Mobile gantry gets the job done

Installing a new overhead travelling crane into an existing factory or workshop can be a tricky business. Sometimes the best option is to take the roof off and bring in a huge mobile crane to lift in the girders and trolley from above. Sometimes this is not so feasible and the work has to be done from below. In such cases a mobile crane may be able to drive into the building and lift the structures into place. But there may not always be space for the boom tip between the roof and top of the new gantry crane.

Bucyrus, a US manufacturer of draglines for open cast mining, faced just this problem last year when it needed to install a new overhead travelling crane. As surface mining equipment has grown in size, so have its components. Bucyrus' 22t (25 US ton) overhead travelling crane, one of three in its assembly factory, could no longer keep up with the larger and larger mining buckets, shovels and dippers that are welded together there. Bucyrus specified a new 72.5t (80 US ton) SWL crane to replace it and awarded the supply and installation contract to Zenar.

It was going to be a tricky installation. A whole new factory was not an option because all the machine tools it needed were there. Bucyrus could not afford to stop production to take off the roof. The factory ceiling was too low to allow a mobile crane - it had only about 4m of open space between the top of the truss and the roof. In addition, in most places that space was filled with ducts, lights and piping that would block access from above. For all of these reasons, the bridge girders of the new 18m span gantry would need to be lifted from below.

Pete Golden, Bucyrus' manager of manufacturing technical services, suggested to Zenar that a TriLifter might be the solution. Golden had seen a TriLifter in action at a local millwrights'.

The TriLifter is produced by US company Riggers Manufacturing. It is a hybrid machine, consisting of a 7m four-stage hydraulic extending gantry and a 2.5m horizontally-extending crane boom, all on a wheeled chassis. Forklift arms or a spreader can be attached to the crane boom. An onboard computer monitors the balance of the load between front and rear cylinders.

Like a forklift (and unlike a hydraulic gantry, for which Riggers is better known) the machine's two back wheels swivel, allowing it to circle with a turning radius of about 5m. With steerable wheels, the lifting rig can raise a bridge girder above the height of its supporting runway and then rotate it into position.

Equally multipurpose is the machine's counterweight. The weight mounts on a rear wheel chassis that, pushed by two hydraulic rams, can extend 1.5m off the back end. With an extended counter- weight and longer wheelbase, the machine can carry heavier loads. When carrying a load on the gantry, however, the TriLifter can also make fine adjustments of position in slow lifts by pushing out or pulling in its counterweight. When the rams push out the counterweight, reaction forces push the TriLifter forward without a jolt that risks unsettling the load.

Approximately 100 of the $220,000 to $300,000 machines have been sold around the world, 70 in the US, since first launched in 1985. Two versions are available: the TL 100, which can lift 45t (100,000 lb) on the hook or 68t (150,000 lb) on top of its gantry; and the TL 150, which can lift 68t (150,000 lb) on the hook or 91t (200,000 lb) on top.

Bucyrus contracted Industrial Erecting to remove the old crane and then Zenar's team moved in with a TriLifter. An existing 40t EOT crane was used to mount the first boom on top of the TriLifter. Then the operator drove it back into the empty bay, extended the hydraulic rams, turned about 90°, lowered the boom into place and repeated the process. Then workers positioned the trolley between the two booms and pinned everything together. The entire lift took a day, and required no special engineering studies.

"The only fuzzy part was if you don't have a real smooth floor, you get wobbles," Golden says. The overhead crane was operational about a week later, after fixing some wiring and load testing. "It has basically become our workhorse," he says.