United States Patent 3789739

A closed center remote control system selectively modulating pressure at either end of a reciprocal spool control valve so as to incrementally position the valve spool with such precision to attain spool metering and speed control. The various embodiments include diverse special operating modes such as float and automatic return-to-neutral. The remote system is equally useful in conventional closed center and open center spool valves essentially identical to manually operated valves.

Krehbiel, Robert D. (Hutchinson, KS)
Graber, Homer R. (Pretty Prairie, KS)
Application Number:
Publication Date:
Filing Date:
Primary Class:
Other Classes:
91/388, 91/461, 91/519, 91/530
International Classes:
F15B13/042; (IPC1-7): F15B13/16
Field of Search:
91/461,358A 137
View Patent Images:
US Patent References:
3400634Vent means for hydraulic circuit1968-09-10Wirtz
3340897Fluid control mechanism1967-09-12Nevulis
3220318Hydraulic system1965-11-30McGuire
3129645Electrically modulated fluid valve1964-04-21Olmsted
2974639Hydraulic bridge servo control system1961-03-14O'Connor et al.

Primary Examiner:
Maslousky, Paul E.
Attorney, Agent or Firm:
Brown Jr., Edward L.
1. In a hydraulic system having valve means controlling communication of a fluid source with a fluid motor and a reciprocal flow control spool in the valve means incrementally shiftable from a neutral flow blocking position towards a position of maximum flow so as to meter flow between said source and said motor to control speed of motor actuation, remote spool positioning means comprising:

2. The system of claim 1 wherein said remote control means includes a manually shiftable element variably and simultaneously restricting communication of said fluid chamber with said pressure source and a reservoir such that the pressure in said chamber varies in direct response

3. The hydraulic system of claim 2 further comprising a plurality of said valve means arranged in a bank and supplied by a common fluid source and common reservoir, each of said valve means controlling flow to separate fluid motors, and each associated with a remote spool control means incrementally positioning the associated spool by modulating pressure in its pressure chamber, said plurality of spool control means arranged in a second bank remote from said first bank whereby said remote pressure

4. The device of claim 2 wherein said shiftable element comprises a longitudinal spool manually shiftable within a bore of said remote spool control means, said bore being intercepted by spaced pressure, work and discharge passages, said spool having metering notches therein adapted to provide simultaneous communication of said work passage with the adjacent pressure and discharge passages, when said spool is shifted out of neutral, said communication incrementally variable with movement of said spool so as to modulate pressure in said work passage, and conduit means

5. The device of claim 1 further including means defining a second fluid chamber at the opposite end of said spool and wherein said remote valve means communicates with both said chambers and selectively modulates pressure in one of said chambers while connecting the other with low pressure to selectively actuate said control spool incrementally in

6. In a hydraulic system controlling communication of a fluid source with a fluid motor; the system having a primary control valve with a longitudinal main bore, a fluid pressure inlet passage and a main work passage intercepting the main bore at spaced apart locations, a main control spool in the bore incrementally movable from a neutral position blocking flow between the inlet and the main work passage to a position directing maximum flow to said work passage, the travel of said main spool away from neutral determining the rate of flow metered to said main work passage, a pressure chamber enclosing one end of said spool and a first spring means biasing the main spool to neutral against the urging of pressure in said chamber, whereby said main spool is incrementally positioned in relation to the magnitude of pressure in said chamber and a second spring means biasing the main spool against the pressure in said chamber only when the main spool has reached said maximum flow position; remote means for incrementally positioning said main spool, including:

7. The means according to claim 6 wherein said main spool is shiftable past said maximum flow position to another operative position in opposition to said second spring means, said remote means further including automatic pressure control means communicating with said output passage and limiting pressure therein to a predetermined value upon positioning said remote spool in said maximum flow position, said remote spool shiftable past said maximum flow position to override said automatic pressure control means and create pressure greater than said predetermined limit in said output passage to overcome the basis of said second spring means and move the

8. The means according to claim 7 wherein said pressure control means includes a pressure relief valve within said remote spool to control flow between first and second cross bores in said remote spool, said cross bores communicating respectively with said remote output passage and remote discharge passage upon positioning the remote spool to said maximum flow position whereby said relief valve limits output passage pressure to said predetermined value, said remote spool blocking flow through one of said holes to permit said remote output pressure to increase to that of the constant pressure source when said remote spool is shifted to said

9. The means according to claim 8 wherein said remote control means includes detent means engaging said remote spool in said maximum flow

10. The means according to claim 6 wherein said main port communicates with said fluid motor to actuate same and said remote control means further include feedback means operatively associated with said motor and said remote spool and adapted to shift said remote spool to neutral to cause said main spool to return to neutral when the motor reaches a

11. The improvement of claim 10 wherein said feedback means includes a piston in a fluid chamber in said remote valve body, said piston when actuated engaging said remote spool and urging same to the neutral

12. The improvement of claim 11 wherein said feedback means further includes an accumulator hydraulically connected with said piston chamber and actuated upon the motor reaching said predetermined position to displace fluid to actuate said piston and urge the remote spool to

13. In a hydraulic system comprising a fluid pump and reservoir communicating with first and second ends of a fluid motor to actuate same, control means interposed between the pump, reservoir and motor having a control element shiftable in opposite directions from a neutral position blocking flow from the pump to both motor ends, to positions directing maximum flow from the pump to said first and second motor ends, said element metering fluid to dictate rate of flow to the respective motor ends as a function of travel of the element away from neutral, means forming first and second pressure chambers at opposite ends of said element whereby pressure in said first and second pressure chambers urges said element in opposite directions to meter flow to said first and second motor ends, and first compressible spring biasing means engaging said element to urge same to neutral and compressing upon travel of said element away from neutral whereby greater element travel requires higher pressure at said pressure chambers, a second compressible spring biasing means engaging said element at positions of maximum flow and urging said element along with said first spring biasing means against any further movement of the element away from the neutral position;


This invention relates to hydraulic longitudinal spool control valves and more particularly to improved remote control systems precisely positioning the valve spool.

Spool type control valves have long found wide acceptance as the most efficient flow control device for hydraulic systems and have reached a highly developed state in reliability, performance and diversity of function. The control spool is incrementally movable to direct and meter flow to hydraulic motors to control direction and speed of motor movement. The ever-increasing power requirements of hydraulic systems limit the use of manually actuated control spools as high flow and high pressure create large forces on the spool making manual actuation difficult and impractical. Accordingly, remote slave actuating systems have been developed to operate the control spools, such systems including pilot valves hydraulically actuating the main spool valve to direct flow, and further devices for metering flow to determine motor speed. None of the prior art pilot systems known are capable of precise positioning of the control spool so that it alone controls both direction and speed of motor movement.


The invention contemplates a remote pilot control system which incrementally positions a spool valve to control and vary direction and speed of motor operation. The manually operated pilot valve modulates pressure exerted at either end of the spool which opposes gradient spring biasing means so as to incrementally position the spool in relation to the developed pilot actuating pressure. The pilot control system is of the closed center, variable flow type with a constant pressure fluid source. The pilot control member is incrementally movable in one direction to meter flow from the constant pressure source into a port connected to a fluid chamber at one end of the main control spool. At the same time, the pilot control member also meters a certain flow from the port to a low pressure drain to create a pressure in the control port and main spool chamber in relation to the position of the pilot control member. The pilot system thus modulates pressure in the main spool chamber in accordance with movement of the pilot control member. One or more spring means bias the main control spool in opposition to the hydraulic force of the pilot actuating pressure. The spring means are of the gradient type, exerting greater force as compressed, such that modulation of the actuating pressure incrementally positions the main control spool in proportion to original movement of the pilot control member. Selective movement of the pilot control member in a second, opposite direction modulates pressure exerted on the other end of the main spool to control spool travel in the opposite direction. A selectively actuable pressure relief valve in the pilot system can be used to position the main control spool to a special function position such a float operation. Automatic return of the pilot control member to neutral can be accomplished by a hydraulic piston selectively engaging the pilot control member.

It is a broad object of the invention to provide improved remote controls for hydraulic valves, particularly those of the longitudinal spool type.

It is a further object of the invention to provide a main spool subject to gradient force biasing means and a remote control system modulating pressure exerted on the spool in opposition to the biasing means so as to incrementally position the main spool.

Another objective in accordance with the preceding is a remote system having both manually actuated and automatic pressure control means for varying pressure at the main spool so as to effect a desired travel of the main spool.

With these broad objectives in view, further more specific objects and advantages of the present invention are set forth or will become apparent from the following detailed description and accompanying drawings of the preferred embodiments.


In the drawings:

FIG. 1 shows in partial cross-section and in part schematically a bank of main control valves and a remote hydraulic actuating system in accordance with the present invention;

FIG. 2 is a cross-section taken along lines 2--2 of FIG. 1 showing a remote pilot valve and the corresponding main spool valve, with portions of the circuitry shown schematically;

FIG. 3 is a partial right angle view of the remote control valve of fIG. 2 showing the spool 90 in a pressure modulating position;

FIG. 4 is a view similar to FIG. 2, taken along lines 4--4 of FIG. 1 showing a main valve and remote control valve with "float" positions, both valves being in a neutral position;

FIG. 5 is a view similar to FIG. 4 with both valves in the "raise" position;

FIG. 6 is a view similar to FIG. 4 with both valves in the "float" position;

FIG. 7 is a partially schematic view of a fluid control system with remote actuation and automatic return-to-neutral provisions, with the remote valve and main valve shown in cross-section taken along lines 7--7 of FIG. 1; and

FIG. 8 is a schematic representation of a closed center flow control system with the remote valve shown in full and the remaining valves shown in cross-section.


Throughout the specification and drawings, like reference numerals refer to corresponding or duplicate elements. Referring now specifically to FIG. 1, depicted is a bank 20 of longitudinal reciprocating spool control valves, whose general construction and operation are well known within the art. A bank 21 of remote control valves includes individual valve sections 22, 24 and 26 sandwiched between end plates 28 and 29. The work ports 30-35 of the remote valves respectively communicate through conduits 56 with control chamber ports 36-41 at opposite ends of the corresponding main valve sections 23, 25 and 27.

Each main valve includes interconnecting central zig-zag passages 42, 43 and 44, those of valve 27 shown in full with the remaining shown by dashed lines, which provide a free flow path in the neutral positions for fluid to flow from pump 45 and port 46 directly to return drain passage 47 in end plate 48. FIG. 1 also shows schematically the interconnected pressure feed ducts 57 of each valve which extend parallel to the zig-zag passages. Ducts 57 connect directly with pressure inlet port 46 of port plate 49 and are dead-headed at the other end plate 48. The individual valves 23, 25, 27 also have interconnected drain passages 50 and 51, and the communication of passage 47 with ducts 50 and 51 carries fluid to the drain port 52 in port plate 49. Each main valve has a pair of work ports 53, 54 which connect with opposite ends of hydraulic motors not shown in FIG. 1. A high pressure relief valve 55 in port plate 49 limits the main valve inlet pressure.

The remote control system includes a constant pressure source of fluid to inlet port 60 of port plate 28 which connects with the communicating pressure ducts 61 of each pilot valve section. The source illustrated is a constant displacement pump 62 limited in pressure, relatively low in comparison to that which can be developed by main pump 45, by a relief valve 63 which normally diverts pressure fluid from inlet 60 to outlet port 64. The remote system is closed center, i.e., end plate 29 blocks pressure ducts 61 so that they continually carry the pressure determined by valve 63. Passage 65 interconnects the through drain passage 66, 67 of each remote valve.

FIG. 2 clearly illustrates the detailed construction of main control valve 23 and its associated pilot actuating valve 22. Main valve 23 has a control spool 70 reciprocal within longitudinal bore 71 that is intercepted by the drain passages 50, 51, the work port passages 53 and 54 and the neutral zig-zag passages 42, 43, 44. The work ports connect through conduits with opposite ends of a working cylinder motor 72. The valve also has pressure feed branch passages 73, 74 intersecting the spool bore adjacent the work ports and whose communication with pressure feed duct 57 is controlled by one-way lift check valve 76 as well known to the art. Spool 70 has several spaced flow control grooves 77-80 of which grooves 78, 79 interconnect the zig-zag grooves 42-44 in the neutral position shown.

The opposite ends of spool 70 extend into interior fluid chambers 81, 82 of the caps 83, 84 affixed to the valve housing. Bolts 88 are secured to the spool ends so that springs 85, 86 act through cup-shaped washers 87 to bias the spool. In the neutral position, only left spring 85 is acting on the spool to hold it in neutral position since right end washer 87 does not engage the right end of the spool in neutral. Control ports 36, 37 respectively connect chambers 81, 82 with remote valve ports 30 and 31.

The small pilot valve 22 similarly has a longitudinal control spool 90 reciprocal within a bore 91 intercepted by port passages 30, 31, pressure passage 61 and drain ducts 66, 67. The right end of the spool extends outside the pilot valve body to facilitate manual actuation, and the other end incorporates a conventional spring centering assembly 92 which lightly resists the manual actuation so as to return spool 90 to the neutral position upon release of the spool. The spool also has metering notches 93 and 94, clearly shown in FIG. 3, respectively controlling pressure flow from inlet 61 to the outlet ports and flow from the outlet ports to the drain passage. For instance, FIG. 3 shows spool 90 moved slightly inwardly so as to meter pressure flow into port 31 and simultaneously meter a predetermined flow out of port 31 to drain 67. Gradual leftward movement of spool 90 uncovers a greater portion of inlet notch 93 while reducing the portion of notch 94 open to drain 67. Pressure in port 31 is accordingly a function of spool movement, continually increasing with farther leftward spool travel. The other outlet port 30 is similarly pressure modulated with spool travel right of neutral.

In operation of control valve 23, travel of spool 70 away from the neutral position blocks communication of central passages 43, 44 with duct 42 so that pressure from pump 45 builds in parallel feeder duct 57 to life check valve 76 and deliver motive fluid to branch passages 73, 74. When spool 70 shifts leftwardly, pressure fluid from branch 74 passes through groove 80 to work port passage 54 and drives the cylinder motor 72 leftwardly. Spool 70 meters pressure flow at the bore between passages 74 and 54 to control motor speed, with further leftward spool travel permitting greater flow and faster motor actuation. Fluid displaced from the motor 72 returns to port 53 to be metered by spool 70 as it flows to drain passage 50 via spool groove 77. Similar operation occurs with rightward spool travel to meter flow from branch 73 to port 53 to dictate rightward movement and speed of motor 72. The foregoing flow control operation of valve 23 is well known within the art and further discussion is not believed necessary.

Main spool 70 moves in response to the balance between the hydraulic forces of control chambers 81, 82 at opposite ends of the spool and the mechanical bias forces exerted by spool springs 85, 86. Spring 85 biases spool 70 to the neutral position when both control chambers 81, 82 communicate, as shown, with low pressure reservoir through conduits 56 and remote spool return passages 66, 67 via ducts 30, 31 and spool bore 91. Upon movement of main spool 70 by introduction of pressure into one of the actuating chambers, spring 85 begins to compress and exerts greater force on the spool in opposition to the pressure force. Sufficient movement of spool 70 picks up spring 86 so that it complements spring 85 in opposing the pressure. For a predetermined pressure in one of the control chambers 81, 82, the spool travels until the springs 85, 86 compress sufficiently to balance the pressure, the spool remaining in such position until the actuating control chamber pressure changes.

With a constant pressure source, pilot valve 22 can reliably modulate pressure in the control chamber 81, 82 so that movement of pilot spool 90 to a predetermined position will repeatedly produce substantially the same pressure in control chamber 81 or 82. Thus, the leftward movement of spool 90 shown in FIG. 3 will create a certain pressure in port 31 and connected end chamber 82, the other end chamber 81 then being at low pressure as a result of being drained through interconnected pilot passages 30 and 66. In response to the pressure magnitude in chamber 82, main spool 70 moves a certain distance and meters a specified rate of flow to port 54 and the motor. The farther leftwardly pilot spool 90 moves, the greater is the pressure in chamber 82 to move main spool 70 farther leftwardly and meter more flow to the motor. It will be apparent that this remote actuating system accurately duplicates manual shifting of main spool 70 so that motor operation can be reliably controlled.

The use of a remote actuating valve 22 permits the location of main control valve 23 closer to the motors. The high pressure circuitry from pump 45 to the motors can be reduced, minimizing the lengths of the pressure carrying conduits and lines. Greater travel of spool 70 is available, and higher forces on spool 70 will cause no problems as the spool is not manually operated. The availability of longer spool travel and higher spool forces permit, of course, the main valve to be larger and to a certain extent makes the valve simpler and more economical to develop as these problems are eliminated as factors in the valve design.

The remote valve 22 may be mounted near the operator for convenience. This pilot valve is miniature in comparison to the main valve, with its short travel of spool 90 and light force of centering spring 92 permitting finger-tip control. The remote valve uses little power, requiring low pressure and flow from its source 62. It may utilize the same or separate fluid reservoir as the main control valve system, whichever is desired. By being closed center and modulating pressure in the control chambers 81, 82, the remote valve produces accurate spool travel of the main valve to provide excellent metering characteristics. Another important advantage derived from the present system is that it requires no substantial alteration in known spool valve design nor does it require extra elements in the system, but rather uses the main valve to control direction and velocity of motor operation. The remote control system of the present invention, due to pressure modulation, also oversteps the various problems of fluid leakage, proper phasing, etc., inherent to other types of remote servo and power assist systems.

FIGS. 4 and 6 show a remote valve 24 and main control valve 25 which have an additional operating position, a "float" position which simultaneously connects both ends of the cylinder motor with drain so that the motor moves freely or floats in response to external forces. The detailed constructions of pilot valve 24 and main valve 25 are quite similar to those of FIG. 2 as shown by like numerals for similar elements. In addition, the pilot spool 100 is provided with an interior ball-type relief valve 102 that controls fluid flow between spool crossbores 103, 104. The spool also has different type of metering means: slightly reduced diametral portions 105 on either side of pressure ducts 61 and similar sections 106 adjacent each drain duct 66, 67. The spool 110 of main valve 25 has additional control grooves 112, 113, 114 and an additional land 115 in the middle of groove 77 defining groove 80. A third spring 116 also biases spool 110, acting between flange 117 fixed to the spool and a washer 118 movable relative to the spool. The spool left end is covered by a cap 83 like that in FIG. 2 while a slightly longer cap 119 accommodating the additional spring 116 covers the right end.

Operation of valves 24, 25 is as described with reference to FIG. 1 upon left and right movement of pilot spool 100 to the "lower" and "raise" positions actuating motor 120 in opposite directions. Displacing pilot spool 100 away from neutral meters communication of one of ports 32, 33 with pressure and drain at sections 105, 106 so as to modulate pressure in the communicating control chambers 121, 122 and accurately position the spool 110 against the bias of springs 85, 86.

FIG. 5 shows remote spool 100 shifted to the maximum velocity "raise" position wherein main spool 110 is positioned to permit maximum flow rate from pressure branch passage 73 to port 53 and the motor. Pressure developed in remote valve port 32 and connected control chamber 121 is now determined by internal relief valve 102 instead of flow metering by sections 105, 106. Spool 100 is so positioned so as to block communication of port 32 with drain 66 through spool bore 101. Flow from port 32 now passes through crossbore 103 to open relief 102 and drain through bore 104 to passage 66. The predetermined opening pressure of relief 102 is less than that of maximum relief 63, but is sufficient to move main spool 110 until washer 118 engages cap 119. The left end of pilot spool 100 has an attached extension 107 with an enlarged lip 108 that engages lightly spring loaded pawls 109 as the maximum speed position is reached. The pawls exert a slight resistance to spool movement when so engaged to give the operator "feel" and signal that the maximum velocity position has been reached.

In FIG. 6 the pilot spool 100 has been displaced farther rightwardly, forcing the pawls 109 to ride over lip 108, past the maximum raise position to the float position. Spool crossbore 104 is now blocked at bore 101 so that pressure builds in port 32 and chamber 121 to the maximum permitted by pilot relief 63 as internal relief 102 is now inoperative. The pressure in chamber 121 is now sufficient to overcome the combined bias of all three springs 85, 86 and 116. Main spool 110 travels to its extreme rightward position abutting the end cover 119. Land 115 on the spool blocks flow between branch 73 and port 53 to isolate the motor 120 from the actuating inlet pressure flow. Spool grooves 112, 113 connect the zig-zag passages 42-44 to re-establish the neutral bypass flow. Port 54 remains connected with drain duct 51 through groove 80, and the other port communicates through groove 114 with drain 50 so that both cylinder ends connects with low pressure reservoir. The piston and rod of motor 120 now move freely in response to any external force imposed therein.

FIG. 7 further illustrates the adaptability of the present remote actuating system for hydraulic controls. The main control valve 27, not shown in cross-section, is similar to valve 23 of FIG. 2 both in structure and operation. Remote valve 26 operates as valve 22 of FIG. 2 and has an additional detent and automatic centering mechanism 125. The main valve work ports 53, 54 connect with opposite ends of a cylinder motor 126 having position sensing means connected by conduits 127, 128 to the centering mechanism 125 on remote valve 26.

The centering device 125 includes a pair of pistons 129 and 130 slidable within bores 131, 132 and biased left-wardly by their respective springs 133, 134. The chamber 135 formed between pistons 129, 130 is vented to atmospheric pressure through breather 136. The left end of remote spool 137 has an attached rod 138 that extends through piston 129. A nut flange 139 on the rod engages piston 129, and washer 140 abuts a shoulder of the spool so that the bias of spring 133 urges spool 137 to the neutral position. In such position the inner ends of pawl detent 141 are intermediate grooves 142, 143. Upon moving spool 137 to its maximum left or right positions the pawls 141 drops into groove 142 or 143 respectively to hold the spool in such position in spite of the bias of spring 133.

Fluid passes from fluid chambers 144, 145 through conduits 127, 128 to ports 146, 147 to actuate pistons 129, 130 respectively. Depression of piston 148, 149 displaces the actuating fluid from chambers 144, 145 to the ports 146, 147. The motor cylinder rod 150 carries actuating cams 151, 152 engaging the cam follower pistons 148, 149.

In operation, spool 137 may be manually moved left-wardly to the maximum speed position where pawls 141 fall into groove 142 so that, unattended, the spool remains in such position. The left end of spool rod 138 is now near piston 130 in a position to be actuated by that piston. Operating as in FIG. 2, valve 26 modulates pressure at main valve control port 41 so that maximum flow rate from work port 54 passes to the motor 126 to drive same leftwardly. As it nears the end of its stroke, the cylinder cam 152 depresses piston 149 to displace fluid to port 147. Piston 130 moves rightwardly, forces the pawls out of groove 142 and returns spool 137 to the neutral position, cutting off flow from the main valve to the cylinder 126 and bringing it to rest. The operation is similar when spool 137 travels rightwardly to actuate the motor 126 rightwardly. The pilot spool detents in groove 143 and automatically returns to neutral upon depression of piston 148 by cam 151 to displace fluid to port 146 and force piston 129 and the spool back leftwardly.

Contrasted to the previous embodiments, FIG. 8 illustrates a closed center main flow control system having a main valve 160 remotely actuated by the same pilot valve 22 fully described above. The closed center system includes a variable volume, constant pressure fluid source 163 from which the remote valve 22 also derives its motive fluid. The pressure reducer valve 161 and flow control valve 162 regulate the input flow from pump 163 to valve 22.

Valve 160 may be considered generally as a standard closed center design to which is added caps 164 covering the protruding spool ends to form the control chambers 165, 166 in which pressure is modulated by pilot valve 22 to actuate the main valve. The pressure compensated pump 163 provides a variable delivery, constant high pressure flow, for instance 2500 psi, to the inlet passage 167 that is blocked by spool 168 in the neutral position. The reciprocating spool has an interior spool bore containing movable check valves 169, 170 that each regulate flow between their respective cross-bores 171-174. Upon rightward spool travel, cross-bore 172 directs the inlet flow from 167 to open and cross check valve 169 and through cross-bore 171 that is communicating with port 175 in order to drive motor 178 rightwardly. In such position the cross-bore 173 connects with port 176 and cross-bore 174 with the right drain passage 177. The fluid displaced from motor 178 upon its rightward travel flows to port 176, through bore 173, to open lift check 170 and continue to drain 177. Cross-bore 172 opens more to inlet 168 as the spool travels farther rightwardly to meter flow and create motor speed control dependent upon the position of spool 168. Similar operation occurs upon leftward spool travel, cross-bores 173, 174 then metering motive flow to port 176 and the motor.

As noted before, the pilot valve 22 is a low power system requiring low pressure and low flow. To utilize a single source 163 for both primary working flow to main valve 160 as well as a pressure source for the remote pilot system, pressure reducer 161 limits pressure in downstream conduits 180, 181 to the maximum required for pilot valve 22, e.g., in this case one-tenth the output pressure of pump 163 in line 179. Flow control 162 limits the rate of flow in remote valve inlet conduit 181 to an acceptable level assuring no impairment of pilot operation. The pilot valve 22 operation is as discussed with respect to FIG. 2, variable and precisely positioning the main spool 168 by modulating pressure in control chambers 165, 166.

The pressure reducer 161 comprises a spool 182 shiftable in bore 183 to variably restrict flow from inlet 184 to outlet 185 at the opening forward between shoulder 186 and spool tapered face 187. Spring 188 biases spool 182 to the full open position shown and is preset in tension to hold the pressure desired in outlet 185 and line 180. Pressure in outlet 185 acts on the left end of spool 182 to oppose spring 188. Excessive pressure in the outlet will drive the spool rightwardly to restrict flow more severely at tapered face 187 to reduce outlet 185 pressure to the desired level. Similarly, spring 188 forces the spool oppositely when outlet 185 pressure is too low, to permit greater flow and increase the outlet pressure. Spring 189 mitigates the gradient effects of the spring 188 which would otherwise require greater balancing force, i.e., higher-than-desired outlet pressure, when in a more compressed position.

This reduced pressure fluid flow enters valve 162 at passage 190 to pass through cross-bores 191, orifice opening 192 and cross-bores 193 of movable spool 194. A constant pressure differential across orifice 192 dictates a constant flow therethrough. Pressure upstream of orifice 192 biases spool 194 to the right in opposition to the combined forces of spring 195 and the downstream outlet pressure communicated to the spring-containing chamber 196 through damping orifice 197. The spool forces will thereby be balanced when the pressure drop across orifice 192 is equivalent to the force exerted by spring 195. The spool moves so as to maintain this force balance, holds a maximum pressure drop across orifce 192 and limits flow in conduit 181 to the required level. The spool shifts to the right in response to a high pressure drop across orifice 192 causes by excess flow, variably closing parts of cross bores 191 to reduce flow rate. The spool moves to the left when flow and the orifice pressure drop are low to open more of cross-bores 191 to passage 190 and increase flow to the required level. From outlet 198 the delivery of pressure and flow regulated fluid to conduit 181 and pilot valve 22. Of course, when the pilot system requires no flow, spool 194 stays in its far left position shown by the urgings of spring 195.