A two-stage hydraulic pump includes a rotor, a ring, and a plurality of rollers disposed in slots formed on the rotor. The pump has two inlet ports and two outlet ports, which are disposed inwardly and outwardly, respectively, of the rollers. A valve mechanism is incorporated which provides for flow directional control of the output flow of the pump between one portion of the discharge ports and a portion of the inlet ports when a low output flow is desired and blocking that interconnection while connecting the inner portion of the suction inlet side with a reservoir.

TECHNICAL FIELD

This invention relates to hydraulic pumps and, more particularly, to hydraulic pumps that are changed in capacity from one operating condition to another.

BACKGROUND OF THE INVENTION

Two-stage hydraulic systems generally employ two hydraulic pumps. One pump is operated for low flow conditions and both pumps are operated for high flow conditions. Other two-stage pumps might control a portion of the output flow to a reservoir and a portion of the output flow to a hydraulic system during low flow conditions and both output portions to the hydraulic system during high flow conditions.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved two-stage hydraulic pump.

In one aspect of the present invention, the hydraulic pump has a plurality of rollers operable in slots on a rotor to provide the movement of hydraulic fluid from an input low-pressure area to an output high-pressure area.

In another aspect of the present invention, the pumps cooperate with the rotor to provide two pumping chambers and two inlet chambers.

In yet another aspect of the present invention, a control valve is incorporated to direct fluid flow from the output chamber to the desired hydraulic location.

In still another aspect of the present invention, the control valve is operable to direct a portion of the output flow to one of the input chambers thereby reducing the amount of flow going to a hydraulic system.

DESCRIPTION OF AN EXEMPLARY EMBODIMENT

As seen inFIG. 1, a hydraulic pump10has a rotor12, a ring14, and a plurality of rollers16. Each of the rollers16is disposed in a respective slot18formed in the rotor12. The pump10has conventional side plates, not shown, in which are formed kidney ports20,22,24, and26.

When the rotor12is rotated in the direction of Arrow A, the rollers16move with the rotor12, such that the ring14will force the rollers16inwardly as they transverse the ports outlet20and22and permit the rollers16to move radially outward as they traverse the inlet ports24and26. The rollers16and the slots18cooperate to form a plurality of spaces or volumes28. As the rollers16move inwardly, the spaces28decreases in size such that fluid therein is discharged through the port22. As the rotor12moves the rollers16from a top dead center30toward a bottom dead center32, a space33between the rotor12and the ring14steadily decreases thereby forcing fluid in this space into the port20. Also, as the rotor12rotates from the bottom dead center32to the top dead center30, the spaces28increase thereby filling with fluid while and the space33also increases thereby filling that space with fluid. Thus, the rotation from bottom dead center32to top dead center30is known as the inlet stroke and from top dead center30to bottom dead center32is known discharge stroke.

As seen inFIG. 2, the ports20and22are connected with a passage34, which in turn is connected with a control valve36. The port24is connected with a reservoir38and through a passage40, which is also connected with the valve36. The valve36is connected with the port26through a passage42. The passage34is in continuous communication with a hydraulic system44. The valve36includes a spool or slide mechanism46, which is urged rightward, as seen inFIG. 2, by a bias spring48and may be urged leftward against the bias spring48by a pressure control signal represented by the arrow50.

In the spring set position shown, the valve36connects the reservoir38with the passage42and therefore the port26. During the spring set position, the passage34is blocked at the valve36. When the valve36is placed in the pressure set position with a control pressure at arrow50, the valve member46moves against the spring48to provide communication between the passage34and the passage42while simultaneously disconnecting communication between the passage40and passage42.

When this occurs, some of the fluid (approximately the amount discharged from the spaces28) in the passage34will be directed through the valve36to the passage42and thence through the port26, which is present at the underside or radially inward side of the rollers16. This portion of the high-pressure fluid will operate to pressurize the radially inward portion of the rollers16such that it is not delivered to the hydraulic system44but is rather recycled through the pump10thereby reducing the high pressure output fluid volume of the pump10, which is delivered to the hydraulic system44. Thus, a two-stage pump is created.

By way of example, the two-stage pump may be designed with the following characteristics in mind. The total pump displacement being 49.26 cubic centimeters per revolution; the low flow pump displacement, that is, the inner port22, being 21.18 cubic centimeters per revolution, thus the ratio of low flow volume to total flow volume is 43 percent. The hydraulic pump10includes thirteen rollers having a roller diameter of 18 millimeters and a roller length of 20 millimeters. The area between the radially outer portion of the rotor12and the ring14is 61.57 square millimeters, and the area of the inner portion between the rollers16and the bottom of slots18is 20.84 square centimeters. These numbers are given by way of example only and are not designed or considered to be limiting of the invention.

The pump10is also designed so that the crossover of both the inner and outer portions at top and bottom dead centers occur at the top of the sine wave, which is conventional. The output flow of a gerotor pump and/or roller type pump is generally a sine wave function. By having the crossover occur at the top of the sine wave, the lowest output flow or change in flow per degree of revolution is encountered. This will aid in quieting the pump. Also, during the crossover of the rollers occur at one-half intervals; that is, when a roller is crossing at top dead center30there is no crossover at bottom dead center32and vice versa.

As seen inFIG. 1, the roller16A is between the ports20,22,24,26, while the roller16B is connected with ports20,22and the roller16C is connected with ports24,26. However, as the rotor rotates in the direction of arrow A, the roller16B will be positioned between the ports and the roller16A will have encountered the ports20and22. This reduces the pressure fluctuations that occur during crossover and provide for a mechanism acting as if it had twenty-six rollers rather than thirteen.