A radioactive service environment provides one of the most difficult challenges for conventional bearings. Steel swells and lubricants break down from radiation damage, causing the bearings to quickly seize up.
These difficulties posed a key challenge for the engineers at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill., who were designing the critical beamline elements for the Main Injector Neutrino Oscillator Search (MINOS) experiment, developed to answer the question of whether neutrinos have mass.
Fermilab mechanical engineers faced this lubrication challenge when designing the two focusing horns in the Neutrinos at the Main Injector (NuMI) beamline at Fermilab. These horns aim the beam of neutrinos at a detector hundreds of miles away. Conventional lubricants wilted quickly under the barrage of radiation, and synthetic lubes also failed.
The problem was solved using graphite-nickel alloy bearings. These trademarked Graphalloy bearings have been in operation for nearly three years at the facility without a single failure. They perform well, according to Fermilabs Kris Anderson, associate department head for engineering at the Accelerator Division/Mechanical Support, because graphite presents a low-profile target for high-energy subatomic particles and the lubricity of graphite enables it to operate without fluid lubrication.
The MINOS Experiment
MINOS is a long-baseline neutrino experiment designed to observe the phenomena of neutrino oscillations, an effect related to neutrino mass. MINOS uses two detectors, one located at Fermilab, at the source of the neutrinos – and the other located 450 miles away at Soudan Underground Mine State Park in Tower-Soudan, Minn. The goal of the experiment is to determine whether the neutrinos oscillate, or change from one of the 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 of each flavor arrive at Soudan. Scientists will also be looking to identify the maximum fraction of the neutrino beam that can change from one flavor to another and the distance required for the beam of neutrinos to change to another flavor then back again to the original flavor.
The NuMI beam production line utilizes protons from the particle accelerator at Fermilab. The protons 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 which is 2 meters in diameter and 675 meters long; here they decay into neutrinos.
The focusing horns themselves weigh more than a ton each, and are suspended by a 20-ton steel positioning module that provides radiation shielding, motion and signal line routing. Each focusing horn generates a magnetic field with peak field of 3 Tesla resulting from a 200 kilo Ampere peak current. Since neutrinos are chargeless, the focusing horns present the last chance to aim the pion beam at the detector hundreds of miles away. Tolerances for the horns are naturally very tight – with a total alignment accuracy of 0.010 inch.
The Bearing Problem
To aim the beam at the detector, mechanical adjustment of the focusing horns is required of course. This means providing precise movement for objects that weigh up to 2,500 pounds. Normally, motion control is provided for an object of this size by mounting it on steel bearings with rotating elements. Gears and a motor are then used to move the object.
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 a radioactive isotopes. Many of these isotopes will quickly break down into other materials and destroy the performance of the bearing.
When this happens, the steel swells up from the radiation damage and locks up the bearings.
In addition, the lubricants most often used in this type of application are organic and will break down quickly in a radioactive environment. Synthetic lubricants cannot be used because of their short life in this environment. Additionally, nitric acid is generated by the interaction of the beam with moist air. This chemical attacks steel and further shortens its life. Anti-corrosion coatings have been tried in this application but tend to flake off quickly due to radiation damage.
As a result of these problems, we long ago concluded that steel bearings will not work in a radioactive environment and began looking for an alternative, Anderson said. Plastic bearings are not the solution because they consist of organic chains that break down very quickly in a radioactive environment. Ceramic bearings are resistant to radiation damage but they are expensive and can easily be damaged if particulates find their way into races or cages.
We were open to finding something new and discovered them a few years back when we heard about Graphalloy bushings. he continued. These bushings offer several key advantages, including resistance to radiation damage, ability to function without lubrication, and resistance to corrosion and particulate matter.
Graphites Advantage
Graphalloy bearings are resistant to radiation damage because the most common isotope of carbon has a lower atomic weight than the most common isotope of iron. The lower atomic weight means that there are far fewer heavy atomic particles in its nucleus, which in turn provides a much smaller cross-sectional target for subatomic particles.
The result is that Graphalloy bearings provide good performance when exposed to radiation. Some of the first Graphalloy bearings at Fermilab have been in use in beamlines exposed to subatomic particle beams for more than a decade without experiencing any damage.
Graphalloy bearings also do not require lubricants due to the special properties of graphite. Its self-lubricating features allow for the elimination of grease or oil that would normally evaporate, congeal or solidify and cause premature bearing failure. The material provides a constant, low coefficient of friction rather than just a surface layer, helping to protect against catastrophic failure.
Because carbon-graphite materials are inherently stable and chemically resistant, Graphalloy is easily able to resist nitric acid as well as other acids, alkalies, hydrocarbons, black liquor, and liquid gases.
Designing the Equipment
Because of his long experience using Graphalloy in applications involving exposure to subatomic particle beams, Anderson had no hesitation about using the bearing material in the critical positioning applications on the NuMI neutrino beam-line. However, Graphalloy is unable to support the full thrust load of the 2,500-pound focusing horns – so Fermilab engineers developed a creative solution. They located the motion control equipment including motors, gearboxes and thrust bearings that suspend the focusing horns on the upper side of the positioning module where they encounter less direct high-energy subatomic particles. Steel bearings are used in this area to handle the thrust loads involved in moving the focusing horns.
The radial loads can only be handled by bearings that are exposed to the particle stream. Fermilab engineers utilized a design in which Graphalloy bushings are press-fit into a register in such a way that they see primarily radial loading. The bushings used on the focusing horns range from 1 inch to 3 inches in outside diameter.
Graphalloy consists of graphite filled with a metal impregnant to enhance the chemical, mechanical and tribological properties of the material. The graphite matrix can be filled with a variety of impregnants to enhance chemical, mechanical and tribological properties. Fermilab used the GM111.3 formulation, which consists of carbon and nickel.
Top-side Uses
Graphalloy bearings are also used in many applications on the top side of the positioning module. This area is further from the primary particle beam, so radiation damage is not a major concern. The ability of Graphalloy bearings to operate without lubrication and resist corrosion damage provides an advantage over steel in many other applications where the bearings are not required to withstand high thrust loading.
Some NuMI bearing applications run on a weekly basis while others may not run for 18 months. Bearings not used for an extended period of time have a tendency to corrode. Graphalloy eliminates this problem because of its self-lubricating and corrosion-resistance properties, and because a thin layer of graphite coats and protects the mating shaft.
Graphalloy bearings are providing the precision positioning needed to focus the neutrino beam on a detector 450 miles away with complete reliability at an economical cost, Anderson concluded. Graphalloy bearings have been used in the NuMI beamline since it began operations in 2005. As with our earlier applications of these bearings, we have not seen a single failure nor have any of these bearings needed to be replaced for any reasons.