Precision bearings on nuclear fusion equipment
run for 16 hours a day for more than 25 years

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

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 or re-lubrication.

EFDA-JET (www.jet.efda.org) is currently the largest nuclear fusion research facility in the world. Situated at the Culham Science Centre in Oxfordshire, 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. JET's unique features enable the facility to investigate nuclear fusion's potential as a safe, clean, and virtually limitless energy source for future generations.

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 providing JET with a unique Tritium and Beryllium capability, respectively.

JET's 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, supplied by The Barden Corporation (UK) Ltd, 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 (ln Sb 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 deg C Dewpoint as in the Antarctica) as 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 were manufactured at Barden's plant in Danbury, USA. The bearings 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 a critical environment is impressive and serves to illustrate the high reliability that our bearings provide."

Barden designs and manufactures super precision bearings to a minimum of P4/ABEC 7 quality standards. Bearing sizes range from 5mm to 250mm outside diameter. Most bearings manufactured are either angular contact or deep groove types. The company is capable of manufacturing bearings to a geometric tolerance of P2 or better, and envelope dimensions to P4 or better. Raceway roundness is better than 0.5µm, with raceway surface finish better than 0.025µm Ra.

Barden Corporation (UK) Ltd in profile
Barden Corporation (UK) Ltd is a recognised world leader in super-precision bearing systems. For more than 60 years, Barden has built on its unrivalled manufacturing expertise and its ability to design bespoke engineering solutions for a variety of difficult applications. The company offers an extensive range of super-precision bearings for a variety of applications, including aerospace, turbomolecular pumps, medical instruments, machine tool and motorsport. Now part of the Schaeffler Group, Barden UK holds an enviable position alongside sister manufacturing plants around the world.

For further information, e-mail: caroline.evans@schaeffler.com or view website: www.bardenbearings.co.uk    Refer to page 75

Load measuring pins customised to suit any mechanical arrangement

A range of load measuring pins is now available from Applied Measurements Ltd, which can be designed and customised to suit almost any existing mechanical arrangement.

A load pin measures the force applied across it, typically via strain gauges that are installed within a small bore through the centre of the pin. Load pins are used in a wide variety of applications, including cranes, winches, hoists and other types of lifting equipment. Typically, the application includes some kind of pulley system or sheave with ropes or wires. The rotating pulley wheel is held in place on a shaft, which can be replaced by a load pin.

In these types of application, the load pins are designed and strain gauged to measure the load in a single direction or axis, since this is usually all that is required to provide the end user with the necessary information.

Other applications require load pins that are designed and strain gauged to monitor the load in two axes simultaneously, normally at right angles to each other. This requires a two-axis load pin.

In most cases, the output signal from the pin is directly from the strain gauges and is normally a low level millivolt output. Often, this low level signal needs amplifying at the pin in order to overcome lengthy cable runs to the instrumentation. Where this is required, Applied Measurements can fit small amplifiers within the head of the load pins to increase the output signal to 0-10Vdc, 0-5Vdc or milliamp, usually 4-20mA.

Peter Lewis, Managing Director at Applied Measurements comments: "We can offer custom load pins at almost the same cost and lead time as our standard load pins. Most of our customers now require us to fit the load pin into their existing mechanical arrangement, which normally requires some sort of customisation. This could be a special IP68 sealing gland for a marine or offshore application, or a two-axis load pin with full instrumentation package to ensure a complete load monitoring solution."

Applied Measurements' standard range of load pins is the DBEP series. This offers capacities from 2.5 tonnes up to 50 tonnes, although the nature of customer applications means that load capacities are often supplied from as low as 200kg up to 1,500 tonnes.

DBEP pins are sealed to IP67 as standard, providing protection from harsh industrial environments. An optional IP68 submersible version is also available. The robust pins are manufactured from precipitation-hardened stainless steel.

For further information, e-mail: info@appmeas.co.uk or view website: www.appmeas.co.uk   Refer to page 87

Self-lubricating polymer slew ring bearings

Plastic bearing expert igus has developed a range of innovative slewing bearings that use two self-lubricating, low friction polymer sliding elements in place of conventional ball bearings. As a cost-effective, low noise alternative, the igus PRT polymer slew ring bearing modules are also highly wear resistant, easy to install and completely maintenance-free, while at the same time are robust and have a very high load capacity.

All housing parts are made of aluminium and use stainless steel screws. The polymer sliding pads in the PRT slewing rings are made of the high performance iglidur J material, which contains a solid lubricant, so it does not require additional lubrication. This material also offers a low coefficient of friction, as well as being resistant to chemical aggressors, featuring low moisture absorption and good vibration dampening properties. In addition, the counter-running surfaces of the iglidur J sliding pads are hard-anodized.

These innovative slewing bearings  have a very high-load capacity.

As explained by Matthew Aldridge of igus UK, the iglidur PRT slewing bearing modules are "rugged, corrosion-resistant units that are particularly well-suited in applications where high loads, extreme overturning torques, alternating ambient media or low to medium surface speeds are prevalent".
Maintenance  and lubrication-free, the iglidur PRT modules are ideal for OEMs looking for a ready-to-install solution, such as in semiconductor and cleanroom applications, assembly lines, wind turbines, production facilities and robots. The iglidur PRT slewing ring bearing system is available in a wide range of sizes, with outside diameters of 20, 30, 60, 100 200 and 300 mm.

For further information, view website: www.igus.co.uk   Refer to next page