US3911942A

US3911942A - Compensated multifunction hydraulic system

Info

Publication number: US3911942A
Application number: US455830A
Authority: US
Inventors: Lanson Becker
Original assignee: General Signal Corp
Current assignee: ICM ACQUISITIONS Inc A DE CORP; Hydreco Inc
Priority date: 1974-03-28
Filing date: 1974-03-28
Publication date: 1975-10-14
Anticipated expiration: 1992-10-14
Also published as: IT1032464B; CA1032063A; SE7503395L; FR2266024A1; FR2266024B1; DE2513919C2; GB1457731A; JPS50130981A; JPS6018844B2; SE402623B; DE2513919A1

Abstract

A compensated multifunction hydraulic system for use with a source of hydraulic fluid includes two system valve means connected with each other and each capable of performing a separate function. One of the system valves includes a pressure compensator communicating with a control valve. A first signal line upstream of said control valve moves a spool member in the pressure compensator to enable the fluid to flow to the other system valve. A second signal line downstream of the control valve moves the spool member to deliver the necessary amount of hydraulic fluid to the control valve in accordance with the full needs thereof. Means are provided in the second signal line for diverting flow in the second signal line towards the spool member in the pressure compensator to enable the hydraulic fluid to flow to the other of the system valves when the control valve is in a stalled condition.

Description

Becker COMPENSATED MULTIFUNCTION HYDRAULIC SYSTEM Primary ExaminerAlan Cohan Assistant Examiner-Gerald A. Michalsky Attorney, Agent, or Firm.leffrey S. Mednick [75] Inventor: Lanson Becker, Galesburg, Mich.

[73] Assignee: General Signal Corporation, 57 ABSTRACT Rochester A compensated multifunction hydraulic system for use [22] Filed; Mar, 28, 1974 with a source of hydraulic fluid includes two system valve means connected with each other and each ca- [211 Appl 455830 pable of performing a separate function. One of the system valves includes a pressure compensator com- [52] US. Cl. 137/117; 91/412; 91 /446; munica ing with a control valve. A first signal line up- 137/1 19; 137/596; 137/596,13 stream of said control valve moves a spool member in [51] Int. C1. F15B 13/06 h pressure compensator to enable the fluid to flow [58] Field of Search 91 /446, 412; 137/1 16.3, to the other system valve. A second signal line down- 137/119, 488, 596, 596 12, 596,13, 117 stream of the control valve moves the spool member to deliver the necessary amount of hydraulic fluid to [56] References Cit d the control valve in accordance with the full needs UNITED STATES PATENTS thereof. Means are provided in the second signal line for diverting flow in the second signal line towards the 3,7l8,l59 2/1973 Tennis l37/596.l2 Spool member in the pressure compensator o na 3,807,447 4/1974 Masuda l37/596.l3 the hydrauic fluid to flow to the other of the System valves when the control valve is in a stalled condition.

10 Claims, 6 Drawing Figures l6 I06 Q 'LT |--'--1' n2 ry----u4 r ---T"P* I08 us l I ue 0 1A 1'- T 5| Lee 33% .1 -J 40 r 1 l 42 a! T F]. 30 T I I 238 l l X Actuator T T J4 T Actuator US. Patent 00:. 14, 1975 Sheet 1 013 3,911,942

Actuator FIG. I

PRIOR ART COMPENSATED MULTIIFUNCTION HYDRAULIC SYSTEM BACKGROUND OF THE INVENTION While the invention is subject to a wide range of applications, it is especially suited for use in a hydraulic circuit including pressure compensated flow regulators and will be particularly described in that connection.

As the need for a more complex hydraulic system has increased, there has been a significant demand for bydraulic systems capable of performing a plurality of functions either in sequence or simultaneously one with the other. In meeting this need, there has been an interest in development of hydraulic systems capable of performing a multitude of functions efficiently.

One of the basic components of many recent hydraulic circuits is a pressure compensated control valve. Pressure compensated control valves not only govern the operativeness of a fluid motor and the direction of its operation, but for a given spool position, also main tain the motor at a constant, selected speed despite variations in the load or the pressure of supply fluid.

An example of the need for such complex hydraulic systems including pressure compensated control valves is found on a crawler-mounted excavator. In using this machine, it is often desirable, and in many instances necessary, to operate a main boom, a stick arm, and a bucket simultaneously. Since each of these operations is controlled by a separate valve section, the operator must be able to control flow in each valve section independently and regardless of variations in the load and pressure. Further, the operator must be able to combine machine operations with no loss of position control of any other operation.

US. Patent Application Ser. No. 392,901, filed Aug. 30, 1973, now US. Pat. No. 3,878,679, to Sievenpiper discloses a hydraulic system including two or more pressure compensated control valves each capable of performing a separate function at the same time. This invention has achieved a high degree of success since it does this While minimizing the output of a variable delivery pump as demands of any one of the hydraulically operated functions vary. In applications using this circuitry, all of the functions do not necessarily require the same pressure during operation. Thus, it is likely that some functions can reach system relief pressure before other functions. The pressure compensator re ceiving flow at system relief pressure is normally in a full flow position and utilizes all of the available fluid for its associated control valve. However, the function is now in a stalled condition, as defined below, and does not actually require flow. Therefore, the pressure builds up and opens a relief valve which by-passes the flow to a reservoir. If the function operating at system relief pressure is upstream of another function operating at a lower pressure, the flow to the downstream function may be stopped.

The term stalled condition, as used in this disclosure, may be defined as a situation where a fluid actuator, such as a piston in a cylinder, is not able to carry out its work function, and causes pressure to build up in the line that supplies fluid to the actuator. This condition may be caused by the piston reaching the end of its stroke or by a load too heavy for the piston to overcome.

A machine which looses one or more of its functions when stalling occurs is not necessarily operating at peak efficiency. Further, a sudden temporary loss of one or more functions may cause a jerkiness in the operation of the machine.

It is an object of the present invention to provide a hydraulic circuit which efficiently operates a hydraulic machine.

It is a further object of the present invention to provide a hydraulic circuit which utilizes the flow of fluid to a stalled function for another downstream function.

It is a further object of the present invention to provide a hydraulic circuit which allows for operation of more than one function at different pressures.

SUMMARY OF THE INVENTION In accordance with the present invention, a compensated multifunction hydraulic system for use with a source of hydraulic fluid is disclosed. It includes two system valves connected one with the other and each capable of performing a separate function. One of the system valves utilizes the necessary amount of hydrau lic fluid to perform its function in accordance with the full needs thereof. The remainder of hydraulic fluid is diverted to the other system valve. Structure is provided for permitting hydraulic fluid to flow from one system valve to the other system valve when the first system valve is in a stalled condition.

To be more specific, the one system valve may include a pressure compensator communicating with a control valve. A first signal line, upstream of the control valve, positions a compensator spool to enable the fluid to flow to the other system valve when the control valve has a sufficient amount of fluid to perform its function. A second signal line downstream of the control valve positions the compensator spool to deliver the necessary amount of hydraulic fluid to the control valve in accordance with the full needs thereof. The improvement comprises a relief in the second signal line for diverting fluid in the second signal line towards the pressure compensator to permit fluid to flow to the other system valve when the control valve is in a stalled condition.

For a better understanding of the present invention, together with other and further objects thereof, reference is made to the following description, taken in connection with the accompanying drawings, while its scope will be pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic illustration of a compensated multifunction hydraulic circuit;

FIG. 2 is a schematic illustration of a first embodiment of the present invention;

FIG. 3 is a schematic illustration of a second embodiment of the present invention;

FIG. 4 is a cross-sectional view of a relief valve used in the present invention;

FIG. 5 is a cross-sectional view of an actuator used in the present invention;

FIG. 6 is a cross-sectional view of a spool-type relief valve used in the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A compensated multifunction hydraulic system includes a source of hydraulic fluid, such as a suitable pump 10. Two system valves 12 and 12' are connected, one with the other, and each is capable of performing of separate work function. System valve 12 utilizes the necessary amount of hydraulic fluid from pump to perform its work function and diverts the remainder of hydraulic fluid to system valve 12. Structure including a relief valve 16 in a second signal line 38 permit fluid to flow from system valve 12 to system valve 12 when system valve 12 is in a stalled condition.

Referring to FIG. 1, there is shown the compensated multifunction hydraulic system disclosed in U.S. Application Ser. No. 392,901. This system controls a plurality of work functions simultaneously. A pump 10, preferably of the variable delivery type such as, for example, a Dynapower Model No. 45, manufactured by the New York Air Brake Co., is illustrated. The delivery of pump 10 is varied by a lever 20 which is actuated against its biasing spring 22 by movement ofa piston 24 located in a cylinder 26. Pump 10 is provided with a pump inlet line 28 leading to a tank T and a fluid output line 29 leading to system valves (also referred to in this disclosure as work function areas) 12 and 12.

In the embodiment shown,

work function areas

12 and 12 may perform separate and distinct functions such as, for example, function area 12 moves a bucket while function area 12' lifts a boom. In this embodiment, work functions area 12 is provided with a conventional control valve 30 and a conventional pressure compensator valve 31. Work function 12 is provided with similar components, and they have been similarily numbered with corresponding primed reference characters.

Compensator valve 31 is connected in series with valve 31 via line 32. Each of the

compensator valves

31 and 31 is connected to its

respective control valve

30 and 30 via

lines

33 and 33 respectively. In addition, the

compensator valves

31 and 31 are normally biased by

springs

34 and 34 to permit flow from

lines

29 and 32 to reach

control valves

30 and 30, through

lines

33 and 33, respectively. A first signal line 36, located upstream of control valve 30, allows fluid to bias a spool member (not shown) in compensator valve 31 in a direction to divert fluid to compensator valve 31' via line 32 when control valve 30 is in a neutral position (as shown in FIG. 1). A load drop check valve 40, located in line 33, prevents fluid from draining out of the actuator in the event that control valve 30 is in a position to deliver fluid to the actuator when pump 10 is not functioning. A second signal line 38 between control valve 30 and pressure compensator 31 allows fluid, downstream from control valve 30 to bias compensator valve 31 in the same direction as a compensator spring 34.

Signal line

36 and 38 sense the pressure drop across a variable orifice 42 in control valve 30 and adjust the flow from compensator valve 31 to control valve 30 so as to maintain a constant differential pressure across variable orifice 42.

Line 32 connects compensator valves 31 and 31' in series (that is to say, line 32 connects the areas). A flow line 44, downstream of compensator valve 31 leads to structure which senses the total requirements of the system, and adjusts the output flow of pump 10 in response thereto. This structure includes a restrictive orifice 46 and a control valve generally illustrated at 48. Valve 48 includes a spool 50 biased by a spring 52, pilot signal ports 54 and 56, a tank port 58, a control port 60, and a pressure port 62. Pilot signal port 56 allows fluid to bias spool 50 against its biasing spring 52 according to the pressure sensed via a line 64 from the upstream end of restricted orifice 46. Similarily, spool 50 is biased in the same direction as the force of spring 52 by a signal from line 66 which senses pressure at the downstream end of restricted orifice 46.

Pressure port 62 receives fluid from pump 10 via

lines

29 and 68. Control port 60 is connected to cylinder 26 via line 70 while tank port 58 is connected to tank T via a line 72. Lines 74 and 74' extend from

control valves

30 and 30 to tank T.

Valve 75, normally referred to as a pressure compensator control valve, limits the maximum pressure generated by pump 10. A pressure port 76 of valve 75 receives fluid from pump 10 via

lines

29 and 78. The pressure in port 76 opposes a spring 80 and may move a spool 83 to allow fluid to flow to cylinder 26 of pump 10 via control valve 48 and line 70.

This system can be more fully understood from the following description of a typical operation. With the system in an idle condition, i.e., the pump is working and no functions are operating, an operator may wish to use a work function such as raising a main boom of an excavator. For example, he may manually actuate control valve 30 to the degree necessary to perform the desired work function. This shift of control valve 30 allows pressurized fluid from pump 10 to pass through line 29 into pressure compensator 31. The spool member (not shown) in pressure compensator 31 is normally biased by spring 34 to allow all flow to pass into the control valve 30 through line 33. Depending upon the setting of variable orifice 42 (determined by position of control valve spool),

signal lines

36 and 38 adjust compensator valve 31 to permit only the required amount of flow to perform a work function, i.e., move a piston in an actuator a desired distance, into control valve 30. Flow returning from the actuator passes through line 74 and into tank T. The remainder of the flow provided by pump 10 is diverted through line 32 and into downstream system valve 12'. The operator may operate another work function using system valve 12 in the manner described for system valve 12.

In the event that more fluid is being provided by pump 10 than necessary, it flows to orifice 46 via .line 44 and causes a pressure differential. Then, the pressure upstream of orifice 46 is sensed by line 64 and transmitted to pilot signal port 56 to bias spool 50 against spring 52. Simultaneously, the pressure downstream of orifice 46 is sensed via line 66 and transmitted to pilot signal port 54 which compensates, to the extent of the pressure drop, for the biasing caused by the higher pressure in port 56. Spool 50 is now adjusted such that fluid from pump 10 is transmitted via

line

29, 68, and 70 into cylinder port 26. The flow moves piston 24 against the bias of spring 22 to adjust pump 10 so that it provides only that amount of fluid necessary to operate both work functions.

A problem may arise when system 12 is operating and performing a,work function, such as raising a main boom of an excavator, while valve system 12' is performing a different work function. A piston, reciprocating in an actuator, used in raising the boom, may reach the end of its stroke. In this case, the actuator can no longer receive hydraulic fluid from control valve 30. At this point, system valve 12 is in a stalled condition.

In the stalled condition, the differential pressure across variable orifice 42 becomes approximately zero and the pressure in signal line 36 approximately equals the pressure in signal line 38. The compensator spring 34 biases the spool member in pressure compensator 31 to send all of the flow to the work function being performed by system valve 12. However, since the actuator no longer requires fluid, pressure builds up in line 29 and passes via line 78 to pressure port 76 of valve 75. The increased pressure biases spool 82 against spring 80 to permit fluid from pump to flow through

lines

29, 78, and 70 and into cylinder 26. This fluid entering cylinder 26 causes piston 24 to move lever against the bias of spring 22 and thereby adjust pump 10 to stop pumping fluid. Thus, when system valve 12 is in a stalled condition, no fluid flows to system valve 12.

The present invention permits fluid to flow from system valve 12 to system valve 12 when the former system valve is in a stalled condition. Referring to FIG. 2, there is shown a first embodiment of the present invention. Except for second signal line 138, this embodiment is similar to the system shown in FIG. 1 and only the system valves have been illustrated. Like parts have received numeral like references. Second signal line 138 includes a control valve line 84 connected at one end to control valve 30 and at the other end to one end of a primary line 86. A compensator line 90 is connected at one end to pressure compensator 31 and at the other end to primary line 86 and to a relief valve line 92. Relief valve 16 is connected to the other end of relief valve line 92 and to an end of tank line 93. A flow restrictor 18 may be provided in primary line 86. A secondary line 88 is shown because it is required in a schematic representation of a pressure relief valve. However, line 88 does function in a spool type relief valve as shown in FIG. 6.

Referring to FIG. 4, relief valve 16 may include a housing 94 with a seat 96 in fluid communication with relief valve line 92. A spring 100 biases member 98 against seat 96 in opposition to fluid pressure from relief valve line 92. The ball 98 moves away from seat 96 to communicate relief valve line 92 with tank line 93 when flud pressure in line 92 increases to overcome the biasing force of spring 100.

The unique diverting features of this invention can be more fully understood by reference to a typical operation of this system when put into use. With the system valve 12 in a stalled condition as explained above, fluid pressure increases in signal line 138 to cause moveable member 98 to move against the bias of spring 100 so as to permit fluid from compensator line 90 to pass through reliefline 92 and relief valve 16 to tank line 93 and tank T. This reduction in pressure on the spring side of pressure compensator 31 permits the higher pressure in first signal line 36 to move the compensator spool to a position where it diverts flow to system valve 12 via line 32. Thus, even though the work function associated with system valve 12 is stopped, the work function being performed by system valve 12' is able to continue.

The flow restrictor 18 may be used to lower the amount of fluid passing from control valve line 84 to tank T. In this way, pump 10 is not required to supply as much fluid as otherwise needed and therefore pump 10 operates more efficiently and at a lower cost. The restrictor 18 may be of any required size such as, for example, 0.060 inches which passes approximately 1 gallon per minute at a pressure differential across flow restriction 18 of 100 psi.

A second embodiment of the present invention, as depicted in FIG. 3 is similar to the first embodiment except for details of second signal line 238. The second embodiment includes a relief valve 16 in second signal line 238 for diverting fluid in second signal line 238 towards pressure compensator 31. Hydraulic fluid can then flow from system valve 12 to system valve 12' when valve 12 is in a stalled condition.

Second signal line 238 includes a control valve line 104 connected at one end to control valve 30 and at the other end to a relief valve line 106 and to one end of a check line 108. A compensator line 110 is connected at one end to pressure compensator 31 and at the other end to control valve line 104. A relief valve 16 is connected to the other end of relief valve line 106 and to one end of an actuator line 112. An actuator 116 is connected to the other end of actuator line 112. Check line 108 includes a check valve 118 for permitting fluid to flow from actuator line 112 to control valve line 104. A secondary line 114 is shown because it is required in a schematic representation of a pressure relief valve. However, line 114 does function in a spool type relief valve of the type shown in FIG. 6.

Referring to FIG. 5 for the details of actuator 116, a cylinder 120 receives fluid from actuator line 112. This fluid causes piston 122 to reciprocate in cylinder 120. Since piston 122 is connected to the spool in compensator 31, it forces the spool against compensator spring 34.

In order to more fully understand the second embodiment of the invention depicted in FIG. 3, assume that system valve 12 is in a stalled condition. The pressure in control valve line 104 is transmitted via relief valve line 106 to move valve 98 of relief valve 16 against a biasing spring 100. This allows fluid to flow through

lines

104 and 106, across seat 96, into line 112 and finally to actuator 116. Actuator piston 122 engages the pressure compensator spool (not shown) and moves the spool against spring 34 to adjust pressure compensator 31 so that fluid-is diverted via line 32 to system valve 12'. Thus, even though system valve 12 is in a stalled condition, another downstream work function can still be performed.

To understand the importance of check valve 118, assume that piston 122 in actuator 116 has moved toward spring 34. When the work function associated with system valve 12 is resumed, the spool in compensator valve 31 must move toward the actuator 116. However, no fluid can pass through actuator line 112 since relief valve 16 is in a closed position. Therefore, check valve 118 which permits flow to control valve line 104 permits the actuator to drain and thus allow pressure compensator 31 to move in the direction of the force of spring 34.

The relief valve 16, as shown in FIG. 4 is only an example of many commercial devices that may be used in the present invention. For example, a spool valve, such as the one shown in FIG. 6, may be used. Spool valve 124 includes a cylinder 126. A spool 128 with two lands 129 is slidably received within cylinder 126. Spool spring 130, in one end of cylinder 126, biases spool 128 so that a fluid line 132 is connected to spool cylinder 126 so as to always be in communication with annular space 131 between lands 129. Line 134 is connected to the end of cylinder 126 containing spring 130. A pressure line 136 is connected to the other end of cylinder 126 so that fluid from line 136 biases spool 128 against spring 130 so as to permit fluid to flow from line 132 to line 134 via space 131. When used as a relief valve, such as at 16 in FIG. 2, line 132 is considered a relief valve line, line 136 is considered a secondary line, and line 134 is a drain to tank.

Although a variable delivery pump has been disclosed, a fixed displacement pump can easily by used in conjunction with the present invention. Further, the description has been limited to two work function areas for convenience only. It should be understood that an unlimited number of areas may be provided in series. The limitation on the number of work function areas is governed primarily by the practical limits placed upon pump 10 in that it usually provides the total requirements of all the functions being performed. Further, in some cases a work function area may consist of a conventional control valve without pressure compensation.

One skilled in the art will realize that there has been disclosed a compensated multifunction hydraulic sys' tem that efficiently operates a hydraulic machine, utilizes the flow of fluid to a stalled function for another downstream function, and allows operation of more than one work function at different pressures.

While there has been described what is at present considered a preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as followed in the true spirit and scope of the invention.

What is claimed is:

1. In a compensated multifunction hydraulic system for use with a source of hydraulic fluid including two system valve means being connected one with the other and each capable of performing a separate function, one of said system valve means including a pressure compensator communicating with a control valve, a first signal line means upstream of said control valve for moving a compensator spool in a first direction in said pressure compensator to enable said fluid to flow to the other of said system valve means, a second signal line means downstream of said control valve for moving said compensator spool in a second direction to deliver the necessary amount of hydraulic fluid to said control valve in accordance with the full needs thereof, the improvement comprising:

relief means in said second signal line means for directing fluid from said second signal line means toward said pressure compensator whereby said compensator spool moves in said first direction when said control valve is in a stalled condition.

2. The hydraulic system defined in claim 1, wherein said second signal line means includes a control valve line connected at one end to said control valve and at the other end to one end ofa relief valve line and to one end of a check line, a compensator line connected at one end to said pressure compensator and at the other end to said control valve line, said relief means connected to the other end of said relief valve line and to one end of an actuator line, and an actuator connected to the other end of said actuator line for engaging said compensator spool so that fluid pressure in said relief valve line actuates said relief means to permit fluid from said control valve line to flow from said relief valve line through said relief means to said actuator line to move said actuator and thereby move said c r sator spool.

3. The hydraulic system defined in claim 2, wherein said actuator comprises a cylinder for receiving fluid from said actuator line, an actuator piston in said cylinder engaging said pressure compensator wherein fluid from said actuator line reciprocates said actuator piston to move said compensator spool.

4. The hydraulic system defined in claim 3, wherein said check line includes a check valve for permitting fluid to flow from said actuator line to said control valve line when said actuator is drained.

5. The hydraulic system defined in claim 4, wherein said relief means comprises a relief valve which includes a housing with a seat in fluid communication with said relief valve line, a moveable member engaging said seat, spring means for biasing said moveable member against said seat in opposition to fluid pressure in said relief valve line, whereby said moveable member moves away from said seat when fluid pressure in said relief valve line increases to a predetermined amount because of the stalled condition of said control valve.

6. The hydraulic system defined in claim 5, wherein said other of said system valve means includes a second pressure compensator and a second control valve.

7. The hydraulic system defined in claim 2, wherein said relief means comprises a spool valve including a cylinder, a spool with two lands, a spool spring in one end of said spool cylinder, one end of a secondary line connected to said check line and the other end to said spool valve, said relief valve line connected to said spool cylinder so as to be in communication with an an nular space between said two lands, said actuator line connected to said one end of said spool cylinder, said secondary line connected to an other end of said spool cylinder wherein fluid from said secondary line biases said spool valve against said spool spring to permit fluid from said control valve line to flow through said relief valve line, through said spool valve and to said actuator line when said control valve is in the stalled condition.

8. The hydraulic system defined in claim 7, wherein said other of said system valve means includes a second pressure compensator and a second control valve.

9. The hydraulic system defined in claim 8, wherein said second signal line means comprises an actuator which includes a cylinder for receiving diverted fluid from said relief means, an actuator piston reciprocating in said cylinder and moving said compensator spool to enable said hydraulic fluid to flow to the other of said system valve means when said control valve is in a stalled condition.

in a stalled condition.

Claims

1. In a compensated multifunction hydraulic system for use with a source of hydraulic fluid including two system valve means being connected onE with the other and each capable of performing a separate function, one of said system valve means including a pressure compensator communicating with a control valve, a first signal line means upstream of said control valve for moving a compensator spool in a first direction in said pressure compensator to enable said fluid to flow to the other of said system valve means, a second signal line means downstream of said control valve for moving said compensator spool in a second direction to deliver the necessary amount of hydraulic fluid to said control valve in accordance with the full needs thereof, the improvement comprising: relief means in said second signal line means for directing fluid from said second signal line means toward said pressure compensator whereby said compensator spool moves in said first direction when said control valve is in a stalled condition.

2. The hydraulic system defined in claim 1, wherein said second signal line means includes a control valve line connected at one end to said control valve and at the other end to one end of a relief valve line and to one end of a check line, a compensator line connected at one end to said pressure compensator and at the other end to said control valve line, said relief means connected to the other end of said relief valve line and to one end of an actuator line, and an actuator connected to the other end of said actuator line for engaging said compensator spool so that fluid pressure in said relief valve line actuates said relief means to permit fluid from said control valve line to flow from said relief valve line through said relief means to said actuator line to move said actuator and thereby move said compensator spool.

7. The hydraulic system defined in claim 2, wherein said relief means comprises a spool valve including a cylinder, a spool with two lands, a spool spring in one end of said spool cylinder, one end of a secondary line connected to said check line and the other end to said spool valve, said relief valve line connected to said spool cylinder so as to be in communication with an annular space between said two lands, said actuator line connected to said one end of said spool cylinder, said secondary line connected to an other end of said spool cylinder wherein fluid from said secondary line biases said spool valve against said spool spring to permit fluid from said control valve line to flow through said relief valve line, through said spool valve and to said actuator line when said control valve is in the stalled condition.

10. The hydraulic system defined in claim 9, wherein said second signal line means further includes a check valve for permitting fluid to flow from said actuator to said control valve other than when said control valve is in a stalled condition.