Jefferson Lab
On Target (October 1995)
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    Riding On Air

    Heavy Hall B Carriages Mobilized By Simple Technology

    The strength of one person and seven inflated inner tubes is about all that's required to move the Forward Carriage-a mere 225 tons-in Hall B. An extraordinary feat no doubt, but how about the fact that once inflated, the tubes and the attached three-story structure all actually float.

    There's no magic involved. Just Newton's laws of motion and seven airpads, which resemble tire inner tubes. Once inflated, and the air pressure inside the tube and the ring become approximately 50 pounds per square inch, air escapes beneath the tube creating a cushion of air. As a result, the structure floats. A winch is then attached so that someone can guide the structure to its final destination.

    "The secret is to have a well sealed floor," says John Robb, Hall B Installation Coordinator, referring to the shiny epoxy coating covering one-fourth of the experimental hall floor.

    Situated under an orange platform, the airpads slide into place-three in the front, four in the back-like a drawer. Wheels and tracks, the method used for moving the detectors in Halls A and C, would have seemed to be a more natural choice. But designers of the project thought differently. "Using airpads keeps the floor clear," says Addison Lake, a mechanical engineer for Hall B and lead designer on the carriage. He explains that rails would have to be bolted to the floor making it difficult for workers and vehicles to maneuver. Robb adds, that the airpads are also the least expensive method and they aren't limited in the direction they can move.

    The technology isn't new, however. In fact, it's the same used to move heavy airplanes under construction or undergoing maintenance. Some high-energy nuclear physics laboratories use airpads to move massive equipment, as well.

    The stately hexagon-shaped Forward Carriage, constructed by Monde Construction in Newport News, hosts a variety of detectors constructed to track particles during experiments. The detector package includes: six triangular electromagnetic calorimeters, which tell how much energy a particle will deposit once completely stopped; six Cerenkov Counters, which separate electrons from heavier particles; and 132 time-of-flight scintillation detectors, which measure the velocity of slow particles and help identify the particle species. All are a part of the CEBAF Large Acceptance Spectrometer, or CLAS-recognized as one of the biggest, most complicated detectors in the world.

    Once all of the detectors are fully installed, the Forward Carriage, now resting near the back wall of Hall B, will be nudged toward the suspended toroidal magnet. Though close, the two structures will never touch. "The Forward Carriage will look almost like a shield," says Robb demonstrating by balling up one hand into a fist and the other looming open beside it. Everything will be located precisely, "within a few millimeters," he adds.

    Each level of the carriage also serves a function. The first level will be used to store all of the delay line cables necessary to make all signals arrive at the electronics at the proper time; the second and third will contain electronics racks; and the fourth contains the transformers which will supply the power.

    Adding to the weight of the carriage is 25,000 pounds of lead buried inside of it to act as a counterweight since the carriage is tall and the center of gravity will be far forward once the detectors are installed. The calorimeters and time-of-flight scintillation detectors will be installed first, starting in December.

    The Forward Carriage isn't the only structure in Hall B being maneuvered by airpads. They will also be used to mobilize two carriages on the sides of the toroidal magnet called Sideward Carriages or "clam shells." These two carriages, containing more calorimeters and time-of-flight scintillation detectors, will close around the toroidal magnet like two cupped hands, says John O'Meara, Chief Engineer for Hall B.

    These two additional carriages, currently under design, will have to be fabricated in the hall, and maneuvered to their positions in an environment that is already crowded.

    "Hall B is looking smaller and smaller to us each day," O'Meara admits. "But everything will fit."

    Testing the effectiveness of the airpads was left up to Hall B lead mechanic, Doug Tilles, and his crew. Once the airpads were inflated by a rented 900-cubic foot per minute air compressor, the empty Forward Carriage-weighing 160,000 pounds-moved with ease.

    O'Meara is optimistic about the effectiveness of the airpads after all of the detectors are installed. He also notes that once filled, all three carriages will only be moved a few times a year for maintenance.

    Now that the obstacles of moving these structures seem to have been overcome, the challenges still aren't over for the team. Now they must figure out how to actually get the calorimeters into the cavernous experimental hall. After each is wheeled down the 320 foot long access ramp, the triangular detectors will have to fit through a square door. "It's going to be tight ... very tight," says Robb. "The calorimeters will have to be tipped up at a 50 degree angle. They will just clear the door."

    "We're all convinced that everything will work. It all has to be just right," says Robb.

    -Deborah Hyman

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