US2818706A - Hydraulic system - Google Patents

Description

Jan; 7, 1958 F. J. WRIGHT v HYDRAULIC SYSTEM Filed April 15. 1953 2 Sheets-Sheet 1 .51 x K w m. m H m E .IIJWS m J D 1 mm M my flu; P2 B NNN a ms 2 0 KL \M N: @3 W3 PS 3N on N- f mu. /& m-

Jan. 7, 1958 F. J. WRIGHT 5 HYDRAULIC sys'rm 7 Filed April 15, 1953 v I 2 Sheets-Sheet 2 I I Maw W6, I a I N it Q 88 INVENTOR; 7 n I I By FEED T 13 g rh-TY 2,818,706 HYDRAULIC SYSTEM Fred J. Wright, Columbus, Ohio, assiguor to The Jeffrey Manufacturing Company, a corporation of Ohio Application April 15, 1953, Serial No; 348,983 2 Claims. (CI. 60-53) This invention relates to hydraulic apparatus and systems.

It is an object of the invention to provide improved hydraulic systems wherein there is a substantially closed hydraulic circuit having high pressure and high volume pump means and hydraulic motor means driven thereby, wherein hydraulic fluid is recirculated and wherein provision is made for minimizing cavitation in the hydraulic circuit caused by any sudden displacement of hydraulic fluid from the low pressure side of the circuit.

Other objects of the invention will appear hereinafter, the novel features and combinations being set forth in the appended claims.

In the accompanying drawings:

Fig. l is a diagrammatic showing of an improved bydraulic apparatus and system including the invention;

Fig. 2 is a diagrammatic showing of another hydraulic apparatus and system including the invention;

Fig. 3 is a diagrammatic showing of still another hy-.

draulic apparatus and system; and

Fig. 4 is a diagrammatic showing of the apparatus and system seen in Fig. 3 as it is applied to drive the coal cutting elements of a mining machine.

The hydraulic apparatus and systemshown in Fig. 1 of the drawings includes a closed high pressure and high volume hydraulic circuit which includes a high pressure variable delivery pump which is capable of being adjusted from a zero output volume to a high output volume of hydraulic fluid under high pressure. The high pressure and high volume hydraulic circuit also includes a hydraulic motor 11. Conduits 12 and 13 connect the high pressure variable delivery pump 10 with the motor 11. The pump 10, is indicated in the drawings, is discharging or pumping hydraulic fluid into the conduit 12 to drive the motor 11. Conduit 13 conducts the hydraulic fluid exhausted from the motor 11 to the intake or suction port of pump 10.

- The hydraulic fluid in the closed circuit, including pump. 10, high pressureconduit 12, motor 11, and low. pressure or return conduit 13, is recirculated, that is, fluid exhaustedby the motor 11 flows back tothe pump 10 throughthe low pressure or return conduit 13 and is pumped by the pump 10 through the high pressure conduit 12 back to the motor 11. The output volume of pump 10 may be: varied from zero to a high volume, and the pump 10 is shown in the drawings as including a rotor 14 having reciprocating vanes 15 that operate in a ring or housing 16, the axis of which may be shifted with respect to the axis of the rotor 14. It. will be seen, of course, that when the axis of the rotor 14 and the axis of the ring or housing 16 coincide, the pump 10 will not be operating to produce any output of hydraulic fluid into the conduit 12. In order to cause the pump 10 to pump hydraulic fluid, the axis of ringor housing 16 may be shifted or offset upwardly, as. seen in the drawings, with respect to the axis of rotor 14, and the amount of oflsetting of these axes determines the output volume of pump 10. The ring or housing 16 is shifted upwardly, as seen in the drawings, to increase its output volume by a piston motor 17, and it is shifted downwardly to decrease its output volume by a piston motor 18. -Although Fig. l of the drawings shows pump 10 as being a variable volume vane type pump, it is to be understood that any type pump having variable or fixed output features may be substituted in the system for the pump 10.

- 26 may be of any well known construction and it may bev 2,818,706 Patented Jan. 7, 1958.

The entire hydraulic system also includes a' tank 19 which is a reservoir for hydraulic fluid used in the hydraulic system. From the hydraulic tank19 a 'supplyconduit 20 leads to a low volume, low pressure constant delivery pump 21. The hydraulic fluid pumped by pump 21 is normally conducted through a conduit 22' into one end: of a manually operablecontrol valve-23f For the momentit will suflice to say of the control valve 23' that hydraulic fluid normally passes through it and is discharged from the opposite end thereof into a conduit24 that leads to the low pressure of returnconduit 13 which connects the exhaust port of motor 11 and the intake or low pressure port of pump 10. Low pressure or return conduit 13 is connected through a conduit 25, a relatively low pressure relief valve 26 and a conduit 27 to the tank 19. *Relief valve sure to hydraulic fluid forced thereinto is connected to the conduit 25 leading to the pressure relief valve 26 and thus to the low pressure conduit 13 of the closed hydraulic circuit including pump 10, high pressure conduit 12, motor 1 1,'an'd low pressure or return conduit 13.

The manually operable control valve 23 is preferably identical to one of the valves shown and described in my Patent No. 2,486,087, dated October 25, 1949,1for a hydraulic valve, but it may be of any other well known design having at least three control positions, namely 'a' central, or neutralposition in which all of the hydraulic fluid being received by it from conduit 22 is directed into the conduit 24 and a position at each side of said central.

or neutral position wherein all of the hydraulic fluid being received by thevalve through conduit 22 is directed to one or the other of a pair of branch conduits 29 and 30 and that conduit 29 or 30 not receiving hydraulic fluid from conduit 22 is connected to the conduit 24 bythe valve.

Branch conduit 29 conducts hydraulic fluid to motor 17 which shifts the ring or housing 16 to increase the output volume of the main pump 10 and conduit 30 conducts hydraulic fluid tomotor 18 which shifts the ring or housing 16 to decrease the output volume of main pump 10. From the foregoing description of manually operable valve 23 it will beseen that when the hydraulic fluid is being directed, for example, to motor 17 that a quantity of hydraulic fluid equal to that supplied to motor 17jwill be expelled from motor 18 into conduit 24 and that thus the volume of hydraulic fluid flowing through conduit 24 low'pressure or return conduit 13 will remain substantiallyconstant regardless of the manual operation of valve 23. It will also be seen that when valve 23 is in its central, neutral or by-passing position both branch conduits 29 and 30 are blocked whereby motors 1'7 and 18 will retain. ring or housing 16 of main pump 10 in that position which it occupied when valve 23 was shifted to'its central, neutral or by-passing position.

in order that the output volume of hydraulic fluid from pump 21 will not be blocked should eithermotor 17 or 13 reach the limit of its travel, a conduit 31 including a pressure relief valve 32, is connectedbetween conduits 22 and 13. v In an apparatus including a system of the above described type the maximum output pressureof .pump 10 was in the range of 2000 tov 3000 p. s. i, and its maximum output volume was in the range of 70-80 G. P. .M. and low pressure, low volume pump 21 operated atan output pressure in the range of 50-150 p. s-..i. and output volume of approximately 8-10 G. P. M. a

Relief valve'26 is setor adjusted to open within a relative low pressure range, for example 50-150' s. i., which, of course, determines the pressure in the low pres sure or return conduit 13, and relief valve 32 is adjusted to open at a pressure slightly greater than that at which valve 26 opens. Relief valve 32 determines the maximum pressure of hydraulicfluid that may beernployed to operate motors 17 andlS.

The above described hydraulic apparatus may be employed,-for example, to drive a coal cutting element'of a miningmachine and when this apparatus is operating to drive said'cutting element the pump circulates hydraulic fluid through the closed circuit including high pressure conduit'1 2, motor 11, and low pressure or return conduit 13. A volume'of hydraulic fluid equal to the output volume of pump 21 is at all times being forced into the conduit 13 through the conduit 24 or 31 and consequently the volume of hydraulic fluid substantially equal to the output volume of pump 21 must be displaced from the closed circuit. This hydraulic fluid is displaced from the closed circuit through the conduit 25, relief valve-26' and conduit 27 to tank 19. Relief valve 26 will, of course, open at a predetermined pressure and consequently the accumulator 28 connected to conduit 25 will at all times be subjected to that pressure at which relief valve 26 is set to open;

The system above described, with the exception of the accumulator 28, is the basic system of the reversible system described in my Patent No. 2,621,479, dated December 16, 1952, and, of course it will be readily apparent to those skilled in the art that the invention herein described may be applied to the system of said Patent No. 2,621,479 and be within the spirit and scope of certain of the appended claims.

It has been found that when the present system is employed, for example, to drive a coal cutting element of a mining machine that when the coal' cutting element is' loaded substantially constantly the system described operates efficiently. However, it has also been found that when, for any reason, the load on the cutting element is relieved suddenly and the load is again suddenly placed upon the coal cuttingelement which, for example, will occur if the'coal cutting element should chatter while working; parts, such as the niotor 11 and pump 10, may be damaged by shocks delivered to them through the hydraulic'systeni. To'prevent these damaging elfects and consequent destruction of expensive elements of the apparatus presented a real and serious problem in' connectionwith hydraulically driven elements such as'the coal cutting" elements above referred to.

Insofar as the above described hydraulic apparatus is concerned, it is-the application of the hydaulic accumulator 28in the hydraulic system that is important. By means of the accumulator 28 it is possible to employ the apparatus described to drive a working element, such as a -coal' cuttin'g'element, and to minimize the destructive action of jerking or chattering of the driven working or cutting element upon the parts of 'the' hydraulic apparatu's; It appears t'obe that in the'describe'd apparatus, not including the accumulator 28', when'the'w'orking'load or resistance met by the driven working element driven by motor 11 is suddenly relieved, th'ere' occurs what may be termed" an elongation' of the column of hydraulic fiuidin the high pressure conduit 12' which may be due to'the expansion and' contraction of the mechanical elements that comprise the high pressure side of the closed circuit, such, for example, as contraction of'the piping that forms the conduit 12, and expansion, etc, of parts of 'both' the pump 10 and motor 11. This so called elongation of the column'of hydraulic fluid in the high pressure" side of the closed hydraulic system causes the motor 1'1 to run momentarily at a speed greater than that at which it was being driven under load, and consequently during this momentary perio'd'r'riotor' 1'1" exhausts more bydraulicfluid into conduit 13 than can be accepted by pump 10. This excess of hydraulic fluid is, of course, discharged from the low pressure side of the closed hydraulic system through the relief valve 26. In other Words, the high pressure s ide, of the closed circuit may be compared with an accumulator and it may be said to function as such.-

It will be seen, of'course, that when this action, herein termed elongation, occurs in' the high pressure side of the closed hydraulic system, the pump 10, lagging behind the sudden discharge ofhydraulic fluid from motor 11, will be required to withdraw s'uddenlya slightly larger quantity of hydraulic fluid from the conduit 13 in the low side of the closed hydraulic system and because the pump 21 cannot supply hydraulic fluid to the low pressure side of the closed system with the rapidity required, cavitation may occur in the "hydraulic system accompanied by pounding or jarring that results in damage and often complete destruction of elements, such as the pump 10 and motor 11 of the hydraulic apparatus.

1 have found that by placing a hydraulic accumulator, such as accumulator 28, in the low pressure side of the closed hydraulic system that the above described pounding or jarring may be minimized. When an accumulator,

such as accumulator 28, is employed in the hydraulic apparatus, as above described, it appears that the accumulator 28 will displace hydraulic fluid under pressure from itself to make up in the low pressure side, and specifically in conduit 13, a quantity of hydraulic fluid substantially equal to that quantity of hydraulic fluid which was discharged from the low pressure side of the hydraulic system through motor 11 when the load thereon was relieved suddenly. This action of accumulator 23, of course, maintains conduit 13 completely filled with hydraulic fluid at all times and it tends to balance the accumulator effect of the high pressure side of the closed circuit, thereby eliminating the above described detrimental cavitation in the hydraulic system.

In Fig. 2 of the drawings there is shown diagrammatically a portion of a hydraulic apparatus and system that includes two high pressure, high volume pumps and 1-10 driven from separate electric motors 114 and 114', respectively. These pumps may, if desired, be driven from a single electric motor. The apparatus ineludes two high pressure, high volume hydraulic motors 111 and 111 driven by hydraulic fluid discharged from high pressure, high volume pumps 110 and 110'. This hydraulic system includes a portion of the system described in connection with Fig. 1 of the drawings, which portion is not duplicated in the drawings but which is clearly described hereinafter, and it includes what is in effect two closed high pressure, high volume hydraulic circuits, one including pumps 110 and motor 111, and the other including pump 110' and motor 111'. Each of these closed circuits correspond for the most part with the closed circuit including pump 10 and motor 11 described in connection with the system shown in Fig. l of the drawings. The closed hydraulic circuit including pump 110 and motor 111 will now be described, it being understood that the closed hydraulic circuit including pump 110 and motor 111' is a substantial duplicate of the described circuit and that the corresponding parts or elements of this circuit carry numbers that correspond with the numbers of the described circuit, except that they carry an exponent. I

Conduits 112 and 113 connect the pump 110 with motor 111, and pump I10 discharges or pumps hydraulic fluid into the high pressure conduit 112 to drive motor 111. Low pressure or return conduit 113 conducts hydraulic fluid exhausted from the motor 111 to the intake or suction port of pump 110. Low pressure or return conduit 113 is connected through a conduit to a relief valve 126 which corresponds with and has the identical function of relief valve 26. Relief valve 126 is con nec'ted to the tank 113 through" a conduit 127, Thefluid is taken from the tank 119 by the punip'121 through the line 120 and fed through line 122 to the hydraulic cir-' or return conduit 113; 'Accum'ulator'128 has the' same function in this system asaccumulator 28 has in the system of Fig. 1, and that portion of the circuit thus far described is capable of functioning as do'es'the high pressure, high volume circuit previously described in connection with thesystem of Fig. 1 of the drawings.

.It is pointed out that the motors 111 and 111' are not interconnected so that the driving effort of one may aid the other and that therefore each motor 111 and 111' may be employedto drive separate working elements, such as coal cutting elements or heads of a mining machine. Because the motors 111 and 111' are not interconnected the working pressures in the high pressure sides, namely conduits 112 and 11 2, of the closed circuits for the two. motors may differ in accordance with the loads placed separately upon the motors 111 and 111- In this system low pressure conduits 113 and 113 are interconnected through the conduit 125. The entire system includes two accumulators 128 and 128', each connected directly through connector conduits 134 and 134' to low pressure or return conduits 113 and 113', respectively, whereby their action in supplying hydraulic fluid to conduits 113 and 113 will be immediate.

It may be pointed out here that the pumps. 110 and 110 are shown as of the constant volume output type and that therefore operating or adjusting motors, such as motors 17 and 18, are not shown in association with pumps 110 and 110. Pumps 110 and 110' may be of the variable delivery type, such as shown in Fig. 1 of the drawings, if desired. I

The hydraulic circuit including pump 110 and motor 111 differs from the system including pump and motor 11 in another respect. Referring again to Fig. 2 of the drawings, it will be seen that a relief valve 135 is interposed in-the high pressure conduit 112. The structure and operation of rcliefvalve 135 is well known and it follows the structure of the relief valve shown and de scribed by G. C. Bonnell in his United States PatentNo. 2,388,820, dated November 13, 1945, and only so much of valve 135 is here described as is necessary to an understanding of the present invention.

Relief valve 135 includes a body or housing 136 having a through passage 137 which may be considered as a portion of conduit 112 through which hydraulic fluid flows'from pump 110 to motor 111. Body 136 has a cylinder in which a'valve core 138 reciprocates. Core 138 includes a piston or head 139 that slides in the above mentioned cylinder, and a hollow stem-140 that closes an exhaust port 141 to prevent fluid in the through passage 137 from flowing from the'valve body 136 through the exhaust port 141. One side of piston 139 is exposed to the pressure of the hydraulic fluid in through passage 137, and the opposite side of piston-.139'is exposed to hydraulic fluidin a chamber 142. Thus the pressure of fluid in the chamber 142 on one side of piston 139 opposes the pressure of fluid on the other side of the piston 139 and in the through passage 137. Piston 139 includes a small bore 143 that interconnects the chamber 142 with the through passage 137. The body of valve 135 includes a head 144 in which there is a passageway that leadsfrom the chamber 142 to a ball check valve that may be adjusted to open at a desired pressure by a spring and? adjusting screw 145. Hydraulic fluid which flows past this check valve is discharged through the hollow amma and out of-thevalve-through the exhaust port 141. Valve'core-138 is'urged in a direction to close port bore 143. When the pressures in the chamber 142, and

through passage 137 are balanced, valve core 138 is urged to close exhaust port 141 by the spring in the hollow stem. It will be seen that if hydraulic'fluid is removed from chamber 142 at a rate "greater than the capacity of bore 143 topermit'fl'uid frorn'through passage 137 to replace it, the pressure of hydraulidfluid'in through passage 137 will move valve core 138 and open the exhaust port 141, or if the pressure in through 'passage 137 should be increased suddenly the same function will occur because hydraulic fluid will be exhausted from chamber 142 through the ball check valve and hollow stern 140, and piston 139 will move hollow stem 140 to open port 141.

In the hydraulic system being described the exhaust port 141 of relief valve 135 is connected with the low pressure conduit 113 by a conduit 146 so that hydraulic fluid discharged from conduit 112 through relief valve 135 is conducted to low pressure conduit 113. It maybe mentioned here that sudden operation of relief valve 135 may cause the same elongation effect in the hydraulic system as is caused when a load is removed suddenly from the motor 111 or 111 as above described.

The head 144 of relief valve 135 also includes a passageway 147 connected with chamber 142. This passageway is connected through a conduit 148 including a check valve 149 that permits hydraulicfluid to flow only out of chamber 142 to a manually controlled venting valve 150 that is connected to exhaust to tank 119 through a conduit 151. It will be seen'that if venting valve 151 is opened relief valve 135 will .be opened to cause hydraulic fluid in conduit 112 to by-pass the motor 111 through valve 135 and conduit 146 directly into con-' duit 113, thus severing the driving connection between the pump and motor 111. Venting valve 150'constitutes a control by which the motor 111 may be effectively and quickly disconnected from the driving effort of the pump 11without stopping the pump. Obviously, when venting valve 150 is opened relief valves and 135 will be operated to disconnect bothmotors 111 and 111' from the driving effort of their respective pump 116 and 110.

Figs. 3 and 4 illustrate a modified hydraulic apparatus and system. Fig. 3 illustrates the apparatus and' system diagramatically, and Fig. 4 illustrates this apparatus and system as it has actually been employed to drive the coal cutting elements of a mining machine.

Referring first to Fig. 3 of thedrawings, the system differs from the system of Fig. 2 above described, in the followingrespects. There is only one accumulators 228 which has the function in this system'of both of the accumulators 128 and 128' of the system of Fig. 2. Relief valves 235 and 235 which occupy positions in the system corresponding with relief valves 135 and 135' of Fig- 2 are modified, motors 211 and 211'1are mechanically interconnected through gearing indicated by the numeral 252 to operate together, and pressure conduits 212 and 212 are interconnected through a conduit 233. All of the remaining parts of this system correspond with the parts of the system of Fig. 2 and they are assigned numbers in the two hundred series, whereas the parts of the system of Fig. 2 are assigned numbersin the one hundred series. Except for the differences ,herein specifically set forth, the description of the system of Fig. 2 is an ac-. curate'description of the system of Fig. 3. V

, Relief, valves 235 and 235 are modified to the extent that their Heads-244' do not include the ball check valve d-by screyv 145, and therefore these valves W any in response to pressure in the all h h prssureconduits 212 and 212', but respond simultano'usly to the opening of a fixed or manuallyff'djustahle pressure relief valve 262. Valve 262 dethe maximum working pressure of hydraulic nl'thepre'ssure" conduits 212 and 212. This system b ineludes a fmanua'lly controlled venting valve 250 wt H has the identical function" in this system as does the ve ting valve 150 in the ystem of Fi .2.

' eeausen otors 211 and 211' are mechanically interconnected,'thissystem will operate efficiently with only one accumulator 228 and this because motors 211 and 2111"msyescn arathe driving effort of the other and because it is; unlikely that the load will ever be relieved suddrily'on'bothirnotors 211 and 211' simultaneously, and ply rarely will the accumulator 228 be required to discharge into both low pressure or return conduits 2 13" and"213" simultaneously although it is capable of doing so through the conduit 225. In this system the conduit 22 5 causes the hydraulic pressures in low pres.- sure or return conduits 2 13 and 213 to balance, and conduit 23'3fcauses the hydraulic pressures in pressure conduits '212' and 212 to balance.

In this last described hydraulic system as applied to one mining machine of which parts are illustrated in Fig f4'of the drawings, each main pump 110, 110' has an" output capacity of '70 G. P. M. and it operates at a maximum pressure of 2000 p. s. i. as controlled by the setting "of valve 262. The low pressure, low volume pump 221 'hasaca'pacity' of 18 G. P. M. and the relief valve 250 is setto open at 125 p. s. i.

It'is ito be' understo'od that all of the pressures in the systems-above set forth are exemplary of high pressure and high volume circuits in which the present invention may be'ernployed with great advantage, but that in other and broader aspects of the invention it may be employed in closed hydraulic systems or circuits wherein the pressuresdiffe'r widely from the examples set forth.

As mentioned above,- Fig. 4 illustrates the systemshown in Fig.3 applied to a coal mining machine to drive the coal cutting elements thereof. The coal cutting elements illustrated in Fig. 4 consist of an upper row of revolvable arms'253 each of which is mounted on the front end of a shaft 254 carri'ed by the housing of an upper cutter head 255. Shafts 254 lie in side by side relation and in a generally horizontal plane and, as indicated in the drawings, are. driven through an obvious gear train so that-some of the shafts 254 rotate in one direction and the others rotate in reverse direction. Head 255 includes a reductiongearing indicated at 256 for which the gear train for shafts 254, and consequently arms 253, is driven by hydraulic motor 211.

Situated directly. below the upper cutter head is a second cutter head 257 which, as shown in the drawings, is a substantial duplicate, though of somewhat dilferent arrangement, of cutter head 255 and which drives a less number of cutter arms 258. Lower cutter head 257 is driven by hydraulic motor 211' through reduction gearing259 similar to that indicated at 256 in the upper cutterhead 255. The reduction gear trains 256 and 259 are interconnected in a manner that is obvious from the drawings'by 'the'gean'ng 252, whereby cutter heads 255 and, 257 are driven simultaneously by motors 211 and 2 11 In Fig. 4 the low pressure low volume pump 221 is ,shown as being driven by a separate motor 260 through a flexible coupling 261, but it is to be understood that pump 22 l may be driven ifdesired in any suitable mannerlby either of themotors 214 or 214.

, Obviously those skilled in the art may make various changes in the details and arrangement of parts without departiiig frorn the spirit and scope of the invention as defined by the claims hereto appended, and applicant therefore wishes not to be festricted to the precise conon rein" unb ased.

of saidmotors and' the other' connecting the highprs-I sure'output p rter th'efotherf f said umps with the inlet port of the other of said motors, means forming two conduits for low pressure hydraulic fluid, 'oi'le connecting the low pressure'intake port of one of said pumps -withthe exhaust port" ofone etfisaidjmotors and the other connecting the low'pres'sure intake'po'rt of the other of said pumps with the iihaust port of the other of said motors, by-p'a'ss conduit mfeans'inclnding a relief valve interconn'ecting 'said' high and low pressure conduithians connected to ea'ch of' said motors, conduit means connecting said pair of'high pressure conduits, conduit means connecting said pair ofjlow pressure conduits,'said last named two conduit means functioning to' cause the hydraulic pressures insaidhigh pressure conduits to remain equal and the pressures in'said low pressure conduits to remain equahineans supplying fluid" to said pair of low pressure conduits, and accumulator means connected to said low pressure conduits for adding fluid thereto'to replace fluid displaced from 'said circuit as a result of sudden increases'in the output" volume of said motors thereby preventing cavitation'in'said circuit. I

2. A hydraulic circuitfor driving two hydraulic motors mechanically interconnected to operate together, said circuit including' two hydraulic pumps and two hydraulic motors, means forming two conduits' for high pressure hydraulic fluid,"oneconnecting the high pressure output port ofone of said pumps with the inlet port of one c-fsaid motors afnd the other connecting the high pressu'reg out'put port of the' other of said pumps with the inlet portjof the other of said motors, means forming twofcon'dui'ts for low"pressure hydraulic fluid, one conmeeting the low pressure intake port of one of said pumps with the eiihaustpo'rt of one of said motors and the other 'conne'cti ng the low pressure intake port of the other of said pumps with the exhaust port of the other of 'said motors, by-pass" conduit'means including a relief valve interconnecting said high and low pressure conduit means connected to each' of said motors, conduit means connecting"'said pair of high pressure conduits, conduit means connecting'said 'pair of low pressure conduits, said last namedtwo conduit means functioning to cause the hydraulic pressures in 'saidhigh pressure conduits to remain equal and the pressures in said low pressure conduits to remain equal, means supplying fluid to said pair of low-"pressure conduits, and accumulator means connected'to 'said'low pressure conduits for adding fluid thereto to replace fluiddisplaced from said circuit as a result of sudden increases in the volume of fluid passing through said last named relief valve means thereby preventing cavitation in said circuit.

References Cited in the file of this patent UNITED STATES PATENTS 958,212 Baab -1 May 17, 1910 991,646 Rich May 9, 1911 1,290,203 Houk Jan. 7, 1919 2,105,824 Simonds Jan. 18, 1938 2,110,428 Simonds Mar. 8, 1938 2,385,069 Ferris Sept. 18, 1945 2,555,427 .Tr'autinan June 5, 1951 2,621,479 Wright Dec. 16, 1952 2,632,999 Balton Mar. 31, 1953 2,687,011 Frankel Aug. 24, 19 54

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