Variable displacement piston pump
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to variable displacement piston pumps and, particularly, to pumps of the type described which are compensated for maintaining constant pressure with variable flow. More particularly, this invention relates to means for controlling the stroke of the pump pistons to provide the aforenoted compensation. 
     2. Description of the Prior Art 
     The pump disclosed is particularly intended for use in electronic actuator diesel fuel injection systems. For this application and for other applications, a minimal pressure ripple or variation with a variation in fluid flow is desirable. Check valve type pumps, which are well known in the art, fulfill this requirement by including check valves for controlling pump discharge and/or inlet functions, and further include an arrangement for varying the stroke of the pump pistons. In pumps of this type, the pressure in the piston cylinders must be slightly greater than the pressure in the discharge manifold before the check valve opens. This is contrary to rotating cylinder block type pumps wherein communication is established between the cylinders and the discharge manifold at some predetermined pump shaft angle, without regard to the respective pressures. The novel feature of the present invention is the arrangement effected by pump discharge manifold pressure and used for varying the stroke of the pump pistons. 
     SUMMARY OF THE INVENTION 
     This invention contemplates a variable displacement piston pump including a shaft having a driven end and an opposite end arranged for supporting a swash plate to pivot about an axis displaced from and normal to the center line of the shaft. A plurality of pistons having hollow cylinders extending therethrough are arranged with corresponding check valves in a pump block. During the &#34;delivery&#34; stroke of the pistons the pressure in the cylinders becomes sufficient to actuate the check valves, whereby fluid is delivered to a common discharge manifold. When the manifold pressure approaches a predetermined value, a force is created which is transmitted to the swash plate and pivots the plate away from a maximum flow position. The pistons are arranged with the swash plate so that when the swash plate pivots, the stroke of the pistons is decreased to reduce fluid flow and pressure. Equilibrium is thus established and a reduced fluid flow at a predetermined substantially constant pressure is maintained. 
     One object of this invention is to provide a variable displacement piston pump having the capability of maintaining a predetermined substantially constant pressure with a variable flow of fluid through the pump. 
     Another object of this invention is to provide a swash plate arranged with the pump pistons so that when the pump discharge manifold pressure approaches a predetermined value, the swash plate is displaced away from a maximum flow position, whereupon the stroke of the pistons is decreased to reduce fluid flow and pressure and to establish an equilibrium state whereby the reduced fluid flow is maintained at a predetermined substantially constant pressure. 
     Another object of this invention is to provide a pump of the type described which contains fewer parts than prior art pumps for similar purposes, and which is inexpensive to manufacture and of rugged design. 
     The foregoing and other objects and advantages of the invention will appear more fully hereinafter from a consideration of the detailed description which follows, taken together with the accompanying drawings wherein one embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for illustration purposes only and are not to be construed as defining the limits of the invention. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectioned front plan view of a pump according to the invention. 
     FIG. 2 is a right end view, relative to FIG. 1, of a swash plate according to the invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     With reference first to FIG. 1, there is shown a shaft designated by the numeral 1, and which shaft 1 is journalled in bearings 2 and 3 suitably mounted in a casing 11. An end 1A of shaft 1 is arranged to be suitably coupled to an engine or other external driving means (not shown) so as to be driven thereby, while the opposite end 1B of the shaft forms a trunnion mounting 4. 
     A swash plate 5 is pivotally supported on trunnion mounting 4 by a pivot pin 5A so as to pivot about a trunnion mounting axis X--X. It will be noted that trunnion mounting axis X--X is displaced from and normal to the center line Y--Y of shaft 1. Swash plate 5 is limited in its counterclockwise pivoting by a shoulder 1C carried by end 1B of shaft 1. 
     The pump of the invention includes a plurality of piston assemblies 6, arranged in a cylinder block 7. The invention will be described in relation to one of the piston assemblies 6 as shown in FIG. 1, with the same description applying to the other piston assemblies as well. 
     Thus, with further reference to FIG. 1, piston assembly 6 includes a piston 6A slideably supported in a bore 6B in block 7, and including a hollow cylinder 6C. A shoe 8 is disposed intermediate a spherical end 6D of piston 6A and swash plate 5. In this connection it is noted that there is a corresponding shoe 8 for each piston assembly 6, with swash plate 5 being common to all of the piston assemblies. 
     It will be understood by those skilled in the art that the bearing unit loading of the shoe/piston and shoe/swash plate arrangements as shown is limited by &#34;pressure balancing&#34;. Piston assembly 6 includes a return spring 9 disposed within piston cylinder 6C which serves to extend piston 6A on the &#34;intake&#34; stroke and to trap shoe 8 between piston end 6D and swash plate 5 when the pressure in piston cylinder 6C is absent or inadequate as will hereinafter become evident. 
     Upon rotation of shaft 1, during the &#34;intake&#34; stroke, fluid flows into the expanding piston cylinder volume through a port 10 in swash plate 5 best shown in FIG. 2. Pump casing 11 is internally flooded through fluid entering an inlet port 12 and, thereupon passing through an aperture 8A in shoe 8 in communication with port 10 and an aperture 6E in piston end 6D in communication with aperture 8A and in communication with piston cylinder 6C. Piston cylinder 6C is arranged with a check valve ball 13 included in piston assembly 6. In this connection it is noted that although a ball type check valve is shown other type check valves may be used as well, as will now be understood by those skilled in the art. 
     When piston 6A has passed top dead center, aperture 8A in shoe 8 no longer communicates with swash plate port 10, and hence piston cylinder 6C is closed and passage of fluid therethrough is blocked. Piston 6A starts its &#34;delivery&#34; stroke, and when the pressure in piston cylinder 6C is sufficient to displace check valve ball 13, lightly loaded by a spring 14, from its seat, fluid is delivered through the piston cylinder to a common discharge manifold 22 including a discharge port 22A and to a conduit or the like 26 which may communicate with a servo valve or the like. With continued reference to FIG. 1, the attitude of swash plate 5 shown therein is indicative of a maximum piston stroke and pump displacement. 
     The function of a regulating valve 21 shown in FIG. 1 is to sense discharge manifold pressure through conduit 26 and to modulate the pressure in a chamber 15 (FIG. 1) through and a conduit 32. Chamber 15 is formed by a differential area piston 16 slidingly arranged within pump casing 11. 
     When discharge manifold pressure approaches a predetermined and substantially desired value, valve 21 permits the pressure in chamber 15 to increase. This increase in pressure exerts a force which urges piston 16 rightward in relation to the arrangement shown in FIG. 1. This force is transmitted through a thrust bearing assembly 17 including a pair of tandem bearings 17A and 17B, a sleeve 18 supporting the thrust bearing assembly, a push rod 19 adjacent the sleeve and a rocker 20 arranged with push rod 19 and swash plate 5. When the force is high enough, it is effective through the aforenoted push rod and rocker arrangement for tilting swash plate 5 from the attitude shown in FIG. 1 to a more nearly vertical attitude. This decreases the stroke of piston 6A to reduce fluid flow and, consequently, to reduce the pressure. Equilibrium is thus established and a reduced flow at a pressure near a predetermined substantially constant pressure is maintained. 
     It will be noted that the thrust forces acting on pump shaft 1 are shared by bearing 3 and thrust bearing assembly 17. Bearing 3 takes the axial thrust on shaft 1 itself, but some of this thrust is relieved by bearing assembly 17. 
     It will also be noted that swash plate 5 is shown in a pivotable arrangement to control the stroke of pistons 6A. Any other hinge type joint would serve the purpose of the invention as well. Further, the invention is disclosed and described with reference to the displacement of swash plate 5 in one direction (clockwise) to decrease the stroke of piston 6A, with displacement occurring in the opposite direction as well as to increase the piston stroke, dependent upon manifold pressure, as will now be understood by those skilled in the art. 
     It will be seen from the foregoing description of the invention with reference to the drawings, that a variable displacement piston pump has been disclosed which is capable of maintaining a predetermined substantially constant pressure with a varying fluid flow. The stroke of the pistons is controlled by the novel arrangement including swash plate 5 being responsive to pump discharge manifold pressure to provide the aforenoted constant pressure with variable fluid flow. 
     Although but a single embodiment of the invention has been illustrated and described in detail, it is to be expressly understood that the invention is not limited thereto. Various changes may also be made in the design and arrangement of the parts without departing from the spirit and scope of the invention as the same will now be understood by those skilled in the art.