Editing Overhead line (section)

== Tensioning ==
[[File:Oberleitung Radspanner IMG10010.jpg|thumb|Line tensioning in Germany]]

Catenary wires are kept in mechanical tension because the pantograph causes mechanical [[oscillation]]s in the wire. The [[wave]]s must travel faster than the train to avoid producing [[standing wave]]s, which could break the wire. Tensioning the line makes waves travel faster, and also reduces sag from gravity.

For medium and high speeds, the wires are generally tensioned by weights or occasionally by hydraulic tensioners. Either method is known as "auto-tensioning" (AT) or "constant tension" and ensures that the tension is virtually independent of temperature. Tensions are typically between {{convert|9|and|20|kN|lbf|abbr=on|lk=on}} per wire. Where weights are used, they slide up and down on a rod or tube attached to the mast, to prevent them from swaying. Recently, spring tensioners have started to be used. These devices contain a torsional spring with a cam arrangement to ensure a constant applied tension (instead of varying proportionally with extension). Some devices also include mechanisms for adjusting the stiffness of the spring for ease of maintenance.

For low speeds and in tunnels where temperatures are constant, fixed termination (FT) equipment may be used, with the wires terminated directly on structures at each end of the overhead line. The tension is generally about {{convert|10|kN|lbf|abbr=on}}. This type of equipment sags in hot conditions and is taut in cold conditions.

With AT, the continuous length of the overhead line is limited due to the change in the height of the weights as the overhead line expands and contracts with temperature changes. This movement is proportional to the distance between anchors. Tension length has a maximum. For most [[25 kV AC railway electrification|25 kV]] OHL equipment in the UK, the maximum tension length is {{cvt|1970|m|ft}}.<ref>{{cite web|url=http://eprints.whiterose.ac.uk/103249/1/Overheadline_paper_ICE_accepted_version.pdf|title=OHLE Modelling}}</ref>

An additional issue with AT equipment is that, if balance weights are attached to both ends, the whole tension length is free to move along the track. To avoid this a midpoint anchor (MPA), close to the centre of the tension length, restricts movement of the messenger/catenary wire by anchoring it; the contact wire and its suspension hangers can move only within the constraints of the MPA. MPAs are sometimes fixed to low bridges, or otherwise anchored to vertical catenary poles or portal catenary supports. A tension length can be seen as a fixed centre point, with the two half-tension lengths expanding and contracting with temperature.

Most systems include a brake to stop the wires from unravelling completely if a wire breaks or tension is lost. German systems usually use a single large tensioning pulley (basically a [[Ratchet (device)|ratchet]] mechanism) with a toothed rim, mounted on an arm hinged to the mast. Normally the downward pull of the weights and the reactive upward pull of the tensioned wires lift the pulley so its teeth are well clear of a stop on the mast. The pulley can turn freely while the weights move up or down as the wires contract or expand. If tension is lost the pulley falls back toward the mast, and one of its teeth jams against the stop. This stops further rotation, limits the damage, and keeps the undamaged part of the wire intact until it can be repaired. Other systems use various braking mechanisms, usually with multiple smaller pulleys in a [[block and tackle]] arrangement.