US1558002A - Hydraulic transmission - Google Patents

Description

w. FERms HYDRAULIC TRANSMISSION Filed Dao. 21. y192s s sheets-sheet 1 Y mVENToR.

` mn TER FMH/5 ATTORNEY.

Oct. Z0, 1925.

w. FERRIS.

HYDRAULIC TRANSMISSION med nec. 21, 1925 3 Sheets-Sheet 2 BY 'Wurm FERR/s A TTORNEY.

W. FERRIS HYDRAULIC TRANSMISSION Filed Dag 21, 1923 s sheets-sheet s Ff IMMER Feng/5 ATTORNEY.

' Patented Oci. 20,4 1925.A

' UNIT-Eo s-'rA'rss PATENT oFFici-z.'

WALTER FERRIS, MILWAUKEE, WISCONSIN, ASSIGNOR T THE OILGEAR COMPANY 0F MILWAUKEE, WISCONSIN, A CORPQRATION 0F WISCONSIN.

HYDRAULIC TRANSMISSION.

Application filed December 21, 1928. SerialNo. 682,056.

To all whom it may concern.'

' Be it known that I, WALTER FRRIS, a

citizen of the United States, residing at Milwaukee, in the county of Milwaukee and State of Wisconsin, haveinvented a certain new and useful Improvement inHydraulic Transmissions, of which the following is a full, clear, concise, and exact description, reference being had to the accompanying drawings, forming a part of this specification.

'This invention relates to hydraulic'transmission. l*

. p The general aim of the present invention, 1s to improve the construction and operation of `devices of this character.

' A more specific object is to improve the construction and arrangement ,of the pum parts' and their connections with the hydraulic circuit thereby to obtain a more convenient control of the -rate and direction of fluid flow in the circuit.

Another object is the provision of improved means for taking care of differences in the volume of uid in the two sides ofthe circuit, which ordinarily occur, for instance, in those systems in which a double' In those instances in which the motor constitutes a verticallyarranged piston and cyl- `inder assembly there `seems to be a tendency for the piston to creep downwardly by gravity.. 'Another object of the present invention is the p rovision of a simple means for avoiding this difficulty. j

Other objects and advantages will appear from the following description of an illus- 'trative embodiment of this invention.

application, Serial No. 483,468, tiled July and as an integral Apart of the Casin 10.

In the drawings: Figure lisa sectional view through the casing of a pump comprising a part of a hydraulic transmission system embodying features of the present invention.

Figure 2 is a sectional view taken substantially along the line 2-2 of Figure 1.

Figure 3 is a view illustrating the circuit connections between the pump discharge and a vertical powercylinder of a typical hydraulic press.

Figure 4 is a -fragmentary sectionall view of the pump shown in section in Figure 1. In the hydraulic transmission shown, the pump and associated parts are enclosed in a casing 10 having a basin or sump 11 in the base thereof containing a reserve supply of working fluid, preferably oil. `A- drive shaft 12, entering the casing 10,is keyed at its inner end 13 to a hub 14 of the pump driving cage 15, and, at its outer end car- 'ries a heavy lywheel-'drivin pulley 16. This shaftis journaled in a Xed bearing 17 in the wall of a housing 18 fixed to the outside of the casing 10, i

The pump shown is similar in many respects to that described in my copending 9, 1921. In this instance, however, the cage 15 rotates about a fixed axis and the pintle 19 is adjustable to vary pump displacement. The cage is journaled at one side in a fixed bearing 20 provided in a removable plate 21 f ormng one part of the casing wall, and at the other side in fixed bearings 22 provided in a rigid structure 23 formed within Thecage 15 l'carries adjacent its perip ery a series of tan er1-tial` reaction plates 24, removably fixe in opposed grooves 25 formed in the peripheral walls 26 of the cage. A cylinder barrel 27 having a radial series of cylinder bores 28 is supported by and rotatesupon the pintle 19. Each cylinder bore has a port 29 adapted to register successively with ports V3() and 31 formed in the upper and lower surfaces of the pintle 19, during rotationof the cylinaction plate within limits defined by a resages taining pinik 37 anchoredin the reaction plate and loosely engaging a slot 38 in the roller cage.

The pintle 19 is fixed in and carried by an upwardly projecting arm 39 having a hub 40 rockahly mounted upon a shaft 41 extending across the casing 10, and having one end 42 fixed in one wall of the casing and the other end fixed in the rigid structure 23 hereinabove mentioned. Upper and lower pairs of passages 43 and 44 in the pintle communicate with the pintle ports 30 and 31, respectively, and with passages 45 and 46, respectively, leading downwardly along the arm 39 to passages opposite sides-of the hub portion 40. Pas- 47 and 48 communicate ,at all times through ports 49 and 50 with two vertical pairs of passages 51 and 52, respectively, extending longitudinally of the .shaft 41. lassages 51 of Vone pair communicate with a port 53 formed in the end 42 of shaft 41 and passages 52 of the other pair communicate with a similar port 54.

A cap 55 covering the projecting end 42 or" shaft 41is fashioned to receive appro- Y priate pipe connections through which the pump may be connected with any form of hydraulic motor desired. (See Figure In this instance, the motor shown is a vertical power cylinder 56 of a typical hydraulic press. A pipe 57 is shown connecting the upper end ot' this'cylinder with the `port 54 while the other port 53 is shown connected Awith the lower end of the cylinder through pipes 58 and 59 and a valve mechanism to be hereinafter described. v

`The arrangement of parts thus far described is Such that when the pintle 19 is in a neutral position in which the axis of rotation of the cylinder barrel 27 is coincident with the axis of rotation of the driving cage 15, rotation imparted to the driving cage 15 by the pulley 16 and shaft 12 will cause the pistons 32 to travel about the pintle 19 without reciprocating in the coi either operating cylinder bores 28 and pump displacement is zero. Any slight rocking of the arm 39 about the shaft 41 so as to shift the pintle 19 substantially horizontally in direction from this neutral osition will cause the pistons to reciprocate' 1n their cylinder bores during rotation of the drivthe piston 60 therein At the same time, iiuid discharged trom the- 47 and 48 inV ing cage. When the pintle 19 is thus shifted in one direction to the eccentric position shown, for instance, in Figure 4, rotation O the cage 15 in a clockwise direction causes the pistons, during the upper half of their circular travel, to drive fluid from their cylinder bores through the ports 29 and pintle port 30'to and through the upper pair ot passages 43 in the pintle. The fluid thus delivered by the pump passes downwardly through the passages 45 and47 in the arm 39, through the passages 51 in the shaft 41, and thence through pipes 58 and 59 to the lower end of the power cylinder 56 causing to travel upwardly.

upper end of this cylinder passes through the pipe 57, passages 52, 48, 46 and the lower passages 44 in the pintle from which it is returned to the cylinder bores in the lower half of their circular travel through the pintle port 31. Similarly, pintle to the other side'ot its neutral position, the cylinder bores during the upper half' of their circular travel receive iiuid from the npper'pintle passages 43 and during the lower halt of their travel discharge iiuid into and through the lower pintle passages 44. ylhus, by swinging the arm 39 the pintle may he shifted so as to vary the direction and rate ot iiuid How delivered by the pump.-

The position of the arm 39 and pintle 19 may be varied and controlled by any appropriate means. 1n this instance, the arm is provided with a pair ot upstanding spaced ears 61 operatively connected through a by shitting the ipo pair of links 62 with a hollow plunger 63,

mounted for lengthwise shifting in a control cylinder 64, secured within a wall ot the casing 10. A piston 65 reciprocable in the cylinder 64 is formed integral with or with the interior of the hollow plunger 63,'

the plunger having a port 76 open at all times to the interior of the pump casing 10. A second longitudinal duct 77 in the valve communicates with the two annular chambers 71 and 73 through ports 78 and 79, respectively. An annular chamber 8O formed in the inner wall of a sleeve portion 81 of the control cylinder 64 .communicates times with the chamber 73 through a port 82 and is supplied with iuid under pressure through a passage 83 from a convenient pressure source, such, for instance, as will be hereinafter described. .Sets of ports 84` at all and 85 through the wall of the plunger 63 positionshown in Figurel 2, so that ports 84 and 85 'are uncovered by heads 68 and 69, luid'under pressure passes from the chamber 80, 4through port 82,`ch'amber 73, and ports 85 into the right end of control cylinder 64. Simultaneously, the left end of control cylinder is opened to exhaust through the ports 8.4, chamber 72, port 75, duct 74 and port 76. This, of course, re-

sultsin movement of `the piston 65 and plunger 63 to the left until the ports 84 and 85 are again covered by the heads 68 and 69.

. The extent of movement of the piston and plunger is, of course, dependent upon the extent to which the pilot valve has been shifted, this movement of the plunger being transmittedto the arm 39, and pintle 19, through the links 62 and ears 61. (Figure 1.) When the pilot valve 66"'is shifted to the right, fiuid under pressure is delivered to the .left end of'control c linder 64 through'the ports 84, chamber 7,1,

port 78,-duct '77, port 79, chamber 73 and port 82 'from chamber 80, andthe right end of cylinder 64 is openedto exhaust through ports 85, chamber 72 and duct 74. This results in movement of the piston'65 and plunger 63 to theright a distance' corresponding to the extent of movement of the pilot valve. The pilot valvev 66 may be shifted and -controlled by any appropriate mea-ns as typilied by the hand wheel 86 onA a shaft 87 having a pinion and rack eonnec tion 88 with the stem 89 of the valve.

It will be noted that a variable displacement pump has been provided Ahaving the desired flexibility without. requiring shifting of the drive shaft 12 or the use -of ilexible pipe connections.

During operation of the machine shown, iuid under substantially constant pressure is maintained in the chamber 80 by mechanism now to be described. Referring to Figure 1, this mechanism includes a gear pump 90 of a well known construction mounted in a casin`g 91 secured to thestructure 23 adjacentthe end ofv shaft 41. This pump is driven by a shaft 92, carrying a vdriving sprocket 93, connected through an appropriate chain 94 with a sprocket 95, keyed' to the pump drive shaft 12. Fluid from the base-of the sump 11 is lifted by the pump through a pipe 96 into the pump casing and is discharged by the pump through a pipe 97 into a passage 98 passing through a wall of the main pump casing 10. Passage 98 communicates with pasdegree of fluid sage 83 leading to the" chamber 80. A

Aspring loaded relief valve determines the pressure existing in the passage 83 and chamber 80. This valve comprises a ball 99 normally seated over a port desired tension in the spring. The quantity of iuid delivered to the passage 83 is preferably always in excess of that demanded by the control mechanism, 'the excess being discharged -past the ball valve 9 9 and through the port 104.

In order to protect the hydraulic transmissionV against excessive pressures advantage is taken of the fact that in a pump ofA the typedescribed, the pintle tends to shift toward'its intermediate or zero pump stroke position by reason of an unbalanced thrust thereon resulting from the eccentricity of the cage 15 and cylinder barrel 27. It has been found that this thrust is substantially proportional to the working pressure in the I tion,- the piston 65 is selected, of such diameter, that with a iven fiuid pressure in thechamber 80, the piston isv capable of producing a thrust upon the .plunger 63suH cient to shift and retain the pintle 19 at any desired position of veccentricity, so long as the pressure., inthe main hydraulic 4circuit is below a desired maximum.. Assuming, for instance, that the transmission is operating atv a pressure below this maximum,

with the pintle 19 in a position offset to the left of the zero pump stroke position, so that the pump thrust upon the pintle tends to stroke position. The pintle is retained in the right endof the cylinder 64 (Figure 2), the plunger 63 and pilot valve 66 being in the relatlve positions shownwith the ports 84 and 85 covered by the heads 68 and 69 on the pilot valve. Now, should the pressure in the main hydraulic circuit increase over the allowed maximum, the pump thrust upon the pintle increases accordingly-until this thrust. transmitted to the plunger 63 and piston 65 is suiiicient to force leakage of fluid from the right end of cylinder 64 and past thehead 69 against the constant pressure of fluid maintained in the chamber 73 from they chamber 80. This leakage of fluid permits the piston 65 to move to the right, openin'f ports 85, and thus permitting further movement thereof and consequently the pintle to the right to reduce the pump stroke. Reduction of the pump Stroke'thus continues u'ntil the pressure in the hydraulic system has been reduced below the allowed lhydraulic circuit. In utilizing this condiloo Y force the pintle to the right toward the zero chamber 80 against the fiuid pressure therein. Since the main circuit pressure at which this action will occur is dependent upon the pressure maintained in the chamber 80, this maximum pressure may be predetermined by the adjusted tensionin the spring 102 of the relief valve. For purposes ot convenience, a pressure gauge 105 may be connected with the passage 83 to indicate the pressure therein and in the chamber 80.

Considerable diiculty has been experience-d in attempting to shift the pintle into zero stroke position with the extreme accuracy required to reduce the iiow in the main hydraulic circuit to absolute zero.

' Anydeviation from this theoretically correct position results in undesirable creeping of the motor due to the slight flow in the circuit. 1n order to avoid this difficulty, provision is made 'for automatically short circuiting the two sides of the circuit as the pintle assumes zero stroke position. In this instance, the mechanism orthis purpose includes a lever 106', depending from a piv'ot pin 107 fixed in appropriate lugs 108, formed uponthe top-wall of the main casing 10. Adjacent its upper pivoted end this lever is formed with an open ended horizontal cross-cut 109. A pair of plates 110, one at each side of the lever, cover the open ends of the cross-cut 109. These plates are carried'by a pair of tension rods 111 exten-ding loosely through the lever above and below the cross-cut. ICoiled compression springs 112 on the rod 111 yieldably retain the plates 110 in contact with the sides of the lever. One of the ears 61 on the pintle supporting arm 39 carries an integral lug 113, projecting loosely into the cross-cut 109, between and in contact with the plates 110, so that as the arm 39 is swung in either direction from the neutral position shown in Figure 2, the lug 113, acting upon one or the other plates 110, tends to shift the lever 106 in the same direction.

The lower free end 114`of1the lever 106 is connected to the stem 115 of a short circuiting valve 116v reciprocable in a valve block 117 secured to a rail 117 in the 'casing. The V valve with three annular chambers 118, 119 and 120. Passages 1 21 and 122 connect the chambers 118 and 120, respectively, with the passages 51 and 52 constituting parts of the two sides of the main hydraulic circuit.

block shown is providedv The valve 116 is provided withan intermediate reduced portion, 123 permitting communication between the chambers 118 and 120 when the valve is in the intermediate position of Figure 2. With the valve in this position, the two sides of the main circuit are inopen communication through the valve block. This is the position of the parts when the pintle is in zero pump stroke osition.

When the arm 39 and pintle are shifted in either direction from this intermediate position, the lug 113, acting upon one of the plates 110, Causes the lever 106 to swing about its pivot 107 Iuntil the lower end thereof has shifted the valve 116 into such position as to close communication between the chambers 118 and 120 in the valve block. Further movement of, the valve 116 is prevented by the engagement or" one of the stop collars 124 or 125. carried thereby, with the outer surface of the valve block. Since the lug 113 is so close to the pivot 107 of the lever as compared with the length thereof, this closing movement of the valve 116 is eli'ected by a very slight movement of the pintle supporting arm. Further movement of the pintle may be continued, without movement of` the lever 106, since either plate 110 is permitted to yield away from the lever against the pressure of the springs 112. it .will thus be understood that the pintle may be shifted in either direction and to any desired extent to obtain the desired pump stroke, and during its initial movement the valve 116 is suddenly shifted so as to'cut oli communication of the twosides of the circuit through the valve block 117.

To insure against building up of pressure in the circuit during the short circuiting position of the valve 116, a discharge duct 126 is provided in communication 1with the intermediate chamber 119 in the valve block 117. This duct is controlled by a check valve 127 of any appropriate type.

Mechanism for taking care of the difference in volume of fluid in the two sides of the main hydraulic. circuit will now be de-. scribed. Such mechanism has particular utility when, as in this instance, the hydraulic motor is in the form of a double acting cylinder. lFor instance, as the piston 60 travels downwardly in the cylinder 56 a greater amount of Huid must be fed into the upper end of the cylinder than is discharged from the lower end thereof to compensate for the displacement of the piston rod 60 in the lower end of the cylinder only. In the pump shown, the mechanism for taking care ot this uid diterential comprises a valve block 129 having a cylindrical bore 130 therein formed with an intermediate annular chamber 131. A piston valve having three piston heads 132, 133 and 134: and reciprocable in the block, divides the lll) .136, 137 and 138.

bore 13() thereof into four chambers 135, The valve is shown in F igure'l in intermediate position in which, the intermediate head-133 closes communication between the chamber 131 and the rest of the bore 130. Chambers 136 and 137 are always in communication .with the passages 45 and 46, respectively, through passages 139 and 140, and chambers-135 and 138 are always in communication with chambers 137 and 136, respectively, through passages 141 and 142. A passage 143 leads from the chamber 131 to the sump 11, preferably through'a two way check valve 14,4 to be later described. The arrangement Vis such that when the passage 46 is carrying the working fluid, the pressure of this lluid is transmitted 'through passages 140 and 141 to chamber 135 causing the piston valve to shift to the right (Figure 2), thereby open-` ing communication between the return side of the circuit and the sump through the pas sage 139, chamber 136, chamber 131 and the passage 143. This is the position of the piston valve when fluid is being delivered tov the upper end of cylinder 56 causing the pis-V t0n 601:0 travel downwardly. As above pointed out, the upper end of the cylinder' D56 demands a greater volume of fluid than that discharged by the lower end thereof. This deficiency of fluid volume is made up by fluid sucked up by the main pump into the passage 45' from the sump through the passage 143, chambers 131 and 136 and passage 139. When the displacement of the .main pump is reversed so that the piston 6() is traveling upwardly, and passage 45 is carrying the'workin'g fluid and passage 46 thereturn fluid, the greater pressure in passage` is transmitted through the passages 139 and 142 to the chamber 138, to therbv shift the piston valve to the left, and thereby open communication between chambers 137 and 131. With the piston valve in this `position, communication is established between what is then the return passage 46 and the sump, through the passages 140, chambers 137 and 131, and the passage 143. The excess luid discharged by the upper end of the cylinder 56 is thus returned to the sump. In order to insure 'sulicient pressure in the chamber 135 or 138 toeifect reversal of' -the piston valve, when the pump is reversed,

check valve 144 is preferably etween the passage 143 and the a two wa provided sump. The checl': valveV shown comprises a A valvecasing 145 communicating with the pipe or passage 143 and with the 'sump 11 l through a duct 146. A tubular valve 147 is yieldably maintained against its seat 148 under the pressure of a light spring 149, and adapted to open under the pressure of Huid passing downwardly through the passage 143. A tubular valve 147 is. rovldedwith'a port 150 normally covered y a ball valver151, arranged to be lifted to open po.v i

sition by flow of fluid upwardly into the passage 143. It will be understood that the return side of the circuit, which is in open communication with the passage 143,.'becomes the working side of the circuit when the pump is reversed, and were it not for the check. valve 144, the fluid in this side of the circuit would be free to escape into the sump without actuating the piston valve. The checkvalveserves to so check the flow of fluid in the passage 143 as to provide,

sutlicient pressure in the work-ing side of the circuit to shift the piston valve when the pump is reversed.`

Provision is preferably made for preventing the piston 60 from creeping downwardly by gravity, as, for instance, when the motor circuitV is short-circuited through the block 117 or otherwise. This is accomplished in this instance by the use of a two 'way check valve interposed between the pipes 58 and 59. This valve comprises a casing-152 -having ports 153 and 154 in communication with pipes 58 and 59, respectively. A hol-l low valve 155 within the casing is yieldably retained in closed position by a spring 156. This valve is provided with a port 157 con- .trolled by a ball check valve 158, arranged to open freely by the flow of fluid from pipe 59 .to pipe 58, and to prevent flow through v the hollow valve 155 in the other direction. The tension in spring 156 is such as to retain the hollow valve on its seat against the pressure of fluid in the pipe 58 due to the weight of piston 60, but to permit this valve to move from its'seat under the fluid pres-- sure in the pipe 58 during normal operation of the'system. Thus, as the piston 60 is' traveling downwardly under the working pressure in pipe 57, the fluid from the lower end of the cylinder passes from pipe 58 through port 153, around the valve 155, and through port 154 into pipe 59.

Various changes may e made in the em-l 'bodiment of the invention hereinabove described, without departing from or sacrifie-- ing any of the vadvantages of the inventionA as defined in the appended claims.

I claim:

1. In a hydraulic transmission the combination of a hydraulic motor, a stationary delivered from said pump,

and motor comprising a stationary member ,1.30

j municating with said pump,

and a second member rockably associated with said stationary member and operable to vary and control the relative displacements of said pump and motor.

3. ina hydraulic transmission the combination oa -variable displacement pump, a stationary member havin a fluid passage therein, and a second mem er rockably associated with said stationary member Jfor regulating pump displacement, said second member having a tluid passage therein communicating with said pump andwith said rst named passage.

4. ln a hydraulic transmission the combination ofY a variable displacement pump, a rockable member for regulating pump displacement having `a Huid passagecomand a pivotal support for said member having a iuid said first passage communicating with named passage 1n all positions of said member.

5. lin a hydraulic transmission the combinationY of a variable displacement pump, a roekable member for regulating pump displacementchaving separate passages for receiving fluid from and delivering Huid to said pump, and a pivotal support for said member having separate passages communieating respectively With said first named passages in all positions of said member.

6. In a hydraulic transmission the combination of a variable displacement pump having a pintle adjustable with changes in pump displacement, said pintle having a passage through which tluid passes to or from saidpump, a shaft having a fluid passage, and a link connecting said pintle and shaft and maintaining communication between said passages in all positions of said pintle.

7. A variable displacement pump or motor having a pintle adjustable with changes in pump displacement, and a support for said pintle roekable about 'an axis oi'set from the axis of said pintle.

8. ln a hydraulic machine the combination of a radial series of iston and cylinder assemblies, a pintle aving passages communicating therewith, and a pintle support having passages communicating With said first named passages andv rockable about an axis otset from the axis. of said pintle to vary the position of saidpintle relative to said assemblies.

9. n a hydraulic machine the combination of a radial series of piston and cylinder assemblies, a pintle substantially centrally disposed within said series, and a support for said pintle rockable about an axis offset therefrom to vary the 4position of said pintle relative to said assemblies.

10. In a hydraulic machine the combination of a pintle, a seriesoi piston and cylinder assemblies grouped about said pintle,

an adjustable support for said pintle, and hydraulic means, for. adjusting said support to vary the Working stroke of said assemblies.

11. In a hydraulic machine the combinato thereby vary the center of rotation o said series of assemblies.

12. In a hydraulic machine the combination of a pintle, a series of piston and cylinder assemblies .rotatable about said pintle, an adjustable support for said pintle, a source ofiuid pressure, and means actuated from` said pressure source for adjusting said support.

ln a hydraulic machine the. combination of a pintle, an adjustable support therefor, a series of piston and cylinder assemblies rotatable about said pintle, a pilot valve, and hydraulic means controlled by said valve for adjusting said support a distance corresponding to the extent of movement of said yalve.

14.1 n a hydraulic transmission the combination of aA variable displacement pump, a hydraulic circuit fed thereby, hydraulic means for regulating pump displacement, said means being of such capacity as to yield under pump thrust to reduce pump displacement when\ the pressure in said circuit exceeds a l predetermined maximum.

15, ln a hydraulic transmission the combination of avvariable displacement pump,

a hydraulic circuit fed thereby, a source of substantially constant Huid pressure, and a piston actuated from said pressure source Jfor regulating .pump displacement and yieldable When the pressure in said circuit exceeds a predetermined maximum to reduce pump displacement.

16. ln a hydraulic tranmission the combination of a variable displacement pump, a hydraulic circuit fed thereby, a pilot valve, and a fluid actuated piston for varying pump displacement in accordance with the extent of movement of said valve and automatically operable to reduce pump displacement When the pressure in said circuit exceeds a predetermined maximum.

17. ln a hydraulic transmission the combination of a variable displacement pump, l

control means for regulating pump displacement, a hydraulic circuit fed by said pump, a by-pass `connecting the sides of said circuit when said control means is substantially l1n zero displacement pos1t1on, and means actuated only by the initial movement of said control means from zero stroke position for closing said by-pass.

18. In a hydraulic transmission the combination of a pump having an adjustable control means therefor, a hydraulic circuit and said control means. v Y

20. In a hydraullc transmission the comtherefor,

fed by said pump, short-circuiting means for said circuit, and means actuated by said element -for controlling said short-circuiting means.

19. Ina hydraulic transmission the combination of a variable displacement pump, control means therefor, a hydraulic circuit fed'by said pump, short-circuiting means,

actuating means therefor', and -yieldable connections between said actuating means binationof a variable displacementpump,

control means therefor, a hydraulie'circuit fed by said pump, a short-circuitlng valve for said circuit, and yieldable connections between said control lmeans and said valve for actuating said valve. q

,21. In a hydraulic transmission the combination of a variable displacement pump,

Vcontrol means therefor, a hydraulic circult fed by said pump, a short-circuiting valve for said circuit, alever for actuating said valve, and connections between said lever and said control means.

22. In a hydraulic transmission the combination-of a variable displacement pump,

control means therefor, a hydraulic circult `fed by said pump, a short-circuiting valve for said circuit, a lever, a pivotal support .connections between lsaid control means and said lever relatively near said support, and connections between said .valve 'and said lever relatively far from said support. i

23. In a hydraulic transmission the combination of a variable displacement pump,

control means therefor, a hydraulic circuit,

a short-eircuitiug valve, a lever, connections betweensaid lever and said'valve,jyieldable connections between said lever and said control means, and a support for said 'lever pivotally connected therewithl at a vpoint relatively near said yieldable connection and far from said first named connection.

a hydraulic circuit fed thereby, means for short cireuiting said circuit when pump displacementis zero, and means'for destroying the pressure in said circuit when said short a Fluid reservoir, and means automatically.'

operable to elfect and maintain communica- -tion between said reservoiriand that side of said circuit containing low pressure uid.

26. In a hydraulic transmission the combination of a reversible flow pump, ahydraulicV circuit communicating therewith Vhaving interchangeable highand low pressure sides of relatively varying capacities, .y

-and means for yieldably resisting the flow of'iiuid through said means in one direction.

27. In a. hydraulic transmission the combination with a hydraulic circuit including a cylinder, a reversible flow pump, and Huid connections between saidpump and the opposite ends of said cylinder, of a piston in said cylinder',` and means including a valve automatically operable to compensate for relative variations in the volumetric capacities of the two sides of said circuit.

28. In a hydraulic transmission the combinationof a hydraulic circuit having interchangeable high and low pressure sides of relatively varying capacities, a Huid reservoir, a valve operated by the high pressure fluid in one side of sa-id circuit for opening communication between said reservoir and the other side of said circuit, and a check valve associated with said rst named valve.

29. In a hydraulic transmission the combipassages, whereby a greater pressure in one side of said circuit will shift said valve to open communication between the other side of said circuit and said port.

30. Ina hydraulic transmission the combi- A nation of a pump, a hydraulic motor, a hy- 24.- Iii a hydraulictransmission the combination of a variable displacement pump,-

draulic circuit connecting @said pump and motor, and means for yieldably resisting the' flow of iiuid in one direction in said circuit to prevent creeping of said motor when said pump is ineffective.

`31. In a hydraulic transmission the combination of a cylinder, a piston therein, a hydraulic circuit connected with said cylinder for operating said piston, and means for yieldably yresisting a flow of fluid from one end of said cylinder to prevent undue creeping of said piston when said circuit is idle.

' 32. In a hydraulic transmission the combination of a substantially vertical cylinder, a piston therein, a hydraulic-circuit connected with said cylinder for actuating said piston, and means for yieldably resisting a `iiow of fluid from the lower end of said nation of a hydraulic circuit having intercylinder to prevent said piston from creeping by gravity therein.

33. In a hydraulic transmission the combination of a reversible pump, a closed hydraulic circuit communicating therewith having interchangeable high and low pressure sides of relatively varying capacities, a Huid reservoir, 'and means responsive to the diti'erence in pressures between the sides of said circuit-for maintaining communication between said reservoir and the low pressure side of said circuit. y

34. In a hydraulic' transmission the combination of a reversible ow pump, a' closed hydraulic circuit having sides `of relatively varying capacities connected to the opposite sides o said pump, a iuid reservoir, and valve mechanism responsive to the dierence in pressures between the sides of said circuit for connecting said reservoir with that side only of said circuit containing low pressure fluid.

35. In a. hydraulic transmission the combination of a reversible flow pump having interchangeable intake and delivery passages, a. closed 'hydraulic circuit having sides of relatively varyin'g capacities connected to said pump passages, respectively, a fluid reservoir, and a valve connected across the sides of said circuit and automatically operable to maintain communication between said reservoir and the intake passage of said pump.

in Witness whereof, hereunto subscribe my name this 5th day of November, 1923.

\ WALTER FERRIS.

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