Lever frame
Encyclopedia
Mechanical railway signalling
Railway signalling
Railway signalling is a system used to control railway traffic safely, essentially to prevent trains from colliding. Being guided by fixed rails, trains are uniquely susceptible to collision; furthermore, trains cannot stop quickly, and frequently operate at speeds that do not enable them to stop...

 installations rely on lever frames for their operation to interlock the signals
Railway signal
A signal is a mechanical or electrical device erected beside a railway line to pass information relating to the state of the line ahead to train/engine drivers. The driver interprets the signal's indication and acts accordingly...

 and points
Railroad switch
A railroad switch, turnout or [set of] points is a mechanical installation enabling railway trains to be guided from one track to another at a railway junction....

 to allow the safe operation of trains in the area the signals control. Located in the signal box
Signal box
On a rail transport system, signalling control is the process by which control is exercised over train movements by way of railway signals and block systems to ensure that trains operate safely, over the correct route and to the proper timetable...

, the levers are operated by the signalman
Signalman (rail)
A signalman or signaller is an employee of a railway transport network who operates the points and signals from a signal box in order to control the movement of trains.- History :...

.

Overview

By the movement of individual levers, signals, points, level crossing
Level crossing
A level crossing occurs where a railway line is intersected by a road or path onone level, without recourse to a bridge or tunnel. It is a type of at-grade intersection. The term also applies when a light rail line with separate right-of-way or reserved track crosses a road in the same fashion...

 gates or barriers and sometimes movable bridges
Swing bridge
A swing bridge is a movable bridge that has as its primary structural support a vertical locating pin and support ring, usually at or near to its centre of gravity, about which the turning span can then pivot horizontally as shown in the animated illustration to the right...

 over waterways are operated via wires and rods. The signaller chooses the correct combination of points, facing point locks and signals to operate, which will control the movement of each train through his or her area of control.

The lever frame contains interlocking
Interlocking
In railway signalling, an interlocking is an arrangement of signal apparatus that prevents conflicting movements through an arrangement of tracks such as junctions or crossings. The signalling appliances and tracks are sometimes collectively referred to as an interlocking plant...

 designed to ensure that the levers cannot be operated to create a conflicting train movement. Each interlocking installation is individual and unique to the location controlled. The interlocking may be achieved mechanically or by electric lever locks, or (more usually) a combination of both.

Signals or points located some distance away from the signal box are sometimes electrically rather than mechanically operated. Movement of the controlling lever operates an electrical circuit controller. In the UK, it is practice to cut short the handles of any levers controlling electrical apparatus, to remind signalmen that little effort is required to move them.

Mechanical lever frames and interlocking have now largely been replaced by modern, much larger electrical or electronic route interlockings located in Power Signal Boxes and more recently Integrated Electronic Control Centre
Integrated Electronic control centre
The Integrated Electronic Control Centre was developed in the late 1980s by the British Rail Research Division for UK-based railway signalling centres, although variations exist around the world...

s which are able to control much larger areas of the rail network.

The world's largest lever frame (192 levers) is thought to be in Spencer Street No.1 signal box in Melbourne, Australia.

Ground frames

A ground frame is a small lever frame located beside a railway, usually at ground level. A ground frame does not normally require any form of shelter since it is usually operated by traincrew and not permanently staffed. A ground frame is typically used to control the points giving access to an infrequently used siding
Rail siding
A siding, in rail terminology, is a low-speed track section distinct from a running line or through route such as a main line or branch line or spur. It may connect to through track or to other sidings at either end...

 or an emergency crossover, where the expense of providing control from a remote signal box cannot be justified. In some situations, signals may be worked from a ground frame.

In most cases, a ground frame has to be released from a signal box before it can be used. Typical methods of release include:
  • Mechanical midway release.
  • Release by key (e.g. Annett's key
    Annett's key
    In railway signalling, an Annett’s key or Annett key is a large key that locks levers or other items of signalling apparatus, thereby functioning as a portable form of interlocking. When not in use, the key is normally held in an Annett's lock that is fixed to the lever or apparatus concerned...

    ).
  • Release by single line token
    Token (railway signalling)
    In railway signalling, a token is a physical object which a locomotive driver is required to have or see before entering onto a particular section of single track. The token is clearly endorsed with the name of the section it belongs to...

    .
  • Electrical release.
  • Track circuit
    Track circuit
    A track circuit is a simple electrical device used to detect the absence of a train on rail tracks, used to inform signallers and control relevant signals.- Principles and operation :...

     release.


Once the ground frame has been released, the other levers controlling the points could be moved. When shunting was completed, the release cannot be restored until all levers are set back to their normal positions, allowing the main line to be used again. The train would then continue on its way.

If it was necessary for the train to be on the sidings controlled by the ground frame for a considerable time, it was desirable to release the running line so other trains could travel through the block section. To achieve this, a token instrument would be provided in a lineside hut near the ground frame. Once the train was off the main line, the points would be set back to normal, allowing the token to be removed from the ground frame. It would be then inserted into the token instrument in the hut, releasing the instruments at both ends of the section, so either signal box could then remove a token for another train to use the line, whilst the shunting train was in the sidings.

The modern equivalent of a ground frame, with switches instead of levers, is called a Ground Switch Panel (GSP). Points or signals worked from a ground switch panel are usually electrically operated.

Power frames

The lever frame principle of operation was perpetuated in the earliest power signalling installations. Since the signals and points worked electrically, no mechanical effort was needed to move the levers, which could therefore be miniaturised. In some cases, the interlocking was still done mechanically, but in others, electric lever locks were used.

Westinghouse Brake & Saxby Signal Co. Ltd. was one of the companies that produced miniature power lever frames. All-electric power lever frames were produced from 1929 onwards; earlier mechanical frames were produced before this date. In the UK, the Style 'L' all-electric miniature power lever frame was the most predominant on UK main line railways.

The Style 'B' & 'K' miniature power lever frames were the historical precursor frames in the development process of the Style 'L' frame.

London Underground railways used miniature power lever frames; initially Style 'B' was used. Later on, Style 'N' was introduced. This is a mechanically locked version of the Style 'L' frame. Style 'M', 'M2' and 'V' were also used.

Identification of lever functions

To assist the operator in determining their functions, each lever in a frame will generally be uniquely labelled, one common method being to number the levers in order from left to right. A lever's identification may be painted on its side or engraved on a badge or plate fitted either to the lever or behind it. This may be accompanied by a description of the lever's function. Usually, a large track diagram is positioned within easy view of the operator, which clearly shows each lever number adjacent to symbols representing the items of equipment that they operate. Levers are commonly coloured according to the type of equipment they control, the code of colours varying between different railway administrations. For example, in British practice, the following code generally applies: a red lever controls a stop signal or shunt signal, a yellow lever controls a distant signal, a black lever controls a set of points, a blue lever controls a facing point lock, and a white lever is spare. Brown levers are used to lock level crossing gates.

Manufacturers

In the UK, larger railway companies developed their own mechanical interlocking systems e.g. the Great Western Railway
Great Western Railway
The Great Western Railway was a British railway company that linked London with the south-west and west of England and most of Wales. It was founded in 1833, received its enabling Act of Parliament in 1835 and ran its first trains in 1838...

 (GWR) and the London and North Western Railway
London and North Western Railway
The London and North Western Railway was a British railway company between 1846 and 1922. It was created by the merger of three companies – the Grand Junction Railway, the London and Birmingham Railway and the Manchester and Birmingham Railway...

(LNWR), whilst smaller independent railway companies sometimes installed commercially obtained signalling products from companies such as The Railway Signal Company (RSC) and The Westinghouse Brake and Signal Company (WB&SCo).

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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