US3678726A

US3678726A - Hydraulic system

Info

Publication number: US3678726A
Application number: US24567A
Authority: US
Inventors: Bernard L Hoffman
Original assignee: FREDRICK A KRAUSE ASSOCIATES I
Current assignee: FREDRICK A KRAUSE ASSOCIATES I; FREDRICK A KRAUSE ASSOCIATES Inc
Priority date: 1970-04-01
Filing date: 1970-04-01
Publication date: 1972-07-25
Anticipated expiration: 1989-07-25
Also published as: CA938196A; GB1342168A; DE2166680A1; DE2116018A1; JPS5215827B1; FR2085777B1; DE2116018B2; FR2085777A1

Abstract

This invention relates to a hydraulic system characterized by a combination pressure and utilization means unit assembly, and a dual low pressure hydraulic piping system between the pressure unit and a hydraulic fluid reservoir, thereby providing continuous circulation of hydraulic fluid during operation.

Description

United States Patent Hoffman I [54] HYDRAULIC SYSTEM [72] Inventor: Bernard L. Hoffman, Trenton, NJ.

[73] Assignee: Fredrick A. Krause Associates, Inc.,

Frenchtown, NJ.

[22] Filed: April I, 1970 [21] Appl.No.: 24,567

511 mu ..B2ld 29/14 [58] Fieldol'Search ..72/3s0,3s1,4s3

[56] References Cited UNITED STATES PATENTS 1,696,140 12/1928 Ferris ..72/3s1 1,696,141 12/1928 Ferris..... ..72/351 2,261,060 10/1941 Giesler ..72/347 1 July 25,1972

Burk .Q. ..72/s7 Williamson 1 ..72/35l Williamson .....72/351 FOREIGN PATENTS OR APPLICATIONS 652,601 11/1937 Germany 1 ..72/351 1,040,488 10/1958 Germany "72/351 Primary Examiner-Richard J. Herbst Attorney-Ward. McElhunnon, Brooks & Fitzpatrick l 5 ABSTRACT This invention relates to a hydraulic system characterized by a combination pressure and utilization means unit assembly, and a dual low pressure hydraulic piping system between the pressure unit and a hydraulic fluid reservoir, thereby providing continuous circulation of hydraulic fluid during operation.

4 Claims, 4 Drawing Figures Patented July 25, 1972 3,678,726

2 Sheets-Sheet l HYDRAULIC SYSTEM This invention relates to hydraulic systems and more particularly it concerns a novel hydraulic pressure unit suitable for use with multiple die punch presses.

Modern presses which draw or otherwise form various intricate shapes from sheet metal utilize complex multiple dies which are hydraulically actuated. These dies are made up of a plurality of die elements, each of which is connected to actuating pistons of various size. The actuating pistons in turn are supplied with hydraulic fluid at a common pressure so that when the die heads of the press are opened, or displaced from one another, the hydraulic pressure maintains the pistons and their corresponding die elements in fully extended position. As the die heads close upon each other with the sheet metal workpiece therebetween, the die elements become retracted in a certain predetermined sequence, depending, at least in part, upon the areas of the pistons actuating them. As the die heads are thereafter opened, the die elements begin to extend again in reverse sequence. It is this specially controlled sequence of retraction and extension of the various die elements which permits various intricate shapes to be produced in a single operation of the press.

The above described die arrangement presents certain requirements of the hydraulic system which supplies fluid under pressure to the actuating pistons. This system, for example, must be capable of supplying a sufficient quantity of hydraulic fluid and at a sufficient pressure to cause full extension of all actuating pistons and their associated die elements when the die heads are opened. This pressure must not be too high, or otherwise, when the press is opened, the die elements will move toward their extended positions with such suddenness that the formed workpiece will be torn violently out of the dies and destroyed or severely damaged. On the other hand, the hydraulic system must be capable of absorbing all of the displaced fluid during the closing of the die heads; and furthermore, it must closely maintain the pressure of the fluid in the system at a very high value during this time so that the workpiece can be properly formed by the extended die elements.

In addition to the above, a further problem arises from the fact that the rapid operation of the press produces very large changes in volume rates of flow of the hydraulic fluid in the system. Such rapid changes in flow rate produce very serious hydrodynamic effects which adversely affect valve operation, and of course, seriously affect die operation. These adverse hydrodynamic effects include fluid foaming and cavitation, pressure reflections and surges, slow valve response time and unstable valve operation.

According to this invention, there is provided a new and improved hydraulic system which employs a combination pressure and utilization means, which may be a die cushion unit, for example. The system employs a low pressure hydraulic supply line and a separate hydraulic return line between a hydraulic reservoir and the pressure and die cushion unit. This allows the hydraulic fluid to circulate between the reservoir and the pressure unit, resulting in the system operating at a low temperature and without foaming. Hence, the system can be operated at increased strokes perv minute as compared to prior art systems.

In view of the foregoing, this invention contemplates the provision of a,novel hydraulic system characterized by a pressure unit assembly, utilization means connected directly to the pressure unit assembly, and a hydraulic fluid reservoir assembly. A first hydraulic line serves to carry fluid from the reservoir to the pressure unit assembly and a second hydraulic line serves to return the fluid from the pressure unit assembly to the reservoir. The pressure unit assembly has a chamber in fluid flow communication with the first hydraulic line and with the utilization means. A check valve is mounted in the first hydraulic line adjacent the pressure unit assembly for permitting fluid flow from the reservoir into the chamber and preventing flow in the opposite direction. An internal passage interconnects the chamber with the second hydraulic line and a valve is mounted in this passage. The valve is biased to closure of the passage and means are provided for opening the valve responsive to variations in static fluid pressure in the chamber, thereby to allow the fluid to return to the reservoir.

There has thus been outlined rather broadly the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described more fully hereinafter. Those skilled in the art will appreciate that the conception on which this disclosure is based may readily be utilized as the basis for the designing of other structures for carrying out the several purposes of the invention. It is important, therefore, that this disclosure be regarded as including such equivalent constructions as do not depart from the spirit and scope of theinvention.

One embodiment of the invention has been chosen for purposes of illustration and description, and is shown in the accompanying drawings, forming a part of the specification, wherein:

FIG. I is a side elevation, partially in section, showing a hydraulic system, adopted for operating a press, constructed in accordance with the concept of this invention;

FIG. 2 is a medial, vertical, sectional view of a combination pressure unit and die cushion unit assembly;

FIG. 3 is a medial, vertical, sectional view of the pressure unit assembly of FIG. 2, but shown in another position thereof; and

FIG. 4 is an enlarged side elevation view of the pressure unit assembly of FIG. 2.

In the embodiment of the invention illustrated, as best seen in FIG. 1, the hydraulic system comprises a press indicated generally at 10, having a die cushion assembly indicated generally at I l, a pressure unit assembly indicated generally at 12, and an oil reservoir assembly indicated generally at 14. According to the present invention hydraulic fluid or oil is recirculated through the entire system with each stroke, and hence, it operates cooler andwith less foaming than prior art noncirculating systems. A hydraulic circuit extends from the oil reservoir 14 through low pressure hydraulic line I6 to the pressure unit assembly 12, and thence internally to the die cushion assembly 11. Further, the oil circuit extends internally back from the die cushion assembly Ilto the pressure unit assembly 12 and then to the oil reservoir 14 via a low pressure hydraulic return line 18, provided for the purpose. The internal circuit will be described more fully hereinafter. The system includes cooling means which comprise a water circuit wherein water enters die cushion l l, as at 20, and is circulated around each cylinder, line 22 serving to carry the water from one cylinder to the next. Then, the water circuit extends internally from the die cushion to the pressure unit assembly 12, and thereafter into the reservoir 14, as at 24, via line 26. Finally, the water enters a cooling coil 28 in the reservoir to cool the oil therein, and then it is discharged, as at 30. A temperature sensing unit 32 is mounted on the pressure unit assembly l2, and it is set to operate at a preselected temperature to open a main electrical circuit (not shown) to deactivate the equipment if the oil temperature exceeds the preselected temperature so as to avoid damage to the equipment as well as preventing the stamping of defective parts in the die. As an example of one embodiment of the invention, the input water temperature is 55 F. and the output water temperature is 60 F., thereby maintaining an oil temperature of 76 F. The temperature sensing unit is set to open the main electrical circuit at I20 F. Such a unit has been operated at 350 strokes per minute.

Still referring to FIG. 1, the reservoir 14 is mounted on a pedestal 34. This reservoir comprises a body 36 having a bottom 38 with a settling ring 40, and a cap 42. A glass sight gauge 44 is mounted on the outside of the reservoir to provide visual indication of the liquid level therein. Hydraulic fluid or oil is supplied to the reservoir via a fluid intake nozzle, not

shown, and for normal operation the reservoir is maintained at about twocthirds capacity. The fluid in the reservoir is maintained at a positive pressure (about I to 135 p.s.i.) during normal operation; and for this purpose an air inlet 46 is employed for supplying air pressure on the upper surface of the oil in the reservoir.

As best seen in FIGS. 2 and 4, the pressure unit assembly 12 is formed with a body 48 having an annular chamber 50 which is connected to the hydraulic line 16, FIG. 2, and a check valve 52 is interposed to permit fluid flow from the reservoir 14 into the chamber 50, but not in the reverse direction. A bias spring 54 in the check valve is quite light and whenever the pressure in the chamber 50 drops below the reservoir pressure, the unbalance will overcome the spring force and allow fluid to flow out of the reservoir 14 through the hydraulic line 16 into the chamber 50. An excess of pressure in the chamber 50, on the other hand, will only serve to set the check valve more tightly so that no fluid can pass back into the reservoir 14, through the hydraulic line 16. The chamber 50 is of annular configuration and is connected to a utilization means, such as the die cushion assembly 11, as will be described more fully hereinafter.

The annular chamber 50 opens directly into a lower chamber 56 which opens directly into a lower connecting passageway 58 which leads to the hydraulic return line I8, FIG. 2. A control valve, indicated generally at 60, is mounted on the pressure body 48 to control the opening of the passageway 58, and thereby control the flow of oil to the return line 18. i

The control valve 60, FIGS. 2 and 3, has a control valve spool 62 which is cylindrical in shape and includes two large diameter end portions 64 and 66, and a smaller diameter intermediate portion 68. The end portions 64 and 66 are hollowed out, to provide a pressure responsive chamber 70 and a spring seating chamber 72. These two chambers are maintained in fluid communication by means of a central passageway 74 extending axially of the spool 62. The fluid flow velocity through the passageway is restricted to a predetermined value by means of a metering orifice insert 76 press fitted into the passageway.

The first larger diameter end portion 64 of the valve spool 62 extends into an opening 78, and the second larger diameter end portion 64 extends into an opening 80 in the body 48. The two openings 78 and 80 are aligned with and of the same 'diameter as the lower connecting passageway 58. The control valve spool 62 is thus reciprocally movable between a passageway closing position as seen in FIG. 2, wherein the second larger diameter portion 66 fully covers the lower connecting passageway 58, and a passageway opening position as shown in FIG. 3, wherein oil can flow from the lower chamber 56 through the lower connecting passageway 58 to the hydraulic return line [8. A valve spool bias spring 82 fits into the spring seating chamber 72 of the valve spool 62, and urges the valve spool toward the left as viewed in FIG. 2, to its valve closing position.

A pressure end cap 84 and an adjustment end cap 86 are bolted to the outer walls of the body 48, and they serve to cover the'valve spool openings 78 and 80. The pressure end cap 84, as shown in FIG. 2, is provided with an internal passageway 88 which, as shown, communicates between the lower chamber 56 and the pressure responsive chamber 70 of the control valve spool 62. Pressure changes in the chamber 56 are communicated via the passageway 88 and act upon the left end of the spool 62, as viewed in FIG. 2, in opposition to the force of the spool bias spring 82 to move the spool to the right as viewed in FIG. 3 and open the lower connecting passageway 58 between the chamber 56 and the hydraulic retum'line 18.

The adjustment end cap 86 is provided with an adjustment needle valve 90 which communicates via an opening 92 with the spring seating chamber 72 of the valve spool 62 and through the central passageway 70 and the metering orifice insert 76, to the pressure responsive chamber 70. The needle valve is biased against a needle valve seat 94 by an adjustment valve spring 96, the stress on which is adjustably controlled by an adjustment screw 98 threaded into the end cap 86. A lock nut 100 is provided on the adjustment screw for securing it at any given setting. As shown in FIG. 2, the region 102 beyond the adjustment valve 90 is in communication, via a feedback passageway 104 with the annular chamber 50. There is also provided safety valve means, not shown, for the chamber 50.

As best seen in FIG. 2, the annular chamber 50 is connected to the die cushion assembly 11. This die cushion assembly includes a die cushion body 104 fixedly mounted on the pressure unit body, as at 106, and a die cushion rod 108 passes therebetween. A cover plate 109 is mounted on the upper end of the body 104. Four piston sleeves 110 (two being shown) are mounted in symmetrically spaced openings 2 in the body 104 for receiving piston assemblies 114. The piston assemblies I14 engage die elements 116. In order to achieve complex working of the metal workpiece in a single pressure operation, certain of the die elements are movable on die shoes 118, FIG. I, and are urged toward protracted or extended positions on the shoes by actuating the piston assemblies 114 which may be of various cross sectional areas. These actuated pistons are in fluid communication with the chamber 50in the pressure unit body 48. As the press closes and the die shoes are moved toward each other with greater and greater force, the die actuating piston assemblies and their associated die elements are forced back into their respective die shoes in predetennined sequence according to the cross sectional area of the pistons.

The press unit 12 operates to receive and transmit via hydraulic return lines 18 to the reservoir 14, the fluid displaced by the retracting pistons during press closure while maintaining the high resistance pressure (in the neighborhood of 4,500 p.s.i.) required for proper metal working, and when the press opens the pressure unit 12 acts to supply oil from the reservoir via hydraulic line 16 to the piston assemblies I14 for repositioning the die elements and ejecting the previously finished workpiece. It will be appreciated that only the pressure unit assembly between the chamber 50 and the spool valve 62 are subjected to the very high pressures, and hence the

hydraulic lines

16 and 18 and their respective fittings may be designed for low pressure operation.

As pointed out hereinbefore, the system is additionally provided with cooling means. The cooling water which enters the die cushion assembly at 20 circulates around the piston sleeves 110, as at 120, FIG. 2. This cooling water also passes through various internal passageways 122 within the body 48 for cooling purposes.

The system operates in the following manner:

When the press opens, and the die shoes 118, FIG. I, are moved apart, there is no retracting force on the actuating pistons 114. Consequently, the pressure in the annular chamber 50, lower chamber 56, passageway 58 and return line 18 falls off toward atmospheric pressure. Similarly, the pressure in the pressure responsive chamber 70 of the control valve spool 62 also decreases because this chamber is in fluid flow communication with the lower chamber 56 via the internal pressure passageway 88. Consequently, the valve spool bias spring 82, receiving no opposition, moves the valve spool 62 to the left, as viewed in FIG. 2, so that the larger diameter end portion 66 of the spool fully covers the lower connecting passageway 58. However, when this decreased pressure falls below the pressure in the reservoir 14, a force unbalance exists across the check valve 52 causing it to unseat so that fluid is forced at about 100-125 p.s.i. from the reservoir 14 via hydraulic line 16 to the annular chamber 50 and thence to the piston assemblies 114 where it forces the actuating pistons to their extended positions.

When the press is closed and the die elements are brought together against a workpiece, they force back against the actuating piston assemblies 114 causing a pressure increase in the annular chamber 50. This increased pressure acts to close the check valve 52. However, it also communicates through the lower chamber 56 and internal passageway 88 to the pres sure responsive chamber 70 in the control valve spool 62 and acts upon that end of the spool in opposition to the valve spool bias spring $2. This causes the valve spool to move to the right, as viewed in FIG. 2, which, in turn, brings about communication between the chamber 56 and the hydraulic return line 18 to the reservoir 14. Accordingly, the fluid which is displaced by the retracting piston assemblies during closure of the press is accommodated through the controlled opening of the lower connecting passageway 58 by movement of the valve spool 62. It will be appreciated that even though the actuating pistons operate in a reciprocating manner, the hydraulic fluid passes in stepwise movement through the entire circulatory path so that cool fluid is continuously brought into the piston area.

It is important in many applications, particularly in the hydraulic press situation described herein, to accommodate the rapid volumetric displacements caused by the retracting piston and yet to maintain the pressure in the system at close to 4,500 psi. Also, because various different pistons begin to retract at different points ,during press closure, the system must be capable of accommodating large surges in both pressure and volume, without the loss of control and valve flutter which often plague conventional valving systems under such conditions.

in the present system, the two larger diameter end portions 64 and 66 of the control valve spool 62 are of equal diameter. Thus, the pressure surges which occur in the chambers 50 and 56 are balanced out at the two end portions of the spool, All control of spool movement occurs as a result of pressure communication through the internal pressure passageway 88 to the end of the spool. Accordingly, the spool does not suffer from the hydrodynamic effect of the fluid rushing through it. Further, the various changes in direction the fluid must take between the chamber 56 and the pressure responsive chamber 70 causes a dissipation of surge effects so that the valve operates only in response to actual pressure changes and not to surge effects.

As pressure increases at the left end of the valve spool 62, it is forced rightward against the valve spool bias spring 82. This forces all fluid in the spring seating chamber 72 out through the opening 92 and against the adjustment needle valve 90, causing it to open when the pressure reaches a sufficient value, i.e., the value corresponding to 4,500 psi. in the chambers 50 and 56. Even while this pressure is sustained, hydraulic fluid passes through the central passageway 70 and the metering orifice insert 76 and into the spring seating chamber 72 where it balances the pressure in the pressure responsive chamber 70. This permits the valve spool bias spring 82 to move the spool 62 back towards the left until the flow through the lower connecting passageway 58 is decreased to a point where the pressure in the system again builds up. The valving arrangement is thus dynamically balanced and in fact is capable of handling pressure and volume variations at rates appreaching 80 cycles per second without causing piston bounce, oil foaming or entrapped air.

It will be appreciated that in certain installations two or more die cushions may be employed per die. In such installations only one reservoir is used but a plurality of oil and return lines are employed to avoid pressure reactions between cushions;

It will thus be seen that the present invention does provide an improved hydraulic system which is superior in simplicity, compactness and efficiency as compared to prior art such systems.

Although a particular embodiment of the invention is herein disclosed for purposes of explanation, various modifications thereof, after study of this specification, will be apparent to those skilled in the art to which the invention pertains,

What is claimed and desired to be secured by Letters Patent is:

l. A hydraulic system comprising a pressure unit assembly, a die cushion assembly connected directly to said pressure unit assembly, and a remotely disposed hydraulic fluid reservoir assembly, water cooling means for circulating cooling water through said die cushion assembly and said pressure unit assembly and said hydraulic fluid reservoir assembly, a first hydraulic line for carrying fluid from said reservoir assembly to said pressure unit assembly and a second hydraulic line for returning fluid from said pressure unit assembly to said reservoir assembly, said pressure unit assembly having a pressure unit body, said body having a chamber in fluid flow communication with said first hydraulic line, a check valve mounted in said first hydraulic line adjacent said body for permitting fluid flow from said reservoir assembly into said chamber and preventing fluid flow in the opposite direction, said body having a lower connecting passageway interconnecting said chamber with said second hydraulic line for receiving substantially all of the flow from said chamber, a control valve spool mounted for movement between a first position closing said lower connecting passageway is open, fluid passage means for exposing one end of said control valve spool to the fluid pressure in said chamber for controlling the movement of said control valve spool, said die cushion assembly having a die cushion body, a piston assembly mounted in an opening in said die cushion body, said piston assembly being in fluid flow communication with said chamber, and a die element mounted for engagement with said piston assembly.

2. A hydraulic system according to claim 1 wherein said control valve spool has two oppositely disposed large diameter end portions and a smaller diameter intermediate portion, one

of said end portions having a pressure responsive chamber and the other of said end portions having a spring seating chamber, a central passageway extending axially of said spool for connecting said pressure responsive and spring seating chambers, said body having oppositely disposed openings for receiving said end portions in slidable contact, a spring mounted in said spring seating chamber for urging said control valve spool in one direction wherein one of said end portions closes said lower connecting passageway, an internal passageway communicating between said chamber and said pressure responsive chamber for communicating pressure changes in said chamber to said pressure responsive chamber to urge said control valve spool to move in opposition to said spring for opening said lower connecting passageway.

3. A hydraulic system according to claim 2 further compris ing an adjustable needle valve assembly mounted on said body adjacent said spring seating chamber, said needle valve assembly having an opening connecting to said spring seating chamber and a passageway connecting to said annular chamber, adjustable spring means for closing said needle valve assembly to fluid flow when the pressure in said opening is below a selected valve and for allowing fluid flow through the needle valve assembly when the pressure in said opening is above said value.

4. A hydraulic system comprising a pressure unit assembly, a die cushion assembly connected directly to said pressure unit assembly, and a hydraulic fluid reservoir assembly, water cooling means for circulating cooling water through said die cushion assembly and said pressure unit assembly and said hydraulic fluid reservoir assembly, a first hydraulic line for carrying fluid from said reservoir assembly to said pressure unit assembly and a second hydraulic line for returning fluid from said pressure unit assembly to said reservoir assembly, said pressure unit assembly having a pressure unit body, said body having an annular chamber in fluid flow communication with said first hydraulic line, a check valve mounted in said first hydraulic line adjacent said body for permitting fluid flow from said reservoir assembly into said chamber and preventing fluid flow in the opposite direction, said body having a lower chamber opening directly into said annular chamber, said body having a lower connecting passageway interconnecting said lower chamber with said second hydraulic line, a control valve spool having two oppositely disposed large diameter end portions and a smaller diameter intermediate portion, one of said end portions having a pressure responsive chamber and the other of said end portions having a spring seating chamber, a central passageway extending axially of said spool for connecting said pressure responsive and spring seating chambers, a metering orifice disposed in said central passageway, said body having oppositely disposed openings for receiving said end portions in slidable contact, said openings being aligned with and of the same diameter as said lower connecting passageway, a spring mounted in said spring seating chamber for urging said control valve spool in one direction wherein one of said end portions closes said lower connecting passageway, an internal passageway communicating between said lower chamber and said pressure responsive chamber for communicating pressure changes in said lower chamber to said pressure responsive chamber to urge 'said control valve spool to move in opposition to said spring for opening said lower connecting passageway, an adjustable needle valve assembly mounted on said body adjacent said spring seating chamber, said needle valve assembly having an opening connecting to said spring seating chamber and a passageway connecting to said annular chamber, adjustable spring means for closing said needle valve assembly to fluid flow when the pressure in said opening is below a selected value and for allowing fluid flow through the needle valve assembly when the pressure in said opening is above said value; said die cushion assembly having a die cushion body, four symmetrically spaced sleeves mounted in openings in said die cushion body, a piston assembly mounted in each of said sleeves, said piston assemblies being in fluid flow communication with said annular chamber, and die elements mounted for engagement with said piston assemblies, respectively.

l l I l Po-ww UNITED; STATES PATENT OFFICE u) CERTIFICATEOF CORRECTION V Patent NC? 8 5 Dated. Ji 25'', 1972 V I Inven t0 r(s) I IBERNARID v I It is eer t if ie d that; efror appears i n' the above-identified patent and that said Letters Patent are'hereby corrected as shdwn bel'owz Col. 3, line 2, ""two ct'hj rde Should read.-two thirds--; Col. 6, line 20, after the wordf'passagew ay insert the words -and a seeofld pos itibn w'h erein said passageway-; Col. 6, line 52 seleted valve"Qsh-ou'ld read --selecte 5l Valuer".

Si g ned and sealed this 2nd day o f January 1972;m

(SEAL) Attest:

" EDWARD M.FLETCHER,JR.

Attesting Officer ROBERT GOTTSCHALK Commissioner of Patent

Claims

1. A hydraulic system comprising a pressure unit assembly, a die cushion assembly connected directly to said pressure unit assembly, and a remotely disposed hydraulic fluid reservoir assembly, water cooling means for circulating cooling water through said die cushion assembly and said pressure unit assembly and said hydraulic fluid reservoir assembly, a first hydraulic line for carrying fluid from said reservoir assembly to said pressure unit assembly and a second hydraulic line for returning fluid from said pressure unit assembly to said reservoir assembly, said pressure unit assembly having a pressure unit body, said body having a chamber in fluid flow communication with said first hydraulic line, a check valve mounted in said first hydraulic line adjacent said body for permitting fluid flow from said reservoir assembly into said chamber and preventing fluid flow in the opposite direction, said body having a lower connecting passageway interconnecting said chamber with said second hydraulic line for receiving substantially all of the flow from said chamber, a control valve spool mounted for movement between a first position closing said lower connecting passageway is open, fluid passage means for exposing one end of said control valve spool to the fluid pressure in said chamber for controlling the movement of said control valve spool, said die cushion assembly having a die cushion body, a piston assembly mounted in an opening in said die cushion body, said piston assembly being in fluid flow communication with said chamber, and a die element mounted for engagement with said piston assembly.

2. A hydraulic system according to claim 1 wherein said control valve spool has two oppositely disposed large diameter end portions and a smaller diameter intermediate portion, one of said end portions having a pressure responsive chamber and the other of said end portions having a spring seating chamber, a central passageway extending axially of said spool for connecting said pressure responsive and spring seating chambers, said boDy having oppositely disposed openings for receiving said end portions in slidable contact, a spring mounted in said spring seating chamber for urging said control valve spool in one direction wherein one of said end portions closes said lower connecting passageway, an internal passageway communicating between said chamber and said pressure responsive chamber for communicating pressure changes in said chamber to said pressure responsive chamber to urge said control valve spool to move in opposition to said spring for opening said lower connecting passageway.

3. A hydraulic system according to claim 2 further comprising an adjustable needle valve assembly mounted on said body adjacent said spring seating chamber, said needle valve assembly having an opening connecting to said spring seating chamber and a passageway connecting to said annular chamber, adjustable spring means for closing said needle valve assembly to fluid flow when the pressure in said opening is below a selected valve and for allowing fluid flow through the needle valve assembly when the pressure in said opening is above said value.

4. A hydraulic system comprising a pressure unit assembly, a die cushion assembly connected directly to said pressure unit assembly, and a hydraulic fluid reservoir assembly, water cooling means for circulating cooling water through said die cushion assembly and said pressure unit assembly and said hydraulic fluid reservoir assembly, a first hydraulic line for carrying fluid from said reservoir assembly to said pressure unit assembly and a second hydraulic line for returning fluid from said pressure unit assembly to said reservoir assembly, said pressure unit assembly having a pressure unit body, said body having an annular chamber in fluid flow communication with said first hydraulic line, a check valve mounted in said first hydraulic line adjacent said body for permitting fluid flow from said reservoir assembly into said chamber and preventing fluid flow in the opposite direction, said body having a lower chamber opening directly into said annular chamber, said body having a lower connecting passageway interconnecting said lower chamber with said second hydraulic line, a control valve spool having two oppositely disposed large diameter end portions and a smaller diameter intermediate portion, one of said end portions having a pressure responsive chamber and the other of said end portions having a spring seating chamber, a central passageway extending axially of said spool for connecting said pressure responsive and spring seating chambers, a metering orifice disposed in said central passageway, said body having oppositely disposed openings for receiving said end portions in slidable contact, said openings being aligned with and of the same diameter as said lower connecting passageway, a spring mounted in said spring seating chamber for urging said control valve spool in one direction wherein one of said end portions closes said lower connecting passageway, an internal passageway communicating between said lower chamber and said pressure responsive chamber for communicating pressure changes in said lower chamber to said pressure responsive chamber to urge said control valve spool to move in opposition to said spring for opening said lower connecting passageway, an adjustable needle valve assembly mounted on said body adjacent said spring seating chamber, said needle valve assembly having an opening connecting to said spring seating chamber and a passageway connecting to said annular chamber, adjustable spring means for closing said needle valve assembly to fluid flow when the pressure in said opening is below a selected value and for allowing fluid flow through the needle valve assembly when the pressure in said opening is above said value; said die cushion assembly having a die cushion body, four symmetrically spaced sleeves mounted in openings in said die cushion body, a piston assembly mounted in each of said sleeves, said piston assemblies being in fluid flow communicaTion with said annular chamber, and die elements mounted for engagement with said piston assemblies, respectively.