Pilot pressure operated directional control valve and an operating cylinder control apparatus

A pilot pressure operated directional control valve is provided in which a housing is formed therein with a spool having a plurality of ports. The spool bore has a spool fittingly inserted therein so as to be slidably displaceable therein and the spool is slidably displaced both by a spring and under a pilot pressure led into a pressure receiving chamber. Also a spring box is attached to an end surface of the housing in a longitudinal direction of the spool. The spring box is provided with a hollow portion which contains the spring while forming a pressure receiving chamber, as well as a pilot pressure fluid inlet that communicates with the hollow portion and is open in the longitudinal direction of the spool.

TECHNICAL FIELD 
This invention relates to a pilot pressure operated directional control 
valve for switching the flows of a pressure fluid by slidably displacing a 
spool under a pilot pressure, and to an apparatus for controlling an 
operating cylinder by using such a directional control valve in which a 
boom cylinder, an arm cylinder or a bucket cylinder of a power shovel or 
the like operating cylinder is supplied with a pressure fluid so as to 
control an operating extension and an operating retraction thereof. 
BACKGROUND ART 
A certain pilot pressure operated directional control valve has hitherto 
been known as disclosed in Japanese Unexamined Patent Publication No. Hei 
3-172602. 
Thus, it is known that in such a directional control valve, as shown in 
FIG. 1 of the accompanying drawings hereof, a housing 1 is formed therein 
with a spool bore 2 to which a pump port 7, a first and a second actuator 
port 8 and 9, and a first and a second tank port 10 and 11 are opened. The 
spool bore 2 has a spool 3 fittingly inserted therein whereas the housing 
1 has a pair of spring boxes 4 disposed at its left hand side and right 
hand side, respectively. The spool 3 is adapted to assume a neutral 
position when it is energized by a spring 5 provided in one of the spring 
boxes 4 and is adapted to be slidably displaced either against, or in a 
cooperation with, a resilient force by the spring 5 to take a first 
position or a second position by supplying a pilot pressure fluid into one 
of pressure receiving chambers 6 formed in the left hand side and right 
hand side spring boxes 4, respectively, to establish and block 
communication between the pump port 7 and the first or second actuator 
ports 8 or 9, and communication between the first or second actuator ports 
8 or 9 and the first or second tank ports 10 or 11. 
This notwithstanding, however, such a pilot pressure operated directional 
control valve is constructed in such a way that the spring 5, the one 
pressure receiving chamber 6 and a pilot pressure fluid inlet 12 are 
formed successively on an axis that is coaxial with the spool 3. Since in 
addition the pilot pressure fluid inlet 12 must be made to have such a 
length that a piping joint such as an elbow can be in threaded engagement 
therewith, the corresponding spring box 4 needs to be proportionally 
lengthened. As a consequence, a problem arises in that the length of the 
entire directional control valve is necessarily increased and the area of 
the site on which it is mounted (i. e. a site area) is so enlarged. 
Here, whilst if the pilot pressure fluid inlet is formed on a plane that is 
orthogonal to the spool longitudinal direction of the spring box 4, the 
length of the spring box 4 can be shortened, then if a plurality of such 
housings 1 are stacked one upon another and connected together, a problem 
is brought about in that a piping joint as mentioned above cannot be 
connected to the pilot pressure fluid inlet since the interstice between 
the adjacent spring boxes 4 is necessarily small in size. 
Furthermore, in such a pilot pressure operated directional control valve, 
it may be noted that in order for a pilot pressure fluid to be supplied 
into the pressure receiving chambers 6, a piping joint assembly such as 
including an elbow must be threaded into and thereby attached to the pilot 
pressure fluid inlet 12 of the spring box 4. 
For example, as shown in FIG. 2 of the accompanying drawings thereof, the 
pilot pressure fluid inlet 12 can be a threaded bore into which the 
threaded portion 14 of a piping joint 13 may be inserted for a mating 
engagement therewith. 
With such a construction adopted, in order for the piping joint 13 to be 
firmly attached to the said spring box 4 and for a fluid leakage from 
their threaded engagement connection to be prevented, the threaded 
connection length between the piping joint 13 and the pilot pressure fluid 
inlet 12 of the spring box 4 needs to be increased. Thus, the threaded 
portion of the pilot pressure fluid inlet 12 of the spring box 4 engaged 
with the piping joint 13 being lengthened, the length of the spring box 4 
will be increased. For this reason, there has hitherto developed the 
problem that the entire size of a directional control valve of this type 
must be increased in length and the area of the site on which it is 
mounted (i. e. the site area) needs to be enlarged. 
Also, if the piping joint 13 has a pipe connecting portion 15 that is not 
aligned linearly with the threaded portion 14 but its alignment therewith 
is L-shaped as shown, it is naturally difficult to orient the pipe 
connecting portion 15 in a desired direction. More specifically, whilst if 
a plurality of such directional control valves are stacked one upon 
another, adjacent pilot pipe lines can readily be interconnected by 
orienting their corresponding pipe connecting portions 15 in an identical 
direction, if a threaded attaching arrangement as mentioned above is then 
employed for a piping joint 13, it is altogether possible that the pipe 
connecting portions 15 may not be oriented uniformly. 
The present invention has been made to obviate such inconveniences and has 
for its one object to provide a pilot pressure operated directional 
control valve which is capable of reducing the length of the entire pilot 
pressure operated directional control valve in the prior art, thereby 
reducing the site area; which, if a plurality of housings are stacked one 
upon another and thereby interconnected, is capable of connecting a piping 
joint to a pilot pressure fluid inlet; and which, if a pipe connecting 
portion of a piping joint is L-shaped, is capable of orienting a plurality 
of such piping connection portions in an identical direction. 
One may also note at this point that a conventional apparatus for 
controlledly supplying a pressure fluid into an operating cylinder is 
typically constructed in such a manner that the pressure fluid discharged 
from a hydraulic pump is supplied into one of a first and a second chamber 
of the operating cylinder under the control by a pilot pressure operated 
directional control valve while a pressure fluid is discharged from the 
other of the first and second chambers into a reservoir, thereby 
performing an extending or retracting operation of the operating cylinder. 
In such an apparatus, the pilot pressure operated directional control valve 
has a valve block that is formed therein with a spool bore to which a pump 
port, a first and a second actuator port and a tank port are opened. The 
spool bore has a spool fittingly inserted therein so as to be slidably 
displaceable therein. The spool can be switched from its neutral position 
to its first or second position to establish a communication between the 
pump port and one of the first and second actuator ports and a 
communication between the tank port and the other of the first and second 
actuator ports, thereby permitting the pressure fluid to be supplied and 
discharged as mentioned above. Also, the spool can be set at its neutral 
position to block each of these ports, thereby preventing a return fluid, 
caused to flow into the first or second actuator port, from flowing out 
into the reservoir. 
It should be noted, however, that between the spool and the spool bore in 
the valve block there exists small clearance which may allow for a fluid 
leakage and that a retention pressure would, owing to an external load, be 
produced in the first or second chamber in the operating cylinder. The 
retention pressure will then cause a portion of the return oil to flow 
through the above mentioned clearance into the reservoir. It follows, 
therefore, that the operating cylinder will undesirably have either an 
extending operation or a retracting operation (hereinafter referred to as 
a "spontaneous fall"). 
For this reason, the prior art has adopted an arrangement in which a 
circuit for interconnecting the retention pressure producing chamber of 
the operating cylinder and the actuator ports of the directional control 
valve is provided with a locking valve which will, when the directional 
control valve is at its neutral position, be closed to act to prevent the 
return fluid, out of the operating cylinder, from flowing into a actuator 
port of the directional control valve and thus to prevent a spontaneous 
fall. On the other hand, when the directional control valve is at its 
first or second position, the locking valve will be opened to allow the 
pressure fluid to flow between the directional control valve and the 
operating cylinder. 
However, if such a locking valve is incorporated as mentioned above, the 
retention pressure will be abnormally elevated when the operating cylinder 
is acted upon by an inertia load or an external force. 
Thence, there has been known an operating cylinder controlling apparatus 
that is designed to resolve this problem in the prior art, as disclosed in 
FIG. 3 of Japanese Unexamined Utility Model Publication No. Hei 2-91201, 
which is provided with a locking valve, called a sequence valve, in a 
circuit for interconnecting one of the actuator ports of the directional 
control valve and the retention pressure producing chamber of the 
operating cylinder, as well as a relieve valve in a circuit between the 
locking valve and the operating cylinder, in such a manner that if the 
retention pressure is elevated when the operating cylinder is acted upon 
by a load of inertia or an external force, the relief valve may provide a 
relieving action for the pressure fluid into the reservoir. 
There has also be known another operating cylinder controlling apparatus, 
as disclosed in FIG. 1 of Japanese Unexamined Utility Model Publication 
No. Hei 2-91201. 
This latter known apparatus, as shown in FIG. 3 of the accompanying 
drawings hereof, is provided with a locking valve 206, then called a logic 
valve, in a circuit 205 for connecting one actuator port 202 of a 
directional control valve 201 and a retention pressure producing chamber 
204 of an operating cylinder 203 together, as well as a pilot valve 209 
for establishing and blocking a communication between a spring chamber 207 
of the locking valve 206 and a reservoir 208. Then, the pilot valve 209 
has a pressure receiving chamber connected to a relief valve 210 and the 
circuit 205 has a connection to a main relief valve 211. Here, if the 
retention pressure in the retention pressure producing chamber 204 of the 
operating cylinder 203 is elevated, the pilot relief valve 210 will act to 
relieve the pressure fluid to bring the pilot valve 209 to its 
communicating position with the pressure fluid relieved, thereby 
communicating the spring chamber 207 of the locking valve 206 with the 
reservoir 208. The locking valve will thus be opened to communicate the 
retention pressure producing chamber 204 with the circuit 205. As a 
result, the retention pressure within the operating cylinder 203 will be 
relieved through the main relief valve 211 and eventually set free. 
Of the above mentioned two operating cylinder controlling apparatuses in 
the prior art, if the former is adopted, it follows that the need to 
handle a large volumetric flow of the pressure fluid requires a relief 
valve to be necessarily large in size and hence any combined unit of the 
relief valve and the locking valve to also be large in size. Furthermore, 
if a plurality of such operating cylinders are involved as is typically 
the case, a like plurality of such units need to be installed, thus giving 
rise to the problematical requirement that the area for their installation 
be necessary which needs to be increased in proportion to the number of 
the units. 
On the other hand, if the latter type of the control apparatus is adopted, 
it follows that the use of the locking valve 206, the pilot valve 209 and 
the pilot relief valve 210, an increased number of components, makes the 
equipment not only costly but also large in size. Therefore, the problem 
of the requirement of an enlarged installation site, here again, arises, 
if a plurality of operating cylinders need to be controlled as in the 
previous case. 
Accordingly, the present invention has been made also to obviate the 
inconveniences discussed in the preceding paragraphs and has for another 
object to provide an operating cylinder controlling apparatus which is 
reduced both in its cost and area of installation for a unit designed to 
prevent any spontaneous fall of an operating cylinder. 
SUMMARY OF THE INVENTION 
In order to achieve the foregoing objects, there is provided in accordance 
with the present invention, in a first aspect thereof, a pilot pressure 
operated directional control valve in which a housing is formed therein 
with a spool bore having a plurality of ports. The spool bore has a spool 
fittingly inserted therein so as to be slidably displaceable therein and 
the spool is adapted to be slidably displaced both with a spring and under 
a pilot pressure led into a pressure receiving chamber, and in which: 
a spring box is attached to an end surface of the housing in a longitudinal 
direction of the spool; and the spring box is adapted to be formed therein 
with a hollow portion containing a spring and forming a pressure receiving 
chamber, and a pilot pressure fluid inlet for communicating with the 
hollow portion and opening in the longitudinal direction of the spool. 
According to the construction mentioned above, it can be seen that the 
pilot pressure fluid inlet will no longer project largely from the hollow 
portion in the longitudinal direction of the spool and, as a result, the 
length of the spring box may be reduced, This will in turn shorten the 
entire pilot pressure operated directional control valve, thus reducing 
its required site area. 
Also, since the pilot pressure fluid inlet is opening in the direction of 
the spool, it can be seen that if a plurality of housings are stacked one 
upon another and thereby interconnected, an individual pilot pressure 
fluid inlet may have a corresponding piping joint connected thereto. 
Preferably, the spring box comprises a first cylindrical body and a second 
cylindrical body which are integrally arranged in a pair and in parallel 
to each other and which are opening to a left hand side and right hand 
side which are opposite to each other. The hollow portion is formed within 
the first cylindrical body, and an interior of the second cylindrical body 
is adapted to be in a communication with an interior of the first 
cylindrical body. Also, the pilot pressure fluid inlet is constituted with 
an opening portion of the second cylindrical body. 
It is also preferred that a piping joint should be connected to the pilot 
pressure fluid inlet. 
It is further desirable that the pilot pressure fluid inlet be formed with 
a piping joint attachment hole, that the piping joint attachment hole be 
constituted of a large diameter hole and a small diameter hole which are 
eccentric to each other, that a piping attachment joint be constituted of 
a fitting portion comprising a large diameter portion and a small diameter 
portion which are eccentric to each other and a pipe connecting portion, 
that the fitting portion be adapted to be fittingly inserted into the 
piping joint attachment hole, and that a pressure plate be bolted with the 
spring box to act to prevent the fitting portion from coming out of the 
piping joint attachment hole. 
According to the construction just mentioned above, it can be seen that the 
length along which the fitting portion of the piping joint is fittingly 
inserted into the piping joint attachment hole of the spring box may be 
shortened. Since the piping joint attachment hole of the spring box can 
thus be reduced in length, the length of the spring box will be shortened, 
thereby reducing the entire length of the pilot pressure operated 
directional control valve, thus making its required site area smaller. 
It can also be seen that with such a pressure plate the piping joint may 
effectively be prevented from coming out of the piping joint attachment 
hole. Also, since a large diameter portion and a small diameter portion 
which are eccentric to each other are fitted, respectively, into a large 
diameter hole and a small diameter hole which are eccentric to each other, 
there will be no rotation of the piping joint and its firm attachment will 
thereby be ensured. If an L-shaped piping joint is adopted, it will be 
noted that its pipe connecting portion can always be oriented in a 
predetermined direction. 
It is also possible that the pilot pressure fluid inlet may be formed with 
the piping joint attachment hole, that the piping joint attachment hole 
may be in the form of a regular polygon, that the piping joint may be 
constituted of a fitting portion in the form of a regular polygon and a 
pipe connecting portion, that the fitting portion may be adapted to be 
fittingly inserted into the piping joint attachment hole, and that a 
pressure plate may be bolted with the spring box to act to prevent the 
fitting portion from coming out of the piping joint attachment hole. 
According to the construction just mentioned above, it can be seen that the 
orientation of the pipe connection portion of the piping joint may be 
altered in accordance with a particular regular polygonal configuration 
and thus by the number of corners of the particular regular polygon. 
It should be noted at this point that it is desirable that an interstice 
between an inner peripheral portion of the joint attachment hole and an 
outer peripheral portion of the fitting portion be adapted to be sealed by 
a sealing material, or that an interstice between a bottom portion of the 
piping joint attachment hole and an end surface of the fitting portion be 
adapted to be sealed by a sealing material. 
The present invention also provides, in a second aspect thereof, an 
operating cylinder control apparatus, which comprises: 
a directional control valve; 
a locking valve which is disposed in a circuit for interconnecting an 
actuator port of the directional control valve and a retention pressure 
generating chamber of the operating cylinder, which has a pressure 
receiving portion and a spring, which is adapted to be thrust in a 
direction of communication by an outlet pressure of the directional 
control valve and the retention pressure within the operating cylinder and 
which is adapted to be thrusted in a blocking direction by the retention 
pressure within the operating cylinder and the spring. The retention 
pressure acts on the pressure receiving portion. 
A switching valve is disposed between the pressure receiving portion of the 
locking valve and a reservoir, which is adapted to be energized by the 
spring to take its blocking position and which is adapted to take its 
communicating position by means of a switching means brought into a 
position for communicating the circuit with a reservoir. 
Also a main relief valve is connected via a check valve to a side to the 
operating cylinder of the rocking valve in the circuit. 
According to the construction just mentioned above, it can be seen that 
since the main relief valve for preventing an abnormally elevated pressure 
within the retention pressure generating chamber of the operating cylinder 
can be placed separately, it will be sufficient to provide only the 
locking valve and the switching valve correspondingly to each of the 
operating cylinders. And yet, since the main relief valve can commonly act 
for a plurality of operating cylinders, the area of the site on which a 
unit for preventing a spontaneous fall of any of the plural operating 
cylinders is mounted may be reduced. Also, since it suffices to provide 
such a single main relief valve alone, it can be seen that the equipment 
may be made less costly. 
In the construction just mentioned above, it should be noted here that it 
is desirable that a valve block equipped with the locking valve and the 
switching valve be connected to a valve block of the directional control 
valve, that an inlet side of locking valve be adapted to communicate with 
the actuator port of the directional control valve, and that each of the 
valve blocks be formed therein with a fluid bore for communicating the 
pressure receiving portion on which the pilot pressure of the directional 
control valve is acting with the pressure receiving portion of a side to 
the spring of the switching valve. 
It should further be noted that an operating cylinder control apparatus as 
mentioned may specifically comprise: 
a directional switch valve which is provided with a pump port, a tank port 
and a first and a second actuator port, which when at its neutral position 
is adapted to block the first and second actuator ports, which when at its 
first pressure fluid supply position is adapted to communicate between the 
pump port and the first actuator port and to communicate between the 
second actuator port and the tank port, and which when at its second 
pressure fluid supply position is adapted to communicate between the pump 
port and the second actuator port and to communicate the first actuator 
port and the tank port. 
A first circuit is provided for connecting the first actuator port to a 
retention pressure generating chamber of the operating cylinder. 
A second circuit is provided for connecting the second actuator port to the 
other chamber of the operating cylinder. 
A locking valve is disposed in the first circuit. The locking valve is 
adapted to be thrust in a direction of communication both under a pressure 
of the first actuator port and under a pressure within the retention 
pressure generating chamber. Also, the locking valve is adapted to be 
thrust in a direction of blocking both by a spring and under the pressure 
within the retention pressure generating chamber acting on a pressure 
receiving portion. 
A switching valve is disposed in a drain path connected to the pressure 
receiving portion of the locking valve. The switching valve is held at a 
blocking position by a spring, and is brought to a communicating position 
under a pressure at the pressure receiving portion. 
Also a main relief valve is connected to a side to the retention pressure 
generating chamber of the locking valve in the first circuit and to the 
second circuit via respective check valves, and may have a construction in 
which the second pressure receiving chamber of the directional control 
valve is connected to the pressure receiving portion of the switching 
valve.

BEST MODES FOR CARRYING OUT THE INVENTION 
Hereinafter, suitable embodiments of the present invention with respect to 
a pilot pressure operated directional control valve and an operating 
cylinder control apparatus using the same will be set forth with reference 
to the accompanying drawings hereof. 
As shown in FIG. 4, a housing 20 is formed therein with a spool bore 21 to 
which a pump port 23, a first and a second load pressure detecting port 24 
and 25, a first and a second actuator port 26 and 27 and a first and a 
second tank port 28 and 29 are opened. The spool bore 21 has a spool 22 
fittingly inserted therein. The above mentioned spool 22 is formed with a 
first and a second small diameter portion 30 and 31 and an intermediate 
small diameter portion 32 so that when located at its neutral position as 
shown in FIG. 4, it may block each of the ports mentioned above. And, when 
the spool 22 is slidably displaced rightwards in FIG. 4 to take its first 
position, communication will be established each between the pump port 23 
and the second load pressure detecting port 25, between the first load 
pressure detecting port 24 and the first actuator port 26, and between the 
second actuator port 27 and the second tank port 29. It should be noted 
here that the first load pressure detecting port 24 and the second load 
pressure detecting port 25 remain communicated with each other at all 
times. It can accordingly be seen that a pressure fluid caused to flow 
into the pump port 23 will flow through the first actuator port 26 into an 
actuator 33 whereas a return fluid out of the actuator 33 will flow 
through the second actuator port 27 into the tank port 29. 
Also, if the spool 22 is slidably displaced from the state of FIG. 4 
leftwards to take its second position, communication will be established 
each between the pump port 23 and the first load pressure detecting port 
24 and between the second load pressure detecting port 25 and the second 
actuator port 27. Since the first load pressure detecting port 24 and the 
second load pressure detecting port 25 remain in communication with each 
other at all times as mentioned above, the pressure fluid in the pump port 
23 will flow into the actuator 33. Also, since the first actuator port 26 
is caused to communicate with the first tank port 28, the return fluid out 
of the actuator 33 will flow into the first tank port 28 mentioned above. 
At this point it should be noted that in FIG. 4, a spool 34 is inserted in 
a check valve bore 20c and constitutes a check valve component 35 whereas 
a spool 36 is inserted in a pressure reduction valve bore 20d and 
constitutes a pressure reduction valve component 37. The check valve 
component 35 and the pressure reduction valve component 37 together 
constitute a pressure compensation valve. 
It is seen that a first spring box 40 is attached to one end surface 20a of 
the above mentioned housing 20 in the longitudinal direction of the spool 
22. As shown in FIGS. 5 and 6, the first spring box 40 comprises a pair of 
a first cylindrical body 42 having an attachment seat 41 and a second 
cylindrical body 43 which is integrally mounted with, and extends in 
parallel to, the first cylindrical body 42 and which is opening to both a 
left hand side and a right hand side which are opposite to each other. The 
first cylindrical body 42 is secured by a bolt 44 to the housing 20 
coaxially with the spool bore 21 and is provided therein with a first 
spring bearing 45 and a second spring bearing 46 so that the first spring 
bearing 45 may be in contact with both the one end 20a of the housing 20 
and a step portion 22a of the spool 22 whereas the second spring bearing 
46 may be in contact with both a bottom wall 42a of the first cylindrical 
body 42 and a step portion 48 of a bolt 47 secured to, or integral with, 
the spool 22. Here, a spring 49 is interposed between the first and second 
spring bearings 45 and 46 so that the spool 22 may be held at its neutral 
position. Note also that the interior of the first cylindrical body 42 
constitutes a pressure receiving chamber 50. Thus, not only does the first 
cylindrical body 42 serve to contain the spring 49 but also it is provided 
therein with a hollow portion 40a that constitutes the pressure receiving 
chamber 50. 
An outer end portion of the above mentioned second cylindrical body 43 is 
configured to open in a direction that is parallel to the longitudinal 
direction of the spool 22, and a portion at which the body 43 is open 
constitutes a pilot pressure fluid inlet 51. Also, an inner end portion of 
the second cylindrical body 43 is configured to communicate via a small 
diameter bore 52 with the interior (i. e., the pressure receiving chamber 
50) of the first cylindrical body 42. Note further that the pilot pressure 
fluid inlet 51 is formed with a threaded portion 53, and that the pilot 
pressure fluid inlet 51 is located at an approximately identical position 
to the end of the pressure receiving chamber 50 in the direction in 
parallel to the longitudinal direction of the spool 22. 
A first threaded portion 55 of a piping joint 54 such as an elbow is 
inserted into the threaded portion 53 of the pilot pressure fluid inlet 51 
of the second cylindrical body 43 to establish a mating engagement 
therewith. A second threaded portion 56 of the piping joint 45 is fitted 
with an interiorly threaded pipe for a pilot pressure fluid to establish a 
mating connection therewith. 
The first threaded portion 55 and the second threaded portion 56 mentioned 
above are configured to be L-shaped as a whole to enable the pipe for the 
pilot pressure fluid to be connected approximately in parallel to the one 
end surface 20a of the housing 20. 
It is also seen that a second spring box 60 is attached to the other end 
surface 20b of the housing 20 in the longitudinal direction of the spool 
22. The second spring box 60 is only formed with a pilot pressure fluid 
inlet 61 and a pressure receiving surface 62 and does not contain a 
spring. 
That is to say, since the spool 22 can be held at its neutral position by 
the spring 49 disposed in the first spring box 42 and is also arranged to 
be slidably displaceable both leftwards and rightwards against the spring 
49, there is no need to provide a spring within the second spring box 60. 
In this connection it should be noted that within the second spring box 62 
there is provided a seat 63 that is designed to regulate a slidable 
displacement rightwards of the spool 22. 
This being the case, it can be seen that the second spring box 60 if 
provided in the form of a cylindrical body may have a length that is 
reduced by the space in which a spring otherwise needs to be contained. 
This notwithstanding, however, it is not definitely objectionable to 
replace the second spring box 60 with the first spring box 40 as one is to 
be attached there. 
FIG. 7 shows a second embodiment of the pilot pressure operated directional 
control valve according to the present invention. In the second 
embodiment, the one end surface 20a and the other end surface 20b of a 
housing 20 have each a first spring box 40 attached thereto and a spring 
49 is contained in a first cylindrical body 42 for each such first spring 
box 40. 
As will be apparent from the foregoing, if either the first or the second 
embodiment mentioned above is adopted in which there is included a pilot 
pressure fluid inlet 51 that is opening in a direction substantially in 
parallel to a hollow portion 40a which contains a spring 49 and 
constitutes a pressure receiving chamber 50 and in which the pilot 
pressure fluid inlet 51 is located at a substantially identical position 
to the end of the pressure receiving chamber 50 in a direction in parallel 
to the longitudinal direction of a spool 22, it can be seen that there 
will be no undesirable, large projection of the pilot pressure fluid inlet 
51 from the hollow portion 40a in the longitudinal direction of the spool 
and, as a result, the length of the spring box 40 may be shortened. This 
will in turn shorten the entire length of a pilot pressure operated 
directional control valve, thus reducing its required site area. 
Also, since the pilot pressure fluid inlet 51 is designed to open in a 
direction in parallel to the spool longitudinal direction, it can be seen 
that even in case a plurality of housings are stacked one upon another and 
thereby interconnected, such an individual pilot pressure fluid inlet 51 
may have a piping joint readily connected thereto. 
FIG. 8 shows a third embodiment of the pilot pressure operated directional 
control valve according to the present invention. 
As shown in FIG. 8, a housing 120 is formed therein with a spool bore 121 
to which a pump port 123, a first and a second load pressure detecting 
port 124 and 125, a first and a second actuator port 126 and 127 and a 
first and a second tank port 128 and 129 are opened. The spool bore 121 
has a spool 122 fittingly inserted therein. The spool 122 is formed with a 
first and a second small diameter portion 130 and 131 and an intermediate 
portion 132 so that when held at its neutral position it may block each of 
the ports mentioned above. And, if the spool 122 is slidably displaced 
rightwards in FIG. 8 to take its first position, communication will be 
established each between the pump port 123 and the second load pressure 
detecting port 125, between the first load pressure detecting port 124 and 
the first actuator port 126, and between the second actuator port 127 and 
the second tank port 129. It should be noted at this point that the first 
load pressure detecting port 124 and the second load pressure detecting 
port 125 remain communicated with each other at all the times. 
Accordingly, it can be seen that a pressure fluid caused to flow into the 
pump port 123 will flow through the first actuator port 126 into an 
actuator 133 whereas a return fluid out of the actuator 133 will flow 
through the second actuator port 127 into the second tank port 129. 
If the spool 122 is slidably displaced from the state of FIG. 8 leftwards 
to take its second position, communication will be established each 
between the pump port 123 and the first load pressure 124 and between the 
second load pressure detecting port 125 and the second actuator port 127. 
Also, since the first load pressure detecting 124 and the second load 
pressure detecting port 125 remain in communication with each other at all 
times as mentioned above, it can be seen that the pressure fluid out of 
the pump port 123 will flow into the actuator 133. Then, also, the first 
actuator port 126 will communicate with the first tank port 128 to allow 
the return fluid out of the actuator 133 to flow into the first tank port 
128. 
It is seen that a first spring box 140 is attached to one end surface 120a 
of the above mentioned housing 120 in the longitudinal direction of the 
spool 122. The first spring box 140 is formed with a piping joint 
attachment hole 141 that is opening to one end surface 140a thereof, a 
spring attachment hole 142 that opens to the other end surface 140b 
thereof and a bore 143 for communicating these attachment holes 141 and 
142 with each other. It is seen that a first spring bearing 144 and a 
second spring bearing 145 are disposed within the spring attachment hole 
142 so that the first spring bearing 144 may be in contact with both the 
one end surface 120a of the housing 120 and a step portion 122a of the 
spool 122 whereas the second spring bearing 145 may be in contact with 
both a bottom wall 142a of the spring attachment hole 142 and a step 
portion 147 of a bolt 146 secured to or integral with the spool 122. It is 
also seen that a spring 148 is interposed between the first and second 
spring bearings 144 and 145 so that the spool 122 may be held at its 
neutral position. Also, the interior of the spring attachment hole 142 
here constitutes a pressure receiving chamber 149. 
As shown in FIGS. 8 and 9, the spring attachment hole 142 comprises a large 
diameter hole 150 and a small diameter hole 151 which are eccentric to 
each other. The large diameter hole 150 is open to the one end surface 
140a of the spring box 140 whereas the small diameter hole 151 has a 
bottom portion thereof which is opening through the bore 143 to the spring 
attachment hole 142. 
As shown in FIG. 10, a piping joint 152 comprises a fitting portion 153 and 
a pipe connecting portion 154 which are configured to be L-shaped as a 
whole. The fitting portion 153 comprises a large diameter portion 155 and 
a small diameter portion 156 which are eccentric to each other. The large 
diameter portion 155 is formed with an annular groove 157 on its outer 
peripheral surface. 
As shown in FIGS. 8 and 9, the piping joint 152 has such a construction 
that the large diameter portion 155 and the small diameter portion 156 may 
be fitted in the large diameter hole 150 and the small diameter hole 151, 
respectively, of the first spring box 140, and thus is attached to the 
latter so that it may not be rotated. Also, an O-ring 158, which is fitted 
in the annular groove 157 of the large diameter portion 155, is pressed 
against, and thereby attached to, the inner peripheral surface of the 
large diameter hole 150 to provide a sealing between the piping joint 152 
and the first spring box 140. Also, a pressure plate 160, which is 
attached by bolts 159 to the one end surface 140a of the first spring box 
140, serves to prevent the piping joint 152 inserted from coming out. 
This being the case, it will be seen that the attachment portion of the 
piping joint 152 to the first spring box 140 is sealed by the O-ring 158, 
the piping joint 152 inserted can be prevented from being removed by the 
pressure plate 160, and the piping joint 152 can be fixed in position so 
as to be not rotatable by means of the large diameter hole 150 and the 
small diameter hole 151 which are eccentric to each other, coupled with 
the large diameter portion 155 and the small diameter portion 156 which 
are eccentric to each other, and yet the orientation of the pipe 
connecting portion 154 of the piping joint 152 can be maintained always 
constant. 
It is also seen that a second spring box 161 is attached to the other end 
surface 120b of the above mentioned housing 120 in the longitudinal 
direction of the spool 122. The second spring box 161, as with the first 
spring box 140, is provided with a piping joint attachment hole, here 
designated at 141, and a spring attachment hole, here designated at 142. 
The piping joint attachment hole 141 has a small diameter hole 151 that is 
directly open to the spring attachment hole 142 in which no spring is 
provided. 
That is to say, since the spool 122 can be held at its neutral position by 
the spring 148 disposed within the first spring box 140 and is also 
arranged to be slidably displaceable both leftwards and rightwards against 
the spring 148, there is no need to provide a spring within the second 
spring box 161. In this connection, it should be noted that as in a 
previous embodiment, here again, a piping joint 152 is attached to the 
piping joint attachment hole 141, in the second spring box 161. 
This being the case, it can be seen that the second spring box 161 may here 
again have a length that is reduced by the space in which a spring must 
otherwise be contained. This notwithstanding, however, it is not 
objectionable to replace the second spring box 161 with the first spring 
box 140 as one is to be attached there. 
It should also be noted that the piping joint 152, as in a fourth 
embodiment of the pilot pressure operated directional control valve as 
shown in FIGS. 11 and 12, may have a fitting portion 153 and a pipe 
connecting portion 154 linearly arranged. 
As will be apparent from the preceding paragraphs, the above mentioned 
third and fourth embodiments of the present invention provide an 
arrangement whereby it is made possible to shorten the length along which 
the fitting portion 153 of the piping joint 152 is fitted into the piping 
joint attachment hole 141 of the spring box 140 while providing a required 
sealing with certainty. Therefore, since the piping joint attachment hole 
141 of the spring box 140 can be shortened, it follows that the length of 
the spring box 140 will be shortened to reduce the entire length of the 
pilot operated directional control valve, thereby reducing its site area 
as required. 
Also, the piping joint 152 inserted can be prevented by the pressure plate 
160 from coming out. Further, since the large diameter portion 155 and the 
small diameter portion 156 which are eccentric to each other are, 
respectively, fitted in the large diameter hole 150 and the small diameter 
hole 151 which are eccentric to each other, it will be seen that the 
piping joint 152, without any fear of its subsequent rotation, can be, and 
does remain, attached firmly in place. In other words, the orientation of 
the pipe connecting portion 154 of the L-shaped piping joint 152 may be 
maintained always constant. 
Also, as in a fifth embodiment of the pilot pressure operated directional 
control valve as shown in FIGS. 13 and 14, a piping joint attachment hole 
141 of each of the first and second spring boxes 140 and 161 may be in the 
form of a regular polygon, and the fitting portion 153 of the piping joint 
152 may also be in the form of a regular polygon that is identical to the 
above mentioned regular polygon. In this case, an O-ring 158 is fitted 
between the bottom portion of the piping joint attachment hole 141 and the 
end surface of the fitting portion 153. Then, the orientation of pipe 
connecting portion 154 of the piping joint 152 may be altered in 
accordance with the configuration of a particular regular polygon and thus 
by the number of corners of the particular polygon. 
In this case, it should be noted that although a bolt 159 becomes necessary 
which is capable of fastening a pressure plate 160 with an intensive force 
in order to squeeze the O-ring 158 to a sufficient degree, such a bolt 
will not give rise to any problem whatsoever since it will not project 
from the piping joint 152. 
According the above mentioned fifth embodiment of the present invention in 
this way, it becomes possible to shorten the length along which the 
fitting portion 153 of the piping joint 152 is fitted into the piping 
joint attachment hole 141 of the spring box 140 while providing a required 
sealing with certainty. Therefore, since the piping joint attachment hole 
141 of the spring box 140 can be shortened, it follows that the length of 
the spring box 140 will be shortened to reduce the entire length of the 
pilot operated directional control valve, thereby reducing its site area 
as required. 
Also, the piping joint 152 inserted can be prevented from being removed by 
the pressure plate 160. Further, since the fitting portion 153 in a 
polygonal configuration is fitted in the piping joint attachment hole 141 
in a polygonal configuration, it will be seen that the piping joint 152, 
without any fear of its subsequent rotation, can be, and does remain, 
attached firmly in place. In other words, the orientation of the pipe 
connecting portion 154 of the L-shaped piping joint 152 may be maintained 
always constant. And yet, it is also possible to alter its orientation as 
desired. 
FIG. 15 is a hydraulic circuit diagram of an operating cylinder control 
apparatus that represents a sixth embodiment of the present invention. As 
shown in FIG. 15, a hydraulic pump 220 has its discharge path 221 that is 
provided with a plurality of pressure compensation valves 222, each of 
which has an output side provided with a directional control valve 223. 
The directional control valve 223 is designed to establish and block 
communications among a pump port 224, a tank port 225, a first and a 
second actuator port 226 and 227 and a load pressure detecting port 228. 
The first actuator port 226 of the directional control valve 223 is 
connected to a retention pressure generating chamber 231 of an operating 
cylinder 230 via a first circuit 229 whereas the second actuator port 227 
thereof is connected to the other chamber 233 of the operating cylinder 
230 via a second circuit 232. 
The above mentioned first circuit 229 is provided therein with a locking 
valve 234, which is designed to assume a thrusting action in the direction 
of communication under a pressure of the first circuit 229 and a thrusting 
action in the direction of blocking both by a spring 235 and under a 
pressure of a pressure receiving portion 236. The pressure receiving 
portion 236 has a pressure of the retention pressure generating chamber 
231 of the operating chamber 230 exerted thereon through a circuit 238 
provided with a throttle 237. Also, the pressure receiving portion 236 is 
connected to a reservoir 242 through a drain path 241 provided with a 
throttle 239 and a switching valve 240, and the switching valve 240 is 
held at its blocking position by a spring 243 and is arranged to assume a 
thrusting action to its communicating position under a pressure of a 
pressure receiving portion 244. 
Numeral 245 represents a main relief valve. A circuit 246 upstream of the 
main relief valve 245 is connected via a check valve 247 to the side to 
the operating cylinder 230 of the locking valve 234 in each first circuit 
229 and is also connected via the check valve 247 to each second circuit 
232. This arrangement is so made that when the highest pressure in each 
first circuit 229 and each second circuit 232 exceeds a preset pressure of 
the main relief valve 245, the latter may operate so as to be relieved. 
With such an arrangement adopted, only a single main relief valve alone is 
made sufficient for use in a circuit assembly provided with a plurality of 
operating cylinders. 
The above mentioned directional control valve 223, when no pilot pressure 
is acting thereon, will act to block both communication between the pump 
port 224 and the first actuator port 226 and a communication between the 
pump port 224 and the second actuator port 227, and is thus held at its 
neutral position A which serves to communicate the load pressure detecting 
port 228 with the tank port 225. It will be switched to a first pressure 
fluid supply position B with a pressure fluid delivered into a first 
pressure receiving chamber 248, and to a second pressure fluid supply 
position C with a pressure fluid delivered into a second pressure 
receiving chamber 249. 
If the directional control valve 223 is switched to assume the first 
pressure fluid supply position B, the pump port 224 will communicate with 
both the first actuator port 226 and the load pressure detecting port 228, 
and the second actuator port 227 will communicate with the tank port 225. 
On the other hand, if it is switched to assume the second pressure fluid 
supply position C, the pump port 224 will communicate with both the second 
actuator port 227 and the load pressure detecting port 228, and the first 
actuator port 226 will communicate with the tank port 225. 
The above mentioned pressure compensation valves 222 are each provided 
therein with a check valve 250 and a pressure reduction valve portion 251. 
The check valve 250 is designed to assume a thrusting action in the 
direction of communication under an inlet side pressure acting on a 
pressure receiving portion a and a thrusting action in the blocking 
direction under an outlet side pressure acting on a pressure receiving 
portion b. Its inlet 252 is connected to the discharge path 221 whereas 
its outlet 253 is configured to communicate with the pump port 224 of the 
directional control valve 223. 
The above mentioned pressure reduction valve portion 251 is designed to 
assume a thrusting action towards the direction in which the inlet 254 and 
the outlet 255 may communicate with each other under a pressure acting on 
pressure receiving portion c, to block a communication between the inlet 
254 and the outlet 255 both by a spring 256 and under a pressure acting on 
a pressure receiving portion d, and to assume a thrusting action towards 
the direction in which the check valve 250 may be blocked. Here, the 
pressure receiving portion c is connected to the load pressure detecting 
port 228 of the directional control valve 223, the pressure receiving 
portion d is connected to the outlet 255, and the inlet 254 is connected 
to the discharge path 221. 
Further, the outlet 255 of each pressure compensation valve 222 is arranged 
to communicate with, and is connected to, a load pressure detecting path 
257 so that when a plurality of directional control valves 223 is at the 
same time operated to simultaneously actuate a like plurality of operating 
cylinders 230, a highest load pressure may cause each pressure 
compensation valve 222 to be brought into its set state, thereby enabling 
the plural operating cylinders 230 with varying load pressures to be 
supplied with the pressurized discharge fluid from a single hydraulic pump 
simultaneously. 
It should be noted that the above mentioned hydraulic pump 220 is of the 
variable capacity type in which its capacity is increased and decreased by 
changing the inclination angle of its swash plate 258. A cylinder 260 for 
controlling the inclination angle of the swash plate 258 is here adapted 
to be supplied with the pump discharge pressure through a control valve 
261, which is arranged to be switchable both under the discharge pressure 
of the pump 220 and under the load pressure of the load pressure detecting 
path 257. 
The pressure compensation valve 222, the cylinder 260 for rotating the 
swash plate 258 with a controlled inclination angle and the control valve 
261, which are mentioned above, are here provided to allow the discharge 
pressure fluid from the single hydraulic pump 220 alone to be supplied 
simultaneously to a plurality of the operating cylinders 230. Thus, it is 
understood that such a combination may be unnecessary either if only a 
single operation cylinder 230 is to be actuated or if there is no need to 
actuate a plurality of such operating cylinders 230 at the same time. 
Further provided is a pilot valve 262 for furnishing a pilot pressure fluid 
into the first and second pressure receiving chambers 248 and 249 of each 
directional control valve 223. The pilot pressure fluid in the second 
pressure receiving chamber 249 is supplied to the pressure receiving 
portion 244 of the switching valve 240. 
An explanation will now be given with respect the operation of the above 
mentioned sixth embodiment of the present invention. 
When a pilot pressure is not applied from the pilot valve 262 to the 
pressure receiving portion 248 or 249 of a directional control valve 223 
and thus the directional control valve 223 is held at its neutral position 
A, the pilot pressure is not applied to the pressure receiving portion of 
the switching valve 240, either, so that the latter may be in a blocking 
state. Then, since the pressure (i. e., the retention pressure) within the 
retention pressure generating chamber 231 of a operating cylinder 230 acts 
on the pressure receiving portion 236 of the locking valve 234 so that the 
latter may be held at a blocking position by the spring 235, the pressure 
fluid may not flow through the directional control valve 223, and thus may 
not leak into any reservoir, thereby effectively preventing any operating 
cylinder from suffering a spontaneous fall. At this stage, however, the 
retention pressure, which acts on the main relief valve 245, is lower than 
a preset pressure therein and hence the main relief valve 245 will remain 
inoperative. 
It should be noted at this point that if the pressure within the retention 
pressure generating chamber 231 tends to be abnormally elevated due to an 
inertia load or an external force in the above mentioned state, any 
elevated pressure will then act on the relief valve 245 through the check 
valve 247 and if it exceeds the preset pressure in the main relief valve 
245, the latter will act to be relieved to permit the excessive pressure 
fluid to flow into the reservoir, thereby preventing the retention 
pressure from rising abnormally. 
Also, when the pilot valve 262 is operated to supply the pilot pressure 
fluid into the first pressure receiving chamber 248 of the directional 
control valve 223, the directional control valve 223 will be switched to 
the first pressure supply position B. Then, the pressure fluid will be 
supplied to the first circuit 229 to cause the locking valve 234 to open. 
Also, when the pilot valve 262 is operated to supply the pilot pressure 
fluid into the second pressure receiving chamber 249 of the directional 
control valve 223, the directional control valve 223 will be switched to 
the second pressure fluid supply position C. At the same time, the 
pressure fluid will be supplied to the pressure receiving portion 244 of 
the switching valve 240 to bring the latter to the position of 
communication. As a result, since the drain path 241 of the locking valve 
234 is brought into a communication with the reservoir 242, the locking 
valve 234 will open to allow the pressure fluid to flow out of the 
retention pressure generating chamber 231 of the operating cylinder 230 
into the tank port 225 of the directional control valve 223. 
An explanation will next be given with respect to a specific structure of 
the directional control valve 223 for use in the present embodiment of 
this invention. 
As shown in FIG. 16, the directional control valve 223 has a valve block 
270 formed therein with a spool bore 271 to which a pump port 224, a first 
and a second load pressure detecting port 228-1 and 228-2, a first and a 
second actuator port 226 and 227 and a first and a second tank port 225-1 
and 225-2 are opened. A spool 272 is fittingly inserted in the spool bore 
271 and is slidably displaceable therein to establish and block 
communications among these ports mentioned above. It should be noted here 
again that the first and second load pressure detecting ports (228-1 and 
228-2) remain in communication with each other, here via a fluid bore 320. 
The above mentioned spool 272 is formed with a first and a second small 
diameter portion 273 and 274 and an intermediate small diameter portion 
275 which serve to block each of the ports mentioned above when the spool 
272 is held at its neutral position as shown in FIG. 16. And, if the spool 
272 is slidably displaced rightwards in FIG. 16 to take its first pressure 
fluid supply position, communication will be established each between the 
pump port 224 and the second load pressure detecting port 228-2 and 
between the first load pressure detecting port 228-1 and the first 
actuator port 226. Then, the pressure fluid caused to flow into the pump 
port 224 will flow into the first actuator port 226. Also, since the 
second actuator port 227 is then brought into a communication with the 
second tank port 225-2, the return fluid will now flow out of the second 
actuator port 227 into the second tank port 225-2. 
If the spool 272 is slidably displaced from the state shown in FIG. 16 
towards the left hand side to take its second pressure fluid supply 
position, communication will be established each between the pump port 224 
and the first load pressure detecting port 228-1 and between the second 
load pressure detecting port 228-2 and the second actuator port 227. Then, 
the pressure fluid introduced into the pump port 224 will flow into the 
second actuator port 227. Also, since the first actuator port 226 is then 
brought into a communication with the first tank port 225-1, the return 
fluid will now flow out of the first actuator port 226 into the first tank 
port 225-1. 
It is seen that a first spring box 276 is attached to one end surface 270a 
of the above mentioned valve block 270 in the longitudinal direction of 
the spool 272. The first spring box 276 is provided therein with a first 
spring bearing 277 and a second spring bearing 278 in such a manner that 
the first spring bearing 277 may be in contact with both the one end 
surface 270a of the valve block 270 and a step portion 272a of the spool 
272 and the second spring bearing 278 may be in contact with a bottom wall 
276a and a step portion 279 of the spool 272. A spring 280 is interposed 
between the first and second spring bearings 277 and 278 such that the 
spool 272 may be held at its neutral position. Also, the interior of the 
first spring box 276 constitutes a first pressure receiving chamber 248. 
It is also seen that a second spring box 281 is attached to the other end 
surface 270b of the above mentioned valve block 270 and its interior 
constitutes a second pressure receiving chamber 249. 
Beneath the above mentioned valve block 270 there lie a check valve section 
250 which is constituted by a check valve bore 270e and a spool 280 
inserted therein, and a pressure reduction valve section 251 which is 
constituted by a pressure reduction valve bore 270f and a spool 283 
inserted therein. The two spools 282 and 283 are aligned so as to oppose 
each other. Also, the check valve section 250 and the pressure reduction 
valve section 251 together constitute a pressure compensation valve. 
A mating surface 270c of the above mentioned valve block 270 is joined and 
connected with a mating surface 290a of a block 290, which is formed 
therein with a first fluid bore 291 that is opening to the first mating 
surface 290a and a second fluid bore 292. The first fluid bore 291 is 
designed to communicate with the first actuator port 226 that is opening 
to the mating surface 270c of the valve block 270 whereas the second fluid 
bore 292 is designed to communicate with the second actuator port 227 that 
is opening to the mating surface 270c of the valve block 270, Further, the 
first fluid bore 291 is provided with a locking valve 234. 
The locking valve 234 includes a poppet 294 which is fittingly inserted in 
a valve bore 293 that is open to a second mating surface 290b of the block 
290 and which is being pushed by a spring 235 in its closing direction. 
The spring 235 is received in a spring chamber 296 (which represents the 
pressure receiving portion 236 in FIG. 15) that is defined by a cap 295 
which is mounted on the second mating surface 290b of the block 290. Also, 
the spring chamber 296 is open to the first fluid bore 291 through an 
axial bore 297 and a narrow bore 298 (which collectively represent the 
circuit 238 including the throttle 237 in FIG. 15) that are formed in the 
poppet 294. Here, the axial bore 297 is designed to communicate with a 
port 300 of a valve bore 293 through a narrow bore 299. 
The above mentioned block 290 is formed therein with a bore 301 that is 
open to both the first mating surface 290a and the second mating surface 
290b. The bore 301 is designed to communicate with the above mentioned 
port 300 through a fluid bore 302, and also to communicate with a 
reservoir 242 through a recess 303 that is formed in the mating surface 
290a of the block 290. Also, the bore 301 has a valve 304 fittingly 
inserted therein which constitutes the above mentioned switching valve 
240. The valve 304 has one end facing a blind hole 305 in the cap 295 and 
is arranged to be energized unidirectionally by a spring 306 so that a 
conical portion 307 formed at its other end may be in contact with a sheet 
308 so as to block a communication between the above mentioned port 300 
and the above mentioned recess 303. Also provided is a spring chamber 309 
(which represents the pressure receiving portion 244 in FIG. 15) that is 
designed to communicate with the above mentioned second pressure chamber 
249 through a bore 310 which is formed in the cap 295, a fluid bore 311 
which is formed in the block 290 and a fluid bore 312 which is formed in 
the valve block 270, so that the pressure fluid in the second pressure 
receiving chamber 249 may flow into the spring chamber 309 through the 
fluid bores 312, 311 and 310 to push up the valve 304 and that the conical 
portion 307 may thereby be departed from the sheet 308 to establish 
communication between the port 300 and the recess 303, thereby permitting 
the pressure fluid within the first fluid bore 291 to flow into the 
reservoir 242. These components here collectively constitute the above 
mentioned switching valve 240. 
The above mentioned valve block 270 is formed therein with a drain port 
313, which is designed to communicate with the first fluid bore 291 
through a first check valve 314 and also to communicate with the second 
actuator port 227 through a check valve 315. The first and second check 
valves 314 and 315 comprise a valve 316 that is attached by a spring 317 
to a sheet 318 under a pressure. Here, the first check valve 314 allows 
the pressure fluid to flow from the first fluid bore 291 to the drain port 
313 whereas the second check valve 315 allows the pressure fluid to flow 
from the second actuator port 227 to the drain port 313. 
As shown in FIG. 17, drain ports 313 as mentioned above, respectively, for 
a plurality of blocks 270, are designed to be each open to adjacent block 
joining surfaces 270d at both sides of each block 270 in the direction of 
its width and are designed to be interconnected by joining together the 
respective blocks 270 of such a plurality of directional control valves 
223. Also, a main relief valve 245 is attached to the valve block 270 that 
is located at an end of a series of such valve blocks 270. 
As will be apparent from the foregoing description, it can be seen that 
according to the sixth embodiment of the present invention in which a main 
relief valve 245 for preventing an abnormally elevated pressure within the 
retention pressure generating chamber 231 of an operating cylinder 230 can 
be placed separately, it will be sufficient to provide only a locking 
valve 234 and a switching valve 240 correspondingly to each of the 
operating cylinders 230. And yet, since the main relief valve 245 can 
commonly act for a plurality of operating cylinders 230, the area of the 
site on which a unit for preventing a spontaneous fall of any of the 
plural operating cylinders 230 is mounted may be reduced. Also, since it 
suffices to provide such a single main relief valve 245 alone, it can be 
seen that the equipment may be made less costly. 
While the present invention has hereinbefore been described with respect to 
certain illustrative embodiments thereof, it will readily be appreciated 
by a person skilled in the art to be obvious that many alterations 
thereof, omissions therefrom and additions thereto can be made without 
departing from the essence and the scope of the present invention. 
Accordingly, it should be understood that the present invention is not 
limited to the specific embodiments thereof set out above, but includes 
all possible embodiments thereof that can be made within the scope with 
respect to the features specifically set forth in the appended claims and 
encompasses all equivalents thereof.