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UNDERSTANDING HYDRAULIC JACKING TECHNOLOGY

Probably the first use of hydraulic jacks in civil engineering was during the construction of Robert Stephenson's revolutionary Brittannia Rail Bridge across the Menai Straits. Work on this complex project started in 1846 and the bridge was opened to rail traffic in March 1850.

This was the first use of box girder construction for bridges and huge steam-powered hydraulic jacks were used to slowly raise the main central 140 m spans (consisting of four sections) of the bridge into their final positions on the huge stone piers, 32 m above high tide level. The pre-constructed wrought iron girders, each weighing around 1800 tonnes, were constructed on the shoreline and floated into position on pontoons. The ends were then attached by chains to the jacks located on the towers. Large grooves in the piers ensured the girders were correctly located. Each time the stroke capacity of the jacks was reached, the girders were locked in place and the chain shortened ready for the next lift. This process was not without its hazards and at one stage one of the huge jacks exploded under the pressure. Despite this the lifts were carried out in a few days.

Sadly the bridge caught fire in 1970, resulting in the severe buckling of the beams. A new structure now carries rail and road traffic, still using the huge stone piers.

The table below gives an overview of typical jack types and capacities. The list is not exhaustive so please contact us for further advice and information.

Strand Jack: A typical strand jack operates like a linear winch. The operating principle is ideal for lifting, lowering and sliding over relatively long distances as well as for tensioning applications. Depending on the application, the hollow design allows one or more steel cables to be guided through the hydraulic cylinder. When the cylinder is operated, wedge type clamps at the top and bottom grip the cable(s) and effect a lifting or lowering movement depending on the stroke direction. The motion of the cylinders is driven by hydraulic power packs, typically controlled via our specialist computer controlled pump systems. Strand jacks operated in the horizontal position are used for sliding applications.
Capacity Range: 15 tonne per single strand (e.g. Triple strand 45 tonne)
Stroke Range: 250 - 500 mm per stroke
Rod Jack: Rod jacks are available in a range of designs/capacities and are typically used for lifting and lowering over relatively short distances (up to 10 metres). Their stroke range is usually up to 1000 mm and designs include standard ram and hollow ram. They can be used singly with a yoke arrangement or in pairs. Linear displacement transducers (LVDTs) are often used in conjunction with this type of jack to maintain precision movement at the multiple jacking points. Unlike strand jacks, the lifting/lowering rod is not gripped by the cylinder – the movement is effected via a support structure. After each stroke length is reached, the rod is locked off and the lifting/lowering structure adjusted.
Capacity Range: 15 – 100 tonnes
Stroke Range: 500 - 1000 mm
Flat Jacks: Flat jacks are ultra-low profile jacks often used in construction and civil engineering jacking applications where space is very limited and relatively high forces are required over short distances. With closed heights of less than 35 mm for up to 860 tonne jacks, they can be used in the tightest of spaces. Applications include lifting, lowering, pre-tensioning and stressing. Flat jacks can be left in situ to provide permenanent support - grout is pumped into the jacking cyclinder and allowed to set.
Capacity Range: 8 - 860 tonne
Stroke Range: 0 - 25 mm
Cylinder (Pot) Jacks: These high capacity - short stroke jacks are used extensively for bridge jacking applications. Both single and double acting ram versions are available in standard and hollow designs. Double acting jacks can be used for both pushing and pulling applications.
Capacity Range: 5 – 1000 tonnes
Stroke Range: 12 – 300 mm
Long Stroke Ram Jacks: These long stroke jacks (up to 1000 mm stroke) are used extensively in the lifting, lowering and supporting of buildings. For optimum stability, they are typically used with additional support structures and restraints.
Capacity Range: 5 – 100 tonne
Stroke Range: 250 – 1000 mm
Weighing Jacks: Specially calibrated hydraulic jacks can be used for weighing large structures. This can be very useful when it is not possible to weigh structures using conventional methods. The jacks carry out the dual role of lifting and weighing. Either tension (lifting) or compression (sitting on) applications can be carried out.
Capacity Range: 5 - 1000 tonnes
Jacking Sledges: These are often used in conjunction with strand jacks in sliding applications. The object being moved (e.g. a bridge section) is supported on jack assemblies which typically have a Teflon or other low friction base. The jacks are used to lift the object clear of the ground and the sledges move on temporary steel rails.
Capacity Range: 50 -500 tonnes

RMDK Megashore Props: In many civil engineering applications, RMDK Megashore Prop frames are used to support structures. Integral jacks extend their versatility and provide additional safety.

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