Aren’t the adjustment switches some really nice devices?
On one side we have rails subjected to thermal stress, tending to seriously expand or contract due to the environment temperature, and on the other side no stress is transferred.
We cut the rails in that funny shape, grease the clamp plates, and we let the rail slide.
Most of them – the subject of this post – are short, with the moving part only on two sleepers. Sometimes they are called breathers or breathing joints – perhaps a better name as these short devices are not built using switch rails.
In English-translated technical literature they are sometimes called expansion joints or compensation devices – direct translation from the original language. The thing in the drawing above is called “dilatation device”. (Note to self: I have to write a post about translations and language barriers, mentioning about a sleeper, a frog and a rabbit foot …)
You see them everywhere.
For quite a long time, every time I saw one, I wondered what is the purpose of the strap rails installed in the fourfoot.
Don’t they transfer part of the thermal force, reducing the efficiency of the device?
How will the rails slide if the sleepers are fixed in place?
Well, they slide, obviously.
But some part of the thermal force is transferred across through the strap rails.
How much force passes from one side to the other?
So, first, why strap rails?
I don’t really know, but I have a theory.
The rails tend to move, very often together with the sleepers – the longitudinal resistance of the movement through the ballast is usually smaller than the one between rail and sleeper.
Sometimes the rails move relative to the sleeper during installation, especially if you use hammers to install fastening clips.
Hence, very often the track looks like this:
Let’s edit that …
… and scale it.
You see it now?! Some will be able to read from this picture the type of fastening and the installation method.
If the sleepers at the adjustment switch will move like this, then there is the chance for the device to be skewed and not to work properly, its sliding movement to be affected by the skewed position of the supporting sleepers, for the device installed on a ballasted track.
This is why, I think, those strap rails are installed – to keep the four sleepers parallel and to allow the device to work as designed. There is a price for this – some stress passes across (not much, the fastening longitudinal resistance of one sleeper).
This is what I though the straps are for, until a saw this, in Brussels – Garre du Midi (on one side CWR, on the other unstrengthened turnout and jointed track):
I was puzzled again. Not only about the strap rails but also about the stress transfer. If we have only one sliding point and the rest of the switch rail length is fastened on 5 sleepers, how does this breath?
Are the switch rail K fastenings without longitudinal restraint?
How much force passes across? 5x fastening longitudinal resistance is something … but doesn’t the switch rail creep relative to the sleeper?
What do you think?