Precision deep-groove ball bearings from Barden have been operating continually for more than 25 years on nuclear-fusion diagnostics equipment at the world's largest nuclear-fusion facility.

These ball bearings, at the EFDA-JET nuclear-fusion centre in Culham, Oxfordshire, will now be replaced.

The bearings have been running for 16 hours a day for more than 25 years without cleaning or re-lubrication.

The JET (Joint European Torus) device is used collectively by EURATOM Associations from more than 20 European countries and is the world's largest Tokamak nuclear-fusion machine.

The JET facilities include plasma-heating systems capable of delivering up to 30MW of power, an active gas handling system and a beryllium handling facility.

The JET diagnostics team uses an 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 hot plasma of approximately one-hundred-million centigrade.

The JET interferometer is a large diagnostics system that contains thousands of optical elements.

The system has an 80m-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, supplied by Barden, 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 (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 about 130mm 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 rotate at 1,800RPM 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, said: '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.' The grating-wheel assemblies are positioned on an enclosed optical table in a very dry atmosphere (-50C dew point as in the Antarctica); this is because the FIR beam is absorbed if the air is humid.

The wheels run from 4:30am until 10:30pm 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, said: '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.' Barden designs and manufactures precision bearings to a minimum of P4/ABEC 7 quality standards.

Bearing sizes range from five to 250mm outside diameter.

Most bearings are angular contact or deep-groove types.

The company can manufacture bearings to a geometric tolerance of P2 or better and envelope dimensions to P4 or better.

Raceway roundness is better than 0.5um, with raceway surface-finish better than 0.025um Ra.

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