Janney coupler

Knuckle 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.

Originally known as Janney couplers (the original patent name) they are almost always referred to as Knuckles in the US and Canada (regardless of their actual official model name, nowadays generally various AAR types in North America), but are also known as American, AAR, APT, ARA, MCB, Buckeye, tightlock (in the UK) or Centre Buffer Couplers.

There are many variations of knuckle coupler in use today, and even more from the past, some variants of knuckle couplers include:

Janney: the American original, a rather finicky coupler; reportedly annoying to make open and close. This design was obsolete by 1900.

MCB: In the latter 1880's the Master Car Builder's Association (MCB) were faced with choosing a standard from the multitude of mutually incompatible automatic coupler designs then on offer. They could not, with any effect, chose a single design, but favored Janney's. The patent holders either proposed, or were persuaded, to release their rights to the Janney coupler's mating profile, and in 1888 a slightly modified profile became the MCB standard. Thus the MCB standard initially specified only the interface between MCB automatic knuckle couplers, leaving all other aspects to open competition between manufactures. There were a number of revisions and additions to the standards over the next two decades, with extensive updates in 1899. By then there were a great many variations of MCB couplers in use - an 1899 Knuckle Identification Chart illustrates 78 mutually incompatible knuckles. Further revisions to the standard followed through 1916, when what is now known as the AAR type D was recommended as the North American standard coupler. However some of the better MCB couplers remained in use for decades, and a few are still manufactured for non-interchange service or export.

The slotted Knuckle variation is a transitional type, used by railroads or in regions that are converting from Link-and-Pin couplers to Knuckle couplers. Here the vertically pivoted knuckle has a horizontal slot through its exposed width, with a vertical pin hole through the knuckle tip. With the knuckle closed and locked, a coupling Link can be inserted through the slot and pined through the vertical hole, allowing coupling to cars fitted with the older Link-and-Pin system. Slotted knuckles were common in North America prior to 1900, in South Africa after 1925, and anywhere else during this conversion. They are rarely found after conversion is complete.

AAR: In the first decade of the 1900s there were upwards of 75 makes of MCB Standard compliant couplers in use on North American railroads. All of these could couple together. Practically none shared internal design or parts. Most were offered with multiple shank patterns to match different draft gears - the Tower coupler had 16. With freight cars freely interchanged throughout the continent's standard gauge lines, the problem of maintaining these couplers fell upon all roads, and prompt repair of damaged couplers was effectively impossible.

Circa 1913 the MCB Coupler Committee, in cooperation with five of the principal coupler manufactures, set out to devise a standard coupler for North American railroads, one that mated with existing MCB standard couplers, was up to the heaviest anticipated service, and of proven operational efficiency and long service life. The participating coupler companies agreed to each submit their best designs for rigorous testing under the MCB committee's supervision, to work together to eliminate weaknesses and combine the best features of each, and to freely share (among themselves) any patented features chosen or developed for the new standard. The result was the American Association of Railroads (AAR, successor to the MCB) Standard "D" Coupler of 1916. This was upgraded to the No. 10 Contour in 1918, which largely eliminated the MCB coupler's tendency to jackknife under buffing forces. A stronger version, the AAR type E was adopted in 1931, the principal change being an increase in knuckle depth from 9 to 11 in (230 to 280 mm).

Both the D and E were essentially freight car couplers, and necessarily provided a degree of slack in their coupling, which is undesirable in passenger service. The type H Tightlock passenger coupler was developed in the 1930's, made an alternative passenger standard in 1937, and the standard for new North American passenger cars in 1947. This design incorporates a pin and socket that flank what is essentially a type E coupler head. While it can still couple with the freight couplers, two Tightlock couplers, when coupled, form a nearly rigid drawbar between their car's draft gear, eliminating the impact associated with slack action.

The surge in North American freight car capacities in the latter 1900's, particularly that of tank cars, emphasized the need to prevent cars uncoupling in the event of a derailment. Several variations of the standard Knuckle coupler have incorporated shelves above and/or below the coupler head, to prevent vertical separation. The development of unit trains for moving coal or ore has led to the substitution of rotary dumped gondolas for traditional hopper cars. These incorporate a rotating coupler and draft gear in one end, to allow the cars to be dumped without uncoupling them.