Bob: This is what they call a jam nut. Jam nuts are about half the size of a regular finish nut in height. Otherwise the circumference and everything else is the same; they use the same wrench to apply the nut and to remove the nut.
Jam nuts, just to give you an idea what these do, you use two nuts together and then you tighten them up against each other. That's why they call them jam nuts: you use them together, you tighten them up, and I'll show you. I'll do a demonstration of that also, but typically that's how they're used. They're also used in tight areas where you can't have a full-size nut.
So I'm going to demonstrate the installation of the jam nuts that I have here. I have two of them. Jam nuts are also called thin nuts. The difference is that they're about half the size of a regular nut. You can see that here.
I'm going to put the first one on and then I'm going to put the second one on. I'm going to use an adjustable wrench to grab the bottom nut. Then I'm going to use the ratchet on the top to tighten it up.
Now I'm tightening them up against each other. That's one of the reasons why they call them jam nuts. Jam nuts are also available in nylon lock nuts where you don't have to use this situation. Obviously jam nuts can be used in many different applications such as if you have a tight area like in a car application where you don't have the room to use a regular sized nut. They'll use jam nuts or a single jam nut to get on the back of an alternator for a bracket; something along those lines. They also use the nylons in that situation.
So there's many different situations that you can use this in. They also sometimes want to put a jam nut like this to wherever it's keeping in place. They want this whole mechanism to be able to turn and move because they might be a moving arm on this bolt holding it in place. They also are used with shoulder bolts. So you'll find jam nuts used with shoulder bolts a lot on mechanical equipment. However you should know that if this is being used in a vibrating machine or something along those lines, these do have a tendency to loosen apart.
So if you want them to stay use some Loctite or some other locking product to keep them in place like they are here. And that's it.