Pumps of this type may be used to pressurize engine oil in a Hydraulic Electric Unit Injector (HEUI) diesel engine fuel system. The invention also relates to methods of making the interface between the piston and slipper.

A slipper-type piston pump is disclosed in U. S. Patent No.

6,427, 663. In these pumps a piston is fitted in a piston bore and is moved back and forth along the bore by a drive member. A slipper is located between the piston and the drive member. A spring holds the piston against the slipper and the slipper against the drive member. The slipper has a recess that receives an end of the piston. Retraction of the piston during an inlet stroke draws fluid into the pumping chamber. Extension of the piston along a pumping stroke flows pumped fluid from the assembly, typically past a spring backed check valve.

In conventional pumps the pistons are commonly made of hardened steel and the slippers are made of softer bronze. The spherical end of the piston and the spherical recess in the bronze that receives the piston end are carefully manufactured to exacting tolerances in order to assure proper engagement between the piston and the slipper. The thickness of the oil film between the spherical surfaces is taken into account in sizing the spherical surfaces. Manufacture of pistons and slippers with exactly mating spherical surfaces is expensive and difficult.

Failure to manufacture the pistons and slippers with mating surfaces increases wear.

Diesel engines using HEUI fuel injectors are well known.

HEUI injectors are actuated by oil drawn from the sump of the diesel engine by the diesel engine oil pump and flowed to a high pressure pump assembly, driven by the diesel engine. The high pressure pump assembly typically uses a swash plate pump with axial pistons and having an output dependent upon the speed of the diesel engine. The pistons have spherical ends that engage spherical slippers with flat faces. The slippers and pistons are extended and retracted by rotation of a cylinder barrel containing the piston bores. The flat faces of the slippers bear and slide against a flat swash plate.

In conventional swash plate pumps the pistons are made of hardened steel and the slippers are made of a softer material, typically bronze. The spherical surface on the end of each piston has a radius only slightly smaller than the radius of the spherical surface in the slipper to permit maintenance of an oil film between the piston and slipper as the slipper moves angularly relative to the piston during each pumping stroke.

Friction, lubrication, and wear between the spherical surface of the piston and the spherical surface of the slipper are complex phenomena, commonly described as contact between the piston and slipper spherical surfaces, although the surfaces are separated by an oil film.

Manufacture of precisely matched spherical surfaces in conventional swash plate pumps is typically accomplished by deforming the softer slipper spherical surface to conform to the harder spherical surface of une piston. Pistons and slippers with spherical surfaces that do not match within the thickness of an oil film have high bearing contact pressure and experience high wear.

Therefore, there is a need for an improved interface between a piston and a slipper and an improved method for manufacturing pistons and slippers with an improved interface. The improved piston and slipper interface and method are particularly useful in a HEUI diesel engine but are also useful in other types of pumps and pumping applications.

Summary of the Invention The invention is an improved slipper and piston interface and an improved method for making the interface.

The high pressure pump includes a drive member which reciprocates pistons in bores. A slipper is positioned between the crank and pistons. A spring in the piston bore holds a spherical end of the piston in a slipper recess and keeps the slipper against the drive member. The piston may be hardened steel and the slipper may be formed from bronze, a material softer than hardened steel. The slipper end of the piston is spherical and extends into a specially shaped, nearly spherical recess formed in the slipper. This nearly spherical recess has a radius of curvature greater than the radius of curvature of the piston end. and has an opening at the top of the slipper that is larger than the piston diameter.

When the piston is first seated in the recess in the slipper the spherical surface on the piston engages the surface in the slipper at a circular line of engagement. During initial operation of the pump the pressure exerted on the slipper by the piston during pumping at the narrow line contact deforms the softer bronze to increase the area of contact and form a wider circular band. The circular band has sufficient width to support the piston and seal the pumping chamber without additional deformation.

The spherical surface on the end of the piston and the near spherical surface on the slipper reduce the cost of manufacturing the piston and slipper. Both surfaces may be manufactured with dimensional tolerances greater than the tolerances required for matching the radii of the pistons and slipper with an allowance for an oil film.

Description of the Drawings Figure 1 is a side view of a pump assembly; Figure 2 is a sectional view taken along line 2--2 of Figure 1 ; Figure 3 is a sectional view taken along line 3-3 of Figure 2; and Figure 4 is a sectional view through the piston, slipper and eccentric of one of the pumps of Figures 1-3 illustrating the improved piston/slipper interface.

Description of the Preferred Embodiment Pump assembly 10 is mounted on a diesel engine, typically a diesel engine used to power an over-the-road vehicle, and supplies high pressure engine oil to a number of solenoid actuated fuel injectors. The pump assembly includes a metal body 12 defining an interior crank chamber 14. Crankshaft 16 is journaled in the body and includes two axially spaced cylindrical drive eccentrics 18 and 20 in chamber 14. The crankshaft extends outwardly of body 10 and supports a drive gear 22 driven by the engine.

The pump assembly includes four high pressure check valve, slipper type piston pumps 24 arranged in two ninety degree oriented banks 26 and 28. Each bank includes two pumps 24. As shown in Figure 2, bank 28 extends to the left of the crankshaft and bank 26 extends above the crankshaft so that the pump assembly has a vee-4 figuration. One pump 24 in each bank is in alignment with and driven by eccentric 18 and the other pump in each bank is alignment with and driven by eccentric 20. The four check valve pumps 24 are identical.

The engine on which assembly 10 is mounted includes a low pressure oil pump which flows engine oil into crank chamber 14.

As described herein, the low pressure oil is flowed into the crank chamber through an inlet port (not illustrated), through the eccentrics and into the pumps 24, is pressurized by pumps 24 and flows outwardly of the pump assembly through an outlet port (not illustrated) and to the fuel injectors. High pressure oil actuates the injectors in response to signals received from the electronic control module for the engine.

The pump assembly 10 may include an inlet throttle valve located in the inlet passage flowing low pressure oil to crank chamber 14 and a control circuit for the inlet throttle valve including an injection pressure regulator (IPR) valve 30 shown in Figure 2. The inlet throttle valve and control circuit for the inlet throttle valve form no part of the present invention. A related pump assembly with an inlet throttle valve and IPR valve is disclosed in U. S. Patent No. 6,427, 663.

Each check valve piston pump 24 includes a piston bore 32 formed in one of the banks and extending perpendicularly to the axis of the crankshaft. A hollow cylindrical piston 34 has a sliding fit within the inner end of each bore 32. The piston has a spherical inner end 36 adjacent the crankshaft. End 36 is fitted in a generally spherical recess in a slipper 38 located between the piston and the eccentric actuating the pump.

Interface 303 between the piston and slipper is illustrated in Figure 4.

The inner concave surface of the slipper is cylindrical and conforms to the surface of the adjacent cylindrical eccentric.

Central passage or opening 40 in the spherical end of the piston and passage 42 in the slipper communicate the surface of the eccentric with variable volume pumping chamber in piston 34 and bore 32. The variable volume portion of the pumping chamber is located in bore 32.

A check valve assembly 46 is located in the outer end of each piston bore 32. Each assembly 46 includes a sleeve 48 tightly fitted in the end of bore 32. A cylindrical seat 50 is fitted in the lower end of the sleeve. Plug or closure 52 is fitted in the sleeve to close the outer end of bore 32. Poppet disc or valve member 54 is normally held against the outer end of seat by poppet spring 56 fitted in plug 52. A piston spring 58 is fitted in each piston 34 and extends between the spherical inner end of the piston 34 and seat 50.

Each eccentric 18,20 is provided with an undercut slot 60 located between adjacent sides of the eccentric and extending about 130° around the circumference of the eccentric. Passage 62 extends from the bottom of slot 60 to two cross access passages 64 extending parallel to the axis of the crankshaft and through the eccentric. The cylindrical eccentrics 18 and 20 are oriented 180° out of phase on the crankshaft so that passages 64 for eccentric 18 are located diametrically across the crankshaft axis from passages 64 for eccentric 20. See Figure 2.

Rotation of crankshaft 16 in the direction of arrow 68 moves the slots 60 in the surfaces of the eccentrics into and out of engagement with slipper passages or openings 42 to permit unobstructed flow of engine oil from the crank chamber 14 into the pumping chambers 44. Rotation of the crankshaft also moves the pistons 34 up and down in bores 32 to pump oil past the check valves and into outlet passages leading to the outlet port. One outlet passage 66 is illustrated in Figure 3. During rotation of the crankshaft the piston springs 58 hold the pistons against the slippers and the slippers against the eccentrics while the slippers oscillate on the spherical ends of the pistons.

During return or suction movement of the piston toward the crankshaft the inlet passage leading from crank chamber 14 to the pumping chamber 44 is unobstructed. There are no check valves in the inlet passage. The unobstructed inlet passage permits available engine oil in the crank chamber to flow freely into the pumping chambers during return strokes. The inlet passage is opened after piston 34 returns sufficiently to allow trapped oil to expand near the beginning of the return stroke and is closed at the end of the return stroke.

Figure 3 illustrates check valve pump 24 in bank 26 at top dead center. High pressure oil in chamber 44 has been flowed past poppet valve 54 and the valve has closed. The closed pumping chamber 44 remains filled with oil under high pressure.

The poppet valve for the pump is held closed during the return stroke by a spring 56 and high pressure oil in the outlet passages. When a piston in pump 24 is at the bottom of its return stroke available oil from the crank chamber has partially or completely filled pumping chamber 44. The inlet passage communicating with the crank chamber and the pumping chamber is closed at bottom dead center. Further rotation of the crank moves the piston through a pumping stroke and flows high pressure oil from the pumping chamber past the check valve and into an outlet passage leading to the outlet port.

Figure 4 illustrates the generally spherical interface 303 between the piston and slipper of each pump 24 and is an enlarged sectional view through the inner end of a hollow cylindrical piston 300, like piston 34, slipper 302, like slipper 38, and crank eccentric 304, like either eccentric 18,20. Spring 58 which biases the lower end of piston 300 against the slipper 302 and the slipper against the eccentric 304 is not illustrated.

Piston 300 is preferably manufactured from hardened steel and includes a hollow cylindrical wall 308 that has a sliding fit in the piston bore of pump 24. The slipper is preferably formed from softer bronze. The spherical end of the piston is fitted in a recess having a nearly spherical surface 328 in slipper 302 to define a generally spherical interface 303 between the piston and slipper. A partial cylindrical surface 312 on the side of the slipper away from the piston engages the cylindrical surface 314 of eccentric 304, as previously described. Central inlet passages 316 and 318 extend through piston end 310 and slipper 302. Rotation of the eccentric past the slipper brings the inlet passage or slot in the eccentric into and out of engagement with passage 318 during pumping movement of piston 300. The inlet passage leading to the pumping chamber is unobstructed during return strokes, as previously described.

Piston end 310 has a convex spherical surface 320 having a center 322 located on central axis 324 and a radius 326 that may be about 0.45 inches. Piston end 310 is fitted in concave nearly spherical surface 328 formed on the side of the slipper away from the eccentric. This surface is symmetrical around the central axis when the piston is at the top or the bottom of its pumping stroke and the slipper and piston are oriented as shown in Figure 4.

Surface 328 is generated by rotating a circular arc located in a plane passing through axis 324 around an arc axis 330, parallel to axis 324, and located in the plane a short distance to the side of axis 324 away from the arc. The axes 330 used to generate the nearly spherical surface 328 lie on a small diameter cylinder 332 surrounding axis 324. Surface 328 is referred to as a revolved positive offset surface. The radius for the nearly spherical surface 328, the distance from point 334 on cylinder 332 and the circular arcs forming surface 328, is slightly greater than the radius 326 of piston spherical surface 320. The radius of curvature of surface 328 is greater than the radius of curvature of surface 320.

When the piston is first seated in the slipper the spherical surface 320 engages nearly spherical surface 328 in a line of contact 324 extending around the piston and slipper in a circle.

The remainder of surface 320 is spaced from surface 328.

During pumping the slipper rotates back and forth relative to the piston to move the circle of contact along spherical surface 320. Pumping exerts consluerable force between the piston and the slipper, resulting in deformation in the softer bronze slipper at the circle of contact. This deformation reduces the radius of curvature of the portion of the slipper contacting surface 320 to conform to the radius 326 of surface 320 and form a partial spherical circular band 336 in surface 328 conforming to the spherical surface 320 of the piston.

During deformation, the width of the initial contact circle increases to form the band. As illustrated in Figure 4, band 336 may extend about 8 degrees to either side of the initial contact circle 324 between the piston and slipper and have a total angular width 338 of about 16 degrees. For a pump having a piston end with a spherical radius of about 0.45 inches, band 336 may extend 1/8 inch or less from top to bottom along surface 328.

Band 336 has sufficient area to support the piston 310 during pumping without appreciable additional deformation.

In pump 24 the arc axes 330 for surface 328 are offset from central axis 324 a small distance of from 0.002 to 0.003 inches and revolved offset surface 328 is very nearly spherical. The radius for surface 328 is only slightly greater than the radius 326 of surface 320. For a piston with a surface 320 having a radius 326 of about 0.45 inches, surface 328 may have a revolved offset radius, as described of about 0.453 inches. In Figure 4, the offset of axes 330 from axis 324 and the divergence of surface 328 from surface 320 have been exaggerated for purposes of clarity.

Manufacture of pistons 300 and slippers 302 with surfaces 320 and 328 as described is facilitated by nearly spherical surface 328 because it is no longer necessary to manufacture nearly identical spherical surfaces for proper seating between the piston and slipper. Tolerances for surfaces 320 and 328 can be relaxed somewhat.

If both surfaces 320 and 328 are spherical, bearing pressure will be distributed over the interface only if spheres are precisely matched. If the piston sphere is slightly larger, bearing pressure will be highest where the cylindrical diameter of the piston contacts the slipper diameter. If the piston sphere is smaller by more than oil film thickness, bearing pressure will be highest at the end of the piston. Tolerances required for spherical piston and slipper surfaces are stricter than for the spherical and nearly spherical surfaces.

In pump 24 the radius of spherical surface 320 may vary slightly and the radius of the nearly spherical recess surface 328 may also vary slightly. The result of these variations is to move the initial point of contact 324 up or down slight distances along surface 328. After initial contact at the line circle, as described, loading of the piston against the slipper will form a deformed band 336 supporting the piston in the slipper. The band should not extend to the end of surface 320 at the top of the interface or to the end of surface 328 at passage 318.

The end of the piston is spherical and fitted into a nearly spherical concave surface in the slipper. This slipper surface has a radius of curvature greater than the radius of curvature of the spherical end of the piston so that initial contact between the piston and slipper is a line circle extending around the two surfaces. During initial operation of the pump loading and relative movement between the piston and the slipper deform the softer slipper material to form a partially spherical band in the slipper, the area of which is'sufficient to allow oil film to carry the piston load.

The invention also includes a pump with a piston-slipper interface where the slipper is formed from a material, such as steel, which is harder than the material forming the end of the piston, which may be bronze. In this pump the concave surface in the slipper is spherical. The convex surface on the end of the piston is nearly spherical having a radius of curvature less than the radius of curvature of the slipper recess. The surface on the end of the piston is generated by rotating a circular arc located in a plane passing through the central axis around an arc axis, parallel to the central axis, and located a short distance to the side of the central axis towards the arc. The axes used to generate the nearly spherical surface lie on a small diameter cylinder surrounding the central axis. This nearly spherical surface is referred to as a revolved negative offset surface.

Initial engagement between the piston and the slipper of his pump is at a circle extending around the central axis. During initial operation of the pump the relatively softer material at the end of the piston is deformed to create a partial spherical band extending around the piston end and providing a continuous surface for support of an oil film to carry the piston load. The band supports the piston during pumping.

The invention is not limited to piston pumps where the slipper engages a cylindrical eccentric, which rotates relative to the slipper to move the piston through pumping and return strokes. The invention includes pumps of the piston and slipper type where the slippers engage a drive member other than an eccentric. For instance, the invention includes swash plate pumps where the plate moves tne slippers and the slippers move the pistons through pumping strokes.