US2922439A - Limit valve - Google Patents

Description

J. A. PALMER Jan. 26, 1960 LIMIT VALVE 2 Sheets-Sheet 1 Filed Feb. 23, 1955 PRIMARY DIRECTION/9L CONTROL VALVE 1 r-E 4 A A Jan. 26, 1960 J, PALMER 2,922,439

LIMIT VALVE Filed Feb. 23, 1955 2 Sheets-Sheet 2 IN I/E/VTO R JOHN a. PAL HER,

LIMIT VALVE John A. Palmer, St. Louis, Mo., assignor to The Emerson Electric Manufacturing Company, St. Louis, Mo, a corporation of Missouri Application February 23, 1955, Serial No. 489,935 19 Claims. (Cl. 137-622) The present invention relates generally to valves, and more particularly to a fluid control valve for automatically stopping and retaining a fluid powered device in a predetermined position. v

The valve of the present invention is particularly useful in connection with fluid operated cylinders or rams where it is desirable that the reciprocating element be accurately positioned, without shock, at a predetermined end point in its stroke. Normally, the valve will be connected between the fluid operated cylinder and the usual primary directional control valve associated therewith. The limit valve has a displaceable fluid controlling element which is mechanically shifted as the reciprocating element of the ram approaches a predetermined position, the arrangement being such that the fluid flow is progressively diminished until the reciprocating element is brought to a halt. If conditions should by any chance be such that the reciprocating element overshoots or drifts past the predetermined position, the fluid flow will be reversed through the cylinder so as to return the reciprocating element to the predetermined position and thus retain it there. Notwithstanding its own ability to reverse the fluid flow to and from the cylinder, the limit valve does not at any time preclude immediate response to a reversal of the primary directional control valve associated therewith.

It is an object of the present invention to provide a limit valve for fluid operated reciprocating devices which functions automatically so as progressively to decelerate a reciprocating element and ultimately to stop it in a predetermined position.

It is another object of the invention to provide a limit valve for use with a fluid operated reciprocator which functions to retain the reciprocable element of the reciprocator in a predetermined position at one end of its stroke.

,It is another object of the invention to provide a limit valve for use with a fluid operated reciprocating device which functions automatically to provide directional control of movement of the reciprocating element of the device so as to compensate for tendency of the reciprocable element to be displaced in either direction from a predetermined position.

It is another object of the invention to provide a limit valve for use with a fluid operated device and a primary directional control valve which avoids any delay in response of the device to manipulation of the primary directional control valve.

The foregoing, along with additional objects and advantages, will be apparent from the following description taken with the accompanying drawings, in which:

Figure l is a schematic diagram showing the limit valve of the present invention installed for positional control of a fluid operated cylinder;

Figure 2 is a cross-sectional view of a dual valve conforming to the teachings of the present invention, the movable parts of the valve being illustrated in normal rest position;

Figure 3 is a view similar to Figure 2, but drawn to reduced scale and showing a valve spool in displaced position; and

Figure 4 is a view similar to Figure 3, but showing the valve spool in a position of further displacement and showing also two ball check valves in displaced position.

Referring to the drawings more particularly by means of reference numerals, the numeral 10 designates alimit valve which is constructed in accordance with the teachings of the present invention. Directing attention to Figure 2 of the drawings, it will be noted that the illustrated valve 10 is actually a dual valve. In other words, the valve 10 could be separated along its horizontal center line, designated in Figure 2 by the numeral 12, to provide two identical limit valves, each of which, incidentally, would conform to the teachings of the present invention. This being true, a description of the upper half of the valve 10 will suflice for the lower half as well, and it will be understood, therefore, that each of the primed reference numerals designates an element in the lower half of the valve 10 which finds its counterpart in the upper half thereof. A valve body 14 is, of course, common to both the upper and lower halves.

Considering now the upper half of the valve 10 as illustrated in Figure the body 14 is provided with a main valve chamber 16 for accommodation of a reciproca-ble valve spool 18. While the chamber 16 general- 1y takes the form of a cylindrical bore 20, it is at the same time provided with a series of annular recesses spaced and sized as clearly indicated in the drawings. Each of the annular recesses in turn communicates with one or more fluid passages formed in the body 14. Thus, working from the left in Figure 2, an annular recess 22 communicates with a fluid passage 24 ported to the outside of the body 14 through a tubular nipple 26. An annular recess 28 communicates with a passage 30 which, if the upper half of the valve 10 constituted a single valve, would also be ported to the outside. In the illustrated dual valve 10, however, the passage 30 is continued in the form of an extension to be described hereinafter.

Besides communicating with the passage 30, the annular recess 28 communicates also with a passage 32 which, in turn, communicates with a branch passage 34 connected into the aforementioned passage 24. The passage 32 includes an enlarged chamber 36 provided with an annular ball seat 38 at the end nearer to the recess 28 and containing a ball valve 40 biased toward the seat 38 by a spring 42.

An annular recess 44 communicates with a passage 46 connected into a passage 48. Like the passage 30, the passage 48 would be ported to the outside if the upper half of the valve 10 constituted a single valve. It too, however, is, in the illustrated device, extended to form a passage to be described. The passage 46 includes an enlarged chamber 50 provided with an annular ball seat 52 at the end farther from the recess 44 and containing a ball valve 54 biased toward the valve seat 52 by a spring 56.

An annular recess 58 communicates with the previously mentioned passage 34. The passage 34 includes an enlarged chamber 6%) provided with an annular ball seat 62 at the end nearer to the recess 58 and containing a ball valve 64 biased toward the seat 62 by a spring 66.

An annular recess 68 communicates with a passage 70 connected into a passage 72 ported to the outside of the valve to through a tubular nipple 74.

An annular recess 76 communicates with the previously described passage 48, and also with a passage 78 connected into the passage 70. The passage 78 includes an enlarged chamber 80 provided with an annular valve seat 82 at the end nearer to the recess 76 and containing a ball valve 84 biased toward the seat 82 by a'spring 86.

Finally, an annular recess 88 communicates with the previously described passage 72.

The cylindrical bore 28, as distinguished from the annular recesses above enumerated, slidably receives the valve spool 18. 'More specifically, the bore 28 is adapted for snug slidable engagement with longitudinally spaced portions of the spool 18, as will appear. Thus, the spool 18 includes portions 90, 92, 94 and 96 spaced from each other as clearly illustrated in Figure 2. It is to be understood that the individual lengths, as well as the spacing of these portions of maximum diameter of the spool 18 bears a particular relation to the spaced arrangement of the aforementioned annular recesses in the body 14. This relationship is also evident from Figure 2 and its significance will appear from a description of the operation of the valve hereinafter.

Considering the valve spool 18 further, it will be observed that the aforementioned portions 90, 92, 94 and 96 are interconnected successively by portions of reduced diameter designated as 98, 108 and 102. There is also a portion 104 and a portion 106, each of reduced diameter, at the outer ends of the portions 90 and 96, respectively. Finally, there is an actuating stem 108 and a guide stem 110 of still further reduced diameter at opposite ends of the spool 18. The actuating stern 108 extends through and beyond a cap 112 secured to the outer surface of the body 14 by means of screws 114. The cap 112 is provided with a central boss 116 which serves, not only as a centering device for the cap 112, but also as a limiting stop for abutment with the portion 104 of the valve spool 18.

The guide stem 110 extends through a cap 118 secured to another external face of the body 14 by screws 120. Thecap 118 has a boss portion 122 which, along with an oppositely extending portion 124 is counterbored so as to provide a spring chamber 126 for accommodation of a compression spring 128 serving to bias the valve spool 18 toward the aforementioned abutment with the boss 116 of the cap 112. It will be noted that the boss 122 of the cap 118 serves as a limiting stop for move ment of the spool 18 in the opposite direction, being engageable with the lateral face of the portion 96, as is clear from Figure 2.

Preferably, both the cap 112 and the cap 118 are provided with conventional sealing devices 130 and 132 for prevention of leakage at the faces of the body 14 and along the valve stems, respectively.

Directing attention now to Figure 1, the schematic representation there depicted shows a typical installation of the dual limit valve 18. As previously indicated, the valve 10 is incorporated between a directional control valve assembly 148 and a fluid operated cylinder assembly 142. The directional control valve assembly 140 is a conventional device provided with a control lever 144 which is effective to connect a fluid pressure line 146 with either the fluid line 74 or the fluid line 74, the lines 74 and 74 being connected into the valve as previously described. The lever 144 is also effective to connect a fluid return line 148 with either of the fluid lines 74 or 74, and it will be understood that with the lever 144 in the illustrated position to the right, the pressure line 146 is connected to the line 74, while the return line 148 is connected to the line 74'. Movement of the lever 144 to the left, however, would be effective to reverse these connections.

The fluid operated cylinder assembly 142 is a double acting device having a reciprocating element 150 movable in either direction relative to a stationary cylinder 152. The element 158 is effective to perform useful work, and may be attached to any suitable device including a device which may impose a constant force tending to move the element 158. Such an arrangement is shown schematically in Figure l in the attachment of a weight W to the element 150 by means of a cable 154 passing over a pulley 156.

A collar assembly 158 having an inclined cam face 160 is adjustably secured to a portion of the reciprocating element 158 as illustrated in Figure 1. Thus mounted, the face 160 is positioned to engage a roller 162 mounted at the free end of an arm 164 of a bell crank assembly 166 pivotally mounted to a stationary bracket 168. An arm 170 of the bell crank assembly 166 has its free end provided with an adjustable screw 172 and retaining nut 174 for abutting engagement with the end of the actuating stem 108 of the aforementioned valve spool 18.

As is clear from Figure l, the depicted arrangement shows a second collar on the reciprocating element 150 and a second bell crank adapted for operative engagement with the actuating stem 108 of the valve spool 18. Inasmuch as these elements are similar in all except dimensional respects to those previously described, the elements of the second set have been designated by the primed numerals of their counterparts.

The tubular line 26, previously described as connected into the passage 24 of the valve 10, is connected at its other end into one end of the fluid cylinder 152. Similarly, the line 26 is connected from the passage 24 in the valve 10 to the other end of the cylinder 152. Thus, it will be understood that the establishment of fluid pressure in the line 26, along with a reduction of pressure in the line 26 will cause the reciprocating element to move to the right, whereas the establishment of fluid pressure in the line 26, along with a reduction of pressure in the line 26, will cause the reciprocating element 150 to move to the left.

Operation Directing attention once more to Figure 2, the valve spools 18 and 18' are shown in their normal rest positions, which is to say that neither is operatively engaged by the bell cranks 166 or 166' described above. Considering once more only the upper half of the valve 10, it is apparent that with the spool 18 in the position mentioned the passage 72 is communicated through the annular recess 88, through that portion of the bore 20 which joins the recesses 88 and '76, and through the latter recess to the passage 48. In similar manner, the passage 24 communicates through the annular recess 22, through that portion of the bore 20 which interconnects the recesses 22 and 28, and through the latter recess to the passage 30. Clearly, the recited intercommunication between the passages 72 and 48 and that between the passages 24 and 38 results, not only from the position of the spool 18, but from the illustrated relationship between the spacing of the annular recesses 22, 28, 76 and 88 combined with the illustrated relative size and spacing of the portions 90, 92, 94 and 96 of the spool 18.

It will be obvious, of course, that intercommunication between passages in the lower half of the valve 10 will correspond to that above described when the valve spool 18 is in its illustrated position of normal rest. It will also be obvious that with both the valve spool 18 and the valve spool 18' in normal rest position, the passages 72 and 24', which is to say the lines 74 and 26', are intercornmunicated, as are also the passages 24 and 72, or the lines 26 and 74'. Finally then, it is apparent that with the valve spool 18' in normal rest position, the common passages 48 and 38 have open communication with the lines 26 and 74, respectively, and, similarly, with the valve spool 18 in normal rest position, the common passages 48' and 30' are in open communication with the lines 26 and '74, respectively. From the illustration of Figure 1, it will be observed that one or the other of the spools 18 and 18 is always in normal rest position, and that, at times, both may be.

Referring to Figure 1 and assuming now that the illustrated position of the lever 144 connects the line 74 with the fluid pressure line 146 and the line 74' with the fluid return. line 148, and. assuming further that the reciproeating element 150 of the assembly 142 is initially in a position somewhat to the left of that illustrated so that both the valve spool 18 and the valve spool 18' are in normal rest positions, fluid pressure will be communicated through the valve to the line 26, and thence to the left end of the cylinder 152. At the same time, the right end of the cylinder 152 will be connected through the line 26, and through the valve 10 to the line 74'. Under these conditions, the reciprocating element 150 will obviously be moved to the right so that eventually the face 160 of the collar assembly 158 will engage the roller 162 of the bell crank assembly 166. The resulting pivotal movement of the bell crank 166 will be reflected in displacement of the valve spool 18 against the action of the biasing spring 128. As the reciprocating element 150 proceeds further to the right, the spool 18 will be simultaneously progressively displaced until it approaches the position illustrated in Figure 3. In the latter figure, it will be observed that the portion 94 of the spool 18 now engages that portion of the bore 20 which separates the annular recesses 76 and 88, thereby obstructing direct communication between these recesses and stopping the flow of fluid under pressure from the line 74 to the line 26. Also, the portion 90 of the spool 18 has now engaged that portion of the bore 20 which separates the annular recesses 22 and 28, thereby closing off communication between the lines 26 and 74 through the annular recess 22 and stopping the return flow of fluid from the line 26 to the line 74'. In neither case, however, is communication between these lines completely eliminated, inasmuch as one-way reversed flow past the ball valves and 84 is still possible. Thus, notwithstanding the obstruction of fluid flow which tends to advance the element 150 to the right, a condition is avoided wherein both ends of the cylinder 152 would be sealed ofl without the possibility of reversing the fluid flow so as to eflect reversed movement of the reciprocating element 150.

In its approach to the position of Figure 3, the portion 94 of the spool 18 gradually constricts the area communicating the annular recess 76 and the adjacent portion of the bore 20 which extends to the recess 88 so that the flow of fluid under pressure from the line 74 through the valve 10 and the line 26' to the cylinder 152 is progressively diminished. At the same time, the portion 90 of the spool 18 progressively constricts, and eventually obstructs, the area communicating the annular recess 22 and the adjacent portion of the bore 20 which extends to the recess 28. As a result of this diminution of flow, the reciprocating element 150 is brought to a gradual, although conceivably a very rapid, halt. Clearly, the relative rapidity with which the reciprocating element 150 is halted is a function of its speed of move ment combined with the angle of inclination of the cam face 160. p

In the event that a work load, such as indicated in Figure l by the weight W, should act upon the reciprocating element 150 so as to bias it to the left, it is obvious that any leftward movement of the element 150 will be accompanied by a corresponding leftward movement of the valve spool 18 such that the fluid pressure connection will be reestablished to the left end of the cylinder 152 to return both the element 150 and the valve spool 18 to mutually balanced condition. If, on the other hand, conditions should arise, either due to loading of the element 158 or to leakage in the fluid flow system, wherein the reciprocating element 158 should be inadvertently moved to the right farther than the above described position of equilibrium, the valve spool 18 would also be moved farther to the right and would occupy a position of further displacement as illustrated in Figure 4. In this figure, it will be observed that the spool 18 has moved to a position wherein the portion 94 has uncovered the annular recess 68 so that the latter is now communicated through the bore 20 with the annular recess 58.

Now, inasmuch as the annular recess '68 is connected by means of the passages 70 and 72 with the line 74, the fluid pressure from the latter is delivered against the ball valve 64 so as to unseat it and allow the fluid pressure to be communicated on through the passages 34 and 24 to the line 26, and thence to the right hand end of the cylinder 152. At the same time, it will be noted that the rightward movement of the portion 92 of the spool 18 has partially uncovered the annular recess 44 so as to communicate the same with the annular recess 28 and, hence, with the line 74'. In other words, not only is the line 74, containing fluid pressure, now connected with the line 26 extending to the right hand end of the cylinder 152, but also the left end of the cylinder 152 is connected by way of the line 26, the passage 24, the intercommunicated recesses 22' and 28', the passages 30 and 46, the intercommunicated recesses 44 and 28, the passages 30 and 48', the intercommunicated recesses 76' and 88', and the passage 72 to the line 74', which is connected for return fluid flow. Obviously, then, the reciprocating element is caused to return to the equilibrium position illustrated in Figure 3.

It will be observed that the return fluid flow just traced is in a direction enabling the ball valve 54 incorporated in the passage 46 to be readily displaced so as to accommodate the described flow. Although the return fluid also engages the ball valve 40, this element is maintained on its seat 38 by the pressurized fluid which occupies the chamber 36 at this time by virtue of the open communication of the latter with the passage 34. If, however, the lever 144 be reversed while the valve 10 is in the condition of Figure 4, the resulting flow of pressurized fluid through the line 74 will clearly act to unseat the ball valve 46 and to seat both the ball valve 54 and the ball valve 64. It is aided in seating the balls 54 and 64, of course, by the fact that the pressure in the line 74 is now reduced for return fluid flow, this reduced pressure being communicated to the ball 64 through the passages 72 and 70, the intercommunicated recesses 68 and 58, and the passage 34, and to the ball 54 through the passages 72, 70, and 78, the recess 76, and the passages 48 and '46. Thus, until the valve spool 18 once more approaches its normal rest position, pressurized fluid will flow upwardly through the valve chamber 36 and return fluid will flow upwardly through the valve chamber 80. r

As soon as the reciprocating element 150 has advanced far enough to the left to enable the spool 18 to return to its normal rest position, and considering still that the lever 144 has been moved to its left hand position, it is obvious that with the line 74' being now connected with the fluid pressure line 146 and the line 74 being now connected with the fluid return line 148, the action of the valve 10 and the cylinder assembly 142 will only be reversed from that previously described. In other words, the reciprocating element 150 *will continue to move to the left until the valve spool 18 is actuated, the spool 18 meanwhile remaining in normal rest position.

Clearly, there has been provided a limit valve and an arrangement incorporating the same which together fulfill the objects and advantages sought therefor.

It is to be understood that the foregoing description and the accompanying drawings have been given only by way of illustration and example. It is further to be understood that changes in the fonn of the elements, rearrangement of parts, or the substitution of equivalent elements, all of which will be obvious to those skilled in the art, is viewed as being within the scope of the present invention, which is limited only by the claims which follow.

What is claimed is:

l. A limit valve comprising, in combination, a body provided with an elongated generally cylindrical valve chamber and a plurality of fluid passages communicating therewith, at'least four of said passages being ported to the outside of said body, a valve spool movably disposed in said valve chamber for selectively interconnecting said fluid passages in different predetermined combinations, and at least three check valves disposed in selected fluid passages for controlling fluid flow therethrough, each of said check valves being disposed between said chamber and a respectively difierent outside port.

2. The limit valve of claim 1 wherein a first and a second ported passage are communicated at a minimum of three longitudinally spaced points each with the main valve chamber, a third ported passage is communicated at a. minimum of two longitudinally spaced points with the main valve chamber, and a fourth ported passage is communicated at a minimum of one point with the main valve chamber.

3. The limit valve of claim 2 wherein at least one path of communication of each of the first, second, and third ported passages with the main valve chamber is provided with a directional check valve.

4. The limit valve of claim 3 wherein at least two of the paths of communication of the first ported passage are provided with directional check valves.

5. The limit valve of claim 4 wherein the fourth ported passage and one of the checked paths communicated with the first ported passage are communicated with the main valve chamber at substantially the same longitudinal point, and wherein the unchecked path communicated with the third ported passage and the checked path communicated with the second ported passage are communicated with the main valve chamber at substantially the same longitudinal point.

i 6. The limit valve of claim 5 wherein at least the respective passages which communicate with the main valve chamber at substantially the same points are permanently communicated with each other.

7. A fluid operated power assembly comprising, in combination, a fluid motor including a reciprocable power transmitting element, a fluid control valve connected to said fluid motor for selectively controlling the reciprocations of said reciprocable element, valve means interconnected between said fluid control valve and said fluid motor for limiting the extent of movement of said reciprocable element, said valve means comprising means defining a plurality of fluid passages therein and a displaceable element adapted to obstruct fluid flow through said passages and thereby effect stoppage of the reciprocable element at a predetermined point, and means for interconnecting said displaceable element with said reciprocable element for corresponding movement therebetween over at least a portion of the range of movement of the reciprocable element.

8. The combination of claim 7 wherein the valve means is provided with a main valve chamber into which each of the fluid passages is communicated, the displaceable element being movable in said chamber to a displaced position whereby to obstruct fluid flow therethrough in at least one direction.

9. The assembly of claim 8 wherein the reciprocable element is movable beyond the aforesaid predetermined point, such movement being efiective through the interconnecting means to move the displaceable element beyond an intermediate position for obstructing fluid flow to a position for diverting fluid flow in the fluid passages and thereby efiect a reverse movement of the reciprocable element.

10. A limit valve for use with a fluid operated cylinder or the like, said valve comprising a body having an elongated chamber and a plurality of fluid passages communicating with said chamber, at least four of said passages being ported to the outside of said body, an elongated valve spool slidably disposed in said chamber for controlling fluid flow through said passages, and means including fluid blocking means for selectively interconnecting each of said four ported passages with either of 8 a predetermined two other of the four ported passages, said fluid blocking means including fixed lands on the valve spool and directional check valves associated with at least three of the four ported passages.

11. The combination of claim 10 wherein at least two of the ported passages have branches communicating at different points along the longitudinal length of the chamber, said valve spool being thereby effective to connect the ported passages in different paired arrangement for different positions of the valve spool.

l2. The combination of claim 11 wherein an annular recess is provided in the Wall of the main valve chamber, said recess having a maximum diameter which exceeds that of the valve spool, at least two fluid passages having communication with said annular recess, at least one of said two fluid passages being provided with a directional check valve.

13. In a fluid operated power assembly, a fluid motor including a reciprocable element, a limit valve having fluid connections with said motor for automatically stopping said reciprocable element in a predetermined position intermediate the ends of its maximum stroke, said limit valve including a displaceable element for controlling fluid flow between the limit valve and the fluid motor, means biasing said displaceable element toward a position of normal rest, and means defining fluid passages in said limit valve for cooperation with said displaceahle element in controlling fluid flow, said fluid passages beingarranged for cessation of fluid flow between the limit valve and the fluid motor upon movement of the displaceable element from its position of normal rest to a first displaced position and for reversal of fluid flow between the limit valve and the fluid motor upon movement of the displaceable element from its first displaced position to a further displaced position.

14. The combination of claim 13 plus means interengaging the reciprocable element and the displaceable element for movement of the latter upon predetermined movement of the former.

15. A fluid operated power assembly comprising, in combination, a reversible fluid motor including an oscillating power transmitting element, a first and a second fluid connection to said motor, a fluid control valve for selectively controlling the oscillations of the power transmitting element, a fluid pressure connection, a fluid return connection, and a first and a second fluid communicating connection to said control valve, a limit valve for limiting the movement of the power transmitting element, a first, a second, a third, and a fourth fluid c0mmunicating connection to said limit valve, fluid conducting means interconnecting said first, second, third, and fourth communicating connections of the limit valve respectively with the first and second communicating connections of the control valve and the first and second fluid connections of the motor, means including movable means in-the control valve for selectively intercommunicating the first communicating connection therein with either the pressure connection or the return connection and the second communicating connection therein with the other of the aforesaid pressure and return connections, and means including movable means in the limit valve for first intercommunicating the second with the third and the fourth with the first communicating connection therein and thereafter inter-communicating the second with the fourth and the third with the first communicating connection therein.

16. The combination of claim 15 wherein the movable means in the limit valve includes a displaceable member for changing the intercommunication between connections to the limit valve, and means for interconnecting said displaceable member with the power transmitting element for automatic self-control of the latter.

17. A device of the kind described comprising: a valve chamber; fluid pressure entrance and exit ports each communicating with a different portion of said chamber,

and fluid exhaust, entrance and exit ports each communicating with a different portion of said chamber; control means in said chamber movable to a first position for providing two-way communication between said pressure ports and between said exhaust ports; a check valve communicating with a fluid pressure port and said chamber and a check valve communicating with a fluid exhaust port and said chamber; said control means being movable to a second position for blocking the two-way communication; said check valves permitting passage of fluid between said pressure ports in one direction only and between said exhaust ports in one direction only when said control means occupies said second position.

18. The device of claim 17 including a second like device wherein the fluid pressure entrance port of the second device is connected to the first-mentioned fluid pressure exit port and the fluid exhaust entrance port of the second device is connected to the first-mentioned fluid exhaust exit port and wherein the check valves of the second device respectively permit one way fluid flow in directions opposite from the first-mentioned check valves when the control means of the second device occupies its second position.

19. A limit valve comprising a body provided with two valve chambers, said chambers being directly intercommunicated by means of at least two independent passages providing two-way communication between the chambers, each chamber being further directly communicated with points outside the body by at least two independent passages, an individual valve spool movably disposed in each valve chamber for controlling fluid flow therethrough, and a directional check valve associated with each of said intercommunicating passages, each of the intercommunicating passages between the chambers including a branch passage containing its directional check valve and communicating with one of the chambers at a different point than the two-way intercommunieating passage.

References Cited in the file of this patent UNITED STATES PATENTS 695,675 Ebel Mar. 18, 1902 2,067,492 Kingsbury Jan. 12, "1937 2,247,141 Twyman June 24, 1941 2,447,968 Trotter Aug. 24, 1948 2,475,298 Sloane July 5, 1949 2,523,665 Morse Sept. 26, 1950 2,540,467 Williams Feb. 6, 1951 2,675,785 Ford Apr. 20, 1954 2,705,971 Dorkins Apr. 12, 1955 2,710,628 Hodgson June 14, 1955 2,757,641 Meddock Aug. 7, 1956. 2,826,258 Livers Mar. 11, 1958 FOREIGN PATENTS 571,979 Germany Mar. 11, 1933 552,832 Great Britain Apr. 27, 1943

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