How a screw coupling is used to connect railroad carriages by thread
Today we are looking at a real fossil among threaded mechanical components. The screw coupling dates back to the early days of the railroad - and that was quite a while ago. As early as the 1840s, screw couplings were used to connect railroad carriages together. And nothing has changed to this day!
It almost sounds as if the engineers of the 19th century had achieved a very special stroke of luck - a clutch so perfect that it was unsurpassed almost 200 years later? Well, it's not quite that clear-cut. The principle of the screw clutch still works perfectly and has, of course, been optimized a little here and there over the course of time. However, there are more bureaucratic reasons why the screw clutch is still in use. In fact, there have been plans to replace the screw coupling with a more modern system since the 1960s. However, as the screw coupling was widespread throughout Europe and for reasons of compatibility the replacement was not only to be carried out regionally, a nationwide changeover would have been quite an effort. It was therefore finally decided that the screw coupling could remain the standard for a few more years - and it was left at that. And because the screw coupling is still so up-to-date despite its advanced age, it's high time we took a look at it in this blog. Because the screw coupling would be nothing without a thread!
How does a screw coupling work?
The heart of the screw coupling is a threaded rod with two opposing threads and two matching nuts. The first nut is firmly connected to a railroad carriage via a joint. The so-called coupling bracket is attached to the other nut. The system also includes the draw hook permanently attached to the wagon.
If two wagons are now to be connected using a screw coupling, the coupling bracket of one wagon is hooked into the draw hook of the other wagon. At this point, the screw coupling is open as far as possible. In the middle of the threaded spindle there is a folding handle with which the spindle can then be turned. Due to the counter-rotating threads, the rotary movement ensures that the two nuts move towards each other. In this way, the two carriages are mechanically connected to each other. Various cables, such as those for the brake system, must then be joined between the carriages. If the carriages are to be uncoupled again, a railroad employee loosens the screw coupling by turning the threaded spindle in the other direction using the handle.
Why is the thread so important in the screw clutch?
The ingenious concept behind the screw coupling is the counter-rotating threads. Left-hand and right-hand threads on the same threaded spindle make it possible to couple two railroad carriages together by means of a rotary movement. The design of the screw coupling makes it possible to produce a rather tight or rather loose coupling, depending on the track layout and requirements.
Where the screw coupling is used everywhere
The screw coupling for railroad wagons is also known as a UIC coupling. The UIC is an international railroad association founded in Paris in 1922, whose main purpose is to facilitate smooth rail traffic across national borders. The most important tool in this endeavour is standardization - including for the coupling devices of railroad wagons.
The screw coupler or UIC coupler is used throughout Europe and North Africa and, due to the former British influence, also in India and some parts of South America. This means that railroad wagons in this area can be coupled together across national borders. It is not only the screw coupling itself that is relevant, the two buffers attached to the left and right of it are also part of the coupling system. A uniform buffer spacing is therefore just as important for interchangeability.
These problems can occur when using a screw coupling
A great deal of responsibility is placed on the screw coupling, and failure would have serious consequences. Accordingly, the most important value when using a screw coupling is the so-called draw hook load. Particularly in the case of freight trains, this must be carefully calculated, as these can significantly exceed the weight of passenger trains depending on the load. The train length is also determined by this upper limit for the load on the screw coupling. This is one reason why trains in the USA and Canada, where a different coupling system is used, can be significantly longer than in Europe.
But what happens if the screw coupling does fail? The result is a train separation, i.e. the unintentional separation of part of the train. Although this phenomenon occurs much less frequently nowadays than it did in the 19th century, this does not mean that screw couplings can no longer fail. Fortunately, this usually has less serious consequences today than it did 100 years ago. On modern trains, the train sections brake automatically if the screw coupling fails, which means that major accidents can be avoided in most cases.
You should therefore leave the screw coupling to the professionals. For all other threading projects, we at BAER have just the right tools for you. Take a look at our online store or ask our customer service team any questions you may have.