Trolley pole
From Wikipedia, the free encyclopedia
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]
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[edit] Origin of the term
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.
The grooved trolley wheel was used on many large city systems through the 1940s and 1950s; it was generally used on systems with "old" style round cross sectional overhead wire. The trolley wheel was problematic at best; the circumferential contact of the grooved wheel bearing on the underside of the overhead wire provided minimal electrical contact and tended to arc (spark) excessively and maximized overhead wire wear . The newer sliding carbon trolley shoe was generally used with a "newer" grooved overhead trolley wire of a figure "8" cross section; the great advantage of the sliding trolley shoe was threefold; it provided far better electrical contact with a great reduction in arcing (sparking), it dramatically reduced overhead wire wear as well. Many systems began converting to the sliding trolley shoe in the 1920s; Milwaukee, Wisconsin converted its large system in the late 1920s. Curiously, Philadelphia did not convert its trolley wheels on its remaining streetcars until 1978. Although a streetcar with a trolley wheel may evoke a look of old fashionedness, the trolley shoe is "modern" and far more practical as well as economical in use.
[edit] Description of the device
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 many 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. Care must be taken to raise the downed pole first, to eliminate the damage caused by arcing between the pole and wire. In the United States, the dual-pole system was the most common arrangement on double end vehicles. However, pushing of the pole (termed "back-poling" in the US or "spear-poling" in Australia, was quite common where the trams were moving at slow speeds, such as at delta (or wye) terminals and whilst backing into the sheds.
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 or "trolley retriever". 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. The similar looking retriever (see photo) adds a spring mechanism that yanks the pole downward if it should leave the wire, pulling it away from all overhead wire fittings. Catchers are commonly used on trams operating at lower speeds, as in a city, whilst retrievers are used on suburban and interurban properties to limit damage to the overhead at speed.
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.
[edit] Single and double pole usage
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 positive "live" current, the other for the negative or neutral return. The tramway system in Havana, Cuba also utilised the dual wire system, as did the Cincinnati (OH) streetcar system. To aid in the reduction of spread-out electrolytic damage to underground pipes and metallic structures, most tram lines operated with the wire positive with respect to the rails.
In Toronto, the single pole is referred to as the Witch's Broom.
[edit] Decline in usage
The use of two trolley poles are common on trolley buses or trackless trolleys worldwide. The trolley coach requires dual poles to complete an electrical circuit, whereas the streetcar (tram) has its electrical circuit between a single overhead wire and completed via steel tires on steel rails. The advantage of double trolley poles, which are longer than those used on trams, is that they allow the bus a degree of lateral steerability which enables the bus to load passengers at curbside as do all buses.
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.
- ^ Dr. Romin Koebel (2005). "Boston Transit Milestones". MIT Open Courseware. http://ocw.mit.edu/OcwWeb/Civil-and-Environmental-Engineering/1-012Spring2002/Readings/detail/green_line_project.htm. Retrieved on 2006-10-24.
