US2572705A - Fluid control valve with selective float position - Google Patents

Description

VALVE WITH SELECTIVE FLOAT POSITION FLUID CONTROL Filed May 4, 1950 Em ia M v 1. M 4 825 Z4; 72, Q 30 6866 4B 34 {6 3z\\\\\& 3\8 g; Z2 70 88 E90 \\z\\\\ g 10 14 122 $1 1&0

113 INVENTOR.

John H.Edman Patented Oct. 23, 1951 FLUID CONTROL VALVE WITH SELECTIVE FLOAT POSITION John H. Edman, Dubuque, Iowa, assignor to Deere Manufacturing Co., Dubuque, Iowa, a corporation of Iowa Application May 4, 1950, Serial No. 160,077

7 Claims.

This invention relates to a fluid-pressure or hydraulic system and more particularly to an improved control valve assembly therefor.

The invention contemplates and has for its principal object the provision of an improved control valve assembly having a valve member selectively operative in either a first status or a second status to secure different operational characteristics of the valve assembly and the means-such as a fluid motor-connected thereto. The valve is preferably symmetrically formed and arranged for angular positioning in a valve bore in the valve casing so that in either status it has capacity to control the transmission of fluid to and the return of fluid from the fluid motor. To this end, the casing has a pair of motor ports, each of which has a check valve therein normally biased to closed position against the return flow of fluid. The valve is constructed to positively open one check valve or the other, depending upon the direction of movement of the valve and further depending upon the desired operational phase of the fluid motor. In at least one status, the valve has means for positively opening both check valves so that, in conjunction with a by-pass passage, the fluid inlet is connected to the reservoir passage, as are both motor ports, whereby the system, with the check valvespositively held open, is enabled to freely circulate fluid at no appreciable pressure, the result being that the motor parts are free to float.

The characteristics outlined generally above are particularly important in the adaptation of the control valve assembly to the hydraulic system of an agricultural tractor and attached implement. In the case of certain implements, it is desired to provide for only two implement positions, such as a ground-working position and a transport or idle position in which the implement is withdrawn from the ground. In the cases of other implements, it may be desirable to provide, in addition to these two positions, for free floatin movement of the implement, that is, a position in which the implement is not hydraulically locked in either position.

Another important object of the invention is to provide in a selective control valve of the type referred to position-retaining means operative when the control valve is in one status but inoperative when the control valve is in its other status. This means enables use of the control valve assembly under ordinary circumstances without bringing in the effectiveness of the position-retaining means, since in some cases it is desired that the control valve be automatically returned to neutral. In other cases, when the valve occupies another status, the position-retaining means is effective to hold the control valve in that position thereof in which both check valves are positively held open and both motor ports are connected for free circulation to the exhaust passage or reservoir.

It is an important object of the invention to simplify the construction of the control valve assembly in general and to provide an assembly of the character specified that is of adequate design, that lends itself to low-cost production, and that is easily serviceable.

The foregoing and other important objects and desirable features inherent in and encompassed by the invention will become apparent as the disclosure of a preferred embodiment of the invention progresses in the following detailed description and accompanying sheet of drawings.

In the drawings:

Figure 1 is a combined sectional and schematic view, illustrating the control valve assembly in longitudinal section and showing schematically the connection thereto of a two-way fluid motor;

Figure 2 is a similar view, but showing the control valve in its second status and in a different position;

Figure 3 is a fragmentary sectional view taken substantially on the line 3-3 of Figure 2; and

Figure 4 is a fragmentary sectional view taken on the line 4-4 of Figure 1.

The control valve assembly includes acasing I0 provided with inlet means in the form of an inlet port l2 and exhaust means in the form of an exhaust passage [4. The inlet port may be suitably connected to any conventional type of fluid pump, and the exhaust or reservoir pas:-

sage may be appropriately connected to a reser-v voir. Neither the pump nor the reservoir has been shown, since these parts may be conventional and form no part of the present invention.

The casing is provided with an elongated valve bore Hi to which the inlet port I2 leads via a pair of axially spaced branches l8 and 20, and to which the reservoir or exhaust passage 14 leads via a pair of exhaust or reservoir ports 22 and 24.

The casing is further provided with a first motor port 26 and a second motor port 28. These motor ports are axially spaced along the valve bore IE to either side of the inlet branches l8 and 20 and are further respectively adjacent to but axially spaced from the first and. second 3 exhaust portions 22 and 24. These motor ports lead to or communicate with the valve bore I6 respectively via substantially annular passages 36 and 32 and further communicate respectively with outlet chambers 34 and 36.

The motor port 26 is controlled by a check valve 38- in the chamber 34, and the motor port 28 is controlled by a similar check valve 40 in the chamber 36. The chamber 34 opens exteriorly of the casing and is internally threaded to receive a fitting 42 which communicates by a fluid-transmitting line 44 to one end of a two-way cylinder 46 comprising one of the parts of a motor repre-' sentative of the type of means that can be controlled by the control valve assembly. chamber 36 for the motor port 28 also opens exteriorly and is internally threaded toreceive a fitting 48 that is connected by a fiuidline 50 to the other end of the cylinder 46. The cylinder. carries for reciprocation a piston 52 having a pistonrod- 54 that" may be appropriately. con-. nectedeto, any type..ofmechanism that it is de-- siredto operate;

The-.chech-valves 38 and 40- are respectivelyspring-loaded. by compressionspring-s 56 and 58 reacting respectively against the fittings-42 and 48 -and normally biasing the check valves to close the motor ports 26 and 28. to return flow of fluid fromthe. cylinder 46; The check valves are respectivelycontrolled by plungers 60 and 62 which respectively,sl-idably-fit the motor ports 26and zaiandzwhich entereradially. into the valve bore I6. to-. be controlled by a control valve member designated generally bythenumeral 64.

' This. .control-valvev member is shownas an elongated, cylindrical member having portions thereof symmetrically .cylindrically and conically reduced pr formed to eifect. connection or disconnection. between certain. passages. and ports a -manner 'to bepresently described. The valve bore l6 extends completely through the casing 10- and ;at onev end thereof the control valve member. 64 has anexteriorly axially projecting. .portion.66. provided. with an opening 68 through 'whichconnection may be made to a suitable actuating means (not shown). The valve 1101 6215 opens-anthe other end of the casing to an outwardly opening internally threaded counterbore -'lll.-which receives a bored closure cap 12 into-which-theright-hand end of thev valve member: Bkprojects. This end ofthe valve member 64 is provided with aninteg-ral portion 14 of reduced diameter surrounded by a centering spring 16 confined between a pair of washers 18 and-.80.- The washer 16 seats against a shoulder- 82-.on .an integralcylindrical spool portion 84 of the valve member 64. Thewasher seats against a. snap'ring 86 carried in a grooveat the extreme right-hand end of the reduced valve member pontion 14.

The closure cap 12 is internally-bored at. 88 to loosely encircle .the. centering spring 1.6.and.

washers l8 and 86 and is further provided with. a reduced counterbore 90-.at the junction of which and the bore 88ris formed a shoulder 92' againstwhich the washer 86 may seat. The right.- hand end of thevalve bore is formed as shown in Figure 4, being relieved at 94 to provide a plu-' rality of intermediate shoulder portions 96 against.

munication of-fluid between the exhaust port 24' and; the interior-of the cap 72. The centering spring lethus operates as means to. maintain.

The:

4 the neutral position of the control valve member 64 as illustrated in Figure 1.

The control valve member 64 has a central or intermediate cylindrical spool portion 98 which, when the control valve member is in its neutral position as shown in Figure 1, blocks off or disconnects thein'let passage 18 from the :valve bore l6. Adjacent and to the right of the spool portion 98, the control valve member has a reduced cylindrical portion I60 which, when the control Valve member is in its neutral position, leads the inlet port I2 to the valve bore l6 via the branch passage 20, whence fluid circulates relatively freely through. a short exhaust passage I02 to 1 .the main-exhaust passage 2 4.

At its left-hand end, the control valve member. 64-has.the cylindrical portion 66 previously described. This portion is carried inwardly to be. received by the left-hand end of the valve bore I6 to provide a cylindrical portion I04 that controls. communication.- between theevalve bore 1 6 and .the first exhaust port-24. The portionriM is similar tothe portion 84rat the; innenend-rofthe control valve member. When the control valve member is in its neutral position, the.por.- tion-Biblocksofi or disconnects the first exhaust. port22.

Thecontrol valve member B l -includes to the rightr-ofthe-reduced portion I00 a conical.por-. tioneifiawh-ich cooperates-with the check valveplunger Gite-open the. checkvalve. 38..whenthe. control valve 64-is shifted. tothe right, or in the, direction. of-the arrow-A in Figure. 1. At the left. of. the.cylindrical. spool portion 98, thecontrol.

valve member includesa. second conical portion l08..v/hich has no. effect on the .check valve plunger-'62 when thecontrol valve memberis. shifted-to-the right but whichoperates to-con-- nect. theinlet port l2 .via the passage [8 tothe. motor. portv 28, .hcnce. efiecting opening ofthe. checkvalve 40 by pump. pressure.

Theconical portion. 1.08 functions; .in. coniunc tion.with the check valve plunger fil-topositively open- ;the .check .valve 40.,when. the control waive. memberv 64. is shifted to the left in the directionlof.

the-cylindrical spoolportion 84. Therefore, when the controlvalve member 64 is shifted to. the.

left or intthe direction of the arrowB, both check valves 38v and .40-are opened.

Another featureof. the invention resides in. the.

provision of .means..ior. selectively connecting. or. lay-passing .one of the motor ports tothe exhaust.

passage......For thispurpose, the casing is provided with a by-passepassage. H2 opening through a valve seat or port H4- tothe exhaust passage |4..

Meansis carried by the casing [9 for controlling. the'port H4. This means comprises a manually controlled. member having a valve head H6 and.

a. threaded control portion ii-connected to the. head.l.l6-by a reduced shank I20. The threaded portiomllfl .is received =by a tapped; bore .122.

In the. preferred embodiment illus.-. trated, this meansincludesa conical portionJlO. reversed with.v respect to, the conical portion I06. and disposed intermediate thisportion E06. and.

formed integral with the casing I0 and further receives a sealing cap I24. The portion H8 has formed. therein a slot I26 which is rendered accessible to a screw driver or equivalent tool upon motor 46-52 is desired, the by-pass passage H2 will be closed, otherwise fluid pressure from the inlet port I2 via the passage 20 would flow di-' rectly to the exhaust passage I4 rather than to the motor port 26 when the control valve mem-;

ber 64 is shifted to the left min the direction of the arrow B.

When the control valve is operated as set in Figure 1, the centering spring I6 operates to, return the control valve member 64 to neutral.

from either of its active positions. However, the assembly is provided with position-retaining means which are effective only when the assem bly is used as set up in Figure 2. Part of thismeans is carried by the casing and comprises a first element in the form of a spring-loaded de tent ball I28 at times cooperative with a second element in the form of a detent-receiving notch or recess I30 provided in the left-hand end por-. tion 66 of the control valve member 64. Since the control valve member 64 is optionally or sellectively angularly positionable in the valve bore I6, the position-retaining means may be ren dered operative or inoperative as desired. Since the position-retaining means is enclosed within the casing, it is desired to have means exteriorly of the casing to indicate to the user the angular position or status of the control valve member. For this purpose, there is provided on the left; hand end portion 6'6 of the control Valve member an indicating member in the form of a short pin I32. This member or indicating means is angularly phased with respect to the detent-receiving notch I30, so that the position of the pin I32 indicates the position of the notch I30.

The left-hand end of the casing is appropriately sealed by a seal I 34.

In addition to the portions of the member 64 previously described, there is a reduced cylindrical portion I36 between the portions I04 and I08; and a short cylindrical portion I38 between the portions I00 and I 06.

OperationFigure 1 In this aspect of the use of the control valve assembly, the control valve member 64 is in its first status in which the position-retaining means is ineffective to retain the control valve member in any position. In short, the detent-receiving notch I30 is disposed downwardly, the indicating pin I32 designating this status of the member 64 exteriorly of the housing.

When the member 64 is in its neutral position as shown, the inlet I2 is connected to the reservoir or exhaust passage I4 via the passages 20 and I021 Both check valves 38 and 40 are closed. The check valve plunger 60 seats or rests at the junction of the reversely conical portions I06 and H0; and the check valve plunger 62 seats-,or rests at the reduced portion of the conical pgrtion I08. Hence, the motor ports 26 and 28 lf'are closed, or disconnected from the inlet I2. The first and second exhaust ports 22 and 24 are blocked off or closed. respectively by means of the valve member portions 84 and I04. The bypass passage I I2 is closed by seating of the valve H6 of the member II6---I20.

. Since both check valves are closed and since inlet pressure is circulated to the reservoir, the position of the piston 52 in the cylinder. 46 is hydraulically locked. .The neutral position of the control valve member 64 is maintained by the centering spring I6.

1 When the control valve member 64 is shifted in the direction A to its first active position, the valve member portion I06 positively opens the first check valve 38. The valve member portion H0 moves into the first exhaust port 22 so that the motor port 26 is connected to the exhaust port 22. At the same time, the valve member portion I 38 maintains the disconnection between the inlet I2 and motor port 26. Further, the valve member portion 90, moving to the right, disconnects the inlet I2 from the exhaust passage I4 (by blocking off the short passage I02), and the portion I08 serves to connect the inlet via the branch passage I8 and bore I6 to the second mo-.

tor port 28, whereupon the check valve 40 is opened by pressure. Fluid is therefore transmitted through the line to one end of the cylinder 46 and fluid returns through the line 44 and through the motor port 26 and exhaust port 22 to the reservoir- I4. Movement of the control valve member 64 to the right is, of course, efiec tive to compress the centering spring I6, so that when the operator releases the control valve member 64, the spring I6 serves to return the control valve member to its neutral position.

When the control valve member '64 is shifted to its second active position in the direction of the arrow B, the reverse conditions obtain; that is, the valve member portion I08 positively opens the check valve 40, the portion I04 moves out of the end of the valve bore I6 and efiects connection between the bore and the second exhaust port 24, whereby fluid returning through the line 50 and motor port chamber 36 and motor port 28 passes to the reservoir or exhaust passage I4. At the same time, the valve member portion I38, moving to the left, disconnects the inlet branch passage 20 from the exhaust passage I02, and the conical portion I06 establishes communication between the inlet I2 and inlet branch passage 20 through the first motor port 26. The valve member portion 84 disconnects the first motor port from the first exhaust passage 22.

In that phase of the operation just described, there is one important difference in operation conditions that obtain when the control valve member 64 is shifted in the direction A. This results from that feature of the invention residing in the provision on the control member 64 of the conical portion H0, which serves as means for positively opening the check valve 38. Hence, when the control valve member 64 is shifted to the left, or in the direction of the arrow B, both check valves 38 and 40 are positively opened. Nevertheless, in this phase of the operation or use of the control valve assembly, the opening of the check valve 38 by the conical portion H0 is relatively unimportant, since this check valve would be opened by pressure from the inlet I2 via the passage 20 to the motor port 26. Although this result (positive opening of the check valve 38) may be said to tbe superfluous in this stage of the operation, it plays an important part in the capacity of the control valve assembly. as will appear below in the description 7 nf theoperationfof."theiassembly when. constituted esasrimtEiguretl I Continuing with E theabove. 'dBSClipti0l1,"it will crbevremembered :that'the. byepassacontrol" valve orepxthezgeneral Junction of 'the assembly :is omexvhatlsimilamto::the conventional two-way szcontrolzvalve. Upon. movement of..:the. control valve member 64 in the direction; Byztheicenter- :ringtspring .16 :iszcompressedz-because. of contact.

filhEtWGBIYth-E washer18...and the portions-.96 (Figemre-A) Therefore; when. the "control valve unem- :daenzis released, :the centering spring: will" return 6417116 :control ivalve: member :.to its neutral p'ostion.

- Operation- -Figure 2.

nspointeitzmay be noted that the. control valve ilzmemherafi liefor purposes 'ofefiiciency in proaductionaissymmetrical about its .principal axis. -ils suggested above, the portion H may be .cona sidered.-.superfluous:.-inthe operation of the:conroL valvevassembly in Figure 1. .;desir.ed;::the:control -valvemember may be asymuanetricalizin this respect. However, this isim- Wma-terialfrom-the inventive :standpoint and-v it is considered'that-the principles :ofthe invention earaapplicable in-either: instance.

.As-shownin'Figure 2, the control Valve member e-fi hiseangularlyapositioned so thatit'occupies a -.-..se.cond -.-status, wherein the --detent-receiving -notch :lfiiszdisposed .upwardly, -as indicated by ....the-.-indicating; pin: i 32. aithe-sneutralzposition of. the control valve. member @154, .-the-:same;-:conditions obtain that-obtain the neutral position as shown in Figure l.

when zthecontrol valve memb-erfi lis in its .;.=neutralvposi-tion,-therposition-retaining means is edisengaged, since .the:detent-receiving :notch 139 twill. ibef. axially rspacedj fromlthe 'detent :ball 1:28. -ii1' kewisemwhen-1themontrol valve-"member 64 :is -zshifted.;.:tofthe:;right. or in .':the:direction of the .earrow A,.the position retainingxmeans' is inefiec- :ative. gfflheeactivezsposition' resulting. from :shiftin; iofxthe" control :ivalvea'member fid. in the direc- .flfiOIl'iAl-fil'f*EigllIBilZi'lS the. same as that in Fig- :znre-l. fI'hatais;.=.the..inlettl2 is disconnected from nthe'rexhaushpassage ti l-Sand is connected to the anti.ve:positiontwhen moved' in the direction A;

efiencexthezcontml valve assembly nowfunctions azone=way:control;.;delivering fluid under pres- ...sure only;.:through;themotor port 28 and line 50? 1t0T-Onefeldl(5f" thecylinder 46; return fluid .sfzomzthepylinder dfi:being carried via the-line 44. J

Milt-#120 closes. the by-passpassage .1112. i'There- Therefore, if-- IllzrSO far as concerns,

:.=.-.port' 2 2 01: .by-pass passage H2.

' The most'fsimportant function;foffthercontiiol "valve. assembly when'r utilized in the ist'atus'r'shown .in Figure. 2 is its ability .to permit floating 'of the motor elements; suchas the piston .52: in thecylinder 4B". or ."the piston 'in-awne-way motor if :trol': =va'lvef'member-portions H 0 and lfllflgwhich providesr-forifree:circulation.of fluid-betweem the cylinder AB -Jand' valve bore l6. Since: the left- I .hand portion [36 establishes communicationme- 'Z' tWeEII the ."second' motor" port -28 and second exhaust. port "2d,' the result will. be. no different ifromzlthat lthat'robtains' when the control valve member 64 is shifted to the left -in Figured.

:However, without athe by-passpassage H 2 and .positiveopening: Of the check valve 38; "there scouldbe: no" free :circulation of fluid at the-rightnha-ndend of the casing. "AS SllOWI1-'ifi Figl-II 2,

xtheispass-ageili 2 directly communicatesthe motor .port.:;28: :toztthe' reservoir or exhaust pass'age I 4.

70ft further importance is the"0pe'r'ati0n now 11 of "the" position retaining means. When the con- .trolivalv i member is shifted to -the position as 'shown rin Figure. 2;' 'the spring-=loa'deddetent ball 15.28 seats in the notch or recess I38, thus holding the control valve. member fi l "in position against i zthe. actionof the loa'ded'spring 'l 6.

- Summary Briefly described,- and for the 'purposes'of "fa- 'cilitating an understanding of the invention as defined in the" appended claims, the control valve assembly may --be considered as having dualcapacity, which results 'from the selective angular positioning 'of 'the'coritrol valve member 64in either a first status (Figure-1) or a-second status '(Figure"2).' In its first status, the control valve member 64 iseffective to positively open the first check va'lve' 38 when'shif-ted to a first active-posi- "tion in the' direction of 'the' arrow A. This 'disconnects' the inlet from the exhaust passage, disconnects the first motor port 26 from'the inlet and connects said first'motor port" to the first exhaust port; and the second motor 'portis connect'ed to' the inlet anddisconnect'ed'fr'om the second exhaust port. *When the control valve member 64 is shifted to its second active-position (in the directiOn' B), theaforesaid conditions are reversed. As mentioned above, it isnot fundamentally necessary that the first check valve 33 be positively opened when the control valve member 64 is shifted to the left. Nevertheless,

1 because :of "the" symmetrical construction offthe control valve member 64'this result follows. ."The by-pass'means |l2-l'l 4.is closed and henceestablishes no communicationbetween the first'motor port 26 and eXhaust'passa-ge 'M. It will benoted that this passage by passes the-disconnection'at 84 between the" first motor port 25 and firstexhaust port 2; although it could readily be otherwise connected to accomplish the sameresult. However, in'the first status of the control valve assembly, the" by-pass is'closed and has no effect on'the operation of the'system.

first chckv'alv-BB-and "the opening of the bypass passage l-|-2-ill4 is-importantfas previously described.

Other-obj;ects'and features of the invention'not specificallyenumerated herein will undoubtedly occur to those versed in the art, as likewise. will various modifications and alterations in the preferred embodiment of the invention disclosed, all of which may be achieved without departing from the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. A fluid-pressure control valve assembly, comprising: a casing having a valve bore, a fluid inlet leading to the bore, first and second motor ports leading from the bore, an exhaust passage, and first and second exhaust ports leading from the bore to the exhaust passage; first and second check valves respectively in the motor ports and normally closed against return fluid; a control valve member in the valve bore having a neutral position in which the check valves are closed and the inlet is led to the exhaust passage; said member being shiftable in one direction from said neutral position to a first active position in which the inlet is disconnected from the exhaust passage, the first motor port is disconnected from the inlet and connected to the first exhaust port, and the second motor port is connected to the inlet and disconnected from the second exhaust port; said member being selectively shiftable in the opposite direction from said neutral position to a second active position in which the aforesaid conditions are reversed; means connected to said member and efiective when the member is shifted to its first active position to positively open the first check valve; means connected to said member and effective when the member is shifted to its second active position to positively open both check valves; means in the casing providing a passage communicating between the exhaust passage and the first motor port and by-passing the fluid disconnection between the first motor port and first exhaust port when the control valve member is in its second active position; and means movably carried by the casing for selectively opening or closing said by-pass passage.

2. The invention defined in claim 1, further characterized in that: means is provided for optionally retaining the control valve member in its second active position, wherein both check valves are positively held open, so that when said by-pass means is opened both motor ports lead to the exhaust passage.

3. The invention defined in claim 1, further characterized in that: a portion of said control valve member has a position-retaining element thereon; the casing is provided with a positionretaining element cooperatively engageable with the position-retaining element of the control valve member when said member is shifted to its second active position; and said control valve member portion is selectively angularly positionable relative to the position-retaining element of the casing to condition said elements for nonengagement when said member is shifted to its second active position.

4. The invention defined in claim 3, further characterized in that: said position-retaining elements are enclosed by the casing; and means is connected to the control valve member portion and projects exteriorly of the casing, including a member indicating the angular position of the position-retaining element on the control valve member portion.

5. The invention defined in claim 1, further characterized in that: the control valve member is selectively angularly positionable in the valve bore to occupy either a first or second control status; said control valve member is symmetrically formed so that in either status it functions as aforesaid to control the motor and exhaust ports and the inlet and exhaust passage; detent means is provided in the casing, including a detent member projecting into the valve bore; and the control valve member is provided with a detent-receiving recess arranged to receive said detent member when the control valve member is in its second position in but one status.

6. A fluid-pressure control valve assembly, comprising: first and second motor port means, respectively having check valves normally biased to closed position against return fluid flow; fluid inlet means; fluid exhaust means; valve means for selectively establishing a neutral condition, in which the inlet means is connected to the exhaust means, or either of first or second active conditions, in the first of which the first motor port is disconnected from the inlet means and connected to the exhaust means and the second motor port is connected to the inlet means and disconnected from the exhaust means, and in the second of which the connections and disconnections are reversed, so that fluid pressure is selectively directed through one motor port while returning through the other; means connected to the valve means for positively opening the first check valve incident to the establishment of said first active condition; means connected to the valve means for positively opening both check valves incident to the establishment of said second active condition; means providing a passage between the exhaust means and the first motor port and by-passing the fluid disconnection between said first motor port and the exhaust means when said second active condition obtains; and means for selectively opening or closing said passage means.

7. A fluid-pressure control valve assembly, comprising: first and second motor port means, respectively, having check valves normally biased to closed position against return fluid flow; fluid inlet means; fluid exhaust means; valve means for selectively establishing a neutral condition, in which the inlet means is connected to the exhaust means, or either of first or second active conditions, in the first of which the first motor port is disconnected from the inlet means and connected to the exhaust means and the second motor port is connected to the inlet means and disconnected from the exhaust means, and in the second of which the connections and disconnections are reversed, so that fluid pressure is selectively directed through one motor port while returning through the other; means for positively opening the first check valve during the establishment of said first active condition; means for positively opening both check valves during the establishment of said second active condition; means providing a passage between the exhaust means and the first motor port and by-passing the fluid disconnection between said first motor port and the exhaust means when said second active condition obtains; and means for selectively opening or closing said passage means.

JOHN H. EDMAN.

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

UNITED STATES PATENTS Number Name Date 2,363,235 Ellinwood Nov. 21, 1944 2,417,278 Van Der Wert Mar. 11, 1947

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