US3596566A

US3596566A - Hydraulic valve

Info

Publication number: US3596566A
Authority: US
Inventors: Robert D Krehbiel; Homer R Graber
Original assignee: Cessna Aircraft Co
Current assignee: Cessna Aircraft Co
Priority date: 1967-05-15
Filing date: 1967-05-15
Publication date: 1971-08-03
Anticipated expiration: 1988-08-03
Also published as: DE1750413B2; GB1177056A; DE1750413C3; FR1562719A; DE1750413A1; SE341532B

Abstract

A manually operated spool-type closed-center flow control valve having two lockout check valves, one for each end of a doubleacting pressure-fluid-actuated motor. The valve affords precise spool metering of pressure fluid returning from either end of the motor to the hydraulic system reservoir because both lockout check valves are fully opened and maintained open prior to, and entirely independent of, any flow of fluid to or from either end of the motor. Thus fluid flowing through either of the fully open lockout check valves can in no way interfere with precise spool metering of flow through the control valve. The flow-metering spool can also be positioned to afford floating movement of the motor plunger and rod in either direction in response to an applied external load.

Description

United States Patent 2} Inventors llobertD.Knhblel Hutchinson; Homer R. Graber. Pretty Prairie, both 0|, Kane. [2! I Appl6 Nov 638,226 [22) Filed May1$,l967 I45] Patented Aug. 3,1971 (73] Assignee The Cesaa Aircraft Company Wichita, Kane.

[54] HYDRAULIC VALVE Claims, 6 Drawing Figs.

152 U.S.Cl .6 91/420, |3v/s9e.2,9| 447 151 lnt.CL ..Fl5bl3/042, FlSbll/OB s01 FieldolSearch .6 137/596,

[56] References Cited UNITED STATES PATENTS 3,274,902 9/l966 Kleckner .6 91/420 3,381.58! 5/!968 Parquet .6 91/420 Primary Examiner-Paul E. Maslousky Anorney-Hubert E Miller ABSTRACT: A manually operated spool-type closed-center flow control valve having two lockout check valves. one for each end of a double-acting pressure-fluid-actuated motor. The valve affords precise spool metering of pressure fluid returning from either end of the motor to the hydraulic system reservoir because both lockout check valves are fully opened and maintained open prior to, and entirely independent of. any flow of fluid to or from either end of the motor. Thus fluid flowing through either of the fully open lockout check valves can in no way interfere with precise spool metering of flow through the control valve. The flow-metering spool can also be positioned to afford floating movement of the motor plunger and rod in either direction in response to an applied external load.

PATENTEDAUG 3mm 113,596,566

sum 1 or 3 31 2| l9 I2 29 1326 3635 22 2sm 2 m I: I O

"a as a L 2s 54 52 57 58 2 2 I23 l8 1 3O 20' 2| g2 23 3| I3 32 ROBERT D. KREHBIEL HOMER R. GRABER ATTORNEY PATENTED AUG 3l97i T',i586 566 SHEET 3 [1F 3 ROBERT D. KREHB'EL HOMER R. GRABER ATTORNEY HYDRAULIC VALVE Prior art spool-type control valves which have included a spring closed lockout check valve for each motor port have depended on pressure fluid flowing from the source outward through one or the other motor port of the valve to open both lockout check valves. In such a valve, when the plunger rod of the controlled motor is subjected to a large external load, there is a large pressure variation in the fluid flow path in which each lockout check valve is interposed. As the plunger rod is accelerated, both by fluid pressure and the external load, the high rate of movement of the plunger rod causes a drop in pressure in the supply flow path. This lowers the pres sure tending to maintain the lockouts open, and both lockouts are momentarily spring closed. Their closing causes an instant stoppage of plunger travel, and a resultant shock to the assembly. When fluid from the source again builds up sufficient pressure. both lockout checks are reopened and the described cycle is repeated. This phenomenon is known in the valve art as lockout chatter or oscillation, and regardless of accuracy efforts the valve operator is incapable of accurately positioning the motor plunger rod and its attached load, or of accurately controlling the rate of movement of the plunger rod, all clue to the above described instability of the lockout checks.

Past efforts to overcome this difficulty and obtain precise control over motor plunger rod movement have involved severe restriction of maximum permitted volume flow both to and from the controlled motor, as well as bulky, intricate and complicated design of lockout check valves. These efiorts at solution created a large operating efliciency loss, and have drastically limited both the speed of movement and the reaction time for plunger operation.

The present invention solves the lockout instability problem by providing a separate pressure chamber and pressure responsive means: (I) for fully opening both lockout check valves before any pressure fluid is permitted to flow from the pressure source through either motor port to the controlled motor; (2) for maintaining both lockout checks fully open independent of any pressure variations which occur while the motor circuit is open; and (3) for affording closing of both lockout checks only after the flow paths to and from the motor have been closed.

This independent opening and closing of the two lockout checks makes them inherently stable in operation, and affords highly precise operator-controlled valve spool metering of fluid returning from the controlled motor, without any interference due to pressure variations which occur in the flow path supplying pressure fluid to the motor, all of which provides highly accurate operator control over the movement and positioning ofthe motor plunger rod and its load.

In valves embodying this invention the lockout check valves are held closed not only by spring pressure, but also by the pressure of fluid in the conduit between each lockout check and the respective adjacent end ofthe controlled motor. In addition, a leakage path is provided to prevent accumulated internal leakage of pressure fluid from unseating or cracking either lockout. This makes it possible for the two lockouts to positively lock the plunger of the controlled motor in a selected position and to maintain it in that position indefinitely.

Prior art control valves having double lockout checks have been incapable of permitting the plunger rod of the controlled motor to "float" in either direction under the influence of an external force because one motor port had to be openly connected to the fluid pressure source in order to open the two lockout checks. In this invention, since the lockout checks are held open independent of source fluid flowing to the motor, a "float position is provided for the flow control spool, and both ends of the controlled motor are connected to a return conduit, thus affording free movement of the motor plunger in its cylinder.

The invention also includes a means for opening both lockout check valves in case the hydraulic system pressure should drop to zero, and the load supported by the plunger rod of the motor happened to be in a dangerous position. The load can then be lowered to normal position.

The invention, and its various features, will be more clearly understood when the following description is read in connection with the accompanying drawings in which:

FIG. 1 is an axial vertical section through a valve embodying the invention, the flow control spool being shown in its neutral position;

FIG. 2 is a transverse longitudinal section taken along the line 1-2 ofFIG. 1;

FIG. 3 is a fragmentary sectional view of the valve of FIG. 1, and shows details in the construction of a different type of lockout check valve than is shown in F I0. 1;

FIG. 4 is a view similar to FIG. I but shows the flow control spool moved slightly to the right of its neutral position to cause fluid pressure opening of both lockout check valves, while flow from the supply passage to either end of the motor remains blocked by the spool;

FIG. 5 is a view similar to FIG. 4 showing the spool moved slightly farther to the right in a position to meter fluid from the right end of the controlled motor, and to simultaneously afford flow of motive fluid from the supply passage to the left end ofthe motor; and

FIG. 6 is an axial vertical section through a flow control valve embodying the invention in slightly different form, and schematically shows a simple hydraulic system in which such valves may be interposed.

Referring to FIGS. 1 and 2, the valve comprises a housing [0 having a fluid supply passage 11, fluid return passages 12 and I3, and two

motor ports

14 and 15, connected to the respective opposite ends of a double acting hydraulic motor 16. Extending through the housing is a spool bore 17 which is encircled by six longitudinally spaced bore communicating ducts 18 through 23, in addition to the

return passages

12 and 13. Ducts 21 and 22 communicate with each other and with the supply or inlet passage 11. Ducts l9 and 22 communicate with each other by means of a longitudinal passage 24.

Mounted in bore I7 is a slidable flow control spool 25 encircled by

spaced grooves

26, 27 and 28 which define

lands

29, 30, 31 and 32.

Lands

29 and 31 are provided with recessed

fluid metering notches

33 and 34, as clearly shown in FIG. 2.

Internally, spool 25 is provided with a ball check valve 35 which closes one end of a duct 36. The ball check chamber also communicates with the spool bore by means of longitudinally spaced

lateral ducts

37 and 38. Spool 25 has a second internal chamber 39 which communicates with the spool bore by longitudinally spaced lateral ducts 40 and 41.

Valve 10 houses opposed spring-closed

lockout check valves

42 and 43, connected respectively to opposed

lockout opening plungers

44 and 45 by means of

spindles

46 and 47, respectively. Lockout valve 42 is interposed between duct 18 and motor port 14, and when closed blocks flow from the motor port 14 into duct 18. Similarly, lockout valve 43 is interposed between duct 23 and motor port l5, and when closed blocks flow from motor port [5 into duct 23.

Lockout opening plungers

44 and 45 are reciprocable in a pressure chamber 48 which communicates with bore communicating duct 20. Internally,

plungers

44 and 45 are provided respectively with

ball check valves

49 and 50, which block communication in one direction between the chamber 48 and the flow ducts I8 and 23, respectively.

Associated with the left end of spool 25 is a conventional spool centering mechanism 51, which includes a coil spring 52, and which functions to return the spool to its neutral FIG. I position when the spool is released by the operator. A spring-pressed ball-type detent 53 mounted in a recess in the housing cooperates with an annular groove 54 near the end of spool 15 to hold the spool in "float" position when the operator desires, as will be later explained in detail.

OPERATION Prior to operation, supply or inlet passage I1 is connected to a hydraulic pump, and

return passages

12 and 13 are connected to a system reservoir, not shown. With the spool in neutral position, as in FIG. I, the supply and return passages are isolated from each other, as are the two motor ports, and the plunger and rod of motor I6 are locked against movement in either direction.

To move the plunger of motor I6 to the right, the operator moves spool 25 to the right. When the spool reaches its FIG. 4 position pressure fluid flows from supply passage II through duct 36, past ball check 35, through ducts 37 38 and 20 into pressure chamber 48, and instantly forces the two

plungers

44 and 45 away from each other, such plunger movement fully opening

lockout checks

42 and 43, as shown in FIG. 4. With the spool in its FIG. 4 position spool land 30 blocks the flow of pressure fluid from duct 19 into duct 18, and spool land 31 blocks flow of fluid from the motor through duct 23 into return passage 13. Air pressure behind lockout checks 42 and 43 is relieved by filter packed

atmospheric vents

55 and 56 respectively.

As movement of spool 25 to the right continues to the FIG. position the fluid metering notches 34 in the left end of spool land 31 afford precise operator controlled metering of fluid from the rod end of motor 16 into return passage I3, and simultaneously the left end face of spool land 30 affords flow of pressure fluid from supply passage 11 through ducts 24 and I8, and out through motor port 14 to the left end of motor I6. The motor plunger thus moves to the right under precise control of the valve operator.

Note that when spool 25 is in its FIG. 5 position, or further to the right if desired by the operator, pressure chamber 48 remains in communication with supply passage II through

ducts

36, 37 and 20, and that lockout checks 42 and 43 are thereby maintained fully open position entirely independent of any flow of fluid to or from the motor 16 through ducts I8 and 23. The lockout checks are thus unaffected by variations in pressure in

ducts

18 and 23, or in motor ports I4 and I5, and thus do not in any way affect precise metering of return fluid by spool 25, and consequent precise control by the operator over the speed of movement and positioning of the plunger of motor 16.

When the operator releases the spool 25, the centering mechanism 51 returns the spool to its FIG. I neutral position. As it moves to such position, flow of pressure fluid to and from both ends of motor 16 is first blocked by the

lands

30 and 31, respectively. Instantly thereafter the two

lockouts

42 and 43 are closed by their respective springs, the fluid in chamber 48 flowing through ducts 41, 39 and 40 (FIG. 1) into return passage I2.

To assure that neither of the

lockouts

42 or 43 are opened by pressure fluid which may leak into

ducts

18 or 23, leakage ducts 57 and 58 (FIG. 1) are provided in the inner ends of

spindles

46 and 47. Accumulated leakage fluid may pass through ducts 57 and 58, past ball checks 49 and 50, and enter chamber 48, from which it may travel to return duct I2, as previously described.

When spool 25 is moved to the left of its FIG. I neutral position to move the motor plunger to the left, the same sequence of events occurs. Pressure fluid first flows through

ducts

36, 37, 38 and into chamber 48, and

plungers

44 and 45

force lockouts

42 and 43 to their fully open positions, as in FIG. 4. Further movement of the flow control spool to the left allows flow of pressure fluid from inlet II through duct 23, through motor port 15 to the rod end of motor l6, and simultaneously begins the metering of fluid from the left end of the motor through metering notches 33 on spool land 29 into the return passage 12, and the motor plunger moves to the left under the precise control of the spool operator.

MOTOR PLUNGER FLOAT CONDITION When spool 25 is moved sufficiently far to the left that ball detent 53 seats in spool groove 54, the detent temporarily holds the spool in that position against the spring force of the spool centering mechanism 51.

In its detentcd position, the spool directs pressure fluid from supply I! to pressure chamber 48 through duct 36, past ball check 35, and through

ducts

38 and 20, thus holding

lockout checks

42 and 43 fully open. Spool groove 26 connects duct I8 with return passage 12, and spool groove 28 connects duct 23 with return passage 13. Spool land 30 blocks flow of pres sure fluid from ducts II and 24 into duct I8, and spool land 3I blocks flow of pressure fluid from passage 11 into duct 23. This detented spool position, then, openly connects both ends of motor 16 with the low-pressure system return ducts 12 and I3, and permits the plunger and rod of motor I6 to be moved in either direction by an external load applied to the rod.

To our knowledge such a plunger float" condition has not previously been possible in a control valve having double lockout check valves because the two lockouts have been opened by pressure fluid flowing from the pressure source to one or the other end of the motor.

While not necessary to the valve invention described above, an auxiliary lockout check valve opening mechanism may be included. It is useful only in case the fluid pressure source should fail, with consequent closing of the two lockouts under spring pressure, which in turn might lock the plunger of the motor at one end of its permitted stroke with the load on the plunger rod in an unstable or dangerous position.

To alleviate such a condition a normally stationary pin 59 is slidably mounted in the valve housing with suitable surrounding packing to prevent leakage. Pin 59 has a generally wedge shaped or conical inner end which is normally positioned adjacent the inner ends of the two

lockout opening plungers

44 and 45, as clearly shown in FIG. I. Should system pressure fall, it is only necessary to tap the outer end of pin 59 to drive its wedge-shaped inner end into contact with the adjacent inner ends of

plungers

44 and 45. The plungers are thereby forced outward, and the two lockouts are unseated, allowing the motor plunger to be moved in either direction by its load. Fluid displaced from either end of the motor 16 during such plunger movement passes through either duct 57 or 58 (FIG. I}, as the case may be, into chamber 48, and thence out of the valve through return passage I2, as previously described.

FIG. 3 LOCKOUT VALVE CONSTRUCTION An alternative lockout check valve and plunger construction is illustrated in FIG. 3. In this construction the lockout valve 62 is mounted in a manner similar to the mounting of lockout 42, but it is not physically connected to the spindle 66 of the lockout opening plunger 64, although plunger spindle 66 opens the lockout in the same manner as does spindle 46. Lockout 62 is reciprocable in a blind bore of a plug but the plug bore is not vented to the atmosphere, as is the case in the mounting of

lockouts

42 and 43.

Lockout 62 is provided with a lateral duct 86 and a communicating axial duct 87 which pressure fluid from motor port I4 to enter the otherwise closed chamber 88 behind the lockout.

With such construction, the closing force exerted on the lockout 62 by the pressurized fluid in chamber 88 is greater than any pressure created force exerted on the other end of the lockout through the lockout seat 89, because the seat exposed area is less than the total cross-sectional area of the lockout. Thus, in this alternative construction the lockout 62 is held in seated position not only by spring force but also by fluid-pressure-created force.

FIG. 6 EMBODIMENT In the valve illustrated in FIG. 6 the leakage ducts 57 and 58 of the FIG. I valve, and the

ball check valves

49 and 50, and their respective leakage passages have been eliminated. The

lockout check

valve opening plungers

44 and 45 have been made solid, and all provision for leakage fluid passage from conduits l8 and 23 into chamber 48 has been omitted.

In the FIG. 6 valve, pressurized fluid which leaks from the supply duct 11 past spool 25 when the spool is in neutral position, and which accumulates in duct 18, is disposed of via an added spool port 90 (FIG. 6), spool chamber 39, port 40, into return passage 12. Similarly, leakage fluid which accumulates in duct 23 is disposed of via an added leakage duct 91, which directly communicates with return passage 13 when spool 25 is in neutral.

Thus in the FIG. 6 valve there is no fluid communication between chamber 48 and the two ducts l8 and 23.

Operation of the FIG. 6 valve is identical to the operation of the valve shown in FIGS. 1 to 5.

Having described the invention with sufficient clarity to enable those familiar with this art to construct and use it, we claim:

1. A hydraulic control valve assembly for controlling the flow of fluid between (a) a source of fluid pressure, (b) a hydraulic motor which has alternate inlet and discharge means, and (c) a reservoir for return fluid, said assembly having supply (I1) and return ports 12 8t 13), and comprising:

first and second passage means (14

8t

18 and 15 8t 23) in the valve body respectively connected to the alternate motor inlet and discharge means;

opposed first and second check valves (42 8t 43) respectively mounted in the first and second passage means between the motor (16) and the supply (11) and return ports (12 81. 13), and biased against return of fluid from the motor;

a pressure chamber (48) in the body located between the opposed first and second check valves;

movable (44 8t 45) in said pressure chamber associated with said first and second check valves, and responsive to pressure in said chamber to fully open and maintain open both said first and second check valves independent of the presence or absence of pressure fluid in said first and second passage means;

and flow control means (25) having (a) a neutral position in which flow of fluid to and from the motor is blocked, (b) a second position in which the supply port is connected to said pressure chamber while flow of fluid to and from the motor remains blocked, and (c) a third position further in the same direction from neutral beyond said second position in which the supply port (ll) is connected to one alternate motor inlet means and the return port (I2--l3) is connected to the motor discharge means while the supply port remains connected to said pressure chamber.

2. The assembly described in claim I in which the flow control means has a fourth position in which the first and second passage means are both connected to reservoir while said pressure chamber is maintained in communication with the supply port.

3. The invention described in claim 1 in which each check valve (62) is in the form of a plunger reciprocably mounted to project from one end of a blind cylinder (88), and the flow passage through the seat on which the projecting head of the check valve seats is of lesser cross-sectional area than the cross-sectional area of the blind cylinder;

and a passageway (86, 87) in each check valve affording communication between the respectively adjacent passage means (l4, l5) and the blind cylinder (88) behind the check valve,

whereby pressure fluid in the blind cylinder (88) behind each check valve, derived from the respective adjacent passage means, urges each check valve onto its seat when the flow control means is in its flow-blocking position.

4. The invention described in claim 1 in which the said movable means is in the form of two separate slidable oppolitely acting plungers (44, 45, 64), and the pressure chamber (48) defines a cylinder for each plunger, the plungers projecting from the opposite ends thereof in close association with the respective opposed check valves (42-43) to open them when the plungers are forced outward by fluid pressure in the cylinder.

5. The invention described in claim 4 in which the two plun gers are connected to and move with the respective check valves.

6. The invention described in claim 4, and

an elongated wedge-type element (59) slidably mounted in the valve assembly for longitudinal movement into said pressure chamber (48) along a line normal to the path of travel of the two check valve opening plungers (44, 45), and at a location and of a size to contact the adjacent inner ends of said plungers to force them to move respectively outward to open both check valves in the absence of pressure in said pressure chamber.

7. The invention described in claim 1, and:

a separate duct (20) in the valve housing independent of said first and second passage means for conducting fluid under pressure into said pressure chamber.

8. The invention described in claim 7, and separate duct means (36, 37, 38) in the flow control means (25) aflording flow of pressure fluid through said separate duct (20) into said pressure chamber (48) prior to and independent of the directing of pressure fluid from the pressure fluid source through either of said passage means, in response to slight movement of the flow control means in either direction from its flowblocking position.

9. The invention described in claim 7, and duct means (39, 40, 41) in the flow control means (25) affording flow between said pressure chamber (48) and a return port (12) only when the flow control means (25 is in its flow-blocking position.

10. The invention described in claim 9, and

a check valve controlled duct (57-58) in each of said plungers (44, 45, 64) effective when the flow control means (25) is in its flow-blocking position to afford flow of leakage fluid from the passage means (18, 23) into the pressure chamber (48), and to block flow of fluid from the pressure chamber into said passage means.

H. A hydraulic system comprising:

a source of fluid pressure;

a hydraulic motor;

conduit means connected to said hydraulic motor;

control valve means operatively connected to the conduit means and to the source of fluid pressure for selectively pressurizing or exhausting said conduit means;

a check valve in the conduit means between the control valve means and the motor, biased against the return of fluid from the motor;

a hydraulic cylinder including plunger means movable therein in response to pressure in said cylinder to open said check valve;

said control valve means having a movable flow control means which has (a) a neutral position in which flow through said conduit means is blocked, (b) a second position in which fluid is directed from the source to said hydraulic cylinder to open said check valve, while flow through said conduit means remains blocked, and (c) a third position further spaced in the same direction from neutral position beyond said second position in which said conduit means is pressurized and said hydraulic cylinder is maintained pressurized independent of pressure fluid in said conduit means.

Claims

1. A hydraulic control valve assembly for controlling the flow of fluid between (a) a source of fluid pressure, (b) a hydraulic motor which has alternate inlet and discharge means, and (c) a reservoir for return fluid, said assembly having supply (11) and return ports (12 & 13), and comprising: first and second passage means (14 & 18 and 15 & 23) in the valve body respectively connected to the alternate motor inlet and discharge means; opposed first and second check valves (42 & 43) respectively mounted in the first and second passage means between the motor (16) and the supply (11) and return ports (12 & 13), and biased against return of fluid from the motor; a pressure chamber (48) in the body located between the opposed first and second check valves; movable (44 & 45) in said pressure chamber associated with said first and second check valves, and responsive to pressure in said chamber to fully open and maintain open both said first and second check valves independent of the presence or absence of pressure fluid in said first and second passage means; and flow control means (25) having (a) a neutral position in which flow of fluid to and from the motor is blocked, (b) a second position in which the supply port is connected to said pressure chamber while flow of fluid to and from the motor remains blocked, and (c) a third position further in the same direction from neutral beyond said second position in which the supply port (11) is connected to one alternate motor inlet means and the return port (12-13) is connected to the motor discharge means while the supply port remains connected to said pressure chamber.

2. The assembly described in claim 1 in which the flow control means has a fourth position in which the first and second passage means are both connected to reservoir while said pressure chamber is maintained in communication with the supply port.

3. The invention described in claim 1 in which each check valve (62) is in the form of a plunger reciprocably mounted to project from one end of a blind cylinder (88), and the flow passage through the seat on which the projecting head of the check valve seats is of lesser cross-sectional area than the cross-sectional area of the blind cylinder; and a passageway (86, 87) in each check valve affording communication between the respectively adjacent passage means (14, 15) and the blind cylinder (88) behind the check valve, whereby pressure fluid in the blind cylinder (88) behind each check valve, derived from the respective adjacent passage means, urges each check valve onto its seat when the flow control means is in its flow-blocking position.

4. The invention described in claim 1 in which the said movable means is in the form of two separate slidable oppositely acting plungers (44, 45, 64), and the pressure chamber (48) defines a cylinder for each plunger, the plungers projecting from the opposite ends thereof in close association with the respective opposed check valves (42-43) to open them when the plungers are forced outward by fluid pressure in the cylinder.

5. The invention described in claim 4 in which the two plungers are connected to and move with the respective check valves.

6. The invention described in claim 4, and an elongated wedge-type element (59) slidably mounted in the valve assembly for longitudinal movement into said pressure chamber (48) along a line normal to the path of travel of the two check valve opening plungers (44, 45), and at a location and of a size to contact the adjacent inner ends of said plungers to force them to move respectively outward to open both check valves in the absence of pressure in said pressure chamber.

7. The invention described in claim 1, and: a separate duct (20) in the valve housing independent of said first and second passage means for conducting fluid under pressure into said pressure chamber.

8. The invention described in claim 7, and separate duct means (36, 37, 38) in the flow control means (25) affording flow of pressure fluid through said separate duct (20) into said pressure chamber (48) prior to and independent of the directing of pressure fluid from the pressure fluid source through either of said passage means, in response to slight movement of the flow control means in either direction from its flow-blocking position.

9. The invention described in claim 7, and duct means (39, 40, 41) in the flow control means (25) affording flow between said pressure chamber (48) and a return port (12) only when the flow control means (25) is in its flow-blocking position.

10. The invention described in claim 9, and a check valve controlled duct (57-58) in each of said plungers (44, 45, 64) effective when the flow control means (25) is in its flow-blocking position to afford flow of leakage fluid from the passage means (18, 23) into the pressure chamber (48), and to block flow of fluid from the pressure chamber into said passage means.

11. A hydraulic system comprising: a source of fluid pressure; a hydraulic motor; conduit means connected to said hydraulic motor; control valve means operatively connected to the conduit means and to the source of fluid pressure for selectively pressurizing or exhausting said conduit means; a check valve in the conduit means between the control valve means and the motor, biased against the return of fluid from the motor; a hydraulic cylinder including plunger means movable therein in response to pressure in said cylinder to open said check valve; said control valve means having a movable flow control means which has (a) a neutral position in which flow through said conDuit means is blocked, (b) a second position in which fluid is directed from the source to said hydraulic cylinder to open said check valve, while flow through said conduit means remains blocked, and (c) a third position further spaced in the same direction from neutral position beyond said second position in which said conduit means is pressurized and said hydraulic cylinder is maintained pressurized independent of pressure fluid in said conduit means.