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Rails to fly on

14/10/2003

According to the Real Academia Española, rails are "railroad highways." However, the Gran Telescopio CANARIAS (GTC) has rails too: they allow the dome to rotate. Its 500 tonnes rest on "carts", rectangular boxes which, with all their constituent components, weigh over a tonne each. They are the dome's wheels and the GTC will have twenty of them, all resting on a rail that is very like those used by trains.

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Thanks to information provided by RENFE, the Spanish national rail network) and GRANTECAN, the public company that is building the telescope), we can compare the means by which the AVE high speed train ‘flies’ and the GTC’s dome rotates.

ABOUT TRAINS AND DOMES

The rails that are now being laid for the Madrid - Barcelona AVE line are type 60E1, which weigh 60kg/ml. In lay terms this means that every metre of rail weighs 60 kg and, in addition, it has to be a specified width.

The GTC’s rails will weigh 101.3 kg/ml - more than railway tracks, because the GTC's rails will be significantly wider.

For railway tracks, rails can be bolted together or soldered with aluminium: the solder makes the join as rigid as the rails themselves, which is advisable for joining rails that need to carry dynamic loads. However, the solder is difficult to make: moulds have to be created and supported, the area has to be prepared and any rough edges have to be removed. At the GTC, all of this would have to be done at an altitude of nearly 2300 metres on top of a 13 metre high wall that is just half a metre wide. Normal electrode soldering is therefore preferable as it is much easier in conditions like these.

The material used in the 90 metres of rail for the GTC is chrome vanadium steel, which has a resistance of 110 kg/mm (every mm of surface area can carry 110 kg). Chrome and vanadium are added to the molten steel to produce an alloy with special properties - mainly in terms of strength.

For train rails, carbon magnesium steel is used. The GTC's rails are harder, stronger and they expand less. Expansion is the change in length experienced in traction tests; in other words, when a small amount of it is pulled with a set weight, the material used in our rails proves to be stronger and to expand less.

In general, although it may not sound like it, one type of rail is much like any other - even if it was not designed for ordinary use. The GTC dome rails are simply stronger and harder to avoid rapid wear caused by abrasion, which would lead to sections of track having to be replaced.

The dynamic load of the rails used in railway tracks is appreciably higher than that which will be produced by the rotation of the dome (trains will travel much faster than the dome will rotate). For this reason, railway tracks are softer to absorb impact energy more efficiently - a harder, more brittle material would be more liable to fracture.

Could you imagine having to replace a section of railway with the train and all its passenges still on the rails? That is what it would be like to have to change a section of rail at the GTC. That is why our rails have been designed to last for 50 years.

GENERAL TECHNICAL PROPERTIES OF THE RAILS:

RENFE: Type 60E1
GTC: rail A-120 as defined by DIN 536/1991

RENFE: Steel grade 260 C-Mn
GTC: Steel grade 110 Cr-V

Most of the steel used is carbon steel: its uses include construction (structures), ship hulls, machinery and vehicle bodywork. The magnesium acts as an antioxidant, making the steel easier to work with as well as stronger and harder. The GTC's rails will be chromium vanadium steel with a resistance of 110 kg/mm2. The percentages by weight of Cr and V are, respectively, 0.23 and 0.115.

Chrome hardens the steel alloy and gives greater wear and corrosion resistance. The vanadium also makes the alloy harder and gives greater resistance to wear and impact. This type of steel has the denomination 1.2842 in the metal and alloy numbering system. In other words it belongs to the "Special steels-tool steels" group, which is made up of steels that are used in tools and which have to be particularly hard.

RENFE: Traction Resistence (N/mm2) Rm > 880
GTC: Traction Resistance (N/mm2) Rm>1080

RENFE: Hardness (HBW) 260 / 300
GTC: Hardness (HBW) 320/350 (HBW: Hardness as defined using the Brinell scake: hardness is directly related to traction resistance - the greater the resistence, the greater the hardness.

RENFE: Expansion 10 %
GTC: Expansion 9%


GTC solder

The solder was manufactured by pouring the molten metal into a mould in situ, so that it conformed to the shape of the rail. The GTC’s dome rails were soldered together using a covered arc and the special ("repairing") welding electrode Magna 303.

Natalia R. Zelman

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