US2504665A - Hydraulic transmission with pressurized reservoir - Google Patents

Description

April 18, 1950 Y J. K. DOUGLAS 2,504,665

HYDRAULIC TRANSMISSION WITH PRESSURIZED RESERVOIR Filed July 1, 1948 2 Sheets-Sheet 1 'l/lllllllllllll/llllllllim v ITIIIIIIIIIIIII 1 3O IIIIIIIIIIIIIIIIII i 5 z 6/ I z 7 I 2 s 35 35 a l' I 34 2 I f F E- [III/11111111111/1111/1/1111i11/11111111/11/11/1/1/1/1111/1/ INVENTOR JAMES K. DOUGLAS Y ATTORNEY P 8, 1950 J. K. DOUGLAS I 2,504,663

' HYEiRAULIC TRANSMISSION WITH PRESSURIZED RESERVOIR Filed July 1, 1948 a V w 2 Sheets-Sheet '2 I 'l/l/I/ I I N q S 3| f r m N N N z 2 s I 5 NE n 3 5 IO & s I! x I, v 8 I, r- Q l ml 1 r ml 0 3 a I 1 I r n I IIIIII IIIIIIIIII'IIII j L I JI 2/ E f I m I I Lu I n i 1." if: p i l 5 92 m l l/l/l/l/l/Il/l/ II/IllIII/l/l E N o z a N t I f" S m Q- N N i v i i v I u a (O x z a z z y INVENTOR JAMES K. DOUGLAS ATTORNEY Patented Apr. 18, 1950 UNITED STATES PATENT OFFICE HYDRAULIC TRANSMISSION WITH PRESSURIZED RESERVOIR Application July 1, 1948, Serial No. 36,400

11 Claims.

This invention relates to hydraulic transmissions of the type which includes a hydraulic motor, a pump for supplying motive liquid to the motor to effect operation thereof, a reservoir containing a supply of liquid for the pump, means connecting the pump and the motor into a hydraulic circuit and means for reversing the flow in the circuit to cause the motor to operate in opposite directions alternately, and in which the motor is so constructed that during operation thereof in one direction the liquid discharged therefrom is in excess of the liquid required to supply the pump and that during operation of the motor in the opposite direction the liquid discharged therefrom is less than the liquid required to supply the pump.

For example, the transmission may be of the type which includes a differential reciprocating motor, such as a cylinder having a piston fitted therein and provided with a piston rod which extends through one end only of the cylinder, and a valve for directing liquid from the pump to the rod end of the cylinder and from the head end of the cylinder to the pump and to the reservoir to eiiect operation of the motor in one direction and for directing liquid from the pump and liquid from the rod end of the cylinder to the head end of the cylinder to efiect operation of the motor in the opposite direction. For the purpose of illustration, the invention will be explained as embodied in a transmission of this type but it is to be understood that the invention is applicable to other types of transmissions.

When the pump delivers liquid to the rod end of the motor cylinder, the head end of the cylinder is connected to the intake of the pump and the-liquid delivered by the pump causes the motor "piston to move toward the head end of the cylinder and to expel liquid from the cylinder to the intake of the pump. Due to the displacement of the piston rod, the liquid expelled from the cylinder is in excess of the liquid required to supply the pump and the excess liquid is exhausted through a back pressure valve into the reservoir.

when the pump delivers liquid to the head end of the motor cylinder, the ends or the cylinder are connected to each other. The liquid delivered by the pump causes the motor piston to move toward the rod end of the cylinder and to expel liquid therefrom to the head end of the cylinder so that at this time the pump m-ust draw its entire supply of liquid from the reservoir through a suitable suction valve. The level of the liquid in the reservoir thus rises and falls during each reciprocation of the motor piston.

It is well known that the motive liquid must I be kept as free as possible from bubbles of air or gas as bubbles in the liquid causes the motor piston to slow down in response to an increase in load and to jump ahead in response to a suddent decrease in load. Theoretically, the joints in the suction valve and the associated parts may be made airtight but it has been found in practice that the joints do not remain airtight and that air seeps through the joints and becomes entrained in the motive liquid during the time that the pump is sucking its supply of liquid from the reservoir. Also, the liquid employed in such transmissions is a mineral oil and, when a pump is sucking oil from a lower level, the pressure at the intake of the pump is below atmospheric pressure which causes gas bubbles to form in the oil.

The present invention has as an object to prevent air from entering the motive liquid through the suction valve assembly.

Another object is to supercharge the pump during at least the greater part of the time that the transmission is in operation.

Other and more specific objects will appear from the following description of the embodiment of the invention shown schematically in the accompanying drawings in which the views are as follows:

Fig. l is a diagram of the hydraulic circuit of a transmission in which the invention is embodied, the flow of liquid to effect movement of the motor piston toward the left being indicated by arrows.

Fig. 2 is a view similar to Fig. 1 but indicating the flow of liquid to effect movement of the motor piston toward the right.

Fig. 3 is a diagrammatic view showing the pump mounted upon the reservoir in the customary manner, the reservoir being shown in vertical longitudinal section and the pump being shown in elevation with a part thereof shown in section and with certain channels shown in positions different from those occupied in actual practice.

As shown, the transmission includes a hydraulic motor I having a piston 2 fitted in a stationary cylinder 3 and provided with a piston rod 4 which extends through one end of cylinder 3 for connection to an element to be reciprocated such as the table of a bed planer.

Liquid for energizing motor I is supplied by a pump 5 which is mounted directly upon reserapoaeoa voir 8 and when the drive is in operation it is The flow of liquid to and from motor I is controlled by a valve 1 which in practice is mounted upon reservoir 6 at the right 01' pump 5 in respect to Fig. 3 and which is connected to opposite ends of pump 5 by two channels 8 and 9 and is connected to opposite ends of cylinder 3 by two channels and ii.

Valve 1 has been indicated as being of a well known type of diiierential control valve which either will connect opposite sides of pump to opposite ends of cylinder 3 as indicated in Fig. 1 or will connect the outlet of pump 5 and the rod end of cylinder 3 to the head end of cylinder 3 as indicated in Fig. 2 but the valve used in practice is a so called combination valve which includes a start and stop valve, a direction control valve and a pilot valve and which is similar to the valve shown in Patent No. 1,943,061 except that the control valve is arranged for differential operation of the motor as explained above.- Also, pump 5 has been shown as being a unidirectional pump which has its output directed to one end or the other of cylinder 3 by valve I but the same result may be obtained by using a reversible pump; and connecting a simple differential valve be-, tween the pump and the motor as shown in Patent No. 2,093,690.

The intake side of pump 5 communicates through a channel [4 with the outlet of a suction valve l5 which is attached to the bottom of the pump casing and extends downward through an opening l6 formed in the upper wall of reservoir 6. The inlet of suction valve l5 has a suction pipe 11 connected thereto and extending downward to a point near the bottom of reservoir 6.

The intake side of pump 5 also communicates through a channel ill with the inlet of a low pressure resistance valve, l9 which functions as a back pressure valve and has its outlet connected by..a channel to a pipe 2| which extends downward through opening it to a point near the bottom of reservoir 6 and then extends laterally so that liquid discharged through pipe 2| will enter reservoir 6 at points remote from suction pipe IT. The pressure created by pump 5 is limited by high pressure relief valve 22 which has its inlet connected to the discharge side of pump 5 and its outlet connected by a channel 23 to the intake of pump 5.

Assuming that the hydraulic circuit is filled with liquid, that pump 5 is running and that ther is an adequate supply of liquid in reservoir 6, the transmission will operate as follows:

When valve I connects opposite sides of pump 5 to opposite ends of cylinder 3 as indicated in Fig. 1, liquid from pump 5 will flow through channel 8, valve 1 and channel Hi to the rod end of cylinder 3 and move piston 2 toward the left on a working stroke. Piston 2 will eject liquid from the head end of cylinder 3 through channel ii, valve 1 and channel 8 to the intake of pump 5. Due to the displacement of rod 4, the liquid ejected from cylinder 3 is in excess of the liquid required to supply pump 5 so that all of the ejected liquid cannot enter pump 5 which will cause pressure to rise and open back pressure valve is and then the excess liquid will flow through valve iii, channel 20 and pipe 2| into reservoir 6. Back pressure valve l9 thus causes back pressure to be maintained upon piston 2 and causes pump 5 to be supercharged at that pressure.

When valve I connects the outlet of pump 5 and the rod end of cylinder 3 to the head end of cylinder 3 as indicated in Fig. 2, pump 5 will draw liquid from reservoir 8 through pipe ll, valve l5 and channel I and will discharge it through channel =8, valve 1 and channel H to the head end of cylinder 3 and thereby cause piston 2 to move toward the right on an idle stroke and to eject liquid from the rod end of cylinder 3 through channel l0, valve I and channel ii to the head end of cylinder 3 so that no liquid is returned to pump 5 which, therefore, must suck its entire supply of liquid from reservoir 3.

The suction valve assembly may be made as air tight as possible such as by inserting'gaskets between the parts thereof and between the valve casing and the casing of. pump 5 but it will not remain air tight and, consequently, when pump 5 sucks its supply of liquid from reservoir 6, air will seep through the joints of the suction valve assembly and be entrained in the liquid unless means are provided to prevent it.

seepage of air through the suction valve joints is prevented by keeping the suction valve submerged in liquid. This is accomplished by arranging the suction valve in a separate chamber and utilizing the flow of liquid into and out oi reservoir 6 to keep the level of the liquid in that chamber above the suction valve.

Reservoir 3 is made as nearly air tight as possible as by welding its joints, by arranging suitable gaskets between its walls and the covers which close such access openings as may be necessary to provide adequate access to the interior of reservoir 6, and by arranging a suitable gasket 25 around opening l5 between the bottom of pump 5 and the top wall of reservoir 6, the thicknesses of the gaskets being exaggerated in the drawing.

Suction valve I5 is arranged within a suction chamber 25 formed by a continuous imperforate wall 26 which extends around opening is and has its lower end spaced a short distance above the bottom of reservoir 6 and its upper end joined as by welding to the top wall of reservoir 6 to form a fiuidtight joint therewith.

In order that the flow of liquid into and out of reservoir 6 may be utilized to keep suction valve i5 submerged in liquid, a small airtight tank 30 is arranged within reservoir 6 outward from chamber 25. Tank 30 is connected as by means of a branched channel 3| to the outlets of two check valves 32 and 33 and by means of a channel 34 to the inlet of a check valve 35 which opens into reservoir 5. The inlet of check valve 32 communicates with the atmosphere as by being connected to a channel 36 which extends through the top wall of reservoir 6 and forms a fiuidtight joint therewith. The inlet of check valve 33 is connected by a channel 31 to the suction chamber at a point above suction valve 15. In the particular pump shown-in Fig. 3, channel 31 extends through wall 26, being welded thereto to form a fiuidtight joint therewith, and then extends upward into a pocket 38 which is formed in the casing of pump 5 and communicates with the interior of suction chamber 25.

Reservoir 6 is made large enough to leave a substantial air space above the liquid when the volume of liquid therein is maximum, that is, when piston 2 is at the extreme end of its movement toward the left, and the volume of liquid supplied to reservoir '6 is great enough to maintain the liquid level above the lower end of suction chamber 25 when the volume of liquid in the reservoir is at its minimum, that is, when piston 2 is at the extreme end of its movement toward the right.

Assuming that piston 2 stopped at the end of its movement toward the left, that pump is running and that the level of the liquid in suction chamber 25 is the same as the level of the liquid in reservoir 5, suction valve l5 will become submerged and kept submerged in the following manner:

When pump 5 draws liquid from reservoir 5 and delivers it to motor I to effect an idle stroke thereof as explained above and as indicated in Fig. 2, the resultant decrease in the volume of liquid in reservoir I will reduce the pressure in the air space which will cause air to flow through channel 35, check valve 32, channel ll, tank 30, channel 34 and check valve 35 into reservoir 5 to fill the space occupied by the liquid withdrawn by pump 5.

Then when liquid flows into reservoir 5 during the working stroke of piston 2 as explained above and as indicated in Fig. l, the incoming liquid will compress the air in the air space for the reason that air cannot escape from reservoir 5 due to the check valves. The pressure created by the incoming liquid will cause liquid to rise in suction chamber 25 and to expel air from the upper part thereof through channel 31, check valve 33 and channel 3| into tank 30 and create a low pressure therein. Also, any air or gas in the incoming liquid will rise into the air space for the reason that pipe 2| discharges the liquid at points outside of suction chamber 25.

Due to the pressure in tank 30, air will escape therefrom through channel 34 and check valve 35 into reservoir 5 the next time pump 5 draws liquid from reservoir 5, thereby preventing the pressure in the air space from dropping below atmospheric pressure. As the level of the liquid in reservoir 6 drops due to pump 5 drawing liquid therefrom, the level of the liquid in suction chamber 25 will start to drop but it cannot drop very far for the reason that the upper end of chamber 25 is sealed.

The liquid in chamber 25 may not be raised above suction valve l5 during the first cycle of operations in which case the liquid will be raised progressively higher each time liquid flows into reservoir 6 until chamber 25 is substantially filled with liquid and thereafter'liquid and/or air will be expelled from chamber 25 through channel 31, check valve 33 and channel 3| into tank 30 each time liquid flows into reservoir 6.

Suction valve I5 is thus kept submerged so that air cannot seep through its joints into the motive liquid. Also, any air or gas bubbles in the hydraulic circuit will be expelled therefrom in a very short time due to the incoming liquid being discharged into reservoir 5 at points spaced from suction chamber 25.

After piston 2 has made one or more reciprocations, the liquid forced into reservoir *5 during each succeeding movement of piston 2 toward the left will create pressure in reservoir 5 and in tank 30 as explained above and, during the latter part of each movement of piston 2 toward the left, the pressure in reservoir 5 and in tank 30 will be increased above the pressure required to raise liquid to the top of suction chamber 25. Then when valve 1 is shifted to cause piston 2 to move toward the right as indicated in Fig. 2, the pressure in reservoir 5 and in tank 30 will force liquid through valve l5 into the intake of pump 5 until suflicient liquid has been expelled from reservoir 5 to drop the pressure therein below that required to raise liquid into the pump intake.

Pump 5 is thussupercharged at a low pressure during the first part of the idle stroke of piston 2 and it must reduce the pressure at its intake below atmospheric pressure only during the latter part of the stroke at which time the pressure at the pump intake need not be reduced as much below atmospheric pressure as would be necessary if suction chamber 25 were not kept substantially filled with liquid at all times. Consequently, gasiflcation of the liquid entering the pump is reduced to a minimum.

The invention has thus far been explained as applied to a transmission having a circuit in which no liquid is returned from the motor to the pump during operation of the motor in one direction. I

However, the invention is equally applicable to a transmission having a circuit in which liquid is returned from the motor to the pump during operation of the motor in both directions but the rate at which liquid is returned to the pump during operation of the motor in one direction is less than the rate of pump delivery.

For example, if valve 1 were an ordinary reversing valve instead of a differential valve, the flow of liquid during movement of piston 2 toward the left would be the same as indicated in Fig. 1 but, during movement of piston 2 toward the right, pump 5 would deliver liquid to the head end of cylinder 3 and it would have liquid returned to it from the rod end of cylinder 3 at a in the opposite direction. Consequently, the pressurized reservoir would function in exactly the same way in either circuit.

The pressurized reservoir may also be employed in a transmission the motor of which has two piston rods of different diameters fixed to opposite ends of its piston and extending through opposite ends of its cylinder" or which consists of two single acting cylinders having separate rams. In either case, the pressurized reservoir would function in the manner described above except.

that the volume of liquid flowing into and out of the reservoir would be equal to the difference between the displacements of the two piston rods or the two rams.

It is to be noted that in the transmissions de scribed but not illustrated herein the valve may be omitted and a. reversible pump substituted for the unidirectional pump.

The invention herein set forth is susceptible of various other modifications and adaptations without departing from the scope of the invention which is hereby claimed as follows:

1. In a hydraulic transmission of the type which includes a reservoir of liquid, a pump having extending from the bottom thereof a suction valve through which said pump may be supplied with liquid from said reservoir, a hydraulic motor and means for directing liquid from said pump to said motor to cause it to perform successive cycles of operation, and said motoris so constructed that during one part of each of said cycles the liquid discharged from said motor is in excess of pump requirements and the excess liquid is discharged into said reservoir and during the other part of each of said cycles the liquid discharged from said motor is less than pump requirements and said pump must draw liquid through said valve from said reservoir so that liquid flows into and out of said reservoir during each of said cycles, the combination with said pump of a fluidtight reservoir containing a supply of liquid and fastened to the bottom of said pump to form a fluidtight joint therewith, a suction chamber having fluidtight side walls arranged within said reservoir around said suction valve and connected to the bottom of said pump to form a fluidtight joint therewith, said chame ber having the lower end thereof open and arranged below the lowest level of liquid in said reservoir, and means responsive to the flow of liquid into and out of said reservoir for raising the .level of the liquid in said chamber above said suction valve.

2. In a hydraulic transmission of the type which includes a reservoir of liquid, 2. pump having extending from the bottom thereof a suction valve through which said pump may be supplied with liquid from said reservoir, a hydraulic motor and means for directing liquid from said pump to said motor to cause it to perform successive cycles of operations, and said motor is so constructed that during one part of each of said cycles the liquid discharged from said motor is in excess of pump requirements and the excess liquid is discharged into said reservoir and during the other part of each of said cycles the liquid discharged from said motor is less than pump requirements and said pump must draw liquid through said valve from said reservoir so that liquid flows into and out of said reservoir during each of said cycles, the combination with said pump of a reservoir adapted to contain a supply of liquid and fastened to the bottom of said pump to form a fluidtight joint therewith, said reservoir being constructed to prevent escape of fluid therefrom except through said suction valve, a suction chamber having fluidtight side walls arranged within said reservoir around said suction valve and connected to the bottom of said pump to form a fluidtight joint therewith, said chamber having the lower end thereof open and arranged below the lowest levelrof liquid in said reservoir, a channel extending from the upper part of said chamber into said reservoir, a check valve in said channel to permit fluid to escape from said chamber but preventing flow in the opposite direction, and means connected to said channel and responsive to flow of liquid into and out of said reservoir for creating sufiicient pres sure in said reservoir during said inflow of liquid to raise the level of the liquid in said chamber above the level of the liquid in said reservoir outside said chamber and thereby cause fluid to flow from said chamber through said check valve so that after the level of the liquid in said chamber has been raised a substantial distance above said suction valve said suction valve will remain submerged in liquid during continued operation of said pump.

3. In a hydraulic transmission of the type which includes a reservoir of liquid, a pump having extending from the bottom thereof a suction valve through which said pump may be supplied with liquid from said reservoir, a hy- 8 said pump to said motor to cause it to perform successive cycles of operations, and said motor is so constructed that during one part of each of said cycles the liquid discharged from said motor is in excess of pump requirements and the excess liquid is discharged into said reservoir and duringvthe other part of each of said cycles the liquid discharged from said motor is less than pump requirements and said pump must draw liquid through said valve from said reservoir so that liquid flows into and out of said reservoir during each of said cycles, the combination with said pump of a reservoir adapted to contain a supply of liquid and fastened to the bottom of said pump to form a fluidtight joint therewith, said reservoir being constructed to prevent escape of fluid therefrom except through said suction valve, a suction chamber having fluidtight side walls arranged within said-reservoir around said suction valve and connected to the bottom of said pump to form a fluidtight Joint therewith, said chamber having the lower end thereof open and arranged below the lowest level of liquid in said reservoir, a channel extending from the upper part of said chamber into said reservoir, a check valve in said channel to permit fluid to escape from said chamber but preventing flow in the opposite direction, means connected to said channel and responsive to flow of liquid into and out of said reservoir for creating sufiicient pressure in said reservoir during said inflow of liquid to raise the level of the liquid in said chamber above the level of the liquid in said reservoir outside said chamber and thereby cause fluid to flow from said chamber through said check valve so that after the level of the liquid in said chamber has been raised a substantial distance above said suction valve said suction valve will remain submerged in liquid during continued operation of said pump, and an exhaust channel communicating with the intake of said pump and extending to a point below said suction valve.

4, In a hydraulic transmission of the type which includes a reservoir of liquid, a pump having extending from the bottom thereof a suction valve through which said pump may be supplied with liquid from said reservoir, a hydraulic motor and means for directing liquid from said pump to said motor to cause it to perform successive cycles of operations, and said motor is so constructed that during one part of each of said cycles the liquid discharged from said motor is in excess of pump requirements and the excess liquid is discharged into said reservoir and during the other part of each of said cycles the liquid discharged from said motor is less than pump requirements and said pump must draw liquid through said valve from said reservoir so that liquid flows into and out of said reservoir during each of said cycles, the combination with said pump of a reservoir adapted to contain a supply of liquid and fastened to the bottom of said pump to form a fluidtight joint therewith, said reservoir being constructed to prevent escape of fluid therefrom except through said suction valve, a suction chamber having fluidtight side walls arranged within said reservoir around said suction valve and connected to the bottom of said pump to form a fluidtight joint therewith, said chamber having the lower end thereof open and arranged below the lowest level of liquid in said reservoir, a channel extending from the draulic motor and means for directing liquid from '7 upper part of said chamber into said reservoir,

escape from said chamber but preventing flow in the opposite direction, means connected to said channel and responsive to flow of liquid into and out of said reservoir for creating suilicient pressure in said reservoir during said inflow of liquid to raise the level'of the liquid in said chamber above the level of the liquid in said reservoir outside said chamber and thereby cause fluid to flow from said chamber through said check valve so that after the level of the liquid in said chamber has been raiseda substantial distance above said suction valve said suction valve will remain submerged in liquid during continued operation of said pump, and an exhaust channel communicating with the intake of said pump and extending into said reservoir a substantial distance laterally outward from said suction chamber.

5. In a hydraulic transmisison of the type which includes a reservoir of liquid, a pump having extending from the bottom thereof a suction valve through which said pump may be sup- :plied with-liquid from saidreservoir, a hydraulic motor and means for directing liquid from said 'pump to said motorto cause it to perform suc- --ing the other part of each of said cycles the liquid discharged from said motor is less than pump requirements and said pump must draw liquid through said valve from said reservoir so that liquid flows into and out of said reservoir during each of said cycles, the combination with said pump of a reservoir adapted to contain a supply of liquid and fastened to the bottom of said pump to form afiuidtight joint therewith, a suction chamber having fiuidtight side walls arranged within said reservoir around said suction valve and connected to the bottom of said pump to form a fiuidtight joint therewith,

said chamber having the lower end thereof 'open and arranged below the lowest level of 7/ liquid in said reservoir, a small tank arranged within said reservoir close to the top thereof and outside said chamber,-a check valve opening from said tank into said reservoir, a check valve opening from the top of said chamber into said tank, a check valve opening from the atmosphere into said tank, said reservoir and said tank being fiuidtight-except -for said check valves, and an exhaust channel communicating with the intake of said pump and extending into said reservoir a substantial distance laterally outward from said suction chamber.

6. In a hydraulic transmission oi the type which includes a reservoir of liquid, a pump having a suction valve and a channel connecting said valve to the intake of said pump, a hydraulic motor and means for directing liquid from said pump to said motor to cause it to perform successive cycles of operations, and said motor is so constructed that during one part of each of said cycles the liquid discharged from said motor is in excess of pump requirements and the excess liquid is discharged into said reservoir and during the other part of each of said cycles the liquid discharged from said motor is less than pump requirements and said pump must draw liquid through said valve from said reservoir so that liquid flows into and out of said reservoir during each of said cycles, the combination with said a check valve in said channel to permit fluid to I pump of a fiuidtight reservoir containing a sup ply of liquid arranged around said suction valve and forming a fiuidtight joint around said channel, a suction chamber having fiuidtight side walls arranged within said reservoir around said suction valve and forming a fiuidtight joint around said channel, said chamber having the lower. end thereof open and arranged below the lowest level of liquid in said reservoir, and means responsive to the flow of liquid into and out of said reservoir for raising the level of the liquid in said suction chamber above said suction valve.

7. In a hydraulic transmission of the type which includes a reservoir of liquid, a pump having a suction valve and a channel connecting said valve to the intake of said pump, a hydraulic motor and means for directing liquid from said pump to said motor to cause it to perform successive cycles of operations, and said motor is so constructed that during one part of each of said cycles the liquid discharged from said motor is in excess of pump requirements and the excess liquid is discharged into said reservoir and during the other part of each of said cycles the liquid discharged from said motor is less than pump requirements and said pump must draw liquid through said valve from said reservoir so that liquid flows into and out of said reservoir during each of said cycles, the combination with said pump of a fiuidtight reservoir containing a supply of liquid arranged around said suction valve and forming a fiuidtight joint around said channel, a suction chamber having fiuidtight side walls arranged within said reservoir around said suction valve and forming a fiuidtight joint around said channel, said chamber having the lower end thereofppen and arranged below the lowest level of liquid in said reservoir, a fiuidtight storage compartment, and means for permitting fluid to flow from outside said reservoir or from the upper part of said suction chamber into said storage compartment and from said compartment into said reservoir but preventing flow in the opposite direction.

8. In a hydraulic transmission of the type which includes a reservoir of liquid, a pump having a suction valve and a channel connecting said valve to the intake of said pump, a hydraulic motor and means for directing liquid from said pump to said motor to cause it to perform successive cycles of operations, and said motor is so constructed that during one part of each of said cycles the liquid discharged from said motor is in excess of pump requirements and the excess liquid is discharged into said reservoir and during the other part of each of said cycles the liquid discharged from said motor is less than pump requirements and said pump must draw liquid through said valve from said reservoir so that liquid flows into and out of said reservoir during each of said cycles, the combination with said pump of a fiuidtight reservoir containing a supply of liquid arranged around said suction valve and forming a fiuidtight joint around said channel, a suction chamber having fiuidtight side walls arranged within said reservoir around said suction valve and forming a fiuidtight joint around said channel, said chamber having the lower end thereof open and arranged below the lowest level of liquid in said reservoir, a tank arranged within said reservoir outside said chamber, a check valve opening from said tank into said reservoir, a check valve opening from the top of said chamber into .said tank, and a check valve opening from the atmosphere into said tank, said reser- 11 voir and said tank being fluidtight except for" 'each of said cycles the liquid discharged from said motor is less than pump requirements and said pump must draw liquid through said valve from said reservoir so that liquid flows into and out of said reservoir during each of said cycles, the combination with said pump of a fluid tight reservoir containing a supply of liquid arranged around said suction valve and forming a fluidtight joint around said channel, a suction chamber having fluidtight side walls arranged within said reservoir: around said suction valve and forming a fluidtight joint around said channel, said chamber having the lower end thereof open and arranged below the lowest level of liquid in said reservoir, a tank arranged within said reservoir outside said chamber, a, check valve opening from said tank into said reservoir, a check valve opening from the top of said chamber into said tank, a check valve opening from the atmosphere into said tank, said reservoir and said tank being fluidtight except for said check valves, and an exhaust channel communicating with the intake of said pump and extending to a point below said suction valve.

10. In a hydraulic transmission of the type which includes a reservoir of liquid, a pump having a suction valve and a channel connecting said valve to the intake of said'pump, a hydraulic motor and means for directing liquid from said pump to said motor to cause it to perform suecessive cycles of operations, and said motor is so constructed that during one part of each of said cycles the liquid discharged from said motor is in excess of pump requirements and the excess liquid is discharged into said reservoir and during the other part of each of said cycles the liquid discharged from said motor is less than pump requirements and said pump must draw liquid or liquid arranged around said suction valve and forming a fluidtight joint around said channel. a suction chamber having fluidtight side walls arranged within said reservoir'around said suction valve and forming a fluidtight joint around said channel, said chamber having the lower end thereof open and arranged below the lowest level of liquid in said reservoir, a tank arranged within said reservoir outside said chamber, a check valve opening from said tank into said reservoir, 2. check valve opening from the top of said chamber into said tank, a check valve opening from the atmosphere into said tank, said reservoir and said tank being fluidtight except for said check valves; and an exhaust channel communicating with the intake of said pump and extending into said reservoir a substantial distance laterally outward from said suction chamber.

11. In a hydraulic transmission of the type which includes a reservoir of liquid, a pump having a suction valve and a channel connecting said valve to the intake of said pump, a hydraulic motor and means for directing liquid from said pump to said motor to cause it to perform successive cycles of operations, and said motor is so constructed that during one part of each of said cycles the liquid discharged from said motor is in excess of pump requirements and the excess liquid is discharged into said reservoir and during the other part of each of said cycles the liquid discharged from said motor is less than pump requirements and said pump must draw liquid through said valve from said reservoir so that liquid flows into and out of said reservoir during each of said cycles, the combination with said pump of a fluidtight reservoir containing a supply of liquid arranged around said suction valve and forming a fluidtight joint around said channel, a suction chamber having fluidtight side walls arranged within said reservoir around said suction valve and forming a fluidtight joint around said channel, said chamber having the lower end thereof open and arranged below the lowest level of liquid in said reservoir, a small tank arranged within said reservoir close to the top thereof and outside said chamber, a check valve opening from said tank into said reservoir, 2. check valve openingfrom the top of said chamber into said tank, and a check valve opening from the atmosphere into said tank, said reservoir and said tank being fluidtight except for said check valves. 7

JAMES K. DOUGLAS.

No references cited.

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