US3500721A - Hydraulic control for two hydraulic motors - Google Patents

Description

March 17, 1970 J. D. ALLEN 3,

HYDRAULIC comm on Two HYDRAULIC mo'roRs Filed July 23, 1968 IN 169N701? Jomv 0. ALEA/ United States Patent 3,500,721 HYDRAULIC CONTROL FOR TWO HYDRAULIC MOTORS John D. Allen, South Euclid, Ohio, assignor to Eaton Yale & Towne Inc, Cleveland, Ohio, a corporation of Ohio Filed July 23, 1968, Ser. No. 746,809 int. Cl. FlSh 13/08, J5/26; F02b 73/00 US. Cl. 91-412 17 Claims ABSTRACT OF THE DISCLOSURE Two hydraulic cylinders have individual lock valves, each directly attached to the respective cylinder and each controlling the return flow from the head end of its cylinder, and a single counterbalance valve connected to both lock valves to regulate both return flows in accordance with the pressure at the rod ends of the cylinders. Each lock valve is pressure-balanced to the fluid pressure at the head end of its cylinder. Each lock valve is also pressurebalanced to the fluid pressure at its connection to the other lock valve so as to prevent undesired interaction between the two lock valves in case one opens ahead of the other When the respective pistons are to be lowered. Both lock valves open ahead of the counterbalance valve when the respective pistons are to be lowered.

This invention relates to a system for controlling the operation of two hydraulic motors, such as cylinder-andpiston units, which may be unequally loaded.

Where two hydraulic motors, such as cylinder-andpiston units, are intended to be operated in synchronism, particularly where they both act on the same load, it is often difficult to maintain the desired synchronism between them when they are unequally loaded, especially when lowering the load. Another serious problem with such systems is to prevent the load from dropping abruptly in case of hydraulic line failure.

The present invention is directed to a novel system in which a single valve controls the return flows from both hydraulic motors so a to maintain them in synchronism while a load is being lowered. The present system also has a pair of lock valves, both connected to the return flowcontrolling valve and each directly attached to the respective hydraulic motor and operable to lock it in position in the event of a hydraulic line failure elsewhere in the system. Each lock valve is pressure-balanced to the fluid pressure at the respective hydraulic motor, so that both lock valves open substantially simultaneously when a load is to 'be lowered. Preferably, also, each lock valve is pressure balanced to the fluid pressure at its connection to the other lock valve, so that there is no undesired interaction between the two lock valves if one opens ahead of the other when the load is to be lowered. The lock valves open ahead of the return flow-controlling valve for smooth lowering of the load.

It is a principal object of this invention to provide a novel and improved system for controlling the operation of two hydraulic motors which may be unequally loaded.

Another object of this invention is to provide such a system which insures smooth lowering of the load and which will lock the hydraulic motors against continued movement in case of a hydraulic line failure, particularly when a load is being lowered.

3,500,7 21 Patented Mar. 17, 1970 ICC Further objects and advantages of this invention will be apparent from the following detailed description of a presently-preferred embodiment, which is illustrated schernatically in the single figure of the accompanying drawmg.

Referring to the drawing, the preferred embodiment of the present system comprises a pair of cylinder-and-piston hydraulic motors 10 and 10a, a pair of lock valves 11 and 11a, one for each cylinder-and-piston unit, a manually-operable four-way directional valve 12, a return flow-controlling counterbalance valve 13 connected between both lock valves 11, 11a and the directional valve 12, a pump 14 and a sump 15.

Each lock valve is pressure-balanced to the fluid pres sure at the head end of its cylinder, so that its opening is not affected by this pressure. Preferably, also, each lock valve is pressure-balanced to the fluid pressure at its connection to the other lock valve, so that undesired interaction between the lock valves is prevented in case one opens before the other, as explained hereinafter. Each lock valve preferably is attached directly to the respective cylinder, so that the possibility of failure of a hose between either lock valve and its cylinder is avoided.

The directional valve 12 preferably is an open-center spool valve which may be operated by a handle 16 to pass the pump output either to a line 17 leading through the counterbalance valve 13 and the respective lock valves 11 and 11a to the head end (the lower end) of both cylinders simultaneously or to a line 18 leading directly to the rod end (the upper end) of both cylinders simultaneously. Return flow from the opposite end of each cylinder passes through the directional valve 12 to a low pressure return line 19 leading back to the sump 15.

COUNTERBALANCE VALVE The counterbalance valve 13 comprises a valve housing or body 20 having a first port 21 connected directly to line 17 and a second port 22 connected by a line 23 to both lock valves 11 and 11a. The valve housing has a passage 26 extending inward from port 21 and a passage 27 extending inward from port 22. A cylindrical bore 28 at the inner end of passage 26 extends to a larger counterbore 29 at the inner end of passage 27. The housing presents an annular shoulder 30 at the juncture between 'bore 28 and counterbore 29.

A sleeve 31 is fixedly positioned in counterbore 29 and bore 28. This sleeve presents an annular external shoulder 32 which abuts against the housing shoulder 30. Just forward of this shoulder the sleeve carries an O-ring 31a which provides a fluid-tight seal between the sleeve and the housing bore 28. The sleeve 31 has an axial opening 35 at its front end.

At the inner end of the housing passage 27, Where the latter intersects the counterbore 29, the sleeve 31 is of substantially smaller diameter than the counterbore, so that an annular space 36 is provided around the outside of the sleeve at this location. The sleeve has a plurality of circumferentially spaced radial openings 37 providing fluid communication between this annular space 36 and the interior of the sleeve. Between these openings 37 and the opening 35 at its front end the sleeve presents an internal annular shoulder 38 defining a valve seat 38a at its inner corner. This valve seat is referred to hereinafter as the outer valve seat.

A plug 39 holds the sleeve 31 in the position shown in the drawing. This plug has an externally screw-threaded inner end which is threadedly received in the valve housing behind the inner end of the passage 27. The plug carries an O-ring which sealingly engages the housing to prevent leakage around the plug. Close to its inner end the plug presents an internal annular shoulder 41 which engages the back end of sleeve 31. Forward of this shoulder, the plug snugly surrounds the sleeve for a short distance.

A hollow outer valve member 42 is slidably disposed in the sleeve 31. At its front end, this outer valve member presents a beveled or tapered annular surface 43 which normally sealingly engages the valve seat 38a provided by sleeve 31. Behind this surface 43 the outer valve member has an outer cylindrical surface 44 of smaller diameter than the inside diameter of sleeve 31, so that an annular space 45 is provided around the outer valve member 42 at this location. As shown in FIGURE 1, this space 45 communicates with the radial openings 37 in the sleeve 31. Behind these openings the outer valve member 42 presents a cylindrical portion 46 which a sliding fit with the inside of sleeve 31.

The outer valve member 42 has an internal cylindrical recess 47, which is open at its back end, and a plurality of circumferentially spaced radial openings 48 which provide fluid communication between this recess and the annular space 45 around the outside of the outer valve member. The recess 47 terminates at its front end in a radially disposed, annular, internal surface 49. A cylindrical modulating passage 50 extends from the inner end of surface 49 to the front end of the outer valve member. An annular corner 51, which is formed at the juncture between the inner end of surface 49 and the back end of passage 50, defines an inner valve seat.

An inner valve member, in the form of a poppet 52 which is coaxial with the outer valve member 42 and the sleeve 31, is provided for cooperation with the inner valve seat 51 and the modulating passage 50 on the outer valve member. This inner valve member comprises a cylindrical stem 52a disposed inside the recess 47 in outer valve member 42, an enlarged head 53 connected to the front end of stem 52a, and a tapered front end portion 55 extending from the enlarged head 53 through the modulating passage 50 in the outer valve member 42. At the corner formed at the juncture between its enlarged head 53 and its tapered end portion '55, the inner valve member 52 is sealingly engageable with the inner valve seat 51 on the outer valve member 42.

The tapered front end portion 55 of the inner valve member has an axial length greater than that of the modulating passage 50 in which it is received. Therefore, when the inner valve member 52 is seated against the inner valve seat 51, this tapered front end portion 55 of the inner valve member projects well beyond the front of the outer valve member 42. The surface of the tapered end portion may be frusto-conical so as to provide a clearance in the modulating passage which increases very gradually according to a predetermind characteristic as the inner valve member is withdrawn from its seat 51.

A relatively stiff coil spring 56 is engaged under compression between the plug 39 and the enlarged head 53 on the inner valve member 52. This spring biases the inner valve member 52 into sealing engagement with the inner valve seat 51 on the outer valve member 42. Due to such engagement, the spring 56 also biases the outer valve member 42 into sealing engagement with the outer valve seat 38a formed on sleeve 31.

The inner valve member 52 has a longitudinal passage 57 extending along its entire length and an inclined cross passage 58 extending between passage 57 and the valve body chamber bore 28.

The valve housing 20 is formed with a bore 60 intersecting the bore 28 coaxially and located to the left of passage 26 in the drawing. A piston 61 is slidably disposed in bore 60. A fitting 62 is threadedly connected to the valve body at the outer end of bore 60 and it has an axial passage 63 which defines a pilot pressure chamber that is connected directly to line 18. Piston 61 constitutes a movable, pressure responsive member which is exposed to the fluid pressures in passages 63 and 26. An O-ring 64 carried by this piston sealingly engages bore 60 to prevent leakage between passages 63 and 26 around this piston. Piston 60 has a reduced diameter extension 65 at its inner end which projects into the inner end of sleeve 31 and is positioned in confronting relationship to the front end of the inner valve member 52. The cross sectional area of the piston which is exposed to fluid pressure in passage 63 is several times the corresponding area of the inner valve member.

In the operation of the foregoing arrangement, assume that selector valve 12 is set by the operator to connect the output of pump 14 to line 17 and to connect line 18 to the return line 19 leading back to the sump 14. This is the selector valve setting for actuating the cylinder- andpiston units 10, 10a to raise their respective loads or to raise the load shared by them. The pump pressure at passage 26 will unseat the outer valve member 42 from the outer valve seat 38a, overcoming the opposing force exerted by spring 56 and permitting fiuid flow to the passage 27 leading to \both lock valves 11 and 11a.

Conversely, when the user operates the directional valve to connect the pump 14 to line 18 for lowering the load or loads, the pump pressure acting on piston 60 will force the latter to the right to unseat the inner valve member 52 to permit return flow from the lower end of each cylinder through the counterbalance valve 13. When piston 60 so moves, it engages the front end of the inner valve member 52 and forces the latter to the right, so that the inner valve member is unseated from the inner valve seat 51 and the tapered end portion 55 of the inner valve member 52 is retracted along the modulating passage 50 in the outer valve member 42. The clearance between the tapered end portion 55 of the inner valve member and the modulating passage 50 determines the rate at which return flow takes place from the lower ends of the cylinders. Due to the taper on this end portion 55, the farther it is retracted, the greater will be this return flow rate. This return flow takes place from valve housing passage 27, through the radial openings 37 in sleeve 31, through the radial openings 48 in the outer valve member 42 into the recess 47 in the latter, past the inner valve seat 51, along the. modulating passage 50 around the outside of the tapered end portion 55 of the inner valve member, and through the opening 35 at the front end of sleeve 31 to passage 26 leading back to the sump 15 through the return flow path provided by the directional valve 12. The rate of this return flow is proportional to the pilot pressure in passage 63 which acts on piston 60. LOCK VALVE The following detailed description of the lock valve 11 is applicable also to the lock valve 11a, which is a mirror image of lock valve 11, with its corresponding parts having the same reference numerals plus an a suffix.

The lock valve 11 comprises a valve body or housing 70 having a first port 71 connected directly to the head end of the cylinder, a second port 72 connected to line 23, and a longitudinal bore 73 between the ports 71 and 72. Valve means in the form of a hollow, generally cylindrical piston 74 is slidably mounted in, and in sealing engagement with, the bore 73. Between the ports 71 and 72 the bore 73 presents a cylindrical land surface 73 which is normally sealingly engaged by a corresponding cylindrical land 74' on the piston 74. A coil spring 75 biases the piston 74 upwardly in the drawing.

The piston 74 has a set of radial openings 76 which normally communicate with the second valve body port 72 and a set of radial openings 77 which normally communicate with the first valve body port 71. Between these sets of radial openings 76 and 77 the piston has an internal passage 78 which presents a valve seat 79 for a ball check valve 80. An end plug 81 is threadedly secured to the upper end of the piston 74, and a coil spring 82 is engaged under compression between this plug and the ball 80 to normally maintain the ball seated on the valve seat 79. When the directional valve 12 is operated to connect the pump 14 to line 17, causing the counterbalance valve 13 to open as described, the pump pressure at the lock valve port 72 will unseat the ball 80 to as to connect the pump to the head end of the cylinder 10.

The upper end of the plug 81 has a central recess 83 which. receives the lower end of a pin 84 whose upper end is engaged by a piston 85. The piston is slidably received in the counterbore 86 formed in an end cap 87 which is threadedly attached to the upper end of the valve body 70. The upper end of the counterbore 86 constitutes a pilot pressure chamber which communicates directly with the line 18 connected to the rod end of each piston, so that the top face of the piston is exposed to the fluid pressure in line 18.

An annular stop member 88 is engaged between the lower end of the end cap 87 and the upper end of the plug 81. This stop member 88 has a vent passage 89 connecting the countertbore 86 below the piston 85 and a recess 90 in the valve body which communicates with the atmosphere through a passage 91. Consequently, the bottom face of piston 85 is exposed to atmospheric pressure at all times.

The pin 84 extends slidably through an axial bore in the fixedly positioned stop member 88. A seal arrangement 92 is engaged between the stop member 88 and pin 84, being held in place by a washer 93 which is acted upon by :a spring 94 whose opposite end engages the bottom of the recess 83 in the plug 81.

An important and advantageous feature of the present invention is that the lock valve is pressure-balanced to the fluid pressure at its port 71 which is connected to the head end of the respective cylinder. With the check valve 80 seated, the valve piston 74 and the plug 81 together present downwardly facing surfaces which are exposed to the fluid pressure at port 71 while the piston 74 and thecheck valve 80 present upwardly-facing surfaces which together are equal in area to these downwardly-facing surfaces and which are also exposed to the fluid pressure at port 71. With this arrangement, the fluid pressure at port 71 exerts equal and opposite forces on the assembly of the piston 74, plug 81 and check valve 80 so that the position of this assembly is unaffected by the pressure at port 71. Consequently, when the pump pressure is connected to the rod end of each cylinder, the opening of each lock valve is determined by the pilot pressure acting on its piston 85 and it is not affected by the fluid pressure at the head end of the respective cylinder, which may be higher for one cylinder than for the other because of unequal loadings on them. Consequently, the lock valves 11 and 11a will open substantially simultaneously to pass return flow from the head ends of the respective cylinders.

Preferably, also, each lock valve is pressure-balanced to the fluid pressure at its second port 72, so that there is no undesired interaction between the two lock valves in case one opens before the other when the pump pressure is connected to the rod ends of the cylinders. The body 70 of the lock valve has an annular chamber 95 extending around the upper end of the valve piston 74 and connected to the second port 72 by a passage 96. In the normal position of the valve piston 74, as shown in the drawing, there is a slight clearance between the upper end of the plug 81 and the bottom face of the stop member 88. The upper end of the plug 81 and the upper end of the piston 74 present upwardly-facing surfaces which are exposed to the fluid pressure at port 72 and which are equal in area to the downwardly-facing surfaces of piston 74 and check valve 80 which are also exposed to the fluid pressure at port 72. Consequently, the position of piston 74 is unaffected by the fluid pressure at port 72.

Each lock valve is directly attached to the head end of the respective cylinder by metal fittings or the like, so that the possibility of dropping of the load because of failure of a hydraulic line between the head end of the cylinder and the lock valve is avoided.

The force of the lock valve spring is relatively small, and when the pilot pressure acting on its piston 85 exceeds the force of this spring the valve piston 74 moves quickly to its fully-open position, in which it does not exert any significant flow-restricting or modulating effect (in contrast to the action of the piston 52 in counterbalance valve 13 which does have this flow-modulating effect).

Each lock valve opens at a lower pilot pressure acting on its piston 85 than the pilot pressure required to open the counterbalance valve 13. Consequently, when the directional valve 12 is actuated to lower the load, the lock valves open first and then the counterbalance valves open to complete the path for return flow from the head ends of the cylinders. This opening sequence of the lock valves and the counterbalance valves insures a smooth and gradual lowering of the load.

OPERATION In the operation of this system, when the used actuates the directional valve to connect the output of pump 14 to line 17, the pump pressure at the counterbalance valve passage 26 will unseat the main valve member 42, permitting fluid to flow through the counterbalance valve to the line 23 leading to the second ports 72, 72a of the respective lock valves 11, 11a. The fluid pressure at these ports will unseat the respective check valves and 80a to pass fluid out through the first ports 71, 71a of the lock valves into the head ends of the respective cylinders. The return flows from the rod ends of the cylinders pass through line 18 and the directional valve 12 to the return line 19 leading back to the sump 15.

Conversely, when the directional valve 12 is actuated by the user to lower the load, the pump pressure is ap plied via line 18 to the rod ends of both cylinders. This pump pressure is applied also to the upper ends of the lock valve pistons 85, a and to the piston 61 of the counterbalance valve. As already explained, the fluid pressure acting on the lock valve pistons 85, 85a causes them to open first, after which the fluid pressure acting on the counterbalance valve piston 61 causes it to unseat the inner valve member 52 to permit a modulated return flow through the passage 55 in the counterbalance valve.

Failure of any hydraulic line in the system while the load is being lowered will result in a drop in the fluid pressure acting on the lock valve pistons 85, 85a so that the lock valves will close and prevent any further lowering of the load.

During normal operation when lowering a load, the pistons in both cylinder-and- piston units 10 and 10a will move downward in synchronism with each other because the return flows from the head ends of both cylinders are regulated by the counterbalance valve.

From the foregoing, it will be evident that the present system combines the features of maximum safety in the event of a hydrualic line failure and precise synchronization of the cylinder-and-piston units when lowering a load.

While a presently-preferred embodiment of this invention has been described in'detail with reference to the accompanying drawing, it is to be understood that various modifications, omissions and adaptations which depart from the disclosed embodiment may be adopted without departing from the scope of the present invention. For example, the lock valves 11 and 11a could be provided on respective hydraulic motors other than the cylinderand-piston units shown. Also, if desired, each lock valve could be connected to the rod end of the cylinder, instead of the head end, or each cylinder could be provided with a lock valve for each end, in which case a double counterbalance valve could be used in place of the single counterbalance valve 13 as shown. Also, if desired, the modulation of the return flows from both fluid motors could be accomplished by providing metering notches on the valve spool of the directional valve 12, so that this valve will control the return flows from the fluid motors and the counterbalance valve 13 may be eliminated.

I claim:

1. In a hydraulic system for controlling the operation of two hydraulic motors, the combination of:

a pair of lock valves, each having a first port for connection to the one side of the respective motor, a second port, and normally-closed valve means controlling the fluid flow between said .ports, said valve means being pressure-balanced to the fluid pressure at said first port, each lock valve having a pilot pressure chamber for connection to the opposite side of the respective motor and a movable pressure-responsive member in said chamber and displaceable in response to fluid pressure thereat to open said valve means;

said second ports of the lock valves being connected directly to each other;

said pilot pressure chambers of the lock valves being connected directly to each other;

and a flow-controlling valve connected to said second port of each lock valve and having valve means for regulating the fluid flows through both said lock valves.

2. The claim 1, wherein said valve means in each lock valve is pressure-balanced to the fluid pressure at said second port of the respective lock valve.

3. The combination of claim 2, wherein said valve means in each lock valve has oppositely-facing surfaces of equal areas which are exposed to the fluid pressure at said first port of the respective lock valve, and said valve means in each lock valve has oppositely-facing surfaces of equal areas which are exposed to the fluid pressure at said second port of the respective lock valve.

4. The combination of claim 1, wherein said valve means in each lock valve has a passage therethrough which is open to said first and second ports of said lock valve and a valve seat in said passage, a check valve in said passage, and spring means normally holding said check valve against said valve seat to prevent fluid flow through said passage, said check valve being exposed to the fluid pressure at said second port of the lock valve and operable by fluid pressure thereat to be unseated from said valve seat to pass fluid from said second port through said passage to said first port of said lock valve.

5. The combination of claim 4, wherein said valve means in each lock valve has oppositely-facing surfaces of equal areas which are exposed to the fluid pressure at said first port of the respective lock valve, and said valve means in each lock valve has oppositely-facing surfaces of equal areas which are exposed to the fluid pressure at said second port of the respective lock valve.

6. The combination of claim 1, wherein said flow-controlling valve is a counterbalance valve having first and second ports and normally-closed valve means controlling the fluid flow between said last-mentioned ports, said second port of the counterbalance valve being connected to said second port of each lock valve, said counterbalance valve having a pilot pressure chamber connected to said pilot pressure chamber of each lock valve, and a movable pressure-responsive member in said pilot pressure chamber of the counterbalance valve and displaceable in response to fluid pressure thereat to actuate said valve means in the counterbalance valve to an open position regulating the fluid flow from said second port to said first port of the counterbalance valve in proportion to the fluid pressure at said pilot pressure chamber of the counterbalance valve;

the movable pressure-responsive member in each lock valve being displaceable to open said valve means in the respective lock valve at a lower fluid pressure in the respective pilot pressure chamber than the fluid pressure required at the pilot pressure chamber of the counterbalance valve to displace the movable pressure-responsive member therein to a position opening said valve means in the counterbalance valve.

7. The combination of claim 6, wherein said valve means in each lock valve is pressure-balanced to the fluid pressure at said second port of the respective lock valve.

8. The combination of claim 7, wherein said valve means in each lock valve has oppositely-facing surfaces of equal areas which are exposed to the fluid pressure at said first port of the respective lock valve, and said valve means in each lock valve has oppositely-facing surfaces of equal areas which are exposed to the fluid pressure at said second port of the respective'lock valve.

9. The combination of claim 6, wherein said valve means in each lock valve has a passage therethrough which is open to said first and second ports of said lock valve and a valve seat in said passage, a check in said passage, and spring means normally holding said check valve against said valve seat to prevent fluid flow through said passage, said check valve being exposed to the fluid pressure at said second port of the lock valve and operable by fluid pressure thereat to be unseated from said valve seat to pass fluid from said second port through said passage to said first port of said lock valve.

10. The combination of claim 9, wherein said valve means in each lock valve has oppositely-facing surfaces of equal areas which are exposed to the fluid pressure at said first port of the respective lock valve, and said valve means in each lock valve has oppositely-facing surfaces of equal areas which are exposed to the fluid pressure at said second port of the respective lock valve.

11. In a hydraulic system having a pair of cylinderand-piston units, each including a cylinder having a head end and a rod end, a reservoir of hydraulic liquid, pump means for pumping hydraulic liquid from said reservoir, directional valve means connected to the output of said pump means and having a return leading back to said reservoir, said directional valve means being selectively operable (1) to connect the output of said pump means to the rod end of each cylinder while passing return flow from the head end of each cylinder to the reservoir or (2) to connect the output of said pump means to the head end of each cylinder while passing return flow from the rod end of each cylinder to the reservoir;

a pair of lock valves connected respectively to the cylinders and each having first and second ports and valve means normally blocking fluid flow between said first and second ports, each of said first ports being connected to the head end of the respective cylinder, each lock valve having a pilot pressure chamber connected to the rod end of the respective cylinder and a movable pressure-responsive member in said chamber operable in response to fluid pressure thereat to displace said valve means to a position for providing fluid flow between said first and second ports, and said valve means in each lock valve presenting oppositely-facing surfaces of equal areas which are exposed to the fluid pressure at said first port whereby said valve means is pressure-balanced to the pressure at the head end of the respective cylinder;

and a counterbalance valve having first and second ports and valve means normally blocking fluid flow between said last-mentioned ports, said second port of the counterbalance valve being connected to the second port of each lock valve, said first port of the counterbalance valve being connected to said directional valve means to be connected by the latter either to the output of said pump means or to said return leading back to the reservoir, and said coun terbalance valve having a pilot pressure chamber connected to the rod end of each cylinder and a movable pressure-responsive member in said last-mentioned chamber operable in response to the fluid pressure thereat to actuate said valve means in the counterbalance valve to a position regulating the return flow from the head ends of the cylinders through the respective lock valves and through the counterbalance valve in proportion to the fluid pressure at said pilot pressure chamber of the counterbalance valve.

12. A system according to claim 11, wherein:

said valve means in the counterbalance valve is movable by fluid pressure at said first port of the counterbalance valve to a position permitting fluid flow from said first to said second port of the counterbalance valve;

and each lock valve has a normally-closed check valve operable to open in response to fluid pressure at the second port of the lock valve to pass fluid from the second port to the first port of the lock valve into the head end of the respective cylinder.

13. A system according to claim 12, wherein said valve means in each lock valve has a passage therethrough which is open to said first and second ports of said lock valve and a valve seat in said passage, and said check valve is normally seated on said valve seat to block fluid flow through said passage.

14. A system according to claim 13, wherein each lock valve is directly attached to the respective cylinder.

15. A system according to claim 11, wherein each lock valve is directly attached to the respective cylinder.

16. A system according to claim 11, wherein said valve means in each lock valve presents oppositely-facing surfaces of equal areas which are exposed to the fluid pressure at said second port of the lock valve whereby said valve means is pressure-balanced to the fluid pressure at said second port.

17. A system according to claim 11, wherein:

said valve means in each lock valve has oppositelyfacing surfaces of equal areas which are exposed to the fluid pressure at said second port of the lock valve whereby said valve means is pressure-balanced to the fluid pressure at said second port;

said valve means in each lock valve has a passage therethrough which is open to said first and second ports of the lock valve and a valve seat in said passage, and each lock valve has a check valve in said passage and spring means biasing said check valve against said seat, said check valve being movable away from said valve seat in response to fluid pressure at said second port of said lock valve to pass fluid from said second port to said first port into the head end of the respective cylinder;

each lock valve is directly attached to the respective cylinder;

and said valve means in the counterbalance valve is movable by fluid pressure at said first port of the counterbalance valve to a position permitting fluid flow from said first to said second port of the counterbalance valve.

References Cited UNITED STATES PATENTS EDGAR W. GEOGHEGAN, Primary Examiner US. Cl. X.R.

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