Briefly on the AAR Coupler Part 1

What makes most trains, trains, is the fact that locomotives and cars are interconnected by some mechanism (of course they no longer had to be since the introduction of the rail diesel car) that allow them to move along the railroad tracks simultaneously.

Before I go further, I wish to introduce, very briefly, buff and draft forces. Two cars are in buff when they push against and are in draft when they pull against each other.

A little bit on history: at the very beginning, this mechanism that connects cars together are made up of separate components that deal with buff and draft forces individually. The link and hook take care of draft forces, and buffers (they look like pancakes sticking out the ends of trains as seen in Thomas the Tank Engine) deal with buff forces. Although this system works well once the coupling is complete, which is why it is still in use in many parts of the world, there are many disadvantages to this primitive coupling system that make it unacceptable in a railroad system where cars need to be coupled and uncoupled on a regular basis. A major disadvantage is that the link-and-hook couplers are very time consuming to manipulate. An individual must be placed between buffers in order to fasten/unfasten the coupling, creating a dangerous work environment. The link-and-hook system also have comparably low capacity and can be difficult to make en route repairs when failure occurs, but these two disadvantages are relatively easy to address than the big one mentioned before.


In late 1873, a gentleman by the name Eli Janney filed a patent on a semi-automatic coupler for railroad use in place of the link-and-hook couplers. This coupler significantly improves safety of coupling/uncoupling cars and is more advantageous to the link-and-hook coupler in almost every way, except in the event that when attempting to couple cars while couplers are misaligned, they can bypass each other and damage the ends of rail cars. This is unlikely to happen on cars with link-and-hook couplers because of the fixed buffers. The use of this Janney Coupler was written into law in America in 1893 and became the standard coupler for the Master Car Builders' Association (former name of the Association of American Railroads, the AAR, the members of which include all major railroads in America, Canada, and Mexico). The AAR coupler is so successful it has become the standard freight car coupler in many countries around the world including Australia, China, Brazil, South Africa, and Japan.

Diagram on Janney's patent submission, you'll notice this coupler has gone through quite an evolution over time
A later but still early version of the MCB (AAR) coupler
This is a Janney coupler used in China with the knuckle closed. Note that the uncoupling pin is on top of the coupler, this is not allowed on freight cars used on AAR Railroads to prevent tripping when crossing over equipment
For this post, I will restrain to the exposed parts of the AAR coupler, in simple terms, the head and guts most of us can see when trains go by in their evolved, modern form. Briefly speaking, the head of the coupler is completed with the knuckle, which opens and closes to make and brake the coupling, this part is easy to replace and is thus designed to be the weakest link in the coupling system. The knuckle swivels around a knuckle pin, which bears no load whatsoever, although traditionally made of steel, plastic replacements are available. The knuckle is locked in the closed position by a lock inside the coupler head, which can be manipulated by the thrower and locklift assembly connected to the uncoupling lever that extends to the side of the rail car on the left hand side of the coupler.

Coupled AAR freight cars with my annotations
I found this very good video on coupling and uncoupling equipment made by the British Railways. Although the details are somewhat different from North America, but it sufficiently shows the difference between the Screw Coupler (hook-and-link) and the Buckeye (another name for Janney, or AAR) Coupler.


Next week I will write about the different types of Janney couplers still in use.

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