Tandem master cylinder

A tandem master cylinder having a small-diameter cylinder formed on the same axis as a cylinder bore in a floating piston, communicating with a primary fluid pressure chamber and a secondary fluid pressure chamber. A small-diameter piston is movably inserted in the small-diameter cylinder, dividing the primary fluid pressure chamber from the secondary fluid pressure chamber. A small-diameter piston spring is mounted in the small-diameter cylinder, pressing the small-diameter piston toward the secondary fluid pressure chamber. A stop is provided in the small-diameter cylinder to stop the forward movement of the small-diameter piston in the small-diameter cylinder. An elastic member is disposed in an unbiased state between the small-diameter piston and a cylinder front end wall. The elastic member has an elastic force in a direction of compression which is greater than that of the small-diameter piston spring. In case of a failure of the primary fluid pressure chamber, the tandem master cylinder immediately produces a fluid pressure in the secondary fluid pressure chamber while compressing the elastic member after making an ineffective stroke, equivalent to the discharge of the primary fluid pressure chamber, until it mechanically contacts the floating piston.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to improvements in a tandem master cylinder 
of a braking system for automobiles in which two mutually independent 
brake fluid pressures are produced. 
2. Description of the Related Art 
In a certain type of conventional tandem master cylinder, e.g., a tandem 
master cylinder disclosed in Japanese Patent Publication No. Sho 60-21098, 
the mounting load of a primary return spring located at the back of a 
floating piston is set greater than the mounting load of a secondary 
return spring located at the front of the floating piston, so that a main 
piston and the floating piston may be moved forward simultaneously at the 
start of brake application to close two return ports simultaneously, 
thereby decreasing an ineffective stroke at the time of braking. A 
small-diameter piston receives a fluid pressure from a primary fluid 
chamber behind the floating piston and from a secondary fluid chamber 
before the floating piston is inserted in a small-diameter cylinder formed 
in the floating piston while the main piston and the floating piston are 
moving forward together as one unit, the small-diameter piston is moved 
backward in relation to the forward movement of the floating piston by a 
fluid pressure built up in the secondary fluid chamber or by a rod member, 
thus restricting a sudden increase in the fluid pressure in the secondary 
fluid pressure chamber and consequently preventing biting of a piston seal 
of the floating piston passing over the return port. This type of tandem 
master cylinders, where the small-diameter piston is moved backward with 
the forward movement of the piston by a rod member has been disclosed in 
Japanese Patent Publication No. Hei 5-25705, which is as shown in FIG. 4. 
In FIG. 4, of the means for restraining a sudden increase in the fluid 
pressure in a secondary fluid pressure chamber 53 by offsetting a relative 
volumetric change resulting from the backward movement of a small-diameter 
piston 56 which is inserted in a small-diameter cylinder 55 provided in a 
floating piston 54. During forward movement of the floating piston 54 when 
the operation of the tandem master cylinder 51 starts, a rigid rod member 
57 prevents the follow-up movement of the small-diameter piston 56 in the 
forward direction of the floating piston 54. Therefore the positional 
relationship between the rear boss 58 of the small-diameter piston 56 and 
the head section 60 of the stopping rod 59 is restricted in design in an 
attempt to satisfy the overall discharge of the tandem master cylinder 51. 
In FIG. 4, therefore, there must be established the relationship Z=X+Y, 
where X is a distance between the front end of the main piston 61 in the 
home position and a cup-shaped spring retainer 62 (equivalent to the 
discharge of the primary fluid pressure chamber 52), Y is a distance 
between the front end 63 of the floating piston 54 in home position and 
the front end wall 64 of the cylinder (equivalent to the discharge of the 
secondary fluid pressure chamber 53), and Z is a distance between the head 
section 60 of the stopping rod 59 and the rear boss 58 of the 
small-diameter piston 56. 
In the tandem master cylinder of FIG. 4, when braking operation is done 
without any fluid pressure built up in the primary fluid pressure chamber 
52 for some reason or other, the floating piston 54 starts moving, and 
after the piston seal 65 has closed the return port 66, the fluid pressure 
is built up in the secondary fluid pressure chamber 53. At this time, a 
fluid pressure increase in the secondary fluid pressure chamber 53 is 
delayed, and accordingly there occurs a delay of brake pressure 
application to the braking equipment, until the small-diameter piston 56 
inserted in the floating piston 54 makes a relative backward movement with 
respect to the floating piston 54 and the head section 60 of the stopping 
rod 59 which is moving forward comes into contact with the rear boss 58 of 
the small-diameter piston 56. A volumetric increase occurs corresponding 
to the volumetric decrease of the secondary fluid pressure chamber 53 
caused by the forward movement of the floating piston 54. Reference 
numeral 67 denotes a push rod. 
SUMMARY OF THE INVENTION 
In view of the above-described problems, the present invention has an 
object to provide a tandem master cylinder which can decrease a time lag 
required for increasing the fluid pressure in the secondary fluid pressure 
chamber in case no fluid pressure is built up in the primary fluid 
pressure chamber, while restraining a sudden increase in a fluid pressure 
in a secondary fluid pressure chamber. 
In a tandem master cylinder of the present invention comprising a main 
piston disposed at a rear end of a cylinder bore and a floating piston 
disposed between the main piston and a front end wall of the cylinder in 
such a manner that they can move into the cylinder; a primary fluid 
pressure chamber defined between the main piston and the floating piston, 
and a secondary fluid pressure chamber defined between the floating piston 
and the front end wall of the cylinder; with a mounting load of a primary 
return spring mounted behind the floating piston set greater than that of 
a secondary return spring disposed in a front of the floating piston; 
return ports which are closed and opened by piston seals mounted on front 
end sections of the main and floating pistons in the cylinder bore; a 
small-diameter cylinder, in the floating piston, communicating with the 
primary fluid pressure chamber and the secondary fluid pressure chamber, 
being formed coaxially in the cylinder bore, and a small-diameter piston 
inserted in the small-diameter cylinder the interior of which is divided 
into the primary fluid pressure chamber and the secondary fluid pressure 
chamber; and a small-diameter piston spring mounted for pressing the 
small-diameter piston toward the secondary fluid pressure chamber; there 
are provided a stopping section for stopping the forward movement of the 
small-diameter piston in the small-diameter cylinder and an elastic 
member, between the small-diameter piston and the front end wall of the 
cylinder, in a state of free length having an elastic force in a direction 
of compression and a greater elastic force than the small-diameter piston 
spring. According to the tandem master cylinder of the above-described 
constitution, on starting ordinary operation, the small-diameter piston 
spring having a low elastic force is compressed with the forward movement 
of the main piston and the floating piston, without compressing the 
elastic member of high elastic force, to move the small-diameter piston 
backward in relation to the forward movement of the floating piston. A 
sudden increase in the fluid pressure in the secondary fluid pressure 
chamber is restrained, and, the main piston makes an ineffective stroke 
proportional to the discharge of the primary fluid pressure chamber until 
it comes into mechanical contact with the floating piston in the event of 
a failure in pressure buildup in the primary fluid pressure chamber. 
Therefore the brake fluid pressure is built up immediately in the 
secondary fluid pressure chamber while the elastic member is being 
compressed. Therefore, in case of a failure in pressure buildup in the 
primary fluid pressure chamber, the time lag required for increasing the 
brake fluid pressure in the secondary fluid pressure chamber can be 
decreased.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a sectional view showing one embodiment of a tandem master 
cylinder of the present invention. 
In the interior of the body 2 of the tandem master cylinder 1, a cylinder 
bore 3 is formed, and a main piston 4 and a floating piston 5 are movably 
inserted. A reservoir 6 is mounted in the upper part of the body 2, in 
which a specific quantity of brake oil is filled. A mounting flange 7 is 
provided; and a push rod 8 is operated by a servo motor or a brake pedal 
which are not shown. 
In the cylinder bore 3, there are formed a primary fluid pressure chamber 9 
between the main piston 4 and the floating piston 5, and a secondary fluid 
pressure chamber 11 between the floating piston 5 and a front end wall 10 
of the cylinder. Reference numerals 12, 13, 14 and 15 denote annular 
piston seals which are mounted on the main piston 4 and the floating 
piston 5 and slide on the inner wall surface of the cylinder bore 3 with 
the movement of the main piston 4 and the floating piston 5. 
In the open left end 16 of the cylinder bore 3 there is fixedly installed a 
stop ring 17, by which the maximum retreat position, i.e., the return 
position, of the main piston 4 is set. 
In the front end section of the main piston 4 one end of a stopping rod 18 
is threadedly inserted, while on the other end of the stopping rod 18 a 
head section 19 is formed. The head section 19 is disposed in an engaged 
state inside a cup-shaped spring retainer 20. Between the main piston 4 
and the cup-shaped spring retainer 20, a primary return spring 21 whose 
mounting load is set by the engagement of the head section 19 with the 
cup-shaped spring retainer 20 is compressibly mounted. 
Reference numeral 22 denotes a return port which is open to release the 
pressure of the primary fluid pressure chamber 9 out into the atmosphere 
in the reservoir 6. A supply port 23 is provided through which the brake 
fluid in the reservoir 6 is drawn into the primary fluid pressure chamber 
9 from between the main piston 4 and the outer periphery of the piston 
seal 13 when the pressure of the primary fluid pressure chamber 9 has 
decreased lower than the atmosphere in the reservoir 6 during brake return 
operation. A an output port 24 connects the primary fluid pressure chamber 
9 to one of the braking systems (not illustrated). A chamber 25 enclosed 
by the cup-shaped spring retainer 20 constantly communicates with the 
primary fluid pressure chamber 9 through a small hole 26. 
The rear section of the floating piston 5 is in constant contact with a 
collar section 27 formed at the front of the cup-shaped spring retainer 
20; when the main piston 4 is in the return position shown in FIG. 1, the 
floating piston 5 is given the maximum separate position from the main 
piston 4, that is, the return position shown in FIG. 1, by the mounting 
load of the primary return spring 21. Reference numeral 28 designates a 
stop bolt which is provided when required to limit the maximum retreat 
position of the floating piston 5. 
Between the front section of the floating piston 5 and the cylinder front 
end wall 10 is installed a secondary return spring 29 having a lower 
mounting load than that of the primary return spring 21; the floating 
piston 5 is pressed by this secondary return spring 29 toward returning. 
Reference numeral 30 denotes a return port for releasing the pressure of 
the secondary fluid pressure chamber 11 into the atmosphere in the 
reservoir 6. A supply port 31 is provided for drawing the brake fluid from 
the reservoir 6 into the secondary fluid pressure chamber 11 through 
between the outer periphery of the floating piston 5 and the piston seal 
15 and the inner wall of the cylinder bore 3 when the pressure of the 
secondary fluid pressure chamber 11 has decreased lower than the 
atmosphere in the reservoir 6 during brake returning operation. An output 
port 32 connects the secondary fluid pressure chamber 11 to the other 
braking system (not shown). 
Within the floating piston 5 a small-diameter cylinder 33 resides coaxially 
with the cylinder bore 3; in this small-diameter cylinder 33 a small 
diameter piston 34 is longitudinally movably inserted. Reference numerals 
35 and 36 denote small-diameter piston seals which are mounted on the 
small-diameter piston 34 and slide on the inner wall surface of the 
small-diameter cylinder 33. The small-diameter piston seal 35 is mounted 
to prevent leakage of the fluid pressure from the chamber 25 communicating 
with the primary fluid pressure chamber 9 through the small port 26 to a 
chamber 37 communicating with the secondary fluid pressure chamber 11, and 
the small-diameter piston seal 36 is mounted to prevent leakage of the 
fluid pressure reversely the chamber 37 to the chamber 25. The chamber 37 
is formed between a stopping section 38 provided on the end wall section 
of the small-diameter cylinder 33 and the front end section of the 
small-diameter piston 34 for the purpose of checking the forward movement 
of the small-diameter piston 34 in the small-diameter cylinder 33. 
At the rear end of the small-diameter piston 34, a boss 39 is formed, 
facing to contact the head section 19 of the stopping rod 18; and at the 
front is formed a spring receiving hole 41 into which the rear end of a 
coil spring 40 as an elastic member having elasticity in the direction of 
compression can be inserted. The small-diameter piston 34 is pressed 
forward into contact with the stopping section 38 by a small-diameter 
piston spring 42. The coil spring 40 is arranged in a free-length state so 
that it will be inserted in a through hole 44 provided in the front end 
section 43 of the floating piston 5, with its rear end set in contact with 
the receiving hole 41 of the small-diameter piston 34 and with its front 
end placed in contact with a recess section 45 formed in the cylinder 
front end wall 10. The elastic force of the coil spring 40 is set at a 
greater compressive elastic force than the elastic force of the 
small-diameter piston spring 42. 
In the return state shown in FIG. 1, the distance Z from the head section 
19 of the stopping rod 18 to the rear boss 39 of the small-diameter piston 
34 is set equal to the distance X between the front end of the main piston 
4 and the cup-shaped spring retainer 20. That is, the length of the 
small-diameter piston 34 is set so that Z will be equal to X. 
Operation of the embodiment shown in FIG. 1 will be explained. First, 
normal operation will be explained. 
When the push rod 8 is not pushed and therefore the main piston 4 and the 
floating piston 5 remain both in their return positions, the primary fluid 
pressure chamber 9 and the secondary fluid pressure chamber 11 communicate 
with the reservoir 6 via the return ports 22 and 30, respectively, being 
open to the atmosphere therein. 
As the push rod 8 is pushed to the right, the secondary return spring 29 is 
compressed primary by the action of the primary return spring 21 having a 
greater mounting load than that of the secondary return spring 29, and the 
main piston 4 and the floating piston 5 move forward simultaneously while 
maintaining the distance shown. Accordingly, the return port 22 is closed 
by the piston seal 13 while the return port 30 by the piston seal 15, thus 
resulting in compression of the secondary fluid pressure chamber 11. 
The coil spring 40 is so set that, when not operating (when not producing a 
fluid pressure), the coil spring 40 is in the state of free length, and 
therefore the elastic force thereof is zero, and as the fluid pressure is 
built up to compress the spring, the elastic force of the spring increases 
greater than that of the small-diameter piston spring 42. Therefore, with 
the advance of the floating piston 5 as described above, the 
small-diameter piston 34 held by the small-diameter piston spring 42 and 
the coil spring 40 having a greater elastic force than that of the 
small-diameter piston spring 42 is moved backward in relation to the 
floating piston 5 while compressing the small-diameter piston spring 42, 
thus decreasing the volume of the chamber 25 while increasing the volume 
of the chamber 37. Therefore, the forward stroke of the floating piston 5 
corresponds to the sum of the quantity of the brake fluid discharged 
through the output port 32 plus the increased volume, that is, becomes 
great as compared with a cylinder without the small-diameter piston 34, 
thus alleviating the increase of the fluid pressure in the secondary fluid 
pressure chamber 11 until the passage of the piston seal 15 over the 
return port 30 and consequently preventing biting of the piston seal 15 by 
the return port 30 (this effect is similar to that of a prior art tandem 
master cylinder shown in FIG. 4). 
In the meantime, the fluid pressure is built up in the primary fluid 
pressure chamber 9 by the volumetric decrease of the chamber 25 even when 
no change is made in the relative separate positions of the main piston 4 
and the floating piston 5. This fluid pressure, however, corresponds to 
the product of the effective surface area of the small-diameter piston 34 
and the amount of backward movement of the small-diameter piston 34 
relative to the floating piston 5. Accordingly, the main piston 4 operates 
in such a manner that the piston seal 13 will not be bitten, allowing 
almost all the fluid pressure to be discharged for consumption through the 
output port 24. 
As described above, when the main piston 4 and the floating piston 5 move 
forward until the piston seals 13 and 15 pass over the return ports 22 and 
30, respectively, the secondary return spring 29 is compressed to decrease 
a load difference between the primary return spring 21 and the secondary 
return spring 29. When the spring load is balanced between the primary 
return spring 21 and the secondary return spring 29, the floating piston 5 
is freely moved back and forth by the fluid pressure and therefore there 
exist balanced fluid pressures between the primary fluid pressure chamber 
9 and the secondary fluid pressure chamber 11 and consequently the same 
fluid pressure is outputted at the output ports 24 and 32, being 
discharged into respective hydraulic braking systems to thereby operate 
the braking equipment. 
FIG. 2(a) and FIG. 2(b) are views showing a difference in the positional 
relationship of the head section 19 of the stopping rod 18 relative to the 
rear boss 39 of the small-diameter piston 34 between the present invention 
and a prior art in the process of operation in which only the secondary 
fluid pressure chamber 11 is operated in case of a failure in fluid 
pressure buildup in the primary fluid pressure chamber 9. FIG. 2(a) shows 
one embodiment of the present invention shown in FIG. 1, and FIG. 2(b) 
shows a prior art shown in FIG. 4. In FIG. 2(a) and FIG. 2(b) are shown 
the tandem master cylinders respectively with the front end of the main 
piston 4 (61) immediately before contacting the cup-shaped spring retainer 
20 (62) in the event of a failure in producing the fluid pressure in the 
primary fluid pressure chamber 9 (52). Reference numerals used in FIG. 
2(b) are the same as those in FIG. 4 described above. 
When the push rod 8 is pushed in case the hydraulic braking system 
connected to the primary fluid pressure chamber 9 has failed in brake 
application for some reason or other, the main piston 4 first makes an 
ineffective stroke for a distance X shown in FIG. 1 until the front end 
contacts the cup-shaped spring retainer 20. As shown in FIG. 2(a), the 
rear end portion of the small-diameter piston 34 is formed long enough to 
reach the position where the head section 19 of the stopping rod 18 
contacts the boss 39 of the small-diameter piston 34; thereafter the 
small-diameter piston 34 moves forward with the advance of the main piston 
4 while compressing the coil spring 40 without changing the relative 
positions of the floating piston 5 and the small-diameter piston 34, 
whereby the fluid pressure in the secondary fluid pressure chamber 11 
increases. The floating piston 5 at this time is stroked by the coil 
spring 40 for the length of compression thereof, and therefore the stroke 
thus made is not included in Z shown in FIG. 1 which is the distance 
between the head section 19 of the stopping rod 18 and the rear boss 39 of 
the small-diameter piston 34. 
That is, since the rod member 57 used in a prior art tandem master cylinder 
is a non-elastic member as shown in FIG. 2(b) and FIG. 4, the distance Z 
between the head section 60 of the stopping rod 59 and the rear boss 58 of 
the small-diameter piston 56 is required, in normal operation, to be equal 
to the sum of the distance X between the front end section of the main 
piston 61 and the cup-shaped spring retainer 62 and the distance Y between 
the front end section 63 of the floating piston 54 and the front end wall 
64 of the cylinder (Z=X+Y). According to one embodiment of the present 
invention shown in FIG. 1, however, the distance Z between the head 
section 19 of the stopping rod 18 and the rear boss 39 of the 
small-diameter piston 34 is sufficient if it is equal to the distance X 
(Z=X) between the front end of the main piston 4 in the return position 
and the cup-shaped spring retainer 20, and therefore it is possible to 
reduce the time lag required for increasing the fluid pressure on the 
secondary fluid pressure chamber 11 in case of a failure on the primary 
fluid pressure chamber 9. 
Furthermore, in FIG. 1, in case of a failure on the secondary fluid 
pressure chamber 11, the fluid pressure in the primary fluid pressure 
chamber 9 rises after the contact (ineffective stroke Y) of the floating 
piston 5 with the cylinder front end wall 10. At this time, the boss 39 of 
the small-diameter piston 34 contacts the head section 19 of the stopping 
rod 18 once, but the small-diameter piston 34 moves forward with the rise 
of the fluid pressure in the primary fluid pressure chamber 9, stopping at 
the stop section 38 of the small-diameter cylinder 33. 
FIG. 3 is a view showing a relationship between the braking pressure in the 
secondary fluid pressure chamber 11 and the moving distance of the main 
piston 4 in case of a failure on the primary fluid pressure chamber 9. 
In the prior art tandem master cylinder, after the main piston 4 has made 
an ineffective stroke for the distance X, the small-diameter piston 56 
further makes a relative backward movement with respect to the floating 
piston 54 which is moving forward, and no fluid pressure buildup takes 
place in the secondary fluid pressure chamber 53 until the boss 58 
contacts the head section 60 of the stopping rod 59 which is moving 
forward. And as indicated by a broken line, a time lag has occurred in the 
fluid pressure rise. According to the present invention, however, the time 
lag is a time equivalent to the distance Z=X and therefore it is possible 
to decrease a time lag of the braking pressure transmitted to the braking 
equipment from the tandem master cylinder 1 in case of failure while 
preventing biting of the piston seals 13 and 15 by the return ports 22 and 
30. 
It is to be noticed that in the embodiment of FIG. 1, the coil spring 40, 
which is employed as an elastic member, is not limited to the embodiment 
and can be replaced with such an elastic member of rubber, synthetic 
resin, etc.