CA1280669C - Energy recovery system for press - Google Patents

Abstract

INVENTION: ENERGY RECOVERY SYSTEM FOR PRESS
INVENTORS: LEON MALASHENKO and RONALD BALLANTYNE
ABSTRACT OF THE DISCLOSURE
A press having a hydraulic mechanism for repeatedly raising and lowering a press platen and incorporating an energy recovery system is described. A
hydraulic cylinder which serves to raise the platen has a vertical chamber, a piston which divides the chamber into upper and lower compartments, and a piston rod which extends upwardly from the upper surface of the piston, centrally through the upper compartment, to the platen. The pressure-effective area of the upper surface of the piston is about one-half that of the lower surface of the piston owing to the comparatively large cross-sectional area of the piston rod. When the platen is to be lowered under gravity, a valve is actuated to place the upper and lower cylindercompartments in communication causing an intensification of fluid pressure in the lower compartment. A second valve is simultaneously actuated to permit hydraulicfluid to be transferred from the lower cylinder compartment to an accumulator pre-charged to receive and store hydraulic fluid only under pressure in excess of that required to actuate the hydraulic cylinder for lifting of the platen. Potential energy associated with the raised platen is thus transformed into energy stored as hydraulic fluid under pressure. When the platen is to be raised, a pump is actuated to apply hydraulic fluid under pressure to the lower cylinder compartment and the accumulator is simultaneously discharged into the lower cylinder compartment to assist in the raising.

Description

1~066~

FIELD OF THE INVENTION
The invention relates generally to hydraulic presses, and more specifically to the reduction of the energy required to repeatedly raise and lower a press platen, such as the massi~e platens commonly associated with high-capacity hydraulic presses.
BACKGROUND OF THE INVENTION
High capacity presses normally have upper and lower platens between which a workpiece can be compressed. In some press designs, it is cornmon to have a stationary lower platen and a movable upper platen which is advanced downwardly to compress the workpiece. If the press is required to generated verylarge compressive forces, the platens tend to be very robust and massive. In presses expected to exert compresses forces in excess of 2500 tons, it would not be unusual for an upper movable platen to weigh as much as 75 tons if the platen is constructed to allow only minimal platen deflection during compression phases of operation.
Considerable energy is required to raise such a platen from a lowered operative position in which a workpiece is compressed to an elevated position in which a molded product can be removed. The potential energy associated with the platen in its raised position, which must be provided by the hydraulic cylinders commonly used to raise such a platen, is dissipated as heat when the platen is lowered under gravity by controlled release of hydraulic fluid from the cylinders. The process of raising and lowering a 75 ton platen may be expected in each cycle of operation to require an expenditure of about .2 kilowatt-hour of energy assuming raising of the platen through about 1 metre. ~ssuming continuous operation of the press, which is quite common, and a cycle time of no more than about one minute, the repeated raising of the platen can be expected to produce energy costs in the order of about $4500.00 per year at current electric energy costs of about 6 cents per kilowatt-hour.
It would accordingly be desirable to provide a convenient, ~, q~

comparatively inexpensive mechanism by means of which the energy required to repeatedly raise and lower such a press platen may be reduced.
BRIEF SUMMARY 0~ THE INVENTIt:)N
In one aspect, in the context of a press having a platen which is mounted for vertical movement, the invention provides an energy efficient apparatus for raising and lowering the platen. A hydraulic cylinder is mounted on the press below the platen, the hydraulic cylinder having a vertical chamber, a piston which divides the chamber into upper and lower compartments, and a piston rod which extends upwardly from the piston through the interior of the upper compartment and externally of the chamber to the platen. Hydraulic actuating means are provided for applying hydraulic fluid under a preselected pressure to the lower cylinder compartment to effect a raising of the platen.
A first fluid path is provided between the upper and lower cylinder compartments. The flow of hydraulic fluid between the upper and lower cylinder compartments is controlled by first controllable valve means. When the platen is to be lowered, under gravity, the first valve means are actuated to permit fluid to flow from the lower compartment to the upper compartment under the weight of the platen.
Assuming, for example, that the pressure-effective area of the upper surface of the piston is one-half the pressure-effective area of the lower piston surface, the pressure generated within the lower compartment under the weight of the platen can be expected to rise to about twice the pressure previously required to raise the platen. Second valve means are contemporaneously actuated with the first valve means to permit a transfer of hydraulic fluid under this head of pressure into accumulator means.
Accordingly, much of the potential energy associated with the platen in its elevated position is stored within the accumulation means as hydraulic fluid under pressure exceeding ~he preselected pressure normally required to raise the platen.
When the platen is to be raised once again, the hydraulic actuating ~28~;6~

means apply hydraulic fluid under the preselected pressure to ~e lower cylinder compartrnent. The second valve means associated with the accumulator means are contemporaneously actuated to discharge hydraulic fluid under pressure from the accumulator means to the lower cylinder compartment. Despite pressure drops associated with the various valve means, a very substantial portion of the energy stored during lowering of the platen is used to raise the platen.
Various other advantages associated with the present invention will be discussed in connection with the description of a preferred embodiment below.
DESCRJPTION OF THE l~RAWII~GS
The invention will be better understood with reference to the drawings illustrating a preferred embodiment, in which:
fig. 1 diagrammatically illustrates a press incorpora~ing platen displacing apparatus embodying the invention and indicating hydraulic fluid flows during raising of the platen; and, ~ lg. 2 is a diagrammatic representation of the press as in fig. 1 indicating fluid flows during lowering of the platen.
DES~I ION OF PREFERRED EMBODIMENT
Figs. 1 and 2 diagrammatically illustrate cornponents of a hydraulic press 10 associated with the present invention. It will be apparent from figs. 1 and 2 that there is a substantial symmetry in the design of the press 10 and the associated platen displacing apparatus. Accordingly, components which are substantially identical have been indicated with like reference numerals except that each components whose function is to be understood with reference to another more fully described has a reference numeral followed by the letter "A".
The press 10 includes a lower stationary platen 12 and an upper movable platen 14 which is guided towards and away from the lower platen 12. A
pair of hydraulic cylinders 16, 1 6A of the rapid advance-retreat variety are mounted to 128~669 the bottom of a bolster plate 18 associated with the lower platen 12 and these cylinders move the upper platen 14 relative to the lower platen 12 between an maximally open position in which, for example, a molded product may be removed from mold halvesattached to the upper and lower platens and a fresh molding charge introduced and an operative position in which, for example, mold halves are proximate to one another and ready to pressurized under large compressive forces. Actual compression of aworkpiece would be performed by more robust compression-phase cylinders which have not been illustrated and which act through only a small range of platen movement. Other press structures not essential to an understanding the invention have also been ornitted. For example, the press might have a crosshead and side slabswhich guide the upper platen 14 or might be constructed with a multiplicity of posts which extend upwardly from the lower platen 12 and on which the upper platen 14 is guided. It should be understood that the present invention is not limited to anyparticular press design.
The hydraulic cylinder 16 is of the conventional double-acting variety. The cylinder 16 has a body member which defines a vertical cylindrical chamber 20, and a piston 22 which is sealed to internal surfaces defining the chamber 20. The piston 22 divides the chamber 20 into a lower compartment 24 and an upper compartment 26. A piston rod 28 extends upwardly from the upper surface 30 of the pis~on 22 fully through the interior of the upper compartment 26 and extends through the top of the body member to a point of attachment at the upper platen 14.
A number of matters respecting the geometry of the hydraulic cylinder 16 should be noted. The piston rod 28 has been provided with a comparatively large cross-sectional area, about one-half the cross-sectional area of the piston 22 itself. The lower piston surface 32 consequently has a pressure effective area which is about twice the pressure-effective area of the upper piston surface 30 to which the piston rod 22 is attached. Owing to the presence of the piston rod 28 in the upper compartment ~806 26 and the relative cross-sectional areas of the piston rod 28 and upper compartment 26, axial displacement of the pislon 22 causes in the lower compaltment 24 abouttwice the incremental change in volume that occurs in the upper compartment 26. The significance of these matters to recovery of energy used to raise the upper platen 14 will be discussed more fully below.
A hydraulic pump 34 serves as a primary source of the hydraulic fluid under pressure required to urge the cylinders 16, 16A to raise the upper platen 14.
The pump 34 receives hydraulic fluid from a reservoir 36, and pumps the fluid through a check valve 38 into lines leading to the lower compartments of the hydraulic cylinders 16, 16A. It will be assumed for purposes of illustration that a pressure of about 2000 psi is required to raise the upper platen 14, which will depend on the weight of the platen (which might for example be 75 tons) and the geometry of the cylinders 16, 16A themselves. The check valve 38 has been provided to prevent reversal of fluid flows through the pump 34 during lowering of the platen 14 under gravity.
A pair of conventional bladder accumulators 40,40A are associated with the hydraulic cylinders 16, 16A. The accumulator 40 has $ its inlet a valvemechanism which is biased by a spring to an open state and which is closed, despite the ~resence of the spring, by expansion of a gas-filled bladder that prevents introduction or discharge of hydraulic fluid at less than some predetermined pressure.
In this particular application, the bladder should be pre-charged to 2500 psi,500 psi above the operating pressure which is applied to the lower compartment 24 of thecylinder 16 to urge the upper platen 14 upwardly. This measure of pre-charging prevents receipt or discharge of hydraulic fluid from the accurnulator 40 at less than the line pressure required to effect a raising of the platen 14 and to accommodate a 500 psi pressure drop expected across a valve described below more fully below. It should be noted that a standard hydraulic accumulator incorporating a piston or any other device ~IL2~0~

capable of accumulating and discharging hydraulic fluid under pressure in excess of a presettable pressure can be substituted.
High-pressure conduits define a first fluid path (not speci~ically indicated) between the upper and lower compartments 24, 26 of the hydraulic cylinder 16. A conventional two-way, two-position valve 42 with flow control is provided to regulate fluid flows along the particular flow path and also fluid flows between the upper compartment 26 and the reservoir 36. The flow control valve 42 has two operating states. In a first operating state, the valve 42 permits fluid flows between the upper and lower cylinder compartments 24, 26 and impedes flows between the uppercompartment 26 and the reservoir 36. Such an operating state is required during lowering of the upper platen 14 under gravity at which time fluid communication between the upper and lower compartments 24, 26 results in a pressure intensification.
In the second state, the flow control valve 42 impedes flows between the upper and lower cylinder compartments 24, 26, but allows fluid flows between the upper compartment 26 and the reservoir 36. This operating state is required primarily in connection with the raising of the platen 14, basically allowing return flows from the upper compartment 26 to the reservoir 36 to permit expansion of the hydraulic cylinder 16.
High-pressure conduits also define a fluid flow path (not specifically indicated) between the lower cylinder compartment 24 and the accumulator 40. A
two-way, two-pvsition valve 44 with flow control is positioned in the flow path. The valve 44 may be actuated to an open or closed state, and the pressure drop in the open state may typically be adjusted to about 500 psi. The purpose of the control valve 44 is to regulate the rate at which hydraulic fluid can be transferred into or out of the accumulator 40. As described more fully below, the flow control valve 44 ensurescontrolled raising and lower of the platen.
There are two pilot-controlled relief valves 46, 46A which selectively place the lower compartrnents of the cylinders 16, 16A in con~nunication with the reservoir 36. These are set to release automatically when a pressure in excess of 4000 psi develops in the lower cylinder compartments or associated hydraulic lines at which time the valves 46,46A automatically vent the lower cylinder compartments to thereseNoir 36. The valves 46, 46A are, however, controllable and can be opened at any time to vent the lower cylinder compartments to the reservoir 36. The valves 46, 40A
can be controlled to open automatically when the accumulators 40, 40A are fully charged, if, for example, the accumulators are sized for a particular range of platen travel and that range is subsequently increased. The valves 46, 46A provide another function depending on the type of press involved, a rnatter which will be discussed more fully below in connection with the operation of the platen displacing apparatus.
A controller 48 regulates the raising and lower of the upper platen 14, and more specifically regulates actuation of the hydraulic pump 34, the flow valves 42, 42A associated with the cylinders 16, 16A, the flow control valves 44,44A associated with the accumulators 40, 40A, and the pilot-controlled relief valves 46,46A. The controller 48 is preferrably microprocessor based with manual switches to trigger specific phases of operation such as raising or lowering of the platen 14. The controller 4B has two principal operating states which are of particular significance to the present invention, the first relating to raising of the platen and the second, to lowering of the platen. There is a third operating state which is important to certain types of press designs, in which the upper and lower compartments of both hydraulic cylinders 16, 16A are vented to the reseNoir 36. The principal operating states will be described with reference to figs. 1 and 2 where the fluid flows associated with these operating states have been indicated with alTows adjacent the conduits schematically illustrated. l`he appropriate programming of the controller 48 will be readily apparent to those skilled in the art from a description of general operation provided below.
The first controller operating state will be described with references to 1~8~669 fig. 1 which illustrates the upper platen 14 in the process of being raised. The controller 48 actuates the valves 42, 42A associated with the hydraulic cylinders 16, 16A to vent the upper cylinder compartments 26,26A to the reservoir 36 thereby permitting extension of the hydraulic cylinders 16,16A under pressure applied to the lower cylinder compartments 24, 24A. The controller 48 actuates the hydraulic pump 34 to apply pressure to the lower cylinder cornpartments 24~ 24A and contemporaneously actuates the flow control valves associated with the accumulators 40,40A so that pressure accumulated during previous lowering of the platen can be discharged into the lower cylinder compartments 24~ 24A. The fluid disch~rged frorn the accumulators 40,40A assists the pump 34 in extending the hydraulic cylinders 16, 16A and raising the pla~en. It should be noted that the pump 34 produces pressure of 2U00 psi and that the accumulators 40,40A discharges from about 3500 psi to 2500 psi. Once pressure in the accumulators 40,40A drops to 2500 psi, the bladders and valve mechanisms asociated with the accumulators 40,40A close the accumulators 40, 40A against further discharge of fluids. Once the upper platen 14 is raised, pumping may be discontinued if desired, tlhe flow control valves associated with accumulators 40,40A may be closed, and the upper platen 14 supported on pressurized fluid in the lower cylinder compartments 24,24A.
The second controller operating state will be described with reference to fig. 2 in which the platen 14 is being lowered under gravity. The conholler 48 actuates the flow valves 42,42A associated with the hydraulic cylinders 16, 16A to permit transfer of fluid from the lower cylinder compartrnents 24,24A to the upper cylinder compartments 26, 26A and to impede fluid flows between the upper cyl;nder compartments 26,26A and the reservoir 36. The platen is effectively supported only by pressure acting on one half of the pressure-effective area of the lower piston surfaces. Owing to coupling of the upper and lower compartments and the relati~e geometry in which the pressure effective area of the upper piston surfaces 30,30A is one-half that of the lower piston surfaces 32,32A, the pressure in the lower compartments rises to about 4000 psi, twice the pressure which must be applied to the lower cylinder compartments 24, 24A to effect a raising of the platen. The controller 48 contemporaneously actuates the flow control valves associated with the accumulators 40,40A thereby allowing flow of hydraulic fluid under this pressurehead into the accumulators 40, 40A. With respect to the cylinder 16, fluid is displaced from the lower compartment 24, and owing to the relahve geometry of the upper and lower compar~nents 24,26, one half the incremental volume displaced under the weight of the upper platen 14 flows into the associated upper compartment 26 and the other half flows into the associated accumulator 40. The accumulator pressure can be expected to rise to about 3500 psi (that is, 4000 psi less the pressure drop expected to cross the flow control valve 44). Accordingly, about 75% of energy required to raise the upper platen 14 is recovered in the form of pressuri~ed hydraulic fluid in the accumulator 40.
On release of the accumulator contents to assist in the raising of the platen, as described above, a further 25% of the energy previously required to raise the platen is dissipated across the flow control valves. This results in a net theoretical energy saving of about 50%. A greater energy saving can be effected by selecting a lower pressure drop across the flow control valves; however, the present arrangement represents a reasonable compromise between energy recovery and well-conholled operation, the pressure drop associated with the valves perrnitting a conhrolled and predictable raising and lowering of the platen.
The third controller operating state is of some importance in the design of contemporary compression molding presses which involve parallelism control. In such designs, as in the press 10 described, light-duty cylinders are often dedicated to the task of displacing the upper platen 14 through the greater part of its operahng range to perrnit removal of molded product or introduction of molding ~Z~ i9 charges. Large-bore, short-stroke compression phase cylinders are comrnonly provided to displace the upper platen relative to the lower platen during the compression phase of operation, over a comparatively small range of operation. In such circumstances, the controller's third mode of operation ensures that the upper and lower compartments of the two rapid advance-retreat cylinders 16, 16A are vented to the reservoir 36 so that they follow movement of the upper platen 14 in response to the compression phase cylinders. In this mode of operation, the controller 48 actuates the flow valves associated with the cylinders 16, 16A so that the upper compartment of each cylinder is vented to the reservoir 36 and contemporaneously actuates the pilot-controlled relief valves 46, 46A to vent the lower cylinder compartments 24, 24A
to the reservoir 36. It will be appreciated that such an operating state would be unnecessary if the two cylinders 16, 16A were adapted to perform the compression phase of operation as well as being responsible for movement of the upper platen 14 through the greater part of its range of movement.
The platen displacing apparatus provides additional advantages in the raising and lower of a press platen. In particular, the assistance provided by the accumulators 40, 40A in raising the platen 14 improves cycle time, permiting the upper platen 14 to be raised more quickly for a hydraulic pump of given size. Alternatively, without impairing cycle time, the size of the hydraulic pump can be reduced to that necessary for other functions associated with the press, such as the operation of compression phase cylinders, providing a cost reduction.
It will be appreciated that a particular embodiment of ~e invention has been described, and that modifications may be made therein without department from the spirit of the invention or the scope of the appended claims. In particular, the invention has application to any press in which a massive press member must be hydraulically raised or lowered on cylinders. The system becomes most advantageous when the press member is, for example, a massive platen which requires considerable ~Z~3066~

expenditure of energy to raise and lower the platen. A single accumulator can beprovided for use with one or more hydraulic cylinders used to raise an obj~t such as a press platen; however, a plurality of accurnulators, one associated with each hydraulic cylinder have been illustrated, as such an arrangement simplifies installation.

Claims (14)

1. A press having a platen which is mounted on the press for vertical movement, comprising:
a hydraulic cylinder mounted on the press below the platen, the hydraulic cylinder having a substantially closed, vertical chamber, a piston slidably mounted within the chamber and dividing the chamber into upper and lower compartments, a piston rod engaged with the upper surface of the piston and extending upwardly through the inferior of the upper compartment and having an upper rod end external to the chamber and attached to the platen;
accumulator means for receiving and storing hydraulic fluid under pressure;
controllable hydraulic actuating means for applying hydraulic fluid under pressure to the lower compartment of the hydraulic cylinder to raise the platen;
means defining a first fluid path, the first fluid path extending between the upper and lower cylinder compartments;
first controllable valve means for controlling flow of hydraulic fluid along the first fluid path between the upper and lower cylinder compartments;
means defining a second fluid path, the second fluid path extending between the lower cylinder compartment and the accumulator means;
second controllable valve means for controlling flow of hydraulic fluid along the second fluid path between the lower cylinder compartment and theaccumulator means;
control means for regulating the raising and lowering of the platen, the control means having a first operating state in which the platen can be lowered under gravity and in which the control means contemporaneously actuate the first valve means to place the upper and lower compartments in fluid communication and the second valve means to permit transfer of hydraulic fluid under pressure from the lower compartment to the accumulator means, the control means having a second operating state in which the platen can be raised at least in part by the hydraulic cylinder and in which the control means contemporaneously actuate the hydraulic actuating means to apply hydraulic fluid under pressure to the lower cylinder compartment and the second valve means for transfer of hydraulic fluid under pressure from the accumulator means to the lower compartment.

2. A press as claimed in claim 1 in which the hydraulic actuating means apply hydraulic fluid under a preselected pressure to the lower cylinder compartment to raise the platen and the accumulator means are adapted to prevent introduction into the accumulator means of and storage of hydraulic fluid under pressure less than thepreselected pressure.

3. A press as claimed in claim 2 in which the piston has a lower surfacewith a lower pressure-effective area and an upper surface with an upper pressureeffective area, the upper pressure effective area being 40% to 70% of the lower pressure effective area.

4. A press as claimed in claim 3 comprising reservoir means in communication with the upper compartment of the hydraulic cylinder for receivingreturn flows of hydraulic fluid applied to the hydraulic cylinder by the hydraulic actuating means.

5. A press as claimed in claim 4 in which the first valve means are adapted to control return flows from the upper cylinder compartment to the reservoir means, the first valve means having a first operating state in which the first valve means permit tansfer of hydraulic fluid between the upper and lower cylinder compartments along the first fluid path and simultaneously impede return flows from the upper compartment to the reservoir means and a second operating state in which the first valve means impede transfer of hydraulic fluid between the upper and lower cylinder compartments along the first fluid path and simultaneously permit return flows from the upper compartment to the reservoir means.

6. A press as claimed in claim 5 in which the lower cylinder compartment communicates with the reservoir means and in which the press comprises third controllable valve means operable by the control means for controlling flow of hydraulic fluid from the lower cylinder compartment to the reservoir means.

7. A press as claimed in claim 6 in which the control means have a thirdoperating state in which the control means contemporaneously actuate the first valve means to permit fluid flows between the upper cylinder compartment and the reservoir means and the third valve means to permit hydraulic fluid flows between the lower cylinder compartment and the reservoir means.

8. A press having a platen which is mounted on the press for vertical movement, comprising:
a plurality of hydraulic cylinder mounted on the press below the platen, each hydraulic cylinder having a substantially closed, vertical chamber, a piston slidably mounted within the chamber and dividing the chamber into upper and lower compartments, a piston rod attached to and extending vertically from the piston through the interior of the upper compartment and having an upper rod end external to the chamber and attached to the platen;
accumulator means for receiving and storing hydraulic fluid under pressure;
controllable hydraulic actuating means for applying hydraulic fluid under pressure to the lower compartment of each of the hydraulic cylinders to raise the platen;
means defining a first plurality of fluid paths, each of the first plurality of fluid path being associated with a different one of the hydraulic cylinders and extending between the upper and lower compartments of the associated cylinder;
first controllable valve means for controlling flow of hydraulic fluid along the first plurality of fluid paths between the upper and lower compartments of each hydraulic cylinders;
means defining a second plurality of fluid paths, each of the second fluid paths being associated with a different one of the hydraulic cylinders andextending between the lower compartment of the associated cylinder and the accumulator means;
second controllable valve means for controlling flow of hydraulic fluid along the second plurality of fluid paths between the lower compartment of each hydraulic cylinders and the accumulator means;
control means for regulating the raising and lowering of the platen, the control means having a first operating state in which the platen can be lowered under gravity and in which the control means contemporaneously actuate the first valve means to place the upper and lower compartments of each hydraulic cylinder in fluid communication and the second valve means to permit transfer of hydraulic fluid under pressure from the lower compartment of each hydraulic cylinder to the accumulator means, the control means having a second operating state in which the platen can be raised by the hydraulic cylinders and in which the control means contemporaneously actuate the hydraulic actuating means to apply hydraulic fluid under pressure to the lower compartment of each hydraulic cylinder and the second valve means for transfer of hydraulic fluid under pressure from the accumulator means to the lower compartment of each hydraulic cylinder.

9. A press as claimed in claim 8 in which the hydraulic actuating means apply hydraulic fluid under a preselected pressure to the lower compartment of each hydraulic cylinder to raise the platen and the accumulator means include regulating means for preventing introduction into the accumulator means of and storage of hydraulic fluid under pressure less than the preselected pressure.

10. A press as claimed in claim 9 in which the accumulation means comprise a plurality of accumulators, each accumulator being associated with a different one of the hydraulic cylinders.

11. A press as claimed in claim 9 comprising reservoir means in communication with the upper compartment of each hydraulic cylinder for receiving return flows of hydraulic fluid applied to the hydraulic cylinders by the hydraulic actuating means.

12. A press as claimed in claim 11 in which the first valve means are adapted to control return flows from the upper compartment of each hydraulic cylinder to the reservoir means, the first valve means having a first operating state in which the first valve means permit tansfer of hydraulic fluid between the upper and lower compartments associated with each hydraulic cylinder along the associated one of the first fluid paths and simultaneously impede return flows from the upper compartment of each hydraulic cylinder to the reservoir means and a second operating state in which the first valve means impede transfer of hydraulic fluid between the upper and lower compartments associated with each hydraulic cylinder along the associated one of the first fluid paths and simultaneously permit return flows from the upper compartment of each of the hydraulic cylinders to the reservoir means.

13. A press as claimed in claim 12 in which the lower compartment each hydraulic cylinder communicates with the reservoir means and in which a press comprises third controllable valve means operable by the control means for controlling flow of hydraulic fluid from the lower cylinder compartment of each hydraulic cylinder to the reservoir means.

14. A press as claimed in claim 13 in which the control means have a third operating state in which the control means contemporaneously actuate the first valve means to permit fluid flows between the upper compartment of each of the hydraulic cylinders and the reservoir means and the third valve means to permit hydraulic fluid flows between the lower compartment of each of the hydraulic cylinders and the reservoir means.