I'm asking this because i need to have the arms of a gripper loose and when i use just a nut i have to unloosen it a little and then the nut get more and more loose.
This problem should not be solved with nuts.
The problem you are having is that you are screwing a joint, that should rotate, really tight. Obviously being tight will make that joint freeze and not work.
The correct solution would be to use a bolt, where the non-threaded region is for your joint, and the threaded region is for your nut. You can also use a small plastic tube and a long screw inside it as a joint, where the nut just holds it all together.