if metals expand when heated, then why does heat remove a stuck bolt? if they expand heat should make it tight

Answer 1

You dont heat the bolt...You heat the nut holding the bolt.

Answer 2

Both the nut (or metal part with the hole) and the bolt expand, but the hole has a larger diameter and since thermal expansion is in the form of some millionths of an inch per degree of heat above nominal the larger part expands more. Also, since it is heated first it expands more rapidly. Also, when metal is placed under force its change in shape often follows the path of least resistance. The bolt is contained by the metal part and tends to grow lengthwise while the hole is more free to move in more directions and expands away from the bolt. Also, often the bolt is frozen by some bonding agent (like oxidation, electrolosis or a chemical reaction from a contaminate), so the heating weakens this and or causes a slight initial movement which makes unscrewing it easier. Whew!

BTW, if my memory still works, steel expands .000057" per inch per degree of temperature change above 75 deg F. So, if you have a disc which measures 2.000" diameter at 75 deg F and raise the temperature to 275 deg F the disc will measure 2.0228".
But this will also depend upon how thick the disc is. Some of the size change will take place in that vector also (depending upon such things as how the metal ahs been processed previously, its grain size, etc).

Answer 3

If you put the heat on the nut or metal that the bolt is screwed into, it will expand thereby giving space between the nut and the bolt. Heating the bolt will cause it to expand and then it will be harder to remove it.

Answer 4

I have one question.
When we heat the nut, the metal of the nut does not expand to all dimensions?
This means that the outer dimension of the nut becomes bigger, and the inner (where the bolt is screwed) becomes smaller, because the metal of the nut expands to both dimensions. Likewise, the nut stucks more with the bolt.

I try to understand how this works. I think that, as mentioned, the heat breaks the rust bonds between the two parts. But I also feel that if it is stuck, first we should heat it to break the rust bonds, then let it cool smoothly and after returning to environment temperature, freeze both parts.

If somebody can give me an answer to this i would appreciate it too much!

Answer 5

1. To screw the bolt into the hole the hole must be slightly larger than the thread.
2. Now its stuck, possibly from rust of other molecular bonding but the metal of the bolt still has a smaller diameter than the hole.
3. Assuming metal with hole in it is same metal as bolt (e.g. mild steel) both will expand with same coefficient when heated. This does 2 things: i) increases gap size
ii) movement of surfaces against each other breaks the bond binding bolt to hole surface

Answer 6

Heating the bolt causes it to expand, but each component expands at a different rate (it is common practice to use dissimilar metals for threaded fasteners, and each metal alloy has a different coefficient of thermal expansion). As each component of the joint expands at a different rate, their relative motion breaks (or greatly loosens) the seal of oxidation that had previously fused them together. The fastener can then be easily removed.

As to why heat does not tighten the joint, manufacturing tolerances are not tight enough to cause this to occur.

Answer 7

Assuming the coefficient of expansion is the same or similar, the much larger piece will expand much more than the smaller bolt - and another important factor is that the heat will "break up" the bond between the rusted surfaces of the bolt and the piece it is threaded into.

Good ol' "smoke wrench" - works every time.

Answer 8

Metals get smaller when they cool. The heating and cooling combo breaks the corrosion that cause the bolt to stick in the first place. The 2nd part of your observation is half witted. Heat does not remove the bolt it only makes it possible to remove, unless of course you use a torch and melt the bolt out of the hole which is the only thing you can do sometimes.

Answer 9

Typically, heating is done with a torch applied directly to the nut. Heat will disperse through the bolt, which has has much more mass, or disperse through the assembly which it is fused to. The nut, unable to disperse heat as efficiently as the bolt will become hotter, expand more and have the possibility of separating and being removed.

Answer 10

Complementing the first answer. Also when you heat a bolt (or nut) There is differential expansion (freezing could work but generally makes iron and steel more brittle). This differential expansion causes whatever keeps the parts stuck to crack, and that "releases" them. Heating metals causes them to loose their "temper/heat treatment" and changes their qualities,properties and resistance.