A buffer is a part of the buffers-and-chain coupling system used on the railway systems of many countries, among them most of those in Europe, for attaching railway vehicles to one another. (In North America, rolling stock instead has draft gear built into the couplers).
Description
Fitted at the ends of the vehicle frames on the buffer beam, one at each corner, the buffers are projecting, shock-absorbing pads which, when vehicles are coupled, are brought into contact with those on the next vehicle. The buffer itself comprises the buffer plates which take the impact.
The draw chain used between each pair of vehicles includes a screw which is tightened after coupling to shorten the chain and keep the buffers pressed together. Such is known as a 'screw coupling'. Historically, coupling chains were no more than that, a short length of heavy chain (typically three links long) with no adjustment. These would result in a 'loose-coupled train' in which the buffers of adjacent vehicles would only touch when the coupling chain was fully slack, such as when being pushed or going down hill.
Design
Wooden buffers
The buffers in the very earliest days of railways were rigid dumb buffers made of wooden blocks with a metal cap at the end. For example, Germany's first commercially successful locomotive, the Adler, built in 1835, had wooden buffers.
In the 1830s the first spring-loaded buffers were introduced on the Liverpool & Manchester Railway and during the mid-19th century sprung buffers were fitted to all coaches and wagons.
The original English buffer plates were the same on each side, so that there was an occasional tendency for them to slide off each other and become locked. German railway buffer plates are flat on one side and convex on the other to reduce this tendency to slide off.
Rod buffers
The rod buffer was the earliest German type of sprung buffer. The first ones consisted of a steel rod that carried the buffer plate at one end. The buffer rod pushed backwards on a spiral volute spring (coil spring), which was supported by a strong sheet metal cylindrical sleeve. Compressing the spring absorbed the impact. The first spring-loaded buffers were invented by John Baillie in 1846.
Rod buffers can only be found on museum vehicles in Europe today. They are still common in Argentina.
Plunger buffers
Since the relatively thin buffer rods often bent during operation and caused major damage in accidents, the German state railways developed the plunger buffer. The buffer plate sits on a steel tube with a larger diameter, which is therefore more resistant to bending.
The volute spring system initially still used for the plunger buffers had the disadvantage that the energy put into compression was completely released again, so that the vehicles were pushed apart again after the impact. This required trains to be braked evenly and, above all, not just at certain points in order to avoid oscillations in the train set, which inter alia can cause the couplings to break. An improvement was made by using ring springs, as these convert most of the input energy into frictional heat.
Centre buffer couplings
With centre buffer couplings, the tasks of buffers are usually transferred to the coupling elements, which are designed to be correspondingly stable and are mounted with interposed spring elements within the vehicle frame instead of on a buffer beam. They are often used on narrow gauge railways and funiculars.
Modern buffers
The buffers fitted to modern locomotives and rolling stock incorporate oleo-pneumatic shock absorbers.
Retractable buffers
Standardisation
In the early days of the railway, each railway company determined the positioning and arrangement of the couplings and buffers at its own discretion. But once the railway network began to develop from the first independently built routes, this hampered the transfer of wagons. Railway vehicles that were to be transferred to other parts of the network, run by other companies, had to be equipped with several buffers or coupling systems. Founded in 1882, the International Conference for Promoting Technical Uniformity on Railways specified the dimensions for buffer positions and distances customary in England and northern Germany as a standard. For all vehicles built from 1 January 1939, the distance according to RIV §38 was set at 1,750 mm ± 10 mm. The centre of the buffer on European railway vehicles must be between 940 and 1,065 mm above the top of rail.