in response to outputs from various 
sensors, not shown, for example, when the vehicle is at rest and when the 
brake pedal continues to be depressed over a given length of time after 
the vehicle has come to a stop. Thereupon, the two valve elements 36, 37 
are driven integrally to the left against the resilience of the springs 
40, 41, causing the valve element 36 to be seated upon the valve seat 33 
to close the solenoid-operated valve 16. 
At the moment the solenoid-operated valve 16 is closed, the brake pedal 
assumes its depressed position, and hence a fluid pressure which depends 
on the degree of depression of the brake pedal is introduced into the 
variable pressure chamber 5. Since the solenoid-operated valve 16 is 
closed, if the brake pedal is now released to establish a communication 
between the variable pressure passage 18 and the constant pressure passage 
15, the brake pressure within the variable pressure chamber 5 cannot be 
displaced into the constant pressure chamber 4. Accordingly, the braking 
action of a magnitude which depends on the fluid pressure is maintained 
even after the brake pedal ceases to be depressed. 
When a fluid pressure of unnecessarily high pressure is confined within the 
variable pressure chamber 5 under such condition, pressure fluid within 
the variable pressure chamber 5 can be relieved into the constant pressure 
chamber 4 through the variable pressure passage 18, the valve mechanism 7, 
the constant pressure passage 15, the bypass passage 50 and the relief 
valve 51, whereby the pressure within the variable pressure chamber 5 can 
be maintained at or below a given value established by the relief valve 
51. 
On the other hand, since it is possible to introduce a fluid pressure which 
depends on the degree of depression of the brake pedal into the variable 
pressure chamber 5, the pressure within the variable pressure chamber 5 
can be increased above the preset value by strongly depressing the brake 
pedal even under the parking maintaining condition, in a similar manner as 
in a usual operation. 
When the controller mentioned above detects the starting operation of the 
vehicle under the parking maintaining condition, it deenergizes the 
solenoid 31 of the solenoid-operated valve 16, whereupon the fluid 
pressure acting upon the two valve elements 36, 37 urges them against the 
valve seats 33, 38, respectively, initially causing the poppet valve 
element 37 having a reduced diameter, and hence having a reduced pressure 
responsive area, to be driven to the right under the resilience of the 
spring 40 to move away from the valve seat 38 on the annular valve element 
36. 
The constant pressure passage 15 is then allowed to communicate with the 
constant pressure chamber 4 through the passage 39 formed in the axial 
portion of the annular valve element 36, whereby the pressure within the 
constant pressure passage 15 decreases until it reaches or reduces below a 
given value, whereupon the annular valve element 36 is readily moved away 
from the valve seat 33 by the spring 41. Thus, the constant pressure 
passage 15 communicates with the constant pressure chamber 4 through an 
increased channel area, rapidly reducing the pressure within the constant 
pressure passage 15 or the variable pressure chamber 5. 
At this time, the pressure within the variable pressure chamber 5 is 
maintained at or below a given value which is established by the relief 
valve 51, and hence is allowed to decrease rapidly, thus eliminating the 
partly braked condition as the parking maintaining condition is to be 
terminated. 
In the embodiment shown in FIG. 2, a bypass passage 50' is formed in an 
axial portion of the poppet valve element 37, with a relief valve 51' 
assembled into the bypass passage 50'. In this embodiment, the pressure 
fluid which is exhausted from the constant pressure passage as the relief 
valve 51' is opened will be discharged toward the constant pressure 
chamber 4 through a bore, not shown, extending through a plug against 
which the left end of the spring 40 abuts. 
FIG. 3 shows a further embodiment of the invention in which a bypass 
passage 50" is formed in a casing 30" of the solenoid-operated valve 16 
and can be opened and closed by a relief valve 51". In either instance, a 
similar effect is achieved as in the embodiment shown in FIG. 1. 
While the invention has been shown and described above in connection with 
several embodiments thereof, it should be understood that a number of 
changes, modifications, and substitutions therein will readily occur to 
one skilled in the art without departing from the spirit and scope of the 
invention defined by the appended claims.