Strand jacks
A strand jack is a hydraulic device used to move heavy loads. The forces are transferred from the piston of the device to the loads to be moved via strands . Strand jacks are mostly used in groups that are operated by a common control unit. Hydraulic pump units are required for the energy supply.
history
The strand jacks were developed by the Swiss construction company VSL International , which specializes in prestressed concrete construction, in the 1970s. For the first time, she used the presses normally used for tensioning prestressing steel for lifting heavy loads, using strands made of prestressing steel for suspending the load . One of the first applications was the lifting of three 490 tonne domes to a height of 28 meters to cover alumina silos in Sardinia . The construction of the roof structure of the Olympic Stadium in Munich was the first major project in which strand jacks were used. Today, strand jacks are used to erect many structures, but strand jacks were also used to erect the capsized cruise ship Costa Concordia .
function
Hollow piston cylinders are mostly used for strand jacks . In these, the cylinder has the shape of a wide annular gap in which the piston moves hydraulically driven. One or more strands run through the hollow axis of the hollow piston cylinder. Both the piston and the cylinder have a clamping device to hold the strands in place. By alternately opening and closing the clamps as well as repeatedly extending and retracting the piston, the strands can be moved gradually relative to the jack.
During normal operations, the cylinder is fixed and the load is attached to the strands. When the piston is retracted, its clamp closes and the load is raised when the cylinder is extended. When the piston is fully extended, the clamp on the cylinder closes and relieves the clamp on the piston. In this way, this can be opened and the piston can move back again, and the cycle can start over.
The same seven-wire strands are normally used as load-bearing elements as for the prestressing of concrete components. One half of the strand bundle is twisted with a left-hand twist and the other half with a right-hand twist in order to avoid twisting when lifting individual points or when lowering the strand bundle.
The advantage of strand jacks compared to winches and pulley blocks is that no rotating elements are loaded and therefore no complex bearings are required. In addition, stiffer cables can be used, as no bends have to be made under load. A challenge when using strand jacks is the clamping, which is why the load is usually distributed over several thinner strands, as more clamping force can be applied. Since strand jacks can be connected in parallel without any problems, individual jacks are only offered up to around 1700 tons of lifting power. With synchronous control, the pressure equalization results in an even load distribution. Safety valves maintain redundancy against failure of a unit.
The lifting speeds depend heavily on the application scenario and can reach several meters per hour.
Applications
Strand jacks are suitable for lifting, lowering and moving individual loads where the use of cranes, winches or other conventional means of transport is not possible for economic or technical reasons. In the ideal case of use, existing support points above the load can be used, such as the bridge piers in the adjacent picture. Supporting it on a component to which the component to be lifted is to be connected allows for a smooth and precise alignment, as is the case with the segment-by-segment assembly of the Edong Bridge . Strand jacks individually achieve the lifting force of a heavy-duty crane, but are easier to coordinate in a network than cranes. It is also possible to assemble the roof structures of halls on the ground and lift them as a whole into their final position.
Web links
Individual evidence
- ↑ Losinger AG VSL International: Lifting technology method for moving large loads . P. 4
- ↑ Heavy lifting. VSL, accessed April 4, 2014 .
- ↑ Barbie Latza Nadeau: Massive Cables Are Slowly Raising the Costa Concordia Shipwreck. In: Scientific American . September 16, 2013, accessed April 5, 2014 .
- ↑ www.e.hebetec.com ( Memento of the original from May 5, 2014 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ VSL Construction Systems, p. 28
- ↑ Losinger AG VSL International: Lifting technology method for moving large loads . P. 2
- ↑ E-Dong cable stayed bridge, China. Dorman Long Technology, accessed February 22, 2014 .
- ↑ Record lifting of a hangar roof . In: Swiss engineer and architect . tape 105 , no. 23 , 1987 ( pdf [accessed April 5, 2014]).
