Flushing valve system in closed circuit hydrostatic power transmission

A flushing valve system suitable for use in a closed circuit hydrostatic transmission including a variable displacement hydraulic pump, a hydraulic actuator and two main lines. The flushing valve system includes poppet valves mounted in passages for respectively communicating the main lines with a fluid tank, passages mounting throttles for causing the pressure in the main lines to act on pressure chambers to keep poppets of the poppet valves from moving to an open position, and a change-over valve responsive to the pressure differential between the main lines to communicate the pressure chamber of the poppet valve connected to the main line on the lower pressure side with the fluid tank. The poppet valve connected to the main line on the lower pressure side opens when a predetermined pressure level is exceeded by the pressure in the main line, to thereby allow working fluid in the main line to be drained to the fluid tank.

BACKGROUND OF THE INVENTION 
This invention relates to closed circuit hydrostatic power transmissions 
comprising a variable displacement hydraulic pump and a hydraulic actuator 
connected to each other in a closed hydraulic circuit, and, more 
particularly, to a flushing valve system used in a closed circuit 
hydrostatic power transmission of, for example, hydraulic power shovels, 
which flushing valve system is operative in cooperation with fluid 
replenishing means, such as a charge pump, to replace the working fluid in 
the closed hydraulic circuit with a fresh supply of fluid. 
Heretofore, a closed circuit hydrostatic power transmission is provided 
with a spool type flushing valve including two inlet ports each connected 
to one of two main lines of the closed hydraulic circuit, one outlet port 
connected to a working fluid reservoir or tank, and a spool having two 
ends each receiving a pressure from one of the two main lines and moved by 
the pressure differential to a position in which one of the two inlet 
ports is allowed to communicate with the outlet port. This type of 
flushing valve permits, when a predetermined pressure differential is 
produced between the two main lines, the inlet port connected to the main 
line of the higher pressure side to be blocked by the spool and the main 
line of the lower pressure side to be communicated with the tank, so as to 
discharge a portion of the working fluid in the main line into the tank. 
In a spool type flushing valve of the prior art, it has been usual practice 
to have a large stroke of the spool, both to prevent the hydraulic motor 
from slipping due to pressure fluid leaks from the main line on the high 
pressure side through a gap between the body and spool of the flushing 
valve, and to increase the area of a passage between the inlet port 
communicating with the main line on the lower pressure side and the outlet 
port connected to the tank. Thus, the spool of the flushing valve of the 
prior art to has a long switching time when shifting from a first 
position, in which it allows the one inlet port to communicate with the 
outlet port, to a second position, in which it allows the other inlet port 
to communicate with the outlet port. It sometimes happens in a closed 
circuit hydrostatic power transmission that the internal pressures of the 
main lines suddenly show a change. For example, when pressure fluid is 
supplied from a hydraulic pump to a hydraulic motor connected to a load of 
high inertia to drive same, the main line connected to the discharge side 
of the hydraulic pump becomes a higher pressure side main line and the 
main line connected to the suction side thereof becomes a lower pressure 
side main line. If an attempt is made to suddenly decrease the volume of 
hydraulic fluid delivered by the hydraulic pump to stop rotation of the 
hydraulic motor, then the hydraulic motor would be driven by the inertia 
of the load connected to the hydraulic motor. This would result in the 
main line connected to the pump suction side that has been a lower 
pressure side main line becoming, for an instant, a higher pressure side 
main line and the main line connected to the pump discharge side that has 
been a higher pressure side main line becoming, for an instant, a lower 
pressure side main line. The flushing valve of the prior art has been 
unable to follow these sudden changes in the internal pressures of the 
main lines, so that even after the lower pressure side main line becomes a 
high pressure side main line, the main line still remains in communication 
with the tank through the flushing valve. Thus, working fluid of high 
pressure would be discharged into the tank and the hydraulic motor would 
continue to rotate without stopping. 
Furthermore, in spite of the fact that the variable displacement hydraulic 
pump has decreased the volume of pressure fluid delivered thereby, the 
hydraulic motor tries to rotate at a velocity at which it has been 
rotating up to then, so that the pressure fluid flowing into the hydraulic 
motor would be reduced in volume and the pressure in the main line 
connected to the pump discharge side would become negative or 
subatmospheric, so that cavitation might develop and cause damage to the 
hydraulic motor. To avoid this accident, it has been usual practice to 
increase the volume of a charge pump for replenishing the main lines with 
pressure fluid to a level high enough to avoid the development of 
cavitation or to decrease the changing rate of the volume of pressure 
fluid delivered by the variable displacement hydraulic pump. However, an 
increase in the capacity of the charge pump is not desirable because it 
increases energy losses and increases limitations to be placed on 
designing. A decrease in the changing rate of the volume of pressure fluid 
delivered by the variable displacement hydraulic pump reduces its 
operational ability. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide a novel 
flushing valve system which is free from the aforesaid disadvantages of 
the prior art, and has a high switching speed and low pressure fluid leak 
level. 
According to the invention, there is provided, in a closed circuit 
hydrostatic power transmission comprising a variable displacement 
hydraulic pump, a hydraulic actuator, first and second main lines 
connecting the variable displacement hydraulic pump and the hydraulic 
actuator together in a closed hydraulic circuit, a fluid tank, and fluid 
replenishing means connected to the first and second main lines for 
replenishing them with working fluid from the fluid tank, a flushing valve 
system interposed between the first and second main lines and the fluid 
tank and operative, when a predetermined level of pressure differential is 
produced between the first and second main lines, to bring the main line 
on the lower pressure side into communication with the fluid tank and the 
main line on the higher pressure side out of communication therewith. The 
flushing valve system comprises a first poppet valve located between the 
first main line and the fluid tank for bringing them into and out of 
communication with each other, a second poppet valve located between the 
second main line and the fluid tank for bringing them into and out of 
communication with each other, and pressure differential responding means 
responsive to the pressure differential between the first and second main 
lines to bring the poppet valve communicating with the main line on the 
higher pressure side to a closed condition, and the poppet valve 
communicating with the main line on the lower pressure side to an openable 
condition. 
Additional and other objects, features and advantages of the invention will 
become apparent from the description set forth hereinafter when considered 
in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings wherein like reference numerals are used 
throughout the various views to designate like parts and, more 
particularly, to FIG. 1, according to this figure, closed circuit 
hydrostatic power transmission, including a prior art flushing valve, 
comprises a variable displacement hydraulic pump 1, a hydraulic actuator 
or a hydraulic motor 2, and two main lines A and B connecting the pump 1 
and the motor 2 together in a closed hydraulic circuit. The two main lines 
A and B have connected thereto a crossover relief valve 3 for regulating 
the internal pressures of the two main lines A and B to keep same from 
exceeding a predetermined allowable maximum pressure, and a flushing valve 
4 for bringing one of the two main lines A and B into communication with a 
low-pressure line C communicating with a fluid tank 10 and having a relief 
valve 5 mounted therein. The two main lines A and B further have connected 
thereto fluid replenishing means comprising a charge pump 6, check valves 
7 and 8 and a charge-pressure-setting relief valve 9. The flushing valve 4 
comprises a body 4a, a spool 4b, springs 4c and 4d, seats 4e and 4f, 
pressure chambers 4g and 4h, an outlet chamber 4i, inlet ports 4j and 4k 
and an outlet port 4l. Assume, for example, that the variable displacement 
hydraulic pump 1 is actuated to drive the hydraulic motor 2 and the main 
line A becomes a main line on the high pressure side, then a pressure 
differential is produced between the pressure chambers 4g and 4h which 
causes the spool 4b to shift to the right in FIG. 1. This brings the inlet 
port 4j out of communication with the outlet port 4l and the inlet port 4k 
into communication with the outlet port 4l through the outlet chamber 4i. 
Thus, the main line A is blocked and the main line B is communicated with 
the low-pressure line C having the relief valve 5 mounted therein. As the 
internal pressure of the main line B becomes higher than the pressure 
level at which the relief valve 5 is set, the pressure fluid in the main 
line B is returned to the fluid tank 10. Meanwhile, as the internal 
pressure of the main line A or B becomes lower than the pressure level at 
which the relief valve 9 is set, pressure fluid is freshly supplied to the 
main line A or B through the check valve 7 or 8 by the charge pump 6. 
Since the pressure level at which the relief valve 9 is set is higher than 
the pressure level at which the relief valve 5 is set, the charge pump 6 
positively supplies pressure fluid to the main line A or B on the lower 
pressure side, while the pressure fluid discharged from the hydraulic 
motor 2 is returned to the fluid tank 10 through the flushing valve 4 in a 
volume equal to the replenishment, thereby effecting a fluid replacement. 
As described hereinabove, due to the spool type flushing valve 4 of the 
prior art having an elongated spool, it has a prolonged switching time 
and, consequently the hydrostatic power transmission has a low response 
characteristic and may develop cavitation. An added disadvantage is that 
it is necessary to use a charge pump of a large size. 
Referring to FIG. 2, a flushing valve system of the present invention 
generally designated by the reference numeral 11 comprises two poppet 
valves generally designated by the reference numerals 12 and 13, a 
change-over valve generally designated by the reference numeral 14 and two 
throttles 15 and 16. The poppet valves 12 and 13 comprises inlet chambers 
12a and 13a connected to the main lines A and B, respectively, outlet 
chambers 12b and 13b both connected to the low-pressure line C, valve 
seats 12f and 13f, poppets 12c and 13c cooperating with the valve seats 
12f and 13f for bringing the inlet chambers 12a and 13a into and out of 
communication with the outlet chambers 12b and 13b, respectively, pressure 
chambers 12d and 13d establishing pressures acting on the poppets 12c and 
13c to urge the same toward the valve seats 12f and 13f, respectively, and 
springs 12e and 13e forcing the poppets 12c and 13c toward the valve seats 
12f and 13f, respectively. The poppets 12c and 13c are arranged such that 
they are urged by the internal pressures of the inlet chambers 12a and 13a 
in a direction away from the valve seats 12f and 13f, respectively. The 
springs 12e and 13e are preloaded in such a manner that, with the 
pressure chambers 12d and 13d being in communication with the fluid tank 
10, the poppets 12c and 13c are kept in contact with the valve seats 12f 
and 13f, respectively, until a predetermined level is exceeded by the 
internal pressures of the inlet chambers 12a and 13a or the main lines A 
and B. Stated differently, the springs 12e and 13e of the poppet valves 12 
and 13 set a maximum pressure for the main line that is a main line on the 
lower pressure side. The pressure at which the poppet valves 12 and 13 are 
set to open is set at a lower level than the pressure at which the relief 
valve 9 of the fluid replenishing means is set to open. 
The change-over valve 14 comprises pressure chambers 14a and 14b connected 
to the main lines A and B, respectively, two poppets 14d and 14e exposed 
to the pressures in the pressure chambers 14a and 14b, respectively to 
jostle each other through a push-rod 14c, weak springs 14f and 14g for 
restoring the poppets 14d and 14e to a neutral position when no pressure 
differential exists between the main lines A and B, a common port 14h 
connected to the low-pressure line C, a change-over chamber 14i, and 
change-over ports 14j and 14k connected to the pressure chambers 12d and 
13d, respectively, of the poppet valves 12 and 13. The push rod 14c has a 
length which is set such that when one poppet 14e is in a position in 
which it brings the adjacent change-over port 14k out of communication 
with the common port 14h, the other poppet 14d is disposed in a position 
in which it brings the adjacent change-over port 14j into communication 
with the common port 14h. The throttle 15 is mounted in a passage 
connecting the main line A with the pressure chamber 12d of the poppet 
valve 12, and the throttle 16 is mounted in a passage connecting the main 
line B with the pressure chamber 13d of the poppet valve 13. The 
change-over valve 14 and the throttles 15 and 16 constitute pressure 
differential responding means responsive to the pressure differential 
between the main lines A and B to control poppet valves 12 and 13. 
Operation of the embodiment constructed as shown in FIG. 2 will now be 
described. In the absence of pressure differential between the main lines 
A and B, the change-over valve 14 is in a neutral position, with the 
common port 14h being in communication with the two change-over ports 14j 
and 14k through the change-over chamber 14i. This brings the pressure 
chambers 12d and 13d of the poppet valves 12 and 13 into communication 
with the fluid tank 10 through the low-pressure line C. Though the 
pressure chambers 12d and 13d are in communication with the main lines A 
and B, respectively, the pressure in the pressure chambers 12d and 13d is 
reduced to substantially the same level as the fluid tank, because the 
throttles 15 and 16 produce suitable pressure drops between the main lines 
A and B and the pressure chambers 12d and 13d, respectively. The condition 
of the poppet valve 12 or 13 in which the pressure chamber 12d or 13d is 
connected to the fluid tank 10 and the pressure in the pressure chamber 
12d or 13d is substantially the same as that of the tank 10 will be 
referred to as an openable condition. In this condition, as the internal 
pressure of the main lines A and B i.e. the internal pressure of the inlet 
chambers 12a and 13a rises above a cracking pressure level set by the 
springs 12e and 13e of the poppet valves 12 and 13, respectively, the 
poppets 12c and 13c are released from contact with the respective valve 
seats 12f and 13f to allow the inlet chambers 12a and 13a to communicate 
with the outlet chambers 12b and 13b, respectively, or render the poppet 
valves 12a and 13a open. Thus, the main lines A and B are communicated 
with the fluid tank 10 to allow the pressure fluid to be released into the 
tank 10 from the main lines A and B. This condition arises when, for 
example, the delivery by the hydraulic pump 1 is reduced to zero (0) and 
the hydraulic motor 2 is shut down while the charge pump 6 alone is 
operating. 
Starting of operation of the hydraulic motor 2 from this condition will be 
described. First, the variable displacement hydraulic pump 1 is actuated 
in such a manner that it delivers pressure fluid to the main line A. 
Delivery of pressure fluid to the main line A by the hydraulic pump 1 
raises the internal pressure of the main line A. Before the internal 
pressure of the main line A reaches the cracking pressure level of the 
poppet valve 12, the change-over valve 14 is caused to shift to the right 
in FIG. 2 by the pressure differential between the main lines A and B, to 
bring the pressure chamber 12d of the poppet valve 12 out of communication 
with the fluid tank 10 while the pressure chamber 13d of the poppet valve 
13 is still in communication with the tank 10. With the pressure chamber 
12d of the poppet valve 12 being connected to the main line A through the 
throttle 15, the internal pressure of the main line A is introduced into 
the pressure chamber 12d. Thus, the poppet 12c of the poppet valve 12 is 
forced against the valve seat 12f by a combination of the internal 
pressure of the pressure chamber 12d or the main line A and the biasing 
force of the spring 12e, thereby preventing the poppet 12c from being 
brought out of contact with the valve seat 12f even if the internal 
pressure of the inlet chamber 12a rises above the cracking pressure level. 
This condition will be referred to as a closed condition. A rise in the 
internal pressure of the main line A immediately brings the poppet valve 
12 associated with the main line A to the closed condition, so that all 
the pressure fluid in the main line A is sent to the hydraulic motor 2 to 
rotate same. Meanwhile, the poppet valve 13 associated with the main line 
B is kept in the openable condition, so that the valve 13 is opened when 
the fresh supply of pressure fluid from the charge pump 6 raises the 
internal pressure of the main line B above the cracking pressure level, to 
allow the pressure fluid to flow from the main line B to the tank 10 
through the poppet valve 13. 
Suppose that the delivery by the hydraulic pump 1 is suddenly decreased in 
order to stop the hydraulic motor 2 when in this condition. As a result, 
the load that has been driven by the hydraulic motor 2 tends to drive the 
hydraulic motor 2 by its inertia. This suddenly raises the internal 
pressure of the main line B connected to the suction side of the hydraulic 
pump 1 and having a lower pressure and changes the main line A to a lower 
pressure side from a high pressure side. These changes in the internal 
pressures of the main lines A and B are immediately transmitted to the 
change-over valve 14, to thereby bring the poppet valve 12 associated with 
the main line A to the openable condition and the poppet valve 13 
associated with the main line B to the closed condition. The poppet valves 
12 and 13 have high switching speed because they can shift from full 
closed position to open position of large fluid passage area with a small 
amount of movement. Thus, the poppet valves 12 and 13 immediately bring 
the main line on the higher pressure side out of communication with the 
fluid tank 10 by quickly following up the sudden changes in the internal 
pressures of the main lines A and B. In this way, deceleration of the 
hydraulic motor 2 can be achieved by quickly responding to a sudden fall 
in the delivery by the hydraulic pump 1, and it is possible to avoid 
cavitation development in the main line on the lower pressure side which 
might otherwise occur due to a low amount of working fluid in the closed 
circuit caused by a discharge of the high pressure fluid through the 
flushing valve from the main line in which a sudden change in pressure 
from a low level to a high level has taken place, as is the case with a 
hydrostatic power transmission using a spool type flushing valve of the 
prior art. Also, since the poppet valves 12 and 13 are capable of securely 
shutting off the communication between the inlet chambers 12a and 13a and 
the outlet chambers 12b and 13b when the valves 12 and 13 are in a closed 
condition, the risk of the pressure fluid leaking from the main line on 
the higher pressure side through the poppet valve and discharged into the 
fluid tank 10 is eliminated. 
FIG. 3 shows still another embodiment of the invention in which 
electrically-operated means is used as the pressure differential 
responding means. More particularly, the pressure differential responding 
means comprises pressure sensors 17 and 18, a control device 19, connected 
to a power source 21, and a solenoid-operated directional control valve 
20. The pressure sensors 17 and 18 are operative to change the internal 
pressures of the main lines A and B to electric signals and supply same to 
the control device 19 which senses pressure differential between the 
inputted electric signals and decides which poppet valve should be brought 
to the openable condition and which poppet valve to the closed condition. 
When the main line A is on the higher pressure side or there is no 
pressure differential between the main lines A and B, the control device 
19 keeps the directional control valve 20 in a position shown in FIG. 3 
and communicates the pressure chamber 12d of the poppet valve 12 with the 
main line A through a check valve 22 while communicating the pressure 
chamber 13d of the poppet valve 13 with the tank 10 via the low-pressure 
line C. When the main line B is on the higher pressure side, the control 
device 19 switches the directional control valve 20 from the position 
shown in FIG. 3 to the other position, to thereby communicate the pressure 
chamber 12d of the poppet valve 12 with the tank 10 through the 
low-pressure line C and the pressure chamber 13d of the poppet valve 13 
with the main line B through a check valve 23. Thus, the poppet valve 12 
or 13 communicated with the main line on the higher pressure side is 
brought to the closed condition and the poppet valve 13 or 12 communicated 
with the main line on the lower pressure side is brought to the openable 
condition so that it opens when the pressure in the particular main line 
rises above the cracking pressure level. This embodiment is capable of 
sensing any pressure differential, no matter how small, and switching the 
directional control valve 20. 
FIG. 4 shows still another embodiment of the present invention, which 
includes, like the embodiment shown in FIG. 2, the poppet valves 12 and 
13, change-over valve 14 and throttles 15 and 16. The embodiment shown in 
FIG. 4 further includes passages connecting the throttles 15 and 16 at 
their outlet or downstream sides to the low-pressure line C, with pilot 
relief valves 24 and 25 being respectively mounted in one of the passages. 
However, the crossover relief valve 3 shown in FIG. 2 is dispensed with in 
FIG. 4. The setting pressure of the pilot relief valves 24 and 25 has a 
value such that a pressure high enough to keep the poppets 12c and 13c in 
closed condition until the main line pressures acting on the inlet 
chambers 12a and 13a reach an allowable maximum pressure is retained in 
the pressure chambers 12d and 13d of the poppet valves 12 and 13 
respectively. Thus, when the internal pressure of the main line A or B 
exceeds the setting pressure of the pilot relief valve 24 or 25, the pilot 
relief valve 24 or 25 opens so that the pressure introduced into the 
pressure chamber 12d or 13d of the poppet valve 12 or 13 is kept at the 
aforesaid setting pressure. Meanwhile, the internal pressure of the main 
line A or B acts on the inlet chamber 12a or 13a of the poppet valve 12 or 
13, so that the poppet valve 12 or 13 opens when the allowable maximum 
pressure is exceeded by the internal pressure of the main line A or B. 
Thus, by letting the flushing valve system 11 have a function like that of 
a balanced piston type relief valve, it is possible to eliminate the need 
to use the crossover relief valve 3 of a large capacity (shown in FIGS. 
1-3) by using the pilot relief valves 24 and 25 of a small capacity. 
In the embodiments shown in FIGS. 2 and 4, the low-pressure lines C and C' 
are connected directly to the tank 10 and are provided with no pressure 
reducing means so that the pressure in the pressure, chambers 12d and 13d 
of the poppet valves 12 and 13, is reduced to the same level as the tank 
10 when the pressure chambers 12d, 13d are in communication with the tank 
10 through the low-pressure line C or C'. However, the invention is not 
limited to this arrangement. A suitable relief valve 5 in FIG. 1 or a 
suitable throttle may be used in the low-pressure line C or C' so that a 
predetermined pressure higher than that in the tank 10 will remain in the 
pressure chambers 12d, 13d of the poppet valve 12, 13 when the pressure 
chambers are in communication with the tank 10. If this is the case, the 
preload of the springs 12e and 13e of the poppet valves 12, 13 may be 
decreased. 
The flushing valve system according to the present invention can be used 
not only in a closed hydrostatic power transmission with a hydraulic pump 
but also in that with a single rod cylinder. FIG. 5 shows an embodiment of 
the invention in which a single rod cylinder 26 is used in place of the 
hydraulic motor 2 of the embodiment shown in FIG. 4. In addition, in the 
embodiment of FIG. 5, the low-pressure line C connected to the poppet 
valves 12 and 13 and the change-over valve 14 is connected to the 
discharge line of the charge pump 6 so that it is connected to the tank 10 
through the relief valve 9. An accumulator 28 is also connected to the 
discharge line of the charge pump 6. 
The operation of the embodiment of FIG. 5 will be described. 
Suppose that the hydraulic pump 1 is discharging a pressure fluid into the 
main line A to move the piston 27 of the hydraulic cylinder 26 to the 
right, and that the main line A is the high pressure side main line, while 
the main line B is the low pressure side main line. At this time, the 
poppet valve 12 connected to the high pressure side main line A is in the 
closed condition in which the pressure in the main line A is introduced in 
the pressure chamber 12d, meanwhile the other poppet valve 13, connected 
to the low pressure side main line B, is in the open condition in which 
the pressure in the pressure chamber 13d is substantially the same as the 
discharge pressure of the pump 6 which is determined by the relief 
pressure of the relief valve 9. When the piston 27 is withdrawn into the 
cylinder 26 (moved to the right in FIG. 5), pressure fluid is discharged 
into the main line B from the cylinder 26 through its bottom side in a 
larger volume then that introduced from the main line A into the cylinder 
through its rod side, due to the difference in area between the rod 
hydraulic side and the bottom side of the single rod cylinder 26. This 
causes an excess to be produced in the working fluid in the closed 
hydraulic circuit including the main lines A and B, resulting in pressure 
rise in the low pressure side main line B. When the pressure in the main 
line B or in the inlet chamber 13a of the poppet valve 13 rises above a 
cracking pressure of the poppet valve 13 determined by the spring 13e and 
the pressure in the pressure chamber 13d, the poppet valve 13 is opened to 
allow the excess working fluid discharged from the low pressure side main 
line B to the low-pressure line C. A part of the excess fluid flowing into 
the low-pressure line C is discharged to the tank 10 through the relief 
valve 9 and the reminder is stored in the accumulator 28. 
When the flow rate of the hydraulic pump 1 is suddenly decreased in order 
to stop the movement of the piston of the hydraulic cylinder 26, there 
would occur sudden changes in pressure in the main lines A and B. As 
explained in the embodiment of FIG. 2, the flushing valve system 11 can 
rapidly respond to the changes in the main line pressures. Thus, the 
hydraulic cylinder 26 is operated rapidly in response to the change in 
flow rate of the discharged fluid from the hydraulic pump. 
On the other hand, during a movement of the piston 27 to the left, there is 
a lack of working fluid in the closed hydraulic circuit. Working fluid is 
supplied from the accumulator and the charge pump 6. The use of the 
accumulator 28 makes it possible to reduce the capacity of the charge pump 
6. 
As is clear from the above description, the embodiment shown in FIG. 5 is 
capable of improving the responsiveness of a hydrostatic power 
transmission and avoiding cavitation development by utilizing the high 
switching speed of the flushing valve system 11, like the embodiments 
shown in FIGS. 2-4. In addition, the embodiment of FIG. 5 has additional 
advantages which will be described. 
The rightward movement of the piston 27 of the single rod cylinder 26 
causes an excess to be produced in the working fluid, which is discharged 
through one of the poppet valves into the low-pressure line C. During the 
time the piston 27 of the single rod cylinder 26 is being withdrawn (moved 
to the right in FIG. 5) into the cylinder 26, it is possible that the 
direction in which a load is applied may suddenly be reversed. For 
example, while the load is being driven by the piston 27, the piston 27 
may inadvertently be pushed by the load. When this happens, the main line 
A which has been on the higher pressure side would become a main line on 
the lower pressure side, and the main line B would become a main line on 
the higher pressure side. If the spool type flushing valve 4 of the prior 
art shown in FIG. 1 were used in place of the flushing valve system 11, 
switching would be effected as the flushing valve 4 passes through the 
neutral position shown in FIG. 1 in accordance with a change in the 
pressure in the main lines A,B. However, when the flushing valve 4 is 
neutral in position, the two inlet ports 4j and 4k are both out of 
communication with the outlet port 4l, so that the excess pressure fluid 
produced by the withdrawing of the piston 27 into the cylinder 26 would 
have no place to go and would be trapped in the closed hydraulic circuit. 
Thus, the piston 27 would be suddenly brought to a stop and an 
inordinately high pressure would be generated in the closed hydraulic 
circuit, thereby giving a shock to the transmission. 
In the embodiment shown in FIG. 5, during the time the flushing valve 
system 11 is being switched or when the change-over valve 14 is brought to 
the neutral position, the pressure chambers 12d and 13d of the poppet 
valves 12 and 13 are communicated with the low-pressure line C. Thus, when 
the pressure in the main lines A and B becomes higher than the cracking 
pressure of the poppet valves 12 and 13, pressure fluid flows from the 
main lines A and B to the fluid tank 10, thereby avoiding trapping of the 
excess pressure fluid in the closed hydraulic circuit. 
In the embodiment of FIG. 5, the low-pressure line C is connected to the 
tank 10 through the relief valve 9. It should be noted, however, that the 
line C may be directly connected to the tank 10. 
In the embodiments shown in FIGS. 2, 4 and 5, the low-pressure line C is 
communicated with the pressure chambers 12d and 13d of the two poppet 
valves 12 and 13 when the change-over valve 14 is in the neutral position. 
However, the invention is not limited to this arrangement and the line C 
may be connected to one of them to achieve the desired effect. 
From the foregoing description, it will be appreciated that according to 
the invention a poppet valve 12, 13 is mounted between each main line A, B 
and the fluid tank 10 and pressure differential responding means is used 
to bring the poppet valve communicated with the main line on the higher 
pressure side to the closed condition and the poppet valve communicated 
with the main line on the lower pressure side to the openable condition. 
By this feature, the switching speed of the flushing valve system 11 can 
be increased and fluid leaks can be minimized. Thus, it is possible to 
avoid cavitation development without increasing the capacity of the charge 
pump 6 or decreasing the changing rate of the delivery by the variable 
displacement hydraulic pump 1.