U.S. patent number 3,753,447 [Application Number 05/202,813] was granted by the patent office on 1973-08-21 for pneumatic valves.
Invention is credited to Christopher Lee Atkins, John David Davis.
United States Patent |
3,753,447 |
Davis , et al. |
August 21, 1973 |
PNEUMATIC VALVES
Abstract
A pneumatic control valve assembly has two valves in a common
housing interconnected by a transfer duct. A first valve controls
communication between an inlet and the transfer duct while the
second valve controls communication between the transfer duct and
an outlet. Operation of the second valve, for example by a lever,
causes input pressure to be delivered to a point of use -- e.g. an
actuator. Back pressure is tapped from the outlet and operates an
actuator in the housing which closes the first valve, so that a
pressure pulse is delivered. The closure of the first valve ensures
that subsequent interruption or failure of the supply pressure has
no effect on the state of the valves.
Inventors: |
Davis; John David (Newton
Ferrers, EN), Atkins; Christopher Lee (Cornwood,
EN) |
Family
ID: |
10477064 |
Appl.
No.: |
05/202,813 |
Filed: |
November 29, 1971 |
Foreign Application Priority Data
|
|
|
|
|
Nov 27, 1970 [GB] |
|
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56,607/70 |
|
Current U.S.
Class: |
137/596; 91/433;
91/448 |
Current CPC
Class: |
F16K
11/168 (20130101); Y10T 137/87169 (20150401) |
Current International
Class: |
F16K
11/16 (20060101); F16K 11/10 (20060101); F16k
011/10 (); F16k 011/14 () |
Field of
Search: |
;137/596,596.1,636.1
;9/356,445,448,433,434 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Klinksiek; Henry T.
Claims
We claim:
1. Pneumatic valve assembly comprising:
a housing having an inlet and an outlet;
first and second valves arranged within the housing and having
respective open and closed positions;
a transfer duct interconnecting said valves within said housing,
said first valve controlling communication between the inlet and
the transfer duct and said second valve controlling communication
between the transfer duct and the outlet;
operating means for said second valve;
a bleed duct within said housing communicating with said outlet,
and
pneumatic actuator means connected to said bleed duct and acting
upon said first valve, said actuator means being responsive to back
pressure at said outlet to close said first valve.
2. A valve assembly according to claim 1, including means
responsive to closure of the second valve to open the first
valve.
3. A valve assembly according to claim 2, wherein the means
responsive to closure of the second valve comprise a further
pneumatic actuator acting on the first valve, and further bleed
duct means tapping pressure from the inlet, flow through said
further bleed duct means being controlled by said second valve so
that then the latter is closed pressure tapped from said inlet acts
on said further actuator to open said first valve.
4. A valve assembly according to claim 2, wherein the means
responsive to closure of the second valve comprise a mechanical
interconnection between the two valves.
5. A valve assembly according to claim 4, wherein the valves have
respective movable valve elements and wherein said mechanical
interconnection between the two valves comprises a lever pivotally
mounted on the housing and engageable with the movable elements of
said valves.
6. A valve assembly according to claim 5, wherein said lever
constitutes the operating means for said second valve.
7. A valve assembly according to claim 1, wherein said first and
second valves comprise respective first and second bores in the
housing and first and second spools arranged for axial displacement
in said bores.
8. A valve assembly according to claim 1, including a plate sealed
to one face of the valve housing the transfer duct being formed, at
least in part, in said plate.
9. A valve assembly according to claim 1, wherein said housing is
further provided with an exhaust port which communicates via the
second valve with the outlet when the second valve is closed and
which when the first valve is closed and the second valve is open
communicates with the said outlet via the said transfer duct.
10. A valve assembly according to claim 1, wherein said pneumatic
actuator means comprise a piston which acts on the first valve and
which is supplied with back pressure through said bleed duct.
11. A valve assembly according to claim 1, including a plate sealed
to one face of the housing, the bleed duct being formed, at least
in part, by a channel in said plate.
12. A valve assembly according to claim 1, including a pressure
relief valve in said bleed duct, said relief valve being normally
closed and being adapted to open, to connect the bleed duct to said
outlet, when the pneumatic pressure at said outlet exceeds a
predetermined value.
13. A pneumatic valve assembly according to claim 1, wherein the
second valve is spring-biased into its closed position.
Description
BACKGROUND OF THE INVENTION
This invention relates to pneumatic valves.
A common type of pneumatic valve is a so-called trip valve having a
valve housing with inlet and outlet ports and a movable valve
element controllable, for example, by means of a displaceable
operating lever, between closed and open positions in which the
outlet port is respectively cut off from and in communication with
the inlet port. Additionally, an exhaust port is commonly provided
in the housing for communication via the valve with the outlet port
when the valve element is in its closed position.
In a typical mode of use of such a known pneumatic trip valve the
outlet port is connected to an actuator, referred to herein as the
controlled actuator, for example a pilot port of a pneumatic
control valve, to pilot the latter into a given position, the inlet
port being connected to a source of fluid pressure, for example a
compressed air supply line. When the valve is operated, or tripped,
to bring the inlet port into communication with the outlet port, a
constant pneumatic pressure is applied to the controlled actuator
so long as the valve element is maintained in its open position.
Consequently the actuator cannot revert to its original state so
long as the pneumatic valve is tripped. In pneumatic systems
employing such control valves it is often necessary to connect the
valve to a further control valve, usually arranged in a "cascade"
with other control valves, which has the function of removing the
pressure signal applied to the controlled actuator when it is
desired to return the actuator to its original state. This,
particularly in a complex pneumatic system, can considerably
complicate the pneumatic circuit of the system.
To avoid such complexity it has been proposed to employ impulse
generating valves in pneumatic control circuits. An impulse
generating valve is responsive to an applied pneumatic pressure to
produce a pressure signal. The back pressure which results when
this pressure signal is applied to a controlled actuator, for
example, a pilot operating piston, is utilised to terminate the
pressure signal, so that the impulse-generating valve produces a
pressure signal of a specific duration, which is then exhausted.
Since the signal is subsequently exhausted it is not necessary to
complicate the pneumatic circuit with further valves for removing
the pressure signal.
The term "pressure signal" as used herein is understood to mean a
flow of air from a high pressure source to a low pressure
region.
The known impulse generating valve of the above-described type
suffers from a drawback which can manifest itself when the
pneumatic pressure supply to the valve is removed, for example in
an emergency or upon shutting down the pneumatic supply overnight,
in that the valve generates a spurious pressure signal when the
pressure supply is subsequently reconnected to the valve. This in
turn may change the setting of the controlled actuator, with
possibly undesirable consequences.
The present invention provides a pneumatic valve assembly of
particularly simple construction which is capable of operating as a
pressure impulse generator.
SUMMARY OF THE INVENTION
According to the invention there is provided a pneumatic valve
assembly comprising first and second valves arranged in a common
housing and interconnected by way of a transfer duct, the first
valve controlling communication between the transfer duct and an
inlet and the second valve controlling communication between the
transfer duct and an outlet, means responsive to closure of the
second valve to open the first valve, and pneumatic actuator means
responsive to pneumatic back pressure at the said outlet when the
second valve is open to close the first valve.
It will be appreciated that back pressure appears at the outlet
only when a component, for example a controlled actuator, is
connected thereto, the back pressure resulting when the pressure in
the component rises to substantially that at the said outlet.
Upon opening the second valve a pressure signal is supplied through
the outlet, and in response to the back pressure generated by this
pressure signal, the first valve is automatically operated to cut
off the inlet from the transfer duct and, therefore, from the
second valve. Consequently, any reduction in the pneumatic pressure
applied to the inlet, for example upon shutting down of the
pressure supply, has no effect whatsoever upon the controlled
actuator connected to the valve outlet.
In one embodiment of the invention the means responsive to closure
of the second valve comprise a further pneumatic actuator acting on
the first valve and operative to open the first valve in response
to pressure tapped from the inlet and applied to said further
actuator through the second valve when the latter is closed.
In an alternative embodiment of the invention the means responsive
to closure of the second valve comprise a mechanical
interconnection between the two valves, preferably in the form of a
lever pivotally mounted on the housing and engageable with
respective movable elements of the respective valves. Preferably
the respective valve elements engage the lever on opposite sides of
the pivot axis of the latter.
The movable elements of the two valves preferably comprise
respective spools arranged for axial displacement in respective
bores in the housing. Thus in a preferred embodiment of the
invention a first port in the bore containing the first valve spool
communicates via the transfer duct with a second port in the bore
containing the second valve spool, the said first and second ports
being in open communication with the inlet and outlet respectively
when the respective valves are open, and being cut off by the
respective valve spools from the inlet and outlet respectively when
the respective valves are closed.
The housing of the valve assembly may comprise a single body, for
example a machined metal block, or two or more bodies
interconnected by bolts or by other means.
The transfer duct may be formed, at least in part, by a channel
provided by a gasket or sealed plate secured to one face of the
valve housing.
The housing is preferably further provided with an exhaust port
which communicates via the second valve with the outlet when the
second valve is closed and which, when the first valve is closed
and the second valve is open, communicates with the said outlet via
the said transfer duct.
Preferably the pneumatic actuator means comprises a piston which is
formed with or bears against the movable element of the first valve
and which is supplied with pneumatic pressure by way of a bleed
duct adapted to communicate with said outlet. The bleed duct may
also be formed, at least in part, by a channel in said gasket or
sealed plate secured to one face of the housing. A pressure relief
valve is preferably provided in said bleed duct, said relief valve
being normally closed and being adapted to open, to connect the
bleed duct to said outlet, when the pneumatic pressure at said
outlet exceeds a predetermined value.
If the valve assembly is so connected that the back pressure at the
outlet does not rise to the predetermined value the valve assembly
acts as a simple trip valve delivering a steady pressure signal
upon operation.
Preferably the second valve is spring-biased into its closed
position.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be more particularly described, by way of
example only, with reference to the accompanying drawings, in
which:
FIG. 1 is a diagrammatic sectional view of a valve assembly
according to one embodiment of the invention;
FIG. 2 is a diagrammatic plan view of the valve assembly shown in
FIG. 1, viewed in the direction of arrow II of FIG. 1;
FIGS. 3 and 4 are diagrammatic sectional views corresponding to
FIG. 1 and showing the valve assembly in two successive stages of
operation;
FIG. 5 is a diagrammatic sectional view, corresponding to that of
FIG. 1, of a valve assembly according to an alternative embodiment
of the invention, and
FIG. 6 is a diagrammatic illustration of a simple pneumatic circuit
incorporating a valve assembly in accordance with the
invention.
DETAILED DESCRIPTION OF THE DRAWINGS
The pneumatic valve assembly according to the invention as
illustrated in FIGS. 1 to 4 comprises first and second valves 1, 2
arranged in a common housing and operatively interconnected as
hereinafter described. The valve housing consists of a body 3
having an inlet port 4, an outlet port 5, and an exhaust port 6,
each of these ports comprising internally screw threaded bores, as
known per se.
The first and second valves 1, 2 comprise respective valve spools
7, 8 movable axially in respective parallel cylindrical bores 9, 10
formed in the body 3 between the inlet port 4 and outlet port 5,
the exhaust port 6 being located between the bores 9, 10. Each of
the valves 1, 2 in the embodiment of FIG. 1 is a three port valve,
and is provided, in a known manner, with three resilient captive
O-rings 11, 12, spaced apart at substantially equal intervals along
the respective bores 9, 10 and positioned therein at fixed axial
positions by respective spacers 13, 14. The three O-rings 11, 12 in
the respective bores 9, 10 are designated 11A, 11B, 11C and 12A,
12B, 12C respectively, starting in each case from the upper (open)
ends of the respective bores 9, 10, as illustrated in FIG. 1.
The respective first and second valve spools 7, 8 are formed with
two axially spaced apart cylindrical lands 7A, 7B and 8A, 8B
respectively, the diameter of each said land being slightly greater
than the internal diameters of the O-rings 11, 12, so that the
respective lands can form effective seals when engaged in the
respective O-rings.
As an alternative to providing the respective valve bores 9, 10
with captive O-rings each valve spool 7, 8 may be provided with
captive O-rings cooperating with respective internal shoulders in
the respective bores.
Inlet port 4 communicates with a port 15 in the bore 9 of the first
valve 1, the port 15 being located between the O-rings 11A and 11B.
The outlet port 5 communicates with a port 16 in the bore 10 of the
second valve 2, the port 16 being located between the O-rings 12B
and 12C.
A transfer duct 17 interconnects the two valves 1, 2. The transfer
duct 17, which is indicated diagrammatically by a broken line in
FIG. 1, may be formed in the body 3 itself, but is in practice
conveniently formed at least in part by a channel in a gasket or
plate secured to one face of the body 3, as hereinafter
described.
The transfer duct 17 interconnects a first port 18 in the first
valve bore 9, between the O-rings 11B and 11C, and a second port 20
in the second valve bore 10, between the O-rings 12A and 12B.
The exhaust port 6 communicates with respective ports 21, 22 in the
respective valve bores 9, 10 at the lower (blind) ends of the
respective bores 9, 10 as illustrated by broken lines.
The valve spools 7, 8 have respective rounded ends 23, 24 which
project axially beyond the open upper ends of the respective bores
9, 10 from one face of the body 3. An actuating lever 25 is
pivotally mounted on this face of the body 3 for rocking movement
about an axis X which is located between and substantially
equidistant from the axes of the two valve spools 7, 8, and is
perpendicular to the said valve spool axes. The operating lever 25
carries at its free end a roller 26 which is adapted to cooperate
with a valve operating member (not shown). The actuating lever 25
bears against the rounded projecting ends 23, 24 of the two valve
spools 7, 8 on either side of the pivot axis X, and substantially
equidistantly therefrom.
A biasing spring 27 is located in a central bore in the second
valve spool 8 and urges the latter upwardly so that the annular
shoulder at the upper end of the land 8A is normally maintained in
contact with a stop ring 28 at the upper end of the bore 10. In
this position of the valve spool 8 the rounded upper end 24 of the
spool 8 is spaced from the lever 25. The lever 25 itself is biased
by a spring 29 coaxially surrounding the upper end of the spool 8
externally of the body 3 into engagement with the first valve spool
7 to urge the latter into its lowermost position (FIG. 1).
A counter-bore 30 is formed as a coaxial extension of the lower end
of the first valve bore 9, the counter-bore 30 having a smaller
diameter than the valve bore 9. The counter-bore 30 houses a
pneumatic actuator comprising a piston 31 formed of nylon which
slides with radial clearance within the counter-bore 30, permitting
slight leakage of fluid between the piston 31 and the counter-bore
30. The piston 31 bears against the lower end of the first valve
spool 7, as illustrated in FIG. 1. Alternatively the piston 31 may
be formed as an integral extension of the first valve spool 7.
A bleed duct 33, shown in broken outline, communicates with the
outlet port 5 of the second valve 2 via a pressure relief valve 34
which is biased by a spring 34a into a normally closed position.
The spring 34a is such that the valve 34 opens when the pressure in
the outlet port 5 is of the order of 48 p.s.i. The bleed duct 33
opens into the lower (blind) end of the counterbore 30, as shown
diagrammatically.
To facilitate manufacture of the valve assembly shown in FIG. 1,
and in particular the formation of the various internal ducts,
including the transfer duct 17 and the bleed duct 33, the body 3 is
formed with a flat machined lateral face 3A (FIG. 2) to which a
cover plate 35 of metal or plastics (e.g. nylon) is secured, by
means of, for example, bolts or adhesive, with the interposition of
a gasket 36. The transfer duct 17 and the bleed duct 33 are formed
in part as respective channels in the face of the plate 35 which is
adjacent the gasket 36, the gasket 36 and the body 3 being formed
with respective apertures (not shown) by which these channels
communicate with respective ports of the first and second valves 1,
2 to effect the desired interconnections, as illustrated
diagrammatically in FIG. 1. A suitable sealing compound may be
interposed between the gasket 36, the body 3 and the plate 35. For
some materials of the body 3 and plate 35 satisfactorily sealing
can be effected without the gasket 36.
The operation of the valve assembly of FIGS. 1 and 2 will now be
described with reference to FIGS. 1, 3 and 4.
Each valve 1, 2 is movable between a closed position and an open
position. The first valve spool 7 is biased into its lower
position, illustrated in FIG. 1 by the action of the spring 29 on
the lever 25. In this position the first valve 1 is open and the
lands 7A and 7B are in sealing contact with the O-rings 11A and
11C. This affords communication, through the O-ring 11B and the
first port 18, between the inlet port 4 and the transfer duct 17.
When the first valve spool 7 is in its upper position the first
valve 1 is closed and the land 7B makes sealing contact with the
O-ring 11B, cutting off the inlet port 4 from the transfer duct 17.
At the same time, the land 7B is removed from contact with the
O-ring 11C and communication is therefore established, through the
O-ring 11C, between the transfer duct 17 and the exhaust port
6.
In the normal or inoperative state of the valve assembly the spool
8 of the second valve 2 is biased by the spring 27 into its
uppermost position, shown in FIG. 1 in which the second valve 2 is
closed. In this position the lands 8A and 8B are in sealing
engagement with the O-rings 12A and 12B, and the transfer duct 17
is cut off from the outlet port 5, the latter being in open
communication, through the port 16 and the O-ring 12C, with the
exhaust port 6. When the second valve spool 8 is in its lower
position the second valve 2 is open and the transfer duct 17
communicates with the outlet port 5 through the O-ring 12B, and the
land 8B makes sealing contact with the O-ring 12C, cutting off the
exhaust port 6 from the outlet port 5.
It will be appreciated that the effect of the lever 25 is to ensure
that, when the second valve 2 is closed, with its spool 8 in its
uppermost position (FIG. 1) the action of the lever 25 on the first
valve spool 7, assisted by the spring 29, is to depress the latter
and ensure that the first valve 1 is opened.
Under normal operating conditions pneumatic pressure from a
convenient source is applied to the inlet port 4, and a controlled
actuator (not shown), for example a pilot operating piston of a
pneumatic control valve, is connected to the outlet port 5. In the
inoperative, normal, position of the valve assembly, illustrated in
FIG. 1, the first valve 1 is open and the second valve 2 is closed.
Consequently the applied pneumatic pressure reaches the port 20 of
the second valve 2, but is prevented from reaching the outlet port
5 to operate the controlled actuator.
When a valve operating member (not shown) engages the roller 26 to
cause rocking movement of the operating lever 25 in a clockwise
direction, as viewed in FIG. 1, the actuating lever 25 depresses
the second valve spool 8 against the action of its biasing spring
27, opening the second valve 2 (FIG. 3). The transfer duct 17 is
then placed in communication with the outlet port 5, as a result of
the land 8B moving out of sealing engagement with the O-ring 12B.
The pneumatic pressure is therefore applied through the inlet port
4, the open first valve 1, the transfer duct 17, the open second
valve 2 and the outlet port 5 to the controlled actuator to operate
the latter.
When the actuator, for example a pneumatic piston, has been
operated, which usually takes place very quickly, the continued
application of the pneumatic pressure will create a back pressure
in the outlet port 5. This back pressure rapidly builds up to a
value, typically 48 p.s.i., sufficient to open the relief valve 34
and the back pressure is then transmitted through the bleed duct 33
to the counter-bore 30. The actuator piston 31 is then displaced
upwardly by the back pressure, displacing the first valve spool 7
upwardly into its closed position (FIG. 4). The upward movement of
the first valve spool 7 is permitted by the earlier clockwise
rocking movement of the lever 25 out of engagement with the
projecting end 23 of the first valve spool 7. The upward movement
of the first valve spool 7 brings the projecting end 23 back into
engagement with the lever 25 and closes the first valve 1, cutting
off the inlet port 4 from the transfer duct 17, while at the same
time opening the transfer duct 17 to the exhaust port 6 through the
O-ring 11C.
It will be seen that in the position shown in FIG. 1, with the
valve 2 closed (in its uppermost position) the outlet port 5
communicates with the exhaust port 6 directly through the O-rings
12C and in the position shown in FIG. 4, with the valve 2 open (in
its lowermost position) the exhaust port 6 communicates with the
outlet port 5 through the O-ring 11C of the first valve 1, the
transfer duct 17 and the O-ring 12B. Thus the outlet port 5 is
connected to the exhaust port 6 both before and after the pressure
impulse has been delivered. Return of the controlled actuator to
its original position is, therefore, permitted after the pressure
impulse has been applied thereto, and no separate control valve is
necessary for removing the pressure signal at the outlet port
5.
Closure of the first valve 1 in response to back pressure developed
after operation of the controlled actuator ensures that the valve
assembly provides a pressure pulse for operating the controlled
actuator, regardless of the length of time for which the operating
lever 25 is depressed. In some cases the valve operating member
which engages the roller 26 assumes a "rest on" position in which
the lever 25 is held in a depressed state so that the valve 2 is
maintained open: regardless of this, the output signal delivered
through the outlet port 5 consists of a short pressure pulse
sufficient to operate the actuator, and after the latter has been
operated the outlet port 5 is vented to exhaust and the first valve
1 is closed, as described previously. This subsequent closure of
the first valve ensures, that, if the supply of pressure is cut off
with the valve operating member "resting on," for example if the
associated pneumatic circuit is shut down, or if the pressure
supply is cut in an emergency, the state of the valve assembly and
its controlled actuator is unchanged. No change in the setting of
the associated controlled actuator therefor takes place upon
subsequently reconnecting the supply of pressure.
FIG. 5 illustrates an alternative embodiment of the invention.
Those parts corresponding directly to component parts of the
embodiment of FIG. 1 have been designated by the same reference
numerals: these parts, and their function, will not require further
explanation.
In the embodiment of FIG. 5, the operating lever 25' is pivoted at
one end of the body 3 and its function is solely to operate the
second valve 2. The first valve 1 is enclosed completely within the
body 3 and has no mechanical connection with the operating lever
25'. In order to open the first valve 1 in response to closing
movement of the second valve 2, as described previously, the
embodiment of FIG. 5 employs a further pneumatic actuator. This
further pneumatic actuator comprises a piston 40 which slides in a
cylinder 41 in a tubular insert 42 located in a counter-bore
communicating with the upper end of the first valve bore 9. The
piston 40 engages the upper end of the first valve spool 7, and may
alternatively be formed integrally with the valve spool 7.
The piston 31 is a plain piston, typically of nylon as in the
embodiment of FIG. 1, permitting slight leakage of fluid between
itself and the wall of the counterbore 30.
The piston 40 is shown in FIG. 5 as having a sliding O-ring seal
with the wall of the cylinder 41, but the piston 40 could
alternatively be made of plastics material, without a sliding
seal.
The second valve 2 in the embodiment of FIG. 5 is a five port
valve, in contrast to the three port valve 2 employed in the
embodiment of FIGS. 1 to 4. The two additional ports are defined
between two further resilient O-rings 12D, 12E in a conventional
manner, and the second valve spool 8 has a further land 8C which
cooperates with the O-rings 12D and 12E. In addition to the bleed
duct 33 there is provided a second internal bleed duct 43 (broken
lines) providing communication between the port 15 of the first
valve, that is, the inlet port, and the bottom (blind) end of the
second valve bore 10. Moreover, in addition to the transfer duct 17
there is provided a second internal transfer duct 44 providing
communication between a port 45 located in the second valve bore 10
between the O-rings 12D and 12E and the upper end of the cylinder
41 in which the actuator piston 40 slides. Both the ducts 43 and 44
are conveniently formed, at least in part, by channels in a plate
or gasket (not shown) attached to a side face of the body 3, as
previously described.
When the second valve 2 is closed, as shown in FIG. 5, cutting off
the transfer duct 17 from the outlet port 5, the O-ring 12D is in
sealing engagement with the land 8C, and communication is afforded
through the O-ring 12E between the inlet port 4 and the cylinder 41
through the internal passages 43 and the second transfer passage
44. Accordingly, piston 40 is forced downwardly, urging the first
valve spool 7 into its lower position, and opening the first valve
1, as shown in FIG. 4.
When the second valve 2 is opened, with its valve spool 8 in its
lower position, by depression of the lever 25', the land 8C makes
sealing contact with the O-ring 12E and cuts off the bleed duct 43
from the second transfer duct 44, at the same time opening
communication between the latter and the exhaust port 6 through the
O-ring 12D and the port 22, the connection to the exhaust port 6
being indicated by broken lines 6a. The first valve spool 7 is then
movable upwardly by means of the actuator piston 31 under the
influence of back pressure, as described previously. It will be
seen, therefore, that the pneumatic actuator piston 40 serves the
same purpose as the mechanical interconnection of the valve spools
7 and 8 by means of the lever 25' in the embodiment of FIG. 1.
FIG. 6 illustrates a typical simple pneumatic circuit, employing a
valve assembly in accordance with the invention. The notation used
in the circuit is the usual notation for pneumatic valves, and the
operation of this circuit will be clear to those skilled in the art
from the drawing. The circuit includes two double-acting pneumatic
rams A, B which are to be operated in an automatic cycle which
consists first of extension and retraction of A followed by
extension and retraction of B. The cycle is commenced by operation
of a manual valve, for example a pedal-operated pneumatic three
port valve 45 which connects the compressed air supply indicated,
conventionally, as a circle with a central dot, to pilot a
double-acting three port control valve 46 into a position in which
it supplied pressure to the ram A to cause extension of the latter.
When the ram A has fully extended it engages a conventional trip
valve which in turn causes piloting of the control valve 46 into
its other position, so as to initiate retraction of the ram A if
the valve 45 has been released. When the ram A has fully retracted
a valve operating member carried thereby engages and `rests on` the
operating lever of a valve assembly V according to the invention.
As a result of this the valve assembly V produces a pressure signal
which is exhausted, after a specific time interval, by the
operation of the valve assembly V in response to back pressure in
the output line of the valve assembly V, leading to a pilot
operating piston of a second control valve 47. This pressure signal
is sufficient to pilot the valve 47 into a position in which it
causes extension of the second ram B. Retraction of the second ram
B is initiated by a second trip valve 48 which pilots the second
control valve 47 to its opposite position, permitting retraction of
the second ram B. This piloting of the second valve 47 into its
opposite position is permitted by the fact that the pressure signal
earlier applied to the valve 47 from the valve assembly V has been
short-lived. Consequently, a cascade system of control valves for
removing pressure signals applied to the various valve piloting
actuators is not necessary when using the valve assembly according
to the invention.
* * * * *