Deep groove ball bearings are about to be replaced on critical diagnostics equipment at the world’s largest nuclear fusion centre based in Culham, Oxfordshire. The bearings have been running for 16 hours a day for more than 25 years without any cleaning orre-lubrication.
EFDA-JET is currently the largest nuclear fusion research facility in the world. Situated at the Culham Science Centre, UK, JET (Joint European Torus) is used collectively by EURATOM Associations from more than 20 European countries. The JET device is currently the world’s largest Tokamak nuclear fusion machine. Its unique features enable the facility to investigate nuclear fusion’s potential as a safe, clean, and virtually limitless energy source for future generations.
The facilities include plasma-heating systems capable of delivering up to 30MW of power, an Active Gas Handling System and a Beryllium Handling Facility providing JETwith a unique Tritium and Beryllium capability, respectively.
The diagnostics team uses a vast array of sensors and other diagnostic systems to constantly monitor the machine. One critical diagnostic is the Far Infrared (FIR) interferometer used for measuring plasma density. This is achieved by sending FIR laser beams through very hot plasma of approximately 100 million degrees Celsius. The JET interferometer is a very large diagnostics system that contains thousands of optical elements. The system has an 80-metre long optical path and a large optical tower that weighs around 70 tonnes and is 15m in height.
In total, six deep groove ball bearings are being used on three diffraction grating wheel assemblies that are critical components of the FIR interferometer. These are used to modulate the FIR laser beams prior to being combined at the cryogenic detectors (lnSb Liquid Helium cooled detectors).
Four of these bearings (i.e. two grating wheel assemblies) have been running for more than 25 years, with the other two bearings operating on another grating wheel since 1992.
The fast-rotating diffraction grating wheels are made of aluminium with micron-sized grooves on their top surface. Each wheel is around 13cm in diameter and 3,600 grooves, with the facets around 100 microns in size. The grating wheel assembly has a shaft with two wheels of the same weight (around 500g) that rotates at 1,800 rpm for 16 hours a day. The system is driven by a belt pulley system and brushless DC motor, which minimise system vibration down to micron levels. Dr Alexandru Boboc, Senior Researcher at EFDA-JET comments: “The bearings are absolutely critical as they are part of one of the two essential diagnostics on JET. If a single bearing fails, the experimental programme of the JET machine would have to be stopped and we would incur costs of several hundred thousand pounds per day. That is without causing frustration to my colleagues all over the world that had to prepare the experiments for that particular day, months in advance.”
The grating wheel assemblies are positioned on an enclosed optical table in a very dry atmosphere (-50°C Dewpoint as in the Antarctica) as the FIR beam is absorbed if the air is humid. The wheels run from 4.30 until 22.30pm each working day during JET operation.
The bearings originally supplied to EFDA-JET had steel balls and were greased for life. The bore size of the bearing was 20mm, with an outside diameter of 42mm and a width of 12mm. The bearings were supplied with a two-piece, fibre-reinforced phenolic-aluminium cage and two, non-contacting shields or closures retained in the outer ring, which help retain the lubrication.
Mark Pritchard, Senior Product Engineer at Barden Corporation (UK) Ltd, comments: “The bearing cages are of a two-piece design, machined from cylindrical segments of phenolic and armoured with aluminium side plates, secured with rivets. The bearings were constructed with double shields, ensuring lubrication retention and prevention of contamination ingress.”
“Although the bearings are from our standard catalogue range, all Barden bearings are manufactured to super precision standards. In that sense, it is not surprising to discover an application in which the bearings have greatly exceeded their expected operating life. However, operating for 25 years in such acritical environment is impressive and serves to illustrate the high reliability that our bearings provide.”