Motion System Design

Special bearings brave heat, moisture, radiation

Do neutrinos have mass? That's the question engineers are trying to answer with the Main Injector Neutrino Oscillator Search (MINOS) experiment, taking place at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill. One of the difficulties engineers faced during the experiment involved bearing selection; radioactive environments present one of the most difficult challenges for conventional bearings. Steel swells and lubricants break down from radiation damage, causing bearings to seize up.

Fermilab engineers faced this challenge when designing the two focusing horns in the Neutrinos at the Main Injector (NuMI) beamline at Fermilab, which aim the beam of neutrinos at a detector hundreds of miles away. They solved the problem by using Graphalloy graphite-nickel alloy bearings from Graphite Metallizing Corp., Yonkers, N.Y. These bearings perform well, according to Fermilab's engineers, because graphite presents a low-profile target for high-energy subatomic particles and the lubricity of graphite enables it to operate without lubrication.

MINOS is a long-baseline neutrino experiment designed to observe the phenomena of neutrino oscillations. MINOS uses two detectors, one at Fermilab and the other located 450 miles away at Soudan Underground Mine State Park in Minnesota. The goal is to determine whether the neutrinos change from one of three families of the ghostly subatomic particles to another. Theory suggests that neutrinos should oscillate, but only if they have mass, and the mass is not identical for the three neutrino flavors. The experiment will determine how many neutrinos of each flavor leave Fermilab and how many arrive at Soudan. The NuMI beam production line uses protons from the particle accelerator at Fermilab, which strike a graphite target, knocking positively charged pions out of the target.

The pions are directed by an electromagnetic field generated by two magnetic focusing horns into a Decay Pipe 2 m in diameter and 675 m long, where they decay into neutrinos. A 20-ton steel positioning module suspends the horns. Since neutrinos are chargeless, the mechanically adjusted focusing horns present the last chance to aim the pion beam at the detector hundreds of miles away. Adjusting the horns requires providing precise movement for objects that weigh up to 2,500 lb.

Normally, an object of this size is moved by mounting it on steel bearings, using gears and a motor to rotate the assembly. In this case, however, the focusing horn is located in the middle of a beam of subatomic particles capable of knocking neutrons out of steel molecules and converting them into radioactive isotopes. Many of these isotopes break down into other materials and destroy bearing performance. When this happens, steel swells from the radiation damage, locking up the bearings.

Heat and moisture are also potential problems in the beamline environment. To keep heat in check, air and water cooling maintain temperatures under 100° C. Moisture is a concern because particle beams interacting with moist air can generate nitric acid. Dehumidification is the answer, keeping humidity at relatively low levels.

After lengthy analysis, Fermilab engineers concluded that steel bearings won't work in a radioactive environment. They also found that plastic bearings won't work either because they consist of organic chains that break down quickly in the presence of radioactivity. Ceramic bearings, although they're resistant to radiation damage, proved to be too expensive and too easily damaged. This left the engineers without many options until they discovered that Graphalloy bushings resist radiation damage, work without lubrication, and stand up to corrosion.

There was one catch, however. Graphalloy bearings weren't able to support the full thrust load of the horns. In answer to that, Fermilab engineers developed a creative solution. They located the motion control equipment that suspends the horns on the upper side of the positioning module where they encounter fewer subatomic particles. Steel bearings are used in this area to handle the large thrust loads involved in moving the horns. The radial loads, on the other hand, can only be handled by bearings that are exposed to the particle stream. Fermilab engineers used a design in which Graphalloy bushings are press-fit into a register in a way that they see primarily radial loading.

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