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Oil-e!

29/08/2005

'Oily' is an unpleasant sounding word but it perfectly describes the quality by which the 300 tonnes of the GTC can be moved with the slightest push. Engineers have tested it. Like an elephant balanced on a spider's web, the world's largest telescope is able to float on on something as simple as a drop of oil.

The ability of a 300 tonne superstructure to move on oil bearings that cause it to float seems surprising. A solution this ingenious makes its inventors worthy of the title engineer.

They could be seen to be as important as the bridge builders of Roman times, given that every manifestation of engineering, be it aqueducts or telescopes, is an extraordinary indicator of the scientific, technological and cultural health of a country.

In recent weeks the Gran Telescopio CANARIAS' movement system, an unassuming technological marvel that not only encompasses the beauty of design applied to technology but also provides an eloquent metaphor for human skill, has been undergoing testing. Generally speaking there are two ways of doing things, easy and difficult, and it is surprising how many people prefer the latter. At the GTC, however, solutions are evaluated by their viability and, most of the time, daring and simplicity are what allow engineers to overcome problems that seem insuperable.

An example of this is the hydrostatic bearings system that makes it possible for the giant mechanical structure of the telescope to move.

The proof

This strange method, which has already been used on other telescopes, allows the GTC to move on a thin layer of oil, so that a slight push is enough for the whole telescope, mirrors and instruments included, to move smoothly and precisely, generating no heat and producing no waste.

The process is achieved by injecting pressurised oil between two surfaces, creating a lubricating film 70 microns thick (a micron is smaller than a speck of dust), which allows the structure to slide. In addition, it is regulated by a system that avoids the GTC losing its precious liquid, so that any lubricant that overflows is chanelled and returned to the telescope at just the right temperature.

With the movement system working, the engineers have been able to prepare for commissioning the GTC's motors. The tube, which weighs 150 tonnes, moved precisely and deliberately and remained perfectly balanced (in the video it has been speeded up) like a drop of oil in a hot frying pan.

Hydrostatics and going back to school

Hydrostatics is a branch of mechanics concerned with liquids at rest and the forces that act through or upon them. Its name comes from the Greek Ydro, "water" and Statikos "that which remains firm or has weight." It is based on two principles that most people will have studied at school and would remember but for the failings of memory: Pascal's Principle and Archimedes' Principal.

Pascal's Principle states that, when pressure is applied to a liquid, the pressure will be equal at all points within that fluid. This means that the resulting pressure is much greater than the pressure applied. This is what happens, for example, in the hydraulic brakes of a car, where the application of the brake pedal exerts equal pressure on each of the four wheels.

The Archimedes Principle dictates that when a body is fully or partially submerged in a liquid, that body will be pushed upwards with a force equal to the weight of the volume of liquid displaced by it. This means that objects submerged in a liquid can 'float', or seem to be much lighter, even without dieting. This is the only reason why, in summer, we do not end up beached on the shore when we go swimming after eating a big meal.

So we cannot help but be impressed by this oily adventure which, using such simple principles, allows us to move an enormous Cyclops like the GTC with a gentle push. Such simplicity recalls an inscription by the Roman engineer Caio Lucia Lacer of 106 AD on the bridge in Alcántara: Ars ubi materia vincitur ipsa sua (a device by means of which matter overcomes itself). A quirk by which the law of gravity, which normally makes objects fall, in this case keeps them in place. In short, its inventiveness and its specialisms will forever be science's keenest allies.

Text: Iván Jiménez

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