US3147723A

US3147723A - Hydraulic control system for die pads in presses

Info

Publication number: US3147723A
Authority: US
Inventors: Floyd M Williamson
Original assignee: DRO ENGINEERING Co DI
Current assignee: DI-DRO ENGINEERING Co; DRO ENGINEERING Co DI
Priority date: 1961-05-24
Filing date: 1961-05-24
Publication date: 1964-09-08
Anticipated expiration: 1981-09-08

Description

Sept. 8, 1964 F. M. WILLIAMSON 3,147,723

HYDRAULIC CONTROL SYSTEM FOR DIE PADS IN PRESSES Filed May 24. 1961 2 Sheets-Sheet 1 W 22 I Z2 M6 1 24 /44- y My IV A W/ A INVENTOR. 2 70 J M M7Z'k7/7J07Z irraFNI/S P 3, 1964 F. M. WILLIAMSON 3,147,723

v HYDRAULIC CONTROL SYSTEM FOR DIE PADS IN PRESSES Filed May 24, 1961 2 Sheets-Sheet 2 i E 222 45 1' if l/ I i INVENTOR.

4: ;j fiy/M 1447527774 E I .52. W 7

United States Patent 3,147,723 HYDRAULIC CONTROL SYSTEM FOR DIE PADS KN PRESSES Floyd M. Williamson, Di-Dro Engineering Company, 2405 E. Grand Blvd, Detroit 11, Mich. Filed May 24, 1961, Ser. No. 112,439 9 Claims. (Cl. 11345) This invention relates to a press having hydraulically actuated die pads for applying pressure against the work during the working stroke of the press and has particular reference to a hydraulic control system for the die pads.

In a press of this type the movable die pad is provided with a hydraulic cushion consisting of a cylinder adapted to be supplied with hydraulic fluid under pressure from a surge tank and a piston reciprocating within the cylinder is urged by the hydraulic pressure fluid against the die pad to exert pressure on the die pad and to provide a predetermined resistance to movement of the die pad as the press closes. The pressure fluid from the cylinder is exhausted during the working stroke of the press back through a pressure relief valve which is set to open when a predetermined pressure is exceeded, thereby providing the desired degree of resistance to movement of the die pad. In some press operations it is desirable that the die pad exert different pressures against the work during different portions of the working stroke of the press. For example, when deep drawing a metal part, relatively high pressure is needed to grip the part at the beginning of the working stroke but, if this high pressure is maintained as the draw continues, the metal is liable to tear. It is therefore desirable to reduce the pressure exerted against the die pad as the working stroke progresses. Prior devices to accomplish this result have not proven satisfactory for many reasons, principal among which is the fact that such prior arrangements unduly complicate the hydraulic system and could not operate accurately in the fraction of a second allowed for their operation. The present invention provides a hydraulic control system which will automatically provide a predetermined degree of resistance to movement of the die pad during the initial part of the working stroke of the press and then either increase or decrease the pressure during the remaining portion of the working stroke as may be required. The valve structure forming a part of this control system includes a timing valve in combination with a pair of pressure relief valves. The pressure fluid in the hydraulic cushion is exhausted first through one of the pressure relief valves and then after the timing valve is actuated through the other of the pressure relief valves to provide different pressures on the die pad at different portions of the working stroke of the press.

A principal object of the invention therefore is to provide a new and improved hydraulic control system for die pads in presses.

Another object of the invention is to provide a hydraulic control system for a die padin a press which is adapted to apply a predetermined pressure against the die pad during the initial portion of the working stroke of the press and a difierent predetermined pressure against the die pad during the remaining portion of the Working stroke of the press.

A further object of the invention is to provide a hydraulic control system of the type described which includes a plurality of pressure relief valves and a timing valve associated therewith for bringing the pressure relief valves into operation at predetermined times during the working stroke of the press.

Other and further objects of the invention will be apparent from the following description and claims and may be understood by reference to the accompanying drawings, of which there are two sheets, which by way of illustration show preferred embodiments of the invention and What I now consider to be the best mode of applying the principles thereof. Other embodiments of the invention may be used without departing from the scope of the present invention as set forth in the appended claims.

In the drawings:

FIG. 1 is a sectional schematic view of a die pad for a press and a hydraulic control system therefor;

FIG. 2 is a sectional view through a die pad for use in performing a different type of operation in the press and which employs the hydraulic control system shown in FIG. 1;

FIG. 3 is a sectional view showing a modified type of control system;

FIG. 4 is a sectional view taken on line 4-4 of FIG. 3;

FIG. 5 is a fragmentary sectional view illustrating a modified type of delay valve which may be used in the control system shown in FIG. 3; and

FIG. 6 is a sectional view of a further modified type of control system.

Referring to FIG. 1, there is illustrated therein a portion of a press including a lower die shoe 10 having a forming die 12 carried thereby and an upper die shoe 14 provided with a punch 16 which is adapted to perform a drawing operation on the work 18 as the press is closed. A movable die pad 20 is provided with a hydraulic cushion consisting of cylinders 22 having pistons 24 therein which engage the die pad 20 to apply pressure against the work 18 as the press closes. The particular arrangement of the die members and the die pad 20 shown in FIG. 1 is for purposes of illustration only and is merely intended to illustrate one type of operation that may be performed in a press in which a relatively high pressure is required to grip the work as the press closes and in which a relatively low pressure is required as the forming operation progresses.

A tank 26 containing hydraulic fluid such as oil is supplied with air under pressure through line 28 to maintain the oil in the tank 26 at a predetermined pressure. The oil is discharged from the tank 26 through a line 30 past a check valve 32 to a line 34 which intersects a conduit 36 from which the oil under pressure is supplied to the cylinders 22 to force the pistons 24 therein to their extended positions. As the press closes the pistons 24 are retracted within their cylinders 22 and the oil in the cylinders is forced out back through conduit 36 to a multistage pressure relief valve indicated generally at 40 to provide a predetermined degree of resistance to movement of the pistons 24 during the working stroke of the press. The check valve 32 prevents a direct return flow from the cylinders 22 back to the tank 26.

The valve 40 is provided with a cavity 42 containing a valve seat member 44 at its upper end and a sleeve 46 engaging the valve seat member 44 and communicating with the inlet passage 48 which communicates with the conduit 36. A cup-shaped valve member 50 reciprocates within the sleeve 46 and is urged toward the lower end of the sleeve 46 by a spring 52 to a position in which it closes off radial ports 54 extending through the sleeve 46. The ports 54 communicate with a cavity 56 defined by a reduced diameter portion of the sleeve 46. A passage 58 communicates with the cavity 56 and with a line 60 which connects into the line 30 leading from the tank 26.

The valve member 50 is provided with a small passage 62 to provide communication between the inlet passage 48 and the interior of the sleeve 46. The valve seat member 44 is provided with an axial passage 64 which communicates with the interior of the sleeve 46 and with a Patented Sept. 8, 1964.

valve chamber 66 containing a pressure relief valve 68 which engages a conical valve seat 70 formed on the valve seat member 44 to normally close the axial passage 64 in the valve seat member. The pressure relief valve 68 is urged to its closed position by a spring 72 which is confined between the valve member 68 and a washer 74 which is engaged by an adjustable screw 76 by means of which the pressure of the spring '72 may be adjusted thereby to regulate the pressure at which the pressure relief valve 68 will open. A passage 78 leads from the valve chamber 66 to the passage 58 and lines 60 and 30 back to the tank 26.

The valve body is provided with a cavity 80 containtaining a valve seat member 82 and a sleeve 84 which defines a valve chamber 86 containing a pressure relief valve member 88 which normally closes a passage 90 in the valve seat member 82. The pressure relief valve member 88 is urged to its closed position by a spring 92 confined between the valve member 88 and a washer 94 which is engaged by a threaded adjusting screw 96 to regulate the pressure at which the valve member 88 will open. The valve seat members 44 and 82 are provided with radial passages 98 and 100 respectively which are connected by a passage 102 in the valve body.

The valve is also provided with a cavity 104 containing a sleeve 106 within which a time delay spool valve 108 reciprocates. A spring 110 urges the spool valve 108 toward the right-hand end of the sleeve 106. A passage 112 communicates with the chamber 86 through a radial port in the sleeve 84 and with the interior of the sleeve 106 through a radial port therein. A passage 114 also communicates with the interior of the sleeve 106 through a radial port therein and the

passages

112 and 114 are normally closed off when the time delay valve 108 is in its right-hand position as shown. A passage 116 provides communication between the passage 100 in the valve seat member 82 and the right-hand end of the time delay cylinder defined by the sleeve 106. A bleeder passage 116' leads from the left-hand end of the sleeve 106 and communicates with a passage 118 which intersects the passage 114 leading back to the cavity 56 around the sleeve 46. An adjustable needle valve 120 regulates the size of the passage 116.

When the press is opened the hydraulic cushion cylin ders are supplied with fluid under the pressure existing in the tank 26 through line 30, check valve 32 and

lines

34 and 36. As the press closes, the hydraulic fluid is trapped in the cylinders 22 to resist movement of the pistons 24 and the trapped fluid flows through line 36 and passage 48 in the valve body 40 to the passage 62 in valve member 50. The oil flows from the interior of the sleeve 46 through

passages

64, 98, 102, 100 and 116 to react against the right-hand side of the timing valve 108. This valve cannot immediately shift to the left, however, because the fluid trapped in the left-hand end of the sleeve must escape past the needle valve 120 which is adjusted to provide a desired time interval before the timing valve 108 shifts far enough to the left to open

passages

112 and 114. The valve member 68 is set to open at a higher pressure than the valve member 88. However, since the

passages

112 and 114 are initially blocked by the valve member 108, the pressure relief valve 88 will be inoperative at the beginning of the working stroke of the press and the excess pressure in the cylinders 22 will at this time be relieved only through the pressure relief valve 68. As soon as the pressure relief valve 68 opens, the pressure on the upper side of the valve member 50 will be reduced, thereby enabling the pressure fluid to shift the valve member 50 upwardly to open the radial ports 54 to return excess fluid through ports 54 and passage 58 to line 60.

The fluid may be bled off past the needle valve 120 at a selected rate so that at a predetedmined time during the working stroke of the

press passages

112 and 114 will be opened and communicate with each other past the reduced portion 122 of the valve member 108. As soon as this occurs, the valve member 88 will open and the consequent reduction in pressure in the system will cause valve member 68 to close, whereupon the pressure exerted on the die pad 20 will be determined by the setting of the pressure relief valve 88. The fluid for the remainder of the cycle of operation exhausts past the relief valve 88 to chamber 86, passage 112 and the interior of sleeve 106 to passage 114, cavity 56 and passage 58 to line 60. When the working stroke is completed, both pressure relief valves 68 and 88 will be closed, valve member 108 will be shifted to its right-hand position by spring and valve member 50 will again close ports 54 under the influence of spring 52. The control system is then set for another cycle of operation.

FIG. 2 illustrates a different type of forming operation on the work 18 in which it would be desirable to cause the die pad to first exert high pressure against the work and then a lower pressure as the working stroke is completed. An example of an operation of this type is a trimming and drawing operation in which the lower die shoe 10 is provided with a trimming die and a forming die 142 while the upper die shoe is provided with a die member 144 which is held in its extended position by a piston 146 reciprocating within a cylinder 148 which communicates through line 150 with the conduit 36 shown in FIG. 1. The pressure on the die pad is regulated by the pressure relief valve arrangement described in connection with FIG. 1 to exert a relatively high pressure on the work during the trimming operation and then a relatively low pressure as the forming operation is carried out.

' In the embodiment of the invention shown in FIG. 3 the control system is designed to provide high pressure during the initial portion of the Working stroke of the press and a lower pressure at a predetermined time during the working stroke as in the previous modification. The construction and operation of the valve are essentially the same in this modification, although the time delay valve is of somewhat different construction. In this modification the inlet passage 48 is in line with the low pressure relief valve 88 rather than the high pressure valve 68. The parts comprising the pressure relief valves 68 and 88 and the valve 50 may be identical to those employed in the FIG. 1 construction and are given the same reference characters. The fluid discharged from the cylinders 22 enters the passage 48 from the line 36 and flows through the passage 62 in valve member 50 and through passages 64 and 98 in the valve seat member 44, passage 102 in the valve body and

passages

100 and 90 to the high pressure relief valve 68. When the pressure for which the valve 68 is set is exceeded, the valve 68 will open to return the fluid to the tank through

passages

78 and 58. The time delay valve includes a valve block 200' having a bore 202 containing a sleeve 204 within which a time delay valve member 206- is slidably disposed. The valve member 206 is urged by a spring 208 downwardly against a stop: plug 210 which is engaged by a threaded adjusting screw 212 which may 'be regulated to adjust the length of the stroke of the of the relief valve 88 communicates with a passage 220 in the valve block 200. The sleeve 204 is provided with radial ports 222 which provide communication through the interior of the sleeve between the passage 220 and a passage 224. The upper end of the passage 224 communicates with the interior of the sleeve 204 above the valve member 206 through a passage 226 and a radial port 228 in the sleeve 204. The lower end of the passage 224 communicates with a passage 230 which is connected to the passage 113 leading to the cavity 56 within which the sleeve 46 is disposed.

When the press begins its working stroke, the valve member 206 blocks the radial ports 222' in the sleeve 204 so that the pressure cannot be relieved through the relief valve 88. Thus high pressure relief valve 68 Will open before valve 88. A passage 232 communicates with the passage 116. in the valve body 40 and with a radial port 234 in the sleeve 204 beneath the valve member 206. The pressure fluid supplied through

passages

64, 98, 116, 232 and port 234 is effective to shift the valve member 206 upwardly against the spring 208 to open the ports 222 as the reduced diameter portion 236 of the valve member is aligned with the ports 222. The pressure fluid then opens the lowv pressure relief valve 88 to reduce the pressure in the system and discharge the fluid through

passages

112, 220,. 224, 230 and 114 tocavity 56 and passage 58 back to the tank 26. When valve 88 opens, valve 68 closes because of the pressure drop.

The fluid trapped in the sleeve 204 above the valve member 206 escapes through port 228 to

passages

226, 224, 230 and 114 to cavity 56 for return to the tank. The rate at which this trapped fluid escapes is controlled by an arcuate slot or groove 240 in the spool 214. The spool 214 may be rotated to a desired position to vary the communication with port 228 thereby to regulate the time delay before opening of the low pressure valve 88.

FIG. 5 illustrates a modified type of timing valve in which the construction is otherwise the same as that shown in FIG. 3. In this form of the invention the timing valve member 250 is constructed so that the ports 222 in the sleeve 204 are open at the beginning of the working stroke so that the low pressure relief valve 88 is effective at the beginning of the working stroke. After a time interval determined by the setting of the timing spool 2'14 andthe adjustment of the screw 212 the valve member 250 will block the ports 222 to render the low pressure valve 88. ineffective and the high pressure reliefvalve 68 will thereupon open to provide a higher pressure in. the cylinders 22 during the remainder of the working stroke of the press.

In the form of the invention shown in FIG. 6, the construction and operation of the valve structure are essentially the same as that shown in FIG. 3 and the parts thereof are provided. with the same reference characters. However, the valve body 300 is of somewhat different construction. In this modification the valve body is provided with a passage 302 which communicates with the interior of sleeve 204 and with the inlet passage 48 so that the timing spool 206 is actuated by pressure fluid from passage 48. The valve member 206 initially blocks the ports 222 in the sleeve 204 to delay the opening of the low pressure relief valve 88 until after the opening of the high pressure valve 68 at the beginning of the working stroke. As soon as the valve member 206 is shifted to the position shown, the ports 222 will be opened, whereupon the pressure fluid will be discharged from the sleeve 46 through passages 64 and 98, passage 304 in the valve body 300, ports 222, around the reduced portion of the valve member 206,

passages

306, 100 and 90 past the low pressure relief valve 88. The fluid is discharged from the low pressure relief valve 88 through a passage 308 to the high pressure relief valve chamber 66 and through passages 310 and 312 back to the supply tank 26. The fluid trapped in the upper end of the sleeve 204 escapes through

passages

314 and 308 on the discharge side of the low pressure relief valve 88 during the time that the latter is effective to control the pressure. A passage 316, which is nornally closed by a check valve 318, communicates with a passage 320 communicating with a port 322 in the sleeve 204 so that after the working stroke is completed the sleeve 206 may be rapidly filled on the return stroke of the press. It will be apparent that the valve member 206 could be modified to provide a lower pressure in the system at the beginning of the Working stroke as in the modification shown in FIG. 5.

While I have illustrated and described preferred embodiments of my invention, it is understood that these are capable of modification, and I therefore do not wish to be limited to the precise details set forth but desire to avail myself of such changes and alterations as fall within the purview of the following claims.

I claim:

1. In a press having a movable die pad engaged by the work upon closing of the press and a hydraulic cushion for said die pad to move said pad to its extended position and to provide a predetermined resistance to movement of said pad to its retracted position upon closing of the press, said hydraulic cushion comprising a cylinder having a piston therein reacting against said pad, a source of fluid under pressure, a fluid connection between said fluid pressure source and said cylinder through which pressure fluid is supplied to said cylinder to move said pad to said extended position, a check valve in said connection to prevent return flow therethrough, a pair of return lines connecting said cylinder to said fluid pressure source through which fluid is returned to said source as said pad is retracted upon closing of the press, a normally closed pressure relief valve in each of said return lines, said relief valves being disposed in parallel relation to each other and set to open at diflerent predetermined pressures, timing means interposed in the return line through the pressure relief valve which is set to open at the lower pressure, said timing means including a valve member adapted in one position thereof to block said one return line and in another position thereof .to open said return line, and means responsive to the hydraulic pressure generated in said cylinder during closing of the press to shift said valve member from one of said positions to the other at a predetermined rate to effect discharge of fluid through each pressure relief valve during. a predetermined portion of the working stroke of the press.

2. In a press having a movable die pad engaged by the work upon closing of the press and a hydraulic cushion for said die pad to move said pad to its extended position and to provide a predetermined resistance to movement of said pad to its retracted position upon closing of the press, said hydraulic cushion comprising a cylinder having a piston therein reacting against said pad, a source of fluid under pressure, a fluid connection between said fluid pressure source and said cylinder through which pressure fluid is supplied to said cylinder to move said pad to said extended position, a check valve in said connection to prevent return flow therethrough, a pair of return lines connecting said cylinder to said fluid pressure source through which fluid is returned to said source as said pad is retracted upon closing of the press, a normally closed pressure relief valve in each of said return lines, said relief valves being disposed in parallel relation to each other and set to open at different predetermined pressures, and timing means interposed in the return line through the pressure relief valve which is set to open at the lower pressure, said timing means being operable responsive to changes in the hydraulic pressure in said cylinder to open and close said one return line at predetermined times during each cycle of operation of the press thereby to eflect return flow past the higher pressure valve during a part of the Working stroke of the press and return flow through the lower pressure valve during the remainder of said working stroke.

3. In a press having a movable die pad engaged by the work upon closing of the press and a hydraulic cushion for said die pad to move said pad to its extended position and to provide a predetermined resistance to movement of said pad to its retracted position upon closing of the press, said hydraulic cushion comprising a cylinder having a piston therein reacting against said pad, a source of fluid under pressure, a fiuid connection between said fluid pressure source and said cylinder through which pressure fluid is supplied to said cylinder to move said pad to said extended position, a check valve in said connection to prevent return flow therethrough, a pair of return'lines disposed in parallel relation to each other and connecting said cylinder to said fluid pressure source through which fluid is returned to said source as said pad is retracted upon closing of the press, a normally closed pressure relief valve in each of said return lines, said relief valves being set to open at different pressures, and timing means operable in response to changes in the hydraulic pressure in said cylinder during closing of the press to etfect return flow of fluid through said pressure relief valves in sequence thereby to change the pressure on the work at a predetermined time during closing of the press.

4. A press according to claim 3 wherein said timing means comprises a valve cylinder having a valve member therein adapted in one position thereof to close the return line through the pressure relief valve which opens at the lower pressure and adapted to open such return line in another position thereof, a passage connecting said hydraulic cushion cylinder to said valve cylinder on one side of said valve member thereby to shift said valve member from one of said positions to the other upon closing of the press, such movement of said valve member being resisted by fluid trapped in the other end of said valve cylinder, and means providing an adjustable bleed orifice through which the trapped fluid escapes from said valve cylinder to control the rate of movement of said valve member and thereby the time at which the pressure on the work is changed.

5. A press according to claim 3 wherein said timing 6. In a press having a movable die pad engaged by the work upon closing of the press and a hydraulic cushion for said die pad to move said pad to its extended position and to provide a predetermined resistance to movement of said pad to its retracted position upon closing of the press, said hydraulic cushion comprising a cylinder having a piston therein reacting against said pad, a source of fluid under pressure, a fluid connection between said fluid pressure source and said cylinder through which pressure fluid is supplied to said cylinder to move said pad to said extended position, a pair of pressure relief valves through which fluid is returned from said cylinder to said source as said pad is retracted upon closing of the press, said relief valves being disposed inparallel relation to each other and set to open at different predetermined pressures, and means to delay the flow of fluid past the pressure relief valve which is setto open at the lower pressure so that fluid is first returned through the higher pressure relief valve during the first portion of the working stroke of the press said means being operable after the higher a pressure relief valve opens and in response to the hydraulic pressure generated in said cylinder during closing of the press to permit fluid flow past said lower pressure valve during another portion of the working stroke of the press thereby to change the pressure on the work during said working stroke.

7. In a press having a movable die pad engaged by the work upon closing of the press and a hydraulic cushion for said die pad to move said pad to its extended position and to provide a predetermined resistance to movement of said pad to its retracted position upon closing of the press, said hydraulic cushion comprising a cylinder having a piston therein reacting against said pad, a source of fluid under pressure, a fluid connection between said fluid pressure source and said cylinder through which pressure fluid is supplied to said cylinder to move said pad to said extended position, a check valve in said connection to prevent return flow therethrough, a pair of return lines connecting said cylinder to said fluid pressure source through which fluid is returned to said source as said pad is retracted upon closing of the press, a normally closed pressure relief valve in each of said return lines, said relief valves being disposed in parallel relation to each other and set to'open at different predetermined pressures'in response to the hydraulic pressure generated in said cylinder during closing of the press, and timing means for eflecting return flow through said pressure relief valves in sequence and at predetermined times during the working stroke of the press, said timing means comprising a cylinder having a valve member therein adapted in one position to close the lower pressure return line and in another position thereof to open such return line, a fluid pressure transmitting passage connecting one end of said cylinder to said hydraulic cushion cylinder to shift said valve member from one of said positions to the other, and a bleed passage leading from the other end of said valve cylinder to regulate the escape of fluid trapped therein upon movement of said valve member thereby to elfect discharge of fluid from said hydraulic cushion cylinder first through one of said pressure relief valves and then through the other at a predetermined time during closing of the press.

8. A press according to claim 7 including means for regulating the size of said bleed passage to control the rate of movement of said valve member.

9. A press according to claim '7 including means for varying the length of the stroke of said valve member.

References Cited in the file of this patent UNITED STATES PATENTS

Claims

1. IN A PRESS HAVING A MOVABLE DIE PAD ENGAGED BY THE WORK UPON CLOSING OF THE PRESS AND A HYDRAULIC CUSHION FOR SAID DIE PAD TO MOVE SAID PAD TO ITS EXTENDED POSITION AND TO PROVIDE A PREDETERMINED RESISTANCE TO MOVEMENT OF SAID PAD TO ITS RETRACTED POSITION UPON CLOSING OF THE PRESS, SAID HYDRAULIC CUSHION COMPRISING A CYLINDER HAVING A PISTON THEREIN REACTING AGAINST SAID PAD, A SOURCE OF FLUID UNDER PRESSURE, A FLUID CONNECTION BETWEEN SAID FLUID PRESSURE SOURCE AND SAID CYLINDER THROUGH WHICH PRESSURE FLUID IS SUPPLIED TO SAID CYLINDER TO MOVE SAID PAD TO SAID EXTENDED POSITION, A CHECK VALVE IN SAID CONNECTION TO PREVENT RETURN FLOW THERETHROUGH, A PAIR OF RETURN LINES CONNECTING SAID CYLINDER TO SAID FLUID PRESSURE SOURCE THROUGH WHICH FLUID IS RETURNED TO SAID SOURCE AS SAID PAD IS RETRACTED UPON CLOSING OF THE PRESS, A NORMALLY CLOSED PRESSURE RELIEF VALVE IN EACH OF SAID RETURN LINES, SAID RELIEF VALVES BEING DISPOSED IN PARALLEL RELATION TO EACH OTHER AND SET TO OPEN AT DIFFERENT PREDETERMINED PRESSURES, TIMING MEANS INTERPOSED IN THE RETURN LINE THROUGH THE PRESSURE RELIEF VALVE WHICH IS SET TO OPEN AT THE LOWER PRESSURE, SAID TIMING MEANS INCLUDING A VALVE MEMBER ADAPTED IN ONE POSITION THEREOF TO BLOCK SAID ONE RETURN LINE AND IN ANOTHER POSITION THEREOF TO OPEN SAID RETURN LINE, AND MEANS RESPONSIVE TO THE HYDRAULIC PRESSURE GENERATED IN SAID CYLINDER DURING CLOSING OF THE PRESS TO SHIFT SAID VALVE MEMBER FROM ONE OF SAID POSITIONS TO THE OTHER AT A PREDETERMINED RATE TO EFFECT DISCHARGE OF FLUID THROUGH EACH PRESSURE RELIEF VALVE DURING A PREDETERMINED PORTION OF THE WORKING STROKE OF THE PRESS.