US3828657A

US3828657A - Piston for swash plate pump

Info

Publication number: US3828657A
Application number: US00267299A
Authority: US
Inventors: M Neuman
Original assignee: FMC Corp
Current assignee: FMC Corp
Priority date: 1972-06-29
Filing date: 1972-06-29
Publication date: 1974-08-13
Anticipated expiration: 1991-08-13

Abstract

A piston assembly for a hydraulic swash-plate pump is provided with a floating bearing which compensates for lateral flexure of the piston to achieve substantially total contact with the cylinder wall regardless of piston flexure and thus minimize the tendency of the piston to wear the open end portion of the cylinder out of round.

Description

United States Patent 1191 1111 3,828,657 Neuman Aug. 13, 1974 [5 PISTON FOR SWASl-l PLATE PUMP 2,068,859 l/l937 Jones et al. 92/258 2,460,948 2/l949 Sander 92/258 [75] lnvemorw" Odde" Valley 2,847,261 8/1958 Cornelius 92/179 3,36l,040 l/l968 Chenault 92/258 X [73] Assignee: FMC Corporation, San Jose, Calif.

Primary Examiner-Irwin C. Cohen 1 Flledi June 29, 1972 v Attorney, Agent, or Firm-R. s. Kelly; c. E. Tripp [21] Appl. No.: 267,299

, [57] ABSTRACT 52 us. Cl 92/258, 91/488, 92/172 A Piston assembly for a hydraulic swash-Plate p p is 51 Int. Cl Fl6j 1/00 provided with a floating bearing which compensates 5 Field f Search 92 172 79 255 257 for lateral flexure of the piston to achieve substantially 92 253; 91 43 total contact with the cylinder wall regardless of piston flexure and thus minimize the tendency of the pis- 5 References Cited ton to wear the open end portion of the cylinder out UNITED STATES PATENTS 680,465 8/1901 Reynolds 92/258 6 Claims, 3 Drawing Figures .OOlS-DO l I l 9 4.0%

PISTON FOR SWASII PLATE PUMP BACKGROUND OF THE INVENTION The field of the present invention concerns hydraulic swash-plate pumps, either driving or driven, which for convenience may be respectively referred to as pumps and motors. Prior art pumps of the swash plate type are used extensively in many heavy duty applications and, due to their mode of operation, have a universal wear problem concerning the pistons and cylinder bores.

More specifically, a typical swash-plate apparatus will include a shaft carrying a cylinder block. A series of spaced pistons operate in parallel relationship to the shaft within cylinder bores in the cylinder block. Each piston has a ball end engaged in the ball socket of an associated bearing shoe that slides over the flat face of a non-rotatable swash plate, the face being in oblique relation to the shaft. Thus, rotation of the cylinder block causes axial movement of the pistons.

In the case of a pump, the shaft will be driven ro rotate the cylinder block. In the case of a motor, fluid under pressure is admitted to the cylinders to cause the pistons to react against the swash plate and rotate the cylinder block and shaft. In either case, rotary movement of the shaft either develops from, or results in, relatively large lateral forces on the pistons, and tends to flex each piston and misalign it relative to its cylinder bore. The end result is that each of the pistons will cause appreciable wear, both to the piston and the cyl inder wall, at the end of its associated cylinder bore where the lateral load is concentrated due to the cocking of the piston.

There are obvious ways to inhibit this wear problem, such as by increasing the physical size of the compo nents to resist such flexurefor by choosing structural alloys which will accomplish a similar objective. It is readily seen, however, that these precautions cannot meet all conditions such as when the structure must be so heavily or expensively constructed that specified cost or weight and size requirements are exceeded.

According to the present invention, there is provided a simple and inexpensive solution of universal application to hydraulic swash plate pumps for the reduction of the usual wear which tends to make the pistons and cylinders out of round at the open end areas of the cylinders.

SUMMARY OF THE INVENTION A pivotable bearing sleeve is mounted on the piston of a hydraulic swash plate pump at about the point where maximum extension of the piston will position the sleeve within the end portion of the cylinder bore. By the provision of means for pivotally mounting the sleeve to the piston and two spaced cylinder contacting surfaces for the piston, localized loading of the piston is avoided and the bearing sleeve retains substantially total contact with the cylinder surface to distribute the lateral load from the piston over a relatively wide bearing area of the cylinder. Since the piston is stressed during operation so that its axis becomes non-linear, the capability of the bearing sleeve to maintain contact with the piston and to maintain substantially coextensive contact with the cylinder surface results in minimal lateral thrust wear of both the piston and the cylinder.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic longitudinal section through a typical hydrostatic transmission of the variable pump displacement, fixed motor displacement type.

FIG. 2 is an enlarged schematic section through a piston and bearing shoe of the motor section of the FIG. I apparatus.

FIG. 3 is a schematic section similar to FIG. 2 wherein the stress forces are exaggerated to illustrate the operational principles of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT One type of known swash plate operated hydrostatic transmission 10 is schematically illustrated in FIG. 1, and includes, within a bolted housing 12, a variable displacement hydraulic pump P and a hydraulic motor M. A power input shaft 18 drives the pump P, and a power output shaft 20 is driven by the motor M. The operating fluid which is pumped between the pump P and motor M is also employed for lubrication.

A cylinder block 22 is secured to the power input shaft 18 and is provided, for example, with nine or some other odd number of cylinder bores 24, only one of which is shown. Each cylinder bore is provided with apiston 26 having a ball end 28 projecting from the cylinder block 22 and being engaged in the complementary socket of a bearing shoe 30. Each bearing shoe slides across a flat face of a non-rotatable swash plate 34 that circumscribes the power input shaft 18. As illustrated, the face of the swash plate 34 is oriented in a plane normal to the shaft 18 and will not, therefore, axially move the pistons 26 as they are rotated by the shaft. This position of the swash plate thus corresponds to a zero output condition of the pump P.

The swash plate 34 can be tiltedobliquely of the axis of the input shaft 18 by swash plate tilt mechanisms 36, whereby rotation of the cylinder block 22 results in axial movement of each piston 26 and pumping of hydraulic fluidat a rate corresponding to the strokeof the pistons as governed by the tilt of the swash plate 34. Fluid is discharged from each pump cylinder through a port 40 which registers with valve ports, not shown,

in a fixed valve plate 42 as the cylinder block 22 rotates.

Similar components are used in the motor M, and include a cylinder block 52 fixed to the power output shaft 20. A series of circumferentially spaced cylinder bores 54 (only one of which is shown) are each provided with a piston 56 and a port 58. At the projecting end of the piston, a ball connector 60 is engaged with a complementary socket ina bearing shoe 62. To energize the motor, the fluid discharged under pressure from the pump P is sequentially routed by the valve plate 42 into the ports 58 to axially force the bearing shoes 62 against a fixed, inclined swash plate 64. The reaction forces thus developed produce a lateral thrust which is delivered to the bearing shoes, pistons and cylinder block to rotate the power output shaft 20 with a rotational torque governed by the displacement of the pump P, the operating pressure, frictional losses and other known parameters.

It can be readily appreciated that swash plate pumps and motors, due to their inherent operating mode, develop appreciable dynamic bending moments in the pistons, especially at maximum stroke. Thus, the piston, in an exaggerated and oversimplified sense, is comparable to a cantilevered beam having a concentrated load near its ball end. Of course, the actual kinematics of rotation, as related to the pistons, is a complex design study, but the net effect in any case is that the pistons exert considerable lateral force upon their respective cylinders, and tend to wear the pistons and wear the cylinders out of round at the open ends thereof where the lateral loads are concentrated. There are obviously many ways of bringing this wear within acceptable limits, such as by the selection of certain alloys for the cylinders and pistons which will more effectively combat wear, or to increase the cross sectional area of the pistons so that they will be more resistant to bending. For reasons of cost, compliance with accepted design factors, meeting a specified efficiency, or for other reasons, no expedient has been found to be entirely satisfactory to overcome the stated wear problem.

In accordance with the present invention, there is an inexpensive, simple and highly efficient solution to the problem of accommodating the lateral thrust in the pistons and cylinders of a swash plate pump or motor. Basically, the invention provides a bearing sleeve 70 (FIG. 2) which is mounted in substantially fixed position on the piston (either piston 26 or 56 in FIG. 1) but is free to rock about a transverse axis by a limited amount.

With more specific reference to FIG. 2, the piston 56 comprises a cylindrical inner body 72 which at one end includes the integral ball connector 60. At its other end, the piston body 72 is counter-bored to provide an annular flange 74 that is swaged against a recess portion 76 of an end cap 78 that forms the head of the piston. An axial bleed passage 80 throughv the body 72 transfers part of the operating fluid applied-against the head of the piston to the ball connector 60, the bearing shoe 62, and the swash plate 64 for lubrication purposes. It should here be noted that .the specific details of the axial passage 80, the bearing shoe 62 and the particular swaged connection of the body 72 to the end cap, orpiston head, 78 are not critical to an understanding of the present invention.

Near the ball connector 60, the piston body 72 is formed with an annular flange 82 and-an adjacent annular land 84. The flange 82 provides for axial retention, with limited endwise movement, of the bearing sleeve 70. The land 84 provides a journal surface for fulcrum engagement of the piston body with the bearing sleeve 70, and has a specific radial clearance from the bearing sleeve in order to permit relative pivotable movement between the piston and the bearing sleeve, as will be presently explained. A tubular spacer sleeve 88 circumscribes the body 72 and extends between the confronting surfaces of the bearing sleeve 70 and the end cap 78, but with a small end clearance which allows verylimited axial movement of the spacer sleeve 88. The spacer sleeve 88 has no contact with the cylinder 54.

The dynamic flexure of the piston 56 can be calculated, or estimated, within reasonably close limits. Upon this basis, the aforementioned operating clearances can be determined in order to allow, or at least not inhibit, such flexure, but at the same time maintain the exterior surface of the sleeve bearing 70 in substantially coextensive contact with the surface of the cylinder 54. Thus, in one specific example, the external clearance between the land 84 and the interior surface of the bearing sleeve 70, the cylindrical space indicated by the reference numeral 90, was the usual 0.00l5 to 0.0020 inches clearance to accommodate flexure of the piston without tilting the bearing sleeve. It was determined that the running clearance at 92 between the exterior surface of the bearing sleeve and'the interior surface of the cylinder 54 should be in the order of 0.0013 inches for maintaining a film of lubricating fluid. Because the land 84 extends radially outward from the body of the piston, a pair of

cylindrical recesses

94 and 95 are provided at the ends of the bearing sleeve. The recess 94 at the outer end of the sleeve is large enough (in practice about 0.015 inches) to accommodate any bending of the piston without permitting contact between the piston and bearing sleeve. Thus, all load transfer between the piston and the bearing sleeve is accomplished through the land 84, and it is desirable to position the land as close to the center 'of the sleeve as possible so that the load will be transferred uniformly between the sleeve and the surrounding cylinder wall over the entire contacting surface of the sleeve. Because the flange 82 on the body 72 will tilt relative to the bearing sleeve 70, the spacer sleeve 88 should have a clearance at one end. This clearance is here illustrated at 93 (FIG. 2) and is about 0.002 inches. The result of the various aforedescribed operating clearances is best understood from a consideration of FIG. 3.

FIG. 3 illustrates the same structure as FIG. 2, but the flexures and operating clearances are further exaggerated to more clearly disclose the mode of operation. One important point to note is that the piston assembly contacts the cylinder bore only via the end cap, or piston head, 78 and the bearing sleeve 70. This fact, and the fact that the bearing sleeve 70 lies approximately midway between the ball connector 60 and the bearing cap 78, allows a generally coextensive flexure of the piston, rather than concentrating the flexure near the bearing sleeve 70.

Because the land 84 is in centered relation to the bearing sleeve 70, is axially shorter than the bearing sleeve, and has the operating clearance at 90, the land 84 can tilt relatively to the bearing sleeve and still maintain substantial contact therewith, while the bearing sleeve maintains coextensive contact with the cylinder 54. Stated otherwise, the overall flexure of the piston body 72 can be substantially greater than a localized flexure at the land 84, and yet the bearing sleeve 70, because of its pivotal connection with the land, will not only maintain coextensive contact with the cylinder bore but will be isolated and unaffected by such flexure with the result that there is less than the usual tendency for wear of the piston andcylinder at the open end of the cylinder.

As the piston is retracted and the bearing sleeve 70 moves inward in the cylinder, the same degree of piston flexure can be accommodated since only the bearing sleeve 70 and the end cap 78 have sliding contact with the cylinder. At the same time, the spaced bearing sleeve and end cap allow the dynamic bending stress of the piston to take placethroughout an extended length of the piston, thereby minimizing localized flexure in the areaof the bearing sleeve 70. Meanwhile, the bearing sleeve 70 provides for improved load distribution between the piston and cylinder, and prevents the generation of high loads at the edge of the cylinder bore to prolong the useful service life of the piston and cylinder.

Although the best mode contemplated for carrying out the present invention has been herein shown and described, it will be apparent that modification and variation may be made without departing from what is regarded to be the subject matter of the invention.

What is claimed is:

1. In a swash plate pump including a cylinder, the improvement comprising a piston assembly for reception in said cylinder in order to minimize lateral dynamic thrust wear of the piston and cylinder, said assembly comprising an elongate piston body having an enlarged piston head for sliding contact with the cylinder, and a cylindrical bearing sleeve mounted on said piston body in axial spaced relation to said piston head for sliding contact with the adjacent portion of the cylinder, means for providing a fulcrum engagement between said bearing sleeve and said piston body to allow overall flexure of said piston body while maintaining said bearing sleeve substantially coaxial relative to the bore surface of said cylinder so that the load from the piston is distributed substantially uniformly over the external bearing area of the sleeve, a predetermined radial clearance being provided between the piston body and the sleeve in order to permit relative pivotal movement therebetween, said means for providing a fulcrum engagement comprising an enlarged land portion on said piston body adjacent said sleeve for contact with the central portion of said sleeve upon lateral movement of said piston body taking up said radial clearance, said land portion having an axial length less than the axial length of the sleeve so that lateral loads on said piston are transferred to said sleeve within the central portion of the sleeve.

2. Apparatus according to claim 1 including a radial flange on said piston body in endwise blocking relation with said bearing sleeve, and a tubular spacer mounted on said piston body between said bearing sleeve and said piston head to limit the axial displacement of said bearing sleeve.

3. Apparatus according to claim 2 wherein said spacer is provided with a diameter less than the cylinder to accommodate flexure of said piston body.

4. Apparatus according to claim 3 wherein said spacer has an end-to-end dimension providing a clearance between said spacer and said piston head and allowing a predetermined axial movement of the spacer between said bearing sleeve and said piston head.

5. Apparatus according to claim 4 wherein said clearance is approximately 0.002 inches, the radial clearance of said bearing sleeve with said enlarged land portion of the piston body being approximately 0.0015 to 0.0020 inches.

6. In a swash plate pump, a piston assembly comprising an elongate piston body, a cylindrical end cap secured to said body and forming the head of the piston, a projecting radial flange on said body in axially spaced relation to said piston head, and a bearing sleeve and a tubular spacer mounted on said body intermediate said flange and said piston head, said bearing sleeve having an external diameter equal to that of said piston head and said spacer having a lesser diameter, said bearing sleeve having limited radial clearance with said piston body in the order of 0.0015 to 0.0020 inches, and said piston having an enlarged portion adjacent said sleeve for contact with said sleeve within the central portion thereof upon lateral movement of said piston body taking up said radial clearance so that lateral loads on said piston will be distributed substantially uniformly throughout the external bearing surface of

Claims

6. In a swash plate pump, a piston assembly comprising an elongate piston body, a cylindrical end cap secured to said body and forming the head of the piston, a projecting radial flange on said body in axially spaced relation to said piston head, and a bearing sleeve and a tubular spacer mounted on said body intermediate said flange and said piston head, said bearing sleeve having an external diameter equal to that of said piston head and said spacer having a lesser diameter, said bearing sleeve having limited radial clearance with said piston body in the order of 0.0015 to 0.0020 inches, and said piston having an enlarged portion adjacent said sleeve for contact with said sleeve within the central portion thereof upon lateral movement of said piston body taking up said radial clearance so that lateral loads on said piston will be distributed substantially uniformly throughout the external bearing surface of the sleeve.