US2901975A

US2901975A - Pumping unit

Info

Publication number: US2901975A
Application number: US478559A
Authority: US
Inventors: Adolf R Grad
Original assignee: Oilgear Co
Current assignee: Oilgear Co
Priority date: 1954-12-30
Filing date: 1954-12-30
Publication date: 1959-09-01
Anticipated expiration: 1976-09-01

Description

Sept. 1, 1959 A. R. GRAD 2,901,975

PUMPING UNIT Filed Dec. 30, 1954 6 Sheets-Sheet 1 I I INVENTOR ADOLF R. GRAD BMW/Mia ATTORNEY A. R. GRAD PUMPING UNIT Sept. 1, 1959 6 Sheets-Sheet 3 Filed Dec. 30, 1954 A R. GRAD PUMPING UNIT Sept. 1, 1959 6 Sheets-Sheet 4 Filed Dec. 30, 1954 INVENTOR ADOLF R. GRAD ATTQRNEY Sept. 1, 1959 A. R. GRAD I 2,

PUMPING UNIT- Filed Dec. 30, 1954 e Sheets-Sheet 5 FIG. 6

INVENTOR 29 ADOLF R. GRAD WMWg/M 'ATTORN Sept. 1, 1959 A. R. GRAD 2,901,975

PUMPING UNIT I Filed-Dec. 30, 1954 6 Sheets-Sheet 6 FIG. 7

INVENTOR ADOLF R. GRAD ATTORNEY United States Patent PUMPING UNIT Adolf R. Grad, Milwaukee, Wis., assignor to The Oilgear Company, Milwaukee, Wis., a corporation of WIS- consin Application December 30, 1954, Serial No. 478,559

1 Claim. (Cl. 103-4) This invention relates to variable displacement pumps of the radial piston type and it has as its object to provide a compact pumping unit comprising a plurality of simultaneously driven variable displacement pumps which are arranged in a single casing and are individually adjustable to vary their displacements independently of each other.

A pumping unit embodying the invention is particularly adapted for energizing the hydraulic motors which rotate the turret and operate the gun elevating mechanism of a military tank and the unit will be explained as employed for that purpose but it is to be understood that it is not limited to such use. For example, pumping units embodying the invention can just as readily be employed for energizing the hydraulic motors which drive the traversing and elevating mechanisms of antiaircraft batteries.

In the present tanks, the turret traversing motor and the gun elevating motor are energized by separate pumps which are geared together and are driven by single electric motor. By employing a pumping unit embodying the invention for energizing these motors, a saving is effected of nearly one-half the space required for the two pumps and the gearing, and the noise and power loss in the gearing is eliminated.

Another object of the invention is to provide a pumping unit comprising a plurality of variable displacement pumps and an auxiliary pump and to connect the pumps in circuit in such a manner that, when the pressure created by any variable displacement pump becomes so high that the pump discharges through its relief valve, the volume of hot liquid thus exhausted through the relief valve is replaced by an equal volume of cooler liquid supplied by the auxiliary pump.

Other objects and advantages will appear from the following description of the pumping unit shown in part in detail and in part schematically in the accompanying drawings in which the views are as follows:

Fig. 1 is a longitudinal section through a pumping unit in which the invention is embodied, the plane of the view being indicated by the line 11 of Fig. 2.

Fig. 2 is a transverse section through one of the variable displacement pumps, the plane of the view being indicated by the line 22 of Fig. 1.

Fig. 3 is a transverse section through the other variable displacement pump, the plane of the view being indicated by the line 33 of Fig. 1.

Fig. 4 is a view, partly in full and partly in section, taken on the irregular line 4-4 of Fig. 1 but drawn to a larger scale and showing the face of the valve which controis the flow of liquid to and from the pump shown in Fig. 2, a part of the pump casing being broken away to expose the ends of the channels through which liquid flows to and from the pump.

Fig. 5 is a sectional View taken on the irregular line 55 of Fig. 4 to show the hold-up motors which keep the face of the valve shown in Fig. 4 in contact with an end of the cylinder barrel shown in Fig. 1.

Fig. 6 is a view, partly in full and partly in section, taken on the line 6-6 of Fig. l but drawn to a much larger scale and showing the face of the valve which controls the flow of liquid to and from the pump shown in Fig. 3, a part of the end head of the pump casing being broken away to expose portions of the channels through which liquid flows to and from the pump.

Fig. 7 is a sectional view taken on the line 77 of Fig. 6 to show the hold-up motors which keep the face of the valve shown in Fig. 6 in contact with an end of the cylinder barrel shown in Fig. 1.

Fig. 8 is a diagram of the hydraulic circuit of the pumping unit when it is connected to hydraulic motors.

The pumping unit includes two reversible, variable displacement, radial piston pumps P1 and P2 which have been shown in Fig. 1 as being of different sizes but they may be the same size. Pumps P1 and P2 are arranged within and carried by a casing comprising a body 1 which is open at its rear end and has an integral front wall 2, an end head 3 which closes the rear end of body 1 and a support 4 which is fastened to the front end of body 1 and is adapted to support an electric motor (not shown) for driving the pumping unit. Casing. 1--4 may be mounted in any desired position and the directional terms used hereinafter refer to the drawings:

and not to the mounted position of the pumping unit.

Pump P1 and pump P2 are reversible variable displacement pumps each having radial pistons in a rotat-- able cylinder barrel. The piston containing barrels for the two pumps are constructed and arranged coaxially of each other for rotation with a driving shaft. As shown, the cylinder barrels for the two pumps are preferably of different size and are integral with each other and comprise a first barrel portion herein designated cylinder barrel 5 and a second barrel portion herein designated cylinder barrel 5 for pumps P1 and P2 respectively.

Pump P1 includes a cylinder barrel 5 having a bank of pistons and cylinders arranged radially therein. As shown in Figs. 1 and 2, cylinder barrel 5 has a plurality of cylinders 6 arranged therein in two annular rows and a piston 7 fitted in each cylinder. One cylinder in each of the two rows communicates with a fluid channel or passage 8 which extends from the inner row of cylinders through the left end of cylinder barrel 5 and, during rotation of cylinder barrel 5, it registers first with one and then the other of two

arcuate ports

9 and 10 formed in an annular flat valve 11 which is held against the left end of cylinder barrel 5 as will presently be explained. Cylinder barrel 5 is rotated by a drive shaft 12 which has been shown as being formed integral therewith. Shaft 12 extends through valve 11 and through front wall 2 into motor support 4 for connection to an electric motor or other prime mover not shown.

Reciprocation of pistons 7 during rotation of cylinder barrel 5 is effected by a thrust member comprising a slide block 15, which is slidable transversely of the axis of cylinder barrel 5 and is restrained from movement in any other direction, and a ball bearing 16 which is fitted in slide block 15 and has two oppositely inclined annular bearing surfaces formed upon its inner face to engage the beveled outer ends of pistons 7. Thus the inner race of ball bearing 16 provides thrust member 15 with an annular internal bearing surface in contact with the outer ends of the pistons 7.

In practice, slide block 15 is mounted between two roller bearings but, in order to simplify the drawing,

it has been shown in Fig. 2 as having a wear plate 17 fixed to its upper and lower ends adjacent to each of its corners and closely fitted between two

bearing plates

18 and 19 which are fixed to the upper and lower walls 1 of casing body 1.

and an annular bushing 20 is fitted in the bored, portion, 1 of body- 1 and holds slide block 15 in contact. with.

the inward extending portions of the wall of, bod y 1'.

The arrangement is suchthat thrust member 15-16 is free tomove in a horizontal plane transversely of cylinder barrel 5. but is prevented from moving inany other direction. Bushing has fixed therein the outer race. of a; radial thrust bearing 21 which rotatably supports; cylinder barrel 5 and has its inner race; fixed; thereon.

Slide block 15 has formed integral therewith on, one of its sides a pair of spaced apart lugs. 22; of' which onlyone appears in the drawing Lugs 22 have arranged therebetween and connected thereto bya pin 23' a link 2.43 for connecting slide block 15 to a controlv 25v which is adapted to shift slide block 15 in one direction or the other to vary the displacement of pump P1 and which is mounted upon a support 26 formed integral with oasing body 1. Since the control used in practice is very complicated, pump P1 has been shown as being: pro.-

vided with a simple hydraulic. control 25 which will be presently described but the control may be either elec.-. trical or mechanical.

As shown in Figs. 4 and 5, valve 111 has arranged therein two hold-up motors 27, each of which. communicates with port 9: through a passage 28, and two hold-up motors 29 each of which communicates with port 10 through a passage 30.

Hold-up

motors

27 and 29 are alike and each includes a cylinder 31 which is formed in valve 11-, a tubular piston 32 which is fitted in cylinder 31, an annular seal ing member 33 which is arranged between piston 32.,and

the inner face of front wall 2, and a spring 34' for ini' tially urging valve 11 against cylinder barrel 5 and for initially urging piston 32 against sealing member 33 and member 33 against wall 2. When pump P1 is: creating pressure, the hold-up motors on the discharge side of the pump will urge valve 11 against cylinder barrel 5' with a force proportional to pump pressure, and if thereis any pressure in the liquid supplied to the pump, the hold-up motors on the intake side of the pump will urge valve 11 against cylinder barrel 5 with a force portionalto that pressure, thereby preventing the pressure in the liquid between the valve and the cylinder barrel from moving the valve away from the cylinder barrel. The contacting surfaces of pistons 32 and members 331 are made spherical and smooth and the contacting surfaces of members 33 and front wall 2 are made flat and smooth to provide substantially liquid tight seals between pistons. 32' and. front plate 2'.

Each hold-up motor 27 communicates through a. passage35 with a main passage 36 which is formed in front wall 2 and is adapted to be connected at one end thereof to one side of a hydraulic circuit,.and each hold-up motor 29 communicates through a passage 37 with. a main passage 38' which is formed in front wall 2 and is adapted to be connected at one end thereof to the other side of the hydraulic circuit.

Main passages

36 and 38. are i also connected

totwo check valves

39 and 40 respectively and to two relief valves 41 and 42, respectively. Check

valves

39 and 46 are both connected to a supply channel 43 and relief valves 41 and 42 are connected to an exhaust channel 44 as shown in Fig. 8.

The arrangement is such that, when slide block 15 is in its central or neutral position, bearing 16 is concern trio with cylinder barrel 5 and rotation of'cylinder barrel 5 will not efiect reciprocation of pistons 7. But when slide block; 15 is olfset to one side of its neutral positionand cylinder-barrel 5 is rotated in a given direction the pistons 7 above the horizontal center line of cylinder barrel 5 will be forced progressively inward by bearing 16 and will. eject liquid from their cylinders 6. through passages 8, valve port 9, passages 28, hold-up motors 27 and passages 35 into main passage 36 and bearing 16 will permit the pistons 7 below the horizontal center line to move progressively outward and liquid to flow into their cylinders 6 through passages 8, valve port 10, passages 30, hold-up motors 29, and passages 37 from main passage 38.

Conversely, when slide block 15. is offset to the other side of its neutral position and cylinder barrel 5 is. rotated in the same direction, the pistons 7 below the horizontal center line of cylinder barrel 5 will be forced progressivel-y inward by bearing16-andwill eject liquid from their cylinders 6 through passages 8, valve port 10, passages 30, hold-up motors 29 and passages 37 into main passage 38 and bearing 16 will permit the pistons 7 above the horizontal center line to move progressively outward and liquid to flow into their cylinders through passages 8, valve port 9, passages 28, hold-up-motors- 27 and passages 35 from main passage 36;.

Pump PI will thus deliver liquid in a direction and at a rate determined by the direction and distance slideblock 15 is offset from its neutral position and the pressure in the hold-up motors on each side of the pump will urge valve 11 against the end of cylinder barrel 5"Wll'b. a force proportional to the pressure prevailing in that side of the pump.

The adjacent ends of

valve ports

9 and 10 are spaced apart a distance equal to the diameter of a passage 8 to providetherebetween two

bridges

45 and 46 which prevent one valve port from communicating with the other valve port as the end of a passage 8 moves from one port to the other, When the pump is performing useful Work, the pressure in the passage 8 in communication with the

valve port

9 or 10 which at that time is the discharge port will. berelatively high while the pressurein the passages Si in communication with the other or intake port will be much lower or negative. Since thereis an odd number of passages 8, the number containing liquid under pressure will vary between odd and even each time a passage 8 crosses one of the bridges, thereby causing a variation in the blow off force which tends to move valve 11 away from cylinder barrel 5.

In order to compensate for the variations in blow-oh force, at least one balancing motor is arranged behind each of

bridges

45 and 46 to assist the hold-up motors in holding valve 11 in contact with cylinder barrel 5'. As shown in Fig. 5, each balancing motor includes a cylinder 47, which is formed in valve 11 behind one of the bridges, and a piston 48 which is fitted. in cylinder 47 and engages front plate 2. A small hole 49 extends from cylinder 47 through the. face of valve 11 so that one hold-up motor is energized each time a passage 8 passes from the discharge port on to one bridge and the other balancing motor is energized each time a passage 8 opens to the discharge port.

Pump P2 is substantially the same as pump P1 except that it has been shown as being smaller. Therefore, corresponding parts have been indicated by corresponding reference numerals with the exponent a added-to the reference numerals applied to pump P2 so that only a brief description of pump P2 is necessary.

Pump P2 includes a cylinder barrel 5 which has been shown as being a part of cylinder barrel 5. Cylinder barrel 5 has a bank of cylinders 6 and pistons 7 arranged radially therein in two annular rows and an equal number of channels or passages 8 formed therein with each passage communicating with one cylinder in each. of the two rows and extending through the right end. of. the cylinder barrel to register during rotation of the cylinder barrel first with one and then. the other of two

arcuate valves ports

9 and 10 formedin a flat valve 11 which engages the end of cylinder. barrel 5 Reciprocation of pistons 7 during. rotation of cylinder barrel 5 is effected by a thrust member comprising a slide block 15 which is slidable transversely of the axis of cylinder barrel 5 and is restrained from movement in any other direction, and a ball bearing 16 which is fitted in slide block 15 and has two oppositely inclined annular bearing surfaces formed upon its inner face to engage the beveled outer ends of pistons 7 In practice, slide block 15 is mounted between two roller bearings but, in order to simplify the drawing, it has been shown in Fig. 3 as being slidably mounted in bushing 20 by means of wear plates 17 and bearing plates 18 and .19. Slide block 15 is restrained from axial movement by a spacer ring 51, which is arranged between it and the outer race of ball bearing 21, and by a flanged spacer ring 52 which is arranged between end head 3 and bushing 20.

Slide block 15 has formed integral therewith on one of its sides a pair of spaced apart lugs 22 of which only one appears in the drawing. Lugs 22 have arranged therebetween and connected thereto by a pin 23 a link 24* for connecting slide block 15 to control 25 which is adapted to shift slide block 15 in one direction or the other to vary the displacement of pump P2 and which is mounted upon support 26. For the purpose of illustration control 25 has been shown as being of a simple hydraulic type and will be presently described. When cylinder barrel 5 is rotated, pump P2 will deliver liquid in a direction and at a rate determined by the direction and distance slide block 15 is offset from its neutral position in the same manner as does pump P1.

As shown in Figs. 6 and 7, end head 3 has arranged therein two hold-up motors 57, each of which communicates with valve port 9 through a passage 58 formed in valve 11*, and two hold-up motors 59 each of which communicates with valve port through a passage 60 formed in valve 11*. Hold-up

motors

57 and 59 urge valve 11 against the end of cylinder barrel 5 and are assisted in doing so by two balancing motors each including a cylinder 47 which is formed in valve 11 and has a small hole 49* extending therefrom through the face of the valve plate at a point between the adjacent ends of the two valve ports, and a piston 48 which is fitted in cylinder barrel 47 and engages end head 3. Hold-up

motors

57 and 59 and balancing motors 47 18 functions in the same manner as the hold-up

motors

27 and 29 and balancing motors 47-48 respectively.

Hold-up

motors

57 and 59 are alike and each includes a cylinder 61 which is formed in end head 3, a tubular piston 62 which is fitted in cylinder 61, an annular sealing member 63 which encircles a

passage

58 or 60 and is arranged between end head 3 and the rear face of valve 11 a chamber 64 which is formed in end head 3 and is concentric with and slightly smaller than cylinder 61, and a keeper ring 65 which is arranged between the end of piston 62 and the shoulder between cylinder 61 and chamber 64 to prevent piston 62 and sealing member 63 from moving a substantial distance away from valve 11 when there is no pressure in chamber 64. Any substantial movement of valve 11 relatively to end head 3 is prevented by two pins 66 (Fig. l) which connects valve 11 to end head 3 and has sufficient clearance to permit valve 11 to adjust itself to the end of cylinder barrel 5 The contacting surfaces of valve 11 and sealing members 63 are made flat and smooth and the contacting surfaces of members 63 and pistons 62 are made spherical and smooth to provide substantially liquid tight joints between valve 11 and pistons 62.

The two chambers 64 of hold-up motors 57 communicate with a main passage 67 which is formed in end head 3 and is adapted to be connected at one end thereof to one side of a hydraulic circuit and the two chambers 64 of hold-up motors 59 communicate with a main passage 68 which is formed in end head 3 and is adapted to be connected at one end thereof to the other side of the hydraulic circuit.

Main passages

67 and 68 are also connected to two

check valves

69 and 70 respectively and to two

relief valves

71 and 72 respectively. Check

valves

69 and 70 are both connected to supply channel 43 and

relief valves

71 and 72 are both connected to exhaust channel 44 as shown in Fig. 8.

The arrangement is such that, when the cylinder barrel is rotated and slide block 15 is offset to one side of its neutral position, the pistons in cylinder barrel 5 above its horizontal centerline will be forced progressively inward and the pistons below the horizontal centerline will move progressively outward. The inward moving pistons will eject liquid from their cylinders through passages 8 valve port 9 passages 58, hold-up motors 57 and chamber 64 into passage 67 and the cylinders containing outward moving pistons will be supplied with liquid through their passages 8 valve port 19 passages 60, hold-up motors 59 and chambers 64 from main passage 68. When slide block 15 is offset to the other side of its neutral position, the flow of liquid will be reversed.

Since valves 11 and 11 are prevented from moving away from the cylinder barrel by the hold-up motors, the pressure in the liquid between valve 11 and the cylinder barrel urges the cylinder barrel toward the right when pump P1 is creating pressure and the pressure in the liquid between valve 11 and the cylinder barrel urges the cylinder barrel toward the left when pump P2 is creating pressure but one pump may be creating a maximum pressure when the other pump is not functioning so that the cylinder barrel is urged axially by the full force of the liquid between one of the valves and the cylinder barrel.

In order to prevent any axial movement of the cylinder barrel, shaft 12 is rotatably supported by a double row preloaded axial thrust bearing 76 (Fig. l) which has its inner race mounted upon shaft 12 and rigidly secured against axial movement in either direction and its outer race closely fitted in support 4 and rigidly secured against axial movement in either direction.

if the pump is of substantial size, bearing 76 may be of the type having two rows of tapered rollers with the rollers in one row tapered oppositely to the rollers in the other row. But if the pump is small, bearing 76 may be of the type having two rows of balls as shown in which case the inner and outer races are so proportioned that the balls in one row bear heavily against the outer race at the right of the center line of the row and bear heavily against the inner race at the left of the center line of the row and the balls in the other row bear heavily against the outer race at the left of the center line of the row and bear heavily against the inner race at the right of the center line of the row so that, when the bearing is pressed into support 4, one row of balls tends to urge shaft 12 in one direction and the other row of balls tends to urge shaft 12 in the opposite direction. The bearing is thus preloaded and positively prevents any axial movement of the cylinder barrel.

In order to supply cool liquid to either of pumps P1 and P2 when it discharges liquid through its relief valve and in order to provide liquid for supercharging pumps P1 and P2 and for control purposes, the pumping unit is provided with an auxiliary pump P3 which has been shown as a gear pump having its driving gear 77 fixed on shaft 12 and its idler gear 78 journaled in front wall 2 and in support 4.

As shown in Fig. 8, pump P3 draws liquid from a reservoir 79 and discharges it into supply channel 43 at a rate in excess of requirements and the excess liquid is exhausted into reservoir 73 through a channel 80 having connected therein a relief valve 81 which enables pump P3 to maintain a constant low pressure in channel 43.

Channel 43 is formed in part by external piping and in part by passages formed in the casing of the pumping unit. It is connected to check

valves

39, 40, 69 and 70, as

7. previously explained, and it is also connected to

controls

25 and 25 which will now be described.

For the purpose of illustration, control 25 has beenshown in Fig. 2 as including a cylinder 82 which is fastened to support 26, a differential piston 83 which is fitted in cylinder 82. and is connected by a pin 84 to the end of link 24 which is arranged within a bore 85 extending axially through piston 83', and a pilot valve 86 which is fitted in bore 85 and may be shifted manually but in practice it is shifted by an electronically controlled apparatus not shown. Valve 86. is also fitted in a bore 87 which is formed in the end head of cylinder 82 and has a port 88 formed in the wall thereof.

Piston 83 has an annular groove or port 89 formed therein around bore 85 and a passage 90 extending from port 89 through the outer end of piston 83. Pilot valve 86 is reduced in diameter near its inner end to provide a cannelure 91 and a valve piston 92 which is the same width as port 89 so that a slight movement of valve 86 in one direction or the other will open port 89 either to cannelure 91 or to the inner part of bore 85. Cannelure 91 is connected to port 88 by a passage 93 formed within valve 86.

Liquid for energizing control 25 is. supplied by pump P3v through channel 43 a branch of which is connected to the inner end of cylinder 82 by a passage 94 formed in the wall of cylinder 82. and is connected to port 88 by a passage 95 formed in the end head of cylinder 82 so that cannelure 91 and the inner end of, cylinder 82 are constantly supplied with liquid at the same pressure.

Piston 83 has the inner part thereof reduced in diameter and extending through the inner end wall of cylinder 82 and it is so proportioned that the piston area upon which the liquid in the outer end of cylinder 82' acts is considerably greater than and preferably is twice as great as. the piston area upon which the liquid in the inner end of cylinder 82. acts.

The arrangement is such that, when pilot valve 86 is moved inward a given distance, liquid will flow from channel 43 through passage 95, port 88, passage 93, cannelure 91, port 89 and passage 9% into the outer end of cylinder 82 and cause piston 83 to move slide block 15 toward the left and to eject liquid from the inner endof cylinder 82 through passage 94- into passage 95 until piston 83 has moved exactly the same distance that valve- 86 was moved and then valve piston 92 will cover port 89' and further movement will cease.

Conversely, when pilot valve 86 is moved outwarda given distance, liquid will fiow from channel 43 through passage 94 into the inner end of cylinder 82 and cause piston: 83 to move slide block 15 toward the right and to eject liquid from the outer end of cylinder 82 through passage 9; port 89 and bore 85 to exhaust until piston 83. has moved exactly the same distance that valve 86 was moved and then valve piston 92' will cover port 89 and further movement will cease.

Control 25 (Fig. 3) is identical to control 25, like parts of the two controls have been designated by like reference numbers, and control 25* functions to control pump P2. in the same way that control 25 functions to control pump Pi. Therefore, a detailed description thereof is unnecessary.

Pump P1 has been shown inFig. 8.as being connected to a rotary hydraulic motor M1 by two

channels

96 and 97 which form therewith a closed hydraulic circuit and pump P2. has been shown as being connected to a-recipro eating; hydraulic, motor M2. by two

channels

98 and 99 which form therewith a closed hydraulic circuit but either pump P1 or pump. P2v may be employed. for energizing either a rotary motor or a reciprocating, motor or for other purposes. The two pumps, the two motors and the interconnecting channels thus constitute two hydraulic transmissions which function independently of each other.

Channels

96, 97, 98 and 919 are formed in part by external piping and in part by passages formed in the 8. casing of the pumping unit. Specifically, channel 96 in cludes passage 36 (Fig. 4) to which check valve 39 and relief valve 41 are connected, channel 97 includes a main passage 38 (Fig. 4) to which check valve 48' and relief valve 42 are connected, channel 93 includes main passage. 67 (Fig. 6') to which check valve 69 and relief valve 7 are connected, and channel 99 includes passage 68 (Fig. 6) to which check valve 70 and relief valve 721 are. connected.

The arrangement is such that, when the pumping: unit is operating, pumps P1 and P2 are supercharged by pump P3. That is, any leakage inthe transmissions is made up by pump P3 which maintains a low pressure at the intake of pumps P1 and P2 when those pumps are dis=- charging liquid in either direction and it maintains a low pressure at bot-l1 sides of either pump P1 or F2 when the pressure created by that pump is less than the pressure created by pump P3.

In a transmission in which the pump and motor is connected into aclosed or substantially closed hydraulic circuit, it is customary to connect the relief valves be, tween the two sides of the circuit so that when the pump discharges through a relief valve in response to the load on the motor becoming excessive, the liquid discharged through therelief valve flows directly to the intake of the pump,. thereby causing the energy dissipated at the reliefvalve to heat the liquid in the circuit.

When pump P1 or P2 creates a pressure in excess of a predetermined. maximum, the pump will discharge liquidthrough the relief valve connected to the dischargeside of the pump and an equal volume of liquid will flow from supply channel 43 into the intake side of the pump through the check valve connected thereto. For example, if pump P1 should discharge liquid through relief valve 41, it will be supplied with liquid through: check valve 40:

Since nearly all of the liquid discharged by pump P3 is exhausted through relief valve 81 during the greater part of the time that the pumping unity is in operation, it is desirable that the. volumetric capacity of pump P3 be considerably less than the capacity of pump P1 as otherwise the liquid in reservoir 79 would become unduly heated by the heat generated at relief valve 81.

If pump P1 or P2 should exhaust liquid through oneof. its relief valves when its slide block was so. adjusted that the rate of exhaust was no greater than the rate at which. pump P 3 delivers the liquid into channel 43:, all of the. liquid exhausted through the relief valve would flow through channels 44 and and relief valve 81 into reservoir 7-9 and an equal volume of cool liquid would be supplied by pump P3 to the intake side of pump P1 or P2 through the check valve connected thereto. But if pump P1 or P2 was so adjusted that the rate at which it exhausted liquid through its relief valvewas greater than the. volumetric capacity of pumpv P3, enough of the exhausted liquid to make up the difference between the.

volumeof liquid exhausted into. channel 44 and thevolume discharged by pump P3 would fiow through channel 80 into channel 43 and the remainder would be exhausted through relief valve 81 in which case pump P1 or P2 would. be supplied. with a mixture of hot and cool liquid through the check valve. connected to its intake side.

Thepumping unit described herein may be modified in various ways without departing from the scope of the invention which is hereby claimed. as follows:

A pumping unit comprising a casing, a drive shaft journaled in said casing, two reversible variable displace.- ment pumps. arranged within said casing. and connected tosaid shaft to. be driven thereby, two main channels connected to opposite. sides of each of said variable displacement' pumps for connecting said variable displacement pumps to separate hydraulic motors, an exhaust.-

to that pump creating a pressure in excess of a predetermined maximum during pump delivery in either direction, an auxiliary constant displacement pump connected to said shaft to be driven thereby, a reservoir connected to the intake of said auxiliary pump for supplying relatively cool liquid thereto, a supply channel connected to the outlet of said auxiliary pump, and a check valve connecting each of said main channels to said supply channel to enable said auxiliary pump to supply to the intakes of said pumps an amount of relatively cool liquid from said reservoir to replace the relatively hot liquid discharged into said exhaust channel, a by-pass channel connecting the outlet of said auxiliary pump to a relatively low pres- 10 sure relief valve that discharges fluid to said reservoir, and said exhaust channel connected to said by-pass channel so that all said relief valves for said pumps are connected to said reservoir through said low pressure relief valve.

References Cited in the file of this patent UNITED STATES PATENTS 2,303,685 Eden et al. Dec. 1, 1942 2,407,013 Ifield Sept. 3, 1946 2,458,985 Ferris et al Ian. 11, 1949 2,660,123 Vlachos Nov. 24, 1953 2,743,582 Wiedmann May 1, 1 956