US3587392A - Means for controlling the operation of hydraulically operated jacks - Google Patents

Abstract

MEANS FOR CONTROLLING THE OPERATION OF A HYDRAULIC JACK HAVING LOAD SENSING MEANS IN WHICH A DUMPING VALVE AND A CHECK VALVE ARE ARRANGED IN THE HYDRAULIC CIRCUIT SO THAT, IN USE, WHEN THE LOAD SENSING MEANS REGISTER A PREDETERMINED VALUE, THE FLOW OF FLUID TO AND FROM THE RAM IS IMMEDIATELY STOPPED.

Description

United States Patent Hugh Jeremy Willis Edwards Cobham, Surrey, England Jan. 21, 1969 June 28, 1971 Cable Covers Limited. Surrey. England Inventor Appl. No. Filed Patented Assignee MEANS FOR CONTROLLING THE OPERATION OF HYDRAULICALLY OPERATED JACKS 9 Claims, 2 Drawing Figs.

US. Cl. 91/1, 91/420, 91/448, 91/459 Int. Cl ..F0lb 25/26, F15b 15/04, E040 3/26 Field of Search 91/459,

[56] References Cited UNITED STATES PATENTS 1,898,228 2/1933 Thompson 9 l/450X 3,074,384 1 1963 Pilch 91/420 3,469,399 9/1969 Krantz 9 l/459X Primary Examiner-Martin P. Schwadron Attorney-Baldwin, Wight and Brown ABSTRACT: Means for controlling the operation of a hydraulic jack having load sensing means in which a dumping valve and a check valve are arranged in the hydraulic circuit so that,

in use, when the load sensing means register a predetermined value, the flow of fluid to and from the ram is immediately stopped.

MlEAWb W'Olii CONTIMJILMNG THE OPERATIION F iiiWlUtliilAUHCALlLY OPEWATEID JACMS This invention relates to means for controlling the operation of hydraulically operated jacks particularly of the kind used for tensioning wires, rods or strands in prestrcssed concrete structures.

it has previously been proposed, for example, in our prior British Pat. Specification No. l,066,7 l 3 to provide prestressing jacks with load cells or other load sensitive devices which, when coupled to a suitable meter, are capable of measuring the stressing load applied by the ram to within gor -1 percent which is the generally accepted degree of accuracy required when carrying out stressing operations. It is also ltnown from the aforementioned specification that switch means may be operatively associated with the load cell in order to close the main control valve of the hydraulic fluid system of the ram when the stressing load reaches a predetermined value.

ln order to achieve the generally accepted accuracy mentioned above, it is necessary to halt the operation of the jack immediately on attaining the predetermined stressing load and this, in a prestressing jaclt under load, means that the supply of fluid to the jack must be accurately controlled. It has been found that, by attempting to control the mains control valve a time lag is encountered in changing the direction of flow of the fluid to cause it to return to the fluid reservoir with the result that the desired degree of accuracy is not always attained.

it is among the objects of the present invention to provide means which avoids or substantially reduces the above mentioned disadvantage.

According to the invention furthermore, means for controlling a hydraulically operated jaclr having, or having associated therewith, a directional control valve through which fluid is passed from a reservoir via a pump to either side of the ram piston, and a load cell or other load sensitive device, which means comprise a dumping valve arranged in the hydraulic circuit between the pump and the control valve, switch means adapted to operate the dumping valve to bypass the pumped fluid when the load cell registers a desired load, and a checlr valve which is positioned in the hydraulic circuit to prevent return flow of pressurized fluid from the jack.

The invention is illustrated, as applied to a prestressing jack for use in tensioning wires, rods or strands in prestressed concrete structures, in the accompanying drawings in which:

FlG. t is a longitudinal cross section partly in elevation through one form of hydraulic jack to which the invention can be applied, and showing schematically the hydraulic circuit for effecting control thereof, and

FIG. 2 is an electric circuit diagram.

Referring to the drawings, there is shown a prestressing ram comprising a stressing tube l at one end of which is provided a piston 2 having seals 3. The other end of the tube 1 has connected thereto, by screw threading a nose cone d within which is slidably received a barrel member 5 having a tapered bore 6. The bore if: has arranged therein a plurality of radially disposed wedge elements 7 which are urged into an operative position by means of a compression spring 8 supported by a wedge cap 9. The nose cone 5 also has arranged therein a wedge release tube M which is adapted to engage the smaller ends of the wedge elements 7.

The nose cone 4 has attached thereto a lock-off cylinder 11 within which is mounted a lock-off plunger 12. The free end of the cylinder it is adapted to receive a stressing nose l3.

Arranged within the stressing tube 1 is a center tube 14 which, at one end, is attached by screw-threading to the barrel member 5 and is locked relative thereto by a locking key 15. The center tube M, which is axially slidable within the tube 1, is provided adjacent said one end with an annular seal 16 held in position by a retaining ring 17.

The center tube M passes through the piston 2 and the other end thereof has attached thereto, by screw threading, an extension tube H8 provided with a rear nut 19.

Suitably mounted on the stressing tube l is a main cylinder 20 one end of which is provided with a sealing ring 2B. The other end of the main cylinder 20 is provided with an end cap 22 and a sealing ring 23, the end cap 22 having flsed thereto, by bolts 24, a center tube retaining cap 25.

An annular load cell 26 is arranged between the cap 25 and the rear nut l9 so that stressing forces applied to the center tube is by movement of the main cylinder 20 are transmitted through the load cell.

The main cylinder 20 is provided with a stressing or extending inlet 27 which communicates with an annular chamber 2b and receives hydraulic fluid under pressure via a line 29. The main cylinder is also provided with a return or retracting inlet 30 to which is attached a fluid line I. The inlet communicates with an annular chamber 32 provided between the main cylinder 20 and the stressing tube 1 and, via a passageway 33, with an annular chamber 3d provided between the stressing tube 1 and the center tube M.

In use, the jack is positioned over the wire or the like to be stressed so that the wire extends through the plunger t2, the wedge release tube 10, the wedge elements 7 and into the center tube M, and so that the stressing nose is in contact with the bearing plate of the concrete structure. Hydraulic fluid is then passed, via the line 29, into the chamber 28 whereby the main cylinder 20 will be moved outwardly relative to the stressing tube 1. This movement will be transmitted, via the load cell 26, to the center tube 14 so that the latter, together with the barrel 5 containing the wedge elements, is also moved outwardly relative to the stressing tube i.

Movement of the barrel 5 will cause the wedge elements 7 to move away from the wedge release: tube 10 and allow the wedge elements 7, under the action of the spring 8, to engage and grip the wire to be stressed, Further movement will tension the wire until a predetennined load, which is governed by the load cell 26, is applied thereto at which stage movement will cease. in this position, wedges associated with the bearing plate are forced home by means of the lock-off plunger 12 so that the tensioned wire is held in position in the concrete structure. in order to release the wire, fluid under pressure from the line 31 is passed into the inlet 30 to move the center tube M and thus also the main cylinder 20 inwardly. This movement causes the wedge elements 7 to engage the release tube 10 and thereby release their grip on the wire.

The operation described above is, in accordance with the invention, controlled as hereinafter described.

As can be seen from FIG. 1, the hydraulic control circuit comprises a pump 35 which passes fluid under pressure from a reservoir 36 through a dumping valve 37 which is operated by a solenoid 3d. The dumping valve is operable to bypass the fluid through an exhaust valve indicated at 39 or to pass it, via line 40, to a directional control valve M.

The directional control valve ill is operable to pass the fluid through line 29 to the stressing inlet 27 to extend the jack, or through line 3! to the return inlet 30 to retract the jack. The spool of the control valve 41 is moved between its operative positions by means of a solenoid 42 which operates against the action of a return spring 43 which biases the spool into a position where the valve is operable to retract the ram.

The lines 29 and 31 have positioned therein a pilot unloaded checlt valve assembly 44! consisting of a check valve d5 arranged in the line 29 and pilot valve as in communication with the line 31, the plunger 417 of the pilot valve as being operable, when required, to unseat the check valve 45.

A pressure relief valve 48 is arranged in communication with the line 40.

As can be seen in FIG. 2, the electric control circuit comprises a low voltage, for example l2 volt, power supply 49 which receives power from a mains input line 50. Low voltage power is passed, via line 51, to one side ofa selector switch 52 of the gang type which is adapted to be moved between off, manual (MAN) and automatic (AUTO) positions as indicated on the drawing.

The other side of the gang switch 52 is coupled to lines 53 and 54 which are returned to the transformer 49 to complete the circuit. Line 53 includes a manual pushbutton switch 55 which is operable to cause extension of the ram and a relay RE which operates a switch SW1. Line 54 includes a manual pushbutton switch 56 which is operable to cause retraction of the ram and a relay R2 which operates a switch SW2.

The switch SWl is connected via line 57 to the solenoid d2 of the hydraulic directional control valve M, and via line 58 to the solenoid 38 of the hydraulic dumping valve 37. The switch SW2 is connected to the mains supply 50.

The transformer 69 also supplies, via line 59, low voltage power to the load cell 26 and, via line 60, to an amplifier 6t connected to a meter 62 which registers the load applied to the load cell 26. The reading registered by the meter 62 may also be recorded on a recorder device of any convenient type indicated at 63.

The system so far described is all that is necessary for manual operation where the operator controls the operation by observing the reading registered by the meter 62.

In order to provide for automatic operation, the automatic contacts of the switch 52 are connected, via a line 64, to a trip device 65 having a switch SW3 and a relay R3. The trip device is also provided with an adjustable trip level controller 66 and is connected to the meter 62 via line 67 so that, when a load value determined by the position of the controller 66 is reached, the relay R3 operates to open the switch SW3 to deenergize the circuit.

In use, for manual operation, the pump 35 is switched on and the selector switch 52 is set at manual. This energizes the relay R2 which closes switch SW2 thereby energizing, via line 58, the solenoid 38 of the dumping valve 37 so that fluid passes through the exhaust valve 39 thereof and no movement of the ram will take place.

In order to extend the ram, the button 55, which is normally open, is depressed so as additionally to energize relay R1 thereby causing changeover of the switch SWil so that, via the line 57, the solenoid 42 of the directional control valve 41 is energized. This automatically deenergizes the solenoid 32 of the dumping valve 37 and causes the previously bypassed fluid to be passed via line 40, to the control valve M. Energization of the solenoid 42 against the action of the spring 413 causes the spool of the valve 45 to move into a position where the fluid passes through the pilot unloaded check valve assembly 44 and via line 29 to the chamber 28 on one side of the ram piston 2. As the ram extends, fluid on the opposite side of the ram piston 2, will return via the line 31 and the valve assembly 44 to the control valve 411. This fluid is not under pressure and therefore no reaction occurs, and the fluid merely passes the exhaust through valve 68 of the directional control valve 4B.

The operator continues to depress the button 55 until the required load is registered on the meter 62. On reaching the required load, the button 55 is released thereby deenergizing the relay R1 and thus energizing the dumping valve solenoid $3 and deenergizing the control valve solenoid 42. This causes the fluid to be bypassed through the exhaust valve 39 and relieves the fluid pressure in the circuit. This lack of pressure causes a drop in pressure in the line 29 and results in immediate seating of the check valve Q thus preventing fluid under pressure from escaping from the chamber 28 of the ram. Thus, the applied load will remain static.

In order to retract the ram, the button 56, which is normally closed, is depressed thereby deenergizing the relay R2 and causing changeover of the switch SW2. This cuts off the power to, and thus deenergizes, the solenoids 38 and 42 with the result that the previously bypassed fluid is passed via line 40 to the directional control valve 431, and with the result that the spool of the control valve 41 is moved to cause the fluid to flow through the line 31 to said opposite side of the ram piston 2.

In this position, fluid pressure will build up in order to retract the ram, but initially, due to the fact that fluid is held on the extend side of the ram by the check valve 435, retraction will not take place. The pressure buildup continues until it reaches a predetermined valve which is sufficient to operate the pilot valve 46. At this point, the plunger 47 of the valve 46 is moved to unseat the check valve 45 whereby fluid flows through the line 29 to the control valve M from which it passes to exhaust through valve 68.

The button 56 is held in its depressed position until the jack is visibly at the end of its stroke at which stage it is released. Release of the button 56 causes the relay R2 to be energized thus energizing the solenoid 33 which results in the fluid again being bypassed through the exhaust valve 39 of the dumping valve 37. The operation can then be repeated.

in use, for automatic operation, the selector switch 52 is set at automatic. Thereafter the operation is basically the same as that described with reference to manual operation.

In this case however, power for relay R1 is routed through line 64 to the relay R3 of the trip device 65 which relay is deenergized at a preset trip point determined by the setting of the trip level controller 66. Thus, as hereinbefore described, depression of the button 55 to extend the ram causes the dumping valve solenoid 38 to be deenergized and the control valve solenoid to be energized. Extension of the ram continues until a given load value determined by the controller 66 is reached. At this point, the relay R3 is energized thereby opening the switch SW3 which cuts off the power to, and deenergizes, relay R1. This causes the control valve solenoid 42 to be deenergized and the dumping valve solenoid 38 to be energized.

The latter operation overrides the button 55 which is still in the depressed position. On releasing the button 55, the dumping valve solenoid 38 would normally be energized but as this has already been effected by the trip controller, there is no further change.

In order to retract the ram, the button 56 is depressed and further operation is as hereinbefore described. The relay R3 is deenergized to close switch SW3 when the applied load decreases below the value set by the controller 66.

Thus, it will be appreciated that, by including the dumping valve 37 in the hydraulic circuit, an immediate response to the signal passed out by the load meter 62 is achieved and the time lag resulting from changing over the main control valve 41 can be ignored from the time factor point of view. Furthermore, the inclusion of the check valve 45 ensures that, as soon as fluid pressure is released by the dumping valve 37, the pressure in the ram remains constant so that the desired predetermined stressing load applied to the wire is maintained and held.

The inclusion of the recording device 63, which may be of the pen-recorder-type makes it possible permanently to record the stressing load attained, it being understood that records ofthis kind would be particularly useful when carrying out large operations such as are required for example in nuclear power stations, and in providing quality control which is necessary when carrying out pretensioning work.

Although the invention has been described with reference to a jack for prestressing the wires or the like in a concrete structure, it will be understood that it is not limited in this respect. Thus, the invention can, without modification, be applied to any hydraulically operated ram having a load cell or other load sensitive device and control means for determining the direction of operation of the ram.

lclaim:

1. Improved control means for controlling the operation of a hydraulically operated jack including a ram comprising a piston, and having associated therewith a directional control valve, a pump for passing hydraulic fluid from a reservoir to either side of the piston of said ram via said directional control valve, and a load cell for sensing the load applied by said ram, the improvement comprising a dumping valve arranged in a hydraulic circuit between said pump and said directional control valve, switch means for operating said dumping valve independently of said directional control valve to bypass fluid being pumped toward said ram, said switch means being controllably coupled to said load cell for actuating said dumping valve in response to said load cell sensing a given desired load, and a check valve positioned in said hydraulic circuit to prevent return flow of pressurized fluid from said jack upon actuation of said dumping valve.

2. Improved control means for controlling the operation of a hydraulically operated jack including a ram comprising a piston, and having associated therewith a directional control valve, a pump for passing hydraulic fluid from a reservoir to either side of the piston of said ram via said directional control valve, and a load cell for sensing the load applied by said ram, the improvement comprising a dumping valve arranged in a hydraulic circuit between said pump and said directional control valve, switch means including a manually operated relayactuated changeover switch for operating said dumping valve independently of said directional control valve to bypass fluid being pumped toward said ram, load indicating means for providing an indication of said load cell sensing a desired load, and a check valve positioned in said hydraulic circuit to prevent return flow of pressurized fluid from said jack upon actuation of said dumping valve.

3. improved control means as claimed in claim 2, wherein said indicating means comprises a meter coupled to said load cell for providing a visual indication of the load sensed by said load cell.

4. Improved control means as claimed in claim 3, including a recording device for providing a record of loads sensed by said load cell during the course of a plurality of operations.

5. improved control means as claimed in claim 2, including a pilot valve for unseating said check valve to permit return flow of pressurized fluid from said jack.

6. Improved control means for controlling the operation of a hydraulically operated jack including a ram comprising a piston, and having associated therewith a directional control valve, a pump for passing hydraulic fluid from a reservoir to either side of the piston of said ram via said directional control valve, and a load cell for sensing the load applied by said ram, the improvement comprising a dumping valve arranged in a hydraulic circuit between said pump and said directional control valve, a manually operated relay-actuated changeover switch for operating said dumping valve independently of said directional control valve, a trip device controllably coupled to said load cell and arranged in circuit with said changeover switch for automatically actuating said changeover switch in response to said load cell sensing a predetermined load, and a check valve positioned in said hydraulic circuit to prevent return flow of pressurized fluid from said jack upon actuation of said dumping valve.

7. Improved control means as claimed in claim 6, in which said trip device comprises a relay-operated cutout switch and an adjustable trip level controller which determines the load value at which said cutout switch becomes operative.

8. Improved control means as claimed in claim 6, including a meter for providing a visual indication of the load sensed by said load cell.

9. Improved control means as claimed in claim 6, including a pilot value for unseating said check valve to permit return flow of pressurized fluid from said jack.

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