US3908519A

US3908519A - Control systems for a variable displacement pump

Info

Publication number: US3908519A
Application number: US515270A
Authority: US
Inventors: Ellis H Born; Alan H Viles; Cecil E Adams; David L Thurston
Original assignee: Abex Corp
Current assignee: Hagglunds Denison Corp
Priority date: 1974-10-16
Filing date: 1974-10-16
Publication date: 1975-09-30
Anticipated expiration: 1992-09-30
Also published as: DE2545362A1; NL178807C; CH617248A5; JPS6127592B2; SE7511568L; SE407101B; MX147117A; NL178807B; AU8478675A; DE2545362C2; NL7510658A; AU472949B2; SU736884A3; GB1522027A; FR2288236A1; IT1047520B; IN142703B; FR2288236B1; ES441550A1; BR7506777A

Abstract

An automatic control system for an across-center axial piston pump which destrokes the pump when fluid pressure or flow exceeds a predetermined maximum and returns the pump to its previous setting when fluid pressure or flow falls below the maximum.

Description

United States Patent Born et a]. Sept. 30, 1975 [54] CONTROL SYSTEMS FOR A VARIABLE 3383,85? 5/1968 Rajchel 60/469 DISPLACEMENT FUN") 3.396.536 8/[968 Miller El ill. 60/464 3.739.974 6/1973 Kiwulle et 11].... 417/222 lnvemorsi Ellis Born: Alan Viles: il 3.788.773 1/1974 Van der Kolk 417 213 E. Adams, all of Columbus; David L. Thurston, Ashley. all of Ohio 7 Primary Examiner-William L. Freeh l73l Asslgnee: Abe) Corporauon New York Assistant E.\'aminerGregory LaPointe 22 u Oct 1 974 Armrney. Agem, ur Firm-Thomas S. Baker, Jr.; David A. Grccnlee [2]] Appl. No.: 515,270

U.S.C|. r 4l7/2l7, 4l7/22Z; 60/452; 60/464 [5| l F013 5/24; FOlB 3/00? F043 H26 An automatic control system forpn acr0ss center axial [58] Field Of Search 4l7/Zl3, 1l7 Zlli. 222; piston pump which destrokes1he pump h fluid l3-ll 452 pressure or flow exceeds a predetermined maximum and returns the pump to its previous setting when fluid l l References Cited pressure or flow falls below the maximum.

UNITED STATES PATENTS 3.257959 6/l966 Budzich 4|7/2l7 19 Claims, 5 Drawing Figures US. Patent Sept. 30,1975 Sheet 1 of4 US. Patent Sepfi. 3&1975 Sheet 2 of4 3,908,519

US. Patent Sept. 30,1975 Sheet 3 of4 3,908,519

CONTROL SYSTEMS FOR A VARIABLE DISPLACEMENT PUMP BACKGROUND OF THE INVENTION 1. Field of the Invention The instant invention relates generally to variable displacement axial piston pumps and more specifically to an automatic control system for such a pump.

2. Description of the Prior Art Automatic control systems for axial piston pumps are well known. In such systems, the pumping mechanism is commonly flow-compensated, pressure-compensated or both. In a flow-compensated system, a control responsive to a change in the pressure drop across an orifree in the discharge line of the pump operates a mechanism which changes the displacement of the pump until a predetermined pressure drop is reached. In a pressure-compensated control system, a control responsive to excessive discharge pressure operates a mechanism to reduce the pump displacement until the pressure reaches a set amount. It is common for a pump to incorporate both flow-compensating and pressurecompensating devices.

A disadvantage of many prior art automatic control devices is their slow response when outlet pressure or flow exceeds a predetermined amount. If a control system does not respond rapidly, momentary high pressure peaks may occur which are harmful to the components in the system.

A further disadvantage of many prior automatic control systems is that these systems are quite complex when utilized in across-center axial piston pumps. Some automatic control systems will not operate in across-center pumps.

A still further disadvantage of prior automatic control systems is that they may have no provision for operating the pump displacement control mechanism to return the pump displacement to a previous setting after the flow-/or pressure-compensator control has operated to reduce the pump displacement. Most control systems simply use a spring to bias the pump control mechanism to a maximum displacement position.

SUMMARY OF THE INVENTION The instant invention provides an automatic control system for an across-center variable displacement axial piston pump which is highly responsive and which elirn inates high pressure peaks in the system.

In the instant invention a variable displacement axial piston pump includes a manual displacement control device for positioning the thrust plate assembly and an automatic displacement control device which overrides the manual displacement control and reduces pump displacement when the discharge fluid exceeds a predetermined maximum pressure or flow-rate. When the pressure or flow-rate falls below the predetermined maximum, the manual displacement control device repositions the thrust plate assembly to the position pre viously set by the operator.

DESCRIPTION OF THE DRAWINGS FIG. 1 is an axial sectional view of an axial piston pump and an automatic control valve block according to the instant invention.

FIG. 2 is an enlarged fragmentary view of a manual control mechanism for the pump showing the fluid motor which operates to change pump displacement.

FIG. 3 is an exploded view of the manual control system shown in FIG. 2.

FIG. 4 is a sectional view of the valve block for the automatic control and a schematic diagram of the hydraulic circuitry for the automatic and manual control systems.

FIG. 5 shows a hydrostatic transmission according to this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIGS. 1 and 2, an axial piston pump has a case 11 which includes a central housing 12, an end cap 13 at one end and a port cap 14 at the other end all fastened together by bolts 15.

Case 11 has a cavity 16 which receives a rotatable cylinder barrel 17 mounted on rollers 18 of a bearing 19 which has its outer race 20 pressed against a housing shoulder 21. A drive shaft 22 passes through a bore 23 in end cap 13 and is rotatably supported in a bearing 24. The inner splined end 25 of drive shaft 22 drivingly engages a splined central bore 26 in barrel 17.

Barrel 17 has a plurality of parallel bores 27 equally spaced circumferentially about its rotational axis. A sleeve 28 in each bore 27 receives a piston 29. Each piston 29 has a ball-shaped head 30 which is received in a socket 31 of a shoe 32.

Shoes 32 are retained against a flat creep or thrust plate 33 mounted on a movable rocker cam 34 by a shoe retainer assembly 35. Assembly 35 includes a shoe retainer plate 36, with a number of equally spaced bores equal to the number of pistons 29, which passes over the body of each piston and engages a shoulder 37 on each shoe 32. Shoe retainer plate 36 has a central bore 38 which passes over a post 39 affixed to rocker cam 34 by a snap ring 40. A spacer 41 is interposed between shoe retainer plate 36 and a snap ring 42 which secures shoe retainer plate 36 on post 39 and prevents shoes 32 from lifting off thrust plate 33.

Each cylinder bore 27 ends in a cylinder port 43 which conducts fluid between a port plate 44 and bore 27. Port plate 44 is positioned between barrel 17 and port cap 14. A pair of kidney-shaped openings, 45, 46, illustrated in FIG. 4, are formed in plate 44. These openings communicate with ports P P in port cap 14. One of the ports contains low pressure fluid and is the intake port while the other port contains high pressure or working fluid and is the exhaust port, depending upon the operating conditions of the pump.

Rotation of drive shaft 22 by a prime mover, not shown, will rotate cylinder barrel 17. If thrust plate 33 is inclined from a neutral position normal to the axis of shaft 22, pistons 29 will reciprocate as shoes 32 slide over plate 33. As the pistons 29 move away from port plate 44, low pressure fluid is received in cylinder bores 27. As the pistons 29 move toward port plate 44, they expel working fluid into the exhaust port.

A stepped shaft is drivingly connected to splined bore 26 of barrel 17 by a splined end 51. Shaft 50 rotates with barrel l7 and drives a fixed displacement servo pump 52 which is keyed to shaft 50. Shaft 50 is rotatably supported by a needle bearing 54 mounted in a port cap bore 55 and by a needle bearing 56 mounted in a bore 57 of a clamp member 58.

Shaft 50 is retained axially in barrel 17 by a snap ring 59, an axially split collar 60 and a spring 61 which biases barrel 17 and port plate 44 against port cap 14. The maximum lift-off distance of barrel 17 from port plate 44 during operation of the pump is the distance between an end face 62 of collar 60 and a facing shoulder 63 on splined end 51. This distance can be varied by adjustment of a nut 64 on the threaded other end 65 of shaft 50.

A spacer 47 is inserted between nut 64 and a thrust bearing 48 which engages clamp member 58. Clamp member 58 engages an outer housing 49 of the servo pump 52 which is a rotor type having a gear rotating within a gear, commonly referred to as a gerotor pump. Pump 52 provides servo pressure to operate a manual pump control mechanism.

Referring to FIGS. 2 and 3, the manual pump displacement control mechanism will next be described. The mechanism on each side of rocker cam 34 is substantially the same. Thus, the description will refer to the left side shown in FIGS. 2 and 3 and identical elements on the right side of rocker cam 34 will be indicated by identical primed numbers. Any differences in structure will be explained.

Rocker cam 34 has an arcuate bearing surface 67 which is received in a complementary surface 68 formed on a rocker cam support 69 mounted in end cap 13. Rocker cam 34 pivots about a fixed axis perpendicular to the axis of rotation of barrel 17. Cam 34, which carries thrust plate 33, is moved on cam support 69 by a fluid motor to change pump displacement.

The fluid motor includes a vane or motor member 70 formed integrally on the side of the rocker cam 34 and is thus movable therewith. Vane 70 extends beyong bearing surface 67 to overlie side 71 of rocker cam support 69 so that the center of vane 70 is at surface 67. Vane 70 has a central slot 72 which receives a seal assembly 73.

A vane housing 74 is located on cam support 69 by dowel pins 75 and is attached to support 69 by bolts 76. One half of vane housing 74 overlies rocker cam 34 so that vane 70 is received in an arcuate chamber 77 which is closed by a cover 78 that is secured by bolts 76 to housing 74. As thus assembled vane 70 and its seal 73 divide chamber 77 into a pair of expansible

fluid chambers

80, 81, shown in FIG. 2, to form a fluid motor.

An elastomeric seal 82 fits in a groove 83 on an inner surface 84 of vane housing 74 which abuts rocker cam 34 as best seen in FIG. 3. This provides a dynamic seal for the fluid motor to prevent leakage when rocker cam 34 is pivoted.

Fluid chambers

80, 81 in the fluid motor on one side of rocker cam 34 are connected by

passages

85, 86 to fluid chambers in an identical fluid motor located on the other side of rocker cam 34. Consequently, both motors are operated simultaneously by supplying pressurized fluid to one of the

chambers

80, 81 and exhausting fluid from the other chamber to move vane 70 within chamber 77.

The operation of the fluid motor is controlled by a servo or follow-up control valve mechanism 87 which regulates the supply of pressurized fluid and which includes a fluid receiving valve assembly comprising a valve plate 88 and a stem 89 which are mounted on rocker cam 34 by double threaded bolts 90. The fluid receiving valve assembly and vane 70 move along concentric arcuate paths when rocker cam 34 is moved. Bolts 91, with heads 92 projecting above valve plate 88', mount the valve plate 88' and stem 89' on the right side of rocker cam 34 and function as described hereinafter.

Valve plate 88 has a pair of

ports

97, 98 which are connected to

respective fluid chambers

80, 81 in the fluid motor through a pair of passageways 99, 100 (shown schematically in FIG. 4). Passageway 99 includes serially connected bore 101 in stem 89, bore 102 in rocker cam 34, a drilled opening, not shown, in rocker cam 34, and bore 103 in vane which opens into fluid chamber 80. Similarly, passageway includes serially connected bore 104in stem 89, bore 105 in rocker cam 34, a drilled opening, not shown, in rocker cam 34, and bore 106 in vane 70 which opens into fluid chamber 81.

For counterclockwise operation of the fluid motor, as viewed in FIGS. 2 and 4, pressure fluid is supplied to port 97 and flows through passageway 99 into chamber 80 to move vane 70 and rocker cam 34 counterclockwise. Expansion of chamber 80 causes chamber 81 to contract and exhaust fluid through passageway 100 out of port 98 and into the pump casing.

For clockwise operation of the fluid motor, the fluid flow is reversed. Pressure fluid is supplied to port 98 and expands chamber 81 to move vane 70 and rocker cam 34 clockwise. Chamber 80 contracts and fluid exhausts through passageway 99 out of port 97 and into the pump casing.

As seen schematically in FIG. 4, check valves 107, I08 and parallel fluid restricting orifices 109, are located in

passageways

99, 100. This arrangement permits a high fluid flow into the expanding chamber, but restricts the exhaust flow from the contracting chamber to limit the rate of movement of fluid motor vane 70. The

check valves

107, 108 and

orifices

109, 110 are of any conventional construction and may be physically positioned in stem 89.

Referring to FIGS. 24, that portion of the follow-up control valve mechanism which selectively supplies fluid to the

ports

97, 98 in valve plate 88 will now be described. A control handle 11 1 is attached to an input shaft 112 which is mounted in a cover plate 114. Cover plate 114 is attached to housing 12 by bolts and includes a fulid port, not shown, which receives servo fluid from servo pump 52. An arm 118 is fastened to the inner end of shaft 112 and moves on a roller bearing 119 sandwiched between arm 118 and cover plate 114.

An input valve assembly includes a pair of

identical valve shoes

121, 122 which are received in a bore 123 in arm 118. Arm 118 pivots about the same axis as valve plate 88. When arm 118 moves, shoe 121 rides on a flat inner surface 124 of cover plate 1 14 and shoe 122 rides on a flat surface 125 of valve plate 88. Each

shoe

121, 122 is continuously fed servo fluid from the cover plate port through a central fluid receiving bore 126 to a rectangular cavity 134 in its bottom. The length of cavity 134 is equal to the distance between

ports

97, 98 and cavity 134 moves along the same are as

ports

97, 98.

O-

rings

127, 128 seated on

shoulders

132, 133 of

respective shoes

121, 122 prevent fluid leakage out of bore 123 and radially position shoes 121, 122 in bore 123 when under pressure. A pair of

flat washers

129, 130 are urged apart by a spring washer 131 to bias 0-

rings

127, 128 against

shoulders

132, 133 and

shoes

121, 122 against

flat surfaces

124, 125.

Manual operation of the fluid motors by control handle 111 will now be described. When the fluid motors are at rest, cavity 134 in valve shoe 122 is between

valve plate ports

97, 98 which are covered by flats 136, l37 on valve shoe 122, as best seen in FIG. 2. To change the displacement of the pump, control handle 111 is moved in the direction rocker cam 34 is to pivot. Movement of handle 111 clockwise, as viewed from the left in FIG. 2, moves shoe 122 clockwise and aligns cavity 134 with port 98, while uncovering port 97. Servo fluid flows from cavity 134 into port 98, through the passageway 100, and into chamber 81. Simultaneously, fluid exhausts from chamber 80 through passageway 99 and out uncovered port 97 and rocker cam 34 pivots clockwise as described above. Rocker cam 34 is pivoted counterclockwise in a similar manner if handle 111 is moved counterclockwise to align cavity 134 with port 97.

Accurate follow-up is provided since angular movement of rocker cam 34 and valve plate 88 is equal to that of control handle 111. When rocker cam 34 and valve plate 88 have moved through the same angle as control handle 111, cavity 134 is centered between

ports

97, 98,

flats

136, 137 on shoe 122

cover ports

97, 98 and the fluid motors stop.

The mechanism on the right side of rocker cam 34 shown in FIG. 3 has a pointer 140 in place of control handle 111 on the left side. Bolt heads 92 which secure valve plate 88' and stem 89 to rocker cam 34 capture arm 118' and force it to move when cam 34 is moved. This moves pointer 140 to indicate the exact angular position of rocker cam 34.

An automatic control system which supplements the manual control system will now be described with reference to FIG. 4. A brief description of the main components of the automatic control system will be presented followed by a detailed description.

The main components of the automatic control system are mounted in a control housing 199 mounted on port cap 14. As previously mentioned. a servo pump 52 provides servo pressure fluid to follow-up valve mecha nism 87 which controls the operation of fluid motors which pivot rocker cam 34. The pressure of the servo fluid is regulated by a pressure modulated servo relief valve 200. Since the force required to pivot rocker cam 34 increases as the pressure of the working fluid increases, relief valve 200 is pressure modulated so that servo fluid pressure vaires directly with changes in working fluid pressure.

Excess servo fluid is by-passed through relief valve 200 into a replenishing circuit to provide make-up fluid for the low pressure port of the main piston pump. Excess fluid in the replenishing circuit is by-passed through a replenishing relief valve 201 into pump case 11. A relief valve 202 is connected between pump case 11 and tank T to maintain the fluid in case 11 under positive, but low pressure. Consequently, if fluid in the replenishing circuit is needed by the main pump at a greater rate than which it can be furnished by servo pump 52, pressurized fluid in case 11 will flow back through replenishing relief valve 201 into the replenishing circuit to the pump inlet.

The hydraulic system is protected from excessive working fluid pressure by a pair of sequence valves 203, 204 which connect pump ports P P with the pump displacement control motors. When the working fluid exceeds a predetermined pressure, working fluid bypassed through a sequence valve is supplied to these motors to override the manual control and reduce the displacement of the main pump until the working fluid pressure levels off at the pressure setting of the sequence valves.

Adjustable orifices

205, 206 in the respective ports P P are connected to sequence valves 203, 204 which sense the pressure drop across the orifices and control the rate of flow of working fluid from ports P P When the working fluid flow rate exceeds a predetermined amount, working fluid is bypassed through a sequence valve and supplied to the pump displacement motors to reduce the displacement of the main pump.

Dual level relief valves 207, 208 bypass excess fluid from sequence valves 203, 204 and limit the pressure of the working fluid supplied to the pump displacement control motors. Fluid bypassed by the dual level relief valves is supplied to the above-mentioned replenishing circuit.

A detailed description of the automatic control system follows. Referring to FIG. 4, fluid in tank T is supplied to the intake side of servo pump 52 through line 209. Servo pressure fluid is exhausted from pump 52 through lines 210 and 211 to the port in cover plate 114 and flows to the manual pump control for operation of the pump displacement control motors as de scribed above.

Lines 212, 213 connect line 210 to pressure modulated servo relief valve 200 where servo fluid acts against a poppet 214 which is biased against a seat 215 by both a spring 216 and a plunger 217 operated by a piston 218. Working fluid is supplied to piston 218 so that the force applied by it to plunger 217 and poppet 214 is modulated by variations in the pressure of the working fluid. For example, at a working fluid pressure of 0 psi, relief valve 200 is set at approximately 300 psi, but at a working pressure of 5000 psi, relief valve 200 is set at approximately 500 psi.

When excessive servo pressure is applied to shoulder 219, poppet 214 lifts from seat 215 and fluid spills into the replenishing circuit which includes

lines

220, 221, 222 and feed line 223 to check valve 224 and feed line 225 to check valve 226. Check

valves

224, 226 are located in

respective lines

227, 228 from main pump ports P P If the low pressure port does not have an adequate supply of fluid, the check valve in that port opens to supply replenishing fluid to prevent cavitation of the pump.

In normal operation, the main pump does not require all of the fluid in the replenishing circuit. The excess fluid flows via line 230 to replenishing relief valve 201 having a poppet 231 which is biased against its seat 232 by a light spring 233 and fluid pressure in cavity 234 above poppet 231. The pressure in cavity 234 is controlled by an adjustable pilot stage 235 which is connected to cavity 234 by line 236. Replenishing fluid in line 230 reaches pilot stage 235 through cavity 237 in valve 201, a bore 238, a cavity 239, an orifice 240, cavity 234 and line 236. When excessive replenishing fluid pressure causes pilot stage 235 to spill, fluid pressure in cavity 234 above poppet 231 has reached its upper limit and any further increase of pressure of the replenishing fluid acting on shoulder 241 raises poppet 231 off seat 232. Poppet 231 is set to lift at approximately 200 psi.

Fluid spilled through valve 201 flows through

lines

242 and 243 to passages 244 between port plate 44 and port cap 14 to cool port plate 44 and then through line 245 to case 11. Fluid leaking past pistons 29, shoes 32 and port plate 44 also accumulates in case 11. Fluid in case 11 flows through relief valve 202 and line 246 to tank T. Relief valve 202 maintains fluid in case 11 at approximately 60 psi. If the main pump should demand more fluid than servo pump 52 or the replenishing circuit can supply, pressurized fluid in case 11 is available to flow in a reverse direction through line 245, passages 244 and

lines

243, 242 back through replenishing relief valve 201 and into the replenishing circuit to provide additional fluid for that circuit. This occurs when a main

pump check valve

224, 226 opens to admit replenishing fluid to the pump and replenishing fluid pressure drops below case pressure. in this instance, fluid pressure in cavity 234 is below case pressure and case pressure fluid can lift poppet 231 from seat 232 to flow into the replenishing circuit.

Sequence valve 203 controls pressure and limits flows rate in main pump port P Working fluid in port P, flows out of the pump through line 227 and orifice 205 to perform desired work.

Lines

227, 248 connect port P, with the bottom of valve 203 and serially connected line 227, orifice 205, line 249 and orifice 250 connect port P, with the top of valve 203. When port P, has working fluid, port P is the inlet port and fluid flows into port P unrestricted through check valve 273 and line 228. Sequence valve 204 controls pressure and limits flow rate in main pump port P Working fluid in port P flows out of the pump through line 228 and ori free 206 to perform desired work. Lines 228, 251 connect port P with the bottom of valve 204 and serially connected lines 228, orifice 206, line 252 and orifice 253 connect port P with the top of valve 204. When port P has working fluid, port P is the inlet port and fluid flows into port P, unrestricted through check valve 272 and line 227.

An adjustable pilot stage 247 which controls the pressure setting of the sequence valves is connected to orifice 250 in the top of valve 203 through a check valve 254, line 255, line 256 and cavity 257 and to orifice 253 in the top of valve 204 thorugh a check valve 258, line 259, line 256 and cavity 257. Working fluid in cavity 257 biases piston 218 which acts on servo relief valve plunger 217 downward as previously mentioned.

Sequence valve 203 includes a poppet 260 biased against a seat 261 by a spring 262. Sequence valve 204 includes a poppet 263 biased against a seat 264 by a spring 265. When port P has working fluid, the fluid passes through an orifice 266 in poppet 260 of valve 203 and orifice 250 to reach pilot stage 247. When port P has working fluid, the fluid passes through an orifice 267 in poppet 263 of valve 204 and orifice 253 to reach pilot stage 247.

When excessive working fluid pressure in port P, spills pilot stage 247, fluid flow through orifices 266, 250 reduces the pressure in cavity 268 above poppet 260 and poppet 260 is lifted from seat 261 by working fluid. Some of the spilled fluid is conducted through line 269 to fluid motor chamber 80 to operate the fluid motor and move rocker cam 34 towards neutral position thereby reducing the displacement of the pump until working fluid pressure is just sustained at the setting of valve 203. Likewise when excessive working fluid pressure in port P spills pilot stage 247, fluid flow hrough orifices 267, 253 reduces the pressure in cavity .0 above poppet 263 and poppet 263 is lifted from .eat 264 by working fluid. Some of the spilled fluid is conducted through line 271 to fluid motor chamber 81 to operate the fluid motor and reduce the displacement of the pump until working fluid pressure is just sustained at the setting of valve 204.

The pressure drop across variable flow control orifice 205 is sensed by sequence valve 203 which limits the rate of flow from port P Line 248 connects the bottom of poppet 260 with the upstream side of orifice 205 and line 249 connects the top of poppet 260 with the downstream side of orifice 205. The pressure drop across variable flow control orifice 206 is sensed by sequence valve 204 which limits the rate of fluid flow from port P Line 251 connects the bottom of poppet 263 with the upstream side of orifice 206 and line 252 connects the top of poppet 263 with the downstream side of orifice 206.

Springs

262, 265 in respective valves 203, 204 determine the pressure drop across

orifices

205, 206 at which the sequence valves will open. The size or flow area of

orifices

205, 206 in turn determine the flow setting of the valves.

When excessive flow causes the pressure differential across one of the

poppets

260, 263 to exceed the force of the

spring

262, 265 acting on the poppet, the poppet will lift from its

seat

261, 264 and working fluid will be by-passed through the valve 203, 204. Some of the fluid by-passed through each sequence valve 203, 204 flows to its connected

fluid motor chamber

80, 81 to operate the fluid motor and move rocker cam 34 towards a neutral position, thereby reducing the displacement of the pump until the rate of fluid flow reaches the setting of the valve 203, 204. The exhaust of each sequence valve 203, 204 is connected to a respective dual level relief valve 207, 208. Dual level relief valve 207 has a stepped poppet 274 urged against a seat 275 by a spring 276. The top surface 277 of poppet 274 has an area twice as great as bottom surface 278. Likewise valve 208 has a stepped poppet 279 urged against a seat 280 by a spring 281; the area of top surface 282 of poppet 279 is twice as great as bottom surface 283. The purpose of having the top areas of the stepped poppets 207, 208 twice as great as the bottom areas is to require twice the fluid pressure on the bottom surface as on the top to spill the valve 207, 208. For example, if fluid at servo pressure acts on top surface 277 of poppet 274, fluid at twice servo pressure must be applied to bottom surface 278 to lift poppet 274. Likewise, if fluid at half servo pressure is applied to top surface 277, fluid at servo pressure must be applied to bottom surface 278 to lift poppet 274.

Dual level relief valves 207, 208 operate differently when they are associated with a sequence valve connected to the working port of the main pump than when they are associated with a sequence valve connected to the low pressure port. Operation of dual level relief valves 207, 208 under both conditions will now be described.

lf port P is the low pressure port, servo fluid flows to a cavity 286 on the top of poppet 279 through

lines

210, 284. The servo fluid flows through a first orifice 285 in line 284 and through a second orifice 287 in a line 288 connected between cavity 286 and the top of a check valve 289. Check valve 289 is connected by line 290 to line 228 from port P Since check valve 289 is exposed to low pressure fluid in port P the servo fluid will flow from line 288 past check valve 289, through line 290 to line 228 from low pressure port P After passing through orifice 285 upstream of top surface 282 and orifice 287 downstream of top surface 282, the pressure of the fluid acting on surface 282 will be reduced to approximately one-half servo pressure. Therefore, fluid at servo pressure must be applied to bottom surface 283 to spill valve 208. Since fluid motor chamber 81 connects to relief valve 208 through line 271, the valve 288 limits the pressure of the fluid in chamber 81 to that pressure (servo pressure) set by servo relief valve 200. Likewise, chamber 80 is connected to dual level relief valve 207 through line 269 and, when P is the low pressure port, the valve 207 limits the pressure of the fluid in the chamber 80 to servo pressure.

lf P is the working port, servo fluid flows through lines 210, 212 to a cavity 293 on the top of valve 207. A first orifice 294 is located in line 212 upstream of valve 207 and a second orifice 295 is located in line 296 connecting cavity 293 and a check valve 297. When P is the working port, working fluid closes check valve 296 and no fluid flows through the orifices 294, 295. Therefore, the fluid acting in cavity 293 and on top surface 277 of poppet 274 is at full servo pressure and fluid at twice servo pressure is required to spill valve 207. This is desirable since this allows the pressure of the fluid flowing through line 269 into fluid motor chamber 80 to reach twice servo pressure. Since the fluid in chamber 81 is limited to servo pressure when P is the low pressure port, a net pressure differential (up to servo pressure) acts on vane 70 to reduce the displacement of the main pump until the volume of working fluid delivered by the pump just sustains the pressure set by sequence valve 207. Likewise, if port P contains working fluid, fluid up to twice servo pressure can flow through line 271 to chamber 81 to act on vane 70 to reduce the displacement of the main pump.

When working fluid above servo pressure enters one of the

chambers

80, 81 and moves vane 70, it overrides the manual control system described above. It is able to override the manual control system since the maximum pressure in that system is servo pressure. As soon as the working pressure falls and the sequence valve closes, the manual control system operates the fluid motors to move the vane 70 back to the position set by the manual control arm 118.

The excess fluid which spills through sequence valve 203 and does not flow fast enough into line 269 spills through dual level relief valve 207 into the replenishing system to make the fluid available to the low pressure port of the pump as described above. Likewise, excess fluid which spills through sequence valve 204 and does not flow into line 271 spills through dual level relief valve 208 into the replenishing system.

The application of the replenishing system of the automatic control system of the instant invention to a pump coupled with a fluid motor to form a hydrostatic transmission is now described. Referring to FIG. 5, it can be seen that ports P,, P of the main pump are connected to

respective ports

301, 302 of a hydraulic motor M, by

respective lines

303, 304. Case 305 of motor M is connected to case 11 of the main pump by line 306. Fluid passing between the main pump and motor M is in a closed loop. Leakage from either of the devices flows to their respective cases. The fluid in the cases flows through line 236, relief valve 202 and line 237 to tank T. Relief valve 202 functions to maintain the fluid pressure in the cases at a predetermined level which may be approximately 60 psi.

It can be seen in FIG. 4 that the inlet of servo pump 52 is connected to tank T through line 209. Since the working fluid in the instant transmission travels in a closed loop system, any internal leakage from either the main pump or motor M must be made up by servo pump 52. Any leakage from either the pump or motor M flows into its case. It is possible that the main pump or motor M could develop a leak greater than the output of servo pump 52. When this occurs, no servo fluid flows into the replenishing circuit. When the main pump is not getting enough fluid in the low pressure port from motor M and servo pump 52, and fluid pressure in the replenishing circuit falls below case pressure, the pressurized fluid in the main pump and motor cases will flow backwards through line 245, passages 244, line 243 and line 242 to lift poppet 231 of replenishing relief valve 201, as previously described, to thereby provide pressurized fluid to the replenishing circuit to meet the demands of the main pump. The effect of this arrangement is to immediately provide ample fluid at case pressure to the low pressure port of the main pump to prevent cavitation.

It should be pointed out that fluid in the cases of the main pump and motor M will only flow into the replenishing circuit when the leakage of the main pump or the motor M is abnormal.

Obviously, those skilled in the art may make various changes in the details and arrangements of parts without departing from the spirit and scope of the invention as it is defined by the claims hereto appended. Applicants, therefore, wish not to be restricted to the precise construction herein disclosed.

Having thus described and shown one embodiment of the invention, what is desired to secure by Letters Patent of the United States is:

1. An automatic control device for a variable displacement fluid energy translating device having a rocker cam pivotally mounted in a rocker cam support for changing the displacement of the device, fluid motor means for pivoting the rocker cam including a first fluid motor member attached to the rocker cam and a second fluid motor member cooperative with the first fluid motor member to define first and second sealed fluid receiving chambers, a servo pump for supplying servo pressure fluid, manual control means for selectively supplying servo pressure fluid to said fluid motor to selectively position the rocker cam between a first position of maximum pump displacement in which said one port is the working port and the other port is the inlet port and a second position of maximum pump displacement in which said other port is the working port and said one port is the inlet port, and the improvement comprising automatic control means for overriding said manual control means including compensating means operable when working fluid exceeds a first predetermined pressure to supply working fluid to said fluid motor to reduce displacement of the device until the working fluid maintains the first predetermined pressure, a first relief valve responsive to working fluid pressure and servo fluid pressure to limit the pressure of the working fluid supplied to said fluid motor to a second predetermined pressure and a second relief valve responsive to inlet fluid pressure and modified servo fluid pressure to maintain and limit the fluid exhausted from said fluid motor to a third predetermined pressure.

2. The automatic control device recited in claim 1, including biasing means for positioning said second relief valve, the force of said biasing means being directly proportional to servo fluid pressure.

3. The automatic control device recited in claim 2, including a third relief valve for limiting the pressure of the servo fluid to a fourth predetermined pressure and bypassing servo fluid when it exceeds the fourth predetermined pressure, said second relief valve biasing means including differential area means having a first area and a second area, means for reducing the pressure of the servo fluid, first passage means interconnecting said servo pump and said first area to supply said servo fluid at reduced pressure to said first area to bias said second relief valve closed to thereby set the second relief valve at the third predetermined pressure, and second passage means connecting said fluid motor exhaust fluid to the second area to bias said second relief valve open and bypass exhaust fluid through said second relief valve when said exhaust fluid exceeds the third predetermined pressure.

4. The automatic control device recited in claim 3, wherein said servo pressure reducing means includes an orifice in said first passage means.

5. The automatic control device recited in claim 3, including replenishing means connected to the pump ports for making excess fluid available to the inlet port, third passage means connecting said third relief valve and said replenishing means to supply servo fluid bypassed through said third relief valve to said replenishing means, fourth passage means connecting the first relief valve and said replenishing means to supply working fluid by-passed through the first relief valve to said replenishing means, and fifth passage means connecting said second relief valve and said replenishing means to supply fluid by-passed through said second relief valve to said replenishing means.

6. The automatic control device recited in claim 5, including a fourth relief valve to limit the fluid in said replenishing means to a fifth predetermined pressure and fifth passage means connecting said fourth relief valve to said pump casing to bypass fluid thereto when fluid in said replenishing means exceeds the fifth predetermined pressure.

7. The automatic control device recited in claim 6, including fifth passage means connecting said casing with a reservoir, and a fifth relief valve in said fifth passage means to maintain the fluid in said casing at a sixth predetermined pressure so fluid in said casing can flow backwards through said fifth passage means, said fourth relief valve and said replenishing means to the inlet port of the pump when the pressure of replenishing fluid falls below case pressure.

8. The automatic control device recited in claim 3, wherein the first relief valve includes biasing means for positioning the first relief valve.

9. The automatic control device recited in claim 8, wherein the first relief valve biasing means includes a differential area means having a first area and a second area, and servo fluid is supplied to said first area to bias the first relief valve closed to thereby set the first relief valve at the second predetermined pressure.

10. The automatic control device recited in claim 9, including third passage means connecting said compensating means with said fluid motor and the second area to operate the fluid motor and bias said first relief valve open when working fluid bypasses said compensating means, the relationship between the first and second areas being such that the bypassed working fluid pressure is reduced to a desired level above servo fluid pressure.

11. The automatic control device recited in claim 10, including replenishing means for making excess fluid available to the inlet port, fourth passage means connecting the first relief valve and said replenishing means and the fluid bypassed through the first relief valve is supplied to said replenishing means, a fourth relief valve to limit the fluid in said replenishing means to a fifth predetermined pressure and fifth passage means interconnecting said fourth relief valve and said pump casing to bypass fluid thereto when fluid in said replenishing means exceeds the fifth predetermined pressure, sixth passage means connecting said casing with a reservoir, and a fifth relief valve in said sixth passage means to maintain the fluid in said casing at a sixth predetermined pressure so fluid in said casing can flow backwards through said fifth passage means, said fourth relief valve and said replenishing means to the inlet port of the pump when said replenishing fluid pressure falls below case pressure.

12. The automamtic control device recited in claim 1, wherein the compensating means includes a flow control orifice in the working fluid port and said compensating means being responsive to a pressure differential across the orifice corresponding to an excessive flowrate to supply working fluid to said fluid motor to reduce displacement of the device until the working fluid maintains a predetermined flowrate.

13. An automatic control device for a variable displacement fluid energy translating device having a rocker cam pivotally mounted in a rocker cam support for changing the displacement of the device, fluid motor means for pivoting the rocker cam including a first fluid motor member attached to the rocker cam, a second fluid motor member cooperative with the first fluid motor member to define first and second sealed fluid receiving chambers, a servo pump for supplying servo pressure fluid, manual control means for selectively supplying servo pressure fluid to said fluid motor to selectively position the rocker cam between a first position of maximum pump displacement in which said one port is the working port and a second position of maximum pump displacement in which said other port is the working port, the improvement comprising automatic control means for overriding said manual control means including compensating means operable when working fluid exceeds a predetermined pressure to bypass working fluid and supply working fluid to said fluid motor to reposition said rocker cam to reduce displacement of the device until the quantity of working fluid maintains the predetermined pressure, a relief valve having differential area means including a first area and a second area, first passage means interconnecting the servo pump and the first area to supply servo fluid to the first area to bias the relief valve closed, second passage means interconnecting the first area and a pump port, a check valve in the second passage means, pressure reducing means for reducing servo fluid pressure acting on the first area when servo fluid flows therethrough, third passage means connecting the compensating means and the second area, fourth passage means interconnecting the fluid motor means and the second area, the relief valve being operable alternatively between a first mode in which the port is the working port and working fluid biases the check valve closed to prevent servo fluid flow through the pressure reducing means whereby servo fluid pressure acts on the first area and bypassed working fluid at greater than servo fluid pressure acts on the second area to bias the relief valve open and provide fluid at greater than servo fluid pressure to said fluid motor and a second mode in which the port is an inlet port and servo fluid flows through the pressure reducing means and the check valve whereby servo fluid at reduced pressure acts on the first area and fluid discharged from the fluid motor acts on the second area to bias the relief valve open.

14. The automatic control device recited in claim 13, wherein said pressure reducing means includes a first orifice in the first passage and a second orifice in the second passage means.

15. The automatic control device recited in claim 13, wherein the relationship of the first and second areas is such that the fluid discharged from the fluid motor at approximately servo pressure acts on the second area to open the relief valve.

16. The automatic control device recited in claim 13, wherein the relationship of the first and second areas is such that the fluid discharged from the fluid motor at less than servo pressure acts on the second area to open the relief valve.

17. An automatic control device for a variable displacement fluid energy translating device having a rocker cam pivotally mounted in a rocker cam support for changing the displacement of the device, fluid motor means for pivoting the rocker cam including a first fluid motor member attached to the rocker cam and a second fluid motor member cooperative with the first fluid motor member to define first and second sealed fluid receiving chambers, a servo pump for supplying servo pressure fluid, manual control means for selectively supplying servo pressure fluid to said one of fluid receiving chambers and simultaneously exhausting fluid from the other of said fluid receiving chambers to selectively position the rocker cam between a first position of maximum pump displacement in which said one port is the working port and the other port is the inlet port and a second position of maximum pump displacement in which said other port is the working port and said one port is the inlet port, and the improvement comprising automatic control means for overriding said manual control means including compensating means operable when working fluid exceeds a first predetermined pressure to supply working fluid to one of said fluid receiving chambers to reduce displacement of the device until the working fluid maintains the first predetermined pressure, said compensating means including a first compensator valve having an inlet connected to the one port and an outlet connected to the one fluid receiving chamber and a second compensator valve having an inlet connected to the other port and an outlet connected to the other fluid receiving chamber, a first relief valve connected to the one fluid receiving chamber, a second relief valve connected to the other fluid receiving chamber, said first relief valve including means for sensing the pressure in the one port and automatically setting the first relief valve at one setting if the one port is the inlet port and setting the first relief valve at another higher setting if the one port is the working port.

18. An automatic control device as recited in claim 17, including first means connecting the top of the first relief valve to servo pressure fluid, second means connecting the top of the first relief valve to the one port, a first orifice in the first connecting means, a second orifice in the second connecting means and a check valve in the second connecting means operable between an open position to permit servo fluid to flow through the check valve and the first and second orifices to provide a first setting for the first relief valve and a closed position to prevent servo fluid flow through the check valve and the first and second orifices to provide an increased setting for the first relief valve and working fluid in the one port operates the check valve to the closed position.

19. An automatic control device as recited in claim 17, wherein said second relief valve includes means for sensing the pressure in the other port and automatically setting the second relief valve at the one setting if the port is the inlet port and setting the second relief valve at another higher setting if the other port is the working port.

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Claims

5. The automatic control device recited in claim 3, including replenishing means connected to the pump ports for making excess fluid available to the inlet port, third passage means connecting said third relief valve and said replenishing means to supply servo fluid by-passed through said third relief valve to said replenishing means, fourth passage means connecting the first relief valve and said replenishing means to supply working fluid by-passed through the first relief valve to said replenishing means, and fifth passage means connecting said second relief valve and said replenishing means to supply fluid by-passed through said second relief valve to said replenishing means.