etpm
Veteran Member
I'm glad I could help. When I posted about a nut not shrinking after heating red hot I was in a hurry. Now I am not so I though I would post here about what happens when a tight nut is heated.Thanks Eric. I first used propane then MAP and then a new to me acetylene torch. My buddy gave me 2 minute tutorial on how to use it and then when I actually went to use I had uTube on replay. I'm handy but never messed with popping noises and serious fire. A bit frightening and I don't look forward to messing with that puppy again. I didn't get anything quite red hot but I did heat the pulley with one hand while I prayed with the other. Cracking that nut wasn't all that bad.
I put it in gear and had to block the wheels and I also engaged the PTO and put a monster pipe wrench on the PTO to stop it from spinning. I was actually moving the tractor with the big cheater bar but it let go fairly easily after all of that.
I'm not a machinist or welder - just a DIY'er so I learn as I go.
Thank you for the info.
Let's talk about steel nuts. When a nut is tight on a bolt the threads are loaded in tension lengthwise. The threads are also loaded in tension diametrically. This means the forces on the nut are trying to make it larger in diameter and trying to stretch it, trying to make it longer. If the nut is rusted these forces are amplified because rust expands quite a bit. If the nut is then made hot enough, and red hot is certainly hot enough, the metal will start to relax and deform enough to relieve the tension. The nut expands when heated any amount but when heated red hot it not only expands so that it gets longer and larger in diameter it also plastically deforms. This means that the nut permanently deforms to the new shape it is in, the shape it became when the tension on the nut was relieved from being heated and it conformed to any rust. As the nut cools slowly it will shrink some just from the cooling but will not shrink enough to become tighter than it was before heating. It will not even try to become tighter than it was before being heated.
There is such a thing as a shrink fit. This shrink fit works because the metal is not made hot enough to plastically deform, it is only made hot enough to expand a certain amount and when it cools it shrinks back to its original size. Or tries to. If a sleeve is to be made a shrink fit onto a shaft it will be bored to a dimension slightly smaller than the shaft. The amount smaller is determined by many factors. But when it is time to fit the sleeve onto the shaft the sleeve is heated, or the shaft cooled, or both, and the sleeve is slipped over the shaft quickly to the proper position and kept there until the shaft and sleeve are at the same temperature.
Eric