Very simple thermo question

[Puissance Motrice]

Hi

I have a really basic thermo question. Be very grateful for any help.

I don't understand why when you put a torch on, say, a seized bolt, it's supposed to free it up.

Heating metal causes it to expand. But surely any such expansion will, within the confines of the area to which the heat is transferred, be regular: I mean to say constant, in three dimensions. What I mean is if a bolt is heated, any expansion will be in all given directions: it's width will increase, and its length. Similarly any housing. So heating up a bolt seized in a hole should just seize it further? I'm always being told heating a seized bolt causes expansion which frees up the bolt. Makes no sense to me at all.

Like I say, nice simple one. Sorry if it's not so interesting. Thanks

 

[LittleInch]

I think the result is that it breaks the bonds between the threads as the bolt expands length ways and circumstantially, allowing it to move if the bolt is rusted in.

If you've got sealing compound it probably either burns it off or makes it liquid again

it also might expand the outer nut or element more than the bolt.

I must admit I've not seen this very often.

A quick googling shows most people try and heat the nut or the area around the bolt, not the bolt itself.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TBP]

Situations like this are as much art as science. If you can heat a nut, it will usually come off while it's hot & expanded. It's it's a blind hole, often heating the bolt and letting it cool will loosen the rust & crud holding the bolt. Other times a couple of raps with hammer will get the job done. Sometimes you need to "heat & beat". And there's no definitive way to know which method - or combination - will work in any particular situation.

And, sometimes nothing works, and you get left with a bolt broken off in a blind hole. Welcome to the art of broken bolt extraction...

 

[Compositepro]

Heating may or may not work due to all sorts of factors, such as how it is done. It is one of the easiest of few options to try. It will burn off any organics that may be acting as an adhesive. Rust can be made more brittle and converted from red to black. After heating and cooling a penetrant should be used and will work more effectively due to more porosity in the corrosion products. Without a penetrating lubricant, the rust may break free initially but will jam the threads again when turning, due to wedging of the powder particle in the small gap. Rusted bolts must usually be worked free by reversing the torque direction after each small movement. What this does is to grind-up the corrosion products into particles that are small enough to not wedge, and remain suspended in the lubricant.

 

[3DDave]

Typically heat is not applied directly to the fastener. Instead it is to the housing where the outside expands and, with luck the fastener comes loose before the heat reaches and equalizes with the fastener.

There is also the slight effect that the gaps also open proportional to the temperature, so if the initial gap is filled with rust then the heated gap will be larger and no longer filled, assuming that rust has a lower coefficient of thermal expansion.

However the best part is that warmth speeds the rate at which oils or other lubricants to more rapidly creep into the rust.

For me, it's probably just a time consumer while the oil does its work and it give me the illusion of doing something rather than just beating the daylights out of whatever is stuck.

 

[georgeverghese]

The diameter of the bolt shank is less than that of the hole in the flange. So it cannot seize radially even though it expands slightly radially. Its length will increase and hence loosen the grip of the nuts on the flange.