Pantograph (rail)
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A pantograph is a device that collects electric current from overhead lines for electric trains or trams. The term derives from the resemblance to pantograph devices for copying writing and drawings.
The pantograph, according to the late rail historian Harre Demoro, was invented by the Key System shops for their commuter trains in the East Bay section of the San Francisco Bay Area in California. They appear in photographs of the first day of service in 1903. For many decades thereafter, the same diamond shape was used by electric rail systems around the world, and remains in use by some today.
However, the most common type today is the so called half-panto (sometimes 'Z'-shaped), with only one pair of interlocking arms, which is simpler to construct but is not generally as robust. The half-pantograph has met criticism because reverse moves require reorienting the pantograph, while the diamond-shaped pantograph moves smoothly in either direction.
The electric transmission system for modern electric rail systems consists of an upper load carrying wire (known as a catenary) from which is suspended a contact wire. The pantograph is spring loaded and pushes a contact shoe up against the contact wire to draw the electricity needed to run the train. The steel rails on the tracks act as the electrical return.
As the train moves, the contact shoe slides along the wire and can set up standing waves in the wires which break the contact and degrade current collection. This means that on some systems adjacent pantographs are not permitted. Pantographs are the successor technology to trolley poles, which were widely used on early streetcar systems and still are used by trolleybuses, whose freedom of movement and need for a two-wire circuit makes pantographs impractical, and by some streetcar systems, although in most cases only heritage lines which use old-fashioned vehicles. A notable exception is Toronto, Canada. Until recently, Melbourne was the largest surviving exception.
Pantographs with overhead wires are now the dominant form of current collection for modern electric trains, because while they are more expensive and fragile than a third-rail system, they also allow for higher voltages.
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[edit] Single- and double-arm pantographs
Pantographs may have either a single or a double arm. Double arm pantographs are usually heavier, requiring more power to raise and lower, but may also be more fault tolerant. For example, "... NJT encountered a whole bunch of wire downings on the Northeast Corridor Branch (New York City - Trenton, NJ) before they decided to replace the pantographs on Arrow-III trains with a more forgiving dual arm design, possibly in 1991...."[citation needed]
On railways of the former USSR, the most widely used pantographs are those with a double arm ("made of two rhombs)", but since the late 1990s there have been some single-arm pantographs on Russian railways. Some streetcars use double-arm pantographs, among them the Russian KTM-5, KTM-8, LVS-86 and many other Russian-made trams, as well as some Euro-PCC trams in Belgium. American streetcars use either trolley poles or single-arm pantographs.
[edit] Metro systems and overhead lines
Most rapid transit systems are powered by a third rail, but some use pantographs, particularly ones that involve extensive above-ground running. (Hybrid metro-tram or 'pre-metro' lines whose routes include tracks on city streets or in other publicly-accessible area must of course use overhead wire, since a third rail would normally present too great a risk of electrocution.) The only current exception to this is the new Bordeaux tram system which uses a system called alimentation par sol which only applies power to segments of track that are completely covered by the tram. This system is used in the historic centre of Bordeaux where an overhead wire system would cause a visual intrusion.
[edit] References
The Key Route, Harre W. Demoro, (2 v.), Interurban Press (1985)
[edit] See also
