US3120816A - Hydraulic pumps and motors - Google Patents

Description

Feb. 11, 1964 FlRTH ETAL 3,120,816

HYDRAULIC PUMPS AND MOTORS Filed April 14, 1960 2 Sheets-Sheet 1 INVENTQRS Donald Firth Roger Harvey Yorke Hancock BY IQRL LOC K5 ATTORNEY Feb. 11, 1964 D. FIRTH ETAL 3,120,816

, HYDRAULIC PUMPS AND MOTORS Filed April 14, 1960 2 Sheets-Sheet 2 INVENTORS Donald Firth Roger Harvey Yorke Hancock awn/44121. (A).T:oc K5 ATTORNEY United States Patent 3,120,816 HYDRAULIC PUMPS AND MOTORS Donald Firth and Roger H. Y. Hancock, East Kiihride, Glasgow, Scotland, assignors to Council for Scientific and industrial Research, London, England, a body corporate of the United Kingdom Filed Apr. 14, 1960, Ser. No. 22,335 Claims priority, application Great Britain Apr. 16, 1959 3 Claims. (Cl. 103-162) This invention relates to hydraulic machines of the positive displacement type, whether pumps or motors, in which the reaction to the thrust exerted on a piston by the fluid pressure in a cylinder is taken through a slipper or shoe on the piston rod working on a tilted or eccentric reaction surface. The machine may be a swash plate machine, in which the reaction surface is constituted by the inclined surface of the swash plate, or a radial cylinder machine in which the reaction surface is a relatively fixed cam or eccentric on the axis of the machine shaft.

One source of loss of efiioiency in positive displacement pumps and motors in which the piston rod bears through a slipper against an inclined or eccentric surface is that, especially at low speeds, the slipper tends to tilt, and this materially increases the risk of metal-to-metal contact and high friction.

A positive displacement hydraulic motor or pump according to the present invention has each piston and slipper assembly designed to operate with a high degree of hydrostatic balance of pressures between the piston and the slipper bearing surfacei.e. the pressure of a lubricant film at the slipper surface is substantially a direct function of the bearing load. in this way, metal-to-metal friction and the tendency of the slipper to tip or tilt are materially reduced or even eliminated.

To this end, the piston and slipper assembly is conveniently designed so that oil at the working pressure in the cylinder at any given inst-ant is available at the Slipper bearing surface which engages the reaction membereccentric, swash plate, or the likeand which controls the reciprocatory motion of the piston.

Preferably the piston and the slipper, together with any intervening connecting rod, are drilled through in register to provide a continuous oil duct which includes a metering restriction at or near the end adjacent the working face of the slipper, and the latter has oil-retaining recesses adjacent its normally leading and trailing edges from which oil can leak. at a controlled rate dependent on the resistance of the metering restriction.

Practical embodiments 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 a swash plate pump or motor, all but one piston and cylinder being omitted for convenience;

FIGURE 2 is an enlarged sectional view of a piston and cylinder;

FIGURE 3 is a plan view of the slipper in FIGURE 2;

FIGURE 4 is a fragmentary sectional view similar to -FIGURE 2 showing a detailed modification;

FIGURE 5 is a view similar to FIGURE 3 of a modified form of slipper; and

FiGURE 6 is a sectional view on the line VI-VI of FIGURE 5.

The pump illustrated in FIGURE -1 consists of a main frame having front and back end plates 1, 2 clamped by four pillars (not shown). Each 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 looked a cyclinder block 8. This block is drawn upon the taper by a back-nut 9 on the shaft. The

3,120,816 Patented Feb. 11, 1964 cylinder block 8 contains a number of cylinders 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 (not shown) by which it can be angularly adjusted on an axis normal to the shaft 6. 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. i

The working face of the swash plate is recessed at 15a 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 FIGURE 1, only one cylinder Iii, piston .11 and slipper 17 is shown. 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 1 6 causes the latter to tend to follow the slippers 17 around the shaft 6, whilst the eccentricity of the plate due to the tilt of the swash plate 12 with respect to the axis of the shaft 6 causes the slipper-s 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 which would be traced by a single slipper 17 if the plate 16 were stationary.

Each piston 11 consists of a hollow sleeve 54 (FIG. 2) closed at its outer end to form a crown 55 and which is a snug fit in the cylinder 1%} and has four external oil control grooves 60, 61, 62, 63. Into the outer or working end 56 of the sleeve 54 is screwed a headed stem 64 having an axial oil feed bore 65 passing therethrough. The outer end of this stem is shouldered at 66 and has a spigot which passes through an oil seal 68 in the piston crown 55. An oil seal 57 prevents leakage of oil between the head 69 of the stem and the end 56 of the sleeve. Coaxially with the bore 65, the crown 55 is drilled through to form an oil duct 67 which opens into a hemi-spherical socket 5B which forms a seating for a hemispherical pivot 76 integral with the slipper 17. The pivot is retained in the socket 5-8 by means of a spring ring 59.

The hemispherical pivot 70 is flattened at 71 to leave a small pocket 72 around the end of the oil duct 67 within the seating 58. A radial oilway '73 is drilled through the pivot and leads into a constriction 74 which communicates with a central recess 75 in the face of the slipper 17 (see also FIG. 3). This recess can thus receive oil from the cylinder 10 through the bore 65 in the stem 64, the duct- 67 in the piston crown 55, the pocket 72, the oilway 73 in the hemispherical pivot 70, and the constriction 74. Thus it will be seen that the pocket 72 must always be proportioned so that the pivot 7tl never closes the duct 67 at a limit position of its angular deflection.

The sliding surface of the slipper 17 also has an annular groove 76 adjacent its periphery, this groove having no communication with the central recess 75, but communicating with the atmosphere through four radial notches 77, 78, 79, 30. The sliding surface of the slipper 17 thus effectively consists of five lands-an inner annulus 81 and four peripheral pant- annuli 82, 83, 84 and 85. These lands are horizontally shaded in FIG. 3.

The purpose of the above-described arrangement of oil recess and grooves is to combat the tendency of a slipper 17 to tip on the swash plate due to friction between the spherical pivot 76) and its seating 58. This tendency increases with increase of thrust between the slipper and the swash plate, which occurs over approximately one half of each revolution of the particular cylinder concerned, and is coextensive with the oil delivery stroke of the piston. Since all the coacting thrust surfaces are fed with oil at delivery pressure (neglecting any pressure drop due to flow resistance in the circuit 65 57, 72, 73, 74) the load will be transmitted at each bearing point through a film of oil at a pressure which is a direct function of the axial thrust on the piston. Thus, there will be a film of oil between the lands 81 84 and the slipper plate 16, and also between the pivot 70 and its seating 58. The whole piston and slipper assembly 11, 17 is thus substantially hydrostatically balanced.

In practice, there will be a pressure drop in the circuit 65 74 due to leakage between the slipper lands 81 84 and the slipper plate 16, but this can be kept to a very low value by careful design and accurate machining of the bearing surfaces. Furthermore if the clearance between the bearing surfaces at any point increases so that the pressure in the central recess 75 falls, the hydrostatic balance of the piston and slipper assembly is upset. The pressure on the piston 11 thereupon tends to reduce the clearance until equilibrium is restored. If the slipper 17 tends to tilt about a point on its leading edge, so that the adjacent arc of the slipper tends to make metal-to-metal contact with the slipper plate 16, the moment of the piston thrust about this point opposes this tilting, so that the system is self-compensating to a considerable degree.

The degree of self-compensation is partly dependent on the cross-sectional area of each notch 77-80 which is presented to the escaping oil, and by careful design of these notches and their interconnecting annular groove 76 a dashpot action can be induced in which the oil pressure changes are slower than the changes in mechanical thrust, so that oscillation of the slipper 17 is minimised.

FIGURES and 6 illustrate a modified oil film control arrangement. Here, there are three symmetrically arranged recesses 75a, 75b, 75c fed with oil through metering ducts 74a, 74b, 740. The recesses are of sector shape, and of equal radius and angle, and are surrounded by a separate concentric groove 76 which communicates with atmosphere through three equiangularly spaced notches 77a, 78:2,7911. This arrangement of recesses and groove forms four lands, the central land 81a resembling in shape a three-spoked wheel and the outer lands 82a, 83a, 84a being part-annular in shape.

The operation of the modified slipper, however is somewhat different from that of the slipper described with reference to FIGURE 3, in that tilting is now opposed by differential hydraulic pressures across the face of the slipper.

For example, let it be assumed that the slipper 17 in FIGS. 5 and 6 is moving over the slipper plate 16 along the line of the section plane marked VI-VI in FIGURE 5, the notch 79a leading. There will be a tendency for the ends of the lands 83a and 84a on either side of the notch 79a to dig into the surface of the slipper plate 16, while points on the lands 81a, 82a diametrically opposite the notch 79a will tend to lift. As soon as any such displacement commences, the clearance between the land 81a and the slipper plate 16 will increase and oil will escape into the annular groove 76 and out through the notches 77a, 78a. Its pressure at this zone will accordingly be reduced, whilst at the diametrically opposite side of the annular land 81a the clearance will be reduced and the oil film pressure will be increased. This change in pressure produces a restoring couple so that the modified system is hydrostatically self-compensating.

In both forms of slipper, the surface areas of the lands,

together with their shapes and positions on the slipper surface, are chosen so that the forces between the slipper 17 and the slipper plate 16 on the swash plate 12 during a delivery stroke of the associated piston 11, which forces tend to promote metal-to-metal contact between the parts, are counterbalanced by forces in the oil films which separate them, these latter forces being directly derived from the instantaneous pressure of oil in the cylinder 10,

A similar hydrostatic balance is not quite achieved by the hemispherical pivot and coacting seating 58 as shown in FIGURE 2, since the projected area of the latter on a plane normal to the piston axis is less than the area of the working face 56 of the piston. Consequently, if hydrostatic balance is to be achieved at this point also of the piston-slipper assembly, the arrangement of FIG- URE 4 is adopted. In this figure, the hemispherical male pivot 7th: is formed on the piston 11 and the coacting socket seating 58a is formed on the slipper 17, the projected area of the seating on a plane normal to the piston axis being made equal to the area of the working face 56 of the piston. By this arrangement the surfaces can be proportioned so that a similar balance between mechanical and hydrostatic forces is achieved as at the working face of the slipper 17.

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

We claim: I

1. In a positive displacerent hydraulic machine, the combination with a reciprocable piston of a slipper articulated to said piston; a piston thrust reaction member on which said slipper is slidable; at least two separate oil retaining recesses symmetrically disposed with respect to the midpoint of the sliding face of said slipper; each of said recesses extending from a point adjacent the center of said sliding face towards the periphery thereof and bounded on all sides by a continuous land; separate ducts communicating between the pressure side of said piston and said recesses; and a metering constriction in each duct for controlling the supply of working fluid to said recess whereby said slipper is restored to its normal operating position by at least one of said recesses when said slipper is moved from said reaction member.

2. The combination as claimed in claim 1 including an annular groove located between said recesses and the periphery of said sliding face, and a plurality of symmetrically disposed notches opening from said groove through said periphery.

3. The combination as claimed in claim 1 wherein said piston thrust reaction member is a swash plate and further including a frame, a shaft journalled in said frame, a cylinder block rigid on said shaft, and said swash plate in said frame.

References Cited in the file of this patent UNITED STATES PATENTS 2,608,159 Born Aug. 26, 1952 2,679,210 Muller May 25, 1954 2,731,308 Wilcock Jan. 17, 1956 2,757,612 Shaw 1 Aug. 7, 1956 2,769,393 Cardillo a a1 Nov. 6, 1959/- 2,901,979 Henrichsen Sept. 1, 1959 2,925,046 Budzich Feb. 16, 1960 FOREIGN PATENTS 246,097 Germany June 10, 1909

Claims (1)

1. IN A POSITIVE DISPLACEMENT HYDRAULIC MACHINE, THE COMBINATION WITH A RECIPROCABLE PISTON OF A SLIPPER ARTICULATED TO SAID PISTON; A PISTON TRUST REACTION MEMBER ON WHICH SAID SLIPPER IS SLIDABLE; AT LEAST TWO SEPARATE OIL RETAINING RECESSES SYMMETRICALLY DISPOSED WITH RESPECT TO THE MIDPOINT OF THE SLIDING FACE OF SAID SLIPPER; EACH OF SAID RECESSES EXTENDING FROM A POINT ADJACENT THE CENTER OF SAID SLIDING FACE TOWARDS THE PERIPHERY THEREOF AND BOUNDED ON ALL SIDES BY A CONTINUOUS LAND; SEPARATE DUCTS COMMUNICATING BETWEEN THE PRESSURE SIDE OF SAID PISTON AND SAID RECESSES; AND A METERING CONSTRICTION IN EACH DUCT FOR CONTROLLING THE SUPPLY OF WORKING FLUID TO SAID RECESS WHEREBY SAID SLIPPER IS RESTORED TO ITS NORMAL OPERATING POSITION BY AT LEAST ONE OF SAID RECESSES WHEN SAID SLIPPER IS MOVED FROM SAID REACTION MEMBER.

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