Fluid flow control system

A fluid flow control system has a capability of operating selectively in a power matching mode, in which power consumption is low, and a maximum flow mode in which high responsivity and high speed features can be appreciated. For this purpose, a mode selector valve is used to switch the system between these two modes.

The present invention generally relates to a fluid flow control system 
suited for use in an injection molding machine or a vehicle and, more 
particularly, to a fluid flow control system operable selectively in a 
power matching mode, in which energy consumption is relatively low, and a 
maximum discharge flow mode in which high responsivity and high speed 
features can be appreciated. 
Recently, injection molding machines, for example, have come to use a fluid 
flow control system operable in a power matching mode, which system 
comprises an orifice disposed on a main line leading from a 
variable-displacement pump and a load sensing valve operable in response 
to the pressure differential between the pressures on respective sides of 
the orifice for controlling a discharge flow control section of the 
variable-displacement pump to adjust the discharge rate of the 
variable-displacement pump to a value required to maintain the pressure 
differential substantially at a constant value. An example of this known 
fluid flow control system is disclosed in the U.S. Pat. No. 2,892,312, 
patented June 30, 1959. 
This known fluid flow control system operable in the power matching mode 
is, since both the discharge rate and the discharge pressure of the 
variable-displacement pump are controlled in response to the load demand, 
advantageous in that the energy consumption is relatively low with no 
possibility of an unnecessary fluid discharge. However, it is 
disadvantageous in the responsivity and high speed feature because the 
discharge rate of the variable-displacement pump is controlled through the 
load sensing valve and a transmission element, such as the discharge flow 
control section, having a delay characteristic. Because of this, where a 
high speed molding operation is desired to be performed in the injection 
molding machine to manufacture highly molded articles, the fluid control 
system having a capability of operating only in the power matching mode is 
not suited for this purpose. 
In addition, since the injection molding machine undergoes a sequence of 
metering, injection, mold clamping and mold release or opening processes, 
it has often been experienced that the high speed controllability is more 
important than the energy saving feature. In such case, the fluid flow 
control system operable only in the power matching mode is not suited for 
this purpose. 
Accordingly, the present invention has been developed with a view to 
substantially eliminating the foregoing disadvantages and inconveniences 
inherent in the prior art fluid flow control system and has for its 
essential object to provide an improved fluid flow control system which is 
operable selectively in a power matching mode and a maximum discharge flow 
mode depending on the demand of a load to be controlled so as to attain a 
low energy consumption and a high responsivity, high speed feature one at 
a time according to the mode. 
According to the present invention, the fluid flow control system having a 
capability of operating selectively under a power matching and a maximum 
discharge flow mode, which comprises a variable displacement pump having a 
discharge flow control section; at least one actuator; a main line 
extending between the pump and the actuator and having a restriction means 
such as orifice or choke disposed therein; a load sensing valve having 
pilot and spring chambers; a first fluid circuit including a power 
matching pilot line and a first pilot line, the differential pressure 
between the upstream and downstream sides of the restriction means being 
supplied to the pilot and spring chambers of the load sensing valve to 
operate said load sensing valve in such a way as to connect the discharge 
flow control section selectively to one of the main line and a fluid 
reservoir to control the discharge rate of the pump such that the 
differential pressure can be controlled to a predetermined value under the 
power matching mode; a branch line extending outwards from a portion of 
the main line upstream of the restriction means; a valve means disposed on 
the branch line and operable, when the load sensing valve is held 
standstill with the pump controlled to give the maximum discharge rate, to 
open; a mode selector valve disposed on the power matching pilot line for 
selectively closing and opening the power matching pilot line; and a 
second pilot line having a feed-in restriction such as an orifice or choke 
and fluid-connecting the spring chamber of the load sensing valve to a 
portion of the main line upstream of the restriction means, whereby, the 
power matching mode can be established when the power matching pilot line 
is opened to permit the load sensing valve and, on the other hand, the 
maximum discharge flow mode can be established when the power matching 
pilot line is closed to make the pressures respectively in the pilot and 
spring chambers of the load sensing valve equal to each other while the 
load sensing valve is held in one position by the action of a spring 
thereof thereby to permit the pump to give the maximum discharge rate and, 
at the same time, to permit the valve means to open.

Referring to FIG. 1, a variable-displacement pump 1, for example, a 
variable displacement piston pump of a type having a swash plate normally 
biased to the maximum permissible angle of inclination for giving a 
maximum discharge rate, has its discharge port fluid-connected with a main 
line 2. The main line 2 having an orifice 3 disposed thereon extends to a 
multi-joint coupling 7 from which branch lines 10, 11 and 12 extend 
outwards. The branch lines 10, 11 and 12 are in turn fluid-connected 
through switching valves 13, 14 and 15 to actuators 4, 5 and 6. The 
actuator 4 may be a hydraulic cylinder used in a mold clamping and 
releasing mechanism of an injection molding machine, the actuator 5 may be 
a hydraulic motor used in the same machine for driving a feed screw, and 
the actuator 6 may be a hydraulic cylinder used in the same machine for 
injecting a plasticized resinous material. 
The fluid flow control system according to the embodiment shown in FIG. 1 
comprises a load sensing valve 17, for example, a three port pilot valve, 
operable to control the discharge rate of the pump 1 in response to the 
pressure differential between the pressures downstream and upstream of the 
orifice 3, a by-pass type pressure compensated valve 18 disposed on a 
branch line 20a extending between a portion of the main line 2 upstream of 
the orifice 3 and a fluid reservoir 19, and a mode selector valve 21, for 
example, a two-position, four-port electromagnetically operated switching 
valve. 
The load sensing valve 17 has three ports, l, m and n and is so designed 
that, when the valve 17 is set in a position shown by V.sub.1, the ports l 
and n are communicated to each other while the port m is closed and, when 
it is set in a position V.sub.2, the ports m and n are communicated to 
each other while the port l is closed. This valve 17 also has a spring 
chamber 40 having therein a spring 41, and a pilot chamber 42, said spring 
41 exerting a biasing force which may correspond to a differential 
pressure of, for example, 6 kg/cm.sup.2 so that, when the differential 
pressure between the pilot and spring chambers 42 and 40 exceeds 6 
kg/cm.sup.2, the valve 17 can be set in the position V.sub.1 whereas, when 
the above described differential pressure is smaller than 6 kg/cm.sup.2, 
the valve 17 can be set in the position V.sub.2. The mode selector valve 
21 is of such a design that, when it is set in a position shown by 
S.sub.1, ports P and A thereof are communicated to each other while ports 
T and B are closed and, when it is set in a position shown by S.sub.2, the 
ports P and B are communicated to each other while the ports T and A are 
closed. 
The port l of the load sensing valve 17 is fluid-connected through a pilot 
line 25 to a portion of the main line 2 upstream of the orifice 3, and the 
port m thereof is fluid-connected to a fluid reservoir 27 through a pilot 
line 26. The port n of the load sensing valve 17 is communicated through a 
pilot line 28 to a discharge flow-control section 30 of the 
variable-displacement pump 1, which section 30 may be constituted by a 
swash plate control cylinder. The pilot chamber 42 of the valve 17 is 
communicated through a pilot line 31 to a portion of the main line 2 
upstream of the orifice 3 and the spring chamber 40 is communicated to the 
port P of the mode selector valve 21 through a pilot line 32. 
The port T of the mode selector valve 21 is communicated to the reservoir 
27 through a pilot line 33 and the port A thereof is communicated to a 
portion of the main line 2 downstream of the orifice 3 through a pilot 
line 34 having an orifice 35 disposed thereon. The port B of the valve 21 
is fluid-connected to the pilot line 25 through a pilot line 37. 
The pilot line 32 and 34 constitute a power matching pilot line which is, 
when the mode selector valve 21 is set in the position S.sub.1, completed 
to introduce a fluid pressure in that portion of the main line 2 
downstream of the orifice 3 to the spring chamber 40 of the load sensing 
valve 17 and which is disconnected when the valve 21 is set in the 
position S.sub.2 at which time a fluid pressure in that portion of the 
main line 2 upstream of the orifice 3 is introduced to the spring chamber 
40 through the pilot line 37. 
On the other hand, the by-pass type pressure compensated valve 18 has a 
spring chamber 45 communicated through a pilot line 46 to a portion of the 
pilot line between the port A of the selector valve 21 and the orifice 35. 
The pilot lines 34 and 46 constitute a pressure matching pilot line 
operable to introduce the fluid pressure in that portion of the main line 
2 downstream of the orifice 3 to the spring chamber 45 of the valve 18 
irrespective of the position of the selector valve 21. A spring 47 housed 
in the spring chamber 45 is so selected as to exert a biasing force which 
may correspond to the differential pressure of, for example, 8 kg/cm.sup.2 
and, therefore, when the differential pressure between the pilot chamber 
49 and the spring chamber 45, that is, the differential pressure between 
the respective portions of the main line 2 upstream and downstream of the 
orifice 3, exceeds 8 kg/cm.sup.2, the pressure compensated valve 18 opens 
to drain an excessive fluid to a reservoir 19 to keep the above described 
differential pressure at a value equal to 8 kg/cm.sup.2. 
The junction between the pilot lines 35 and 34 is also communicated to a 
reservoir 52 through a line 51 having disposed thereon an 
electromagnetically operated proportional pilot relief valve 50. 
It is to be noted that the variable-displacement pump 1 is shown as having 
an adjustment screw 55 for adjusting the maximum discharge rate thereof 
for avoiding any possible overload which would otherwise occur during the 
control operation under the pressure matching mode. 
The fluid flow control system of the construction described hereinbefore 
with reference to FIG. 1 is caused to operate is the following manner. 
At the start-up, a warm-up is performed in the following manner to increase 
the temperature of a fluid medium, for example, oil, thereby to reduce the 
viscosity of the fluid medium. For this purpose, the switching valves 13, 
14 and 15 are closed and the mode selector valve 21 is set in the position 
S.sub.2 to close the power matching pilot line. In this condition, the 
variable-displacement pump 1 is driven. 
As the pump 1 is driven, the fluid pressure in that portion of the main 
line 2 upstream of the orifice 3 is introduced to the spring chamber 40 of 
the load sensing valve 17 through the pilot lines 31, 25, 37 and 32 and, 
at the same time, to the pilot chamber 42 thereof through the pilot line 
31. Since the load sensing valve 17 is set in the position V.sub.2 by the 
action of the spring 41, the discharge flow control section 30 of the pump 
1 is communicated to the reservoir 27 so that the swash plate (not shown) 
of the pump 1 is inclined to assume the maximum angle of inclination 
determined by the position of the adjustment screw 55. In this condition, 
since the power matching pilot line (32, 34) is closed by the selector 
valve 21, the load sensing valve 17 remains set in the position V.sub.2 
irrespective of the differential pressure between the respective portions 
of the main line 2 upstream and downstream of the orifice 3 and, 
therefore, the variable-displacement pump 1 delivers a maximum discharge 
rate at all time during this condition. 
On the other hand, the fluid pressure in the pressure matching pilot line 
(34, 46) extending between the spring chamber 45 of the valve 18 and the 
orifice 35 is equal to the set pressure of the pilot relief valve 50 and, 
accordingly, the valve 18 opens to such an opening as required to render 
the fluid pressure in that portion of the main line 2 upstream of the 
orifice 3 to be higher than said set pressure by 8 kg/cm.sup.2 with any 
excessive fluid drained to the reservoir 19. 
Thus, since the variable displacement pump 1 delivers the maximum discharge 
flow determined by the position of the adjustment screw 55 in 
correspondence to the set pressure of the pilot relief valve 50 and the 
fluid delivered from the pump 1 is totally drained to the reservoirs 19 
and 52 through the valves 18 and 50, respectively, the pressure energy can 
be converted into the heat energy and, accordingly, the fluid flow control 
system is effective to quickly increase the temperature of the fluid 
medium, that is, the oil temperature, with the warm-up time consequently 
reduced. In other words, since the pressure control is effected at a high 
pressure while the pump 1 is permitted to deliver the maximum discharge 
flow under a pressure matching mode which is an example of the maximum 
discharge flow mode, high pressure energies can be converted into heat 
energies with the consequently reduced time required to perform the 
warm-up operation. 
Subsequently, the selector valve 21 is set in the position S.sub.1. When 
the valve 21 is so set, the discharge rate given by the pump 1 becomes 
zero and is in position to be controllable by the pressure determined by 
the pilot relief valve 50. If the switching valve 14 is opened to drive 
the hydraulic motor 5, while the above described condition has been 
established, so that the feed screw (not shown) can be rotated to effect a 
metering of a plasticized resinous material, the fluid pressure in that 
portion of the main line 2 upstream of the orifice 3 and that downstream 
of the orifice 3 are respectively supplied to the pilot chamber 42 of the 
valve 17 through the pilot line and to the spring chamber 40 of the same 
valve 17 through the selector valve 21 and the power matching pilot line 
(32, 34). Accordingly, the load sensing valve 17 operates in the following 
manner to maintain the differential pressure between the respective 
portions of the main line 2 upstream and downstream of the orifice 3 at a 
predetermined value by controlling the pump 1 to increase the discharge 
rate from the zero value to the value at which a necessary amount of fluid 
can be discharged therefrom. That is to say, the discharge rate of the 
variable-displacement pump 1 starts from the zero value and, in the case 
where the differential pressure between the respective portions of the 
main line 2 upstream and downstream of the orifice 3 is lower than the 
predetermined value (6 kg/cm.sup.2), the differential pressure between the 
pilot and spring chamber 42 and 40 of the valve 17 to which the pressures 
upstream and downstream of the orifice 3 are respectively transmitted is 
of a value smaller than the biasing force of the spring and, therefore, 
the load sensing valve 17 is set in the position V.sub.2. In this 
condition, the discharge flow control section 30 of the pump 1 is 
communicated to the reservoir 27 through the pilot line 28, the ports n 
and m of the valve 17 and the pilot line 26 so that the swash plate of the 
pump 1 can be inclined to assume the maximum angle of inclination to 
increase the discharge rate of the pump 1 for increasing the differential 
pressure in the respective portions of the main line 2 upstream and 
downstream of the orifice 3. It is to be noted that the response of the 
swash plate may be delayed at this time. On the other hand, if the 
pressure in the portion of the main line 2 upstream of the orifice 3 
increases and the differential pressure in the respective portions of the 
main line 2 upstream and downstream of the orifice 3 subsequently becomes 
a value higher than the predetermined pressure (6 kg/cm.sup.2), the 
differential pressure between the pilot and spring chambers 42 and 40 of 
the valve 17 becomes higher than the biasing force of the spring 41 in the 
spring chamber 40, thereby setting the valve 17 in the position V.sub.1. 
Once this condition is established, the discharge flow control section 30 
of the pump 1 is communicated to the main line 2 through the pilot line 
28, the ports n and l of the valve 17 and the pilot line 25, so that the 
swash plate can be so inclined as to decrease the discharge rate of the 
pump 1, thereby reducing the differential pressure in the respective 
portions of the main line 2 upstream and downstream of the orifice 3. 
Summarizing the above, the load sensing valve 17 is set selectively in the 
positions V.sub.1 or V.sub.2 depending on the differential pressure in the 
respective portions of the main line 2 upstream and downstream of the 
orifice 3 so that the discharge rate of the pump 1 can be controlled to 
maintain said differential pressure at the predetermined value (6 
kg/cm.sup.2). At this time, although the pressure in that portion of the 
main line 2 downstream of the orifice 3 is transmitted to the spring 
chamber 45 of the by-pass type pressure compensated valve 18 through the 
pressure matching pilot line (34, 46) the valve 18 remains closed because 
the spring 47 is so selected as to exert a biasing force required to open 
the valve 18 when the differential pressure between the pilot and spring 
chambers 49 and 45 attains a value higher than 8 kg/cm.sup.2. In other 
words, the by-pass type pressure compensated valve 18 is in an inoperative 
position and no control operation under the pressure matching mode occur. 
Accordingly, under this condition, the flow control takes place under the 
power matching mode wherein both of the discharge rate and the discharge 
pressure of the pump 1 is matched to the load demand with energy 
consumption being low. It is to be noted that, while the fluid control 
system operates under the power matching mode in the manner as 
hereinbefore described, the pressure in the main line 2 downstream of the 
orifice 3 is lower than the cracking pressure of the pilot relief valve 50 
and, accordingly, the valve 50 remains closed. It is also to be noted that 
the metering speed can be varied by varying the opening of the orifice 3. 
However, even in this case, care must be taken to the delay in fluid flow 
control occurring as in the previously described case. 
After the metering has completed, the switching valve 14 is closed and the 
selector valve 21 is set in the position S.sub.2 to temporarily establish 
the same condition as under the pressure matching mode, and thereafter, 
the switching valve 15 is switched over to cause the injection cylinder 6 
to advance for the purpose of filling the plasticized resinous material 
from a heated barrel (not shown) into a mold assembly in the injection 
molding machine. 
At the instant that the selector valve 21 is set in the position S.sub.2, 
the power matching pilot line (32, 34) which has been operated to transmit 
the pressure in that portion of the main line downstream of the orifice 3 
to the spring chamber 40 of the load sensing valve 17 is closed by the 
selector valve 21 and, consequently, the fluid pressure in that portion of 
the main line 2 upstream of the orifice 3 is transmitted to the spring 
chamber 40 of the valve 17 through the ports P and B of the valve 21 and 
the pilot lines 37 and 32. Because of this, the load sensing valve 17 is 
set in and remains in the position V.sub.2 as is the case during the 
warm-up operation and the pump 1 is locked at a position effective to 
discharge the fluid medium at a predetermined rate determined by the 
position of the adjustment screw 55. On the other hand, since the fluid 
pressure in that portion of the main line 2 downstream of the orifice 3 is 
transmitted to the spring chamber 45 of the valve 18 through the pressure 
matching pilot line (46, 34), the valve 18 selectively opens and closes 
depending on the differential pressure in the respective portions of the 
main line 2 upstream and downstream of the orifice 3 after the opening of 
the switching valve 15, with the excessive fluid consequently drained to 
the reservoir 19, thereby controlling the differential pressure in the 
respective portions of the main line 2 upstream and downstream of the 
orifice 3 to the predetermined value of 8 kg/cm.sup.2. Because of this, 
incident to the filling of the resinous material into the mold assembly, 
the injection pressure, that is, the fluid pressure in that portion of the 
main line 2 downstream of the orifice 3 increases in a manner as shown in 
the graph of FIG. 2 with high responsivity and, therefore, with the flow 
control performed accurately. Especially, since this flow control takes 
place under the pressure matching mode in which the excessive fluid is 
drained to the reservoir 19 through the by-pass pressure compensated valve 
18, that is to say, with a valve control at reduced transmission delay, 
the responsivity of the fluid flow during the opening of the switching 
valve 15 and the change of the orifice 3 is very good. 
Upon completion of the filling of the resinous material into the mold 
assembly, the injection cylinder 6 which has been advanced to effect the 
injection of the resinous material returns to a standstill position at 
which no motion take place. And, the injection pressure abruptly increases 
as shown by a portion of the curve shown in FIG. 2 at a right-hand portion 
from the axis of the time t.sub.1, thereby creating a primary pressure 
P.sub.1a. This primary pressure P.sub.1a is determined by the set pressure 
of the pilot relief valve 50 which serves to maintain the fluid pressure 
at its input port at the set pressure. Therefore, the pressure in the 
spring chamber 45 of the valve 18 is equal to the set pressure thereof. In 
view of the above, the by-pass type pressure compensated valve 18 performs 
a pressure control at high responsivity under a control system wherein the 
fluid pressure in the main line 2 upstream of the orifice 3 correspond to 
the pressure P.sub.1a which is higher than the pressure in the spring 
chamber 45 by a value corresponding to the biasing force (8 kg/cm.sup.2) 
of the spring 47. It is to be noted that the pump 1 is at this time 
operating to discharge the fluid medium at the maximum rate. 
The selector valve 21 is, at the time t.sub.2 shown in FIG. 1, set in the 
position S.sub.1 in response to a switching signal derived in any suitable 
manner from the cylinder displacement, timer or fluid pressure, to effect 
the pressure control under the power matching mode. At this time, since 
the spring chamber 40 of the load sensing valve 17 is fluid-connected to 
the input port of the pilot relief valve 50 through the pilot lines 32, 34 
and 51, the pressure inside the spring chamber 40 is controlled to the set 
pressure of the pilot relief valve 50 after the discharge rate of the pump 
1 has been decreased. For this reason, the load sensing valve 17 is 
selectively set in the positions V.sub.1 and V.sub.2 so as to make the 
differential pressure between the pilot and spring chambers 42 and 40 
equal to the biasing force of the spring 41, thereby controlling the fluid 
pressure in that portion of the main line 2 upstream of the orifice 3 to 
the primary pressure P.sub.1b which is somewhat lower than the primary 
pressure P.sub.1a. At this time, the by-pass type pressure compensated 
valve 18 is nevertheless closed and held standstill. Normally, the load 
sensing valve 17 is set in a position intermediate between the positions 
V.sub.1 and V.sub.2 to control the swash plate in such a way as to 
maintain the discharge rate at a small value. Accordingly, even at this 
time, no loss of power occurs substantially. It is, however, to be noted 
that the differential pressure between the pilot and spring chambers 42 
and 40 is established because of the flow of the fluid medium from the 
pump 1 to the pilot relief valve 50. 
When a predetermined time passes from the time t.sub.2 to the time t.sub.3 
as shown in FIG. 2, the current supplied to the pilot relief valve 50 is 
caused to decrease to lower the set pressure thereof to establish a 
filling pressure, i.e., a secondary pressure shown by P.sub.2 in FIG. 2. 
The pressure control to establish the secondary pressure P.sub.2 is carried 
out by the load sensing valve 17 in a manner similar to that to establish 
the primary pressure P.sub.1b. In other words, the valve 17 operates in 
such a way as to make the differential pressure between the pressure 
inside the pilot chamber 42 and the pressure inside the spring chamber 40 
which is the set pressure of the pilot relief valve 50 correspond to the 
biasing force of the spring 41, so that the filling pressure can be 
maintained at the secondary pressure P.sub.2. It is to be noted that the 
timing at which the pressure matching mode is to be switched over to the 
power matching mode may not be always limited to the time t.sub.1, but it 
may be the time t.sub.2 or t.sub.3. It is also to be noted that the speed 
control of the injection cylinder may be carried out under the power 
matching mode as is the case of the control of the metering motor used in 
the injection molding machine for metering the resinous material and, in 
such case, the by-pass type pressure compensated valve 18 serves 
concurrently to absorb an abnormal pressure increase (surge pressure) 
which would take place in that portion of the main line 2 upstream of the 
orifice 3 when the opening of the orifice 3 is abruptly reduced or as a 
result of a delay in decreasing the discharge rate of the pump 1 which 
would occur during the transfer from a speed control region to a pressure 
control region (subsequent to the time t.sub.1). 
As hereinabove described, the fluid flow control system shown in FIG. 1 is 
such that it operates under the power matching mode when the selector 
valve 21 is set in the position S.sub.1 to open both the power matching 
pilot line (32, 34) and the pressure matching line (34, 46) to bring the 
by-pass type pressure compensated valve 18 and the load sensing valve 17 
into inoperative and operative positions, respectively, depending on the 
difference between the set pressure of the springs 41 and 47, thereby 
coping with the demand for the reduced energy consumption feature and also 
such that it operates under the pressure matching mode when the valve 21 
is set in the position S.sub.2 to close and open the power matching pilot 
line (34, 32) and the pressure matching pilot line, respectively, to 
introduce the fluid pressure in the main line 2 upstream of the orifice 3 
into the spring chamber of the load sensing valve 17 through the pilot 
line 37 for bringing the valve 17 into the inoperative position at the 
maximum discharge rate and, at the same time, to bring the by-pass type 
pressure compensated valve 18 into the operative position, thereby coping 
with the demand for the high responsivity feature. It is to be noted that 
the pressure matching mode referred to herein is effective even in a 
servo-mechanism (not shown. Closed control) wherein the velocity and the 
pressure are fed back to the orifice 3 and the relief valve 50. 
FIG. 3 illustrates another embodiment of the present invention. The fluid 
flow control system shown in FIG. 3 differs from that shown in FIG. 1 in 
the following points which will now be described. 
In the fluid flow control system shown in FIG. 3, a pilot valve 75 for the 
pressure control of the variable-displacement pump is additionally 
utilized for the purpose of improving the pressure control preciseness of 
such as a pressure override characteristic. In addition, instead of the 
mode selector valve 21 used in the embodiment shown in FIG. 1, a mode 
selector valve 21a is used for effecting the switching between the modes 
irrespective of the relationship in biasing force between the spring 41 in 
the load sensing valve 17 and the spring 47 in the by-pass type pressure 
compensated valve 18. Yet, a feed-in orifice 36 is utilized on the pilot 
line 37 extending between the pilot lines 25 and 32. 
The pilot valve 75 is of a construction indentical with the load sensing 
valve 17. The pilot valve 75 has a port m communicated to the port n of 
the load sensing valve through a pilot line 76, a port n communicated to 
the discharge flow control section 30 of the variable-displacement pump 1 
through a pilot line 77, and a port l communicated through a pilot line 78 
to that portion of the main line 2 upstream of the orifice 3. This pilot 
valve 75 also has a pilot chamber 80 communicated through a pilot line 81 
to that portion of the main line 2 upstream of the orifice 3 and a spring 
chamber 82 fluid-connected to the port B of the mode selector valve 21a 
through a pilot line 83 and also to the pilot line 78 through a pilot line 
85 which has a feed-in orifice 84 disposed thereon. 
The mode selector valve 21a is so designed that, when it is set in the 
position S.sub.1, the ports P and A and the ports B and T are communicated 
to each other, but when in the position S.sub.2, the ports A and T are 
communicated to each other while the ports B and T are disconnected from 
each other. 
The valve 21a has the port A communicated through the pilot line 34 to that 
portion of the main line 2 downstream of the orifice 3, and the port P 
communicated through the pilot line 32 to the spring chamber 40 of the 
load sensing valve 17. The pilot lines 34 and 32 altogether constitute a 
power matching pilot line. The pilot line 32 is also fluid-connected to 
that portion of the main line 2 upstream of the orifice 3 through the 
pilot line 37 having the feed-in orifice 36 disposed thereon. The port T 
of the mode selector valve 21a is fluid-connected to the spring chamber 45 
of the bypass type pressure compensated valve 18 through the pilot line 46 
having an orifice 90 disposed thereon. The pilot lines 46 and 34 
altogether constitute a pressure matching pilot line. The pilot relief 
valve 50 is disposed between the orifice 90 and the spring chamber 45 of 
the valve 18. 
The fluid flow control system shown in FIG. 3 is such that, when the mode 
selector valve 21a is set in the position S.sub.1, the power matching 
pilot line (32, 34) is opened while the pressure matching pilot line (34, 
46) is closed. Since during the flow control the fluid pressure in that 
portion of the main line 2 upstream of the orifice 3 is fed to the spring 
chamber 82 of the pilot valve 75 through the pilot line 81, 78, 85 and 83, 
the pilot valve 75 is set in the position V.sub.2 and held thereat. Also, 
since the fluid pressure in that portion of the main line 2 upstream of 
the orifice 3 is fed to the spring chamber 45 of the valve 18 through the 
pilot line 83, the ports B and T of the selector valve 21a and the pilot 
line 46, the bypass pressure compensated valve 18 is closed and held in an 
inoperative position. On the other hand, since the fluid pressure in that 
portion of the main line 2 downstream of the orifice 3 is fed to the 
spring chamber 40 of the load sensing valve 17 through the pilot line 34, 
the ports A and P of the selector valve 21a and the pilot line 32, the 
load sensing valve 17 is selectively set in the position V.sub.1 and the 
position V.sub.2 to control the discharge flow control section 30 of the 
pump 1 for maintaining at the constant value the differential pressure 
between the respective portions of the main line 2 upstream and downstream 
of the orifice 3. In this condition, the flow control system operates in 
the energy-saving power matching mode in which the discharge rate and the 
discharge pressure of the variable displacement pump 1 match with the load 
demand. In addition, since the fluid medium is supplied from the main line 
25 to the spring chamber 40 of the load sensing valve 17 through a 
relatively short fluid path which is constituted by the pilot line 37 
having the feed-in orifice 36, the load sensing valve 17 can respond 
quickly. 
When the flow control is switched over to a pressure control wherein no 
discharge flow is required and the injection cylinder 6 is held stand and 
wherein the pressure in a portion of the pilot line 46 between the spring 
chamber 45 of the bypass type pressure compensated valve 18 and the 
orifice 90 attains a value equal to the set pressure of the relief valve 
50, the pilot valve 75 is caused to be in the position V.sub.1 to make the 
differential pressure between the pilot chamber 80 and the spring chamber 
82 correspond to the biasing force of a spring 95 in the spring chamber 
82, thereby setting the swash plate of the pump 1 at a neutral position. 
Even at the initial stage of this operation, the system can operate to 
maintain the differential pressure between the respective portions of the 
main line 2 upstream and downstream of the orifice 3 at the constant value 
and, therefore, the system according to the embodiment shown in FIG. 3 can 
exhibit a better pressure override characteristic than that according to 
the embodiment shown in FIG. 1. In other words, since in the embodiment 
shown in FIG. 1, the load sensing valve 17 is set in a substantially 
intermediate position between the positions V.sub.1 and V.sub.2, a slight 
amount of the fluid medium flows through the relief valve 50 when the load 
pressure increases to a value equal to the cracking pressure of the relief 
valve 50. When this occurs, the differential pressure is developed between 
the pilot and spring chambers 42 and 40 of the load sensing valve 17, 
where for the load sensing valve 17 is immediately switched over to the 
position V.sub.1 to reduce the discharge rate of the variable displacement 
pump 1. On the contrary thereto, in the embodiment shown in FIG. 3, even 
though a slight amount of the fluid medium flows through the relief valve 
50 in the manner as hereinabove described, the pilot valve 75 remains set 
at the position V.sub.2 before the differential pressure between the pilot 
and spring chambers 82 and 80 of the pilot valve 75 attains a value 
greater than the biasing force of the spring 95. Accordingly, since there 
is no possibility that the discharge rate of the pump 1 is reduced by the 
pilot valve 75 and since the load sensing valve 17 serves to maintain the 
differential pressure between the upstream and downstream rides of the 
orifice 3 at the predetermined value until the discharge rate of the pump 
1 is reduced by the pilot valve 75, an accurate fluid flow control can be 
appreciated. 
On the other hand, when the mode selector valve 21a is set in the position 
S.sub.2 during the flow control, the power matching pilot line (34, 32) is 
closed while the pressure matching pilot line (34, 46) is opened. 
Since the ports P and B of the selector valve 21a are disconnected from 
each other, the pressure in the spring chamber 40 of the load sensing 
valve 17 is equalized by the pilot line 37 having the feed-in orifice 36, 
to the pressure in that portion of the main line 2 upstream of the orifice 
3 and, at the same time, the pressure in the spring chamber of the pilot 
valve 75 is equalized by the pilot line 85, having the feed-in orifice 84, 
to the pressure in that portion of the main line 2 upstream of the orifice 
3. Because of this, the load sensing valve 17 and the pilot valve 75 are 
set in the respective position V.sub.2 and held thereat, thereby 
communicating the discharge flow-control section 30 of the pump 1 to the 
reservoir 27 through the pilot lines 77, 76 and 26. Therefore, the pump 1 
discharges at its maximum discharge rate determined by the adjustment 
screw 55. 
Also, since the fluid pressure in that portion of the main line 2 
downstream of the orifice 3 is fed to the spring chamber 45 of the bypass 
type pressure compensated valve 18 through the pilot line 34, the ports A 
and T of the selector valve 21a and the pilot line 46, the valve 18 
operates to drain the excessive fluid medium to the reservoir 19 to 
maintain the differential pressure between the respective portions of the 
main line 2 upstream and downstream of the orifice 3 at the constant 
value. Thus, the system operates in the pressure matching mode wherein the 
responsivity is high. 
Hereinafter, the fluid flow control system according to a third preferred 
embodiment of the present invention will be described with reference to 
FIG. 4. 
Referring now to FIG. 4, a variable-displacement pump 1 has its discharge 
port fluid-connected with a main line 2. The main line 2 has a pressure 
reduction type pressure compensated valve 101 and a first orifice 3a on 
one side of the valve 101 opposite to the pump 1. This main line 2 extends 
to a multi-joint coupling 7 from which branch lines 10, 11 and 12 extend 
outwards. These branch lines 10, 11 and 12 are in turn fluid-connected 
through switching valves 13, 14 and 15 to actuators 4, 5 and 6, 
respectively. The actuators 13, 14 and 15 may be a hydraulic cylinder used 
in a mold clamping and releasing mechanism of the injection molding 
machine, a hydraulic motor used in the same machine for driving a feed 
screw, and a hydraulic cylinder used in the same machine for injecting a 
plasticized resinous material. A portion of the main line 2 between the 
valve 101 and the coupling 7 has a bypass line 102 having a second orifice 
3b arranged in parallel to the first orifice 3a. 
The fluid flow control system shown in FIG. 4 comprises a load sensing 
valve 17, a pressure control pilot valve 75 of a construction identical 
with the valve 17, a mode selector valve 21a which may be a two-position 
four-port switching valve, an accumulator 105 communicated through a line 
20b to a portion of the main line 2 upstream of the valve 101, a shut-off 
valve 18a which may be a pilot check valve disposed on the line 20b, and a 
surge pressure absorbing valve 106 disposed on a line 108 which connects a 
portion of the main line 2 between the valve 101 and the first orifice 3a 
with a reservoir 107. 
The load sensing valve 17 is so designed that, when it is set in a position 
shown by V.sub.1, ports l and n thereof are communicated to each other 
while a port m thereof is closed and that, when it is set in a position 
shown by V.sub.2, the ports m and n are communicated to each other while 
the port l is closed. This valve 17 has a spring chamber 40 having therein 
a spring 41 capable of exerting a biasing force .DELTA.P.sub.L such that, 
when the differential pressure between a pilot chamber 42 thereof and the 
spring chamber 40 exceeds the biasing force .DELTA.P.sub.L, the valve 17 
is set in the position V.sub.1, but when said differential pressure is 
smaller than the biasing force .DELTA.P.sub.L, it is set in the position 
V.sub.2. The pilot valve 75 has a spring chamber 82 accommodating therein 
a spring 95 capable of exerting a biasing force .DELTA.P.sub.P. The mode 
selector valve 21a is of such a design that, when it is set in a position 
shown by S.sub.1 , ports P and A as well as ports B and T thereof are 
communicated to each other and, when it is set in a position shown by 
S.sub.2, the ports A and T are communicated to each other while the ports 
P and B are disconnected from each other. 
The load sensing valve 17 has a port l fluid-connected to a portion of the 
main line 2 upstream of the valve 101 through a pilot line 25, a port m 
fluid-connected to the reservoir 27 through a pilot line 26, and a port n 
fluid-connected to a port m of the pilot valve 75 through a pilot line 76. 
The pilot valve 75 has, in addition to the port m, a port n 
fluid-connected to a discharge flow control section 30, which may be 
constituted by a swash plate control cylinder, of the pump 1 through a 
pilot line 77 and a port l fluid-connected to a portion of the main line 2 
upstream of the valve 101 through a pilot line 78. 
The load sensing valve 17 also has a pilot chamber 42 communicated through 
a pilot line 31 to a portion of the main line 2 upstream of the valve 101, 
and a spring chamber 40 communicated to the port P of the selector valve 
21a through a pilot line 32. The pilot line 32 is in turn connected to a 
portion of the main line 2 upstream of the valve 101 through a pilot line 
37 having a feed-in orifice 36 disposed thereon. 
The pilot valve 75 also has a pilot chamber 80 communicated to a portion of 
the main line 2 upstream of the valve 101 through a pilot line 81 whereas 
the pilot valve 75 has a spring chamber 82 communicated to the port B of 
the selector valve 21a through a pilot line 83. The pilot 83 is in turn 
communicated to a portion of the main line 2 upstream of the valve 101 
through a pilot line 85 having a feed-in orifice 84 and also to the 
reservoir 130 through a pilot line 51 having a pilot relief valve 128 for 
controlling the charge pressure of the accumulator. 
The port A of the mode selector valve 21a is communicated to a portion of 
the main line 2 downstream of the first orifice 3a through a pilot line 34 
which, together with the pilot line 32, constitutes a power matching pilot 
line. The pilot line 34 has an orifice 111 disposed thereon, a portion of 
this pilot line 34 between the orifice 111 and the port A of the selector 
valve 21a being communicated to the spring chamber 114 of the valve 101 
through a pilot line 113. The spring chamber 114 for a spring 115 capable 
of exerting a biasing force .DELTA.P.sub.G and the differential pressure 
between the upstream and downstream sides of the first and second orifices 
3a and 3b is controlled to a value equal to the biasing force 
.DELTA.P.sub.G. 
The port T of the selector valve 21a is communicated to the spring chamber 
126 of the surge pressure absorbing valve 106 through a pilot line 125. 
The spring 127 in the spring chamber 126 of the valve 106 exerts a biasing 
force .DELTA.P.sub.R and the valve 106 opens when the differential 
pressure between the spring and pilot chambers 126 and 128 exceeds the 
biasing force .DELTA.P.sub.R. It is to be noted that the biasing force 
.DELTA.P.sub.R is selected to be larger than the biasing force 
.DELTA.P.sub.G. The pilot line 129 is fluid-connected to a reservoir 52 
through a pilot line 129 having an electromagnetic proportional pilot 
relief valve 50a disposed thereon. 
The pilot valve 18a which is an example of the shut-off valve has a port 
151 adapted to be communicated to a reservoir 157 through pilot lines 155 
and 156 when an electromagnetic switching valve 152 is set in a position 
shown by S.sub.11, thereby to avoid any possible reverse flow of the fluid 
medium, and also to be communicated to a branch line 20b through pilot 
lines 155 and 158 when it is set in a position shown by S.sub.12, thereby 
to open the check valve 18a to permit the fluid medium stored in the 
accomulator 105 to be discharged therethrough. 
The pump 1 has an adjustment 55 for regulating the maximum discharge rate 
by positioning a swash plate at a maximum angle of inclination so that any 
possible overload condition of a prime mover (not shown) can be avoided. 
However .DELTA.P.sub.R &gt;.DELTA.P.sub.P must be established. 
The flow control system of the construction described above with reference 
to FIG. 4 operates in the following manner. 
(1) In the case where the biasing force .DELTA.P.sub.G is so selected as to 
be larger than the biasing force .DELTA.P.sub.L : 
In this case, the selector valve 21a and the switching valve 152 are set in 
the respective positions S.sub.1 and S.sub.11, the second orifice 3b is 
completely closed, and the current applied to the pilot relief valve 50a 
is increased from a zero value to a predetermined value while, at the same 
time, the switching valve 15 is caused to open to advance the injection 
cylinder 6. It is, however, to be noted that the metering of the resinous 
material inside the feed screw by the operation of the hydraulic motor 5 
is assumed to have already been completed. 
When this is done, the discharge rate of the pump 1 increases from a zero 
value. However, since the pressure at the discharge port of the pump 1 is 
lower than the set pressure of the pilot relief valve 50a during the 
advance of the injection cylinder 6, the fluid pressure upstream of the 
valve 101 is fed to the pilot and spring chambers 80 and 82 of the pilot 
valve 75 through the respective pilot lines 81 and 85. Therefore, the 
pilot valve 75 is set in the position V.sub.2. At this time, the fluid 
pressure upstream of the valve 101 is fed to the pilot chamber 42 of the 
load sensing valve 17 through the pilot line 31 and, on the other hand, 
the fluid pressure downstream of the first orifice 3a is fed to the spring 
chamber 40 of the valve 17 through the ports A and P and the power 
matching pilot line (32, 34). Because of this, the valve 17 is, when the 
differential pressure between the pilot and spring chambers 42 and 40 is 
of a value smaller than the biasing force .DELTA.P.sub.L, set in the 
position V.sub.2 to communicate the discharge flow control section 30 of 
the pump 1 to the reservoir 27 through the pilot line 77, the pilot valve 
75, the pilot line 76 and the pilot line 26 to cause the swash plate of 
the pump 1 to be so inclined as to give the maximum discharge rate. It is 
to be noted that the responsivity of the swash plate is somewhat delayed 
at this time. On the other hand, when the differential pressure between 
the pilot and spring chambers 42 and 40 increases to a value larger than 
the biasing force .DELTA.P.sub.L, the load sensing valve 17 is set in the 
position V.sub.1 to communicate the discharge flow control section 30 of 
the pump 1 to a portion of the main line 2 upstream of the valve 101 
through the pilot line 77, the pilot valve 75, the pilot line 76 and the 
pilot line 25, thereby causing the swash plate of the pump 1 to be 
inclined towards a neutral position to decrease the discharge rate. In 
this way, the valve 17 serves to control the discharge rate of the pump 1 
to make the differential pressure between the upstream and downstream 
sides of the first orifice 3a equal to the biasing force .DELTA.P.sub.L. 
This operation of the load sensing valve 17 takes place quickly because 
the fluid medium is introduced to the spring chamber 40 through a short 
path 37 having the feed-in orifice 36 disposed thereon. 
On the other hand, since the fluid pressure downstream of the first orifice 
3a is introduced to the spring chamber 114 through the pilot lines 34 and 
113, the valve 101 tends to control the differential pressure between the 
upstream and downstream sides of the first orifice 3a to a value equal to 
the biasing force .DELTA.P.sub.G. However, since the biasing force 
.DELTA.P.sub.G is so selected as to be larger than the biasing force 
.DELTA.P.sub.L as hereinbefore described, and since the valve 17 is 
operating to control the differential pressure between the upstream side 
of the valve 101 and the downstream side of the first orifice 3a to a 
value equal to the biasing force .DELTA.P.sub.L, the valve 101 does not 
operate while remaining opened. 
On the other hand, it is assumed that the accumulator 105 is filled with 
the fluid medium under a pressure higher than the pressure of the fluid 
medium discharged from the pump 1 during the advance of the injection 
cylinder 6. Because the switching valve 152 is set in the position 
S.sub.11 the check valve 18a is closed relative to the above described 
high pressure fluid medium and, accordingly, the accumulator 105 neither 
discharge nor receive any fluid medium. 
Accordingly, at this time, the pump 1 is controlled to match the discharge 
pressure and the discharge rate with the load demand and the power 
matching mode in which the energy consumption is low is performed to the 
first orifice 3a. The relationship between the opening of this first 
orifice and the output flow is shown by a curve a in the graph of FIG. 5. 
It is to be noted that, by varying the opening of the orifice 3a, the 
injection speed can be varied. Even in this case, a delay occurs in the 
responsivity of the fluid flow control. 
In addition, the pilot relief valve 50a is assumed to be closed during the 
above described operation. 
Assuming that the above described flow control is switched over to the 
pressure control in which the filling of the resinous material performed 
by the injection cylinder 6 has completed, the pressure inside the pilot 
lines 83, 125 and 129 between the valve 50a and the feed-in orifice 84 is 
controlled by the valve 50a to a value equal to its set pressure. At this 
time, the pilot valve 75 is selectively set in the position V.sub.1 or in 
the position V.sub.2 to make the differential pressure between the 
pressure inside the pilot chamber 80 and the pressure, i.e., the set 
pressure, inside the spring chamber 82 to be a value equal to the biasing 
force .DELTA.P.sub.P and is normally held in a position intermediate 
between the positions V.sub.1 and V.sub.2. Therefore, the discharge flow 
control section 30 of the pump 1 is communicated to the main line 2 
through the pilot lines 77, 78 and 81 so as to position rapidly the swash 
plate at the neutral position whereby with a very slight discharge rate, 
the pressure inside the main line 2 can be controlled to the predetermined 
value. It is to be noted that, although the flow decreases abruptly during 
the transit period in which the speed control region shifts to the 
pressure control region, the operation of the swash plate is delayed at 
this time and, therefore, the surge pressure tends to occur in the main 
line 2. However, any possible surge pressure occurring in the main line 2 
can be absorbed by the surge pressure absorbing valve 106. If the shift 
from the above described flow control to the pressure control takes place 
slowly at the initial stage of the transit period, the load sensing valve 
17 can operate to control the differential pressure between the upstream 
and downstream sides of the first orifice 3a to a value equal to the 
biasing force .DELTA.P.sub.L and therefore, the pressure override 
characteristic of the fluid flow control system is very good. In other 
words, even when a slight fluid flow takes place through the relief valve 
50a as a result of the increase in the load pressure, an accurate flow 
control can be performed maintaining at the constant value the 
differential pressure between the upstream and downstream sides of the 
first orifice 3a before the pilot valve 75 is completely switched. 
The flow control under an accumulator mode which is an example of a maximum 
discharge flow mode takes place in the following manner. 
In the first place, after the selector valve 21a has been set in the 
position S.sub.2, the electromagnetic switching valve 152 is set in the 
position S.sub.12 and, at the same time, the switching valve 15 is 
switched. 
When this has been done, the port P of the selector valve 21a is closed 
and, therefore, only the fluid pressure in the portion of the main line 2 
upstream of the valve 101 is fed to the spring chamber 40 of the valve 17 
through the pilot line 37 having the feed-in orifice 36 thereon. For this 
reason, the valve 17 is set in the position V.sub.2 and held standstill 
thereat. Similarly, since the port B of the selector valve 21a is closed, 
the spring chamber 82 of the pilot valve 83 is applied with the fluid 
pressure in the portion of the main line 2 upstream of the valve 101 
through the pilot line 85 having the feed-in orifice 83 and, accordingly, 
the pilot valve 75 is set in and held at the position V.sub.2. Therefore, 
the discharge flow control section 30 of the pump 1 is communicated to the 
reservoir 27 with the discharge rate locked at a maximum value as 
determined by the position of the adjustment screw 55. 
However, the pilot port 151 of the check valve 18a is, since the switching 
valve 152 is set in the position S.sub.12 at this time, communicated to 
the branch line 20b through the pilot lines 155 and 158. For this reason, 
the check valve 18a is opened with the fluid medium in the accumulator 105 
consequently discharged to the main line 2 through the check valve 18a and 
the line 20b. 
At the same time, since the fluid pressure downstream of the first orifice 
3a is fed to the spring chamber 114 of the valve 101 through the pilot 
line 34, the orifice 111 and the pilot line 113, the valve 101 operates to 
reduce the pressure of a combined stream of the fluid medium, discharged 
in a maximum discharge rate from the pump 1 then operating at a fixed 
volumetric pump, with the fluid medium discharged from the accumulator 
105, so that the differential pressure between the upstream and downstream 
sides of the first orifice 3a can be controlled to a value equal to the 
biasing force .DELTA.P.sub.G of the spring 115 of the valve 101. Since the 
fluid flow control in which the differential pressure between the upstream 
and downstream sides of the first orifice 3a is controlled to the 
predetermined value by the use of the valve 101 in a so-called valve 
controlled scheme, a high responsive, highly accurate fluid control can be 
achieved at a moderately high speed at the beginning of the injection. In 
addition, since the fluid medium discharged from the accumulator 105 is 
utilized, the actuators 4, 5 and 6 can be driven at high speed. The 
relationship between the opening of the first orifice 3a and the output 
flow is shown by a curve b in the graph of FIG. 5. It is to be noted that, 
since the biasing force .DELTA.P.sub.G is larger than the biasing force 
.DELTA.P.sub.L, the differential pressure between the upstream and 
downstream sides of the first orifice 3a which is attained during the 
accumulator mode of operation in which the valve 101 operates is larger 
than that attained during the power matching mode of operation. 
Accordingly, as shown by the curves a and b in the graph of FIG. 5, the 
output flow during the accumulator mode is larger than that during the 
power matching mode. 
It is also to be noted that, during this accumulator mode, the pilot relief 
valve 128 operates as s safety valve or a charge pressure control valve 
for the accumulator and the load pressure control is performed by the 
relief valve 50a. Where the pressure control is initiated under the 
accumulator mode, the surge pressure on the side of the accumulator is 
absorbed by the surge absorbing valve 106. 
(2) In the case where the biasing force .DELTA.P.sub.G is so selected as to 
be smaller than the biasing force .DELTA.P.sub.L : 
In this case, the selector valve 21a and the switching valve 152 are set in 
the respective positions S.sub.1 and S.sub.11, the second orifice 3b is 
completely closed, and the switching valve 15 is caused to open 
simultaneously with the relief valve 50a to advance the injection cylinder 
6. 
When this is done, as is the case with the previously discussed case (1), 
the fluid flow control system operates under the power matching mode. 
However, since the biasing force .DELTA.P.sub.G is smaller than the 
biasing force .DELTA.P.sub.L, the valve 101 operates even during the power 
matching mode to control the differential pressure between the upstream 
and downstream sides of the first orifice 3a to a value equal to the 
biasing force .DELTA.P.sub.G. That is to say, the valve 101 and the first 
orifice 3a altogether constitutes a flow control valve. That is to say, in 
addition to the control of the differential pressure between the pressure 
upstream of the valve 101 and that downstream of the first orifice 3a to a 
value equal to the biasing force .DELTA.P.sub.L, the differential pressure 
between the upstream and downstream sides of the first orifice 3a is 
controlled by the valve 101. Accordingly, a high responsivity can be 
appreciated even during the operation under the power matching mode. This 
is because the control is performed by the valve 101 at this time 
irrespective of the pump 1. 
When the selector valve 21a and the switching valve 152 are subsequently 
set in the respective positions S.sub.2 and S.sub.12, the fluid flow 
control system operates under the accumulator mode as is the case with the 
previously discussed case (1). Therefore, the valve 101 controls the 
differential pressure between the upstream and downstream sides of the 
first orifice 3a to a value equal to the biasing force .DELTA.P.sub.G. 
Accordingly, under the condition in which the biasing force .DELTA.P.sub.G 
is smaller than the biasing force .DELTA.P.sub.L, the relationship between 
the opening of the first orifice 3a and the output flow is such as shown 
by a curve c in the graph of FIG. 6 which is substantially identical with 
that exhibited during the power matching mode. It is to be noted that a 
curve d shown in the graph of FIG. 6 illustrates the relationship between 
the opening of the first orifice 3a and the output flow which is attained 
when the second orifice 3b is opened during the accumulator mode, from 
which curve d, it is clear that the output flow is increased by an 
increment determined by the second orifice 3b. Nevertheless, the 
differential pressure between the upstream and downstream sides of the 
second orifice 3b is also controlled by the valve to a value equal to the 
biasing force .DELTA.P.sub.L. 
The embodiment shown in FIG. 7 differs from the thrid embodiment shown in 
FIG. 4 in that the function of the pilot valve 75 used in the third 
embodiment is taken by the load sensing valve 17 and in that, instead of 
the selector valve 21a used in the third embodiment, the same selector 
valve 21 as in the first embodiment is used. 
The fluid flow control system shown in FIG. 7 operates in such a manner 
that, when the selector valve 21 is set in the position S.sub.1, the power 
matching pilot line 34 is opened to permit the fluid pressure downstream 
of the first orifice 3a to be fed to the spring chamber 40 of the load 
sensing valve 17. And, during the flow control, the valve 17 operates in 
response to the differential pressure between the pressure upstream of the 
valve 101 and that downstream of the first orifice 3a to control the 
discharge rate of the pump 1 under the power matching mode. Where the 
biasing force .DELTA.P.sub.G of the spring 115 in the valve 101 is smaller 
than the biasing force .DELTA.P.sub.L of the spring 41 of the valve 17, 
the valve 101 is under operating condition during the power matching mode 
as is the case in the third embodiment shown in FIG. 4. On the other hand, 
where the biasing force .DELTA.P.sub.G is larger than the biasing force 
.DELTA.P.sub.L, the valve 101 will not operate. In addition, where the 
actuators 4, 5 and 6 do not operate at all such as occurring, for example, 
subsequent to the completion of the filling of the resinous material and 
during the pressure control in which the pilot relief valve 50 attains the 
set pressure and, therefore, should operate, the valve 17 is held at a 
position intermediate between the positions V.sub.1 and V.sub.2 to set the 
swash plate of the pump 1 at the neutral position so that the pressure 
control can be performed while the discharge rate is zero. However, in the 
embodiment shown in FIG. 7, the valve 101 operates when the biasing force 
.DELTA.P.sub.G is smaller than the biasing force .DELTA.P.sub.L, but does 
not operate when the biasing force .DELTA.P.sub.G is larger than the 
biasing force .DELTA.P.sub.L. 
On the other hand, when the selector valve 21 is set in the position 
S.sub.2, the power matching pilot line (34, 32) is closed and, at the same 
time, the spring chamber 40 of the valve 17 is communicated to a portion 
of the main line 2 upstream of the valve 101 through the pilot line 37 
having the feed-in orifice 36. Accordingly, the pressure inside the spring 
chamber 40 of the valve 17 becomes equal to the pressure inside the pilot 
chamber 42, thereby setting the valve 17 to the position V.sub.2. Then, 
the swash plate is inclined to a position determined by the adjustment 
screw 55 so as to increase the discharge rate and, therefore, the pump 1 
operates as a fixed volumetric pump which discharges a predetermined 
amount of fluid medium. When the electromagnetic switching valve 152 is 
set in the position S.sub.12 during this condition, the check valve 18a is 
opened to permit the accumulator 105 to discharge the fluid medium 
therefrom. The valve 101 operates to reduce the pressure of the fluid 
medium from the accomulator 105 combined with the fluid medium from the 
pump 1 then operating as a fixed volumetric pump. That is to say, the flow 
control is performed under the accumulator mode as is the case in the 
third embodiment. 
The fifth embodiment shown in FIG. 8 differs from the third embodiment 
shown in FIG. 4 in the following points. In other words, in the embodiment 
shown in FIG. 8, a bypass type pressure compensated valve 18 is disposed 
on the main line 2 between the first orifice 3a and the valve 101, having 
its spring chamber 45 fluid-connected to the pilot relief valve 50a. In 
addition, a pilot switching valve 202 is utilized as shown in association 
with the mode selector valve 21a. 
In this embodiment shown in FIG. 8, the biasing force .DELTA.P.sub.L of the 
spring 41 of the valve 17, the biasing force .DELTA.P.sub.G of the spring 
115 of the valve 101 and the biasing force .DELTA.P.sub.R of a spring 47 
of the bypass type pressure compensated valve 18 are respectively set to 
10 Kg/cm.sup.2, 6 Kg/cm.sup.2 and 8 Kg/cm.sup.2. 
And, the mode selection valve 21a is set in the position S.sub.1, the pilot 
switching valve 202 is set in a position shown by X.sub.1 in FIG. 8, and 
the switching valve 152 is set in the position S.sub.11. 
When this is done, the valve 18 is closed and remains standstill with no 
fluid medium discharged from the accumulator 105, but the pump 1, the 
valve 17 and the valve 101 operate and, therefore, the system shown in 
FIG. 8 operates under the power matching mode. 
When the selector valve 21a and the valve 152 are respectively set in the 
position S.sub.2 and S.sub.12, the valve 17 is set in and held standstill 
at the position V.sub.11 with the pump 1 consequently operating as a fixed 
volumetric pump which gives the maximum discharge rate, the valve 18 is 
closed and held standstill and the accumulator 105 and the valve 101 
operate, whereby the system of FIG. 8 is operated under the accomulator 
mode. 
When the valve 202 and the valve 152 are subsequently set in the respective 
positions X.sub.2 and S.sub.11, the valve 101 remains opened, the 
accumulator 105 does not operate, the pump 1 operates as the fixed 
volumetric pump giving the maximum discharge rate, and the valve 18 
operates to permit the excessive fluid medium to be drained into the 
reservoir 19 to control the differential pressure between the upstream and 
downstream sides of the first orifice 3a to the predetermined value. In 
this way, the system operates under the power matching mode. 
Accordingly, the fluid flow control system according to the fifth 
embodiment shown in FIG. 8 can be selectively operated under the power 
matching mode and the maximum discharge flow mode one at a time. In 
addition, during the maximum discharge flow mode of operation, the system 
can also be selectively operated in the accumulator mode and the pressure 
matching mode one at a time. 
It is to be noted that the bypass type pressure compensated valve 18 used 
in the embodiment of FIG. 8 may be replaced with the surge absorbing valve 
106 shown in FIG. 4 as employed in the third embodiment. 
From the foregoing, it has now become clear that the fluid flow control 
system according to the present invention has two selective capability of 
operating under the power matching mode and the maximum discharge flow 
mode one at a time. Specifically, the switching over from one mode to the 
other can be performed merely by changing the valve position of the pilot 
switching valve and the valve at an appropriate time required by the 
actuators. Accordingly, while the single variable-displacement pump is 
used for all of the operating modes of the system, the power consumption 
can be minimized by switching over the system so as to operate under the 
power matching mode on the one hand and, on the other hand, by switching 
over the system so as to operate under the maximum discharge flow mode is 
including the pressure matching mode and the accumulator mode, not only 
can the high responsivity and the high speed controllability be 
appreciated, but the feed-back control of the speed and/or pressure can 
also be achieved. In addition, when the system operates under the maximum 
discharge flow mode, the fluid medium can be discharged from the pump 
under high pressure in a maximum discharge rate and then fed back to the 
reservoir with the pressure energy efficiently converted into the heat 
energy to rapidly heat the fluid medium. Therefore, the time required to 
perform the warm-up operation can advantageously be reduced. 
Moreover, if a manifold is formed to suit with the fluid flow control 
system of the present invention, the fluid flow control system of the 
present invention can be used exclusively for the operation under any one 
of the pressure matching mode or the accumulator mode and the power 
matching mode only by employing a simple modification, for example, 
closing the manifold by the use of a cover or a plug. Accordingly, the 
system of the present invention effectively contributes to the 
standardization of the manifold and, also, effectively and readily meets 
the optional requirements. 
Especially, the system of the present invention is effectively applicable 
to the injection molding machine in view of the fact that the various 
hydraulic cylinders and their associated fluid circuit components are 
operated under different modes one at a time. 
Although the present invention has fully been described in connection with 
the preferred embodiments thereof with reference to the accompanying 
drawings, it is to be noted that various changes and modifications are 
readily apparent to those skilled in the art. By way of example, although 
the pump has been described as being of a type of which the discharge rate 
decreases as the fluid pressure supplied to the discharge flow control 
section 30 increases, it may be of a type whose discharge rate increases 
as the fluid pressure supplied to the control section 30 increases. In 
addition, instead of the use of the orifice 3a, either an orifice 
switching valve shown by 3c in FIG. 9 or an orifice switching valve, shown 
by 3d in FIG. 10, having a neutral unload passage 300 may be employed. 
Moreover, the mode selector valve may be in the form of a multi-port 
switching valve including a two-port switching valve. In the case of the 
two-port switching valve, it may be combined with another two-port 
switching valve. 
Therefore, such changes and modifications are to be understood as included 
within the spirit and scope of the present invention unless they depart 
therefrom.