US2269765A - Power transmission - Google Patents

Description

Jan. 13, 1942- F. T. HARRINGTON ET AL 2,269,765

I POWER TRANSMISSION Filed July 13, 1959 s Sheets-Sheet 1 ATTORNEY Jan. 13, 1942. T. HARRINGTON ET AL I 3 POWER TRANSMISSION Filed July 15, 1939 3 Sheets-Sheet 2 FERRIS T. HARRINGTON 8- RALPH L. TWEEDALE ATTORNEY Jan. 13, 1942. F. T. HARRINGTON ET AL 2,259,765

POWER TRANSMI SS ION .Filed July 15, 1939 3 Sheets-Sheet 3 ozammmm INVENTORS FERRIS T. HARRINGTON &

QmnEOhw RALPH L.

Y TWEEDALE fiw 1.

ATTORNEY Patent ed Jan. 13, 1942 POWER TRANSMISSION Ferris T. Harrington and Ralph L. Tweedale, De-

troit, Mich., assignors to Vickers Incorporated, Detroit, Mich., a corporation of Michigan Application July 13, 1939, Serial No. 284,206

7 Claims.

This. invention relates to power transmissions and particularly to those of the type comprising two or more fluid pressure energy translating devices, one of which may function as a pump and another as a motor.

The invention is more particularly concerned with a hydraulic transmission system for operating a hydraulic press or analogous device.

Presses of this character are frequently provided with-a surge tank at the top of the press and with the hydraulic control mechanism and pump also mounted at the top of the press either in or adjacent to the surge tank. In very large presses the necessary manual controls must be located within reach of the operator near the bottom of the press, and it has been common practice to provide an electric circuit for operating the control'mechanism at the surge tank by means of push button switches located within reach of the operator at the bottom of the press and arranged to selectively energize solenoids which are connected to operate the hy-.

draulic controls overhead.

It is also desirable in many press installations to provide for reversing the press platen when the operating pressure in the main cylinder reaches a. predetermined peak value. In presses of this character a commonly used expedient is to provide a solenoid for pulling the hydraulic control member into pressing position which solenoid is energized by a momentary-contact, normally-open, push button switch and to provide a holdng relay for the solenoid, the holding circuit of which is arranged to be opened by a pressure responsive electric switch when the main ram pressure builds up to the desired peak. The control operating solenoid is thus .deenergized permittng a spring or other biasing means to throw the control over to platen reversing position. Usually a mechanical or other connection between the platen and the control is provided for shifting the control me nber to neutral position when the platen is fully returned.

Control circuits of this character have presented certa'n d fficulties centering around the pressure switch and holding circuit. If the hydraulic control member happens to be a fourway valve, the sudden shifting of the same from its pressing position to its platen returning position at the time when the pressure switch opens the hold'ng circuit causes a sudden release of the extremely large amount of energy which which is operated entirely hydraulically so far I as the operation of shifting the control member from pressing position to returning position is concerned and which still permits the use of I electric push button controls positioned at the operator's station to provide the necessary manual control of the press. This is preferably accomplished by the provision of a holding circuit which is arranged to be initially energized by a starting push button, and when energized, to be electrically held, not for the duration of the pressing stroke, but for the duration of a complete cycle consisting of a pressing strokeand a returning stroke, and to then be deenergized by a suitable limit switch operated by the platen when it reaches its fully returned position.

Since hydraulic presses are almost always manufactured to order with individual features to suit each user's requirements, it becomes desirable to standardize as many of the parts of the press as possible without sacrificing the manufacturers ability to provide a press control system which will meet any user's requirements. Another object of the present invention is, therefore, to provide a hydraulic transmission system for operating a prss wherein there is utilized a main control panel which may be selectively combined with auxiliary control panels of various types to provide either an electrically operated control system or-a. direct mechanically operated one.

Another problem which arises in the construction of hydraulic presses concerns those of the type wherein a booster cylinder is provided for producing a positive rapid advance stroke of the press. It is desirable, of course, to have substantially the same speed during the rapid advance stroke and during the return stroke of the platen. This requirement has heretofore necessitated the use of a separate booster cylinder and a separate is stored in the .main cylinder and causes delepull-back cylinder having substantially equal areas. It is an object of the present invention, therefore, to provide a power transmission system for a hydraulic press wherein a single differential area cylinder and piston may be utilized for acting both as a booster cylinder and as a pull-back cylinder while retaining substantially equal speeds on the rapid advance and pull-back strokes.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawings wherein a preferred form of the present invention is clearly shown.

In the drawings:

Figure 1 is a diagrammatic view of a hydraulic power transmission system embodying a preferred form of the present invention.

Figure 2 is a fragmentary view corresponding to a portion of Figure 1 showing an alternative construction.

Figure 3 is a diagrammatic view of the main and auxiliary control panels forming a portion of the circuit illustrated in Figure 1.

Figure 4 is a diagrammatic view of the alternative auxiliary control panel illustrated in Figure 2. n

, Figure 5 is a diagrammatic layout of the hydraulic circuit of Figure 2 with a tabulation of the positions of the various valves at different stages of the operating cycle.

Referring now to Figure 1, there is illustrated a press having a main cylinder I0 in which is slidably mounted a main ram I2 carrying a press platen I4. Secured to the top of the cylinder I0 is a combined booster and pull-back cylinder I6 aving a diiferential piston I8 therein and con ected to the main ram I2 by the rod 20. Mounted on top of the cylinder I0 is a surge tank 22 which encloses the cylinder I6 as well as a pair of surge valves 24 of any suitable construction.

A pump 26 which is shown as of the variable displacement, unidirectional delivery type is adapted to be driven by a suitable prime mover, such as an electric motor 28. The pump 26 is preferably flange-mounted in one wall of the tank 22, the motor 28 being supported outside the tank, but for convenience in illustration is shown at the lower part of the figure. A suction conduit 30 connects the suction port of the pump with the surge tank 22 while the delivery conduit- 32 connects the delivery port of the pump with a main control panel 34.

A pump stroke regulator 36 is provided for shifting the yoke 38 of the pump toward neutral position in response to a predetermined increase in pressure in the delivery line. For this purpose the regulator 36 is connected by a conduit 40 with an auxiliary control panel 42. conduit 44 also connects the regulator with the tank 22 through a tank conduit 46. It will be understood that where the pump 26, regulator 36 and control panels 34 and 42 are positioned in the tank 22 that the tank conduits 30, 44 and 46 may be omitted or at least be very short.

The main control panel 34 is provided with a pressure port 48 and a tank port 50 to which the conduits 32 and 46 are respectively connected. It is also provided with a booster cylinder port 52, a main cylinder port 54 and a pull-back cylinder port 56 which are connected with their respective cylinders by conduits 58, 60 and 62.

The auxiliary control panel 42 is provided with three push- type solenoids 64, 66 and 68 adapted to operate certain valves within the panel and which will be later described. An electric control circuit is provided for controlling the energization of the solenoids and comprises a normally-open, two-circuit, starting push button switch 10 which, when depressed, establishes a A drain connection from line L to the solenoid 64 by a conductor I2 and thence to line L. The switch I0 also establishes a circuit from line L by a energized to establish a branch circuit from line L through a normally closed limit switch 18, conductor 80, normally closed emergency stop switch 82, and conductor 84. This circuit is in parallel with the circuit I2 and when completed permits the release of the starting switch 10. The limit switch 18 is arranged to be opened by a cam 86 carried by the platen I4 when the platen reaches the upper limit of its return stroke. A normally open, momentary contact, emergency reverse switch 88 is adapted when depressed to establish a circuit from line L by a conductor 90 to the solenoid 68 and thence to line 1?. p

7 Referring now to Figure 3, the internal construction of the main panel 34 and the auxiliary panel 42 is there illustrated. The pressure port 40 of panel 34 is in communication by a conduit 92 with a relief valve 94 of any suitable type serving as a maximum pressure limiting valve in the main pump delivery line. The valve illustrated is similar to that described in the patent to Harry F. Vickers, 2,043,453, and operates to open a by-pass 96 leading to the tank port 50 whenever a predetermined pressure is exceeded in the conduit 92. The maximum pressure limiting action of the valve is controlled by a pilot relief valve 98 connected to the control chamber I00 of the valve 94.

The conduit 92 extends to a spring-loaded resistance valve I02 from which a conduit I04 extends. The valve I02 is adapted to maintain a predetermined smiall pressure difference between conduits 92 and I04 such that a small pressure is always available at the conduit 92 for the purpose of operating pilot control valves in the panel 34.

The conduit I04 extends to the pressure port I06 of a main four-way valve !08. Valve I00 is provided with a centering spring IIO of usual construction and is adapted to be shifted by the admission of pilot pressure to the end chambers H2 and H4 through pilot control conduits H6 and H8. The valve is illustrated in its central or neutral position. A conduit I20 extends from the tank port 50 of the panel 34 to a tank port I22 of the valve I08.

Cylinder ports I24 and I26 of valve I08 are connected to conduits I28 and I30. The former leads through a pressure responsive sequence valve I32, which is similar in construction to the relief valve 94, to the booster cylinder port 52. The valve I32 is adapted to open when a predetermined pressure exists in conduit I28 and, when open, to connect that conduit with a branch. conduit I34 leading through a resistance valve I36 to the main cylinder port 54. A parallel branch conduit I38 extends through a check valve I40 permitting free flow from the port 54 to the port 52. The conduit I30 extends through a check valve I42 and through a pressure responsive foot valve I44 to the pullback port 56. The valve I44 is also similar in construction to the valve 94 and serves to normally close a bypass conduit I46 in parallel with check valve I42. When the pressure in conduit I30 exceeds a predetermined value which may be slightly above the gravity load of the platen I4, the valve I44 will open permitting flow from port 56 to port I26 through the by-pass I46.

The spool I41 of valve I08 is provided with a cut-away land I49 which registers with port I24 in central position of the spool but is of insufllcient width to block flow between ports I06 and I22 in this position. A blocking land II registers with port I26 in central position and seals this port. End lands I53 and I55 serve to isolate the end chambers H2 and H4 from any communication with the main ports of the valve and act as pistons for shifting the valve. A central bore I69 connects the spaces or opposite sides of land I5I permitting flow from port I26 to port I 06 when the spool is shifted to the left.

For the purpose of operating the surge valves there is provided a branch conduit I51 leadin .from conduit I30 to a port I59. The latter is connected by a conduit I6I to the opening cylinders of the surge valves 24. A branch conduit I63 leads from conduit I34 to a port I65 and thence by a conduit I61 to the closing cylinders of surge valves 24.

The auxiliary control panel 42 comprises two pilot valves I48 and I50 for controlling the flow of fluid to the pilot connections I I6 and I I8 forthe main four-way valve I08. The pilot valve I48 has a shiftable spool I52 normally spring pressed downwardly and adapted to be lifted by the solenoid 64 whenever the same is energized. The valve I50 has a spool I54 adapted to be shifted upwardly by the solenoid 66 when energized or downwardly by the solenoid 68 when energized. A spring-pressed detent arrangement I55 maintains the spool in either one .of its two possible positions.

The valve I48 constitutes in effect an on-off valve for the supply of pilot fluid while the valve I50 is connected to operate as a four-way reversing valve. Accordingly the valve I48 is provided with a pressure port I56 which is connected by a conduit I58 with the pressure conduit 92 in the main panel 34. A second port I60 connects by a conduit I62 with the pressure port I64 is connected to the conduit 40 (see Figure 1) for actuating the pump regulator 36 at the same time that valve I50 is shifted downwardly.

For the purpose of permitting return flow from branch conduit 204 connects the port I88 with the drain conduit I14 through a restriction 206 which is not suflicientlylarge to handle the en-.

tire discharge therefrom when the valve lifts.

For the purpose of introducing a variable time delay in the operation of reversing the press, the lower end chamber 208 of valve I50 is drained through an adjustable restriction 2I0 leading to the drain conduits I12 and I14. A check valve 2'I2 permits free flow around the restriction 2I0 in the opposite direction when valve spool I54 is of valve I50. The lower cylinder port I66 of a valve I is connected to conduit H6. The upper cylinder port I68 is connected to conduit II8 which for sake of clearness is only partially illustrated in Figure 3. The lower tank port I10 of valve I50 is connected by a conduit I12 with a second conduit I14 leading to the tank conduit I20 in the main valve block. Conduit I12 extends beyond the port I10 b,v branch portions I16 and I18 to the tank port I19 and the lower end chamber I8I, respectively, of the valve I48. The upper tank port I of valve I50 is connected by a conduit I82 with theconduit I28 in the main valve block which during thepressing stroke is under pressure and during the return stroke is connected to tank.

The upper end chamber I84 of the .ralve I50 serves as a pilot operating chamber to shift the spool I54 downwardly at the time of reversal of the press. For this purpose the chamber I84 is connected by a conduit I86 with the discharge port I88 of a pressure responsive valve I90 which may be similar. in construction to the valve 94. The valve I90 is connected by a conduit I92 with the main cylinder port 54 of the main panel 34. Thus whenever the pressure in the main cylinder rises to a value de ermined by the setting of the pilot valve I96 of valve I90, pressure oil is admitted through conduits I92 and I86 to the pilot chamber I84 of valve I50. A branch conduit I98 lifted by solenoid 66. The adjustable restriction 2I0 may be provided with a sprocket wheel 2 I 4 for remote adjustment adjacent the operator's station by means of a chain and sprocket drive 2 I6 (see Figure 1). A similar chain and sprocket drive 2I8 may be provided for the adjusting screw 220 for the pilot valve I96 ofthe pressure responsive valve I90.

Since the valve I90 determines the pressure or tonnage at which reversal of the press takes place and since this tonnage requires adjustment for different jobs which a given press may be required to do, convenient means for adjusting the same are provided. Likewise the variable restriction'2I0 determines the length of'dwell of the press before reversal takes place and requires frequent adjustment for-various jobs. The adjustments for the pressure responsive valve 94, I32 and I44 normally do not require change after the press has been once set up.

In operation, starting with the position illustrated in Figures 1 and 3 with the motor 28 driving the pump 26, fluid is withdrawn from the tank through conduit 30 and delivered by the pump through the conduit 32 to port 48 of the main panel 34. The entire delivery of the pump is by-nassed through resistance va ve I02, conduit I04, port I06, and across the cut-away land I49 to port I22 from which the flow returns to tank through conduit I20, port 50 and conduit 46. in the lower end of cylinder I6 from escaping to tank through conduit 62, port 56, and conduit I30. The check valve I42 and the pressure responsive valve I44 also prevent any outflow from thelower end of cylinder I6 except at a pressure sufliciently high to open the valve I 44, which pressure is not created by the normal gravity load of the platen.

' If now the starting switch 10 be depressed. s0- lenoid 64 will be energized to lift valve spool I52, and soleno d 66 will be energized to lift valve spool I54. Under these conditions pressure fluid is delivered from conduit 92 ahead of theresistance valve I 02 through conduit I58. ports I56 and I60,

conduit I62. ports I64 and I68, and conduit 8 to the end chamber II4 of the main valve I08. The valve spool is accordingly shifted to the left blockingthe by-pass path between port I06 and I22 and admitting full pump delivery from conduit I04 to conduit I28 and through port 52 and conduit 58 to the head end of cylinder I6. The piston I8 and main ram I2 accordingly descend at a rapid rate, the surge valve 24 operating as The blocking land I 5| locks the fluid trapped check valves and freely opening to admit oil from the surge tank 22 to the main cylinder 50.

Oil discharged from the lower end of cylinder I6 flows through conduit 62, port 56 and conduit I30 to valve I44. As soon as suflicient pressure is built up, valve I44 opens permitting the flow to continue through conduit I46 to port I26 and through the central passage I69 to joint with the pump delivery at port I06. Thus the entire discharge from the rod end of cylinder I6 is returned to the head end thereof making the effective area of cylinder I6 equal to the area of the rod 20 only. Thus if equal speeds on th rapid advance and return strokes are desired, the piston I8 is made with an area double that of the rod 20. At the time the valve spool I41 shifted to the left, oil was discharged from end chamber II2 through conduit II6, port I66, port I10, and conduits I12, I14 and I20 to tank.

As soon as the platen meets resistance sufiicient to open the pressure responsive valve I32, the latter lifts admitting pressure oil to the conduit I36. Before the resistance valve I 36 is opened, oil is admitted through branch conduit I69, port I66, and conduit I61 to the closing chambers of surge valves 24 thus positively closing the same. As soon as these valves are closed, pressure is built up further to open the resistance valve I36, thus admitting the pump delivery to the main cylinder l in addition to the cylinder I6.

The main pressing stroke, thus begun, continues until sufficient pressure builds up in the pump delivery line to open the valve I90. This pressure is transmitted from port 54 through conduit I92 to the valve I90 which then lifts admitting oil through conduit I86 to the pilot operating chamber I84 of valve I50. The latter is accordingly shifted downwardly at a rate determined by the setting of the adjustable restriction 2I0 which determines the rate at which oil can be discharged from the lower end chamber 208 to the tank.

At this point it may be noted-that the solenoid 86 is no longer energized since the starting switch will normally be released by the oper. ator as soon as the platen I4 starts downwardly and lets cam 86 ride out from under the limit switch 18, thus permitting the latter to close. At this point the holding circuit 84 is now completed, thus permitting the solenoid 64 to remain energized without continued depression of the starting switch 10.

When the valve I50 is shifted fully downwardly, both end chambers II 2 and H4 are momentarily connected to equal pressures. The chamber H2 is connected to full pump delivery pressure from conduit 92, through conduit I58, valve I48, conduit I62, ports I64 and I66, and conduit II6. Chamber H4 is connected to the same source of pressure from conduit 92, through resistance valve I 02, conduit I04, valve I08, conduit I28, conduit I82, ports I80 and I68, and conduit II8. In this connection it will be noted that the resistance valve In: has a substantial area which is exposed to the pressure in conduit I04 so that whenever the pressure in conduit I04 builds up to a significant value, the resistance valve I02 opens fully to impose no restriction on the flow therethrough. Thus the resistance valve operates as such only when the conduit I04 is at substantially zero pressure. Accordingly, since the pressures on both ends of the valve spool I41 are equal, even though high, the spring IIO takes assures control and brings the spool to its central position.

At the time that pressure responsive valve I opened, pressure oil was also admitted through conduit I86, branch conduit I98 and conduit 40 to the pump stroke regulator 36. The regulator piston accordingly is moved up to bring the adjustable yoke 30 of the pump nearly into neutral position. Thus, the rate of pump delivery is reduced to correspond with the requirements during the dwell at the end of the pressing stroke.

As soon as valve spool I41 approaches center position, the land I49 begins to open to flow from port I24 across to port I22. The tremendous energy which is stored in the main cylinder I0 is thus gradually relieved to tank through conduit 60, port 54, conduit I38, check valve I40, conduit I28, ports I24 and I22 and conduit I20 to tank. As soon as the pressure at port I24 is fully relieved which in actual practice requires only a fraction of a second, the pressure in end chamber H4 is relieved correspondingly, thus permitting the superior pressure in end chamber II2 to complete the movement of spool I41 to the right to cause the return stroke of the press. Under these conditions pump delivery is directed from port I06 to port I26 and through conduit I30, check valve I42, port 56 and conduit 62 to the lower end of cylinder I6. The piston I8 is accordingly lifted discharging oil from the head end of cylinder I6 through conduit 58, port 62, conduit I26, ports I24 and I22 and conduit I20 to tank. Pressure oil from conduit I30 is also delivered through branch conduit I51, port I59, and conduit I6I to the opening chambers of surge valves 24 thus positively opening the same permitting oil to be displaced from main cylinder I0 back to the surge tank 22.

As soon as the platen reach its fully returned position, cam 86 opens limit switch 18 thus deenergizing solenoid 64 permitting valve I48 to move downwardly under its spring bias cutting off the pressure supply from the end chamber H2 and connecting the same to tank through conduit I I6, ports I66 and I64, conduit I62, ports I60 and I19, and conduits I16, I12, I14 and I20. Since the right-hand end chamber I I4 remains connected to tank as before, the spring IIO again brings the spool I41 to center position, the parts having been thus restored to the position of rest.

If, while the platen is descending on a pressing stroke, an emergency condition should demand immediate reversal of the press, the emergency reverse switch 88 may be depressed to energize solenoid 68, thus shifting the valve spool I54 downwardly before the full working pressure is built up in the press. Should it be desired merely to stop the press without reversal in the middle of a pressing stroke, the emergency stop switch 82 may be depressed, opening the holding circuit 84 for solenoid 64 and permitting the valv spool I52 to drop, thus shifting the valve I08 to neutral position in the manner previously described.

The four pressure responsive valves 94, I44, I32 and I90 have been described as similar to one another, but there are certain differences in their operating connections which should be pointed out.

The valve 84 has the discharge side of its pilot valve 90 connected to tank through the interior bore of the main valve 84. Thus the pilot valve 98 is responsive to pressure difference between control chamber I00 and tank. The valve 94 is arranged to be responsive also to the pilot valve I06 for pressure responsive valve I90 since it has been found that this assists in .obtaining a smooth reversal of the press and insures against momentary pressure peaks in the pump delivery line due to the small but appreciable time interval required for shifting the pump regulator 36. For this purpose a conduit 222 extends between the control chamber I and the inlet side of pilot valve I96 thus connecting together the control chamber of both valves 94 and I90. The adjustment of pilot valve 98 is normally the highest adjustment of any of the pilot relief valves in the system and serves merely as an over-all safety pressure limiting control in case of accidental derangement of the system. l

The valves I32.and I44, since they discharge to passages which are not always at'tank pressure, have their pilot valves arranged with their discharge sides connected to tank by drain conduits 224 and 226 which lead by a connection228 to the tank conduit I20. Thus the valves I32 and I44 open when the pressure in their control chambers reaches a .predetermined level above tank pressure which level is ordinarily relatively low compared to the opening pressures of valves I90 and 94. Since at the time thesevalves are open their discharge conduits I34 and I46 respectively connect to points of high pressure, it will be seen that the action of these two valves is to open wide as soon as they begin to lift and to remain open until the pressure in their respective discharge conduits falls below the value at which the valves opened.

The action of pressure responsive valve I90 is similar to that of valves I32 and I44 since its pilot valve I96 discharges to tank rather than to the main discharge port I88 of the valv I90.

able'displacement pump 26 a fixed displacement pump 254 is illustrated as being connected in between the suction conduit 30 and the delivery conduit 32. The spool 232 may be operated by a .depending rod 256 extending downwardly along side the platen and having a collar 258 adapted to be lifted bya projecting member 260 on the platen when the latter reaches the upper limit of its stroke. The lower end of the rod is connected for hand operation with a lever 262.

The operation of the device with the alternative auxiliary panel provided is the same as that' previously described except that the operator manually depresses the lever 262 to raise the rod 256 and valve spool 232, thus establishing the same connections'as those described when valve v engages the collar 258, thus, lifting the rod 256 In Figures 2 and 4 there isillustrated an alternative auxiliary control panel 230 which may be used in place of the control panel 42 for installations where electric push button control is not desired and a hand lever control is satisfactory. In the alternative panel 230 only a single valve 232 is provided which is arranged to combine the functions of valves I48 and I50. For this purpose the main pressure port 234 connects by a conduit 236 with the pump delivery conduit 92 in a manner analogous to the conduit I58. The cylinder. ports 238 and 240 connect with the conduits H6 and H8 respectively. The lower tank port 242 connects with the tank conduit I12 while the upper tank port 244 connects with the conduit I82.

The spool 232 has its two middle lands provided with notches 248 which connect lines H6 and II8 with the tank ports 242 and 244 when the spool is in central position illustrated, the pressure port 234 being blocked. When the valve is shifted either upwardly or downwardly, the same connections are established as those established by the valve I50 previously described. The upper pilot operating chamber 250 corresponds with the operating chamber I84 and is connected with the conduit I86, the branch I98 being plugged where a fixed displacement pump is utilized for driving the system. The lower chamber 25I corresponds to the chamber 208 in Figure 3. The valve spool is provided at its upper end with the usual spring-loaded detent mechanism 252 for lightly retaining the valve spool in any of its three possible positions.

As shown in Figure 2, the alternative control panel 230 cooperates with the main panel 34 to provide a control for the main hydraulic circuit in the same manner as previously described in connection with Figure 1. In place of the variand spool 232.to center position in which both end chambers of the main valve are connected to tank through the slots 248.

Figure 5 is a diagrammatic layout of the hydraulic circuits shown using the alternative control panel 230 wherein the various valves are represented merely as circles with the flow conditions therethrough at various stages of the operating cycle being indicated in the table immediately below each valve.

While the form of embodiment of the invention as herein disclosed constitutes a preferred form, itis to be understood that other forms might be adopted, all coming within the scope of the claims which follow.

What is claimed is as follows:

1. In a hydraulic powertransmission system for operating a press or the like the combination with a source of pressure fluid, and a reversible fluid motor of a main valve for controlling the direction of fluid flow to the motor and having a pair of pilot-operating chambers for selectively shifting the valve in'opposite directions between forward and reverse positions, spring means for shifting the valve to a neutral position when the pressures in said chambers are equal, said valve in' neutral position being arranged to exhaust one end of the motor, and pilot valve means for selectively directing pressure fluid from said source to either of said pilot-operating chambers and connected to simultaneously exhaust the other of said pilot-operating chambers, the exhaust line from one of said chambers being direct while the exhaust from the other pilot-operating chamber is connected to said one end of the motor whereby, when the pilot valve means is shifted to reverse the main valve,'the main valve may be shifted to neutral position by said spring and momentarily prevented from shifting to redirection of fluid flow to the motor and having a pair of pilot-operating chambers for selectively shifting the valve in opposite directions between forward and reverse positions, spring means for shifting the valve to a neutral position when the pressures in said chambers are equal, said valve in neutral position being arranged to exhaust one end of the motor, pilot valve means for controlling the flow of fluid to and from said chambers to shift the main valve selectively to forward or reverse position, and means connecting saidpilot valve means to said chambers and to said source and motor to provide momentarily balanced pressures in both the chambers when the pilot valve means is shifted to cause the main valve to shift to reverse position, whereby the main valve may be shifted to neutral position by said spring.

3. In a hydraulic power transmission system for operating a press or the like the combination with a source of pressure fluid, and a reversible fluid motor of valve means for controlling the direction of fluid flow to the motor, a separate valve for selectively causing the delivery of fluid from the source to the motor or interrupting such delivery, electric means including a holding circuit for operating the second valve to start a cycle of forward and reverse motor movements and to stop the motor at the end of the cycle, electric means for shifting the first valve means to cause a forward stroke of the motor, and hydraulic means responsive to pressure built up at the end of a forward stroke of the motor and acting directly on said first valve means for initiating a reverse stroke of the motor independently of either electric means.

4. In a hydraulic press of the type having a surge tank located near the top thereof the combination of a pump, and a directional flow control assembly mounted adjacent the tank, said control assembly including self-contained, electrically-actuated means for initiating, maintaining and terminating a complete cycle of operation including a pressing stroke and a return stroke, self-contained, hydraulically-actuated means for reversing the flow to the press in response to the building up of pressure at the end of a pressing stroke, self contained, electrically actuated means for reversing the flow to the press independently of pressure, and switch means positioned remote from'the tank and adjacent an operator's station for controlling said electricallyactuated means.

5. In a hydraulic press of the type having a surge tank located near the top thereof the combination of a pump, and a directional flow control assembly mounted adjacent the tank, said control assembly including self-contained, electrically-actuated means for initiating, maintaining and terminating a complete cycle of operation including a pressing stroke and a return stroke, self-contained, hydraulically-actuated means for reversing the flow to the press in response to the building up of pressure at the end of a pressing stroke, switch means positioned remote from the tank and adjacent an operator's station and a circuit for initially energizing said electricallyactuated means, a parallel circuit for maintaining said electrically-actuated means energized independently of said switch means, and a second switch means controlled by the return of the ram to its idle position for interrupting said parallel circuit.

6. A pressureregulating system for a hydraulic power transmission circuit having a pump, a

' hydraulic motor, a shiftable control member for determining the flow between the pump and the motor, and a pilot-controlled relief valve for bypassing the fluid delivered by the pump, said system comprising a servomotor for operating said member, a second pilot-controlled relief valve for controlling the flow of fluid to operate said servomotor, a pilot relief valve and means connecting said pilot relief valve to both pilot-operated relief valves for joint control thereof.

7. Ice hydraulic power transmission system for operating a press or the like the combination with a source of pressure fluid, and a reversible fluid motor of means for controlling the direction of fluid flow to the motor including a valve arranged to exhaust one end of the motor in neutral position of said means, and pilot control mechanism for shifting said means to a first position in which pressure fluid is directed to said one end of the motor, and to a second position in which pressure fluid is directed to the opposite end of the motor, said pilot control means including a fluid pressure connection from the pump eifective to cause shifting tosaid second position, another fluid pressure connection to said one end of the motor effective to oppose such shifting, and means for centering said firstnamed means in neutral position when pressures in both connections are acting in opposition to one another, said other connection acting to prevent shifting past neutral position until the pressure in said one end of the motor has fallen to a predetermined value by exhausting through said valve.

FERRIS T. HARRINGTON. RALPH L. TWEEDALE.

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