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Trolley pole

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A trolley pole is a tapered cylindrical pole of wood or metal, used to transfer electricity from a "live" overhead wire to the control and propulsion equipment of a tram or trolley bus. The use of overhead wire in a system of current collection is reputed to be the 1880 invention of Frank J. Sprague.[1]

Origin of the termEdit

The term 'trolley' predates the invention of the trolley pole. The earliest electric cars did not use a pole, but rather a system in which each car dragged behind it an overhead cable connected to a small cart that rode on a 'track' of overhead wires. From the side, the dragging lines made the car seem to be 'trolling' as in fishing. Later, when a pole was added, it came to be known as a trolley pole.

The term trolley is also used to describe the pole or the passenger car using the trolley pole is derived from the grooved conductive wheel (trolley or troller) attached to the end of the pole that "trolls" the overhead wire. It was first used by an experimental tramway in Toronto, Ontario in 1883, having been invented by John Joseph Wright, brother of the mining entrepreneur Whitaker Wright. Modern "trolley cars" usually don't use the grooved trolley wheel at all, but use a grooved sliding "shoe" containing a carbon insert.

Description of the deviceEdit

The trolley pole sits atop a sprung base on the roof of the trolley vehicle, the springs maintaining the tension to keep the trolley wheel or shoe in contact with the wire. If the pole is made of wood, a cable brings the electrical current down to the vehicle. A metal pole may use such a cable, or may itself be electrically "live," requiring the base to be insulated from the vehicle body.

On systems with double ended railway cars capable of running in both directions, the trolley pole must always be pulled behind the car and not pushed, or dewiring is very likely, and it can also cause damage to the overhead wires. At terminus points therefore, the conductor must turn the trolley pole around to face the correct direction, pulling it off the wire either with a rope or a pole and walking it around to the other end. In some cases, two trolley poles are provided, one for each direction, so in this case it is just a matter of raising one and lowering the other. Since the operator could raise the pole at one end whilst the conductor lowered the other, this saved time and was much easier for the conductor.

Trolley poles are usually raised and lowered manually by a rope from the back of the vehicle. The rope feeds into a spring reel mechanism, called a trolley catcher. The trolley catcher contains a detent, like that in an automotive shoulder safety belt, which "catches" the rope to prevent the trolley pole from flying upward if the pole is dewired.

On some older systems, the poles were raised and lowered using a long pole with a metal hook. Where available, these may have been made of bamboo due to its length, natural straightness and strength, combined with its relative light weight and the fact that it is an insulator. Trolleybuses usually carried one with the vehicle, for use in the event of dewirement, but tram systems usually had them placed along the route at locations where the trolley pole would need reversing.

Single and double pole usageEdit

When used on a trolley car or tram, i.e., a railway vehicle, a single trolley pole usually collects current from the overhead wire, and the steel rails on the tracks act as the electrical return. Trolleybuses, on the other hand, must use two trolley poles and dual overhead wires, one pole and wire for the negative "live" current, the other for the positive or neutral return. The tramway system in Havana, Cuba also utilised the dual wire system.

In Toronto, the single pole is referred to as the Witch's Broom.

Decline in usageEdit

Trolley poles are still common on trolley buses, because they provide the most straightforward means of keeping separate the electrically live and ground contacts. A few locations, such as Philadelphia, Pennsylvania continue to use trolley poles, even on new street railway vehicles, because of the difficulty and expense of modifying long stretches of existing overhead to prevent other types of current collectors from fouling wire hanging hardware. Toronto, Ontario, also continues to use this system, and has recently built new track sections using the same technology.

On most railway vehicles using overhead wire, however, the trolley pole has given way to the bow collector or the pantograph, a folding construction of metal that presses a wide contact pan against the overhead wire. While more complex than the trolley pole, the pantograph has the advantage of being almost free from dewiring, being more stable at high speed, and being easier to raise and lower automatically. Also, on double ended trams, they eliminate the need to manually turn the trolley pole when changing direction, and eliminate the need for wire frogs to make sure the pole goes in the correct direction at junctions.

See also Edit

References Edit

  1. Dr. Romin Koebel (2005). Boston Transit Milestones. MIT Open Courseware. Retrieved on 2006-10-24.

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