Jack for prestressed concrete structure

Abstract

 A stressing jack is of light weight because limiting means (19) are provided to reduce the axial tension generated in the rearward end of the jack casing (1) at or towards the limit of the rearward movement of the jack pulling cylinder (8).
Preferably the limiting means is a safety pressure limiting shuttle valve (19) having a plunger (22). Because of the limiting means the outer cylinder wall can be of low thickness which saves weight and creates stability.

Description

[0001] The invention relates to the prestressing of concrete structures and in particular to a jack for use in stressing the elements, eg. wires, strands, cables, of the tendon in a prestressed concrete structure. Such jacks are well known see eg. _our British patents 1066713 and 1375203 (Agents ref:7097 and ₇₁₄₁ respectively).

[0002] It is one object of the invention to provide for the purpose specified a jack which is light in weight, stable in use, and simple and economic in operation.

[0003] According to one aspect of the invention there is provided a stressing jack for use in stressing a plurality of elongate stressing elements for a concrete structure, the jack comprising a main casing including a forward nose portion adapted to bear on one end face of the concrete structure, a pulling cylinder within the casing and arranged to move axially of the casing and pull the elements towards the rearward end of the casing, lock off means arranged to operate with the pulling cylinder, and pulling cylinder, and limiting means to reduce the axial tension generated in the rearward end of the casing at or towards the limit of the rearward movement of the pulling cylinder.

[0004] By virtue of the limiting means the axial tension in the casing is reduced or eliminated as a result of which the wall thickness of the rearward parts can be reduced so saving weight and cost of metal. Most preferably, the limiting means comprises a shuttle valve. Most preferably, the valve is arranged to be opened by mechanically contacting an end plate at the limit of the rearward movement of the pulling cylinder. For this purpose the valve preferably includes a projecting plunger.

[0005] According to a preferred aspect of the invention the stressing jack comprises a main casing having a thrust plate from a forward end of which extends a lock off assembly and a forward nose portion adapted to bear on an end face of the concrete structure, a pulling cylinder disposed-within the casing and in engagement with a piston disposed rearwardly of the thrust plate, the cylinder being arranged to move axially of the casing and pull the elements towards the rearward end of the casing, a source of pressurised fluid for moving the pulling cylinder and actuating the lock off assembly, and limiting means in association with the pressurised fluid acting on the thrust plate to reduce the axial tension generated in the rearward end of the casing at or towards the limit of the rearward movement of the pulling cylinder.

[0006] In order that the invention may be well understood, it will now be described with reference to the accompanying diagrammatic drawings, in which

Figure 1 is a perspective view, parts being cut away, of a hydraulic jack,

Figure 2 is a longitudinal section of the jack of Figure 1,

Figure 3 is a rear end view of the jack

[0007] The jack of Figures 1 to 3 comprises a main casing having an outer cylinder 1. A thrust plate 2 is connectable by a thread connection to the forward end of the cylinder 1 and from the front of the thrust plate 2 extends a lock off assembly comprising a lock off cylinder 3, a lock off piston 4 and a lock off thrust plate 26. A nose portion 5 extends forwardly of the lock off assembly and is adapted to engage a bearing plate 6 which engages an anchorage A of a concrete structure, Figure 2. A piston cylinder 7 is present in the outer cylinder rearwardly of the thrust plate 2. The inner surfaces of the lock off piston 4, the thrust plate 2 and the piston cylinder 7 contact an inner cylinder 8, the rearward end of which has annular shoulder 9 which threadingly engages a'corresponding recess 10 in the piston cylinder 7. The inner cylinder 8 houses a nest of tubes 11 corresponding in shape and -number to the strands or wires of the anchorage element A. The tubes are held at their forwards ends in a forward locating ring 23. A set of gripping wedges 12 is present in a rear gripping plate 13 to engage the strands or wires and the plate 13 is connected to the piston head by clamps 14. The tubes 11 also locate in the rear gripping plate 13 so that rotation of the plate 13 also rotates the tubes 11. The tubes 11 and the plate 13 are free to-rotate with respect to the cylinder 8. As seen in Figure 3, the plate 13 has handles 25 so that it may easily be mounted in the jack. A retract cylinder 15 extends from the piston head 7 and the movement thereof is restricted by an end plate 16 depending from the rearward end of the outer cylinder 1.

[0008] The jack includes a supply of hydraulic fluid, not shown. Pressurised fluid may be admitted via inlet 17 and exit via outlet 18 to move an assembly of the piston head 7, inner cylinder 8 and retract cylinder 15 rearwardly. According to the invention a safety pressure limiting shuttle valve 19 is present in the path of the pressurised fluid through the piston head 7 and serves to direct the fluid back to the reservoir when a pressure limit has been reached, so avoiding the generation of axial tension in the outer cylinder 1. As shown in Figure 3, there is a pair of valves 19-mounted on the plate 13 diametrically opposite each other. Pressurised fluid may be admitted via the lock off inlet and exit via the lock off exit to actuate the lock off system. Seals and rings are provided between working surfaces, as shown.

[0009] Two bands 20 pass about the outer cylinder 1 and have lifting hooks 21 so that the jack may be moved as required. The bands are rotatable so that the hoses carrying the pressure fluid to and from the jack can be -positioned anywhere on a circumference of 360 relative to the lifting points.

[0010] In use, the parts of the jack are assembled by threading the thrust plate 7 into the forward end of the outer casing 1, and the hydraulic lines are connected. The jack is advanced to a concrete body to be stressed and the ends of the wires or strands are then fed through the lock off thrust plate 26 and into the tubes 11. The wedges 12 are engaged with the wire or strand ends. Pressurised fluid is then admitted to urge the piston head 7 to the rear so stressing the wires or strands. Rearward movement of the piston head 7 cannot go past the end plate 16 and when the piston head approaches the end plate the valve 19 opens so directing fluid to the reservoir and preventing axial tension being generated in the outer cylinder 1. The valve 19 communicates between the extend side of piston cylinder 7, and the retract side. When extending the jack oil tries to flow from the extend side of the piston through the valve to the retract side. The flow of the oil closes the valve which is held shut by the pressure generated on the extend side. The shuttle valve has a plunger 22 which projects from the retract side of the piston cylinder. When the jack reaches the limit of extension the plunger 22 strikes the end plate 16 which opens the shuttle valve 7, and. allows oil to pass from the extend side of the piston cylinder into the retract chamber on the retract side of the piston cylinder, and then back to the tank through the retract hose.

[0011] When there is no load in the jack then the valve 19 is opened fully, the pressure created across it being very small, and hence the axial load generated in the outer cylinder is negligible.

[0012] When the jack reaches full extension and is also loaded the valve 19 will not open far enough for the pressure in the extend side of the piston to be eliminated, as this would allow the jack to retract, the valve closing again immediately. The valve is arranged to open just sufficiently to take the full flow of hydraulic oil from the pump, but with a pressure generated across the valve just sufficient to maintain load in the stressing elements. This means that the force in the jack is balanced exactly by the force in the stressing elements, and negligible force is transmitted into the end plate or outer cylinder.

[0013] On retraction of the jack, the flow of oil from the retract side to the extend side of the piston cylinder closes the valve (in the opposite direction to the extend operation) and the jack retracts. The lock off system is energised either at full stroke or at a predetermined load. Pressurised fluid closes the shuttle valve and acts to retract the inner cylinder, in known manner. The strands or wires are then held in position against the bearing plate 6 by the wedges W and excess lengths are cut off.

Claims

1. A stressing jack for use in stressing a plurality of elongate stressing elements for a concrete structure, the jack comprising a main casing including a forward nose portion adapted to bear on one end face of the concrete structure, a pulling cylinder within the casing and arranged to move axially of the casing and pull the elements towards the rearward end of the casing and lock off means arranged to co-operate with the pulling cylinder, characterised in that limiting means (19) are present to reduce the axial tension generated in the rearward end of the casing (1) at or towards the limit of the rearward movement of the pulling cylinder (8).

2. A stressing jack according to Claim 1, characterised in that the limiting means (19) comprises a shuttle valve.

3. A stressing jack according to Claim 2, characterised in that the valve (19) includes a projecting plunger (22) arranged to be actuated by mechanically contacting an end plate (16) at the limit of the rearward movement of the pulling cylinder.

4. A.stressing jack according to any preceding Claim characterised in that the valve (19) is arranged to communicate between the extend and retract side of a piston (7) connected to the pulling cylinder(8).

5. A stressing jack according to any preceding Claim, characterised in that the jack includes a thrust plate (2) from a forward end of which extends the lock off assembly (3,4,26) and the cylinder (8) is arranged to move axially of the casing (1) and pull the elements towards the rearward end of the casing and a source of pressurised fluid is present for moving the pulling cylinder (8) and actuating the lock off assembly (3,4,26).

6. A stressing jack according to Claim 4 or 5, characterised in that when the pulling cylinder (8) reaches full extension and is loaded, the valve (19) is arranged to open just sufficiently to allow pressure fluid to pass therethrough, with the pressure generated across the valve (19) being sufficient to maintain the load in the stressing elements.

7. A stressing jack according to any preceding Claim characterised in that the elements to be stressed are received in tubes (11) connected to a rear gripping plate (13) which is located to the rear of and is free to rotate with respect to the pulling cylinder.

8. A stressing jack according to any preceding Claim characterised in that at least one rotatable band (20) passes about the outer cylinder (1) whereby the lightweight jack may be moved.

Drawing