Well, this may be a 'Dear John' note from your 'Armstrong'... Pictures would help a lot.
Usually fans are NOT threaded.. would be surprised.
So, one 'option' if you can't pull it off would be to apply some 'heat'.. carefully... If you are not proficient with a torch, a word of warning... heat the part.. do not melt it.
Can you get a 'puller' on it? Get some 'S-OK' and some PB Blaster.. those are good 'solvents' and may help things release.
Again.. a picture(s) would help..
Peace and Blessings..