US2142500A - Control for hydraulic machines - Google Patents

Description

Jan. 3, 1939i .1 K. DOUGLAS CONTROL FOR HYDRAULIC MACHINES Filed March l2, 1957 2 Sheets-Sheet l` A INVENToR JAMES K Dlzmlsms BY 7 Mmmm J. K. DOUGLAS CONTROL FOR HYDRAULIC MACHINES Jan. 3,- 1939.

2 Sheefs-Sheet 2 Filed March 12, 1937 -INVENTOR. JAMES K DDUBLAS B. 2-

TOKNEY Param-edi Jaaa, 193s CONTROL FCR HYDRAULIC MACHINES James K. Douglas, Shorewood, Wis., assignor toV The Oilgear Company, Milwaukee, Wis., a corporation oi' Wisconsin Application March 12, 1937, Serial No. 130,427

20 Claims.

This invention relates to hydraulic machines capable of maintaining a holding pressure substantially constant, such as the machine shown in my copending application Serial No. 28,068 of which this application is a continuation as to matters common to both applications and which has since matured into Patent No. 2,080,810.

The machine shown in the above application includes a variable displacement pump for supplying motive liquid to a hydraulic motor to operate the same and a control which permits the pump to deliver liquid to the motor at a predetermined maximum rate until the motor meets suiiicient'resistance to cause pump pressure to rise to a predetermined maximum and then the control reduces pump displacement until the pump is delivering just sulcient liquid to maintain that maximum pressure constant. The control may be adjusted to vary the maximum pressure but a different4 maximum pressure cannot be maintained without adjustment of the control.

The present invention has as an object to provide a hydraulic machine which is capable of maintaining any one of a plurality of predetermined pressures constant.

Another object is to provide means whereby any one of a plurality of predetermined maximum pressures may be selected at a point remote from the pump and its control.

Another object is to provide a reversible pump which is capable of maintaining any one of a plurality of predetermined pressures constant.

Other objects and particular advantages of the invention will appear from the following description of the hydraulic machine shown schematically in the accompanying drawings in which the views are as follows:

Fig. l is a circuit diagram of a hydraulic press to which the invention has been applied.

Fig. 2 is a schematic drawing showing the mechanism which controls the operation of the pump shown in Fig. 1.

Fig, 3 is a view showing the selector valve in a dierent characteristic position.

Fig. 4 is a section through the selector valve taken on the line 4`4 of Fig. 2.

Fig. 5 is a view showing an adjustable resistance valve which may be substituted for the selector valve and associated resistance valves shown in Figs. 1 to 4.

The press shown in Fig. 1 includes a ram I, which is tted in a stationary cylinder 2 and connected at its lower end to a movable press head or platen 3, and two pistons 4 and 4*iL which (Cl. Gli-52) are tted, respectively, in stationary cylinders 5 and 5 and connected by rods 6 and 6 to platen 3 to move it toward and from a stationary press head '1.

Cylinder 5 has its upper end connected by a channel 8 to the upper end of cylinder 5,and by a channel 9 to one port of a pump I0 and its lower end connected by a channel I I to the lower en-d of cylinder 5EL and by a channel I2 to the other port of pump I so that, when pump I0 delivers liquid into channel I2, pistons 4 and 4*I will be moved upward simultaneously and move platen 3 and ram I upward and, when pump I0 delivers liquid into channel 9, pistons 4 and 4l, platen 3 and ram l will move downward.

Ram cylinder 2 is connected through a surge valve I3 and a channel I4 to a surge tank l5 which contains a supply of liquid and is connected by an overow channel I6 to a reservoir I1 which is ordinarily formed in the base of pump I0.

Surge valve I3 has not been shown in detail for the reason that such valves are well known and in extensive commercial use. It is deemed sufficient to state that it has one of its ports connected to channel 9 by a channel I8, another of its ports connected to channel I2 by a channel I9 and that it operates as follows:

When pump I0 delivers liquid into channel I2 to raise pistons 4 and 4a,.pressure extends through channel I9 and shifts surge valve I3 to a position in which cylinder 2 is open to channel I4 so that, as platen 3 and ram I are raised by pistons 4 and 4a, ram I may eject liquid from cylinder 2 through valve I3 and channel I4 into surge tank I5. If the liquid ejected from cylinder 2 into surge tank I5 raises the level of the liquid therein above the entrance to drain channel I6, liquid will flow through channel I6 into reservoir I1.

When pump I0 delivers liquid into channel 9 and has liquid returned to it through channel I2, either gravity or liquid supplied to the upper ends of cylinders 5 and 5a at a low pressure by pump I0 will cause ram I, platen 3 and pistons 4 and 4a to move downward until further downward movement thereof is arrested by head I or by work arranged between it and platen 3.

As ram I,moves downward, liquid will ow from tank I5 through channel I4 and valve I3 to cylinder I and keep it lled. Also, if ram I and pistons 4 and 4EL move downward by gravity and tend to cause a deficiency of liquid in channel 9 and the upper ends of cylinders 5 and 5a, such deficiency will beI made up'by liquid which flows from tank I5 into channel 9 through a check valve which permits liquid to flow in that direction but preventsit from flowing in the opposite direction.

When downward movement of platen 3 is arrested by head 1 or by the work, pressure will rise in channels 9 and I8 until it is high enough to cause liquid to flow through channel I8 and shift valve I3 to a position in which cylinder 2 is closed to channel I4 and open to channel I8 so that the upper end of ram I is exposed to pump pressure. Then platen 3 is urged downward by a force equal to the product of the pump pressure and the combined areas of ram I and pistons 4 and 4.

The movement of platen 3 may be reversed by reversing the pump or by means of a valve to reverse the iiow of liquid in channels 9 and I2. For the purpose of illustration. pump I9 has been shown as being reversible by operating a rod 2l which is slidable horizontally in a stationary bearing 22 and urged toward the right by a spring 23.

Rod 2| has a roller 24 arranged upon the right `end thereof and held by spring 23 in contact with the beveled face of a cam 25 fixed upon the upper end of a rod 2l which is slidable vertically through at least one guide 21 and extends downward through an arm 28 carried by platen 3. The lower end cf rod 28 is connected to one end oi' a control lever 29 which is pivoted intermediate its ends to a stationary bracket III.

When the free end of lever 29 is depressed to a position below a central or neutral position as shown in full lines, cam 25 will hold rod 2I in a position to cause pump I0 to deliver liquid into channel 9 and thereby cause platen 3 to descend.

When thc free end of lever 29 is raised to a neutral position as indicated by dotted lines at 3i, cam 25 will permit spring 23 to shift rod 2| to a position to cause the displacement of pump III to be reduced to zero so that the press remains idle.

When the free end cf lever 29 is moved to a position above neutral as indicated by dotted lines at 32, cam 25 will permit spring 23 to shift rod 2i to a position to cause pump I0 to deliver liquid into channel I2 and thereby cause platen 3 to rise.

Arm 28 is provided in order that upward movement of platen 3 may be arrested at a predetermined point. To this end, a collar 33 is fixed upon rod 29 in such a position that, as platen 3 appreaches the point at which its upward movement is to be arrested, arm 28 will engage collar 33 and raise it and rod 75 until cam 25 has shifted rod 2I far enough to reduce pump displacement to zero at which time further upward movement of platen 3 ceases.

I f platen 3 should then tend to descend due to leakage or other causes, a spring 34 arranged around rod 29 between guide 21 and collar 33 would move rod 25 and cam 25 downward until spring 23 had shifted rod 2i far enough to cause pump I0 to deliver just suilicient liquid into channel I2 to hold up the platen.

Pump I8 has been shown in Fig. 2 as being of the rolling piston type fully illustrated and described in Patent No. 2,074,068. It is deemed sufficient to state that the pump has its cylinders and pistons arranged radially in a cylinder barrel 38 which rotates upon a central valve shaft or pintle 48 through which liquid flows to and from the cylinders.

Cylinder barrel 39 is arranged inside a thrust member or slide bloclf. 4I which is closely fitted in the casing 42 of pump I0. Slide block 4I is provided with an inner circular reaction surface 43 which engages the outer ends of heads of the pistons and causes the pistons to reciprocate in the cylinders whenever the cylinder barrel is r0- tated and the axis of the reaction surface is offset from the axis oi' the cylinder barrel. The stroke of the pistons and consequently the volumetric delivery of the pump is proportional to the distance that the axis of the reaction surface is offset from the axis of the cylinder barrel.

If the cylinder barrel 39 is rotated in a counterclockwise direction as viewed in Fig. 2, pump I0 will deliver liquid into channel 9 when the axis of reaction surface 43 is offset toward the left from the axis of pintle 40 as shown, and it will deliver liquid into channel i2 when the axis of reaction surface 43 is offset toward the right from the axis of pintle 40.

Slide block 4I is adapted to be moved toward the right by a hydraulic servo-motor consisting of a stationary cylinder 44 and a piston 45 which is tted in cylinder 44, connected to slide block 4I and operated by liquid supplied to cylinder 44 under the control of mechanism to be presently described.

Slide block 4I may be urged toward the left by springs as shown in Patent No. 2,080,810, particularly if the movement of platen 3 is to be reversed by reversing the ow of liquid in channels 9 and I2 by means of a reversing valve. For the purpose of illustration however. pump I0 has been shown as being reversible and as having slide block 4I urgedtoward the left by a hydraulic servo-motor consisting of a stationary cylinder 4B and a piston 41 which is fitted in cylinder 45 and connected to slide block 4I by a stem 49.

Liquid for operating servo-motor 48-41 is supplied by a gear pump 50 which is ordinarily driven in unison with cylinder barrel 39`and arranged Within pump casing 42 according to the usual practice and as shown in Patent No. 2,074,068. Gear pump 50 draws its supply of liquid from reservoir I1 through a channel 5I and discharges it into a channel 52 having connected thereto a low pressure relief valve 53 through which all liquid discharged by the gear pump in excess of requirements is exhausted and which enables the gear pump to maintain a low pressure in channel 52.

Operation of servo-motor 48--41 is controlled by a follow-up valve which, in practice, is of the rotary type but, in order to illustrate the operation of both the follow-up valve and the servomotor in a single view, the follow-up valve has been shown as being of the reciprocating type.

As shown, cylinder 46 is provided upon its outer end with a head 54 of substantial thickness having a bore 55 extending therethrough. A bore 55 is formed in piston 41 and in stem 48 in axial alinement with bore 55, and a plunger type follow-up valve 51 is fitted in bores 55 and 55 to reciprocate therein.

Gear pump discharge channel 52 communicates with an annular groove 58 which is formed in the wall of bore 55 and to which gear pump 50 continuously supplies liquid at a constant pressure as determined by the resistance of valve 53. Liquid which may escape from groove 58 outward along valve 51 is collected in an annular groove 59 which is formed in the wall of bore 55 and discharges into reservoir l1 as by means of a drain channel 80. l

Valve 51 is provided with an axial passage 83 which is closed at each of its ends and is at all times in communication with groove 58 through a plurality of Vradial ducts 64 formed in valve 51. Passage 63 is connected near its inner end by a plurality of radial ducts 55 to an annular supply groove 56 which is formed in the peripheral sur- 75 face of valve l1. Consequently, groove 66 is continuously supplied with liquid from gear pump l.

An annular drain groove 61 is formed in the peripheral surface of valve 51 a short distance from supply groove 66 and connected to the inner end of bore 58 by a plurality .of ducts 68 which extend from groove 61 through the end of valve 51. A plurality of openings 69 formedin the inner part of stem 48 permit liquid discharged through groove 61 and ducts 66 into the inner end of bore 56 to escape therefrom into pump casing 42 from which this liquid may escape into drain channel 66.

When piston 41 and valve 51 are in their normal relative positions, grooves 66 and 61 are arranged upon opposite sides of an annular groove 16 which is formed in the wall of bore 56 in piston 41 and communicates with the interior of cylinder 46 through a plurality of ducts 1| formed in piston 41.

The width of groove is ordinarily equal to the distance between grooves 66 and 61. 'I'hat is. in the normal positions of valve 51 and piston 41, the edges of groove 16 are ordinarily in alinement, respectively, with the adjacent edges of grooves 66 and 61 so that there is no communication between groove 10 and groove 66 or groove 61 but a slight movement of valve 51 in one direction or the other will open groove 1l) to groove 66 or to groove 61.

Movement of val-ve 51 toward the left opens supply groove 66 to groove 10 and thereby permits liquid to flow therethrough and through ducts 1| to the interior of cylinder 46 and move piston 41 and slide block 4| toward the left against the resistance of piston 45 provided the pressure in cylinder 44 is not too high.

When piston 41 has moved exactly the same distance that the valve was moved, groove 16' passes out of registry with groove 66, thereby stopping any further movement of the piston by cutting off the supply of driving liquid and trapping the liquid then contained in cylinder 46. The liquid trapped in the cylinder 46 then holds piston 41 and slide block 4| stationary until valve 51 is again shifted or until the pressure in cylinder 44 becomes high enough to move piston 45 and slide block 4| toward the right against the resistance offered by piston 41.

In order that slide block 4| may be moved toward the right when valve 51 is stationary, a relief valve 12 is connected vbetween cylinder 46 and drain channel 60 and adjusted to open at a pressure a few pounds higher than the pressure at vwhich gear pump relief valve 53 opens so that, when the pressure in cylinder 44 becomes high enough to cause slide block 4| to be shifted to.

ward the right as will be presently explained. piston 41 may eject liquid from cylinder 46 through relief valve 12.

Movement of valve 51 toward the right opens drain groove 61 to groove 10 so that liquid may escape therethrough and through ducts 68 from the interior of cylinder 46 and permit piston 45 to move slide block 4| and piston 41 toward the right. When piston 41 has moved toward the right the same distance that valve 51 was moved. groove 10 passes out of registry with groove 61, thereby stopping any further movement of the piston by cutting olf the escape of liquid and trapping the liquid then remaining in cylinder 45. The liquid trapped in cylinder 46 then holds piston 41 and slide block 4| stationary until valve 51 is again shifted or until the pressure in cylin der 44 becomes high enough to move piston 45 and slide block 4| toward the right against the resistance offered by piston 41.

Valve 61 is shifted through the medium of rod 2| which is connected thereto so that, when the free end of control lever 29 is depressed, cam 25 will rise and shift rod 2| and valve 51 toward the left and, when the free end of lever 29 is raised, cam 25 will descend and permit spring 28 to shift rod 2| and valve 51 toward the right.

Piston 45 is urged toward the right by liquid supplied to cylinder 44 from gear pump 50 when the pressure developed by pump I0 is below a predetermined value and by liquid supplied to cylinder 44 from pump l0 when the pressure developed by pump l0 is above a predetermined value. The effective pressure area of piston 45 is smaller than the effective pressure area. of piston 41 so that, when cylinder 44 isl being supplied with liquid from gear pump 56 and valve 51 is shifted toward the left to admit liquid from gear pump 56 into cylinder 46, the force exerted by the liquid upon piston 41 will be greater than the force exerted by the liquid upon piston 45 se that slide block 4| will be shifted toward the left and, when valve 51 is shifted toward the right to open cylinder 46 to drain, the force exerted by gear pump liquid upon piston 45 will be ample to shift slide block 4| toward the right. When liquid is supplied to cylinder 44 from pump I0 at a predetermined pressure which is higher than gear pump pressure, the force exerted by this liquid upon piston 45 is ample to shift slideblock 4| toward the right regardless of whether valve 51 is open or closed.

The delivery of liquid from pump I0 to cylinder 44 is under the control of a pressure responsive control valve 15 which is closely fitted in a bore 16 formed in a control unit casing 11. Bore .16 is connected at its lower end to pressure channel 9 by a channel 18 so that the lower end of valve 15 is exposed at all times to the pressure created by pump l0.

Valve 15 controls communication between the lower part of bore 16 and a port 19 which is formed in the wall of bore 16 intermediate the ends thereof, the lower end of valve 15 being provided with one or more notches 80 for the purpose of throttling the flow of liquid into port 19.

Valve 15 is fixed at its upper end to a piston 8| closely fitted in a counterbore 82 which is considerably larger in diameter than bore 16 and is closed at its upper end by a cover plate 83 at tached to the top of casing 11.

Piston 8| controls communication betwe'en counterbore 82 and an annular port 84 which is formed in the wall thereof, a plurality of notches 85 being formed in the upper edge oi' piston 8| to throttle the ow of liquid between counterbore 82 and port 84.

I-f pump I0 were employed to deliver liquid inbut one direction so that the only functionof piston 45 would be to vary pump displacement, port 84 would be connected to drain. However, in order that piston 45 may also function to reverse the direction of pump delivery, port 84 has been shown as being connected by a channel 86 to gear pump supply channel 52 so that gear pump 50 may deliver liquidto cylinder 44 to effect reversal of pump I0 as will be presently explained.

Notches 8|) and 85 are preferably so arranged that the vertical distance between them is equal to the vertical distance between the bottom of port 19 and the upper edge of port 84 so that, when control valve 15 has been raised until the bottom of notch is even with the bottom of port 18, the bottom of notches will be even with the upper edge of port 84. Then, a very slight movement of control valve 15 in either direction will open either port 18 to pressure channel 8 or counterbore 82 to port 84.

Valve 15 is urged upward or toward open position by the pump pressure in the lower end of bore 15, and it is also urged upward by the pressure prevailing ln an annular groove 81 arranged around counterbore 82 at its junction with bore |8.` Groove 81 is connected by a channel 88 to a channel 89 which connects port 18 to cylinder 44 so that the pressure in groove 81 is ordinarily the same as pressure in cylinder 44.

Valve 15 is urged downward or toward closed position by a helical compression spring 9| arranged in counterbore 82 between piston 8| and a plunger 82 which extends through cover plate 88 and is closely tted therein. The upper end of plunger 82 engages a piston 93 which is closely fitted in a cylinder 84 shown as being attached to cover plate 83. Piston 88 is adapted to be urged downward by liquid supplied to cylinder 94 at any one of a plurality of pre-selected pressures as will be presently explained.

The arrangement is such that valve 15 is held closed until pump pressure rises to a value great enough to cause the liquid in bore 18 to exert upon the lower end of valve 15 a force greater than the force exerted upon piston 93 by spring 8| at which time valve 15 will be moved upward and permit liquid to ow from pressure channel 9 through channel 18, bore 16, port 19 and channel 88 to cylinder 44 and move piston 45 and slide block 4| toward the right to reduce the displacement of pump I8 until pump l0 is delivering just sumcient liquid to maintain pump pressure at that value.

The pump pressure at which valve 15 will open is thus proportional to the pressure in cylinder 84' provided the pressure in cylinder 94 is great enough to compress spring 9|. In order to vary the pump pressure at which valve 15 opens, the pressure in cylinder 94 is varied by mechanism to be presently described. If the pressure in cylinder 84 is lower than the pressure required to compress spring 8|, piston 93 will stall against the head of cylinder 84 and then the pump pressure required to open valve 15 will be the pressure required to compress spring 8|.

Due primarily to the mechanism which controls the pressure in cylinder 94, there would be a small amount of lag in the operation of valve 15 if plunger 82 acted directly upon piston 8|. By

interposing spring 9| between plunger 92 and piston 8|, piston 8| is free to move at any instant without waiting for a corresponding 'movement of piston 88, and valve 15 opens substantially instantly in response to pump pressure reaching a predetermined maximum.

When valve 15 opens. liquid flows through channel 88 to cylinder 44 and shifts slide block 4| toward the right as previously explained. In order to prevent this liquid from moving the slide block so fast that the momentum thereof would cause it to move too far and thus cause hunting, a choke 85 is arranged in channel 89 tc limit the rate at which liquid may enter cylinder 94 and thereby limit the rate at which the slide block may move.

The ilow of liquid through channel 89 is also restricted by a throttle valvei88 which offers a small resistance to the ow of liquid past it so that the pressure in channel 88 and groove 81 is higher than the pressure in cylinder 44 when liquid is flowing through channel 88 to cylinder 44.

In order that gear pump 50 may deliver liquid to cylinder 44 to operate piston 45 and in order that liquid may be exhausted from counterbore 82 by the upward movement of piston 8| after port 84 is closed, a channel 91 has one of its ends connected to counterbore 82 and its other end connected through a check valve 98 to a channel 98 which is connected to channel 88.

In order that liquid may be expelled from cylinder 44 by piston 45 when slide block 4| is shifted toward the left to increase pump displacement. a resistance valve |00 and a bleeder choke 0| are connected between channels 81 and 98 in parallel with check valve 88 and with each other. Check valve 88 permits liquid to flow freely from channel 81 into channel 89 but prevents it from flowing from channel 89 into channel 81 except through resistance valve |00 or choke |0| Choke |0| prevents liquid from flowing therethrough except at a very limited rate so that, when slide block 4| is moved toward the left and causes piston 45 to expel liquid from cylinder 44, nearly all of this liquid must pass through resistance valve |00 which is adjusted to open at a low pressure.

Resistance valve |00 resists the flow of liquid therethrough and thereby resists movement of slide block 4| toward the left and also causes the pressure in channel 99 to be higher than the pressure in channel 91 which at this time is open to gear pump supply channel 52. Moving slide block 4| toward the left increases pump delivery which ordinarily results in an increase in pump pressure. Since valve 15 is closed at this time so that there is no flow of liquid through channel 89, the pressure prevailing in channel 99 can extend past throttle valve 96 into groove 81 and act upon the lower end of piston 8|, thereby assisting pump pressure to raise valve 15 which is thus caused to start upward just before pump pressure reaches a predetermined maximum.

As valve 15 moves upward, notches 85 throttle the flow of liquid from counterbore 82 to port 84 and thereby cause the pressure to rise in channel 81 until the difference between the pressures in channels 91 and 88 is less than the pressure required to open valve 80 which will then close so that the liquid expelled from cylinder 44 by piston 45 during further movement of slide block 4| must pass at a slow rate through bleeder choke |8|. The result is that slide block 4| is moved rapidly during the first part of its movement and is decelerated during the last part of its movement so that it cannot overrun, thereby prevent,- ing hunting.

When pump pressure is caused to rise substantially instantly, as by platen 3 stalling, the inertia of slide block 4| will prevent pump displacement from being reduced quickly enough to prevent pump pressure from exceeding the predetermined maximum. In order to relieve the pump under such a condition, the apparatus is provided with a high pressure relief valve |05 and. in order that the pressure at which relief valve |85 opens shall bear a definite relation to the pressure at which pump displacement is reduced, the operation of relief valve |05 is placed under the control of valve 15.

As shown, relief valve |05 is closely fitted in a bore |88 which is formed in casing 11 and connected intermediate its ends to bore 16 by a channel |01. The lower end of valve |05 normally engages an annular valve seat |08 formed in the ameter than valve |05 and fitted in a counterbore ||2 formed in valve casing 11 concentric with bore |06. Counterbore ||2 is closed at its upper end by cover plate 83 and connected at a point below piston to port 19 by a channel ||3 having a choke ||4 arranged therein.

A spring ||5, arranged in counterbore ||2 between piston and cover plate 83, urges piston l|| downward and normally holds valve |05 in contact with valve seat |08 to close communication between channel |01 and relief channel |09. In order that operation of piston may not be impeded by entrapped gas or liquid, drain channel 60 has been shown in Fig. 2 as being connected to counterbore ||2 above piston When valve 15 opens and permits liquid from pump to enter port 19, pressure will extend through channel 3 and choke ||4 to the lower end of counterbore 2 and act upon the lower face of piston When the pressure in the lower end of counterbore |2 becomes high enough to overcome the resistance of spring l| |5, piston will move upward and raise valve away from seat |08, thereby permitting pump I0 to discharge through channels 9, 18 and |01 into relief channel |09. Since relief valve |05 is not subjected to pump pressure butis opened by pressure which acts upon piston only after valve opens, it will open at a pressure which is always the same number of pounds higher than the pressure at which valve 15 opens regardless of the amount of pressure required to open valve 15.

As previously explained, the pressure prevailing in cylinder 94 determines the pump pressure required to open valve 15. Cylinder 94 is connected through a channel |20, an orifice choke |2I, a channel |22, a second orice choke |23 and a channel |24 to gear pump supply channel 52 so that gear pump 50 may supply liquid under pressure to cylinder 94.

In order to vary and to limit the pressure in cylinder 94, liquid may be permitted to escape from channel |22 through either a free channel, or through any one of a plurality of resistance valves as shown in Figs. l and 2, or through a single adjustable resistance valve as shown in Fig. 5.

Channel |22 has beenshown in Figs. 1 and 2 as being connected by a channel |25 to the inlet of a selector valve |26 through which it may be connected to any one of a plurality of resistance valves or to drain. For thel purpose of illustration, the apparatus has been shown as having four resistance valves |21, |28, |29 and |30 arranged in a casing |3| having formed therein a bore |32 in'which selector valve |26 is closely tted to rotate therein.`

of valve |26. It is to be understood that vertical duct |35 is closed below groove |33 and above duct |36.

Resistance valves |21, |28, |29 and |30 are adjusted to open at different pressures and have the inlets thereof open to bore |32 so that the resistance valves are controlled by selector valve |26. The outlets of the resistance valves are connected to each other as by means oi ducts |38, |39 and |40, and the outlet o1 resistance valve |30 is connected by a duct |4| to a drain channel |42 which communicates with bore |32 and discharges -into reservoir |1 as by being connected to drain channel 60. For the purpose of illustration, the selector valve unit has been shown in Fig. 4 as having its casing |3| closed at its upper end by a removable head |43 and valve |26 as being provided with a stem |44 which extends through head 43 and l has fixed thereon a handle |45 by means of which valve |26 may be rotated.

In order that selector valve |26 may be turned promptly toand stopped exactly in each of its six operative positions, handle |45 may have a pointer |46 xed thereto in alinement with the center of port |31. and head |43 may be provided with six markers |41 one of which is arranged directly above drain channel |42, one of which is arranged directly above the inlet of each of resistance valves |21, |28, |29 and |30, and one of which is arranged midway between channel |42 and the inlet of resistance valve |21.

The arrangement is such that, when handle |45 is turned until pointer |46 registers with the marker above resistance valve |29 so that port |31 registers with the inlet of resistance valve |29 as shown in Figs. 2 and 4, liquid from gear pump supply channel 52 may ow through channel |24, choke |23, channels |22 and |25, groove |33, ducts |34, |35 and |36, port |31, resistance valve |29 and ducts |40 and |4| into drain channel |42.

Choke 23 limits the flow of liquid therethrough and'thereby enables gear pump 50 to maintain in channel 52 a constant pressure equal tothe resistance of relief valve 53. Resistance valve |29 resists the ilow of liquid therethrough and thereby causes pressure to be created in port |31. This pressure will extend through ducts |36, |35 and |34, groove-|33, channels |25 and |22, choke |2| and channel to cylinder 94 and act upon piston 93 so that the pressure required to open valve 15 is determined by the pressure required to open resistance valve |29. v

If handle |45 should be turned until port |31 registered with the inlet of any one of the other three resistance valves, valve 15 would open at a diierent pressure for the reason that resistance valves |21, |28, |29 and |30 open at different pressures.

If handle 45 should be turned until port |31 registered with channel |42 as shown in Fig. 1, the liquid passing through choke |23 could escape freely into drain channel |42 so that the pressure in cylinder 94 would be reduced substantially to zero with the result that the pressure required to open valve 15 would be determined by the initial tension of spring 9|.

If handle |45 should be turned until pointer |46 registered with the marker |41 arranged midway between drain channel |42 and resistance valve |21, port |31 would be blocked as shown in Fig. 3, and no liquid could escape through valve |26. Consequently, gear pump pressure would extend through channel 24, choke |23, channel |22, choke |2| and channel |20 to cylinder 94 and act upon piston 95 so that the pressure required to open valve 15 would be determined by the resistance of relief valve 53.

It is thus apparent that any one of a plurality of predetermined maximum pump pressures may be selected by simply turning valve |29 which may be arranged remote from the pump at any convenient point. With the structure shown, any one of six maximum pressures may be selected, namely, a low maximum pressure determined' by the resistance of spring 9|, a `high maximum pressure determined by the resistance of relief valve 53, or any one of four intermediate maximum pressures determined by the resistances oi' valves |21, |29, |29 and |30 each of which may be individually adjusted.

l Instead of varying the pressure in cylinder 94 by means of a plurality of resistance valves as shown in Figs. `l to 4, it may be varied by means of a single adjustable resistance valve such as the valve shown in Fig. 5. This valve has its casing |50 closed at one end by a head |5I, connected intermediate its ends to drain channel 50, and its other end provided with an inlet |52 which is controlled by a valve |53 and connected to channel |25.

Valve |53 is urged against its seat by a spring |54 arranged within casing |50 between two spring retainers |55 and |59 one of which engages valve |53 and the other of which engages an adjusting screw |51 threaded through head |5I.

Screw I 51 is adapted to be rotated by a handle I 59 which is fixed upon the outer end thereof and provided with a pointer |59 to register with suitable graduations arranged upon head |5| but not shown in the drawings. i

The characteristics of spring |54 and the pitch of screw |51 are preferably such that less than a complete revolution of screw |51 is required to vary the tension of spring 54 between zero and a i predetermined maximum which is great enough to hold valve 53 closed against a pressure at least as great as the gear ,pump pressure.

When the tension of spring |54 is substantially zero, little if any pressure is required to open valve |53 and liquid may flow substantially freely from channel |22 through channel |25 and casing |50 into drain channel 90 so that the pressure in cylinder 94 will be substantially zero and the pressure required to open valve 15 will be determined by the initial tension of spring 9|, that is, when piston 93 is stalled against the head of cylinder 94.

When the tension of spring |54 is between zero and maximum, valve |53 resists the flow of liquid from channel into drain channel 50 and thereby enables gear pump 50 to maintain in cylinder 94 a pressure corresponding to the tension of spring |54 as adjusted.

When the tension of spring |54 has been increased to the predetermined maximum, valve |53 prevents liquid from flowing through casing |50 and thereby enables gear pump 50 to maintain in cylinder 94 a pressure equal to the pressure required to open gear pump relief valve 55.

The graduations on head |5| indicate various pressures from zero to maximum and are so arranged that the operator, by turning handle |59 `until pointer |59 registers with the graduation representing the desired pressure, may cause to be maintained in cylinder 94 any pressure between and including zero and the pressure required to open gear pump relief valve 53.

If the apparatus is constructed as shown and the free end of control lever 29 is depressed to thereby shift valve 51 toward the left and open groove 55 to groove 15, liquid from gear pump 55 will enter cylinder 45 and move piston 41 and slide block 4| toward the left until groove 15 is out of registry with groove 55 as shown in Fig. 2, at which time pump l5 is at a predetermined maximum stroke and discharging liquid into channel 5.

Liquid from pump I0 will flow through channels 9 and 5 to the upper ends of cylinders 5 and 5 and move pistons 4 and 4*, platen 5 and ram downward at high speed, and liquid will now from tank I5 through channel I4 and surge valve I5 to cylinder 2 and keep it filled.

When platen 3 stalls, pressure will rise in channel 9, extend through channel I5 and shift surge valve l5 to close cylinder 2 to tank I5 and open it to channel 5 and then platen 5 will be urged downward by pump pressure acting upon the combined areas of ram and pistons 4 and 4.

When pump pressure reaches the maximum determined by the adjustment of selector valve |25, valve 15 will open and permit liquid to enter port 19. If platen 3 stalls suddenly, pump pressure will rise almost instantly but, since pump displacement cannot be reduced instantly due to the inertia of slide block 4| and the resistance offered by choke 95 and throttle 95, pump pressure will momentarily exceed the predetermined maximum value determined by the adjustment of spring 9| and a part of this liquid will flow through choke ||4 and channel I |3 to the lower end of counterbore I|2 and'raise piston' thereby raising relief valve |05 from seat |55 so that pump can discharge through channels 9, 19 and |01 and valve seat |05 into relief channel I 99, it being understood that the 'pressure required to open relief valve |05 is only a few pounds higher than the pressure required to open valve 15.

At the same time, liquid will ilow from port J9 through choke 95 and channel 99 to cylinder 44 and move piston 45 and slide block 4I toward the right to reduce pump displacement. The pressure in cylinder 44 will extend through channel 99,. choke |0| and channel 91 to the upper part of counterbore 92 but no liquid can escape therefrom for the reason that piston 5| moved upward with valve 15 and closed port 54.

When pump displacement has been reduced enough to cause pump pressure to drop to a value slightly higher than the pressure required to open valve 15, relief valve |05 will close and slide block 43 will continue to move toward the left until pump I0 is delivering just sulcient liquid to maintain the predetermined maximum pressure constant, and then valve 15-will oat in an intermediate position with the bottoms of notches 90 and 95 substantially in alinement, respectively, with the adjacent edges of ports'15 and 94, thereby enabling platen 5 to maintain a constant holding pressure upon any work arranged between it and stationary press head 1.

After the holding pressure has been maintained for the desired length of time, the operator may raise the free end of control lever 2, thereby lowering cam 25 and permitting spring 25 to shilflt rod 2| and follow-up valve 51 toward the r g t.

Movement of valve 51 toward the right opens drain groove 91 to groove 10 so that liquid can escape from cylinder 49, thereby permitting the position and yieldingly retained in a position pressure in cylinder 44 to move piston 45 and slide block 4I toward the right to further reduce pump displacement with the result that pump pressure drops and permits valve 15 and piston 8| to descend.

Piston 8| in descending uncovers port 84 so that liquid from gear pump 5I may flow through channels 52 and 86, port 84, counterbore 62, channel 91, check valve 98 and channel 99 to cylinder 44 and continue the movement of slide block 4I toward the right until valve 51 has stopped and groovel 10 has passed out of registry with groove 51 at which time pump I0 is delivering liquid into channel I 2.

The liquid discharged by pump I into channel I2 will flow therethrough and through channel I9 to surge valve I3 and operate it to open cylinder 2 to channel I4. Then the liquid will flow through channels I2 and I I to the lower ends of cylinders and 5B and raise pistons 4 and 4a which will raise platen 3 and lram I, liquid being expelled from cylinder 2 by ram I through surge valve I3 and channel I4 into surge tank I5.

When platen 3 approaches the upper limit of its movement, arm 28 will engage collar 33 and raise rod 26 and cam 25 which Will move rod 2l and valve 51 toward the left, thereby opening groove 66 to groove 10 so that liquid from gear pump 50 may enter cylinder 46. Since the effective area of piston 41 is greater than the eifective pressure area of piston 45 which at this time is also subjected to gear pump pressure, the liquid entering cylinder 46 will move piston 41 and slide block 4I toward the left until the displacement of pump I0 has been reduced to zero at which time upward movement of platen 3 ceases and the free end of lever 29 is in its neutral position.

The platen will then remain idle until the free end of lever 29 is again depressed below its neutral position to raise rod 26 and cause cam 25 to shift valve 51 farther toward the left, thereby opening groove 66 to groove 10 so that gear pump liquid may enter cylinder 46 and shift piston 41 and slide block 4I farther toward the left to cause pump I0 to deliver liquid into channel 9 and start another cycle of operation.

The invention herein set forth is susceptible of various modifications and adaptations without departing from the scope thereof as hereafter claimed.

The invention is hereby claimed as follows:

l. The combination, with a hydraulic actuator, of a variable displacement pump for supplying liquid to said actuator to operate the same, said pump having displacement varying means movable toward and from maximumdisplacement position and yieldingly retained in a position corresponding to a predetermined pump displacement, a fluid servo-motor for moving said displacement varying means toward zero displacement position, means for supplying motive uid to said servo-motor tooperate the same including a valve urge-d by pump pressure in a direction to open said valve and admit motive fluid to said servo-motor, means for exerting a force in the opposite direction upon said valve and tending to hold it closed to thereby enable said pump to supply liquid to` said actuator at a pressure proportional to said force, and hydraulic means for varying said force.

2. 'The combination, with a hydraulic actuator, of a variable displacement pump for supplying liquid to said actuator to operate the same, said pump having displacement varying means movable toward and from maximum displacement corresponding to a predetermined pump displacement, a fluid servo-motor for moving said displacement varying means toward zero displacement position, means for supplying motive fluid to said servo-motor to operate the same including a valve urged by pump pressure in a direction to open said valve andadmit motive fluid to said servo-motor, a spring urging said valve in the opposite direction and tending to hold it closed to therebyenable said pump to supply liquid to said actuator at a pressure proportional to the tension of said spring, and hydraulic means for varying the tension of said spring.

3. The combination, with a hydraulic actuator, of a variable displacement pump for supplying liquid to said actuator to operate the same, said pump having displacement varying means movable toward and from maximum displacement position and yieldingly retained in a position corresponding to a predetermined pump displacement, a uid servo-motor for moving said displacement varying means toward zero displacement position, means for supplying motive iiuid to said servo-motor to operate the same including a Valve urged by pump pressure in a direction to open said valve and admit moti've fluid to said servo-motor, means including a piston and a cylinder for urging said valve in the opposite direction and tending to hold it closed, means for supplying motive fluid to said cylinder, resistance means for permitting uid to escape from the last mentioned means upon the pressure in said cylinder reaching a predetermined maximum to thereby enable said pump to supply liquid to said actuator at a pressure proportional to the pressure in said cylinder, and means for adjusting said resistance means to thereby vary the pressure in said cylinder.

4. The combination, with a hydraulic actuator, of a variable displacement pump for supplying liquid to said actuator to operate the same, said pump having displacement varying means movable toward a'n-d from Vmaximum displacement position and yieldingly retained in a position corresponding to a predetermined pump displacement, a fluid servo-motor for moving said displacement varying means toward zero displacement position, means for supplying motive fluid to said servo-motor to operate the same including a valve urged by pump pressure in a direcsaid servo-motor, means including a piston and a cylinder for urging said valve in the opposite direction and tending to holdit closed, means for supplying motive fluid to said cylinder to cause said piston to urge said valve toward closed position and thereby enable said pump to supply liquidto said actuator at a pressure proportional to the pressure in said cylinder, and a resistance valve communicating with said cylinder to permit the escape of fluid supplied to said cylinder and thereby enable the last mentioned means to maintain in said cylinder a pressure equal to the pressure required to open said resistance valve.

5. The combination, with a hydraulic actuator, of a variable displacement pump for supplying liquid to said actuator to operate the same, said pump having displacement varying means movable towardv and from maximum displacement position and yieldingly retained in a position corresponding to a predetermined pump displacement, a fluid servo-motor for moving said displacement varying means toward zero displacement position, means for supplying motive fluid to said servo-motor to operate the same including a valve urged by pump pressure in a direction to open said valve and admit motive iluid to said servo-motor, means including a piston and a cylinder for urging said valve in the opposite direction and tending to hold it closed. means for supplying motive liquid to said cylinder to cause said piston to urge said valve toward closed position and thereby enable said pump to supply liquid to said actuator at a pressure proportional to the pressure in said cylinder, a plurality of resistance valves, and means for providing communication between said cylinder and the inlet oi' any one of said resistance valves to thereby enable said liquid supplying means to maintain in said cylinder a pressure equal to the pressure required to open the resistance valve in communication with said cylinder.

6. The combination, with a hydraulic actuator, of a variable displacement pump for supplying liquid to said actuator to operate the same, said pump having displacement varying means movable toward and from maximum displacement position and yieldingly retained in a position corresponding to a predetermined pump displacement, a iluid servo-motor for moving said displacement varying means toward zero displacement position, means for supplying motive iluid to said servo-motor to operate the same including a valve urged by pump pressure in a direction to open said valve and admit motive uuid to said servo-motor, a spring urging said valve in the opposite direction and tending to hold it closed to thereby enable said pump to supply liquid to said actuator at a pressure proportional to the tension-,of said`spring,`means including a piston and a cylinder for determining the tension of said spring, means for supplying motive liquid to said cylinder, and resistance means communicating with said cylinder and permitting the escape of said motive liquid upon the pressure in said cylinder reaching a predetermined maximum.

'7. The combination, with a hydraulic actuator, of a pump for supplying liquid to said actuator to operate the same and having a displacement varying member, a large servo-motor connected to said member, a source of low pressure liquid, means for supplying liquid from said source to said servo-motor to cause it to urge said member in a direction to increase pump displacement, a smaller servo-motor connected to said member for moving it in a direction to decrease pump displacement, means for supplying liquid from said pump tosaid small servo-motor to operate the same including a valve urged by pump pressure in a direction to open said valve and admit liquid to said small servo-motor, means for exerting a force in the opposite direction upon said valve and tending to hold it closed to thereby enable said pump to supply liquid to said actuator at a pressure proportional to said force, and hydraulic means for varying said force.

8. The combination, with a hydraulic actuator, of a reversible pump for supplying liquid to said actuator to operate the same, said pump having a displacement varying member shiftable in one direction or the other from a neutral position to cause said pump to deliver liquid in one direction or the other, means for shifting said member in a direction to cause said pump to deliver liquid in a given direction and for yieldlngly retaining said member in a position corresponding to a predetermined pump displacement, a iiuid servo-motor for moving'said displacement varying member in the opposite direction, means for supplying liquid from said pump to said servo-motor to operate the same including a control valve urged by pump pressure in a direction to open said valve and admit liquid to said servo-motor, means for exerting a force in the opposite direction upon said valve and tend- `ing to hold it closed to thereby enable said pump to supply liquid to said actuator at a pressure proportional to said force, an auxiliary pump, and means for supplying liquid from said auxiliary pump to said servo-motor including a valve movable with said control valve to open said servo-motor to said auxiliary pump when said control valve closes and to close said servo-motor to said auxiliary pump when said control valve opens.

9. Ihe combination, with a hydraulic actuator, of a reversible pump tor supplying liquid to said actuator to operate the same, said pump having a displacement varying member shiitable in one direction or the other from a neutral position to cause said pump to deliver liquid in one direction or the other, means for shifting said member in a direction to cause said pump to deliver liquid in a given direction and for yieldingly retaining said member in a position corresponding to a predetermined pump displacement, a iluid servo-motor for moving said displacement varying member in the opposite direction, means for supplying liquid from said pump to said servo-motor to operate the same including a control valve urged by Dump pressure in a direction to open said valve and admit liquid to said servo-motor, means including a piston and a cylinder for urging said valve in the opposite direction and tending to hold it closed, an auxiliary pump, means ior supplying liquid from said auxiliary pump to said cylinder, resistance means for permitting liquid to escape from the last mentioned means upon the pressure in said cylinder reaching a predetermined maximum to thereby enable said reversible pump to supply liquid to said actuator at a pressure proportional to the pressure in said cylinder, means for adjusting said resistance means to thereby vary the pressure in said cylinder, and means for supplying liquid from said auxiliary pump to said servomotor including a valve movable with said control valve to open said servo-motor to said auxiliary pump when said control valve closes and to close said servo-motor to said auxiliary pump when said control valve opens.

10. The combination, with a hydraulic actuator, of a reversible pump for supplying liquid to said actuator to operate the same, said pump having a displacement varying member shiitable in one direction or the other from a neutral position to cause said pump to deliver liquid in one direction or the other, means for shifting said member in a direction to cause said pump to deliver liquid in a given direction and for yieldingly retaining said member in a position corresponding to a predetermined pump displacement, a fluid servo-motor for moving said displacement varying member in the opposite direction, means for supplying liquid from said pump to said servo-motor to operate the same including a control valve urged by pump pressure in a direction to open said valve and admit liquid to said servo-motor, a spring urging said valve in the opposite direction and tending to hold it closed to thereby enable said pump to supply liquid to said actuator at a pressure proportional to the tension of said spring, an auxiliary pump, and means for supplying liquid from said auxiliary pump to said servo-motor including a valve movable with said control valve to open said servo-motor to said auxiliary pump when said control valve closes and to close said servomotor to said auxiliary pump when said control valve opens.

11. The combination, with a hydraulic actuator, of a reversible pump for supplying liquid to said actuator to operate the same, said pump having a displacement varying member shlftable in one direction or the other from a neutral position to cause said pump to deliver liquid in one direction or the other, means for shifting said member in a direction to cause said pump to deliver liquid in a given direction and for yieldingly retaining said member in a position corresponding to a predetermined pump displacement, a iluid servo-motor for moving said displacement varying member in' the opposite direction, means for supplying liquid from said pump to said servo-motor to operate the same including a control valve urged by pump pressure in a direction to open said valve and admit liquid to said servo-motor, a spring urging said valve in the opposite direction and `tending to hold it closed to thereby enable said pump to supply liquid to said actuator at a pressure proportional to the tension of said spring, means including a piston and a cylinder for determining the tension of said spring, an auxiliary pump, means for supplying liquid from said auxiliary pump to said cylinder, resistance means communicating with said cylinder and permitting the escape oi' liquid upon the pressure in said cylinder reaching a predetermined maximum, and means for supplying liquid from said auxiliary pump to said servo-motor including a valve movable with said control valve to open said servo-motor to said auxiliary pump when said control valve closes and to close said servo-motor to said auxiliary pump when said control valve opens. y

12. The combination, with a hydraulic actuator, of a variable displacement pump for supplying liquid to said actuator to operate the same, said pump having displacement varying means movable toward and from maximum displacement position and yieldingly retained in a position corresponding to a predetermined pump displacement, a hydraulic servo-motor for moving said displacement varying means toward zero displacement position, means for supplying liquid from said pump to said servo-motor to operate the same including a valve urged by pump prespressure in a direction to open said valve and admit liquid to said servo-motor, means for exerting a force in the opposite direction upon said valve and tending to hold it closed to thereby enable said pump to supply liquid to said actuator at a pressure proportional to said force, and hydraulic means for varying said force.

13. The combination, with a hydraulic actuator, of a variable displacement pump for supplying liquid to said actuator to operate the same, said pump having displacement varying means movable toward and from maximum displacement position and yieldingly retained in a position corresponding to a predetermined pump displacement, a. hydraulic servo-motor for moving said displacement varying means toward zero displacement position, means for supplying liquid from said pump to said servo-motor to operate the same including a valve urged by pump pressure in a direction to open said valve and admit liquid to said servo-motor. a spring urging said valve in the opposite direction and tending to portional to the tension of said spring, and

hydraulic means for varying the tension of said spring.

14. The combination, with a hydraulic actuator, oi' a variable displacement pump for supplying liquid to said actuator to operate the same, said pump having displacement varying means movable toward and from maximum displacement position. and yieldingly retained in a position corresponding to a predetermined pump displacement, a hydraulic servo-motor for moving said displacement varying means toward zero displacement position, means for supplying liquid from said pump to said servo-motor to operate the same including a valve urged by pump pressure in a direction to open said valve and admit liquid to said servo-motor, means including a piston and a cylinder for urging said valve in the opposite direction and tending to hold it closed, means for supplying motive liquid to said cylinder, resistance means for permitting liquid to escape from the last mentioned means upon the pressure in said cylinder reaching a. predetermined maxirnum to thereby enable said pump to supply liquid to said actuator at a pressure proportional to the pressure in said cylinder, and means for adjusting said resistance means to thereby vary the pressure in said cylinder.

15. 'Ihe combination, with a hydraulic actuator, of a variable displacement pump for supplying liquid to said actuator to operate the same, said pump having displacement varying means movable toward and from maximum displacement position and yieldlngly retained in a position corresponding to a predetermined pump displacement, a. hydraulic servo-motor for moving said displacement varying means toward zero displacement position, means for supplying liquid from said pump to said servo-motor to operate the same including a valve urged by pump pressure in a direction to open said valve and admit liquid to said servo-motor, means including a piston and a cylinder for urging said valve in the opposite direction and tending to hold it closed, means for supplying motive liquid to said cylinder to cause said piston to urge said valve toward closed position and thereby enable said pump to supply liquid to said actuator at a pressure proportional to thepressure in said cylinder, a plurality of resistance valves, and means for providing communication between said cylinder and the inlet of any one of said resistance valves. to thereby enable said liquid supplying means to maintain in said cylinder a pressure equal to the pressure required to open the resistance valve in communication with said cylinder.

16. The combination, with a hydraulic actuator, of a variable displacement pump for supplying liquid to said actuator to operate the same, said pump having displacement varying means movable toward and from maximum displacement position and yieldingly retained in a position corresponding to a predetermined pump displacement, a hydraulic servo-motor for moving said displacement varying means toward zero displacement position, means for supplying liquid piston and a cylinder for urging said valve in the opposite direction and tending to hold it closed, means for supplying motive liquid to said cylinder to cause said piston to urge said valve toward closed position and thereby enable said pump to supply liquid to said actuator at a" pressure proportional to the pressure in said cylinder, and aresistance valve communicating with said cylinder to permit the escape of liquid supplied to said cylinder and thereby enable the last mentioned means to maintain in said cylinder a pressure equal to the pressure required to open said resistance valve.

17. The combination, with a hydraulic actu- .at01, 0f a Variable displacement pump fOr Supplying liquid to said actuator to operate the same, said pump having displacement varying means movable toward and from maximum displacement position and yieldingly retained in a position corresponding to a predetermined pump displacement, a hydraulic servo-motor for moving said displacement varying means toward zero displacement position, means for supplying liquid from said pump to said servo-motor to operate the same including a valve urged by pump pressure in a direction to open said -valve and admit liquid to said servo-motor, a spring urgingsaid valve in the opposite direction and tending to hold it closed to thereby enable said pump to supply liquid to said actuator at a pressure proportional to the tension of said spring, means including a piston and a cylinder for determining the tension of said spring, means for supplying motive liquid to said cylinder, and resistance means communicating with said cylinder and permitting the escape of said motive liquid upon the pressure in said cylinder reaching a predetermined maximum. l

18. The combination, with a hydraulic actuator, of a power pump for supplying liquid to said actuator to operate the same and having a displacement varying member shiftable in one direction or the other to vary pump displacement and to cause said pump to deliver liquid in one direction or the other, a large servo-motor and a small servo-motor for moving said member in opposite directions, an auxiliary pump, means including a first valve for either connecting said large servo-motor to said auxiliary pump to enable it to move said member in a given direction or connecting it to drain to permit said small servo-motor to move said member in the `opposite direction, means for supplying liquid from said power pump to said small servo-motor including a second valve urged by pump pressure in a direction to open said second valve and admit liquid from said power pump to said small servo-motor, means for exerting a force in the opposite direction upon said second valve and tending to hold it closed to thereby enable said power pump tosupply liquid to said actuator at a pressure proportional to said force, and means including a third valve i'or directing liquid from said auxiliary pump to said small servo-motor. said third valve being movable with said second valve and opening and closing oppositely to said second valve.

19. An apparatus as defined in claim 18 where- 'in Vsaid first valve is a follow-up valve and operates to direct liquid to said large servo-motor to cause it to move said displacement varying member in one direction, or to permit liquid to escape fromlsaid large servo-motor to thereby permit said small servo-motor to move said member in the opposite direction when both servomotors are supplied with liquid at the same pressure, or to trap liquid in said large servo-motor and thereby cause said large servo-motor to hold said member stationary until the' pressure in said small servo-motor reaches a predetermined maximum.

20. An apparatus as deilned in claim 18 including a spring for exerting a force upon said second valve to hold it closed, a piston and a cylinder for controlling the tension of said spring, means for directing liquid from said auxiliary pump to said cylinder at a limited rate, and resistance means for permitting liquid to escape from said last mentioned means upon the pressure in said cylinder reaching a predetermined maximum.

JAMES K. DOUGLAS.

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