US2460440A - Hydraulic system - Google Patents

Description

Feb. 1, 1949. Q R, ALDEN 2,460,440

HYDRAULIC SYSTEM Filed oct. 11, 1943 y Patented Feb. 1, 191,49

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2,460,440 HYDRAULIC SYSTEM Carroii R. Alden, Detroit, Mich., assignor to Ex- Ceil- Corporation, Detroit, Mich., a corporation of Michigan Applica-tion October 11, 1943, Serial No. 505,799

(El. Gil-52) 12 Ciaims.

The invention pertains to improvements in hy- V draulic systems, and more particularly to a novel control arrangement for the same.

One object of the invention is to provide in a hydraulic system having'high and low'pressure sides, 'a novelarrangement forimaintaining the high pressure side of the system either under a. .predetermined relatively high pressure or, alternatively, dropping the same to `a low value, as may be desired.

'More speciiicallyy it is an object of the invention toprovide in ahydraulic system embodying a pumpas a source Vof pressure fluid and a main control valve vfor controlling the flow of fluid to a main actuator, a novel arrangement for optionally maintaining the pump discharge under substantial pressureA or not, as may be desired, when the main control valve' is shifted to its neutral or oi position.

The optional maintenance of pump discharge L under predetermined'pressure or atsubstantially low pressure when Vthe main actuator is stopped very substantially enhances the eiciency and potential iields of use of the system under different operating conditions. In the'event that the pump is being used not only asV a source of fluid pressure Yfor a main actuator, 'but also as such source for some auxiliary device or other actuator, it is requisite that the pump output pressure be maintained if continued use of the auxiliary devioeis to be possible during those intervals during which the main actuator is stopped. On the other hand, mere maintenance of pump output pressure at all times, irrespective of system demand, is inexpedient and highly inefficient becauseof the frequent waste of power input for the pump which results. Thepresent invention oomprehends the use of va simple selector for conditioning the system either for pressure maintenance or not, just as may be desired, during the oit periods of the main actuator. I

` .A more specific object oi the invention is to provide a hydraulic system in whicha'pressure operated diverting or governor valve is arranged notionly to divert pump discharge in accordance with the pressure drop across a restricted orice in the actuator exhaust line during normal operation-of the actuator, but in which the governor valve may also be conditioned at will to divert iiuid in the course of maintenance of a predetermined discharge pressure for the pump while the actuator is stopped.

vAnother object is toprovide ina system employing a governor valve operable as noted vin the preceding statement of object during normal operation, an arrangement effective at will to condition the governor valve for use in maintaining the pump discharge pressure at a preselected value whenever the main actuator stops, whether such stoppage results from stalling or from shifting of the main control valve to'its oir position.

Still another object is to provide a hydraulic system embodying a novel safety arrangement for disabling the normal controls in the event an actuator stalls and in such manner that the supply of viluid to the system must thereafter be? stopped before the actuator can be restarted.

Further objects and advantages of the invention will become Vapparent as thefollowing description proceeds, taken in connection with the accompanying drawing, in which:

l Figure l is va generally schematicY representation of a hydraulic system embodying the present invention.

Fig. 2 is similar to Fig. 1, but shows a modified form of hydraulic system -also embodying'the present invention. g'

VWhile the invention is susceptible of various e2 modications and alternative constructions, I

have shown in the drawings and will herein describe in detail certain preferred embodiments, but it is to be understood that I do not'thereby intend to limit the invention to the speciiic forms y disclosed, but intend to'cover all modifications and alternative constructions falling within the spirit and scope of the invention as expressed in the appended claims.

Referring more particularly to the embodiment i of the invention illustrated in Fig. l, the system there shown comprises a pump l as a source of pressurefuid, such as oil. This vpump is desirably arranged to deliver fluid at la substantially constant volumetric rate and may be of f either the well-known vane type or gear type.

Fluid from they pump is delivered, under the control of a main valve ll, to a main actuator l2 consisting of a cylinder I3 and lpiston Ill. Such actuator may drive any movable element desired, and in the present instance it'is contemplated that the cylinder i3 be stationary, the pisten l being shown as rigidly connected to a movable machine tool table l5.

`The main valve li may take' various forms. 1n the particular construction illustrated, it serves not only to control the stopping and starting of the actuator l2 but also its direction of move-V ment. The valve shown is of the sliding plunger type `and comprises a plunger it slid-able ,endwise within a suitably ported housing li. Atthe center of the plunger I6 `are two lands or collars I8, I3 separated by an annular channel 26, while at the opposite sides of these two central collars are two further collars 2I and 22 separated from the adjacent central collars by respective annular channels 23, 24. Annular end channels 25, 26 separate the collars 2|, 22, respectively, 'from the end portions of the plunger I6. Pressure iiuid is delivered by the pump lll into its discharge or outlet line or conduit 21 from which it passes through branch conduits 28, 29 and 30 so that :duid from the pump is supplied to the main valve II through ports at the spaced termini of the three conduits 21, 29 and 33.

Opposite ends of the acuator cylinder I3 are connected to the main valve II at spaced points through conduits 3l, 32, while a return or exha-ust line 33 to the reservoir 34 is connected to the bore of the main valve through three spaced passages or branches 35, 36 and 31.

When the main valve plunger I6 is in the neutral or off position shown, midway in its path of travel, the flow of fluid to the actuator I 2 is blocked by collars 2I, 22 and the fluid discharged from the pump lows through conduits 21, 28 and 30, thence through the central channel 20 in the valve, and through the passage 35 into the exhaust line 33, through which it returns to the reservoir 34. The inlet 38 of the pump I is also connected to this reservoir.

Upon shifting the main valve plunger i6 to the right a distance substantially equal to the width of the land 22, pressure fluid discharged from the pump I0 is supplied through conduits 2?, 23, 29, and thence through groove 25 and conduit 3I to the left end of the cylinder I3. At the same time, :duid is exhausted from the opposite end of theY cylinder through conduit 32 and groove 24, into passage 31 and exhaust line 33 so that the piston I4 moves to the right. Conversely, when the valve plunger I6 is shifted to the left from its mid position shown, pressure uid is supplied to the righth-and end of the cylinder I3 (through 21, 2B and 32), while uid is exhausted fromthe lefthand end of the cylinder'I3 (through 3|, 23, 36 and 33) so that the piston i4 moves` in the opposite direction, that is, to the left. The collar I8 or I9, as the case may be, blocks the passage 35 upon shifting of the valve plunger I6 to either right or left. If desired, a back pres-- sure valve 39 of conventional spring loaded type may be interposed in the exhaust line to the reservoir.

The speed of the actuator I 2 in either direction of movement is controlled by an adjustable restricted orifice 40 operating in conjunction with a governor valve 4I. The restricted orifice 40 is interposed in the exhaust or return line 33 from the actuator and meters the fluid out of the actuator during either forward or reverse movement of the latter. The governor valve 4I, on the other hand, comprises a valve plunger l2 having an annular channel 4.4 therein and slidable endwise within -a casing 43. A diversion or bleed passage 45 leads to the governorvalve Iii from the pressure line 28 and connection to the exhaust line 33 is completed through a passage 46. A compression spring 41 yieldably biases the valve element 42 toward a position in whichit blocks connection between the passages 45, 46. Upon application of sucient pressure to the left end of the valve element 42, it is shifted to the right against the bias of the spring 41 for connection of the passages 45, 46 through Vthe channel 44, such connection between the passages 45,

45 becoming less and less restricted by the gov- -rnor valve as the latter is shifted progressively to the right. To subject the valve element 42 to the pressure drop across the restricted orifice de, connection is established from the exhaust line 33 on the upstream side of the orice 40 through conduit 43 having a restriction 49 therein, an annular passage 5l? inV a pilot valve 5I, whose purpose will later appear, and a passage 52.

' With the orilice and governor valve 40, 4I arranged as describedany tendency of the actuator ii'. to slow down will result in a diminution in the new of iiuid through the orifice 4S and consequent drop in pressure across the same. This l cause the governor valve element 42 to shift to the left under the urge of the spring 41, theretending to diminish the amount of the fluid diverted through lines 45, 46, so that the delivery oi fluid from the pump to the actuator is correspondinglyincreased, thus increasing the actuator speed. Conversely, if the actuator i2 tends to over-speed,l the amount of huid exhausted iron; it wili tend to increase so that the pressure drop through the orifice 43 is correspondingly increased and the governor valve element thus shifted io the right to divert a corfiondinglyincreased amount of fluid from the pressure line 23. This results in a slowing down the actuator by reason oiA the diversion of uid which would otherwise pass through it. ln this manner the speed ofthe actuator is precisey and accurately governed with a high degree of sensitivity despite fluctuations of load which may be encountered.

In the system as so far described, when the valve li is in its neutral position shown, fluid from the pump lll is circulated back to the reservoir 34 with substantially no restriction to ilow except that oifered by the orifice 4D and the small amount caused by the back pressure valve 39; In the event that the governor valve 4i is made sensitive through the use of a light spring 41, as is desirable for maximum sensitivity in its governing operation, the pressure drop across the restricted orice i0 will be quite small. In such circumstances, the pressure at the pump outlet 1 will fall to a low value when the main valve I l is shifted to neutral. Although such diminution in pump discharge pressure is desirable from the standpoint of minimizing power consumption for:

the pump while the main actuator I2 is idle, it may in some circumstances be highly undesirable as, for example, when a branch line 53 is led oil' times for operation of the auxiliaries, while at other times when the auxiliaries are not in use efficiency is best served by permitting the discharge pressure of the pump to fall to a low value.

in accordance withv one aspect of the present invention, provision is made for selectively conditioning the system to maintain, at the option oi the operator, either a relatively high predetermined pump discharge pressiu'e or to drop such pressure to a negligible value when the main valvey li is in neutral. VIt is for this purpose that ahand operated selector valve 54 and the pilot valve 5I are included in the system.

In such case, it may be'requisitek '.Tlie Agenera-1' iun'ction. lof V4itl're selector valve' lill-k tortue' upstream sideo the orifice-60, 'and :to: com f neet theigovernor -valve directly ato'fthe `disclria'rge side foi thepump throughlinesf'land 28v when tion. l

Thepilotvalve 5l .coniprisestaf. longer Vlil having therein the annular "channel 250 I noted, such plunger being urged byfa compressionbspringl' inte theposition vshown' :in --which itlieiclianiiel.-v l251i. interconnects the lines Miami 52, The ipurnp discharge pressure is applied directlyr to the righthandf endoi'thezplungerf from the jlin'e '28.

AWhen thefselector valve 1554, which is :a `two- `position*oil-'foi valve, Sis .shifted from its fopen position Vshoivri to `its 'optionally 'available closed positiomit` blocks the -i'ovvy offrit-iid from theoutlet'of thepump into fthe exhaust fline :33. In consequence, lznressure-ibuilds-rup on` .the .discharge side sof'tlie pump l0 untilitfrea'ches 'a' value sufficiently high to-ove'rcomethe-'spring 56 :and thrust thea-pilot valve plunger 55to y.the let. .'Suchfleftward 'shift 4of.tlf-1epilot -valvesplunger :blocks the line 48 vand opens Wcolrin'umication'between 'lines ZE-and 52. The resultant-application fofipressure to 'the .-leitend -of the governor valve-plunger 142. shifts -`the latter tothe-right vso "thatlfluid fis .idivertedfrom the ipump :outlet through passages 55,46. In this way duid .isfdiverted 'from trie Y outlet fofthe .pump through the governor valve Vbutat a .pressure ydterniined -by the springisetting of the..pilot'valvef5l. Byadjustrnentfofthe springfloading for ltheupilot va1ve,discharge pressure of fthe'fpump :l0 -may'thus be--maintainedat any desired value suitable :for actuation of auxiliaries orv the like :connected :to Ilinef53. j

It will, be observed "that-in utilizing the 1arrangement-described above, thel selector-"valve 5t may be thrown to its blocking positioniertfarit7 time, as,for example, while .the factuator "I2 is in motion, without inany :way -interfe`ring with the normal operationioffthe governor `vvalve '41 vin maintaining acoristant feed .rate `for theiactuator. Whenr the i main vvalve il is i either Y injits norinal right'or le-ithan'dposition:for-movement of the actuator 1l2,"the passage V-35 is Y'b1oched-fanyhovv so :that .the opening or closing pf ,the --selector valve 154 `rwill have `no immediate feifect'on "the operation of thesystem'so'flong as the'ac'tuator continues to :move normally. Closing oi? 'the selector "vali/eat such time'does, however, preliminarily condition the :system iso that upon sub sequent stopping of the actuator "l2, #either by stal-ling or `restoration ofthe mainivalve 1 I to neutral, the pilot valve 5| 'wil-liminediately "become-effective to control fthe opening' foffthe, governor Avalve :for maintaining 'fa Ypredetermined high pressure fin k.-tlie-puirip .discharge line. Should Ithe :actuator l2 Vstall whilevthe'selector va1ve-54 is closed, the-.governor'valve-llfl 'will-tend to close since-there is no lfurther pressure 'drop across the orifice 4-0, and pressure Will-build up in :the vpump discharge line. "Such pressurek tends i to shovethe V7actuator aheadt'o 'overcome what= everobstruction caused it '-tostall. The pump dischargepressure is,.however, limited toa safe value by thespilot valve-5I l-for as soonifas the idischargepressure Vreaches the-'value for which"V the Y pilot Valve-is set "the latter opens, thereby in turn opening thegovernor'r valve to divert fluid, so that the pump-pressure ris limited to the value predeterminedby the spring loading of the pilot valve. Such safe pressure, Aand no more, is thus applied to `move the stalled actuator.

From the foregoing, it will be seen that by merel-y throwing the selector valve 58, the system'canbe Conditioned at'will either to cause the pump l0' to continue to deliver ilu-id at high pressurefffor use in auxiliaries or the like when the main valve ll is inneutral, or alternatively conditioned. to cause the pump voutput to be recirculated 'at lov.7 pressure with consequent low power consumption in the event that there is no occasion to use anyoutput pressurev from it when themain valve is in neutral. Moreover, in -either position ofthe selector valve 5, safely limited limited pressure is 'available to break loose the jam should the actuator stall.

VBy way yof analysis, it Will be noted that there are in effect rst and second alternatively available relief passages from the outlet of the pump when-the main valve il is in neutral. One such to .the'governoivalve 4l.

passage returns to the reservoir 36 by way of the exhaust line 33 :and the other by way of the governor valve under the control of the pilot valve. Closing the=selector valve 5d cuts off the riirstmentioned .passage by Way of the exhaust line and compels the fluid to go through the other passage by Way of the governor valve which is controlled Vby 'the pilot valve to maintain the desired highpressure. Y

-In Fig.' 2 is shown a modified hydraulic system also embodying the present invention The systemor Fig. -2`difers in a structural sense from that-lof Fig-ffl primarily in that the pilot valve 5ta, corresponding Yto the pilot valve 5l, is adapted 'to-cut oli positively the iiow oi huid to tli'erestricted lorifice g1ML-in addition to perl-Tormin'g i'tlriefotl-ier 'functions noted for the pilot'valve 5l'heretofore-described, whenever the pump discharge 'pres'su're builds up suiiiciently to actuate the pilot'valve. lIn an operational sense, this 'restilts inY substantially diierent characteristics tliantliose heretofore described when the linain adtuat'oi' -2 is stalled, all as detailed below. Since the remaining :parts of the system are identical With'those employed in the system shown in Fig, i, the 'saine reference numerals have been used for v'theiri'dentication, Vand a repetition of the detailed :description of f the same is unnecessary. 11n-this instance, the pilot valve 5in. has a slidable Yplun-ger V55a vwith two annular channels therein, 50a and-5501), the plunger being urged tothe 'rlgh't'as ybefore by the compression spring this instance, thechannel 59h is interposedin 'the exhaust 1ine3'3 on the upstream side of 'thefres'tfricted orice 40, while the channel lais interposed' between'a branch line 33a from the eXhauStTlirieand the passage 52 leading in consequence, when the Apilotvalve plunger 55a is shifted'to the left under 'the inu'enceof pressure on Ythe discharge side of'tliepuinp lll, it cuts oil vany Vflow of Vexhai-'ist uid from the line 33 into the restricted orifice '46, as -Well as interrupting the flow of fluid from tlieflihe-SSto the governor valve and connecting "thelatter lto the pump discharge pressure.

v When `the actuator I2 is moving normally, the restricted orifice 4U and governor 'valve 4! coact to govern its speed. in precisely the saine inaniier-Iasdescrihed with reference to Fig. l, irrespective ofltlie position :oi-thc kselector valve 54.

Similarly, when theactuator l2 'is'stopped by shift of the mainY valve il to neutral', fluid'vvill be circulated from the outlet of the pump back to the reservoir'tt with only the small restriction of the orifice ii? in the event that the selector valve d is in the open position shown. On the other handit the selector 5t is closed,.when the main valve is shifted to neutral, the pilot valve 51a will subject 'the governor vaivev tl to the pump discharge pressure and maintain such discharge pressure at the value determined by the setting of the spring 56 of the pilot valve, also just as in the case of the system shown in Fig. I.

The difieren-ce in operation between the systems of Figs. l and 2 occurs when the actuator l2 is stalled with the selector valve 5e closed. As has heretofore been described in connection with Fig. l, stalling of the actuator l2 when employing the latter system causes the pilotA valve 5l to render the governor valve d! operative to divert uid from the pump outlet in such manner as to maintain the pump discharge pressure at a safe value determined by the setting of the bias spring 5t oi the pilot valve. In contrast, the arrangement in Fig. lis suchthat when the actuator l2 is stalled with theselector valve 54 closed, the pressure on the actuator piston I4 will be equalized and the actuator cannot be restarted until the pump ill has been stopped and the pilot valve 5in permitted to return to its position shown in the drawings. Since the operator is thus compelled to give special attention to the machine by stopping the pump in the event that the actuator becomes stalled, the safety of the system is obviously enhanced.

Choice of the arrangement of Fig. 1 or Fig. 2 will depend upon Whether the particular machine in which the system is installed makes it more desirable to provide the safety stoppage arrangement of Fig. 2 when the actuator stalls, or the continued application of limited pressure as in Fig. l to free the actuator under like condition.

As to just what takes place in the system of Fig. 2 when the actuator l2 stalls while the selector 54 is closed, it Will be observed that in such case the pressure will build up in the pump discharge lines 21, 28 due to the immobility of the actuator piston le. In consequence, the pilot valve plunger 55a is shifted to the left, thereby blocking the 110W of uid from the exhaust line 33 to the restricted orice 4B, as well as to the governor valve lil, and at the same time subjecting the governor valve 4l to the pump discharge pressure. Under such circumstances, the pressure on opposite sides of the actuator piston I4 is very quickly equalized and the pump discharge pressure is limited toI and maintained at, a value determined by the setting of the pilot valve spring 5E. Once the pilot valve 51a has thus shifted, any subsequent shift of either the main valve ll or Aselector valve 5d will be ineffective to change the pressure conditions within the system, wherefore the actuator l2 remains at rest. To again set the system in operation, it is necessary that the operator stop the pump lil and thereby drop the pump discharge pressure to a value such that the pilot valve 5Ia is restored to its initial position shown in Fig. 2. The operator is thus compelled to give his attention to the machine and is apprised of the fact that the actuator has been stalled so that he may clear the obstruction which caused the stalling.

v I claim as my invention: Y

1. In a hydraulic system, the combination of a substantially constant volume pump, an ac- 81? tuator having an inlet' and an outlet, means'for directinguid from .the pump to said actuator inlet, a restricted orice interposed in the path of uid exhausting `from the actuator outlet. means dening a iluid diversion passage from the discharge side of the pump, a movable governor valve member interposed in said diversion passage'and'spring biased into a position to block the same, means for applying to said governor valve member the pressure acrosssaid orifice in a direction to oppose said spring bias, and means including a pilot valve for interrupting the application to saidgovernor valve member of the pressure across said oriceand applying to .said

Vgovernor valve member Vthe discharge pressure of said pump, saidpilot. valve including a spring loaded valve member subjected to the discharge pressure of said pump.

2. In a hydraulic system, the combination of a substantially constant volume pump, an actuator having an inlet and an outlet, means including a main valve having on'and off positions in which it respectively directs fluid from the pump to said actuator inlet and to exhaust, a restricted orifice interposed in the path of iiuid exhausting from the actuator outlet, means denning a iluid diversion passage from the discharge side of the pump, a movable governor valve member interposed in said diversion passage and spring biased into a position to block the same, means for applying to said governor valve member the pressure across said orice in a direction to oppose'said spring bias, means including a pilot valve operable upon shifting thereof from a rst to a second position to interrupt the application to said governor valve member of the pressure across said orifice and to apply to said governor valve member the-discharge pressure of said pump, said pilot valve being spring urged to said Viirst position, means for applying the discharge pressure of the pump to said pilot valve in a direction urging it toward said second position, and a selector valve for optionally blocking or opening the flow of fluid from the main valve to exhaust when the latter is in said off position therefor.

3. In a hydraulic system, the combination of a substantially constant volume pump, an actuator having an inlet and an outlet, means for directing fluid from the pump to said actuator inlet, a restricted orice interposed in the 'path of uid exhausting from the actuator outletmeans defining a fluid diversion passage from the discharge side of the pump,a movable valve member interposed in said diversion passage and spring biased into a position to block the same, means for applying to said valve the pressure across said orifice in a direction to oppose said spring bias, and means for interrupting the application to said valve member of the pressure across said orice and applying to said valve member the discharge pressure of said pump. 4. In a hydraulic system, the combination of a iluid delivery pump and an actuator, a main valve for cutting off the flow of iiuid from said pump to said actuator when the 'valve isin an off position therefor, means including a pressure operated valve for maintaining the discharge pres,- sure of the pump at a predetermined value, and selector means operable only when said main valve is in its oil. position to render said Vfirst mentioned means operative or inoperative at will. `5'. In a hydraulic system,` the combination of a iiuid delivery pump and an actuator, a main valve for cutting off the new ci duid from said y able at will to prevent to said actuator when the valve is in an oi position therefor, and means operable only when the main valve is in said ofi position to retain the pump discharge pressure either high or low.

6. In a hydraulic system, the combination of a substantially constant volume pump, an actuator having an inlet and an outlet, means for directing iluid from the pump to said actuator inlet, an exhaust line leading from said actuator outlet and having a restricted oriiice interposed therein, means defining a fluid diversion passage leading from the discharge side oi said pump, a movable governor valve member interposed in said diversion passage and spring biased into position to restrict the same, and ,means including a pilot valve operable in alternate first and second positions thereof to apply respectively the pressure across said orifice and the pumpdischarge pressure to said governor valve member in a direction such as to urge the same in each case toward open position against said spring bias, said pilot valve being adapted when'in said second position thereof to block said exhaust line on the upstream side of said orice.

7. In a hydraulic system, substantially constant volume pump, an actuator pump the combination of a having an inlet and an outlet, an exhaust line leading from said actuator outlet and having a restricted orifice interposed therein, means including a main valve operable in alternate posi tions thereof to direct the now of uid from the pump to said actuator inlet or to block such inlet and direct the pump discharge to said exhaust line, means dening a fluid diversion pas- Sage leading from the discharge side of said pump, a movable governor valve member interposed in said diversion passage and spring biased into position to restrict the same, means including a pilot valve operable in alternate first and second positions thereof to apply respectively the pressure across said orifice and the pump discharge pressure to said governor valve member and in each case in a direction to urge said governor Valve member toward open position against said spring bias, said pilot valve being adapted when in said second position thereof to block said exhaust line on the upstream side of said orifice, and means including a selector valve operthe diversion of uid by said main valve into `said exhaust line.

8. In a hydraulic system, the combination of a substantially constant volume pump, an actuator having an inlet and an outlet, means for directing fluid from the pump to said actuator inlet, a restricted orice interposed in the path of fluid exhausting from the actuator outlet, means dening a fluid diversion passage leading from the discharge side of said pump, a movable valve member interposed in said diversion passage and spring biased into a position to restrict the same, means for applying to said valve the pressure across said orifice in a direction to oppose said spring bias, and means for substantially simultaneously interrupting not only the exhaust of fluid from said actuator to said orice, but also the application to said valve member of the pressure across said orifice and for applying to said 10 valve member the discharge pressure of said pump.

9. In a hydraulic system, the combination with an actuator and means including a pump for Supplying pressure fluid thereto, of means operable automatically upon stalling of the actuator for not only preventing the supply of further uid to the actuator from said pump but for also thereafter retaining said actuator at rest until the pump has been stopped and restarted.

10. In a hydraulic system, the combination with an actuator and means including a pump for suppiying pressure fluid thereto, of means in cluding a valve operable in response to discharge pressure of the pump for effecting the diversion of fluid from the pump outlet to disable the actuator against further movement when the pump discharge pressure reaches a predetermined maximum value and for simultaneously blocking the discharge oi fluid from said actuator.

11. In a hydraulic system, the combination with an actuator and a source of pressure'luid therefor, of means operable in response to the application of a predetermined maximum pressure to said actuator for thereafter automatically maintaining a substantially equalized pressure across the actuator to prevent its further movement until the delivery of pressure uid from said source is stopped.

12. In a hydraulic system, the combination of a fluid delivery pump and a reversible actuator, a conduit having branches, an exhaust line having branches, a main valve shiftable to at least three selectively available positions therefor in as two or which said pump is connected to said actuator for movement of the latter in corresponding opposite directions and in the third of which said positions said pump is disconnected from said actuator and connected to a .branch of said exhaust line through a branch of said conduit,

means including a pressure operated valve for maintaining the discharge pressure of said pump at a predetermined value, means for utilizing pressure in said conduit for rendering said pressure operated valve eiective to maintain the pump discharge pressure at said predetermined value when sufcient pressure is made available in said conduit by shift of said main valve to said third position, and a selector Valve located in said branch of said exhaust line and connected when said main valve is in said third position to a branch of said conduit for optionally rendering said pressure operated valve effective or ineiective by retaining pressure in or releasing pressure from said conduit.

` CARROLL R. ALDEN.

REFERENCES CITED The following references are of record in the fue of this patent:

UNITED STATES PATENTS Number

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