Screw Coupling
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Buffers and chain couplers (also known as "buffers and screw", "screw", "screwlink", and "English" couplers) are the de facto UIC standard railway stock coupling used in the EU and UK, and on some surviving former colonial railways, such as in South America and India, on older rolling stock. Buffers and chain couplers are an assembly of several devices: buffers, hooks and links, or turnbuckle screws. On the modern version of the couplers, rail vehicles are mated by manually connecting the end link of one
chain A chain is a serial assembly of connected pieces, called links, typically made of metal, with an overall character similar to that of a rope in that it is flexible and curved in compression but linear, rigid, and load-bearing in tension. A c ...
which incorporates a turnbuckle screw into the towing
hook A hook is a tool consisting of a length of material, typically metal, that contains a portion that is curved or indented, such that it can be used to grab onto, connect, or otherwise attach itself onto another object. In a number of uses, one e ...
of the other wagon, drawing together and slightly compressing the buffer pairs, one left and one right on each headstock. That limits slack, and lessens shunting shocks in moving trains. By contrast, vehicles fitted with the semi-automatic Janney Type E coupler can experience significant jarring during mating and shunting. Very early rolling stock had "dummy buffers", which were simple rigid extensions of the frame, but they were improved with the use of enclosed mechanical, then hydraulic, springs to damp possible jarring. Each chain incorporates both a hook and a turnbuckle.


Characteristics

The standard type of coupling on railways following the British tradition is the buffer and chain coupling used on the pioneering Planet class locomotive of the
Liverpool and Manchester Railway The Liverpool and Manchester Railway (L&MR) was the first inter-city railway in the world. It opened on 15 September 1830 between the Lancashire towns of Liverpool and Manchester in England. It was also the first railway to rely exclusively ...
of 1830. Those couplings followed earlier
plateway A plateway is an early kind of railway, tramway or wagonway, where the rails are made from cast iron. They were mainly used for about 50 years up to 1830, though some continued later. Plateways consisted of "L"-shaped rails, where the flange o ...
or
wagonway Wagonways (also spelt Waggonways), also known as horse-drawn railways and horse-drawn railroad consisted of the horses, equipment and tracks used for hauling wagons, which preceded Steam locomotive, steam-powered rail transport, railways. The t ...
practice, but were made more regular. The buffers and chain coupling is still the standard in European countries, except for the former Soviet Union, where the SA-3 automatic coupler is used. Coupling is done by a worker who must climb between the cars, who winds the turnbuckle to the loose position, and then hangs the chain on the hook. After that, the turnbuckle handle is stowed on the idle hook to prevent damage to itself, the vehicle, or the brake pipes. A dangling chain is only permitted during shunting. Disconnected brake pipes must be stowed on dummy connectors, to allow proper operation of the brakes. (The picture shows two coupled cars, with a single brake pipe.) The hooks and chain hold the carriages together, while the buffers keep the carriages from banging into each other so that no damage is caused. The buffers can be " dumb" or spring-loaded. That means there are no run-in forces on the coupler. The other benefit compared with automatic couplers is that its lesser slack causes smaller forces on curves, so there is less probability of a broken coupler on a curve than with automatic couplers. The disadvantage is the smaller mass of the freight that can be hauled by hook and chain couplers, the maximum being . Early rolling stock was often fitted with a pair of auxiliary chains as a backup if the main coupling failed. That made sense before the fitting of continuous
fail-safe In engineering, a fail-safe is a design feature or practice that in the event of a specific type of failure, inherently responds in a way that will cause minimal or no harm to other equipment, to the environment or to people. Unlike inherent safe ...
braking systems. On railways where rolling stock always pointed the same way, the chain might be mounted at one end only, as a small cost and weight saving measure. On
German German(s) may refer to: * Germany (of or related to) **Germania (historical use) * Germans, citizens of Germany, people of German ancestry, or native speakers of the German language ** For citizens of Germany, see also German nationality law **Ger ...
and Scandinavian railways, the left buffer is flatter than the slightly more rounded right one, which provides better contact between the buffers than would be the case if both buffers were slightly rounded.


Variants


Three-link couplings

A peculiarly British practice was the "loose-coupled" freight train, operated by the locomotive crew and a "guard" at the rear of the train, the successor to the brakesman of earlier times. That train type used three-link chain couplings for traction and side buffers to accept pushing forces but, since such trains were not fitted with an automatic through-train braking system, there were no pipes to connect between the vehicles. The last vehicle of the train was a heavily ballasted guard's van with its brakes controllable by a handwheel operated by the guard. The slack between vehicles was very convenient when starting heavy trains with a relatively low-powered locomotive on the level or on a rising gradient. On the driver's command the guard would apply his brake as hard as possible. The driver would then gently reverse to close up the wagons onto their buffers. Then the locomotive was driven ahead, picking up the load wagon by wagon, thus giving an easy start up the gradient. Wagons of that era didn't have roller bearings and the grease-lubricated plain bearings exerted considerable resistance, especially on a cold day, so starting wagon-by-wagon greatly reduced the traction force required from the locomotive. The disadvantage of that convenience was that the guard could get badly thrown about as the train changed speed due to the inter-wagon gaps opening or closing. In the worst case, the jerks could break a coupling or cause a derailment. A skilled guard would observe or listen to his train and apply or release his brake to keep the last few couplings reasonably taut, acting as a shock-absorber. The same effect occurred when the route changed gradient. When going over a hill the rear of the train would catch up with the wagons held back by the locomotive, but the guard could minimise that. That method of train working was why the guard, just like the driver, was required to prove his route knowledge before being given charge of a heavy train. Loose-coupled trains travelled at low speeds and were phased out in the 1970s. An improvement on the loose-coupled train is the "Instanter" coupling, in which the middle link of a three-link chain is specially triangular shaped, so that when lying "prone" it provides enough slack to make coupling possible, but when the middle link is rotated 90 degrees, the length of the chain is effectively shortened, reducing the amount of slack without the need to wind a screw. The closeness of the coupling allows the use of inter-vehicle pipes for train brakes. Three-link and Instanter couplings can be operated entirely from the side of the wagons, using a shunter's pole, which has a hook on the end, and is safer when shunting is being done. Similarly, the screw-adjustable coupler can be connected by a shunter's pole once it has been unscrewed. Ordinary three-link couplings have been superseded by instanter, screw or buck-eye couplers in UK freight trains today.


Center-buffer-and-chain(s)

On some narrow-gauge lines in
Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...
, and on the
Paris Metro Paris () is the capital and most populous city of France, with an estimated population of 2,165,423 residents in 2019 in an area of more than 105 km² (41 sq mi), making it the 30th most densely populated city in the world in 2020. Sin ...
, a simplified version of the loose-coupler is used, consisting of a single central buffer with a chain underneath. Sometimes there are two chains, one on each side of the coupler. The chain usually contains a screw-adjustable link to allow close coupling. These variants are also used elsewhere. On sharp curves, a single centre buffer is less likely to be subject to buffer-locking. The
Eritrean Railway The Eritrean Railway is the only railway system in Eritrea. It was constructed between 1887 and 1932 during the Italian Eritrea colony and connects the port of Massawa with Asmara. Originally it also connected to Bishia. The line was partly damag ...
also uses a centre buffer and chain coupler.


Buffer-and-chain on narrow gauge railways

Buffer-and-chain couplers are not suitable on lines with very sharp curves, because there is a buffer-locking problem if pushing the limit. Because of that, and because of
Carl Pihl Carl Abraham Pihl (16 January 1825 – 14 September 1897) was a Norwegian civil engineer and director of the Norwegian State Railways (NSB) from 1865 until his death. Pihl was one of the main architects of the use of narrow-gauge railways in Norw ...
's successful promotion of the single-buffer
Norwegian coupler A Norwegian coupling (or meat chopper) or claw hammer coupling or pickaxe coupling is a manual coupling consisting of a central buffer with a mechanical hook that drops into a slot in the central buffer. The system is only found on narrow gauge ra ...
that he designed to overcome the problem, conventional buffers-and-chain coupling is rarely employed on narrow-gauge systems, notable exceptions being the railway networks of
Senegal Senegal,; Wolof: ''Senegaal''; Pulaar: 𞤅𞤫𞤲𞤫𞤺𞤢𞥄𞤤𞤭 (Senegaali); Arabic: السنغال ''As-Sinighal'') officially the Republic of Senegal,; Wolof: ''Réewum Senegaal''; Pulaar : 𞤈𞤫𞤲𞤣𞤢𞥄𞤲𞤣𞤭 ...
/
Mali Mali (; ), officially the Republic of Mali,, , ff, 𞤈𞤫𞤲𞥆𞤣𞤢𞥄𞤲𞤣𞤭 𞤃𞤢𞥄𞤤𞤭, Renndaandi Maali, italics=no, ar, جمهورية مالي, Jumhūriyyāt Mālī is a landlocked country in West Africa. Mali ...
,
Tunisia ) , image_map = Tunisia location (orthographic projection).svg , map_caption = Location of Tunisia in northern Africa , image_map2 = , capital = Tunis , largest_city = capital , ...
and
Côte d'Ivoire Ivory Coast, also known as Côte d'Ivoire, officially the Republic of Côte d'Ivoire, is a country on the southern coast of West Africa. Its capital is Yamoussoukro, in the centre of the country, while its largest city and economic centre is ...
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Burkina Faso Burkina Faso (, ; , ff, 𞤄𞤵𞤪𞤳𞤭𞤲𞤢 𞤊𞤢𞤧𞤮, italic=no) is a landlocked country in West Africa with an area of , bordered by Mali to the northwest, Niger to the northeast, Benin to the southeast, Togo and Ghana to the ...
in Africa, and
Queensland ) , nickname = Sunshine State , image_map = Queensland in Australia.svg , map_caption = Location of Queensland in Australia , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , established_ ...
and
Tasmania ) , nickname = , image_map = Tasmania in Australia.svg , map_caption = Location of Tasmania in AustraliaCoordinates: , subdivision_type = Country , subdi ...
in Australia. Narrow gauge railways are often isolated from other railways, so standardization is not as important.


Problems with buffers and chain


Maximum load

The buffers and chain coupling accommodates much lower maximum load than that of the
Janney coupler Janney couplers are a semi-automatic form of railway coupling that allow rail cars and locomotives to be securely linked together without rail workers having to get between the vehicles. They are also known as American, AAR, APT, ARA, MCB, knuck ...
. Buffers and chain couplings allow a total train weight of around , depending on the how they are constructed, whereas the Janney coupler is sometimes built to allow for train weights of .


Buffer-locking

On sharp
reverse curve In civil engineering, a reverse curve (or "S" curve) is a section of the horizontal alignment of a highway or railroad route in which a curve to the left or right is followed immediately by a curve in the opposite direction. On highways in the ...
s, the buffers can get buffer-locked by slipping over, and onto the back of, an adjacent buffer. Although careful track design makes that a rare occurrence, a series of derailments in the Stuttgart main station in 2012 were caused by buffer-locked wagons. Buffer-lock could occur on very sharp switches on rolling stock with the older, rounded buffers. The newer buffers are rectangular and are wider than they are tall. They are not as flat, so they rarely suffer from buffer-locking. Buffers and chain couplers allow curves to have around radius, but sharp S-curves are not allowed. If it weren't for the couplers, much sharper curves could be allowed, on the condition the train is not too long. Tramways exist with , or less curve radius, with center couplers.


Variation with gauge

The width between the buffers tends to increase as the
track gauge In rail transport, track gauge (in American English, alternatively track gage) is the distance between the two rails of a railway track. All vehicles on a rail network must have wheelsets that are compatible with the track gauge. Since many d ...
increases and decrease as the track gauge decreases, which means that if wagons are changed from one gauge to another, the buffers might no longer match. That is because the buffers are originally extensions of the frames, which are spaced according to the gauge of the track. As well, the height of the buffers is usually lower on
narrow gauge railway A narrow-gauge railway (narrow-gauge railroad in the US) is a railway with a track gauge narrower than standard . Most narrow-gauge railways are between and . Since narrow-gauge railways are usually built with tighter curves, smaller structur ...
s, corresponding to the generally lower height of the rolling stock. Therefore, narrow gauge railways often use centre couplers without buffers. However, in the case of Iberian broad gauge railways, the height and spacing of the buffers are the same as for
standard gauge A standard-gauge railway is a railway with a track gauge of . The standard gauge is also called Stephenson gauge (after George Stephenson), International gauge, UIC gauge, uniform gauge, normal gauge and European gauge in Europe, and SGR in Ea ...
railways in Europe including
Great Britain Great Britain is an island in the North Atlantic Ocean off the northwest coast of continental Europe. With an area of , it is the largest of the British Isles, the largest European island and the ninth-largest island in the world. It is ...
, in order to allow through-running of rolling stock by the use of
bogie exchange Bogie exchange is a system for operating railway wagons on two or more gauges to overcome difference in the track gauge. To perform a bogie exchange, a car is converted from one gauge to another by removing the bogies or trucks (the chassis cont ...
.


Dimensions

Buffers and chain couplers tend to have the two buffers spaced according to the gauge, but especially in Europe this is modified to the standard gauge value to allow interrunning by means of
bogie exchange Bogie exchange is a system for operating railway wagons on two or more gauges to overcome difference in the track gauge. To perform a bogie exchange, a car is converted from one gauge to another by removing the bogies or trucks (the chassis cont ...
. Dimensions showing variation of spacing by gauge.


Gallery

File:Mh eisenbahn schraubenkupplung.jpeg, Buffer and chain screw couplers File:Togo Eisenbahn 086 1990-02.jpg, Buffer and twin chain German-heritage couplers in
Togo Togo (), officially the Togolese Republic (french: République togolaise), is a country in West Africa. It is bordered by Ghana to the west, Benin to the east and Burkina Faso to the north. It extends south to the Gulf of Guinea, where its c ...
File:Railway tanker at Washford Station - geograph.org.uk - 943849.jpg, Buffer and chain three-link coupling on a tank wagon File:Balancierhebelkupplung-außen.JPG, Balance lever in front of buffer support ( balance lever coupling) File:Balancierhebelkupplung-innen.JPG, Balance lever behind buffer support (" balance lever coupling") File:CH FO HGE44-105-2.JPG, Electric locomotive of the
Furka Oberalp Bahn The Furka Oberalp Railway (german: link=no, Furka Oberalp Bahn) is a narrow gauge mountain railway in Switzerland with a gauge of . It runs in the Graubünden, Uri and Canton of Valais. Since January 1, 2003, it is part of the Matterhorn Gott ...
(today MGB ) with balance lever coupling File:Puffer-02.jpg, Buffers and chain coupler on goods wagon. The chain hangs on the towing hook File:BB 1056 Cambodia0092.jpg, Center-buffer-and-chain used on the Royal Railway Cambodia BB 1056 diesel locomotive in
Cambodia Cambodia (; also Kampuchea ; km, កម្ពុជា, UNGEGN: ), officially the Kingdom of Cambodia, is a country located in the southern portion of the Indochinese Peninsula in Southeast Asia, spanning an area of , bordered by Thailand t ...


See also

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Gangway connection A gangway connection (or, more loosely, a corridor connection) is a flexible connector fitted to the end of a railway coach, enabling passengers to move from one coach to another without danger of falling from the train. Origins: Coaches in Br ...
*
Railway coupling A coupling (or a coupler) is a mechanism typically placed at each end of a railway vehicle that connects them together to form a train. A variety of coupler types have been developed over the course of railway history. Key issues in their desig ...
*
Railway coupling by country The railcar couplers or couplings listed, described, and depicted below are used worldwide on legacy and modern railways. Compatible and similar designs are frequently referred to using widely differing make, brand, regional or nick names, which ...


References


External links

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Buffer and chain coupling
{{Railway coupling Couplers