I live in an old apartment and the door knob on my front door has been having problems for a while. In theory, the door locks from the outside automatically, but can always be opened from the inside. But recently the knobs came loose, so that the door was almost impossible to open from the inside - you had to pull the knob all the way towards you before turning it, and even that only worked sometimes.
This morning, the knobs were so loose that I gave up on opening the door and just went out the back. When I got home I tightened the set screws on both the inside and outside knobs so that both knobs were flush with the door, but then the door couldn't be opened from either the inside or outside.
So I took everything apart to try to figure out how it worked, but no matter how I put it back together I just couldn't fix it.
As shown below, the shaft is squared and has a slight twist in the center, presumably so that the knob can be turned from the inside but not the outside. The second image shows the place where the shaft is inserted. Though there's one hole, there are actually two separate metal plates with square holes in them, one (the outside) which is stationary, and the other (the inside) which can be turned in order to open the door. I can unlock the door by inserting the shaft (or any small rigid object, like a screwdriver) partway, so that it only contacts the inside hole, but if it's properly inserted I can't turn it at all in any direction.
I just don't fully understand the mechanism - if one of the plates is stationary, how can the knob ever be turned at all? More importantly, if it was working before, and all the parts are still there, why has it stopped working? Please help - I really miss being able to use my front door!