US3547007A - Hydraulic control circuit - Google Patents

Description

United States Patent Reuter 2,628,731 2/1953 60/52HE 2,988,891 6/l 961 Hemings 60/52HE 3,003,472 l0/l96l Ferris et a]... 91/4] lX 3,170,379 2/1965 Dempster.... 60/97H 3,407,947 10/1968 Valla 91/41 1X Primary Examiner- Edgar W. Geoghegan Attorneys-Thomas O. Kloehn, Arthur H. Seidel and Ray G.

Olander l ONT 0L IRCUIT [54] HYDBAUL C C c ABSTRACT: The first embodiment powers two hydraulic ac- 5 Claims, 3 Drawing Figs.

' tuators In a loader bucket tilt mechanism. A directional valve UaS- connects two mains to a pump and to a reservcir The mains 60/52 60/97 feed the rod ends of the actuators directly. A selector valve ll". Cl. onnects the end of the actuators either to the resern yoir or to the mains The other two embodiments power two 91/411 pairs of actuators. The third embodiment uses four selector 56 R r C ed valves connected to the directional valve. Two valves al- I 1 2 ounces I 1 temately connect the blind ends to the mains, or together. The UNITE STATES PATENTS other two valves alternately connect the blind ends of the ac- 2,157,240 5/ l 939 Keel 60/52X tuators to the mains, or to the reservoir.

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"iii-j: INVENTOR v HERMAN 6. PETERS ATTORNEY SHEET 2 OF 3 INVENTOR HERMAN 5. PETERS ATTORNEY PATENT ED use 1 5 mo SHEET 3 OF 3 l m- MIM- INVENTOR HERMAN G. PETERS ATTORNEY 1 HYDRAULIC CONTROL cmcur'r' BACKGROUND on THE INVENTION The copending application, Ser. No. 770,129 of Rupert, filed on Oct. 23,1968, now U.S. Pat. No.,3,506,l45, for Wrist Action Mechanism for a material handling implement on a Power Shovel is owned by the same assignee as the present application and illustrates an environment in which the present invention may be initially used. That copending application discloses a quarry shovel in which a front-end loading bucket is pivotally mounted on the end of a bucket handle and the wrist action for the bucket onthe end of the bucket handle is provided through levers and links by two pairs of linear hydraulic actuators, one pair being mounted on each side of the bucket handle. Each pairof hydraulic linear actuators drives a compound lever that'isfpivotally mounted on a fulcrum between the linear hydraulic actuators and a load end of the compound lever is connected to third-class lever which is pivotally mounted on the bucket handle between the bucket and the compound lever. The power unit of the present invention gives that system of levers and links multispeed operation.

In addition to achieving the objective of the multispeed drive, however, the present invention solves the problem of varying fluid flows within a closed system that inheres in the structure of the above mentioned application. Because of the mounting of both linear hydraulic actuators on the same side of the compound lever, the linear displacements of the pistons in the cylinders differ from one another and vary through the strokes of the two linear hydraulic actuators as the compound lever moves from one extreme position to-the opposite extreme position. This variation in displacemen'tis in addition to the inherent variation and displacement between the blind side and rod side of the piston. In prior'art closed hydraulic circuits, such a variation and displacement would cause destructive fluid pressures in some portions of the circuit and SUMMARY oF THE INVENTION The present invention relates to a hydraulic control circuit for operating at least two double-acting hydraulic linear motors; and more specifically, the invention resides in the combination of a pair of double-acting linear hydraulic motors, a reservoir of hydraulic fluid, a hydraulic pump with a suction port connected to said reservoir, a directional control valve that is adapted to alternately connect the pressure port of the pump to either of two main conduits and connect the reservoir to the other of two main conduits, the main conduits being connected to the rod ends of the two hydraulic linear actuators, and a second valve means that alternately connect the blind ends of the two linear hydraulic motors, either in common to the reservoir or separately to the two main conduits.

By the foregoing combination, it ispossible to drive the two double-acting linear hydraulic actuators in' both directions. Also, the foregoing combination makes it possible to drive just one of the hydraulic actuators or to drive both of them simultaneously, or to drive the hydraulic actuators either from the blind end or from the rod end. Thus a plurality of speeds may be obtained with corresponding .variations in mechanical advantage. In addition to those advantages, the present invention automatically compensates for any variation in displacement of fluid as the two double-acting linear hydraulic motors operate, so as to expel excess fluid to thereservoir without developing excessive fluid pressure and at'the same time, to ensure that adequate fluid pressure is maintained at all times in all parts of the circuit. i

BRIEF DESCRIPTION OF THE DRAWING effecting the wrist action of a bucket on a front end loader;

FIG. 2 is an adaptation of the embodiment shown in FIG. I to operate two pairs of linear hydraulic motors simultaneously; and

FIG. 3 is a circuit diagram of a third embodiment of the invention connected to operate the two pairs of linear hydraulic motors simultaneously.

DESCRIPTION OF THE PREFERRED EMBODIMENT The first embodiment of the invention shown in FIG. 1 operates a pair of double-acting linear hydraulic motors 1 and 2 that provide a multispeed drive to apply wrist action to a bucket 3 that is pivotally mounted at a joint 4 on the end of a bucket handle 5 of a quarry shovel (not shown) which is disclosed in the copending application of Rupert, Ser. No. 770,129, filed on Oct. 23, 1968, for Wrist Action Mechanism for Material Handling Implement on a Power Shovel and assigned to the same assignee as the present invention and application. A more complete showing of other aspects of the quarryshovel is made in the copending application of Learmont, Ser. No. 663,277, filed Aug. 26, 1967, now U.S. Pat. No. 3,452,890 granted July l, 1969 for Power Shovel" and also assigned to the same assignee as that of the present application.

The double-acting linear hydraulic motors l and 2 have cylinders 6 and 7, respectively, which define bores 8 and 9, respectively, and in which pistons 10 and 11, respectively, are reciprocably mounted. From the pistons 10 and 11, respective piston rods 12 and 13 project outwardly through seals in the ends of the cylinders 6 and 7, respectively, and hence the cylinder bores 8 and 9 and the cylinders 6 and 7 can be said to have blind ends 14 and 15, respectively, and rod ends 16 and 17, respectively. The blind ends 14 and 15 of the respective cylinders 6 and 7 are pivotally mounted to the bucket handle 5 adjacent one another, and the double-acting linear hydraulic motors l and 2 diverge from the blind ends to pivotal fastenings l8 and 19 of the piston rods 12 and 13, respectively, at opposite sides of a fulcrum 20 on a compound lever 21. The compound lever 21 has a load end 22 to which one end of a lever link 23 is pivotally connected, and a force end opposite the load end 22. Hence the upper hydraulic motor 1 may be referred to as the force end hydraulic motor 1 and the lower hydraulic motor 2 can be called the load'end hydraulic motor 2. The other end of the lever link 23 is pivotally fastened to a third-class lever 24 between its fulcrum 25 on the bucket handle 5 and its free swinging, load end 26. A bucket link 27 has one end pivotally fastened to the load end 26 of the third-class lever 24 and an opposite end pivotally fastened to a rib 28 on the bucket 3 to join the third-class lever 24 to the bucket 3.

The control circuit for driving and controlling the doubleacting linear hydraulic motors 1 and 2 has a reservoir 29 and a constant displacement, nonreversible pump 30. The pump 30 has a suction port 31 that is connected to the reservoir 29, and it has a pressure port 32 that is connected through two branches 33 and 34 of a pressure line 35 to two pressure inlet ports 36 and 37 of a directional control valve 38. Two exhaust ports 39 and 40 of the directional control valve 38 are connected in common to the reservoir 29. Two line ports 41 and 42 of the directional control valve 38 are respectively connected to two main conduits 43 and 44. The one main 43, which may be called the dumping main 43 because when it is connected to the pressure port 32 of the pump 30 the bucket 3 is pivoted toward its dumping attitude, has one branch 45 connected directly to the rod end 16 of the bore 8 in the cylinder 6 of the upper double-acting linear hydraulic motor 1, and a second branch connected to a port 48 of a selector valve means 47. The other main conduit 44, which may be analogously called the loading main 44, has a first branch 49 that is connected directly to the rod end 17 of the bore 9 of the cylinder 7 of the lower double-acting linear hydraulic motor 2,

and a second branch 50 that is connected to a port 51 of the selector valve means 47. Two other ports 52 and 53 are connected in common to the reservoir 29 and the remaining two ports 54 and 55 are respectively connected to the blind ends 14 and of the bores 8 and 9 of the cylinders 6 and 7 of the linear hydraulic motors 1 and 2.

The second embodiment of the invention illustrated in FIG. 2 is a modification of the first embodiment such that the second embodiment will simultaneously drive two pair of double-acting linear hydraulic motors 56, 57, 58 and 59. One pair of hydraulic motors 56 and 57 is connected at opposite sides of a fulcrum 60 of a compound lever 61 to pivot it about the fulcrum 60 in the directions indicated by the arrows at its load end 62. Similarly, the other pair of hydraulic motors 58 and 59 is connected at opposite sides of a fulcrum 63 of the second compound lever 64 to pivot it in the directions indicated by the arrows at its load end 65. As is indicated in the mentioned copending application, the levers and linkages 21, 23, 24 and 27 illustrated on the bucket handle 24 of FIG. 1 are duplicated on the opposite sides of the bucket handle 24, and hence it is necessary that the hydraulic power unit be capable of driving and controlling two pairs of hydraulic motors, such as the hydraulic motors 5659 in FIG. 2 simultaneously. Much of the circuitry and components illustrated in FIG. 2 are duplicates of those shown in FIG. 1, and insofar as that is the case the same reference numerals are used in FIG. 2 and the description of these elements will not be repeated. As in the first embodiment each of the motors 56-59 in the second embodiment has a cylinder 66, 67, 68 and 69, respectively, defining bores 71, 72, 73 and 74, respectively. A piston 75, 76, 77 and 78, respectively, is mounted in each of thebores 7l74, and each piston 75-78 has a piston rod 79, 80, 81 and 82, respectively, that are fastened, as mentioned above, to the compound lever 61 and 64, respectively. Each of the cylinders 6070 and their respective cylinder bores 7174 has a blind end 83, 84. 85 and 86 and a rod end 87, 88, 89 and 90, respectively, on opposite sides of the respective pistons 75-78.

Since the two inner hydraulic motors 57 and 58 are connected respectively to the load ends 62 and 65 of the respective levers 61 and 64, they may be called the load end hydraulic motors 57 and 58. Accordingly, the outer hydraulic motors 56 and 59 may be differentiated from the inner or load end hydraulic motors 57 and 58 by calling them the force end hydraulic motors 56 and 59.

The dumping main 43, in addition to having the branch 46 connected to the port of the selector valve 47, has a pair of branches 91 and 92 that are connected directly to the rod ends 88 and 89, respectively, of the cylinders 67 and 68 of the load end motors 57 and 58. Similarly, the other loading main 44, in addition to having the branch 50 connected to the port 51 of the selector valve 47, has a pair of branches 93 and 94 which are respectively connected to the rod ends 87 and 90 of the cylinders 66 and 69 of the force end motors 56 and 59. The blind ends 83 and 86 of the cylinders 66 and 69 of the force end motors 56 and 59, respectively, (which are mounted on opposite sides of the fulcrums 60 and 63 from the load ends 62 and 65 of the two levers 61 and 64) are connected in common to the other blind end port 55 of the selector valve 47.

The third embodiment, which is illustrated in FIG. 3, employs the same reservoir 29, pump 30, directional control valve 38, main conduits 43 and 44 and interconnections as were described in connection with the first and second embodiments. The same reference numerals will be used with those elements in the third embodiment and the description of those elements will not be repeated. The same is true of the double-acting linear motors 5659 and the component parts of them. However, the selector valve means for the third embodiment consists of four, two- position valves 95, 96, 97 and 98.

The dumping main 43 is connected by branches 99 and 100, respectively, to the rod ends 87 and 90 of the cylinder bores 71 and 74 of the force end hydraulic motors-56 and 59. In addition, the dumping main 43 has two more branches 101 and 102 which are connected, respectively, to ports 103 and 104 of the selector valves 95 and 98. The remaining two branches 105 and 106 of the dumping main 43 are connected to respective ports 107 and 108 of the other two selector valves 96 and 9 Similarly, the other main conduit 44 has six branches 109, 110, 111, 112, 113, and 114. Two branches 110 and 113 of the loading main 44 are connected directly to the rod ends 88 and 89 of the cylinder bores 72 and 73 of the load end hydraulic motors 57 and 58. Another pair of branches 109 and 114 of the loading main 44 are connected to ports 115 and 116, respectively, of the selector valves 95 and 98, and the remaining two branches 1 11 and 1 12 are connected to ports 117 and 118 of the other two selector valves 96 and 97. The selector valves 95 and 96 which control the pair of motors 56 and 57 on the one compound lever 61 have ports 120, and 121, 122 and 123 connected respectively to the blind ends 83 and 84 of the cylinder bores 71 and 72 of the hydraulic motors 56 and 57. Similarly, the pair of selector valves 97 and 98 that are associated with the hydraulic motors 58 and 59 that drive the other compound lever 64, have ports 123 and 124, 125 and 126, that are respectively connected to the blind ends 85 and 86 of the cylinder bores 73 and 74 of the hydraulic motors 58 and 59. In addition, the selector valve 96 associated with the first two hydraulic motors 56 and 57 has two ports 127 and 128 that are connected in common to the reservoir 29, and the selector valve 97 has two corresponding ports 129 and 130 that are connected in common to the reservoir 29.

The selector valve 95 controlling the left lever 61 is a twoposition valve, which in one position connects the blind ends 83 and 84 of the cylinder bores 71 and 72 together, and in its other position connects the blind end 83 of the force end motor 56 to the loading main 44 and the blind end 84 of the load end motor 57 to the dumping main 43. The corresponding selector valve 98 controlling the other lever 64 is also a two position valve that either connects the blind ends 85 and 86 of the motors 58 and 59 together, or connects the blind end of the motor 59 on the force end of the lever 64 to the loading main 44 and the blind end of the load end motor 58 to the dumping main 43.

The directional control valve 38 is adapted to connect the pressure port 32 of the pump 30 to the loading main 44 and to connect the dumping main 43 to the reservoir 29, or, in the alternative, it may connect the pressure port 32 of the pump 30 to the dumping main 43 while connecting the loading main 44 to the reservoir 29. In its third position, the directional control valve 38 connects the pressure port 32 to the pump 30 back to the reservoir 29 while terminating the two main conduits 43 and 44. The third position is the holding position. The other two operating positions of the directional control valve 38 alternately connect the pressure port 32 of a pump 30 to the rod end of one of each pair of motors while connecting the rod end of the other motor to the reservoir 29. By determining which of the linear motors 5659 will have its rod end 87- -89 connected to the pressure port 32 of the pump 30, the directional control valve 38 governs the direction in which the load driven by the linear motors will be moved.

In the first embodiment the selector valve 47 is a two-position valve that in one position (not shown) connects the blind ends 14 and 15 of the cylinders 6 and 7 of the linear hydraulic motors 1 and 2 to the reservoir 29. In this position fluid pressure applied only to the load end of hydraulic linear motor 2 will drive the compound lever 21, and the compound lever 21 will drive the actuating element of the force end hydraulic linear motor 1, extending the piston rod 12 to draw the piston 10 up toward the rod end of the cylinder 6 and exhausting the hydraulic fluid in the rod end 16 back through line 45 and the dumping main 43 and the directional control valve 38 to the reservoir 29. Since both blind ends 14 and 15 of the linear motors 1 and 2 would be connected to the reservoir 29 neither can contributed any operating force to the movement of the lever 21.

If the selector valve 47 is now actuated to its other position, which is shown in FIG. 1 of the drawings, the blind end 14 of the cylinder 6 of the force end motor 1 will be connected through the loading main 44 to the pressure port 32 of the pump 30, and the blind end of the cylinder 7 of the load end hydraulic motor 2 will be connected through the selector 47, the branch 46 of the dumping main 43, the dumping main 43 and the directional control valve 38 to the reservoir 29. As a result, the compound lever 21 will be driven in the same direction as previously, (because the position of the directional control valve 38 is unchanged) but now the compound lever 21 will be driven by both hydraulic motors 1 and 2 instead of by just the one. Since the pump 30 is a constant displacement pump, the fluid flow remains the same, the speed of the drive will be reduced by about a half and the force available will be multiplied correspondingly. Fluid from the pressure port 32 of the pump 30 will flow through the directional control valve 38, the loading main 44, and from there through one of its branches 49 to the rod end of the load .end linear motor 2 and through the other branch 50 and the selector valve 47 to the blind end 14 of the cylinder 6 of the force end hydraulic linear motor 1. The blind end 15 of the cylinder bore 9 on the load end motor 2 is connected through the selector valve 47 to the branch 46 of the dumping main 43, and from there through the directional control valve 38 to the reservoir 29. The rod end 16 of the upper" linear motor 1 is connected directly through the branch '45 of the dumping main 43, the dumping main 43 and the directional control valve 38 to the reservoir 29. I g

If the directional control valve 38 is now turned toa second operating position, fluid from the pressure port 32 of the pump 30 will be pumped into the dumping rnain 43, and the loading main 44 will serve as a return line. in these circumstances, fluid will be driven directly from the dumping main 43 through its one branch 45 to the rod end 16 of the force end linear motor 1 while fluid will also be driven from the other branch 46 of the dumping main 43 through the selector valve 47 and into the blind end 15 0f the lower .linear hydraulic motor 2. Thus the direction of the rotation of the compound lever 21 about its fulcrum 20 is the reverse 'of that described above, and with the setting of the selector valve 47 as shown, the compound lever 21 is being driven by} both hydraulic linear motors 1 and 2 to achieve a greater mechanical advantage and lower speed of operation. if the selector valve 47 is now switched to its other position, the blind ends 14 and 15 of both linear hydraulic motors 1 and 2 will be connected to the reservoir 29 as will the rod end 17 of the cylinder 7 of the load end linear hydraulic motor 2. With'the directional control valve 38 and the selector valve 47in this position, the upper linear hydraulic motor 1 will act alone to drive the upper compound lever 21 about its fulcrum 20 and to drive the piston 11in the lower linear hydraulic motor 2 through its stroke in the bore 9 of the cylinder 7. n

The operation of the second embodiment illustrated in FIG. 2 differs oniy slightly from the operation 'of'the first embodiment described above. lnstead of being connected to the rod end of a single cylinder, the loading main 44 has a pair of branches 93 and 94 that connect simultaneously to the blind ends 87 and 90 of the cylinder bores 71 and 74 of both force end linear hydraulic motors 56 and 59, and similarly the other dumping main 43 has a pair of branches 91 and 92 that are connected to the rod ends 88 and 89 of the cylinder bores 72 and 73 in both load end linear hydraulic motors 57 and 58. The port 55 of the selector valve 47 is connected to the blind ends 83 and 86 of both force end motors 56 and 59, while the other port 54 of the selector valve 47 is connected to the blind ends 84 and85 of both load end motors 57 and 58.

When both the directional control valve 38 and the'selector valve 47 are in the positions shown in FIG; 2, all four linear hydraulic motors 56-59 will be driven simultaneously by the pump 30. The rod ends 87 and 90 ofthe respective cylinder bores 71 and 74 of the force end linear hydraulic motors 56 and 59 will be connected directly to the pressure port 32 of the pump 30 through the branches 93 and 9,4 of the loading main 44 and the directional control valve 38, while the blind ends 84 and 85 of the load end linear hydraulic motors 57 and 58 will be connectedto the pressure port 32-of the pump 30 through the other branch 50 of the loading main 44 and through the selector valve 47. Since the dumping main 43 is connected to the reservoir 29 through the directional control valve 38 the rod ends 88 and 89 of the load end motors 57 and '58 are connected to the reservoir 29 through the branches 91 and 92 of the dumping main 43 and directional control valve 38, and the blind ends 83 and 86 of the force end motors 56 and 59 are connected through the selector valve 47 and theother branch 46 of the dumping main 43 to the directional control valve 38 and thus to the reservoir 29.

If the selector valve 47 is actuated to its other position, it connects the blind ends 83-86 of the cylinder bores 71-74 of all four linear hydraulic motors 56-59 directly to the reservoir 29 through the ports 52 and 53 leaving only the rod ends 87 and 90 of the force end linear hydraulic motors 56 and 59 connected directly to the pressure port 32 of the pump 30 through the branches 93 and 94 of the loading main 44 and the directional control valve 38. The rod ends 88 and 89 of the load end linear hydraulic motors 57 and 58 exhaust to the reservoir 29 through the branches 91 and 92 of the dumping main 43'and through the directional control valve 38. If the directional control valve 38 is actuated to its other operating position, the pressure port 32 of the pump 30 will be connected to the dumping main 43, and the loading main 44 will be connected to the reservoir 29 so that the direction of rotation of the levers 61 and 64 about the fulcrums 60 and 63 will be reversed. The operator can control the speed and mechanical advantage of the drive of the levers 61 and 64 by actuating the selector valve 47 to connect each to be driven either by the rod end of one cylinder, or by the rod end of one and the blind end of the other simultaneously.

The third embodiment illustrates an alternative way to control the four linear hydraulic motors 56-59 to drive the two levers 61 and 64 simultaneously. In placeof the single selector valve 47 that appeared in the first two embodiments, the third embodiment employs four selector valves 95,96, 97 and 98, two 96 and 97 of which act in unison to alternately connect the blind ends 83-86 of the cylinder bores 71-74 of the hydraulic linear motors 56-59 to the reservoir 29 or to the respective branches 113 and 114 of the loading main 44 and branches 105 and 106 of the dumping main 43. The selector valves 96 and 97 are two-position valves, and in one position they connect the blind ends 84 and of the respective cylinder bores 72 and 73 of the load end motors 57 and 58 through the branches 105 and 106 to the dumping main 43, and they connect the blind ends 83 and 86 of the cylinder bores 71 and 74 of the force end motors 56 and 59 through the respective branches 111 and 112 to the loading main 44. Similarly to the previous embodiments, the loading main 44 is also directly connected to the rod ends '88 and 89 of the cylinder bores 72 and 73 of the load end motors 57 and 58 through respective branch lines 110 and 113. Similarly the rod ends 87 and of the cylinder bores 71 and 74 in the two force end motors 56 and 59 are connected directly to the dumping main 43 through the respective branches 99 and 100. The other two selector valves and 98 of the selector valve means of the third embodiment alternately join the blind ends 83, and 84, 85 and 86, respectively, of the linear motors 56 and 57,58 and 59 on respective levers 61 and 64, or connect the blind ends 84 and 85 of the respective load end motors 57 and 58 to the dumping main 43 and the blind ends 83 and 86 of the force end motors 56 and 59 to the loading main 44.

In the third embodiment, the directional control valve 38 operates in the same fashion as was described in connection with the first and second embodiments. Similarly the selector valve means95-98 serve to connect the blind ends 83-86 of the cylinder bores 71-74 in the linear motors 56-59 either through one of the mains 43 and 44 to the reservoir 29 so that the compound levers 61 and 64 may be actuated by pairs of linear motors 56 and 57, 58 and 59in parallel, or each can be operated asa first-class lever or a third-class lever by the rod ends 87, 89 or 88, 90 of the linear motors 56, 58 or 57, 59, each acting alone on its own respective lever 61 and 64.

In each of the embodiments here disclosed, a two-speed hydraulic control circuit for driving a pair of linear hydraulic motors is shown. In each, this is accomplished by connecting the linear motors to the same load and then driving the load either with both linear motors connected in parallel or with a single linear motor. In addition, each embodiment provides a means for accommodating unequal volumes of fluid flow due to unequal distances of travel in the two linear motors being driven. All of this is accomplished with a closed system by directing the return fluid back through the reservoir and utilizing the valve structure disclose. Only three of many possible embodiments of the invention are illustrated here and hence none of these is alone definitive of the scope of the invention, which is instead particularly pointed and distinctly claimed in the claims that follow.

Iclaim:

1. A hydraulic control circuit capable of simultaneously driving at least two double-acting linear hydraulic motors, the combination comprising:

a reservoir of hydraulic fluid;

a pump having a suction port connected to said reservoir to draw hydraulic fluid from said reservoir and having a pressure port;

a pair of hydraulic motors, said hydraulic motors being linear double-acting hydraulic motors and having a cylinder bore with a blind end and a rod end;

a first main conduit connected to said blind end of a first of said pair of hydraulic motors;

a second main conduit connected to said blind end of a second of said pair of hydraulic motors;

a directional control valve being connected to said pressure port of said pump and to said reservoir and to said first and second main conduits, and being adapted to alternately connect each of said first and second conduits to said reservoir and to said pressure port of said pump; and

a selector valve means adapted to alternately connect said blind ends of said hydraulic motor cylinder bores to said reservoir and to one of or another of said first and second main conduits.

2. A hydraulic control circuit as set forth in claim I wherein said pair of hydraulic motors have piston rods connected to a compound lever on opposite sides of a fulcrum for said lever.

3. A hydraulic control circuit as set forth in claim 2 wherein said pair of hydraulic motors are connected to said compound lever so that movement of said lever extends a movable element of one of said pair of motors and retracts a movable element of the other of said pair of motors.

4. A hydraulic control circuit for controlling fluid flow in a closed system between a reservoir, a pump and at least two double-acting linear motors connected to a load and having cylinder bores with blind ends and rod ends an and flow connections between the reservoir pump and motors to altemately drive some or all of said motors, the combination comprising:

a pair of main conduits;

a directional control valve having ports connected to said pump and to said reservoir and to each of said pair of main conduit conduits to alternately connect said pump and each of said main conduits to said reservoir;

and a selector valve means having ports connected to each of said pair of main conduits, to said blind ends of said cylinder bores of said motors, and to said reservoir, and being adapted to alternately connect said main conduits to said blind ends of said cylinder bore of one of said motors to one of said main conduits and to alternately connect said blind ends of said cylinder bores to said reservoir; and

said pair of main conduits being connected to said rod ends of said cylinder bores of said motors.

5. A hydraulic control circuit as set forth in claim 4 wherein said selector valve means includes a valve utilizing two operating positions, in one of said 0 crating positions a said selector va ve passes fluid from one 0 said mam conduits to said blind end of one of said pair of motors and from said blind end of the other of said pair of motors to the other of said main conduits, and in the other operating position it passes fluid from said blind ends of said fluid motors to said reservoir.

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 54:7 007 Dated December 15, 1970 Invented!) Herman G. Peters It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

r column 4, line 70, change "contributed" should be --contribute-- column 5, line 2, after "selector" insert --valve-- column 6, line 6, after "and" insert the-- column 7, line 40, "of" first instance should be deleted column 8, line 13, after "ends" delete "an" column 8, line 20, after "main" delete "conduit" column 8, line 26, "ends" should be -endcolumn 8, line 34, after "positions" delete "a" Signed and sealed this 23rd day of March 1971.

(SEAL) Attest:

EDWARD M.FLETGHER,JR. WILLIAM E. SGHUYLER, J'R Attesting Officer Commissioner of Patents

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