An Old Timers Trick to Get Unstuck
I once had an old commercial blower assembly on a 20 ton package unit and the bearings and pulleys were seriously shot.
I was 19 or 20 and as much as I tried, I just couldn't get the rusted hunk of metal apart. I tried penetrating oil, pullers, pry bars ….. I knew better than to beat on it with a hammer but I was getting close.
I did what many young techs do, I called a guy who had “been there and done that”. He laughed at me and then said
“grab your torches and heat the the thing up a little, see if that breaks it free”
He wasn't very specific about how to do it and I think I started a mini fire, but I got it loose (and back together) by some miracle.
I've used a torch to get things loose a few times since then and it's always gotten the job done. Since that time a learned a few more things.
A lot of the “where you put the heat” advice is urban legend. Yes, it is better if you can apply heat to the “outside” part you are trying to get off because the part you heat is the part you will expand more. In the case of a nut the entire nut diameter will increase slightly and “loosen”.
In my experience a bit of heat cycling can help to break it free no matter where you apply the heat.
Now, it's still better to heat the outside because at least in that case it can move away. If you heat the inside (bolt, shaft etc…) it will expand in a trapped, confined space.
What heating and cooling does regardless of where it's directed is to force the metal to move which breaks up the bonds of the rust and corrosion. If you have ever pulled off a really nasty blower wheel or fan blade you know that sometimes forcing it the wrong way a bit can help you loosen it up to pull it off the right way.
When something is really stuck, any movement that doesn't damage or mushroom the parts is good movement. Heat is a type of movement that works all the way down to the molecular level.
Don't Be Careless
When you are ticked off it's easy to get parts too hot that can mess with the hardness of the metal, damage wires or insulation or cause a general mess.
You don't need to get the parts cherry red, just use some soft, even heat and protect all of the parts and wires in the area.
You need to know what sort of metal you are dealing with, steel, stainless, aluminum, copper, tin and brass all have different melt temperatures and characteristics, this approach is mostly for steel or cast iron with a decent amount of thickness to it.
In many cases you will have already tried various lubricant and penetrating oil sprays. Make sure to wipe that all up before you bring the heat. Have a fire extinguisher and wet rags ready and wear good goggles and gloves when using a torch, especially when there may be grease or oil present.
You may find that attempting to get it apart is easier once it is cool or mostly cool. Never spray anything on HOT metal but once you in the range you can touch spraying penetrating oil on the area while still warm can help get it deeper into threads.
Most of all be patient and don't try to “muscle” it without taking all the proper precautions, using proper backing wrenches etc…
Getting big hunks of metal apart is one of the most satisfying parts of the trade, and like most things one approach doesn't fit all situations.
Wrenches, wire brushes, sand cloth, penetrating oil, pullers and occasionally a torch… are all tools in your toolbox.
Use them wisely.