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Like an oil pocket

29/04/2003

As usual, we will make everything as clear to you as water...except that this time the subject is oil. This superstructure, with all of its components, weighs 400 tonnes - and its entire weight must be spread across bearings so that it 'floats' on an oil pocket.

Escalators, windmills, sewing machines, vehicles, Apollo II, vacuum cleaners, roller skates...what do they all have in common? They all depend on the same thing - bearings. The average home has up to 150 of them and the GTC will have just as many - but the bearings the GTC uses will be very different.

They will be hydrostatic bearings and they will make the GTC ‘float’ on a film of pressurised oil. These bearings will make the GTC so sensitive to movement that just a touch of the hand will move the whole telescope structure, mirrors and instruments included - and it will move smoothly, accurately and with minimum effort.

What is hydrostatic lubrication?

Hydrostatic lubrication is the injection of pressurised lubricant into the space between two surfaces, so that it forms a film thick enough to allow the surfaces to slide across one another without the friction and damage that could make them heat up and so wear away.

8 elevation bearings in the elevation axis will hold up the tube, whilst 8 azimuth bearings, in axial-radial formation between the azimuth ring and each of the GTC’s ‘legs’, will hold up the mount. The entire bearing for the GTC will be made up of the hydrostatic bearings and the azimuth ring - in other words, the azimuth ring itself will be a component of this gigantic bearing.

To make the system work, pressurised oil will be injected over the washers to create a lubricating film 70 microns (0.07 mm) thick. As it overflows, the oil will collect in trays along the sides of the azimuth ring before being propelled through pipes by gravity into the 2,700 litre oil tank located in the pump room. Next the oil will pass through the pumps, filters and refrigeration unit before being recirculated into the telescope. The refrigeration unit will cool the oil to ten degrees below the ambient temperature, compensating for the heat it will pick up as it returns to the telescope so that, when it arrives back, it will be at exactly the same temperature as the telescope chamber.

Currently under construction by the Swedish company SKF, the bearings will spew out 70 litres of oil per minute at a pressure of 120 bars.

When completed, the bearings will allow 400 tonnes to move at the touch of a hand - and all so that life at the GTC floats along on an oil pocket.

Natalia R. Zelman

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