Exercises in Heavy Lowering
Driving up to Point 5, one can't help but notice the impressively enormous red structure climbing up and over Building 3585. This unusual bridge-like construction is CMS'new crane, otherwise known as 'the gantry'.
The 'gantry'was constructed by VSL, a Swiss company that is used to building such uniquely designed cranes used for major construction jobs, including lifting the roofs of various stadiums and the huge Airbus A380 assembly hall in Toulouse or bridge foundation caissons.
CMS's crane was custom-built to sustain up to 2000 tonnes of machinery and detectors and slowly lower them down into the experimental cavern.
The feature of two towers, one on either side of the building, each 24 m high, is the reason behind the nickname of this amazing crane. Two large beams, 28 m long and 3.4 m high, run along the width of the roof and four openings, each 5 m long, in the ceiling will allow the cables to pass through into the gallery.
Unlike typical cranes that use coils to lift materials, this one uses a 'strand-jacking'hydraulic system that will gently lower each of the 15 CMS elements individually down 100 m into the experimental cavern, a process called 'heavy lowering'. A concrete plug, 2.2 m thick, will be moved in place over the 20 m diameter pit and a detector element will be shifted on top of it and then lifted slightly. Once hanging on the 'gantry'crane, the plug will be reopened and the lowering can start.
The heaviest element is the central yoke barrel, which weighs 1920 tonnes. The 'gantry'will be attached at four points to the detector element with 55 strands, each finger-thick that will pass through a 35 cm diameter steel plate. The holes have a mechanism with conical pieces inside that can influence the clamp for the hydraulic system and slowly lower the pieces down. 'It will take about 10 hours to lower a single element', explained Hubert Gerwig, Project Leader from PH-CMI. 'It's the same principle as if you were to lower someone down by hand using a cable.'
A dead weight test will be performed in April 2006 using the equivalent of 340 CERN standard shielding blocks to test the sustainability of the plug. Then in June, cables will be fixed from the 'gantry'onto the plug to pull with 25% more than nominal force in order to test the crane and its foundation. Survey engineers will be on hand to check the foundations and the crane's deflection and strain gauges will observe the stresses in the large beams.
Finally, a 'dummy'load test will be conducted to check the movement of the cables. 'It is important to move the load down and up to see if the mechanism is working correctly and make sure to keep the load in a horizontal position', says Gerwig. If the load starts to tip to one side it could be a dangerous situation for CMS. However, Gerwig is assured that if one cylinder fails it is not a disaster, it can be fixed. However, this has never happened to VSL in its 30 years'experience in the business.
The first element scheduled to be lowered after the tests are performed is the hadronic forward calorimeter, which is currently in Meyrin. Of all the pieces, it is the lightest at a mere 250 tonnes.
