US3839866A

US3839866A - Fill and pressurizing system

Info

Publication number: US3839866A
Application number: US00420762A
Authority: US
Inventors: L Seidel
Original assignee: Jergens Inc
Current assignee: Jergens Inc
Priority date: 1971-06-30
Filing date: 1973-12-03
Publication date: 1974-10-08
Anticipated expiration: 1991-10-08

Abstract

A hydraulic booster is disclosed in which a generally cylindrical housing is divided by a separator into a reservoir chamber above the separator and an air motor cylinder below the separator. A differential area air piston located in the air cylinder drives a plunger along a hydraulic cylinder extending axially within the reservoir. Compressed air supplied to the reservoir causes hydraulic fluid to flow into the hydraulic cylinder pressurizing the system to a pressure approaching the air pressure. Valved means subsequently open to operate the air driven piston to further pressurize the hydraulic fluid within the hydraulic cylinder to the high required pressure.

Description

nited States Patent [191 Seidel cti,i974

[ FllLlL AND PRESSURIZING SYSTEM Inventor: Lawrence H. Seidel, Cleveland,

Ohio

Assignee: .lergens, inc, Cleveland, Ohio Filed: Dec. 3, 1973 Appl. No.: 420,762

Related US. Application Data Continuation of Ser. No. l58,429. June 30, l97l U.S. Cl 60/547, 60/574, 60/593 Int. Cl. F15b 7/00 Field of Search 60/547, 560, 574, 576,

References Cited UNITED STATES PATENTS Primary Examiner-lrwin C. Cohen Assistant Examiner-A. M. Z apcic Attorney, Agent, or FirmMcNenny, Farrington, Pearne & Gordon [5 7] ABSTRACT A hydraulic booster is disclosed in which a generally cylindrical housing is divided by a separator into a reservoir chamber above the separator and an air motor cylinder below the separator. A differential area air piston located in the air cylinder drives a plunger along a hydraulic cylinder extending axially within the reservoir. Compressed air supplied to the reservoir causes hydraulic fluid to flow into the hydraulic cylinder pressurizing the system to a pressure approaching the air pressure. Valved means subsequently open to operate the air driven piston to further pressurize the hydraulic fluid within the hydraulic cylinder to the high required pressure.

13 Claims, 5 Drawing Figures 747 12/1948 Fischer et al. 60/547 ,766 12/1954 Hufford 60/560 .l60 4/l960 Van Wart et al.. v. 188/357 1.792 ll/l970 Ellis 60/534 PMENIED 8W4 3.839.866

SHEU 1 BF 2 INVENTOR.

4 Ala/196M676 56/054 PATENIEU 3W4 3.839.866

SHEEI 20? 2 /0/ l we 86 /03 l A 1 FILL AND PRESSURIZING SYSTEM This is a continuation, of application Ser. No. 158,429 filed June 30, 1971.

BACKGROUND OF THE INVENTION PRIOR ART It is often desirable to use relatively low pressure compressed air, which is available in many manufacturing plants or the like, to power a relatively high pressure hydraulic system. In such instances pressure boosters are often used, which employ differential area of piston means, so that the hydraulic output pressure substantially exceeds the compressed air pressure available to power the device. Examples of such devices are illustrated in the US. Pat. Nos. 2,877,624 and 3,473,328, the latter of which is assigned to the assignee of the present invention.

In both of these patents, hydraulic booster systems are illustrated for supplying relatively high pressure hydraulic fluid to work clamping devices or the like. Further, in both of these systems, a reservoir is provided which contains hydraulic fluid and provides an air space thereabove..Compressed air supplied to such reservoirs pressurizes the hydraulic fluid before the differential area piston is operated. The hydraulic fluid, which is directly pressurized by the compressed air, flows to the clamping device to take up any clearances and to cause the clamping devices to be operated to a preliminary clamping condition on the work piece. After the preliminary clamping is completed and the pressure builds up, the differential piston operates to increase the pressure of the hydraulic fluid supplied to the clamping device to the desired high pressure.

SUMMARY OF THE INVENTION In the illustrated embodiment of the present invention, a simple structure is utilized in which valved means connect the air chamber of the air motor portion of the differential area piston to the air space within the reservoir only after the pressure in the hydraulic chamber portion of the pump reaches a predetermined pressure to insure that the air motor will not be actuated until the pressure of the hydraulic fluid supplied to the system approaches the pressure of the compressed air supplied. Such valved means then functions to connect the air space in the reservoir to the air chamber of the air motor to cause the piston to further pressurize the hydraulic fluid. A simple ball valve element is actuated by a plunger exposed to hydraulic pressure to create this operation. The plunger of the differential air motor which pressurizes the hydraulic fluid in response to operation of the air motor operates to automatically isolate the reservoir and the pumping chamber as soon as the plunger commences to move under the influence of compressed air.

The various components of this system are arranged so that substantially all of the hydraulic fluid in the system is available for the operation of the clamping devices. The illustrated system provides structural integrity, high reliability, low maintenance, and low initial manufacturing costs.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a side elevation partially in section illustrating a pressure booster in accordance with the preferred embodiment of this invention;

FIG. 2 is an enlarged fragmentary section of the valved system illustrating the structural detail thereof;

FIG. 3 is a schematic illustration of the booster system illustrating the position the elements assume prior to pressurization;

FIG. 4 is a schematic illustration similar to FIG. 3 but illustrating the system after pressurization has commenced;

FIG. 5 is a schematic illustration similar to FIGS. 3 and 4 illustrating the elements of the booster while the booster is being exhausted.

DETAILED DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a booster in accordance with the preferred embodiment hereof. Such booster includes a base 10, a head 11, and a divider or separator 12. A tubular member 13 extends between the base 10 and separator 12 and cooperates therewith to define the cylinder of an air motor portion of the device. Draw bolts 14 extend between the base 10 and the separator member 12 and cause the ends of the tubular member 13 to tightly engage radial surfaces 16 and 17 formed on the separator 12 and base 10, respectively. O-

ring type seals

18 and 19 are provided on the base 10 and separator 12 to provide a fluid tight joint with the inside diameter of the tubular member 13.

A second tubular member 21 extends between the separator member 12 and the head 11. Here again, draw bolts 22 clamp the ends of the tubular member 21 against

radial surfaces

23 and 24 formed on the separator member 12 and head 11. Seals 26 and 27 are engaged by the ends of the tubular member 21 to provide a fluid tight joint between the tubular member 21 and the head 11 and separator member 12, respectively. The tubular member 21 is preferably formed of a transparent material to provide a visual indication of the level of liquid therein.

A hydraulic cylinder member 28 is threaded at its lower end into the separator member 12 and extends at its upper end through a central opening 29 in the head 1 l. A hydraulic cylinder bore 31 is formed in the cylinder member 28 which extends to an outlet port 32 at its upper end. A sea] 33 is located at the lower end of the cylinder member 28 to provide a fluid tightjoint between the cylinder member 28 and the separator 12 and a pair of spaced seals 34 and 36 provide a fluid tight joint between the upper end of the cylinder mem' ber 28 and the head 11 at two spaced locations. A reservoir chamber 37 is defined by the cooperating separator 12, head 11, and tubular member 21 around the cylinder member 28. The lower portion of the reservoir chamber 37 is adapted to be filled with hydraulic fluid and the upper portion above the hydraulic fluid is an air space adapted to be pressurized with compressed air. Normally, compressed air is used to power the device. However, other compressed gases may be used, and the phrase compressed air or the term air is intended to include other compressed gas or gases.

A differential area piston assembly includes a piston head 41 and a plunger 42. The piston head 41 is sized to extend to the inner wall of the tubular member 13 and divides the air motor portion of the device into a lower air chamber 43 and an upper air chamber 44. A seal 46 prevents leakage past the piston head 41. The plunger 42 which is driven by the piston head extends through a seal 47 in the separator member 12 and into a hydraulic pumping chamber 48 defined by the cylinder bore 31 and an extension thereof 49 formed in the separator member 12 below the seal 33. An inclined passage 51 connects the lower end of the reservoir chamber 37 and the extension 49. The various elements are proportioned so that fluid communication is provided between the liquid portion of the reservoir chamber 37 and the hydraulic pump chamber 48 when the piston is in the retracted position illustrated in FIG. 1, but also so that as the plunger is moved upward, the upper end engages the seal 33 to isolate the pump chamber 48 from the reservoir after the plunger 42 engages the seal 33.

The head 11 is provided with an inlet port 52 through which compressed air may be supplied to or exhausted from the upper portion of compressed air space of the reservoir chamber 37. Similarly, the separator member 12 is provided with a port 53 through which compressed air may be supplied to or exhausted from the chamber 44 of the air motor. These two ports are connected to a four-way valve 54 (illustrated in FIG. 4) which can be operated to selectively connect the reservoir chamber 37 or the air motor chamber 44 to a source of compressed air such as a compressor schematically illustrated at 56. The valve is arranged so that when the reservoir chamber 37 is pressurized, the air motor chamber 44 is exhausted through an exhaust 57. Operation of the four-way valve 54 to the other position pressurizes the air motor chamber 44 while exhausting the reservoir chamber 37.

Mounted in the head 11 is a valve assembly 61 which controls the operation of the differential piston. This valve assembly includes a pressure operated valve and a back check valve 62. These valves both communicate with a passage 63 which is connected through a pressure line 64 to the lower chamber 43 of the air motor. Both of the valves also communicate with the upper side or air space portion of the reservoir chamber 37 through a passage 66. Referring to FIG. 2, the back check valve 62 includes a valve body 67 held in a valve bore 68 by a cap member 69. Seals 71 and 72 prevent leakage along the bore past the valve body 67 and cap 69, respectively. The valve body 67 is provided with a central bore 73 open to the passage 63 and provides a valve seat 74 at its inner end. A valve disc 76 is normally seated against the valve seat 74 by a spring 77. Communication between the spring side of the valve disc 76 and the air space portion of the reservoir chamber is provided by a passage 78 and the passage 66 which both open into the bore 79 in which the valve assembly 61 is mounted.

The spring 77 is sized to provide a relatively light force urging the valve disc 76 against the seat 74. Consequently, whenever the pressure in the passage 63 exceeds the pressure in the passage 78, and in turn the pressure in the reservoir chamber 37, the disc is lifted away from the seat to allow flow. On the other hand, flow in the opposite direction is positively prevented since the existence of pressure in the passage 78 higher than in the passage 63 urges the disc toward its seated position.

The valve assembly 61 functions to connect the reservoir chamber 37 with the lower air motor chamber 43 only after a predetermined pressure exists in the cylinder 31. This is accomplished by providing a valve body 81 which is threaded into the bore 79 and is pro vided with spaced seals 82 and 83 which provide a fluid tight joint with the bore at spaced locations. The portion inwardly of the body 81 is in communication with the passage 63, and the portion intermediate the seals 82 and 83 is in communication with the passages 78 and 66.

An annular valve seat 84 is positioned in the valve body 81 and is engaged by a ball valve element 86. A spring 87 extends between the ball and the adjusting screw 88 which is threaded into the body 81. The force of the spring 87 on the ball 86 is varied by adjusting the screw 88 inward or outward to either increase or de crease the force of the spring 87 on the ball. An operat ing plunger 89 is slidable in a bore 91 and is provided with an extension 92 movable through the valve seat 84 into engagement with the ball 86 to overcome the action of the spring 87 and open the valve. The opposite end of the plunger 89 is in communication with the pressure within the cylinder 31 through an annular groove 93 and radial ports 94. Spaced seals 96 are provided on the plunger 89 on opposite sides of a port 97 which communicates with the reservoir chamber 37. The port 97 cooperates with the spaced seals to prevent any leakage of hydraulic fluid out of the cylinder 31 past the first seal 96 from reaching the valve section.

The effective area of the plunger 89 is greater than the effective area of the ball 86. Consequently, even though the plunger 89 on the ball may be exposed to the same pressures on opposite sides, the force developed by such pressure on the plunger is greater than the force developed on the ball, and sufficient force will be developed to cause the plunger to slide to the right as viewed in FIGS. 1 and 2 and open the valve 61 when a predetermined pressure is reached in the cylinder bore 31 corresponding to a given differential from the pressure in air space 107. The value of such predetermined pressure can be adjusted by adjusting the force of the spring 87. A pressure gauge mounted on the head 11 is connected through porting in the head to the pump chamber 48 and indicates the hydraulic pressure developed.

The operation of the booster system can best be understood by referring to FIGS. 3 through 5. In these figures, work clamping devices are schematically illustrated at 101 and 102. Each of these devices is a piston and cylinder actuated type device having a piston 103 which is extended against the action of a spring 104 when hydraulic fluid under pressure is supplied thereto. The two clamping devices are connected in parallel to the outlet port 32 of the booster. FIG. 4, as previously discussed, illustrates the connection of the booster through a four-way valve 54 to a compressor 56.

FIG. 3 illustrates the condition of the system before operation thereof. At this time, the pistons 103 are retracted by their associated springs 104 and the reservoir chamber 37 is substantially filled with liquid 106 leaving an air space 107 thereabove. When compressed air is supplied to the air space 107 through the inlet port 52, it pressurizes the liquid 106 and causes it to flow through the passages 51 into the hydraulic pump chamber 48. From there, the liquid under the pressure of the compressed air passes to the clamping

units

101 and 102 causing the pistons 103 to extend into engagement with the work piece to be clamped as schemati cally represented at 108. During this operation, the ball valve 86 remains seated because the force of the spring 87 thereon is sufficient to overcome the force of the plunger 89. After the work piece 108 is engaged, a resistance to further flow is developed causing the pressure of the hydraulic fluid 106 to increase and approach the pressure of the compressed air supplied to the air space 107 by the compressor 56.

The spring 87 is preferably adjusted so that when the pressure within the pump chamber 48 reaches a value of about 90 percent of the pressure of the supplied compressed air, a sufficient force is developed on the plunger 89 to cause it to move the ball valve 86 off of the seat. For example, if the pressure of the compressed air supplied to the system is in the order of 100 psi, the spring should be adjusted so that the ball valve 86 is lifted away from its seat at about 90 psi.

As soon as the ball valve 86 is lifted away from its seat, the air motor chamber 43 is supplied with compressed air. At this time, the air motor chamber 44 above the piston head 41 is connected to exhaust by the four-way valve 54 so the piston assembly starts to raise causing the plunger 42 to pass the passages 51 and isolate the pump chamber 48 from the reservoir chamber 37. Upward movement of the piston continues until a pressure is reached in the pump chamber 48 which is substantially equal to the supply compressed air pres sure times the ratio of effective area of the piston head 41 and plunger 42. In the illustrated embodiment, the area of the piston head 41 is about 30 times the effective area of the plunger 42. Consequently, if the compressed air is supplied at a pressure in the order of 100 psi, the final operating pressure of the hydraulic fluid will be approximately 3,000 psi. The increase in pressure will increase the clamping force of the

clamps

101 and 102 on the work piece 108. Because the pressure in the pumping chamber 48 is higher at this time than the pressure of the compressed air supplied to the system, the plunger 89 maintains the ball valve in the unseated position. During this phase of operation, the check valve disc 76 remains closed, since the pressure thereacross is either higher on the spring side thereof or equal on both sides.

When it is desired to release the clamping force, the four-way valve 54 is shifted to supply compressed air to the chamber 44 above the piston 41 and the reservoir is connected to the exhaust 57. When this occurs, the piston 41 is driven downwardly to its retracted position as illustrated in FIG. 5. This causes a decrease in pressure and when the plunger 42 is retracted beyond the passages 51, communication is re-established between the reservoir chamber 37 and the pump chamber 48. Since the reservoir chamber is connected to exhaust at this time, the hydraulic pressure returns to atmospheric pressure. Consequently, the springs 104 return the associated pistons 103 to the retracted position releasing the work piece. During the downward stroke of the piston, the disc valve element 76 is lifted off of its seat so that the lower air motor chamber 43 is exhausted even though the ball valve element 86 reseats prior to full retraction of the piston.

Since the initial clamping is accomplished before the differential piston functions, it is not necessary to provide a large volumetric capacity in the hydraulic pump even when a relatively large number of clamping units is utilized. Further, all of the liquid 106 within the reservoir is available for the operation of the clamping devices, and none of it is required for the operation of the air motor. With such a system, the amount of compressed air required to operate the system through a cycle of operation is relatively small. Further, the illus trated structure provides complete operation with only two very simple valves which function with reliability and are low in cost.

Although a preferred embodiment of this invention is illustrated, it is to be understood that various modifications and rearrangements of parts may be resorted without departing from the scope of the invention disclosed and claimed herein.

What is claimed is:

1. A hydraulic booster which cycles back-and-forth through a two-stage feeding phase and a return-toreservoir phase comprising a pistonless reservoir providing a lower portion adapted to receive hydraulic fluid with an air space thereabove adapted to be supplied with compressed air, an air motor providing a piston defining a portion of an expansible air chamber, a hydraulic pump providing a plunger driven by said piston and defining a part of an expansible hydraulic chamber connected at an output end to a hydraulic outlet port, said piston and plunger being movable between a retracted position and an extended position in response to compressed air supplied to said air chamber, the effective area of said piston being substantially larger than the effective area of said plunger, flow con-' trol means connecting said lower portion to said hydraulic chamber when said piston is substantially in said retracted position for feeding; of reservoir fluid in a first stage of said feeding phase and isolating said lower portion and said hydraulic chamber in a second stage of said feeding phase when said piston is moving from said retracted position toward said extended position, said last named connection being at an input end at the opposite end of said hydraulic chamber from said output end, valved means exhausting said air chamber when said air space is exhausted and maintaining said air chamber and air space at substantially the same pressure when the pressure in said hydraulic chamber exceeds a predetermined difference from the pressure in said air space, and means for self-purging of air from said booster upon back-and-forth cycling of said booster, said means including direct interfacing between the hydraulic fluid and the air space in said reservoir and the constituting of said hydraulic chamber as an elongate and relatively narrow passageway within which substantially all points are on-line with respect to fluid flow between said output end connection and said input end connection.

2. A hydraulic booster as set forth in claim 1 wherein said plunger closes said flow control means and isolates said hydraulic chamber except when said plunger is in said retracted position.

3. A hydraulic booster as set forth in claim 2 wherein said valved means includes a valve member, and a mov able surface exposed to pressure in said hydraulic chamber operable to move said valve member to an open position when such pressure reaches said prede termined pressure difference.

4. A hydraulic booster as set forth in claim 3 wherein said valved means includes adjustable spring means operable to adjust said predetermined pressure difference.

5. A hydraulic booster as set forth in claim 4 wherein said booster is operated by a supply pressure, and said predetermined pressure is adjusted according to said supply pressure.

6. A hydraulic booster as set forth in claim 5 wherein said valved means connects said air space and said air chamber.

7. A hydraulic booster as set forth in claim 6 wherein said valved means provides a check valve in'parallel with said valve member.

8. A hydraulic booster as set forth in claim 1 wherein said valved means connects said air space and said air chamber 9. A hydraulic booster as set forth in claim 1 wherein said piston is double-acting.

10. A pressure multiplying apparatus which cycles back-and-forth through a two-stage feeding phase and a returnto-reservoir phase comprising a pistonless fluid tight reservoir containing liquid with an air space thereabove adapted to receive air under pressure, a differential area pump means having a piston reciprocable between a retracted and an extended position, said piston having a first surface defining part of an expansible air chamber and a second surface substantially smaller than said first surface defining a part of an expansible liquid chamber connected at an output end to a liquid outlet port, passage means connecting the liquid in said reservoir to said liquid chamber for feeding of reservoir fluid in a first stage of said feeding phase only when said piston is in said retracted position, said last named connecting being at an input end at the opposite end of said liquid chamber from said output end, valve means connecting said air chamber and said air space when the pressure in said air chamber exceeds the pressure in said air space and when the pressure in said liquid chamber exceeds a predetermined difference from the pressure in said air space, means for adjusting said predetermined difference, air pressure in said air chamber operating to move said piston from said retracted position toward an extended position in a second stage of said feeding phase and pressurizing the liquid in said liquid chamber to a pressure substantially higher than said air pressure, said piston remaining in said retracted position when air under pressure is supplied to said air space until the pressure in said liquid chamber reaches said predetermined difference of pressure and thereafter moving toward said extended position and pressurizing the liquid in said liquid chamber to a pressure substantially higher. than the pressure of air supplied to said air space, and means for self-purging of air from said apparatus upon back-and-forth cycling of said apparatus, said means including direct interfacing between the hydraulic fluid and the air space in said reservoir and the constituting of said liquid chamber as an elongate and relatively narrow passageway within which substantially all points are on-line with respect to fluid flow between said output end connection and said input end connection.

11. A hydraulic booster which cycles back-and-fo'rth through a two-stage feeding phase and a return to reservoir phase comprising a housing assembly provided with a partition separating said assembly into an air cylinder portion on one side of said partition and a hydraulic reservoir portion on the other side of said partition, the hydraulic reservoir defined by said hydraulic reservoir portion being pistonless, a hydraulic cylinder in said housing defining a separate pump chamber connected at an output end to a liquid output port, differential area pump means including a piston reciprocable in said air cylinder and a plunger reciprocable in said pump chamber, said piston defining a part of an air chamber, pressure in said air chamber causing movement of said pump means from a retracted position toward an extended position, passage means connecting the lower portion of said reservoir and said pump chamber for feeding of reservoir fluid in a first stage of said feeding phase only when said pump meansis in said retracted position, said last named connection being at an input end at the opposite end of said pump chamber from said output end, movement of said pump means from said retracted position toward said extended position in a second stage of said feeding phase causing closing of said passage means and isolation of said pump chamber from said reservoir, and valved means connecting the upper portion of said reservoir and said air chamber when the pressure in said pump chamber exceeds a predetermined difference from the pressure in said air space, and means for self-purging of air from said booster upon back-and-forth cycling of said booster, said means including direct interfacing between the hydraulic fluid and the air space in said reservoir and the constituting of said pump chamber as an elongate and relatively narrow passageway within which substantially all points are on-line with respect to fluid flow between said output end connection and said input end connection.

12. A hydraulic booster as set forth in claim 11 wherein said housing is generally cylindrical and said partition extends laterally thereacross, said reservoir being located above said partition and said air cylinder being located below said partition. I

13. A hydraulic booster as set forth in claim 12 wherein said hydraulic cylinder extends axially along the interior or said reservoir.

Claims

1. A hydraulic booster which cycles back-and-forth through a two-stage feeding phase and a return-to-reservoir phase comprising a pistonless reservoir providing a lower portion adapted to receive hydraulic fluid with an air space thereabove adapted to be supplied with compressed air, an air motor providing a piston defining a portion of an expansible air chamber, a hydraulic pump providing a plunger driven by said piston and defining a part of an expansible hydraulic chamber connected at an output end to a hydraulic outlet port, said piston and plunger being movable between a retracted position and an extended position in response to compressed air supplied to said air chamber, the effective area of said piston being substantially larger than the effective area of said plunger, flow control means connecting said lower portion to said hydraulic chamber when said piston is substantially in said retracted position for feeding of reservoir fluid in a first stage of said feeding phase and isolating said lower portion and said hydraulic chamber in a second stage of said feeding phase when said piston is moving from said retracted position toward said extended position, said last named connection being at an input end at the opposite end of said hydraulic chamber from said output end, valved means exhausting said air chamber when said air space is exhausted and maintaining said air chamber and air space at substantially the same pressure when the pressure in said hydraulic chamber exceeds a predetermined difference from the pressure in said air space, and means for self-purging of air from said booster upon back-and-forth cycling of said booster, said means including direct interfacing between the hydraulic fluid and the air space in said reservoir and the constituting of said hydraulic chamber as an elongate and relatively narrow passageway within which substantially all points are on1line with respect to fluid flow between said output end connection and said input end connection.

3. A hydraulic booster as set forth in claim 2 wherein said valved means includes a valve member, and a movable surface exposed to pressure in said hydraulic chamber operable to move said valve member to an open position when such pressure reaches said predetermined pressure difference.

7. A hydraulic booster as set forth in claim 6 wherein said valved means provides a check valve in parallel with said valve member.

8. A hydraulic booster as set forth in claim 1 wherein said valved means connects said air space and said air chamber

9. A hydraulic booster as set forth in claim 1 wherein said piston is double-acting.

10. A pressure multiplying apparatus which cycles back-and-forth through a two-stage feeding phase and a return-to-reservoir phase comprising a pistonless fluid tight reservoir containing liquid with an air space thereabove adapted to receive air under pressure, a differential area pump means having a piston reciprocable between a retracted and an extended position, said piston having a first surface defining part of an expansible air chamber and a second surface substantially smaller than said first surface defining a part of an expansible liquid chamber connected at an output end to a liquid outlet port, passage means connecting the liquid in said reservoir to said liquid chamber for feeding of reservoir fluid in a first stage of said feeding phase only when said piston is in said retracted position, said last named connecting being at an input end at the opposite end of said liquid chamber from said output end, valve means connecting said air chamber and said air space when the pressure in said air chamber exceeds the pressure in said air space and when the pressure in said liquid chamber exceeds a predetermined difference from the pressure in said air space, means for adjusting said predetermined difference, air pressure in said air chamber operating to move said piston from said retracted position toward an extended position in a second stage of said feeding phase and pressurizing the liquid in said liquid chamber to a pressure substantially higher than said air pressure, said piston remaining in said retracted position when air under pressure is supplied to said air space until the pressure In said liquid chamber reaches said predetermined difference of pressure and thereafter moving toward said extended position and pressurizing the liquid in said liquid chamber to a pressure substantially higher than the pressure of air supplied to said air space, and means for self-purging of air from said apparatus upon back-and-forth cycling of said apparatus, said means including direct interfacing between the hydraulic fluid and the air space in said reservoir and the constituting of said liquid chamber as an elongate and relatively narrow passageway within which substantially all points are on-line with respect to fluid flow between said output end connection and said input end connection.

11. A hydraulic booster which cycles back-and-forth through a two-stage feeding phase and a return to reservoir phase comprising a housing assembly provided with a partition separating said assembly into an air cylinder portion on one side of said partition and a hydraulic reservoir portion on the other side of said partition, the hydraulic reservoir defined by said hydraulic reservoir portion being pistonless, a hydraulic cylinder in said housing defining a separate pump chamber connected at an output end to a liquid output port, differential area pump means including a piston reciprocable in said air cylinder and a plunger reciprocable in said pump chamber, said piston defining a part of an air chamber, pressure in said air chamber causing movement of said pump means from a retracted position toward an extended position, passage means connecting the lower portion of said reservoir and said pump chamber for feeding of reservoir fluid in a first stage of said feeding phase only when said pump means is in said retracted position, said last named connection being at an input end at the opposite end of said pump chamber from said output end, movement of said pump means from said retracted position toward said extended position in a second stage of said feeding phase causing closing of said passage means and isolation of said pump chamber from said reservoir, and valved means connecting the upper portion of said reservoir and said air chamber when the pressure in said pump chamber exceeds a predetermined difference from the pressure in said air space, and means for self-purging of air from said booster upon back-and-forth cycling of said booster, said means including direct interfacing between the hydraulic fluid and the air space in said reservoir and the constituting of said pump chamber as an elongate and relatively narrow passageway within which substantially all points are on-line with respect to fluid flow between said output end connection and said input end connection.

12. A hydraulic booster as set forth in claim 11 wherein said housing is generally cylindrical and said partition extends laterally thereacross, said reservoir being located above said partition and said air cylinder being located below said partition.