Railway coupling
A coupling or coupler is a mechanism, typically located at each end of a rail vehicle, that connects them together to form a train. The equipment that connects the couplers to the vehicles is the draft gear or draw gear, which must absorb the stresses of the coupling and the acceleration of the train.
Throughout the history of rail vehicles, a variety of coupler designs and types have been developed worldwide. Key design considerations include strength, reliability, easy and efficient handling, and operator safety. Automatic couplers engage automatically when the cars are pushed together. Modern versions not only provide a mechanical connection, but can also couple brake lines and data lines.
Different countries use different types of couplers. While North American railroads and China use Janney couplers, railroads in the former Soviet Union use SA3 couplers and the European countries use Scharfenberg and screw couplers. Challenges and complications arise when coupling vehicles with different couplers. Barrier cars, also called match cars, cars with dual couplers, or adapters are used to accomplish this task.
Nomenclature
Compatible and similar couplings or couplers are frequently referred to using widely differing make, brand, or regional names, or nicknames, which can make describing standard or typical designs confusing. Dimensions and ratings noted in these articles are usually of nominal or typical components and systems, though standards and practices also vary widely with railway, region, and era.Buff: when the consist of cars is in compression; opposite of tension.
Buffers and chain
The basic type of coupling on railways following the British tradition is the buffer and chain coupling. A large chain of three links connects hooks on the adjoining wagons. These couplings followed earlier tramway practice but were made more regular. Buffers on the frame of the wagon absorbed impact loads, as the train overran a slowing locomotive.The simple chain could not be tensioned, and this loose coupling allowed a great deal of back and forth movement and bumping between cars, as well as jarring when trains started. While acceptable for mineral cars, this coupling made for an uncomfortable ride in passenger coaches, so the chain was improved by replacing the center link with a screw with a left-hand thread on one side and a right-hand thread on the other. In the center of the screw is the handle housing with a hinged ball handle attached. This turnbuckle style arrangement allows the vehicles to be pulled together by tightening the screw with the attached handle. Typically, the screw is tightened until there are two threads left next to the handle housing. A support is attached to the trunnion nut on the coupling link side to rest the handle of the screw to prevent loosening of the screw while the coupling is in use. The official name of this type of coupling is screw coupling or UIC coupling according to the European standard EN 15566 Draw gear and screw coupling.
A simplified version of this, quicker to attach and detach, still used three links but with the centre link given a T-shaped slot. This could be turned lengthwise to lengthen it, allowing coupling, then turned vertically to the shorter slot position, holding the wagons more tightly together.
Higher speeds associated with fully fitted freight made the screw-tensioned form a necessity.
The earliest 'dumb buffers' were fixed extensions of the wooden wagon frames, but later spring buffers were introduced. The first of these were stiff cushions of leather-covered horsehair, later steel springs and then hydraulic damping.
This coupling is still widespread in Western and Central Europe and in parts of Northern Africa, the Middle East and South Asia.
Link and pin
The [|link-and-pin] coupling was the original style of coupling used on North American railways. After most railroads converted to semi-automatic Janney couplers, the link-and-pin survived on forest railways. While simple in principle, the system suffered from a lack of standardization regarding size and height of the links, and the size and height of the pockets.The link-and-pin coupler consisted of a tube-like body that received an oblong link. During coupling, a rail worker had to stand between the cars as they came together and guide the link into the coupler pocket. Once the cars were joined, the employee inserted a pin into a hole a few inches from the end of the tube to hold the link in place. This procedure was exceptionally dangerous and many brakemen lost fingers or entire hands when they did not get them out of the way of the coupler pockets in time. Many more were killed as a result of being crushed between cars or dragged under cars that were coupled too quickly. Brakemen were issued with heavy clubs that could be used to hold the link in position, but many brakemen would not use the club, and risked injury.
The link-and-pin coupler proved unsatisfactory because:
- It made a loose connection between the cars, with too much slack action.
- There was no standard design, and train crews often spent hours trying to match pins and links while coupling cars.
- Crew members had to go between moving cars during coupling, and were frequently injured and sometimes killed.
- The links and pins were often pilfered due to their value as scrap metal, resulting in substantial replacement costs.
- When a car happened to be turned 180 degrees one would have to look for a link.
- Railroads progressively began to operate trains that were heavier than the link-and-pin system could cope with.
The Panama Canal mules, the locomotives used to guide the ships through the locks of the Panama Canal, have link and pin couplers and side buffers. This design was chosen so that these normally solo operating locomotives could be coupled to another locomotive in the event of a breakdown. On straight track, the link and pin coupler is used. Since the vertical curve between the straight track sections and the ramp between the lock chambers has a very small radius, the difference in height would be too great for a link and pin coupler, so the locomotives must be pushed through these sections uncoupled by using the side buffers. They have an extra high buffer plate to prevent the buffers from buffer-locking in tight vertical curves.
Balance lever coupling
The balance lever coupling, also central buffer coupling with two screw coupling, is a coupler commonly used on narrow gauge railroads with tight curves. By swapping the pulling and pushing devices, the standard screw coupling used on standard gauge railroads became a center buffer coupling with one screw coupling on each side of the buffer. The screw couplers are connected to a compensating lever that pivots on a vertical trunnion on the center buffer rod, allowing an even distribution of tractive forces between the two screw couplers.Albert coupler
To avoid safety issues, Karl Albert, then director at the Krefeld Tramway, developed the Albert coupler during 1921. The Albert coupler was created as a key and slot coupler with two pins. Vehicles to be coupled were pushed together, both couplings moving to the same side. One pin was inserted, then the vehicles were pulled to straighten the coupling and the other pin inserted. This operation required less exact shunting. Due to the single-piece design, only minimal slack was possible. The system became quite popular with tram systems and narrow gauge lines.During the 1960s most cities replaced them with automatic couplers. But even in modern vehicles, Albert couplers get installed as emergency couplers for towing a faulty vehicle.
Miller hook and platform
The link and pin was replaced in North American passenger car usage during the latter part of the 19th century by the assemblage known as the Miller platform, which included a new coupler called the Miller hook. The Miller platform was used for several decades before being replaced by the Janney coupler.Norwegian
The [|Norwegian coupler] consists of a central buffer with a movable hook that drops into a slot in the central buffer. There may also be a U-shaped safety catch on the opposite buffer that is flipped over the top of the hook to secure it. The safety device may also be a chain with a ball-shaped weight at the end that is thrown over the hook to hold it in place. On railways where the rolling stock always face the same direction, the mechanical hook can be on one end of the wagon only. Not all Norwegian couplers are compatible with one another as they vary in height and width, and may or may not be limited to one hook at a time. The traction force limit is typically 350 kN. Sometimes the Norwegian coupler is supplemented with auxiliary chains.The Norwegian coupler is also known as the Lloyd coupler named after its British manufacturer F.H. Lloyd & Co. Ltd near Wednesbury or as the meat chopper coupler named after the shape of the movable hook. The Norwegian coupler allows sharper curves than the buffer and chain coupler, which is an advantage on narrow gauge railways where low speeds and reduced train loads allow a simpler system. The Norwegian coupler is found only on narrow gauge railways of, or less in Great Britain and its former colonies. For example, it is used on the Isle of Man Railway, the Western Australian Government Railways, in Tanzania, on the Ffestiniog Railway, on the Lynton and Barnstaple Railway, and on the Welsh Highland Railway,