US3267231A

US3267231A - Pressure switch and relief valve combination

Info

Publication number: US3267231A
Application number: US422713A
Authority: US
Inventors: James K Douglas
Original assignee: Oilgear Co
Current assignee: Oilgear Co
Priority date: 1964-12-31
Filing date: 1964-12-31
Publication date: 1966-08-16
Anticipated expiration: 1983-08-16

Description

.1.` K. DOUGLAS 3,267,231

PRESSURE SWITCH AND RELIEF VALVE COMBINATION Filed nec. s1, 1964 Aug. 16, 1966 United States Patent O 3,267,231 PRESSURE SWITCH AND RELIEF VALVE l COMBINATION .lames K. Douglas, Milwaukee, Wis., assignor to The Olgear Company, Milwaukee, Wis. Filed Dec. 31, 1964, Ser. No. 422,713 4 Claims. (Cl. 200-82) This invention relates to a pressure switch of the micro-movement type that is utilized in combination with a fluid pressure relief valve which are constructed and arranged to be fast acting and operatively coordinated with respect to each other.

For an application such as a mechanical blanking press that employs a hydraulic cylinder and a pressure relief valveto limit the pressure in the cylinder for protection of the press, it is advantageous to utilize a pressure switch to shut down the press. Such an application requires that the pressure switch be fast acting and coordinated with the pressure setting of the relief valve.

Such a mechanical' blanking press may operate, for example, with an eight-inch stroke and at thirty strokes per minute on 1/32 inch material, may have a safety cylinder precharged to 85 percent of the pressure setting of the relief valve, which may be sufficient so that the blank forming operation does not raise the pressure sufficiently to open or blow the relief valve. However, more than normal displacement of the safety cylinder, as occurs when more than one blank is simultaneously fed into the die of the press, would cause the work to be engaged earlier by the extra thickness of the work and displace the plunger or cylinder accordingly during the work portion of the stroke.

Normally, the work portion of the stroke, between engagement with the blank and completion of the stroke occurs within an interval of 40 milliseconds. In the event of two blanks superposed on the die, the work engage- 40 milliseconds for completion of the work stroke.

It is desirable that the press be stopped before another vwork forming operation takes place so that the defective blank material can be removed from the die and the error causing the difficulty corrected. A relay controlling the press must therefore be operated within about 40 milliseconds. Since suitable relays require on the order of l milliseconds to drop out when disconnected from an energization circuit, the switch for disconnecting the relay must therefore` be operated within about 30 milliseconds of the blowing of the relief valve, for the reason that upon a return stroke of the press the work force on the safety -cylinder is removed and the excess pressure relieved in extending the safety cylinder.

According to 4an embodiment of the invention a differential pressure switch is responsive to differential pres- ',sure between the line or inlet of a relief valve and the control pressure for the relief valve so as to be effective either to operate the switch immediaely upon occurrence of a predetermined differential pressure or after a time delay. Including a short time delay is preferable and permits coordinating the switch operation with the blowing of the relief valve, anticipating the blowing of the relief valve, or occurring a short interval of time after blowing of the relief valve. In each of these cases the pressure switch remains automatically operative for different pressure settings of the relief valve.

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It is an object of the invention to provide a pressure relief valve and a control actuator responsive to a differential pressure of the relief valve to effect a control operation.

Another object of the invention is to provide a combination of pressure relief valve and a pressure switch whose operations are coordinated with each other for different pressure settings of the pressure relief valve.

Another object of the invention is to provide a fast` acting pressure switch in combination with a pressure relief valve both responsive to differential pressure across a control piston for the relief valve. A further object thereof is to make the pressure switch responsive to a slightly lower differential pressure than is required to operate the relief valve. And an additional object is to provide a delay in the response of the pressure switch to the predetermined differential pressure of the relief valve.

Another object of the invention is to provide a differential pressure responsive switch actuator responsive to a predetermined differential pressure and adapted for connection across a control piston of a pressure relief valve, and including means limiting the rate of iiow of line pressure fluid to the actuator. It is also an object to provide, in combination with the means limiting the rate of flow of line pressure fluid to the actuator, an actuator stroke adjustment to selectively vary the time interval for operation of the pressure switch upon occurrence of the predetermined differential pressure across the control piston of the pressure relief valve.

Other objects and advantages will be apparent from the description of an embodiment of the invention taken wit the drawings, in which:

The figure is a schematic illustration of a differential pressure responsive switch and a relief valve embodying the combination of the invention.

Referring to the drawing, by characters of reference, la combination of a differential pressure responsive switch 1 and a differential pressure responsive relief valve 2 is shown operatively connected for controlling an electric circuit 3. Electric circuit 3 is open when switch contacts R-2 are open. A control circuit therefor comprises a relay R which when energized closes lcontacts R-1 ina holding circuit for relay R and closes switch contacts R- Z in circuit 3. Relay R is connected in series with switch contacts C-l of a micro-movement switch 4.

The micro-movement switch 4 has a snap-acting switch member that carries a switch contact C-2 which would normally be in contact with fixed contact C-3 but the switch member is displaced by the actuator 5 to normally hold contacts C-2 and C-1 in engagement.

The micro-movement switch 4 employed is characterized in having a normal pretravel of .015 inch, a normal travel of .005 inch, and a normal overtravel of .060 inch, in the arrangement shown in the figure. The micro-movement switch thus requires displacement of a plunger or piston 6 of the actuator 5 of a few thousandths of an inch to cause the switch member to be displaced from one position to another.

The actuator S comprises a cylinder 7 with a substantially hydrostatically balanced piston 6 urged by a spring 8 toward one end of the cylinder so that a plunger pin 9 holds the switch contacts C-1 and C-2 closed. The piston 6 is subject to displacement by differential fluid pressure thereacross that overcomes the force of the bias spring 8 to permit opening of switch contacts C-1, C-Z and thereby de-energizng relay R in the control circuit. The actuator piston 6 is displaced, lifted in the figure, by a differential pressure on the order of 15 p.s.i.

The actuator cylinder 7 has a so-called high pressure chamber 11 at the plungerl end of the piston 6 which is connected by a port 12 adapted for connecti'on'to a high pressure or inlet chamber of the relief valve 2. A choke f chamber 14 connected to a port 15 adapted for connection to a low pressure or control chamber of the relief valve without any flow limiting restriction therebetween.

The actuator cylinder 7 has a removable end head 16 .fitted with a screw plug 17 for easy access to a stroke adjusting screw 18 which is threaded in the bottom of the cup shaped piston 6 which abuts the end wall of the `actuator cylinder. The plunger pin 9 is secured to the screw 18 so as to provide a narrow longitudinal extension thereof that projects through the lower end of the cylinder 7 in a suitable manner.

The micro-movement switch 4 has a movable contact member C-2 that is normally held in displaced position with contacts C-1 and C-2 closed by the plunger pin 9. A housing for the switch 4, illustrated in outline form, is suitably mounted in a known manner to the actuator cylinder 7, with the plunger pin extending through and in sealing engagement with a diaphragm, not shown, to provide friction free movement of the plunger pin and to isolate uid from the micro-movement switch 4.

The stroke adjusting screw 18 may be tensioned by a spring, not shown, to maintain the screw in a fixed setting. The screw 18 is threaded so as to provide a travel or stroke adjustment within the range of the piston travel which is 1/16 inch. The switch 18 is turned inward to a position where the micro-movement switch indicates that contacts C-1 and C-2 are closed. Turning the screw 18 inward an additional amount increases the required lift movement of the piston 6 to open the switch. Turning the screw 18 outward decreases the required piston move- `ment to open the switch. The pressure switch is constructed and arranged, taking into consideration the stroke adjustment and the flow restriction provided by choke 13, so that using a starting position of the screw turned inward until the switch contacts Cel and C-Z are closed, the stroke adjusting screw 18 should be turned inward by an angular rotation of 15 to 360 degrees to obtain a desired range of time delay. At least 15 degrees inward rotation from the starting point is required to obtain reliable repeat action. At this setting the switch will be very sensitive and will open on a sudden rise in line pressure preceding the blowing of the relief valve. At 360 degrees of inward rotation of the screw 18 from the starting point, the switch will open about one-tenth of a second after the relief valve blows. The preferred adjustment is a setting of the adjusting screw of about 180 degrees of inward rotation from the startin-g point, but different settings between 15 and 360 degrees will be suitable for diierent applications.

Thus the adjusting screw 18 is adjusted to utilize an overtravel characteristic of a conventional micro-movement switch whose normally open contacts C-l and C-Z are normally closed for the normally depressed position of the actuator piston 6, and, vice versa, whose normally closed contacts C-2 and C-3 are normally open in this arrangement.

The relief valve l2, is a hydrostatically pressure balanced and pilot operated type such as is shown and described in applicants United States Patent 3,083,727. Relief valve 2 comprises a valve body 20 with a valve chamber connected by an inlet port 21 and an outlet port 22. A valve member comprises a control piston 23 in the valve cha-mber and an additional small piston 24 in a balance charnber beyond the valve seat on the discharge port side thereof. The small piston 24 is integrally connected by a stern and plunger 25 to the control piston and has a diameter that is substantially equal to the plunger diameter. A passage 26 extends through the valve member so that opposite end areas of the control piston and small piston are interconnected so as to provide hydrostatic balance to the valve member. The plunger 25 is held against a valve seat 27 by a spring 28 that acts on the control piston 23 so as to normally interrupt communication between the inlet and

outlet ports

21, 22 of the relief valve 2. vThe spring 28 requires a differential pressure across the control' piston of 20 p.s.i. to overcome the force of the spring that holds the valve closed.

The control piston 23 divides the valve chamber into a high pressure or inlet chamber 29 open to inlet port 22 and a low pressure or control chamber 30 in which fluid pressure acts to hold the valve closed. A choke or flow restriction passage such as is formed by an axial slot in the periphery of the control piston 23 interconnects the control Vchamber 30 with the inlet chamber 29 for'admitting inlet pressure uid at a limited rate, so that a differential pressure will develop across the control piston for a sudden rise of inlet pressure or upon inlet pressure exceeding the pressure setting of a pilot valve 32, which includes an adjustable tensioned spring for varying the pressure setting thereof. The pilot valve normally blocks communication between control chamber 30 and a drain or return passage 33.

The relief Valve is provided with a port 34 open to inlet chamber 29 and adapted for connection to port 12 of the actuator cylinder, and is provided with a port 35 open to the control chamber and adapted for connection to the port 15 of the actuator cylinder. The relief valve body 20 and the actuator cylinder 7 are constructed and arranged for mounting together so that their

respective ports

34, 35 and 12, 15 are aligned and interconnected, so that the actuator cylinder 7 and piston 6 are hydraulically connected across the control piston 23 of the relief valve 2; whereby pressure fluid 0f a line to which the relief valve is connected is admitted through choke 13 to the lift chamber 11 of the accumulator and through choke 31 to the control chamber 30 of the relief valve and to hold-down chamber 14 of the actuator.

In the operation of the combination relief Valve and pressure switch, the relief valve is adapted to have its inlet port 21 connected to a hydraulic device, line, or system, such as a safety cylinder, not shown, for a blanking press, and to have its outlet port 22 connected to a return line or drain, so thatthe relief Valve limits line pressure in accordance with its pressure setting. With the pilot valve set forth 1000 p.s.i., for example, the pilot valve will open to limit pressure in the control chamber 30 to y 1000 p.s.i., but under static conditions a differential pressure of 20 p.s.i. across the control piston to open the relief valve occurs when line pressure is 1020 p.s.i. Under such static conditions, the actuator piston would be actuated when line pressure attained 1015 p.s.i., and would therefore be actuated always before the relief valve opens for substantially static conditions or for gradually increasing line pressure that exceeded 1015 p.s.i. For such gradual changes in line pressure, the actuator and the relief valve can be coordinated to operate simultaneously or nearly simultaneously only if the spring 8 for the actuator is changed to require the same differential pressure to displace it as is required for the spring 28 for the relief valve.

Usually, however, operating conditions requiring relief valve protection include rather sudden rises in line pressure for which the stated 20 p.s.i. and 15 p.s.i. differen- `tial pressure necessary to displace the control piston of the relief valve and the actuator piston, respectively, are

preferred to provide better control of the timing of their- Vsure switch less sensitive to transient pressures while operatively responsive to positive pressure rises. While with- 4out choke 13 the pressure switch would always operate before the relief valve could open, the choke 13 is chosen with a flow limiting characteristic relative to that of choke 31 for the relief valve so as to generally delay pressure switch opening within milliseconds of opening of the relief valve for line pressure increasing rapidly.

It should be noted that timing of pressure switch operation with respect to blowing of the relief valve is substantially independent of changes in the pressure setting for blowing of the relief valve, so that their operation remain coordinated in accordance with a predetermined timing for any pressure setting of the relief valve.

It should also be noted that in the embodiment of the invention illustrated that the slight hydrostatic unbalance of the valve member for the relief valve and the slight hydrostatic unbalance of the actuator piston, due to the plunger being slightly larger than the valve seat and the actuator piston having a small cross-section pin, respectively, are each unbalanced in the same operative directions and in the same ratio of lift area to hold-down area of their respective pistons, so that the above referred to timing is maintained for different pressure settings of the relief valve.

I claim: 1. A pressure switch in combination with a pilot operated relief valve operable to limit line pressure to different selected values:

the relief valve comprising a valve chamber having inlet and outlet ports and a valve member normally interrupting communication between said ports, said valve member comprising a differential area control piston forming an inlet pressure chamber at one end and a control pressure chamber at the other end of said valve chamber, a control spring urging the control piston against the inlet pressure toward valve closed position, and a rst restriction passage interconnecting the opposite ends of the valve chamber for limiting the rate of flow of pressure uid between the inlet pressure chamber and the control pressure chamber; the pressure switch comprising a hydraulic cylinder with an electrical switch mounted on one end of the cylinder and being a precision micro-movement type having a movable contact, an actuator piston in said cylinder for operating the switch, a spring normally holding the actuator piston toward the switch end of the cylinder for holding the switch closed, means selectively adjusting the stroke of the actuator piston necessary for opening the switch, and means connecting the upper end of said cylinder to the lower end of said valve chamber and connecting the lower end of said cylinder to the lower end of said valve chamber, said connecting means including a second flow restriction passage for conducting line pressure fluid at a limited rate to the switch end of said cylinder,

whereby, said second flow restriction passage and said actuator piston stroke adjusting means provide adjustable time delay means for relating time of switch opening to the operation of the relief valve.

2. A pressure switch in combination with a pilot operated relief valve:

the relief valve comprising a valve chamber, a valve member having a control piston within said valve chamber and defining an inlet pressure chamber and a control pressure chamber on opposite sides of said control piston, inlet and outlet ports to said inlet pressure chamber adapted for connection respectively to high and low pressure lines of a hydraulic system, said valve member normally interrupting communication between said ports, a restriction passage continuously interconnecting the inlet pressure chamber and the control pressure chamber, .a control spring urging the valve member toward a normally valve closed position, a pilot valve responsive to the pressure in said control chamber and operative to drain fluid from the control chamber to limit the pressure therein to a predetermined value; the pressure switch comprising a hydraulic cylinder with an actuator piston therein for moving a contact of an electrical switch, the switch end of said cylinder connected for continuous communication with the inlet pressure chamber of the relief valve, the other end of said cylinder connected for continuous communication with the control chamber of the relief valve, a spring urging the actuator piston toward the switch end of the cylinder, the hydrostatic unbalance of said actuator piston when both sides are subject to the same pressure having substantially the same ratio of lift area to hold-down area as that of the valve member of the relief valve, and said spring for said actuator piston providing a bias force per unit lift area for said actuator piston with respect to a bias force per unit lift area of the relief valve provided by the control spring thereof such that the ratio of the bias force per unit lift area of the relief valve to that of the actuator is greater than one which is therefore the ratio of the pressures required to raise the control piston and the actuator piston, whereby the timing of the operation of the pressure switch relative to the operation of the relief valve is independent of a pressure setting for the relief valve.

3. A pressure switch in combination with a relief valve as defined in claim 2 including a flow restriction limiting the rate of flow of line pressure fluid into the hydraulic cylinder tending to actuate the same.

4. A pressure switch in combination with a relief valve, as defined in claim 2, including a flow restriction limiting the rate of flow of line pressure fluid into the hydraulic cylinder tending to actuate the same, and means for adjusting the stroke of the actuator piston to effect operation of the switch so as to relate time of operation of the switch and the relief valve.

References Cited by the Examiner UNITED STATES PATENTS 3,133,475 5/1964 Barkan 200-82 X BERNARD A. GILHEANY, Primary Examiner.

G. MAIER, Assistant Examiner.

Claims

1. A PRESSURE SWITCH IN COMBINATION WITH A PILOT OPERATED RELIEF VALVE OPERABLE TO LIMIT LINE PRESSURE TO DIFFERENT SELECTED VALUES: THE RELIEF VALVE COMPRISING A VALVE CHAMBER HAVING INLET AND OUTLET PORTS AND A VALVE MEMBER NORMALLY INTERRUPTING COMMUNICATION BETWEEN SAID PORTS, SAID VALVE MEMBER COMPRISING A DIFFERENTIAL AREA CONTROL PISTON FORMING AN INLET PRESSURE CHAMBER AT ONE END AND A CONTROL PRESSURE CHAMBER AT THE OTHER END OF SAID VALVE CHAMBER, A CONTROL SPRING URGING THE CONTROL PISTON AGAINST THE INLET PRESSURE TOWARD VALVE CLOSED POSITION, AND A FIRST RESTRICTION PASSAGE INTERCONNECTING THE OPPOSITE ENDS OF THE VALVE CHAMBER FOR LIMITING THE RATE OF FLOW OF PRESSURE FLUID BETWEEN THE INLET PRESSURE CHAMBER AND THE CONTROL PRESSURE CHAMBER; THE PRESSURE SWITCH COMPRISING A HYDRAULIC CYLINDER WITH AN ELECTRICAL SWITCH MOUNTED ON ONE END OF THE CYLINDER AND BEING A PRECISION MICRO-MOVEMENT TYPE HAVING A MOVABLE CONTACT, AN ACTUATOR PISTON IN SAID