Update on the small wheels

3 March 2008

The first small wheel descends

“She’s the last big piece of ATLAS, so there’ll definitely be a celebration on Friday after she’s gone down,” said Ariella Cattai last Tuesday, ahead of the lowering of the second small wheel. It’s hardly surprising that Ariella – the Small Wheels Project Coordinator – refers to the wheel as “she”, considering the intensity of her fifteen-month relationship with it and its sister, which was lowered without a hitch on February 15th.

The wheels were transported from building 191 to Point 1 using a special 128-wheel hydraulic-bed truck, which made sure that the pieces were kept level as the truck negotiated corners and bumps in the road; a journey which US ATLAS Muon Manager, Frank Taylor, described as “quite a parade” (see eNews Feb 4th). The wheels made their journeys on February 8th and 14th respectively, each taking around seven hours to be transported over the road.

So far, so good. But the engineers were forced to think on their feet in order for the wheels to reach their final destination, as Frank explained: “The bottom line is that the building isn’t big enough. Or, of you like, we built ATLAS too large for the building up above. At one point, the plan was to remove the roof, and hire an enormous crane to lift the larger pieces over, but instead, the door on the Jura side was made larger about a year ago.”

The engineers’ ingenuity was called upon again when it came to delivering the pieces into the cavern itself. The wheels were transported, and due to be lowered, in specially designed protective frames. But when it transpired that the straps that were going to be used to lower the wheel-and-frame configurations wouldn’t provide enough manoeuvrability to sidestep the end cap toroids, which were sitting in the way in the pit, they had to come up with a new plan.

Their solution to this freedom-of-motion dilemma was to attach a rigid bar across the top of each of the lifting frames, but doing this meant that the frames would then stand taller than the crane that would be lifting them into the pit. The final procedure saw each of the wheels perform a complicated dance, first being lowered into a shallow 30cm trench next to the access shaft, before the new crossbar pieces were fitted. This left just enough room for the crane to be able to pass over the top and collect its loads, ready for their hour-long 93m descents.

“All this is a very delicate operation, because the weight in this game is massive,” explained Ariella last Tuesday. “The whole lot, including the frame, weighs 130 tonnes, and she has to go around some very delicate pieces of the experiment, with only centimetres of clearance. It’s a huge – and remarkable – piece of engineering. We owe a lot to our engineers; their work has been so impressive, and has been done under strict timescales too.”

The wheels were lowered onto airpads on top of the ATLAS rails, which had been coated in oil to minimise friction. Using a set of pistons, the first wheel was then “walked” into position in the detector, in a series of small jumps. The second wheel will follow in the next few days. Over the coming weeks, the small wheels’ gas, water, and power cables will all be connected, ready for the commissioning phase to begin soon after.

Ariella described the experience of witnessing the final steps of the first wheel’s journey as “moving”, but admitted to being a little nervous about the international press presence at the lowering of the second wheel. “There are going to be TV cameras everywhere! But we’re confident,” she smiled. “The crew are great – these people are really professional. A lot of work, a lot of people, and a lot of countries have been involved in this last large piece of ATLAS.”


Colin Barras


Ceri Perkins