Bow collector

A bow collector is one of the three main devices used on tramcars to transfer electric current from the wires above to the tram below. While once very common in continental Europe, it has now been largely replaced by the pantograph.

Origins

When the bow collector was first conceived by German inventor Ernst Werner von Siemens in the late 1880s, American inventor Frank J. Sprague of Virginia had just patented his trolley pole system of current collection from an overhead wire. To avoid contravening this patent, the Siemens company was forced to design its own, unique form of current collection, namely the bow collector. The bow collector was first used by the Siemens electric company in its early electric tramcars in either the late 1880s or early 1890s. The Hobart electric tramway system - the first of its kind in the Southern Hemisphere, opened in 1893 - used Siemens cars with very early bow collectors. Many other continental European and some English tramway systems also used this method.

Construction

The bow collector is one of the simplest and most reliable methods of current collection used on tramways. The very earliest versions were simply very heavy-gauge wire or steel bars bent into a rectangular shape and mounted long-side-down on the tramcar roof. The height of the collector was such that its top edge would scrape along the wire above. The top section is made of a 1 inch broad (or thereabouts) steel rod, machined to have a bow-shaped cross section, hence the name. This bow shaped rod is referred to as the 'collector plate', and in later models may be up to several inches wide. Unlike many trolley poles, the bow collector does not have a revolving base, but is rather fixed centrally to the tramcar roof.

In the late 1900s the simple framing methods mentioned above were gradually replaced by more complex and sophisticated methods, but the general mode of operation remained the same. The changes of design are most noticeable on systems where both double- and single-deck cars were used on the same system. Single deck trams usually have tall and lightly constructed collectors with complicated frames to support the heavy cast-steel collector plate, while double deck cars usually have heavier collectors with less complicated frames.

To maintain good electrical contact, the bow collector must exert quite strong pressure on the wire above, and so complicated systems of springs or weights were put into use to ensure good electrical contact, and hence efficient operation was maintained.

The steel rails on the tracks act as the electrical return.

Operation

Properly the bow collector should be mounted in such a way so that the top edge of the collector plate would rise several inches above the wire when the collector frame is standing straight up. Thus the collector usually leans opposite to the direction of travel; when the time comes to travel in the opposite direction, the collector must be swung over. To allow this to happen, the overhead wire must be raised by several inches at places where the bows are swung over, such as terminals and turn-outs. This operation is usually achieved by ropes and pulleys. The collector is folded down to a horizontal position when the car is not in use.

Some early cars had no means to swing the bows over. It was thought that this would happen automatically when the tramcar started travelling the other way, but collectors such as these were a failure.

Most Soviet trams (of which some are still in use in ex-USSR) had no means to swing the bows over. These trams were not designed to travel two ways. Another example is KTV-55-2 tramcar which had two bow collectors for the two directions of travelling.

Advantages and Modern Usage

The bow collector has fewer moving parts than the trolley pole, but is heavier and sometimes more complicated to construct. The construction of overhead wires for bow collectors is simpler than trolley pole wiring. As bow collectors do not have revolving mountings, the collector cannot jump off the wire or follow the wrong one at intersections, as trolley poles sometimes do. Thus overhead 'frogs' and guides for trolley poles are not necessary with bow collectors. Bow collectors are, however, much noisier than trolley poles.

The overhead wires for bow collectors are stretched tighter than for trolley poles, and straight sections are 'staggered', that is, the wire does not run completely straight down the centreline of the track, but rather zig-zags slightly across a small distance. This distributes wear across the bow collector's collector plate, and extends the collector's life.

In addition to some vintage tramways, bow collectors are still used in some tram systems in ex-USSR, e. g., in Kazan, Minsk, Dzerzhinsk, Daugavpils.

See also

* Pantograph
* Trolley pole
* Third rail
* Overhead lines