US2926496A

US2926496A - Hydraulic pump

Info

Publication number: US2926496A
Application number: US712471A
Authority: US
Inventors: Edward B Heckenkamp
Original assignee: Allis Chalmers Corp
Current assignee: Allis Chalmers Corp
Priority date: 1958-01-31
Filing date: 1958-01-31
Publication date: 1960-03-01
Anticipated expiration: 1977-03-01

Description

MalCh l, 1950 E.l1.Hr-:CKENKAMF 2,925,496

HYDRAULIC PUMP 3 sheets-sheet 1 Filed Jan. 31, 1958 March 1, 1960 E. B. HECKENKAMP 2,926,496

HYDRAULIC PUMP Filed Jan. 51, 1958 3 Sheets-Sheet 2 vMarcrhl, 1960 E. B. HECKENKAMP HYDRAULIC PUMP Filed Jan. 31, 1958 3 Sheets-Sheet 3 States HYDRAULIC PUMP Edward B. Heckenkamp, Wauwatosa, Wis., assignor to vl'lis-Chalmers Manufacturing Company, Milwaukee,

Application January 31, 1958, Serial No. 712,471

9 Claims. (Cl. 60-52) This invention .relates to hydraulic control apparatus in general and more particularly to apparatus for controlling the flow of pressure fiuid to and from a hydraulic motor.

This invention is quite specifically an improvement of apparatus disclosed and claimed in U.S. Patent 2,611,319, issued to W. F. Strehlow et al., September 23, 1952. The preferred embodiment of the invention is concerned with a hydraulic apparatus having a plurality of constant delivery pumps and including a control valve which is capable of selectively connecting one or more of the pumps to a fluid motor. In hydraulic apparatus of this sort it is customary to provide a relief device which bypasses the pressure fluid back to a fluid reservoir after the Vhydraulic motor has accomplished the desired result Aor when the pressure has become excessive. When the motor, in this instance a single acting hydraulic cylinder and piston unit, has reached the fully extended position, pressure is built up until the pressure limit of the relief device is reached and the fluid is then routed back to the reservoir. The piston is maintained in the fully extended position by a check valve which traps the uid in the cylinder.

In some applications this type of system is sufficient and the fluid leakage which is inherent in most systems is not critical. However, in many cases this leakage is critical because of the resulting settling of the piston. Since in many cases it is not possible and in other cases it is impractical from a monetary standpoint to overcome this leakage it has been a practice to maintain pressure in the lines by replenishing the fluid lost by leakage. This, however, presents another problem which is overheating of the fluid. Excessive heating results in low operating lefliciency and damage to the system. Heating of the fluid is caused by prolonged pumping of a reasonable volume of fluid at a reasonably high pressure. The word reasonable is used as different pressures and volumes may be critical under different circumstances.

Applicant proposes to provide a hydraulic apparatus having a plurality of pumps which are capable of delivering a large volume of high pressure fluid. Applicant further proposes the provision of a means to insure a controlled bypass of a major portion of the pressure fiuid to the fluid reservoir while maintaining a predetermined pressure in the system by pumping only a small quantity of fluid. In this manner sufficient pressure is maintained in the system to insure the continued extension of the hydraulic piston. However, because of the small volume of fluid being moved, the pressure fluid will not heat to a point that will damage the system.

It is therefore an object of the invention to provide a hydraulic apparatus having a plurality of high pressure pumps with control apparatus constructed and arranged in a manner so that a major portion of the pressure uid can be returned tothe fluid reservoir and a minor portion of the pressure fluid can be delivered to the hydraulic motor.

Another object of the invention is to provide a hydraulic apparatus of the hereinbcfore described type with more than one pressure fluid passage to one side of a fluid motor.

A further object of the invention is to provide a hydraulic apparatus having a plurality of pumps and two pressure fluid passages to one side of a hydraulicmotor with control mechanism constructed and arranged in a manner so that pressure fluid is delivered through both of the passages to the hydraulic motor or to a fluid reservoir or through 'one of the passages to the fluid reservoir and through the other passage to the hydraulic motor.

Another object of the invention is to provide a hydraulic apparatus of the above mentioned type with a plurality of pumps wherein the fluid delivery capacity of the pumps is unequal.

It is a further object `of the invention to provide a hydraulic apparatus of the hereinbefore described type with pressure relieving apparatus constructed and arranged in a manner to relieve the small capacity pumps at a lower pressure than the larger capacity pumps.

The foregoing and other objects and advantages of the invention will become more fully apparent from the following description of the invention shown in the accompanying drawings.

Referring to the drawings in which like reference characters designate the same or similar parts of the various views:

Fig. l is a plan view of apparatus embodying the inventicn with parts shown in phantom View;

Fig. 2 is a sectional view taken on line II-II of Fig. l and showing the valve in a full lift position;

Fig. 3 is a view of the valve structure shown in Fig. 2 indicating a full lower position;

Fig. 4 is a schematic showing partly in section of apparatus embodying the invention; the valve is in a hold position;

Fig. 5 is a sectional view taken on line V-V of Fig. 2;

Fig. 6 is a broken sectional view taken on line VI-VI of Fig. 1; and

Fig. 7 is a sectional View taken on line VII-VII of Fig. 1.

Referring to Fig. 1 the preferred embodiment of the Ainvention comprises a housing 11 having three large pump signed to be submerged in oil and the plungers are openA to the oil through individual intake passages 16 (shown in Fig. 7). Fluid flows into the housing when the plungers are in the raised position and as the plungers are driven downward by the cam followers the intake passages are closed off and the fluid is pressurized.

Referring to Fig. 2 the pressurized uid is delivered to a control valve chamber 17 by pump discharge passages 18. Adjacent to each discharge passage is a bypass passage 19 which connects the control valve chamber with the fluid reservoir. housing 11 also includes a first fluid conduit or pressure manifold 21 which is disposed above valve chamber 17 and in generally parallel relation thereto. The manifold is connected with the valve chamber 17 through a group of transfer passages 22 corresponding in number to the number of large pump plungers 12. Each transfer passage is provided with a spring biased ball check valve 23. These valves require only a minimum amount of pressure to open the transfer passages from the chamberV Ptented Mar. 1,

In the preferred embodimentv 3 ing fluid passage 24. Communication between the working fluid passage and the pressure manifold is controlled by another spring biased ball check valve preventing a backow of uid from the working uid passage into the manifold. i' i" 'I 'i In diametrically opposed relation to the small pump plunger discharge passage 18 isV another transferpassage 26`fin communication with the control valve chamber 17. The transfer passage 26 does not communicate with` the pressure manifold '21 but it is connected to the working fluid passage 24 through a second fluid conduit or linking passage 27. A ball check valve 25 is provided iii'the linking passage to block the return flow of uid from the linking passage 27 to the control valve chamber. It should now be apparent that there are in effect two separate routes of pressure fluid to the working iiuid passagev 24. One source being the three large plungers 12 which empty pressure iluid into the pressure manifold 2.1 which is in turn connected withthe working passage 24. The second source is from the small plunger 13 through the linking passage 27 to the working passage 24. Each of the two routes contains ball check valves' to block the back ow of fluid to the valve charnbe'r`17.

"The housing 11 is also provided with a cylindrical unloading valve chamber 28 (shown in Fig. 7) extending therethrough in laterally spaced parallel' relation with respect to cylindrical control valve chamber 17. Referriiig'fto Figs. 4 and 7 the left hand end of the pressure manifold 21 communicates with the adjacent end ofthe unloading valve chamber 28 by means of a connecting passage 29. The right hand portion of the unloading valvechamber has two discharge ports 30 and 31 opening through the side surface of the housing 11 and connecting the right hand portion of the unloading valve chamber with the fluid reservoir.

The left end portion of unloading valve chamber 2S is somewhat enlarged to provide an internal shoulder 32 against which is positioned an elongated cylindrical plug33.' The center portion of the plug is of a smaller diameter than the ends thereof resulting inan annular chamber 34 between the center portion of the plug and the inner walls of the unloading valve chamber 28. The ends of the plug are of a diameter providing a close fit with the inside wall of the unloading valve chamber andare provided with oil seals so as to insure an oil tight relationship between the annular chamber 34 and the'remaining portion of the unloading valve chamber 28; A transverse bore 36 connects diametrically opposed portions' of annular chamber 34 with a central bore 37 which extends to the right from the transverse bore 36 and coaxially throughv the enlarged right hand end portion of the plug 33. Central bore 37 is enlarged from'apoint adjacent transverse bore 36 to the discharge side' thereof to provide a seat for a ball valve 38 and to receive a projection of cylindrical valve element 39. Thel cylindrical valve 'element is slidably contained within the reduced right hand portion of the unloading valve chamber '28. A coil spring 41 contained within the unloading valve chamber resiliently urges the cylindrical valveelement 39 `against the ball valve 3S which is in turn urgedagainst the valve seat. The cylindrical valve element 39 is provided with a longitudinally extending bore 42 from the ball valve end thereof approximately to its midpoint where it communicates with a circumferential groove 43. A small opening 44 is provided between the end of longitudinal bore 42 and the right hand end of the unloading valve chamber so that any fluid which leaks into the longitudinal bore when the ball valve is seated may pass through this small opening and return to the uid reservoir through the discharge port 31.

V`The left hand end of unloading valve chamber 28 is closed'by a cover plate 46 secured in fluid sealing relation'to the housing 11 by means of cap screws 47. rlfhje maar,

4 right hand end of unloading valve chamber 28 is closed by means of a cover plate 48 secured to the housing 11 by one or more cap screws 49. A rod 51 secured to the cover plate 48 extends axially through the coil spring 41 and acts as a stop against which the valve element 39 abuts in the open position. The coil spring is seated against spacing washers 52 which are positioned in abutting relation against the inside surface of the cover plate 48. The number of washers 52 is varied to vary the pressure the spring 41 exerts against the valve element 39 and therefore the pressure required to unseat `the ball valve 38.4

Referring to Figs. 2, 3 and 4, it will be seen that a cylindrical' liner comprising 'axially

separable sections

52, 53, 54 and 56 is disposed in cylindrical control valve chamber 17 with the right hand section 56 abutting cover plate 48. A sleeve 58 abuts cover plate 46 and holds left hand section 52 in place through spacer 57. The number of spacerscan'be varied toinsure proper positioning' ofthe cylindrical liner sections. The cylindrical sections are provided with circumferential grooves 61 placing pump discharge passages 18 in communication with the diametrically opposed transfer passages 22 and 26. In addition, each section is provided with a trans- Verse'bore 62 vplacing the interior of the liner in communication with'the grooves 61. The ends of the liner sections which abut l"one another are complementary shaped to form annular grooves 63. These grooves together with the relieved left hand end abutting portions of the

liner sections

53, 54 and 56 combine to place the interior of the cylindrical liner in fluid communication with the bypass passages 19. In addition, the right hand liner sectionV 56 is' provided with two circumferential grooves 64 and 66, the lgroove 66 communicating with working uid'passage 24. A transverse bore 67 through theliner section 56 places diametrically opposed portions of groove'66 in communication with the interior of the liner section 56. The groove 64 communicates'with'a main return passage '68' 'and with the' interior of the cylindrical liner through transverse bore 69.

A cylindrical control valve is positioned in the cylindri'call liner'sections for sliding movement relative thereto. This valve is preferably' formed by a series of separable

cylindrical valve elements

71, 72, 73 and 74 disposedin end abutting relation. VThe end abutting portions of the valveelernents are reduced to form annular spaces between same 'andthes'urrounding internal surfaces of the correspondingliner sections. In this connection it should be noted that the right hand valve element 74 comprises an inner reduced portion uniting enlarged cylindrical portions which in turn terminate in reduced oppositely projecting end portions 77 and 78. The end portion 77 abuts'the reduced portion on adjacent valve element 73 andthe other end portion '78 projects through a coaxial opening in cover plates 48. This right hand end of valve element 74 has an enlarged striking plate 79 which limits the left hand movement of the valve element. The left hand end valve element 71 has a piston rod 81 connected thereto which abuts a piston 82 slidably received in the sleeve 58. The valve elements are held in end abutting relation by means of a compression spring 83 having one end seated on the piston 82 and its other end in abutting relation to a cap 84 (see Fig. 2) which is secured to' the housing 11 by means of cap screws. Besides loading the valve elements 71 through 74 the piston 82 and coil spring 83 serve the further purpose of acting as a shock absorber for sudden loads imposed on the control valve. Inasmuch as the preferred embodiment of the subject apparatus is to be submerged in fluid when Vthe'v'alve is in the extreme right hand position (shown in Eig. 3) the piston 32 has moved' to` the'right'bey'ond an opening 86 in the liner v58v allowing'fluid toV pass from the left hand bypass passage 19 to the left of the pistonl 82. When the valve is moved to the left this fluid isf't'ra'pped as the piston`82'shuts off the opening 86 thereby compressing the fluid. A small opening 87 in the cap allows the fluid to seep through and in so doing cushions any shock that may be applied to the control valve.

Referring to Fig. 1, a relief valve 88 is positioned in the ho'using 11 in communication with the discharge side of the small plunger 13. This relief valve extends at right angles to the major axis of the small plunger and is positioned intermediate the unloading valve chamber 28 and the control valve chamber 17. Referring to Fig. 6 a cylindrical relief valve chamber 89 has a reduced portion adjacent the small plunger which provides a shoulder against which abuts a plug 91. The plug has an axial bore 92 therein which connects with a bore in communication with the discharge side of the small plunger. A ball valve seat is formed in the plug concentric with the bore 92 and remote from the plunger 13. A ball valve 93 is shown seated on the ball Vvalve seat and held there by a coil spring 94 acting through a positioning rod 96. The positioning rod has a complementary concave surface mating with ball valve 93 and a shoulder which is acted upo'n by one end of the coil spring 94. The other end of the coil spring abuts the plate 48 on the housing 11.

In downstream relation, that is in the direction of pressure fluid flow, to the pressure manifold 21 and the linking passage 27 in the working fluid passage 24 there is a manually operable ball check valve 97. This check valve or hold positioning valve is contained in an auxiliary housing fastened to the main housing 11 in any conventional manner. The hold positioning valve consists of a stepped cylindrical chamber 98 in which is contained a plug 99 which provides a valve seat for a ball valve 101. The ball valve 101 is resiliently contained on the seat by a compression spring 102. A plunger 103 slidably contained in the auxiliary housing is manually movable to force the ball valve 101 from its seat allowing fluid to flow in either direction. A lluid motor 105 is connected to the discharge side of the hold positioning valve 97 by passage 106.

In application the subject hydraulic apparatus is used in conjunction with a weight transfer system and the general hydraulic control system on a farm tractor. If it is desired to' quickly actuate the fluid motor 105 the main contol valve elements 71 through 74 are shifted to the extreme left as shown in Fig. 2. In this position the return passage 68 is blocked from the working fluid passage 24 and the bypass passages 19 are blocked off and the three large plungers 12 deliver pressure fluid to the working fluid passage 24. At the same time the small plunger 13 also delivers pressure fluid to the working fluid passage 24. From the working fluid passage the pressure fluid is carried through the ram passage 106 and out to the hydraulic motor 105. After the piston has extended to its maximum limit the fluid pressure in the system builds up to the critical point as determined by the setting of the unloading valvel and the pressure relief valve 88. Once the unloading valve pressure has been reached the ball valve 38 is forced from its seat and the pressure fluid then acts against the larger surface of the valve element 39 by passing the fluid from the three large plungers. It should be noted that although a large pressure is required to remove the ball valve from its seat a `considerably smaller pressure is required to retain the unloading valve in the unloading position due to the large surface of the Valve element 39. This allows the three large plungers to pump fluid back to the fluid reservoir at a considerably lo'w pressure thereby insuring only a minimum amount of heating of the fluid.

The small plunger 13, once the piston has reached its limiting position, is pumping through the relief valve 88 back to the fluid reservoir. In most cases it will be found that the relieving pressure limit of the relief valve 88 will be less than the pressure required to force the ball valve 38 fromY its seat and above the pressure required to keep the unloading valve open due to the action of the valve element 39. This is because normally the low pressure at which the large volume of fluid will not heat is much lower than the pressure required to actuate the hydraulic motor. Furthermore, in most cases, it is desirable to be able to' pressurize the fluid beyond that minimum pressure required to perform most of the requirements of the systemso that unusual situations can be handled. As an example, it has been determined that in the application of a particular embodiment of applicants invention the maximum safe or first pressure in the system is approximately 3500 pounds per square inch.

. The ball valve 38 is therefore adjusted to unseat when a pressure o'f 3500 pounds per square inch is built up in the system. It has further been determined that in the application of this particular embodiment of applicants invention serious heating in the system will not occur if the pressure is reduced to a lower pressure of approximately 60 pounds per square inch while the volume of fluid delivered by the three plungers is being moved through the opening pro'vided therefor. It has furthermore been found in the application of this particular embodiment of applicants invention that the application of approximately 1500 pounds per square inch in the fluid motor will cause the motor to exert a fo'rce sufficient to accomplish the majority of requirements placed on it. With this in mind the relief valve 88 has been set to relieve the system when a 1500 pounds per square inch pressure exists in the small plunger passage. From the above, it is seen that when the main control valve elements are placed in a position shown in Fig. 2, a maxi- Y mum amount of fluid is delivered to the fluid motor until the piston has extended to its maximum limit and the pressure thereby starts to increase. When this pressure has reached 1500 pounds per square inch the relief valve relieves the small plunger and when the pressure has reached 3500 pounds per square inch the unloading valve unloads the three large plungers. At this point the three large plungers are pumping against a pressure head of 60 pounds per square inch and the small plunger is pumping against a pressure head of 1500 pounds per square inch. Because of the smallvolume delivered by the small plunger excessive heating does not occur when a pressure head of 1500 pounds per square inch is maintained in the system. With this arrangement any normal leakage in the system does not affect the position of the piston as the 1500 pounds per square inch pressure maintained by the small plunger is sufficient to overcome any normal load on the liquid motor. Furthermore, in the event that so'me damage should result to the unloading valve or the three large plungers would not function, the

separate small plunger system is sufficient to actuate the hydraulic motor, although at a slower rate.

It should be understood that the pressure values mentioned above are only used for the sake of clarity and it is not intended that they should limit the scope of applicants invention.

Applicant has in effect provided a hydraulic apparatus with two independent coacting systems. In most cases both the three large plunger system and the small plunger system will act together. However, should either system fail to function the other is capable of actuating the hydraulic motor until it is possible to repair the damaged of said pumps having a larger fluid delivery vcapacity than the other; a uid responsive motor; a rst fluid con-v duit connecting the larger capacity pump and one side of said motorin pressure fluid communication; a fluid pressure reducing means connected in fluid communicaf tion with said first conduit and operable to connect said first conduit to said reservoir at a first pressure and maintain connection therewith at a pressure lower than said first pressure, said lower pressure being such that excessive heating of the fluid will not occur; a second fluid conduit connecting the smaller capacity pump and said one side of said motor in pressure fluid communication; a fluid pressure relief means connected in fluid communication with said second conduit and operable to connect said second conduit to said reservoir at a pressure intermediate said first pressure and said lower pressure; fluid return means selectively positionable to connect said one side of said fluid motor and said reservoir in fluid cornmunication; and control means operatively associated with each of said pumps fo'r selectively directing the flow of fluid to said conduits and said reservoir.

2. In a hydraulic system for operating a fluid motorsaid motor comprising: walls defining a working fluid pasf sage in said housing; walls defining a first passage in said housing connecting the larger of said pumps and said working fluid passage in fluid communication; a fluid pressure reducing means in fluid communication with said first passage and operable to connect said first passage to said reservoir` upon the attainment of a first pressure and to, maintain connection therewith at a pressure lower than said first pressure, said lower pressure being such that excessive heating of the fluid will no't occur; a check valve. in said first passage in downstream relation to said reducing means; walls defining a second passage in said housing connecting the smaller capacity pump and said working fluidpassage in fluid communication; a relief valve in fluid communication with said second passage and operable to connect said second passage with said reservoir when the fluid in said second passage attains a pressure intermediate said first and lower pressures; and a checkvalve in said second passage in downstream relation to said relief valve. Y

3. In a hydraulic system for operating a fluid motor including a housing, a fluid reservoir, a pair of pumps having their intake sides in communication with the fluid reservoir, one ofthe pumps having a larger fluid delivery capacity than the other, anda control valve chamber in thehousingin communication with each of the pumps for receiving pressurelfluid therefrom, the combination for controlling the quantity and pressure of fluid delivered to said motor comprising: walls defining a working fluid passage in said housing; walls defining a first passage connecting saidcontrol valve chamber and said working fluid passage in fluid communication; a fiuidfpressure reducing meansin fluid communication with said first passage and operable to connect` the. first passage to the reservoir upon the attainment of afirst pressure and to maintain connection therewith at a pressure lo'wer than said first pressure, said.lower pressure being such that excessive heatingfofthe fluid will not occur; a check valve in said first passage in downstream. relation to said reducing means; walls defining. asecondpassage connecting `said working passage and sadfchamber in fluid communication; pressure reliefhmean's in iluid communication with said,s econd passage and,v connecting said seco'nd passage tofsaid,rescrvoirlwhenthe. fluid in said second passage attainsafpre'ssure. intermediate said first and lower pressures; `azchecllsvalve in 4said vsecond passagein downstream relatie te.

flisfmsasn-flla. Camiel veli/s. mais.

chamber selectively positionable to connect the larger of said pumps to said first passage and the smaller o'f said pumps to'said second passage.

4. In a hydraulic system for operating a fluid motor including a housing, a fluid reservoir, a pair of pumps having their intake sides in communication with the fluid reservoir, one of said pumps having a larger fluid delivery capacity'than the other, and a control valve chamber in the housing in separate communication with each of the pumps for receiving pressure fluid therefrom, the combination for controlling the quantity and pressure of fluid delivered to said motor comprising: walls defining a working fluid passage in said housing; walls defining a return passage connecting said chamber `and said working passage in fluid communication; walls defining a first passage connecting said chamber and said working fluid passage in fluid communication; a fluid pressure reducing means in fluid communication with said first passage and operable to connect said first passage to said reservoir upon the attainment of a first pressure and lto maintain connection therewith `at a pressure lower than said first pressure, said lower pressure being such that excessive heatingV of the fluid will not occur; a check valve in said first passage in downstream relation to said reducing means; walls defining a second passage connecting said working passage and said chamber in fluid communication; pressure relief means in fluid communication with said second passage and connecting said second passage to said reservoir when the fluid in said second passage attains a pressure intermediate said first and lower pressures; a check valve in said second passage in downstream relation to said relief means; and a control valve in said chamber selectively positionable to connect the larger of said pumps to said first passage and the smaller of said pumps to saidv second passage and said return passage to said reservoir.

5. In a hydraulic system for operating a fluid motor including a housing, a fluid reservoir, a pair of pumps having their intake sides in communication with the fluid reservoir, one of said pumps having a larger fluid delivery capacity than the other, and a control valve chamber in the housing in separate communication with each of the pumps for receivingpressure fluid therefrom, the combination for controlling the quantity and pressure of fluid delivered toV said motor comprising: a pair of bypass passages in. said housing connecting said valve chamber with said reservoir; walls defining a working fluid passage in said housing; walls defining a first passage in said housing connecting said chamber and said working fluid passage in fluid communication; a fluid pressure reducing means in fluid communication with said first passage and operable to connect said first passage to said reservoir upon the attainment of a first pressure and to maintain connection therewith at a pressure lower than said first pressure, said lower pressure being such that excessive heating of the fluid will not occur; a check valve in the rst passage in downstream relation to the reducing means; walls definingy a second passage in said housing connecting said working passage and said chamber in fluid communication; pressure relief means in fluid cornmunication with said second passage and connecting said second passage to said reservoir when the fluid in said second passage attains a pressure intermediate said first and lower pressures; a check valve in said secondpassage in downstream relation to said relief means; and a control valve in said chamber selectively positionable to connect the larger of said pumps to said first passage and one of said bypass passages, and the smaller of said pumps to said second passage and the other of said bypass passages.

in a hydraulic system for operating a fluid motor including a housing, a fluid reservoir, a plurality of pumps having theirintalte sides in communication with the fluid reservoir, one of said pum-ps having a smaller fluid delivery capacity than the remainderof said pumps, and a control Yvalve Achamber in the housing in separate comm-unica` 9 tion with each of the pumps forreceiving pressure fluid therefrom, the combination for controlling the quantity and pressure of fluid delivered to said moto-r comprising: walls defining `a `working' fluid passage in said housing; walls defining -a return passage in said housing connecting saidvworking passage land said chamber in fluid communication; walls defining a pressure manifold in said housing connecting said chamber and said working fluid passage-in fluid communication; a fluid pressure reducing means in fluid communication with said pressure manifold and operable to connect lsaid manifold to said reservoir upon the attainment of a first pressure and lto maintain connection therewith at a pressure lower than said first assenso pressure, said lower pressure being such that excessive l heating of the fluid will not occur; a check valve in said manifold in downstream relation to said reducing means; a plurality of bypass passages in said housing severally connect-ing said chamber to said reservoir; walls defining a passageway in said housing connecting `said working passage and said chamber in fluid communication; pressure relief means in fluid communication with said passageway andl connecting said passageway to said reservoir when the fluid in said passageway attains a pressure intermediate said first and lower pressures; `a check valve in said passageway in downstream relation to said relief means; and a control valve in said chamber selectively positionable to connect said remainder of said pumps to said manifold and to -a bypass passage, and said one of said pumps to said passageway and a bypass passage and saidV return passage to said reservoir.

7. In a hydraulic system for operating a fluid motor including a fluid reservoir the combination for limiting the quantity andpressure olf fluid delivered to said motor comprising: a pair of pumps `having their intake sides in communication with said reservoir, one of .said pumps having a larger fluid delivery capacity than the other; a working fluid conduit in fluid communication with said motor and said reservoir; a manually operablercheck valve in said working fluid conduit selectively.positionable to allow fluid flow in opposed directions; a first fluid conduit connecting the larger of said pumps and `said working fluid conduit in pressure fluid communication; -a fluid pressurereducing rneans in fluid communication with said first conduit and operable to connect'sad first co-nduitr to said reservoir lat a first pressure Iand maintain connection therewith at a pressure lower than said first pressure, said lower pressure being such that excessive heating of the fluid does not occur; a check valve in said first conduit in downstream relation to said pressure reducing means; a second fluid conduit connecting the other of said pumps and said working fluid conduit in fluid corna munication both of said conduits beingconnected to said working fluid passage in upstream relation to said manually operable check valve; a fluid pressure relief means in fluid communication with said second conduit and operable to connect said second conduit to said reservoir when the fluid in said second conduit attains a pressure intermediate said first and lower pressures; a check valve in said second conduit in downstream relation to said pressure relief means; and control means operably asclosed within said housing and including an intake pas sage in communication with said reservoir, one ofrsaid pumps having a larger fluid delivery capacity than the other; a valve chamber in communication with the discharge'side of each of said pumps; walls defining a first passage in said housing connecting said chamber and said working fluid passage' in fluid communication; a

fluid pressure reducing means in fluid-communication' with said first passage and operable to connect said first passage to said reservoir upon attainment of a first pressure and to maintain connection therewith at a pressure lower than said first pressure, said lower pressure being such that excessive heating of the fluid will not occur; a check valve in said first passage in downstream relation to said reducing means; walls defining a second passage in said housing connecting said valve chamber and said working fluid passage in fluid communication; fluid pressure relief means in said second passage operable to connect said second passage to said reservoir when the fluid in said second passage attains a pressure intermediate said first and lower pressures; a check valve in said second passage in downstream relation to said relief means; and a control valve in lsaid chamber selectively positionable to connect the larger of said pumps to said first passage and the other of said pumps to said second passage.

9. In a hydraulic system for a farm tractor including a fluid motor, a fluid reservoir, a housing, a pair of pumps having their intake sides in communication with the fluid reservoir, one of the pumps having a larger fluid delivery capacity than the other, and control means operably associated with the pumps to selectively deliver pressureY fluid to the reservoir and the housing, the combination for controlling the quantity and pressure of fluid delivered to said motor comprising: walls defining a working fluid passage in said housing and operatively connected to one side of said motor; walls defining a first passage in said housing connecting the larger capacity pump and said working fluid passage in fluid communication; a fluid pressure reducing means in iluid'communication with said first passage and operable to connect said first passage to said reservoir upon the attainment of a first pressure and to maintain connection therewith at a pressure lower than said first pressure, said lower pressure being such that excessive heating of the fluid will not occur; a check valve in said first passage in downstream relation to said reducing means; walls defining a second passage in said housing connecting the smaller capacity pump and said working fluid passage in fluid communication; a relief valve in fluid communication with said second passage and operable to connect said second passage with said reservoir when the fluid in said second passage attains'a pressure intermediate said first and lower pressures; and a check valve in said second passage in downstream relation toI said relief valve.

References Cited in the file of this kpatent UNITED STATES PATENTS Towler et al. May 15,