US3089427A - Hydraulic machines - Google Patents

Description

May 14, 1963 D. FIRTH ETAL 3,089,427

HYDRAULIC MACHINES Filed April 14. 1960 7 Sheets-Sheet 1 Donald Firth 8 Roger Harvey Yorke Hancock BY R'RL- C/Q-IFLOCKS ATTORNEY May 14, 1963 D. FIRTH ET AL HYDRAULIC MACHINES 7 Sheets-Sheet 2 Filed April 14. 1960 INVENTORS Donald Firth 8 Roger Harvey Yorke Hancock BY ,(RRL Ulq'Io CKS ATTORNEY May 14, 1963 D. FIRTH ET AL HYDRAULIC MACHINES 7 Sheets-Sheet 3 Filed April 14. 1960 INVENTORS Donald Firth 8 Roger Harvey Yorke Hancock BY I QRL (Alma/ 5 ATTORNEY y 1963 D. FIRTH ET AL 3,089,427

HYDRAULIC MACHINES Filed April 14. 1960 7 Shets-Sheet 4 INVENTORS Donald Firth a Roger Harvey Yorke Hancock av 6721. (4.).1LocK5 ATTORNEY May 14, 1963 Filed April 14. 1960 7 Sheets-Sheet 5 LIE-z 22a 30 27 T T L \1 9 2 29 l -/0" l 6 i Z2 6/ /a v I7 INVENTORS Donald Firth 8 Roger Harvey Yorke Hancock BY MW ROCKS ATTORNEY y 1963 D. FIRTH ET AL 3,089,427

HYDRAULIC MACHINES Filed April 14. 1960 '7 Sheets-Sheet 6 INVENTORS Donald Firth 8\ Roger Harvey Yorke Hancock ATTORNEY United States Patent 3,689,427 HYDRAULIC MACHWES Donald Firth and Roger H. Y. Hancock, East Kilbride, Glasgow, Scotiand, assignors to Council for Scientific and industrial Research, London, England, a corporation of the United Kingdom Filed Apr. 14, 196i), Ser. No. 22,334 Claims priority, application Great Britain Apr. 16, 1959 13 Claims. (Cl. 103-462) This invention relates to hydraulic machines, and particularly to swash plate type pumps and motors. Hitherto, in pumps of the swash plate type in which the cylinder block is rotated, considerable difliculty has frequently been encountered in developing sutliciently high pressures without risk of distortions or deflections of the cylinder block sufficient to cause seizure between it and the port or valve block or plate if the working clearances are kept to the very small values necessary to avoid undue leakage. The usual steps taken to combat these difliculties have hitherto been to increase the overall diameter of the cylinder block so as to impart a higher resistance to distortion, and to employ a flexible drive shaft or spherical seatings between the cylinder block and the port or valve plate for maintaining the required small working tolerances. The former leads to high inertia, and correspondingly sluggish response to signals, and the latter to mechanical complication.

Furthermore, it has usually been found necessaryin order to maintain the necessary fine working tolerances and to reduce friction losses-to support the cylinder block in comparatively large ball or roller bearings.

It is an object of the present invention to avoid or reduce to a minimum the difficulties hitherto experienced in the design and operation of swash plate pumps with rotary cylinder blocks without the disadvantages of high inertia and friction or substantial risk of seizure at high output pressures.

A further object is to provide a construction which embodies a high degree of automatic hydrostatic balance between the mechanical and oil lubricant pressures at the working surfaces of the port plate so that friction losses are kept very low.

By arranging the parts according to the present invention so as to make use of the principle of hydrostatic balance by means of oil films between the various working surfaces, it has been found possible to produce a design of pump which is not only of relatively small overall dimensions with low frictional drag, but also one which embodies simple and inexpensive plain journal bearings for supporting the shaft of the cylinder block.

A principal feature of the present invention is the interposition, between the working face of the cylinder block and a ported mounting block or end plate of the pump carrying the inlet and delivery connections, of an annular port plate concentric with the cylinder block shaft and whose opposite faces are at all times substantially supported on oil films the pressure of which is a function of the mechanical load imposed on the plate by the compression of the oil or other hydraulic working fluid in the cylinder block.

The port plate has a limited degree of radial float which is controlled by radially opposed hydrostatic pressures applied between the port plate and a relatively stable radially locating surface on the machine so as to vary differentially with variation of the clearance between the port plate and the said reference surface.

Conveniently, the opposed hydrostatic forces governing the radial fioat of the port plate act between the inner periphery of the annular plate and the radially stable surface of the shaft, the said periphery of the plate being formed with oil retaining cavities or pockets symmetrical- 1y arranged and connected to a source of oil under pressure so as to produce radially opposed stabilising oil pressures between the port plate and the shaft the difference between which at any instant is a function of any radial force tending to displace the floating port plate radially and cause metal to metal contact between it and the shaft.

If preferred, the radially opposed stabilising oil pressures on the port plate may be applied at the periphery of the port plate by locating the plate in a recess in the adjacent surface of the frame or end cover of the machine, and forming the pressure oil cavities or pockets either in the circumferential edge of the port plate or in the surrounding wall of the recess.

All the oil cavities or pockets may conveniently be interconnected by a common channel or .groove which in turn is fed with oil under pressure from the high pressure port of the machine.

The necessary radial floating action of the port plate depends on the achievement of a significant reduction in oil pressure in a cavity or pocket whenever the clearance between the port-plate and its relatively stable radially locating surface increases in the vicinity of the cavity or pocket Thus, each pocket must be connected to the source of oil under pressure through a narrow duct or constriction across which a rapid pressure drop is built up as the rate of flow tends to rise.

A hydraulic swash plate pump according to the present invention is capable of high power outputs-cg. 200 H.P.-at speeds which are variable over a wide range and in which rapid response can be obtained to input control signals. The reductions possible in bearing surface areas reduce frictional loss, and. hence friction heat, in the working parts.

A preferred embodiment of the invention will now be particularly described, by way of example only, with reference to the accompanying drawings in which:

FIGURE 1 is an axial cross section of the complete pump assembly;

FIGURE 2 is an elevation of the discharge end of the FIGURE 3 is a fragmentary section on the line HIIII of FIGURE 2;

FIGURE 4 is a fgragmentary side elevation as seen on the arrow IV in FIGURE 2;

FIGURE 5 is a section on the line V--V of FIGURE 4;

FIGURE 6 is a diagrammatic sectional view of the port plate arrangement;

FIGURE 7 is a cross section on the line VII-VII of FIGURE 6;

FIGURE 3 is a sectional view on the line VIIIVIII of FIGURE 7 and FIGURES 9 and 10 are sections similar to FIGURE 7 showing modified arrangements.

The pump illustrated consists of a main frame or body having front and ported back end plates 1, 2 clamped by four pillars 3 (FIGS. 2, 3 and 5). Each end plate 1, 2 carries a journal bearing 4-, 5 respectively for a short rigid drive shaft '6. Adjacent the bearing 5 in the back end plate 2, the shaft 6 is formed with a locking taper section 7 on which is locked a cylinder'block 8. This block is drawn up on the taper by a back-nut 9 on the shaft. The cylinder block 8 contains a number of cylinde'rs 10 whose axes are mutually inclined inwards towards the back end plate 2. A piston 11 in each cylinder is reciprocable under the control of a normally fixed swash plate '12 carried on trunnions 13 (FIG. 5 The swash plate 12 has a central conical aperture 14 through which the shaft 6 passes, the dimensions of this aperture being sufficient to allow for adjustment of the angle of the swash plate to the shaft 6.

The working face of the swash plate is recessed to accommodate an annular bearing pad 15 and an annular slipper plate 16. The latter is free to rotate under the frictional drag of slippers 17 each of which is engaged with a respective piston 11. For clarity of illustration in FIG. 1, only one cylinder 10, piston 11 and slipper 17 is shown.

The back face 18 of the cylinder block 8 (see FIG. 6) is pierced by inlet/ outlet ducts 19, one to each cylinder 10, the openings to which register successively with arenate inlet and outlet ports 29, 21 in a floating port plate 22. This port plate is not anchored to the back end plate 2 except for angular orientation about the shaft 6 with respect to the swash plate 12. This angular orientation is controlled by means of a threaded rod 23 which is slidable in a bore 24 in the back end plate 2 and which engages, at its inner end, a locating peg 25 fastened to the port plate 22 and projecting into the bore 24. The rod 23 is axially adjustable by means of a captive capstan nut 26. The ports 20, 21 communicate with an external circuit by way of flared ducts 27, 28 respectively (see FIGS. 1 and 2) and inlet and outlet sockets 27a, 28a in the back end plate 2. The ports 27, 28 open through a bearing surface 29 on the back end plate to register with the arcuate ports 20, 21, respectively in the port plate 22.

The port plate 22 forms a kind of washer between the mutually opposed faces 18 of the cylinder block 8 and 29 of the back end plate 2, and has a limited freedom to float radially between the two. This radial floating action is hydrostatically controlled by means of internal ducts and cavities.

As shown best in FIGS. 7 and 8, the port plate 22 has a central insert or hub 22a which fits over the shaft 6 with slight radial clearance or float. Around the internal periphery of the hub 22a are four equally spaced oil pockets 30, 31, 32, 33. Each pocket communicates with an outer peripheral groove 34 around the hub 22a by means of small-bore radial ducts 35, 36, 37, 38 respectively, and this peripheral groove is in turn directly connected to the main arcuate outlet port 21 'by means of a radial bleed passage 39. Thus, each oil pocket 30-33 is fed with oil at pump delivery pressure, and since the pockets are cquiangularly spaced around the internal pe riphery of the hub 22a, the radial forces exerted by the oil pressures therein tend to balance out and maintain the port plate 22 centralised with respect to the shaft 6. In this floating position, there is a narrow leakage path for oil between the inner periphery of the hub 22a and the radially stable locating surface of the shaft 6, and all leakage paths are of the same nominal size and hence are of equal flow resistance. There is, therefore, a continuous flow of oil through the bleed passage 39 into the groove 34, and from this groove along each radial duct -38. The pressure drops along these parallel paths are equal.

Let it now be assumed that the plate 22 is radially displaced with respect to the shaft 6 in the vertically downward direction in FIG. 7. The leakage paths from the pockets 30 and 31 are then restricted whilst those from the pockets 32 and 33 are increased. Less oil will then escape from the pockets 30 and 31, and more will escape from the pockets 32 and 33. Hence, the pressure in the pockets 30 and 31 will rise and that in the pockets 32 and 33 will fall, tending to restore the plate 22. to its centralised position in which friction with the shaft 6 is at a minimum.

Instead of being radially located on the shaft 6, the port plate 22 may be located at its outer periphery by an annular wall on the back end plate 2 of the machine casing, which thus constitutes the relatively stable or reference surface. An increased radial clearance is then provided between the port plate 22 and the shaft 6 so as to avoid any risk of binding of the plate on the shaft. Alternative constructions of this modified arrangement are illustrated in FIGS. 9 and 10.

In FIG. 9, the port plate 22 is shown as having a peripheral groove 34a machined around its circumference which is connected to the pressure port 21 by an internal duct 39a drilled radially in the thickness of the plate. This groove is closed by a ring 122 shrunk onto the plate 22 to form a rim. The ring has four pockets 30a, 31a, 32a and 33a at spacings around its circumference, these pockets being connected to the groove 34a by ducts 35a, 36a, 37a and 38a and corresponding in function to the pockets 3033 described above with reference to FIGS. 7 and 8. The periphery of the rim 122 is located with small clearance within an annular wall 2a bounding the surface 29 on which the port plate 22 rests. The action of the oil in the pockets 30a-33a and the narrow clearance from the wall 2a results in the same kind of self-centerin g action as occurs at the shaft 6 in the arrangement of FIGS. 7 and 8.

FIG. 10 illustrates the same principle as FIG. 9 except that the pockets 30b, 31b, 32b and 3312 are now formed in the wall 2a which constitutes the inner periphery of a ring 222 which is a force fit in a recess in the back end plate 2. A circumferential groove 34b is machined around the outer circumference of the ring 222 and com municates with each pocket 30b, 31b, 32b and 33b by ducts 35b, 36b, 37b and 38b. It also registers with a supply duct 39b leading from the high pressure port 28. The action of this arrangement is identical with that of FIG. 9, as will be understood.

When the pump is working, half the total number of pistons 11 are being forced inwards against high oil pressure by the swash plate 12 whilst the other half are being forced outwards by low pressure oil. Since the thrust on each piston is taken through a slipper 17 on an inclined surface '16, there will be a lateral component of reaction to each thrust which will be exerted on the respective cylinder wall and since these reactions are unequal over the two diametrically opposed arcs defined by the outlet and inlet ports 21, 20 respectively, the cylinder block 8 experiences a tilting couple about the horizontal plane in FIG. 1. This couple tends to close the gaps at G G (FIG. 6) between the port plate 22 and the adjacent faces 18 of the cylinder block 8 and 29 of the back end plate 2 over an are on the inlet side and at the same time to open thediamet-rically opposite gaps at G G This couple is resisted by the short, stout shaft 6, and its magnitude is kept to a minimum by the reduction in overall diameter of the port face 18 of the cylinder block 8 which results from the inclination of the axes of the cylinder 10, so that a frequent source of seizure or loss of output in pumps of this general kind, is accordingly minimized.

The floating port plate 22 itself also assists in mitigating possible trouble due to distortion of the cylinder block 8 under load. Although gaps are indicated in FIG. 6 at 6 -6 it will be understood that their dimensions are of the order of a few ten-thousandths of an inch. However, they exist physically and serve partly to cushion or accommodate cylinder block deflections and partly to combat distortion of the port plate 22 under fluctuating load conditions. The gaps are initially formed with the aid of capillary action.

Referring again to FIG. 6, the gaps G G are intersected by that part of the inlet circuit comprising the passage 27, port 20, and the relevant cylinder inlet/outlet ducts 19. Similarly, the gaps G G are intersected by that part of the outlet circuit comprising the relevant cylinder inlet/outlet ducts 19, port 21, and outlet passage 28. In each of these cases, the oil in the circuit is at a pressure above ambient pressure in the pump casing, the cylinders 10 being charged by oil at a positive inlet pressure from an auxiliary pump. Hence, leakage will tend to take place at both surfaces of the port plate 22, as indicated by the arrows L in the gaps G G and L in the gaps G G Since the ports 20, 21 are very large compared with the dimensions of the gaps G G the leakage pressure drop across the plate 22 is, for all practical purposes, zero, so that at all points around the plate the pressures on opposite faces are equal, although the pressures on either face of the plate at different points around the shaft 6 5. vary from maximum pump delivery pressure to minimum inlet pressure. This range of variation may be from 2000 lbs/in. to 50* lbs/m Provided that there is radial clearance or float between the port plate 22 and the shaft 6, as described with reference to FIGS. 7 and 8, the port plate 22 floats axially irrespective of the actual value of pump delivery pressure, and the system is self-stabilising. Most of the couple tending to cause tilt of the cylinder block 8 due to uneven pressure loading is resisted by the short, rigid drive shaft 6, and any residual deflection can be accommodated by compression of the leakage oil layers in the gaps G and G The aperture surface areas on opposite sides of the floating port plate 22 are proportioned so as to leave a relatively small residual unbalanced axial thrust on the shaft 6 which serves to overcome any tendency for the port plate 22 and cylinder block 8 to float axially in an uncontrolled manner leading to instability, and breakdown of the oil films. Due to the cyclic nature of the operation of successive pistons in each revolution of the cylinder block, a certain degree of pumping action is generated which tends to keep the films continuous, and this can, by careful design, be utilised to keep the mean unbalanced load between the plate 22 and the end plate 2 sufficiently low to prevent the port plate from binding on the end plate.

The combination of inclination of the cylinder axes and a fully floating port plate 22 materially reduces the frictional drag on the shaft 6. The radial width of the annular face 18 of the cylinder block 8 determines the total force due to the pressure of leakage oil acting on the cylinder block. This force is balanced by the force acting on the area of the end wall of each pumping cylinder 1% around the outlet duct 19.

The smaller the pitch circle diameter of the ports 20, 21, the less is the effective radius at which leakage oil in the gaps G and 6;; of FIG. 6 is sheared, and the lower is the power loss absorbed thereby. On the other hand, if the amount of leakage at these gaps is tobe kept within practicable limits, the difference in radius of this pitch circle and the outer circumference of the face '18 and port plate 22 must be as large as possible in order to give along leakage path of high resistance.

All these factors influence the design of the cylinder block 8 and port plate 22, and determine the angle of inclination of the cylinders to the axis of the shaft 6.

The provision for angular adjustment of the port plate 22 enables smoother operation of the pump to be obtained than is possible with conventional swash plate pumps in which no such adjustment is possible. The reason for this is that, since the pistons 11 cannot suck oil into their cylinders lil, charging of the cylinders through the inlet port 20 must be effected under positive pressure. This must necessarily be low in comparison with the delivery pressure, so that at T.D.C. of the piston 11 (i.e. the completion of the charge stroke), the cylinder 10 contains oil at low pressure. If now the common inlet/outlet duct 19 is opened to the delivery port 21, a pressure surge will be experienced in the cylinder due to the much higher pressure in the load circuit, and a series of shock waves would be set up in the system each time a piston commenced its delivery stroke. Hence, a small angular la-g must be provided between T.D.C. and the opening of the duct 19 to the delivery port 21 to allow the oil to be pre-compressed in the cylinder 16 to at least approximately line pressure. The correct setting of the angular adjustment rod 23 which controls the port plate setting may be obtained by trial and erroras by means of the captive capstan nut 26or may be determined by a servo mechanism responsive to load on the pump. In this latter event, the fully floating feature of the port plate 22 considerably reduces the effort required by the servo motor, so that a more rapid response to changes in load can be achieved, with consequent improvement in the smoothness of running.

The swash plate 12, as already noted, is mounted on trunnions 13, and these are carried in bushes 42, 43 (FIG. 5) mounted in side walls 44, 45 of the pump casing. The bushes 42, 43 are locked in position by means of cap screws 46.

Since the swash plate '12 takes the whole of the reaction of each piston 11, it will experience both the cyclic variations in load on each piston and also any shocks due to mis-timing of the outlet or delivery port 21. Vibrations will, therefore, be set up in the swash plate 12 which are preferably absorbed by some convenient damping means (not shown) and operating either internally (i.e. within the structure of the swash plate assembly) or externally on any of the conventional principles, such as a dashpot, boundary lubrication, viscous or dry friction, inertia, or a combination thereof.

The angle of tilt of the swash plate 12 is adjusted by means of an arm 47 FIG. 4) locked on the projecting end of one trunnion 13, the extremity of which is clamped between a pair of adjustment screws 48 threaded through brackets 49 bolted on the side wall 42. Each screw 48 has a knurled head 50 and carries a knurled locking nut 51. A pointer 52 projecting from the boss of the arm 47 registers with a scale 53 graduated in degrees of tilt of the swash plate 12.

The working face of the swash plate is constituted by the freely rotatable circular slipper plate 16 which beds against the bearing pad 15 in a circular recess in the swash plate. The bearing pad 15 is locked against rotation by means of a dowel. Thus, the friction between the piston slippers 17 and the slipper plate 16 causes the latter to tend to follow the slippers 17 around the shaft 6, whilst the eccentricity of the plate due to the angle of tilt of the swash plate 12 causes the slippers 17 to trace a path over the working surface of the slipper plate which is not of constant configuration. Thus wear of the plate is distributed over an area greater than the annulus traced by a single slipper 17 if the plate 16 were stationary.

In a practical hydraulic transmission system embodying the invention, the speed ratio between the drive for the pump and the motor operated by the pump is in finitely variable over a relatively wide range by adjustment of the angle of tilt of the swash plate 12. The arm 47 (FIGS. 4 and 5) in such an arrangement is linked to a speed control instead of being clamped in any given position of angular adjustment by the screws 48. This speed control may be either manual or automatic in response to some external variable associated with the motor output, according to the requirements of the case.

It is to be understood that, with appropriate design modifications, the pump described above may be used as a motor.

We claim:

1. A swash plate type hydraulic machine comprising a stationary swash plate, a rotatable cylinder block, a plurality of pistons reciprocable in cylinders in said cylinder block under control of the swash plate, ports in a flat face of the cylinder block communicating with the cylinders therein, a stationary port block having a flat face, inlet and outlet ports in said fiat face of the port block registrable with the ports in the cylinder block, a port plate interposed beween the flat faces of the cylinder block and the port block and having apertures permitting the passage of fluid between the ports in the cylinder block and the ports in the port block, said port plate having opposite flat faces of substantially equal area and clearances being provided between said opposite faces and the flat faces of the cylinder block and the port block within the limits of which clearances the port plate is free to move axially, whereby pressure fluid from said ports maintains a fluid film on each flat face of the port plate giving substantially equal and opposite axial loadings, a radial locating member having an annular surface spaced with radial clearance from an annular surface on said port plate within the limits of which clearance the port plate is free to move radially, a plurality of pockets disposed symmetrically around one of said annular surfaces and open toward the other annular surface and a constricted fluid-supply connection to each of said pockets.

2. A machine according to claim 1 wherein the fluidsupply connection comprises a passageway having constricted branches each leading to one of said pockets.

3. A swash plate type hydraulic machine comprising a stationary swash plate, a rotatable cylinder block, a plurality of pistons reciprocable in cylinders in said cylinder block under control of the swash plate, ports in a flat face of the cylinder block communicating with the cylinders therein, a stationary port block having a flat face, inlet and outlet ports in said flat face of the port block registrable with the ports in the cylinder block, a shaft carrying said cylinder block, an annular port plate disposed coaxially around said shaft and interposed between the flat faces of the cylinder block and the port block and having apertures therethrough registering with the ports in the port block, said port plate having opposite flat faces of substantially equal area and clearances being provided between said opposite faces and the flat faces of the cylinder block and the port block within the limits of which clearances the port plate is free to move axially, whereby pressure fluid from said ports maintains a fluid film on each flat face of the port plate giving substantially equal and opposite axial loadings thereto, a plurality of pockets spaced symmetrically around the inner periphery of said port plate and open toward the shaft, a radial leakage clearance being provided between the shaft and the port plate within the limits of which clearance the port plate is free to move radially, and a pressure-fluid connection from at least one of the aforesaid ports in the port block to said pockets through constrictions each associated with a separate pocket.

4. A swash plate hydraulic machine comprising a stationary swash plate, a rotatable cylinder block, a plurality of pistons reciprocable in cylinders in said cylinder block under control of the swash plate, ports in a flat face of the cylinder block communicating with the cylinders therein, a stationary port block having a flat face, inlet and outlet ports in said flat face of the port block registrable with the ports in the cylinder block, a port plate interposed between the flat faces of the cylinder block and the port block and having apertures permitting the passage of fluid between the ports in the cylinder block and the ports in the port block, said port plate having opposite fiat faces of substantially equal area and clearances being provided between said opposite faces and the flat faces of the cylinder block and the port block within the limits of which clearances the port plate is free to move axially, whereby pressure fluid from said ports maintains a fluid film on each flat face of the port plate giving substantially equal and opposite axial loadings, a radial locating member having a circular recess into which the port plate fits with a radial clearance between the circumferential surfaces of the port plate and the recess, a plurality of pockets spaced symmetrically around one of said circumferential surfaces and open toward the other circumferential surface, and a constricted pressure-fluid connection to each of said pockets.

5. A machine as claimed in claim 4 wherein said pockets are disposed in the port plate.

6. A machine as claimed in claim 4 wherein said pockets are disposed in the radial locating member.

7. A swash plate type hydraulic machine comprising a. stationary swash plate, a rotatable cylinder block, a plurality of pistons reciprocable in cylinders in said cylinder block under control of the swash plate, ports in a flat face of the cylinder block communicating with the cylinders therein, a stationary port block having a flat face, inlet and outlet ports in said flat face of the port block registrable with the ports in the cylinder block, a port plate interposed between the flat faces of the cylinder block and the port block and having apertures permitting the passage of fluid between the ports in the cylinder block and the ports in the port block, said port plate having opposite flat faces of substantially equal area and clearances being provided between said opposite faces and the fiat faces of the cylinder block and the port block within the limits of which clearances the port plate is free to move axially, whereby pressure fluid from said ports maintains a fluid film on each flat face of the port plate giving substantially equal and opposite axial loadings, a radial locating member having an annular surface spaced with radial clearance from an annular surface of said port plate within the limits of which clearance the port plate is free to move radially, a plurality of pockets at equally spaced points around the annular surface of the port plate and opening toward the annular surface of the radial locating member, and a constricted pressure-fluid connection to each of said pockets from an aforesaid port.

8. A swash plate type hydraulic machine comprising a housing having an end plate with a flat surface, inlet and outlet ducts in said end plate for conveying hydraulic fiuid, a rotatable shaft mounted in said housing on an axis normal to said flat surface, a swash plate mounted in said housing on said axis, a cylinder block fixed on said shaft and located between said end plate and said swash plate, cylinders in said block disposed around said axis, a port communicating with each of said cylinders and opening through a fiat end face of said cylinder block opposite to said flat surface of the housing endplate for conveying said fluid to and from the cylinder, a reciprocable piston in each cylinder displaceable by said swash plate on relative rotation of said swash plate and said cylinder block, an annular port plate whose opposite sides have smooth flat surface of equal area, the port plate being mounted to float axially between said flat surface on the housing end plate and said fiat end face of the cylinder block and to float radially with respect to said axis, pressure balancing means for limiting radial movement of the port plate, spaced arcuate transfer ports in said port plate respectively registering with said inlet and outlet ducts in the housing end plate and registrable in sequence with said ports in the cylinder block on rotation of the cylinder block, whereby fluid can be transferred by reciprocation of said pistons from the inlet duct, via said transfer ports and said ports and cylinders in the port block, to the outlet duct, said pressure balancing means comprising a plurality of pockets opening through an annular surface of the port plate and symmetrically disposed about said axis, a pressure-fluid connection to each pocket, a restriction in each pressure-fluid connection whereby the pressure in each pocket depends on the rate of escape of fluid therefrom, a radial locating member radially fixed relative to said axis and having a circular reference surface adjacent to and spaced from said annular surface of the port plate for controlling the rate of escape of fluid from said pockets in dependence on the spacing of said annular surface from the reference surface.

9. A machine according to claim 8 wherein said radial locating member comprises said shaft.

10. A machine according to claim 8 wherein said radial locating member comprises said casing and said circular reference surface is provided by the wall of a recess in the casing.

11. A machine according to claim 8 wherein said pressure-fluid connections lead to one of said arcuate transfer passages.

12. A machine according to claim 8 wherein the port plate is angularly adjustable relative to the swash plate.

13. A swash plate type hydraulic machine comprising a housing having an end plate with a flat surface, inlet and outlet ducts in said end plate for conveying hydraulic fluid, a rotatable shaft mounted in said housing on an axis normal to said flat surface, a swash plate mounted in said housing on said axis, a cylinder block fixed on said shaft and located between said end plate and said swash plate, cylinders in said block disposed around said axis, a port communicating with each of said cylinders and opening through a flat end face of said cylinder block opposite to said flat surface of the housing end plate for conveying said fluid to and from the cylinder, a reciprocable piston in each cylinder displaceable by said swash plate on relative rotation of said swash plate and said cylinder block, an annular port plate Whose opposite sides have smooth flat surfaces of equal area, the port plate eing mounted to float axially between said flat surface on the housing end plate and said flat end face of the cylinder block and to float radially with respect to said axis, pressure balancing means for limiting radial movement of the port plate, and spaced arcuate transfer ports in said port plate respectively registering with said inlet and outlet ducts in the housing end plate and registrable in sequence With said ports in the cylinder block on rotation of the cylinder block, whereby fluid can be transferred by reciprocation of said pistons from the inlet duct, via said transfer ports and said ports and cylinders in the port block, to the outlet duct, said pressure balancing means comprising means for distributing pressure fluid in opposed relationship through an annular surface of the port plate symmetrically about said axis, a pressurefluid connect-ion to said means for distributing pressure fluid, restrictor means in said pressure-fluid connection for controlling the pressure in said means for distributing pres sure depending on the rate of escape of fluid therefrom, a radial locating member radially fixed relative to said axis having a circular reference surface adjacent to and spaced from said annular surface of the port plate for controlling the rate of escape of fluid from said means for distributing pressure fluid in dependence on the spacing of said annular surface from the reference surface.

References Cited in the file of this patent UNITED STATES PATENTS 1,867,308 Durner July 12, 1932 2,546,583 Born Mar. 27, 1951 2,571,377 Olah Oct. 16, 1951 2,642,809 Born, et al June 23, 1953 2,646,754 Overbeke July 28, 1953 2,733,666 Poulos Feb. 7, 1956 2,753,802 Omohundro July 10, 1956 2,847,938 Gondek Aug. 19, 1958 2,948,229 Brundage Aug. 9, 1960 2,956,512 Brund-age Oct. 18, 1960 2,972,962 Douglas Feb. 28, 1961

Claims (1)

1. A SWASH PLATE TYPE HYDRAULIC MACHINE COMPRISING A STATIONARY SWASH PLATE, A ROTATABLE CYLINDER BLOCK, A PLURALITY OF PISTONS RECIPROCABLE IN CYLINDERS IN SAID CYLINDER BLOCK UNDER CONTROL OF THE SWASH PLATE, PORTS IN A FLAT FACE OF THE CYLINDER BLOCK COMMUNICATING WITH THE CYLINDERS THEREIN, A STATIONARY PORT BLOCK HAVING A FLAT FACE, INLET AND OUTLET PORTS IN SAID FLAT FACE OF THE PORT BLOCK REGISTRABLE WITH THE PORTS IN THE CYLINDER BLOCK, A PORT PLATE INTERPOSED BETWEEN THE FLAT FACES OF THE CYLINDER BLOCK AND THE PORT BLOCK AND HAVING APERTURES PERMITTING THE PASSAGE OF FLUID BETWEEN THE PORTS IN THE CYLINDER BLOCK AND THE PORTS IN THE PORT BLOCK, SAID PORT PLATE HAVING OPPOSITE FLAT FACES OF SUNSTANTIALLY EQUAL AREA AND CLEARANCES BEING PROVIDED BETWEEN SAID OPPOSITE FACES AND THE FLAT FACES OF THE CYLINDER BLOCK AND THE PORT BLOCK WITHIN THE LIMITS OF WHICH CLEARANCES THE PORT PLATE IS FREE TO MOVE AXIALLY, WHEREBY PRESSURE FLUID FROM SAID PORTS MAINTAINS A FLUID FILM ON

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