Control valve means for hydraulic press
United States Patent 3916766

A hydraulic molding press comprising a main cylinder in which a main ram is mounted for forward and reverse movement and the main ram has a booster cylinder therein into which a hollow stationary booster ram extends. A control valve comprising a valve housing is provided at the end of the main cylinder and the booster ram extends therethrough. The valve housing includes a removable portion called a booster ram cap which threads onto and supports the end of the booster ram. The booster ram cap is detachably secured to the valve housing and the latter is detachably secured to the end of the main cylinder by four studs. A prefill tank connects to the valve housing for supplying prefill fluid to the main cylinder as the main ram is moved rapidly forward by booster fluid supplied through the booster ram from a port in the booster cap. A prefill valve piston slideably mounted in the valve housing is movable to close off prefill fluid flow from the prefill tank. The piston has a central opening through which the booster ram extends. The opening in the piston also serves as a fluid passage for supplying high pressure fluid to the main ram after the piston is moved forward to closed position. Fluid to this passage is supplied through a pair of prefill chambers in the booster ram cap which opens when the prefill pins are moved forward by pilot pressure to close the prefill valve piston. The prefill valve piston is controlled by a two-position remotely operated valve. The prefill pins are continually under pilot pressure. Forward and reverse movement of the main ram is controlled by a three-position remotely operated valve. To effect reverse movement of main ram, the two-position valve operates to decompress the high pressure fluid behind the main ram and opens the prefill valve piston to allow free fluid flow to the prefill tank. Then the three-position valve operates to move the main piston in reverse.

Application Number:
Publication Date:
Filing Date:
Primary Class:
Other Classes:
91/441, 91/527
International Classes:
B29C45/67; (IPC1-7): F15B11/18; F15B13/06
Field of Search:
91/441,412,411A 60
View Patent Images:
US Patent References:
3473543CHIP-FREE EXPLOSIVELY ACTUATED VALVE1969-10-21Haefner et al.
3084512Hydraulic molding machines1963-04-09Huelskamp
2878648Molding machine ram and valve construction1959-03-24Norman et al.
2805447Safety circuit for molding machine1957-09-10Voges
2539361Hydraulic circuit for operating hydraulic motors1951-01-23Cannon
2423120Valve arrangement1947-07-01Sedgwick
2190755Hydraulic press1940-02-20Dinzl
2152837Prefill valve for hydraulic presses1939-04-04Cannon

Primary Examiner:
Cohen, Irwin C.
Attorney, Agent or Firm:
Burns, Doane, Swecker & Mathis
Parent Case Data:

This is a continuation, of application Ser. No. 237,085, filed Mar. 22, 1972 now abondoned.
I claim

1. In a hydraulic press:


This invention relates generally to hydraulic presses such as hydraulically actuated molding machines of the type having a mold clamp assembly for quickly closing, clamping and opening a mold. In particular it relates to control valves therefor, such as prefill valves.


My U.S. Pat. No. 3,084,512 issued Apr. 9, 1963 for "Hydraulic Molding Machines" discloses an injection molding machine wherein an initial high-speed movement of the main ram to mold closing position is accomplished by means of a smaller booster ram. The advantage of this is that a given amount of pressure fluid supplied to the booster cylinder will effect a much faster and greater ram movement than it would if supplied to the main ram cylinder. This construction however requires that fluid prefill means be provided for keeping the main ram cylinder filled with fluid as the main ram is moved under the action of the booster ram. It also requires that valve means be provided for shifting the flow of pressure fluid from the booster ram to the main ram when the latter reaches that point in its travel at which full ram pressure is desired. This construction further requires that valve means be provided for quickly closing the large prefill line to the main cylinder just before the shiftover of the high pressure fluid which is commonly at a pressure of 3000 lbs./sq. in. or higher.

In practice these machines are quite large and require much floor space for installation and to enable servicing. Also, since they are supplied with high pressure hydraulic fluid for operation, pressurized components therein, such as valves, require strong castings, numerous carefully fitted O-rings and other types of costly seals, and numerous studs or bolts to secure components together. Furthermore, most valve assemblies and components thereof are relatively complex and are difficult and costly to machine, assemble and service. In some instances construction of specially designed intricate fluid passageways in valve elements tends to weaken the part and necessitates the use of specially hardened expensive metals, as well as special tools and manufacturing techniques. Also, in some machines, special additional valve spools, choking devices and time-delay devices are required to effect decompression of the fluid in high pressure chamber before reverse movement of the main ram can occur. However, these means are only partially successful in effecting smooth decompression and sometimes allow pipe breakage and other problems to occur.


A hydraulic press, such as a hydraulic molding machine, in accordance with the invention comprises a main ram cylinder, a main ram reciprocably movable in the ram cylinder, a booster ram cylinder formed in the main ram, and a hollow booster ram extending into the booster ram cylinder. A flow of pressurized hydraulic fluid through the hollow booster ram into the booster ram cylinder effects rapid closing movement of the main ram. Subsequent flow of pressurized fluid into the main ram cylinder behind the main ram effects a slow, powerful, final closing movement and holding of the main ram. A fluid reservoir or prefill tank supplies prefill fluid by gravity to the main ram cylinder as rapid closing movement occurs.

A control valve is provided to control fluid flow to and from the booster ram cylinder and the main ram cylinder and valve control means, including a hydraulic circuit, and multiposition valves, are provided to operate the control valve.

The control valve comprises a valve housing including a valve body, having a chamber therein and open at both ends, and an end portion. One end of the valve body lies adjacent the open end of the main ram cylinder. The other end of the valve body is provided with the aforesaid end portion which serves as a closure means and as a booster ram cap. The booster ram cap supports and seals one end of the hollow booster ram by making threaded engagement with the hollow booster ram. A plurality (preferably four) large bolts secure the booster ram cap to the valve body and the latter to the main ram cylinder.

A prefill piston is slidably mounted within the valve body and has a central opening through which the booster ram extends. The booster ram does not touch the prefill piston but cooperates therewith to provide an annular fluid passage. A prefill fluid passage is provided in the valve body and serves as a means by which the resevoir is connected to the chamber in the valve body. The prefill piston is movable from an open position to a closed position wherein it bears against a valve seat provided in the main cylinder and closes off fluid flow between the prefill reservoir and the main ram cylinder. One of the fluid inlet ports in the valve body connects to a space behind the prefill piston and is pressurizable to move the prefill piston to an open position.

The booster ram cap is provided with a first fluid passage which communicates with the hollow interior of the booster ram. The booster ram cap is also provided with a second fluid passage which communicates between the first fluid passage and a pair of cylindrical prefill pin valve chambers in the booster ram cap, each of which has a slidably movable grooved prefill pin valve therein. A pilot pressure port is connected to each prefill pin valve chamber to move the prefill pin valves.

In a typical cycle of operation, the pilot pressure ports for the prefill pins are pressurized but pressurization of the fluid inlet port in the valve body causes the prefill piston to be maintained in open position against the force of the prefill pins. When the first fluid passageway in the booster ram cap is pressurized by operation of a forward-reverse valve, the main ram is moved forward rapidly by pressurized fluid which flows through the hollow booster ram into the booster ram cylinder. Since the prefill piston is open, prefill fluid can flow by gravity from the prefill reservoir to the main ram cylinder. After the main ram has traveled a predetermined amount, a shifter valve is operated to cause a shift to occur whereby the fluid inlet port in the valve body is depressurized. This allows the prefill piston to be moved to closed position by the prefill pins as the latter advance under the force of pilot pressure. As the prefill pins advance, pressurized fluid is able to flow from the first fluid passage, past the grooves in the prefill pins, and through the annular passage between the prefill piston and booster ram into the main ram cylinder. When the prefill piston is moved to fully closed position, flow of prefill fluid from the prefill reservoir is cut off and fluid pressure builds up behind the main ram to slowly move it to clamped position and to hold it closed for a predetermined length of time, depending on the setting of the control valve means. To reverse the movement of the main ram and effect unclamping, the shifter valve is actuated to cause a second fluid port in the valve body to be depressurized and effect decompression of the fluid acting on the main ram by allowing fluid to flow back from the main ram cylinder, through the annual fluid passageway in the prefill piston, out through the second fluid port. This is essentially a metered flow and allows for a smooth transition to decompression. The prefill valve also returns to open position as this occurs. Shortly after operation of the shifter valve, the forward-reverse valve operates to cause rearward movement of the main ram and, with the prefill valve open, to allow flow from the main ram cylinder directly to the prefill reservoir as the prefill piston moves in reverse.

The valve control means includes hydraulic circuitry, electrically operated solenoid valves and limit switches, all hereinafter described in detail.

A hydraulic press, the control valve therefor and valve control means in accordance with the invention has the following advantages over the prior art apparatus. Valve construction and mode of operation allows for a substantially larger prefill fluid capacity. Fewer studs are required to secure the valve body and its end cover means (namely, the booster ram cap) to the main ram cylinder. The size of the working pressure surface area of the prefill piston is substantially reduced with an attendant reduction in the operating force required therefor. The valve seat on the prefill piston and on the main ram cylinder are move accessible for initial machining and subsequent inspection. At least one high pressure fluid seal is eliminated and no back-up rings are required. Numerous presicion-machined surfaces and intricate passageways are eliminated thereby reducing production efforts and costs while providing inherently more rugged valve components. Construction of the booster ram and its cap allow removal of the cap without removal of the ram itself, thereby allowing a reduction in installation and service space. Pilot pins are easily replaceable with others of a different type to allow for desired changes in shift-over positions. Other advantages will hereinafter appear.


FIG. 1 is a side elevation partially in section of a molding machine having control valve means embodying the present invention; with portions of the valve control means therefor shown schematically;

FIG. 2 is an enlarged cross-sectional view of the control valve means showing the prefill valve piston in open position before the main ram is moved by the booster ram;

FIG. 3 is a view similar to FIG. 2 but showing the prefill valve piston in closed position when high pressure fluid is first supplied to the main ram; and

FIG. 4 is an end view of control valve means in accordance with the present invention.


In FIG. 1 of the drawing, there is shown an injection molding machine embodying the present invention and having a frame 10 with an injection mechanism assembly 11 and a mold actuating or clamp mechanism assembly 12 on adjacent portions of the frame. The injection mechanism assembly 11 is of conventional form and includes an injection nozzle 13 through which plastic material passes in a wellknown manner into a cavity defined by mold halves 14, 15.

In order to hold the mold halves in face-to-face contact during the injection of the plastic material and to permit subsequent removal of the molded piece, a horizontally movable platen 16 carries one mold half 15 toward and from a stationary die head 17 which supports the second mold half 14. For this purpose the mold clamp assembly 12 is provided with a main ram 18 which is horizontally reciprocable in a ram cylinder 20 and which has secured to its outer end the movable platen 16.

Means are provided for operating the main ram 18 to give rapid initial closing movement of main ram 18 with a slow but powerful final approach and clamping action. Such means comprises a main ram assembly, a booster ram assembly, a control valve and control means for the control valve.

The main ram assembly comprises a ram cylinder 20 which takes the form of a generally cylindrical housing having a longitudinally positioned, open-ended cylindrical bore 21. To actuate ram 18 in the forward direction, pressure fluid is applied to a pressure chamber 22 behind the ram. To actuate ram 18 in the reverse direction, pressurized fluid is applied to an annular cavity 23 surrounding the ram. The forward or outer end of ram cylinder 20 has suitable seals, such as ring seals 24, to prevent escape of pressurized fluid, and to permit reciprocation of main ram 18. The rear or inner end of main ram 18 has an enlarged portion or flange 25 forming an annular area against which the pressurized fluid acts to return the ram. The enlarged portion 25 has suitable fluid seals, such as piston rings 26, which likewise permit reciprocation of main ram 18. Return movement of main ram 18 is limited by an internal abutment or shoulder 27 formed in the inner wall of the main ram cylinder 20. Forward movement of main ram 18 is limited by engagement of the mold halves 14, 15.

To provide for rapid closing movement of main ram 18, and a slow, powerful final approach and clamping action thereof, fluid pressure is applied to differential areas on the rear or inner end of main ram 18.

The booster ram assembly for rapid traverse applies fluid pressure to a relatively small area of main ram 18. The booster ram assembly includes a stationary booster ram 28 of cylindrical shape which projects forwardly into an axial bore or compartment 29, called the booster ram cylinder, formed centrally in the rear or inner end portion of main ram 18. The booster ram cylinder 29 is of sufficient length to allow axial clearance between the forward end of booster ram 28 and the closed end of the cylinder 29 when main ram 28 is fully retracted. The forward closed end wall of booster cylinder 29 provides a first differential, or booster, area against which pressurized hydraulic fluid acts to force main ram 18 forwardly in rapid forward or closing movement. A second differential pressure area on main ram 18 is provided by the remaining annular area on the rear end face surrounding booster ram cylinder 29, and includes the annular retaining plate 32. The fluid applied to the second differential pressure area of main ram 18 is received in chamber 22 formed by the rear section of main ram cylinder 20 and the forward end of valve body 35. The chamber, of course, includes the interior portion of the main ram cylinder 20 as ram 18 moves forward.

Referring also to FIGS. 2 and 3 of the drawings, sealing the open rear end of booster cylinder 29 and maintaining the booster and main rams in substantially coaxial sliding relationship, is a tubular collar 30 seated in a counterbore 31 in the rear end portion of main ram 18. The tubular collar 30 has an inner diameter which permits sliding engagement with booster ram 28 and is held in position by an annular plate or retaining ring 32. Retaining plate 32 is secured to the rear face of main ram 18 and has its inner edge portion extending radially inwardly over the exposed end of collar 30 blocking rearward displacement of the latter.

A control valve 34 is provided to control fluid flow to and from the main ram and booster ram assemblies.

As FIGS. 2 and 3 show, control valve 34 comprises a valve housing including a valve body 35 having a chamber 36 therein and open at both ends. The valve housing also comprises an end portion 37 thereinafter described. One end of valve body 35 lies adjacent the open end of main ram cylinder 20. The other end of valve body 35 is provided with end portion 37 which serves as a closure means for chamber 36. Portion 37 takes the form of a booster ram cap which makes threaded engagement as at 38 with one end of booster ram 28 and supports the latter. The booster ram 28 can be unthreaded from booster ram cap 37 when dissassembly of the valve is required. Thus the booster ram tube 28 can be left in the machine during servicing and the allowance of extra floor space for tube removal is not required. As FIGS. 2, 3 and 4 show, a plurality (preferably four) large studs 39 having nuts 40 thereon secure booster ram cap 37 to valve body 35 and the latter to main ram cylinder 20. The studs 39 extend through suitable holes in end cap 37, past valve housing 35 and take into threaded holes in main cylinder 20. O-ring seals 42 and 43 are provided between valve body 35 and cylinder 20 and between valve body 35 and booster ram cap 37, respectively. Two auxiliary studs 44, one of which is shown in FIG. 1, are used to hold valve body 35 against main cylinder 20 and are for convenience in holding the valve body in position when booster ram cap 37 is removed. The four main studs 39 resist the high pressure forces applied in the interior of the valve housing.

As FIGS. 2 and 3 show, a prefill valve piston 45 is slidably mounted within valve body 35 and has a central opening 46 through which booster ram 28 extends. Prefill valve piston 45 has a bevelled valve surface 47 at one end which seats against a bevelled valve seat 76 provided in the end of main ram cylinder 20. Valve seats 47 and 76 are thus accessible for ease of machining and exposed for inspection when valve body 35 is detached. The prefill piston 45 is precision-fitted to valve body 35 but does not touch booster ram 28. Therefore, the inside bore or hole 46 in piston 45 may be rough machined to a size sufficient to provide a fluid passage 49. The machining of booster ram 28 in the prefill valve area is also rough turning with no precision required. Piston 45 preferably is made of cast iron requiring no heat treatment, and the bursting strength of which is adequate since no thru-drilled holes are provided therein. Prefill piston 45 is provided with suitable piston rings 48 on the exterior thereof and is movable from an open position, shown in FIG. 2, to a closed position, shown in FIG. 3, wherein it bears against valve seat 76 provided in main cylinder 20 and closed off fluid flow from a prefill fluid reservoir 50, shown in FIG. 1, which otherwise can flow through valve body chamber 36 and main ram cylinder pressure chamber 22.

First and second fluid ports 52 and 53 are provided in valve body 35 and connect to spaces 54 and 55 ahead and behind prefill piston 45, respectively.

The booster ram cap 37 is provided with a fluid passage 60 which communicates with a hollow interior or passage 61 in booster ram 28. Passage 60 is provided with a fluid supply by means of a fluid supply fitting 63 attached to the exterior of booster ram cap 37. The booster ram cap 37 is also provided with another fluid passage 62 in the form of a cross-drilled hole which communicates between fluid passage 60 and a pair of cylindrical prefill pin valve chambers 64 and 65 in the booster ram cap. Each chamber 64 and 65 has a slidably movable prefill pin valve 66 therein. Each pin 66 has a circumferential groove 67 therein. Passage 62 directs shiftover fluid to the prefill pin chambers 64 and 65. The outer end of each prefill pin 66 extends into the pilot pressure chamber 69 of a pilot pressure housing 70 which is secured to the outer face of booster ram cap 37, as FIGS. 2, 3 and 4 make clear. Each pilot pressure chamber 69 is provided with a pilot port 71. The two prefill pins 66 are movable by pilot pressure in the chamber 69 to bear against and close prefill piston 45. The pins 66 have the grooves or reduced diameter portions 67 which are used to conduct shiftover fluid from passages 60 and 62 when the pins are shifted forward. Because of the bored construction of booster ram cap 37, pins 66 of different type can be installed easily if experiment shows that the shifover position should be changed.

For conducting pressure fluid from passage 60 in booster ram cap 37 to the operative ram areas, booster ram 28 is provided with the conduit means or passage 61 extending axially from the rear to the forward end thereof. Thus, when pressurized fluid is applied to passage 61 the pressure is applied continuously to the first or booster pressure area of booster cylinder 29 in main ram 18.

Because of the relatively small cross-sectional area of booster ram 28 and booster ram cylinder 29, only a small volume of pressurized fluid need be supplied to move main ram 18 forward. Consequently, for a given rate of supply of pressure fluid, main ram 18 is pushed forward rapidly to provide rapid traverse for closing the mold. It will be appreciated, however, the force moving the ram is relatively small because of the small pressure area at the end of booster ram cylinder 29.

As FIG. 1 shows, prefill tank 50 is mounted above clamping assembly 12 and contains hydraulic fluid of the same kind as that to be used in the high pressure conduits. A short pipe 72 leads from the bottom of the prefill tank to a suitable opening 73 in the top of valve housing 35. The prefill fluid flows by gravity from the prefill tank through pipe 72 into chamber 36 in valve housing 35. The forward end of chamber 35 is open for communication with chamber 22 at the rear face of main ram 18. Consequently, as main ram 18 moves forwardly under the action of booster ram 28, prefill fluid flows rapidly into the expanding space behind the main ram keeping that space full of fluid at little or no pressure. This arrangement minimizes the amount of pressure fluid which must be delivered when applying clamping pressure, and, at the cessation of rapid traverse, rapidly permits increasing the pressure in the main ram chamber to clamp the molds together. Thereafter pressure fluid is supplied to main ram 18 through fluid passage 49 between booster ram 28 and the inside surface of the prefill valve piston 45 from passage 60 in booster ram cap 37.

As FIGS. 1 and 2 show, chamber 36 in valve housing 35 is provided with a fluid port 75 and main ram cylinder 20 is provided with port 78 which communicates with cavity 23 therein.

The control means for control valve 34 and for the booster ram assembly and the main ram assembly of the injection molding machine or press in accordance with the invention comprise the multiposition valves 80 and 81 and hydraulic circuitry shown in FIGS. 1, 2 and 4 as well as limit switches (not shown) which effect operation of these multiposition valves.

FIG. 1 shows valve 80 to be a conventional hydraulic control valve having three positions (neutral, forward and reverse) which is pilot operated in response to hydraulic signals. Operation of valve 80 effects forward and reverse movement of main ram 18. Valve 80, which is shown in neutral position, is supplied with hydraulic fluid through a supply line 82 connected to a hydraulic pump 83. Valve 80 is also provided with a fluid return line 84 which connects to a reservoir 85. Valve 80 is connected by a fluid line 87 to port 78 in cavity 23 of main cylinder 20. Valve 80 is also connected by a fluid line 88 to fluid supply fitting 63 on booster ram cap 37. Valve 80, pilot operated, is controlled by another valve, solenoid operated, not shown.

FIG. 1 also shows valve 81 to be a conventional hydraulic control valve having two positions (normal and shift) which is solenoid operated and effects shifting of fluid to main cylinder 20. Valve 81, which is shown in normal position, is supplied from a pilot pressure fluid line 94 having a check valve 95 therein. Valve 81 is also connected by a fluid line 96 having a check valve 97 therein to port 75 in control valve 34. Valve 81 is also connected by fluid lines 98 and 99 to ports 52 and 53, respectively, in control valve 34. Line 99 has a check valve 100 therein. A fluid line 101 is connected between the fluid line 94 and the ports 71.


Operation of a typical cycle of the apparatus shown in FIGS. 1 through 4 will now be described. First assume that all operative components, including valves 80 and 81, are in the position or condition shown in FIGS. 1 and 2. Also assume that fluid line 94 is pressurized, and that pump 83 is in operation and that fluid line 82 is pressurized. In this condition, pilot pressure is acting on the rear of the pins 66. However, because valve 81 is in the position shown, fluid line 98 pressurizes chamber 54 in valve housing 35. Therefore, valve piston 45 is held open because the pressurized area on piston 45 is larger than the pressurized area of the pins 66.

To advance main ram 18 rapidly forward toward mold clamping position, operating valve 80 is moved leftward from the neutral position shown to its "forward" position to effect pressurization of fluid line 88, port 63 of control valve 34, fluid passages 60 and 62 in booster ram cap 37, pin chambers 64 and 65 and passage 61 in booster ram 28. Pressurization of passage 61 causes main ram 18 to advance rapidly forward. Prefill fluid flows rapidly and unimpeded from prefill tank 50, through pipe 72, through prefill port 73 in valve body 35, through chamber 36 in the valve body, and into region 22 of the main ram cylinder 20 behind main ram 18.

When main ram 18 has advanced a predetermined distance and is near the end of its stroke, it effects tripping of a limit switch (not shown) which operates the solenoid of shift valve 81 and causes valve 81 to shift upwardly from the position shown in FIGS. 1 and 2 to a position wherein fluid line 94 is connected to and blocked against check valve 100 and fluid line 98 is connected to line 96. This permits fluid to flow from chamber 54 in valve body 35 to chamber 36 thereof and to prefill tank 50. Under these conditions, the main cylinder shift occurs, i.e., the shiftover from rapid operation of the main ram 18 by booster ram 28 to slower, high powered clamping movement and operation.

More specifically, with chamber 54 depressurized, the prefill pins 66 move forwardly under the force of the pilot pressure applied thereto effect forward movement of prefill valve piston 45 toward its valve closed position. As the prefill pins 66 advance, the grooved regions 67 thereof extend into chamber 55 in valve body 35 and allow pressurized fluid to flow from port 63, through the passages 60 and 62, through the prefill pin chambers 64, into the region or chamber 55 behind prefill valve piston 45, as shown in FIG. 3. Such fluid flow continues from chamber 55, through the annular passages 49 into chamber 22 behind main ram 18.

When prefill valve piston 45 advances to its fully closed position, as shown in FIG. 3, its seating surface 47 is in fluidtight engagement with its mating valve seating surface 76 on main ram cylinder 20. This causes fluid flow from prefill tank 50 into chamber 22 to be cut off. It allows pressurized fluid flow directly through passage 49 into chamber 22 and, with valve piston 45 closed, pressure builds up in chamber 22 to move main ram 18 forward slowly for its final length of travel. Such condition is maintained for whatever predetermined length of time is desired to maintain ram 18 in stationary clamping position.

To reverse main ram 18 and unclamp the press, the valves 80 and 81 are actuated simultaneously by suitable manual or automatic operating means (not shown). However, it is to be noted that effective operational movement of valve 80 is slower than that of valve 81, resulting in effects explained hereinafter.

Upon release of the solenoid of valve 81 to effect movement thereof back to the normal position shown in FIGS. 1 and 2, the biasing spring means in valve 81 carry out valve movement quickly. With valve 81 in the position shown in FIGS. 1 and 2, chamber 55 in valve body 35 is connected through port 53, check valve 100, fluid line 99, valve 81, fluid line 96, check valve 97 and port 75 in valve body 35 to chamber 36 in the valve body which is connected to prefill tank 50. The immediate effect of this connection is to decompress or relieve the high pressure condition in the body of fluid directly in chamber 22 directly behind main ram 18. Since the tubing involved is of relatively small diameter, there is essentially a metered flow which transfers the stored energy behind the main ram 18 smoothly and without shock to the fluid in chamber 36 of valve housing 35 and prefill tank 50. Pilot pressure is still being applied to the prefill pins 66. Since chamber 54 ahead of prefill valve piston 45 is now pressurized, piston 45 begins to move rearward toward to its open position. The effect of this is to further relieve pressure conditions in the fluid behind main ram 18.

Subsequent to operation of valve 81, slower-acting valve 80 is moved rightward from "forward" to the "reverse" position. In "reverse" position, valve 80 connects pressurized fluid line 82 from pump 83 to fluid line 87 which is connected to port 78 for chamber 23 in main ram cylinder 20. Valve 80 also connects fluid line 88 from port 63 on booster ram cap 37 (and the passages 60 and 62 pin chamber 64 and 65 therein, as well as passage 61 in booster ram 28) to fluid return line 84 for reservoir 85. Under these conditions, reverse movement of main ram 18 occurs as pressurized fluid enters chamber 23 of main cylinder 20 ahead of the main ram. Since prefill valve piston 45 is now open, fluid behind main ram 18 is able to flow freely under the action of ram movement from chamber 22 in cylinder 20, through chamber 36 of valve body 35, through port 73 therein and through prefill pipe 72 back into prefill tank 50. When rearward movement of ram 18 is completed, limit switches (not shown) effect movement of forward and reverse valve 80 to its neutral position shown in FIG. 1.

This completes one cycle of operation and the apparatus is in readiness for the next cycle of operation.