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A two-stage air powered hydraulic pump has a portion of the air pressure that operates the pump diverted directly to the reservoir. The air pressure on the reservoir presses hydraulic fluid in the reservoir out of the reservoir, through the pumping chamber and to the load to quickly advance the load under low load conditions and to pre-charge the pumping chamber.

Digman, Micah L. (Beaver Dam, WI, US)
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Primary Examiner:
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I claim:

1. In an air powered hydraulic pump of the type having an air pressure input port, a hydraulic pressure port and a reservoir of hydraulic fluid, the improvement wherein a portion of the pressurized air input to the pump is diverted to the reservoir so as to pressurize the fluid in the reservoir and press it out of the pump through the hydraulic pressure output port.

2. The improvement of claim 1, wherein hydraulic fluid is pressed into a pumping chamber of the pump by air pressure that is diverted to the reservoir.

3. The improvement of claim 1, wherein an air valve is provided to turn the pump on and off.

4. The improvement of claim 3, wherein the air valve also turns off the air pressure to the reservoir when it is off.

5. The improvement of claim 1, wherein a valve is provided to enable adjustment of the air pressure on the reservoir without affecting the air pressure supplied to the pump.

6. The improvement of claim 5, wherein the valve can be operated to turn off the air pressure on the reservoir.



This claims the benefit of U.S. Provisional Patent Application No. 60/743,820 filed Mar. 27, 2006.


Not applicable.


This relates to hydraulic pumps, and in particular to air powered hydraulic pumps.


Air powered hydraulic pumps such as the Enerpac PA-133 available from Enerpac, an Actuant company, having a place of business in Glendale, Wis., are well-known. In such pumps, air pressure drives the hydraulic pump. The air pressure input to the hydraulic pump is typically line pressure, which is normally about 60-120 psi, and the pump intensifies the pressure so that the hydraulic fluid pressure output of the pump is considerably higher than the air pressure, for example up to 10,000 psi. Typically, the air pressure cyclically drives a piston linearly, which drives a smaller diameter piston that pressurizes the hydraulic fluid in these types of pumps.

These pumps are typically used to power single-acting hydraulic cylinders, for example to lift a load. In some applications, the cylinder may have to extend for a distance before any significant resistance to further extension is encountered. In these applications, the initial extension of insignificant load can be time-consuming, since the hydraulic flow rate from the pump results in a relatively slow advance.

Also, typically, in these pumps the reservoir is at or near atmospheric pressure. If the pump motor is operated too fast, cavitation occurs and the pump chamber is not completely filled.


The invention provides an air powered pump with two-stage operation that supplies a higher flow of hydraulic fluid at lower pressures and a lower flow at higher pressures. A pump of this invention accomplishes this by routing a portion of the air supply used to power the pump directly to the reservoir of the pump. The air pressure acting on the reservoir pressurizes the reservoir to a relatively low pressure, for example 5-120 psi, which presses the hydraulic fluid out of the reservoir to the pump outlet port, to be delivered directly to the load, for example a hydraulic cylinder.

Preferably, the air pressure is directed to the reservoir outside of a rubber bladder that contains the hydraulic fluid. This keeps the air separate from the hydraulic fluid, as it is undesirable to contaminate the hydraulic fluid with contaminants that may be in the air or introduce air directly into the hydraulic fluid.

In another preferred aspect, the fluid is pressed from the reservoir through the pumping chamber of the pump, so as to pre-charge the pumping cavity to prevent cavitation and enable running the pump faster and at a higher volumetric efficiency.

Preferably, a shutoff valve or pressure-reducing valve is provided by which the air pressure to the reservoir may be shut off and/or reduced to a lower pressure below line pressure. For example, if line pressure is 120 psi, the pressure-reducing valve could be provided to adjust the pressure exerted on the reservoir to a value of approximately 5-120 psi. Thus, the pump could be operated as a single-stage pump, or if operated as a double-stage pump, the pressure of the first stage could be controlled. For example, a 5 psi positive pressure in the reservoir would be desirable if only for the purpose of filling the pumping chamber faster on the suction stroke of the pump, for better hydraulic efficiency that results from pre-charging the pumping chamber with even this small pressure.

Existing air powered pumps typically have a three-way foot operated switch or valve that in one position advances the cylinder, in another position holds the cylinder, and in a third position retracts the cylinder by permitting fluid to flow back into the reservoir. In the retract mode, this actuator would be operable to retract the cylinder until the pressure on the reservoir is reached in the cylinder, in which case retraction would stop, unless the pressure on the reservoir was shut off or reduced. The foot actuator could be modified to do this, in other words to shut off the air pressure to the reservoir in the retract position. Preferably, pressure on the reservoir would also be off in the neutral or hold position, and when off the reservoir would be vented to the atmosphere. Alternatively the pressure reducing or relieving valve in the input line to the reservoir could be used to shut off the pressure to the reservoir.

Thus, an air powered pump would benefit from using the invention in any or all of a number of ways, including providing a rapid first-stage flow at relatively low pressure, better efficiency at high pressure, and allowing the pump to cycle faster and at a higher volumetric efficiency by pre-charging the pumping chamber.

These and other advantages of the invention will be apparent from the detailed description and drawings.


FIG. 1 is a side elevation view of a pump incorporating the invention with a fragmentary portion shown in section along the line 1-1 of FIG. 4;

FIG. 2 is a top plan view of the pump of FIG. 1;

FIG. 3 is a bottom plan view of the pump of FIG. 1;

FIG. 4 is a left end plan view of the pump of FIG. 1; and

FIG. 5 is a schematic view of the pump of FIG. 1 connected to a single acting hydraulic cylinder.


FIGS. 1-4 are views of an Enerpac Model PA-133 air powered hydraulic pump modified to incorporate the invention. The pump 10 has a three-position foot pedal 12 for operating the pump to either advance the cylinder, hold it, or retract it. The pump also has a reservoir 14 which is typically lined by an elastomeric bladder 16 that contains the hydraulic fluid 18 (FIG. 5). The pump 10 has an air inlet 20 and hydraulic outlet port 22 (shown capped in FIGS. 1-4 with plug bolt 21). A passageway 29, 34, 30 connects the air inlet 20 when valves 32 (FIG. 5) and 34 are turned on to the reservoir 14 so as to pressurize the bladder 16 and therefore the hydraulic fluid 18. Valve 34 may be an on-off valve and/or a pressure adjusting valve.

Referring particularly to FIG. 5, the pump 10 includes an air motor 40 and a hydraulic pump 42 that is driven by the air motor 40. As illustrated in FIG. 5, the air motor 40 may be a rotary motor that drives pump 42 rotationally, which may be with a crank or cam drive connection. However, typically in prior art air powered hydraulic pumps, the air motor has been a linear type of air motor, in other words a piston with a surface area which is relatively large in comparison to the surface area of the driven piston that pumps the hydraulic fluid. Advantages of the invention can be realized using a linear pump as well.

As illustrated in FIG. 5, hydraulic fluid from the reservoir 14 is supplied to the hydraulic motor through a one-way valve 44 and is expelled from the pumping chamber of the pump 42 through another one-way valve 46. Foot pedal 12 operates three-way, three-position valve 48, which in the center position as illustrated blocks the hydraulic pressure line 50, the reservoir line 52 and the port line 54 to which the cylinder 56 is connected via port 22. In this position, valve 32 is illustrated as off, with the air motor 40 and reservoir 14 vented to atmosphere represented by dashed line 23, but valve 32 could be left on in this position of the foot pedal 12. In the forward position of the pedal 12, the reservoir line 52 is blocked and hydraulic pressure is applied to the cylinder 56, with the air supply turned on via valve 32 to the air motor 40 and to the reservoir 14 to drive the hydraulic pump. A mechanical connection between actuator pedal 12 and valve 32 is illustrated by dashed line 62, for operating valve 32 with actuator pedal 12. In the upward or release position of the pedal 12, the cylinder is connected to the reservoir line 52 and the pressure line 50 is blocked, which can result in pressure being diverted to the reservoir via pressure relief valve 60. In the release position, the pedal 12 would typically turn off the valve 32 so as to stop the air motor 40 and terminate pressure on the reservoir, venting it to atmosphere through valve 32, to allow fluid to flow back to the reservoir under atmospheric or very low pressure. Also, a one-way check valve 59 may be used in line 52 to allow flow in that line only in the direction from the cylinder 56, and prevent fluid from being pressed from the reservoir 14 through the line 52 toward the cylinder 56.

Valve 32 is optional as the air supply port 20 could have a direct communication to both the air motor 40 and to line 29. In that case, whenever air was supplied to motor 40, pump 42 would be operated, and if line 50 was blocked, hydraulic pressure would be diverted back to the reservoir 14 via pressure relief valve 60. In that case, valve 34 could be used to relieve pressure from the reservoir and/or vent it to atmosphere.

Whenever air pressure is applied to the reservoir 14 via line 30, a relatively low pressure, for example 5-120 psi of air pressure, is applied to the hydraulic fluid 18 in the bladder 16, which presses the fluid through fluid supply line 70 and past valve 44 into the pumping chamber of pump 42. Even if pump 42 is not being operated, the fluid can flow through the pumping chamber to line 72 and past valve 46 to line 50. If the treadle 12 is in the blocked position as illustrated, the fluid will stop there. If the treadle 12 is placed in the press, or forward, position, the hydraulic fluid pressure will be communicated to outlet port 22 and to the cylinder 56 to rapidly advance the cylinder 56 as long as there is no significant resistance to advance of the cylinder 56. When the load is encountered, thereby creating a significant resistance to the flow, it will be necessary to apply air pressure to the air motor 40 to operate pump 42 to increase the hydraulic pressure and thereby advance cylinder 56, albeit more slowly. When cylinder 56 is desired to be retracted, the pedal 12 is placed in the release, or upward, position, to block line 50 and communicate line 54 with line 52 so that fluid can be pressed out of the cylinder 56 back to tank 14.

Valve 34 may be used to turn the air pressure off to tank 14 and/or to adjust it to any desired value up to the value of the air line pressure. A relatively low pressure, for example 5 psi, may be all that is needed to pre-charge the pumping chamber of pump 42 to thereby achieve improved hydraulic pumping efficiency, and perhaps be able to operate the pump 42 at a faster cycle rate. It may be necessary to apply higher air pressures at the reservoir 14 to more quickly advance the cylinder 56, depending upon the load to which the cylinder 56 is subjected.

A preferred embodiment of the invention has been described in considerable detail. Many modifications and variations to the embodiment described will be apparent to those skilled in the art. For example, rather than the hydraulic fluid being separated from the air pressure by a rubber bladder, it could be separated by a piston, a diaphragm, a bellows or the invention could be applied to any sealed reservoir. Therefore, the invention should not be limited to the embodiment described, but should be defined by the claims which follow.