Liquid pressure booster

An improvement of a liquid pressure booster provided with a reaction piston is disclosed. A guide formed on a housing (plug) is caused to project into a tubular section of a power piston. A reaction piston is slidably supported by the guide, and an input shaft is slidably supported by the reaction piston. A spring which urges the reaction piston is disposed in a clearance defined between the outer periphery of the guide and the inner periphery of the tubular section of the power piston. In this manner, the guide, a sliding portion of the reaction piston, a sliding portion of the input shaft and the spring can be disposed radially of each other while overlapping each other in the axial direction to minimize the axial size of the liquid pressure booster.

FIELD OF THE INVENTION 
The invention relates to a liquid pressure booster as may be used in a 
brake booster or the like, and more particularly, to a liquid pressure 
booster which is provided with a reaction piston. 
DESCRIPTION OF THE PRIOR ART 
A liquid pressure booster is known in the art including a power piston 
slidably disposed in a housing and including a tubular section in its rear 
portion, a power chamber defined in the housing adjacent to the rear 
portion of the power piston, an input shaft slidably extending into the 
housing through the rear portion of the power piston, a control valve 
disposed between the tubular section of the power piston and the head of 
the input shaft for supplying a liquid pressure to the power chamber in 
accordance with an input load applied to the input shaft, a reaction 
piston slidably disposed around the input shaft and responsive to a liquid 
pressure within the power chamber to be retracted with respect to the 
input shaft for abutment against a stopper mounted on the input shaft, and 
a spring disposed between the power piston and the reaction piston for 
maintaining the reaction piston resiliently at an advanced position spaced 
from the stopper (see Japanese Laid-Open Patent Applications No. 
60,871/1990 and No. 74,456/1990). 
In these liquid pressure boosters as disclosed, an advancement of the input 
shaft causes the control valve to supply a liquid pressure to the power 
chamber in accordance with an input load applied to the input shaft, 
thereby driving the power piston forward for providing a servo action. The 
liquid pressure supplied to the power chamber acts upon the input shaft to 
provide a reaction thereto, and also acts upon the reaction piston to 
cause it to retract relative to the input shaft against the resilience of 
the spring. 
The liquid pressure solely acts upon the input shaft having a relatively 
small pressure-responsive area to produce an increased output at a large 
servo ratio until the reaction piston abuts against the stopper mounted on 
the input shaft, whereupon the reaction piston and the input shaft move 
together, and the pressure-responsive area increases, so that the output 
load increases at a reduced servo ratio. The large servo ratio which 
prevails during the initial phase of operation assures a favorable feeling 
of operation. 
Liquid pressure boosters provided with reaction pistons are also known in 
other different arrangements, as disclosed in Japanese Laid-Open Patent 
Applications No. 44,095/1980 and No. 90,675/1981. 
In a conventional liquid pressure booster as mentioned above, it is 
necessary that the input shaft be formed with a portion which slides with 
respect to the housing and another portion which slides with respect to 
the reaction piston and that these portions are disposed in series, 
resulting in a disadvantage that the axial size becomes increased. 
SUMMARY OF THE INVENTION 
In view of the foregoing, it is a feature of the invention that the 
reaction piston is assembled into the liquid pressure booster in a 
sophisticated manner to minimize the axial size of the booster. 
Thus, in a liquid pressure booster which is constructed in a manner 
mentioned above, in accordance with the invention, the housing is provided 
with a guide which projects into the tubular section of the power piston 
so that the reaction piston may be slidably fitted with the guide so as to 
be slidably supported thereby, and the input shaft is slidably fitted into 
an axial section of the reaction piston so that the input shaft may be 
slidably supported also by the reaction piston, with the spring disposed 
in a clearance defined between the outer periphery of the guide and the 
inner periphery of the tubular section of the power piston. 
With this construction, the guide on the housing projects into the tubular 
section of the power piston to slidably support the reaction piston, which 
in turn slidably supports the input shaft, and the spring which urges the 
reaction piston is disposed in a clearance formed between the outer 
periphery of the guide and the inner periphery of the tubular section of 
the power piston. In this manner, the guide, the sliding portion of the 
reaction piston, the sliding portion of the input shaft and the spring are 
radially telescoped within each other while overlapping each other in the 
axial direction to minimize the axial size of the liquid pressure booster. 
Above and other objects, features 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 EMBODIMENT 
Referring to the drawings, an embodiment of the invention will now be 
described. Referring to FIG. 1, a liquid pressure booster includes a 
housing 1 having a bore 2 formed therein, in which a power piston 3 is 
slidably fitted, the power piston including a tubular section 3A at its 
right-hand or rear portion. A push rod 4 which is formed at the left end 
of the power piston 3 slidably projects externally of the housing 1 in a 
liquid tight manner, with its head connected to the piston of a master 
cylinder, not shown. 
At its right-hand end, the opening of the bore 2 is sealed by a plug 5, 
which forms part of the housing 1. The plug 5 is integrally secured to the 
housing 1 by a nut 6 which is threadably engaged with the housing 1. A 
power chamber 8 is formed between the plug 5 and the power piston 3 for 
admission of a pressure oil therein. A low pressure chamber 9 is formed on 
the opposite side of the power piston 3 from the power chamber 8 and 
receives a spring 10 therein. The resilience of the spring 10 normally 
maintains the power piston 3 at its inoperative position, shown, where it 
abuts against the plug 5. The chamber 9 communicates with a reservoir, not 
shown, through a passage 11 formed in the housing 1. 
An input shaft 17, mechanically coupled to a brake pedal, not shown, 
slidably extends through the plug 5, and a control valve 18 is disposed 
between the left or head of the input shaft 17 and the inside of the 
tubular section 3A of the power piston 3. 
The control valve 18 comprises a first valve seat 20 formed on a plate 19 
disposed inside the tubular section 3A of the power piston 3, a ball valve 
22 seated upon the first valve seat 20 in the opposite direction from the 
power chamber 8 under the resilience of a spring 21, an annular pin 23 
formed on the head of the input shaft 17 for urging the ball valve 22 away 
from the first valve seat 20, and a second valve seat 24 formed on the 
head of the annular pin 23 and on which the ball valve 22 is adapted to be 
seated. The plate 19 is secured to the power piston 3 by means of 
retainers 56, 56' and a set screw 58 which is threadably engaged with the 
tubular section 3A of the power piston 3. 
In the inoperative position shown, the ball valve 22 is seated upon the 
first valve seat 20 under the resilience of the spring 21, thus 
interrupting a communication between the power chamber 8 formed to the 
right of the first valve seat 20 and a pressure chamber 29 formed to the 
left of the first valve seat 20. The pressure chamber 29 communicates with 
a pump, not shown, through a supply passage 30, the pump acting to 
maintain a pressure oil of a given pressure in the pressure chamber 29. 
The supply passage 30 comprises a radial passage 31 formed in the power 
piston 3, an annular groove 32 formed around the outer peripheral surface 
of the power piston 3, a radial passage 33 formed in the housing 1 and a 
conduit, not shown, which connects the passage 33 and the pump together. 
In the inoperative position shown, the second valve seat 24 formed on the 
head of the annular pin 23 is spaced from the ball valve 22 which is then 
seated upon the first valve seat 20, and under this condition, the power 
chamber 8 communicates with the reservoir through a discharge passage 38. 
The discharge passage 38 comprises a passage 39 formed in the axial 
portion of the annular pin 23, a passage 41 formed in the axial portion of 
a shim 40 interposed between the annular pin 23 and the input shaft 17, a 
passage 42 formed in the axial portion of the input shaft 17, a passage 43 
formed in the plug 5, and a passage 44 formed in the housing 1, the latter 
passage 44 being connected to the passage 11 to allow a communication of 
the discharge passage 38 with the reservoir, not shown. 
The left end of the ball valve 22 which constitutes the control valve 
slidably extends through a collar 48 in a liquid tight manner, defining a 
balance chamber 4a to the left of the collar 48. The balance chamber 49 
communicates with the power chamber 8 through a communication path 50 
formed in the power piston 3 and a through-opening 51 formed in the plate 
19, whereby the pressure-responsive area of the ball valve 22 which faces 
the balance chamber 49 is made greater than the pressure-responsive area 
of the ball valve 22 which faces the power chamber 8, or the inner area 
within the first valve seat 20 from which the inner area of the second 
valve seat 24 is subtracted. By choosing the pressure-responsive areas in 
a manner mentioned above, as the input shaft 17 and the annular pin 23 are 
driven forward to move the ball valve 22 away from the first valve seat 20 
to allow a pressure rise within the power chamber 8, the ball valve 22 
which had been seated upon the second valve seat 24 on the annular pin 23 
is prevented from being spaced from the second valve seat 24 to cause the 
occurrence of a liquid leakage. 
The plug 5 which forms part of the housing 1 is provided with a tubular 
guide 5A which extends into the tubular section 3A of the power piston 3, 
and a reaction piston 55 is slidably fitted in the guide 5A to be 
supported thereby in a slidable manner. The input shaft 17 is slidably 
fitted into the axial portion of the reaction piston 55, whereby the input 
shaft 17 is slidably supported by the reaction piston 55 which is in turn 
supported by the guide 5A. 
The left end of the reaction piston 55 is formed with a flange 55A which 
extends radially outward, and a spring 57 is disposed between the outer 
portion of the flange 55A and the retainer 56 which is located within the 
tubular section 3A of the power piston 3, thus normally maintaining the 
reaction piston 55 in its inoperative position shown where it abuts 
against the plate 19. 
Under this condition, the reaction piston 55 is located at its advanced 
position with respect to the input shaft 17, and is spaced from a stepped 
stopper 17A formed on the input shaft 17, and as will be described later, 
when the force exerted by the oil pressure within the power chamber 8 and 
acting upon the reaction piston 55 becomes equal to or exceeds the set 
forth of the spring 57, the reaction piston 55 will retract with respect 
to the input shaft 17 to abut against the stopper 17A against the 
resilience of the spring 57. 
As a result, while the force exerted by the oil pressure within the power 
chamber 8 and acting upon the reaction piston 55 has been reacted by the 
power piston 3 through the spring 57 and the retainer 56 before the 
reaction piston 55 abuts against the stopper 17A, the force will be 
transmitted to the input shaft 17 as a reaction force after the abutment 
of the reaction piston 55 against the stopper 17A, thereby allowing a 
servo ratio to be changed between before and after the abutment of the 
reaction piston 55 against the stopper 17A. 
The spring 57 and the retainer 56 are disposed in a clearance defined 
between the outer periphery of the guide 5A and the inner periphery of the 
tubular section 3A of the power piston 3. Accordingly, in the inoperative 
position shown, it will be seen that the guide 5A, a portion of the 
reaction piston 55 which slides relative to the guide 5A, a portion of the 
input shaft 17 which slides relative to the reaction piston 55, and the 
spring 57 which urges the reaction piston 55 are located radially relative 
to each other while overlapping in the axial action. As a consequence, the 
axial size of the liquid pressure brake booster can be reduced as compared 
with the conventional arrangement in which the sliding portions of the 
reaction piston 55 and the input shaft 17 have been disposed in series in 
the axial direction. 
A mechanism which reduces a loss stroke of the input shaft 17 during the 
initial phase of operation will be described. The head of the input shaft 
17 is formed with a step 17B to provide a reduced diameter, and the head 
thereof which is located forward of the step 17B is inserted into a 
radially inner flange 61A which is formed on the right end of a tubular 
stop member 61, with the flange 61A abutting against the step 17B. An 
annular member 62 is disposed as a press fit into a clearance defined 
between the outer periphery of the input shaft 17 which is located forward 
of the step 17B and the inner periphery of the tubular stop member 61, 
thus maintaining a liquid tightness between the input shaft 17 and the 
tubular stop member 61 while securing the latter to the input shaft 17. 
As shown in FIG. 2, the left or head of the tubular member 61 is integrally 
formed with diametrically located, radially outwardly extending stoppers 
61B. Each stopper 61B extends outward of the axial portion of the reaction 
piston 55 through a diametrically extending slits 55B formed in the flange 
55A of the reaction piston 55. 
The shim 40 and the annular pin 23 are sequentially inserted into the 
tubular stop member 61, and a spring 63 is disposed between the annular 
pin 23 and the plate 19, so that in the inoperative position shown, the 
stopper 61B is disposed in abutment against the left end face of the guide 
5A to limit the retreating movement of the input shaft 17. On the other 
hand, the left end of the retainer 56 extends to a position adjacent to 
and located to the right of the stopper 61B, whereby during the retreating 
movement of the power piston 3, the retainer 56 is effective to limit the 
retreating movement of the input shaft 17. 
A liquid tightness of a space located radially outward of the power chamber 
8, which is defined within the bore 2, is maintained by a seal member 66 
which is disposed around the outer periphery of the plug 5, and a seal 
member 67 which is disposed around the outer periphery of the power piston 
3. On the other hand, a space located radially inward of the power chamber 
8 is sealed against the discharge passage 38 by seal means disposed at 
four locations. Specifically, a seal member 68 disposed around the inner 
periphery of the guide 5A seals a clearance between the reaction piston 55 
and the guide 5A and communicating with the power chamber 8; a seal member 
69 disposed around the outer periphery of the annular pin 23 seals a 
clearance between the tubular stop member 61 and the annular pin 23 and 
communicating with the power chamber 8; and a combination of the annular 
member 62 and an axially relatively elongate sliding surface 70 between 
the reaction piston 55 and the input shaft 17 seals a clearance between 
the tubular stop member 61 and the reaction piston 55 and communicating 
with the power chamber 8. 
Seal members 71, 72 disposed around the inner and the outer periphery of 
the plug 5 seal between the discharge passage 38 and the atmosphere. 
The operation of the described arrangement will now be described. In the 
inoperative position shown when a brake pedal, not shown, is not 
depressed, the annular pin 23 is urged to the right by the spring 63 which 
is interposed between the pin 23 and the plate 19 and thus is spaced from 
the ball valve 22, whereby the power chamber 8 communicates with the 
reservoir through the discharge passage 38. 
When the brake pedal is depressed to drive the input shaft 17 forward, the 
second valve seat 24 formed on the head of the annular pin 23 abuts 
against the ball valve 22 to interrupt the communication between the 
discharge passage 38 and the power chamber 8, and also the annular pin 23 
causes the ball valve 22 to be moved away from the first valve seat 20 
against the resilience of the spring 21 (see point A in FIG. 3), whereby 
the pressure oil which is normally introduced into the pressure chamber 29 
is now admitted into the power chamber 8 through a clearance between the 
outer periphery of the annular pin 23 and the inner periphery of the plate 
19. 
When the pressure oil is introduced into the power chamber 8, the power 
piston 3 is driven forward or to the left against the resilience of the 
spring 10 while the reaction piston 55 is displaced to the right against 
the resilience of the spring 57. However, during the initial phase of such 
operation, the reaction piston 55 is maintained as spaced from the stopper 
17A on the input shaft 17 by the spring 57. 
Under this condition, the force exerted by the oil pressure within the 
power chamber 8 and acting upon the reaction piston 55 is reacted by the 
power piston 3 through the spring 57, retainer 56 and set screw 58, and 
thus is not transmitted to the input shaft 17. Accordingly, a reaction 
force which is transmitted to a driver through the input shaft 17 is 
obtained by the oil pressure within the power chamber 8 which directly 
acts upon the input shaft 17. Since the prevailing pressure-responsive 
area of the input shaft 17 is small, the output increases with a large 
servo ratio (see line B in FIG. 3). 
As the oil pressure within the power chamber 8 rises to continue to drive 
the power piston 3 to the left or forward, thus providing an effective 
braking action, the reaction piston 55 abuts against the stopper 17A on 
the input shaft 17 (see point C in FIG. 3), whereupon the force which acts 
upon the reaction piston 55 is transmitted through the stopper 17A to the 
input shaft 17 to reduce the servo ratio, and accordingly the output 
subsequently rises with a reduced servo ratio (see line D in FIG. 3). 
A resistance presented upon the input shaft 17 by seal means when the brake 
pedal is depressed comprises resistances presented by the sliding surface 
70 between the reaction piston 55 and the input shaft 17, and also by the 
seal member 71 disposed around the inner periphery of the plug 5. Since 
seal means which seal between the power chamber 8, which is subjected to 
an increased pressure, and the discharge passage 38 is formed by the 
sliding surface 70, the magnitude of the resistance presented upon the 
input shaft 17 by the seal means can be sufficiently reduced as compared 
with the resistance presented by a rubber seal member as used in the prior 
art, thereby providing a light feeling during the initial phase of 
depression of the brake pedal. 
It will be recognized that during the reciprocating motion of the input 
shaft 17, the communication between the passage 42 formed in the input 
shaft 17 and the passage 43 formed in the plug 5 must be maintained, and 
accordingly the seal members 68 and 71 must be axially spaced by a 
distance in excess of the reciprocating stroke of the input shaft 17. On 
the other hand, the sliding surface 70 must be axially long enough to 
provide a satisfactory sealing action. In the present embodiment, the 
sliding surface 70 is formed in an interval as represented between the 
seal member 68 and 71 in the inoperative position shown, thus effectively 
preventing the axial size of the liquid pressure booster from being 
increased as a result of an increased axial length of the sliding surface 
70. 
When the brake pedal is now released and the annular pin 23 is displaced to 
the right under the influence of the spring 63, the ball valve 22 will be 
seated upon the first valve seat 20, thus interrupting the communication 
between the pressure chamber 29 and the power chamber 8. The second valve 
seat 24 on the annular pin 23 continues to move away from the ball valve 
22, and the power chamber 8 communicates with the reservoir through the 
discharge passage 38, so that the oil pressure within the power chamber 8 
decreases, causing the power piston 3 to retract to the right. 
The resulting retreating movement of the input shaft 17 is limited by the 
abutment of the stopper 61B of the tubular stop member 61 which is 
integral with the input shaft against the retainer 56 which is integral 
with the power piston 3. Under this condition, the ball valve 22 which is 
seated upon the first valve seat 20 is largely spaced from the second 
valve seat 24 on the annular pin 23, securing a large enough channel area. 
Immediately before the right end of the power piston 3 abuts against the 
plug 5, the stopper 61B of the tubular stop member 61 which is maintained 
in abutment against the retainer 56 abuts against the guide 5A. The power 
piston 3 retreats through a given stroke from this condition and comes to 
a stop as a result of the abutment of its right end against the plug 5, 
whereupon the stopper 61B of the tubular stop member 61 is located at an 
advanced position relative to the power piston by the given distance 
mentioned above, thus locating the second valve seat 24 on the annular pin 
23 adjacent to the ball valve 22 which is seated upon the first valve seat 
20. 
Accordingly, when the brake pedal is depressed for the next time, the 
second valve seat 24 will be immediately seated upon the ball valve 22 to 
interrupt the communication between the power chamber 8 and the reservoir, 
thus reducing the loss motion during the initial phase of depression of 
the brake pedal. 
It will be noted that the seal member 68 may be disposed around the outer 
periphery of the reaction piston 55 at its right end rather than around 
the inner periphery of the guide 5A. Alternatively, the seal member 68 may 
be omitted, and the sliding surface between the guide 5A and the reaction 
piston 55 may be utilized such as seal means. 
While the invention has been described above in connection with an 
embodiment thereof, it should be understood that a number of changes, 
modifications and substitutions therein will readily occur therein to one 
skilled in the art from the above disclosure without departing from the 
spirit and scope of the invention defined by the appended claims.