US 3698286 A
Description (OCR text may contain errors)
United States Patent 1151 3,698,286 Ellis et al. [451 O t. 17 1972  POWER TRANSMISSION 3,289,606 12/1966 Bosch ..91/485 721 t:Gld.ll, nven ors gir 2 i ir ifi John A Primary Exammer-Wdham L. Freeh Assistant Examiner-Leonard Smith  Assignee: Sperry Rand Corporation, Troy, Attorney-Theodore Van Meter Mich.
221 Filed: June 28,1971  ABSTRACT A hydraulic pump or motor of the axial piston type  Appl 157511 having a cylinder barrel rotating against a fixed valve plate perpendicular to the shaft and free to find its 52 us. c1 ..91/486, 417/201 Own seat against the valve Plate has an improved we 511 1111. c1 ..F0lb 3/00 ing which Produces a non-uniform velocity in the fluid  Field of Search ..91/486, 487, 489, 499, 504, circulating by frictional drag of the cylinder barrel- 91/505 506 484 417/203 This produces Bemouli effects which react upon the sides of the cylinder barrel and conteract the Bemouli 56] References Cited effects acting on the back end face of the cylinder barrel which are produced by the rotary motion of the UNITED STATES PATENTS projecting pistons. This increases the maximum speed t h h th t t 3,181,477 5/l965 Matthews ..91/485 3 w eum can operae 2,064,299 12/ 1936 Ferris et al ..91/484 3 Claims, 3 Drawing Figures PATENTEDncm m2. 3,698,286
. I N VENT GAYLORD C).v ELL JOHN A HALAT ATTORNEY POWER TRANSMISSION In the copending patent application of Edward J. Chondzinski for Power Transmission, Ser. No. 101 ,647, filed Dec. 28, 1970, there is disclosed an axial piston pump or motor unit of the type having a revolving cylinder barrel mounted to find itsown seat against a stationary valve plate in which means are provided for reducing the volume of fluid which circulates with the rotating cylinder barrel and pistons by reason of frictional drag. This reduces the magnitude of the fluid pressure forces acting on the cylinder barrel and which tend to tip it off from the valve plate and results in a significant increase in the maximum operating speed of a given unit. While this is a step in the right direction and significantly improves the performance of units using that invention, such units are nevertheless subject to residual fluid pressure forces tending to tip the cylinder barrel, even though those forces are of reduced magnitude.
The present invention aims to further improve the performance of axial piston pump or motor units by so directing the circulating fluid in the casing of the unit as to produce fluid pressure forces acting on the cylinder barrel in opposite directions and of substantially equal magnitude, thus substantially eliminating tipping forces on the cylinder barrel resulting from hydrostatic pressure of the circulating fluid.
This is achieved in a hydraulic pump or motor of the swash plate type having a revolving cylinder barrel carrying axial pistons which transmit the driving torque as cantilevers and which has a drive shaft and a valve plate perpendicular thereto with the cylinder barrel connected to the shaft with freedom to find its seat against the valve plate, by the provision of the improvement which includes a casing having an interior configuration surrounding the cylinder barrel which iseccentric in relation to the cylinder barrel and provides a narrower clearance for fluid circulation at the region of maximum piston extension than the clearance at the region of minimum piston extensionln the drawing:
FIG. 1 is a longitudinal cross section of a piston pump or motor unit incorporating a preferred form of the present invention.
FIG. 2 is a sectional view taken along line 2-2 of FIG. 1.
FIG. 3 is a diagrammatic view illustrating the principal forces acting on the cylinder barrel.
Referring to FIG. 1, a typical axial piston pump or motor unit has a casing and a valve plate 12 within which a drive shaft 1'4 is journalled on bearings 16 and 18. Inlet and outlet connections 20 are provided in the valve plate 12 which lead to the usual arcuate commutating ports in the flat surface 22 of the valve plate 12. The casing 10 includes a fixed swash plate 24 around which piston shoes 26 slide, carrying the pistons 28 which project from their respective cylinder bores 30 in a revolving cylinder barrel 32. The barrel 32 is connected to the shaft 14 by a spline 34 which allows a slight amount of universal action which permits the barrel 32 to find its own seat against the flat valving surface 22 of the valve plate 12. A spring 36 acts to hold the cylinder barrel in contact with the valve plate with a force additional to the hydraulic force generated within the shouldered ends of the cylinder bores 30.
The foregoing description is typical of axial piston units as heretofore designed. Referring now to FIG. 3,
this illustrates diagrammatically the couples acting upon the cylinder barrel which either assist or oppose its tendency to find a seat flat against the valve plate in its freedom to move about the center A of the spline 34. Thus, the moment generated. by centrifugal force acting on the pistons with their varying degrees of projection is represented by the couple B. This couple tends to lift the cylinder barrel off from the valve plate as it may pivot about the Center A. counteracting this tendency is the couple C, generated by the spring 36. These couples may be readily calculated and the force of spring 36 so chosen as to hold the cylinder barrel in contact with the valve plate up to any desired maximum speed. It has been found in practice, however, that particularly at the higher speeds, a much stronger spring than the one calculated to be necessary must be used if the cylinder barrel is to be maintained from tipping. It is, of course, undesirable to use high holddown pressures either from the spring or from the hydraulic forces within the cylinders 30 if undesirable friction and wear are to be avoided at the valving surfaces.
In pump or motor units as heretofore constructed, when operated with a flooded casing, the high speed rotation of the cylinder barrel and pistons causes a rotary circulation of the body of fluid within the casing 10. This body of fluid, as it passes through the area represented by the top portion of FIG. 1, occupies an area whose axial extent is represented by the dimension D; whereas, when it passes through the bottom portion of FIG. 1, it has a much smaller area represented by the axial dimension E. This results in a substantial difference in velocity between these two areas. The total energy being substantially constant throughout the body of circulating liquid, it follows that the dynamic or velocity pressure of the fluid passing through the bottom area of FIG. 1 of a conventional unit would be very high and that passing through the top area would be relatively low. The static pressures, however, are just the opposite and it is the static pressures which exert forces upon the cylinder barrel 32, in accordance with Bernoulis principle. Thus, in a conventional unit, the static pressure forces would be highest at the top of FIG. 1 and produce a couple acting about center A tending to tip the barrel.
These effects can be substantially eliminated by providing an interior configuration of the casing 10 which is eccentric to the shaft in an axial zone opposite the side walls of the cylinder barrel. This eccentricity is so located that the clearance between the casing and the cylinder barrel is small at the top of FIG. 1 and large at the bottom, and results in the production of hydrostatic forces F and G, (FIG. 3) which are respectively small and large. The fluid circulating at the rear end face of the cylinder barrel and caused by the rotation of the projecting pistons 28, because of the differing axial dimensions at the top and bottom, results in hydrostatic pressure forces H and I which are respectively large and small.
In order to separate the two bodies of circulating fluid acting upon the side walls and upon the end face of the cylinder barrel respectively, a baffle 38 of circular form and having a very small clearance with the cylinder barrel is provided. Thus, in FIG. 3 on the righthand side of the baffle 38, the fluid circulates rapidly at the top of the Figure and slowly at the bottom; while on the leftlhand side of the baffle 38, the relative velocities of the circulating fluid are just the opposite. The degree of eccentricity of the interior wall of the casing may then be made just sufficient so that the couple produced by the algebraic sum of the forces F and G will be substantially equal and opposite to that produced by the sum of forces H and I.
In a particular unit, this may be determined by a trial and error approach. It has been found, for example, that a typical conventional hydraulic motor unit would experience cylinder barrel tipping at speeds in the neighborhood of 3,500 rpm. A unit of the same design, except incorporating the present invention, was found to operate without tipping at speeds above 4,500 rpm.
We claim 1. In a hydraulic pump or motor of the swash plate type, having a revolving cylinder barrel carrying axial pistons which transmit the driving torque as cantilevers and having a drive shaft and a valve plate perpendicular thereto, with the cylinder barrel connected to the shaft with freedom to find its seat against the valve plate, that improvement which includes a casing having an interior configuration surrounding the cylinder barrel which is eccentric in relation to the cylinder barrel and provides a narrower clearance for fluid circulation at the region of maximum piston extension than the clearance at the region of minimum piston extension.
2. A pump or motor unit as defined in claim 1 having a barrier of concentric circular configuration closely spaced from the cylinder barrel at a substantial distance from the valve plate.
3. A pump or motor unit as defined in claim 1 having a barrier separating the fluid which circulates by frictional drag of the cylinder barrel from that which circulates with the projecting pistons.