cobalt expansion on fishplate joints

[Archie]

what is the purpose of cobalt expansion on the holes on a joint.

Cheers

[Peter]

what is the purpose of cobalt expansion on the holes on a joint.

Cheers

I am guessing you have only heard of this and not seen it written because I think what you are asking about is "Cold Bolt-Hole Expansion". I don't understand the physics of it, but when you drill a metal, you are likely to get tiny cracks (drilling after all is basically chipping away at the metal in tiny stages quite fast). Pulling something slightly bigger through the hole after you have drilled it helps to stop this. I am guessing it compresses the metal around the hole, so when you do it, the metal it being malleable (able to be hammered or pressed permanently out of shape without breaking or cracking), you undo any cracks left in the hole and hence stop them propagating.

In simple answer to the question, it helps stops cracking around the holes.

[Archie]

Yeah that sounds good, sorry I did not word that properly, I have actually done a lot of it myself on track and rather embarrassingly never knew the meaning of it, I did ask the question but to no avail.

[Peter]

Yeah that sounds good, sorry I did not word that properly, I have actually done a lot of it myself on track and rather embarrassingly never knew the meaning of it, I did ask the question but to no avail.

There is no shame in asking a question and no worries on the naming of it.
The point you raise about asking the question has been the subject of a lot of debate recently - understanding why you are doing something is as important (in my view) as knowing how to do something. If the person asking you to do something does not know why either, that it even worse!