I'm interested in the usage of a self-Locking hex nut with a nylon insert (ISO 7040 or DIN 985). How is it used correctly? Is there a prescribed usage, or a norm that describes how it should be used?
The biggest consideration is that the thread must fully engage the nylon ring. If your bolts are too short, the nylon won't do it's job by deforming to the shape of the threads. Depending on how critical your application is, in many industries they are considered single-use as the nylon gets less effective the more times the nut is installed.
For very long studs, power driving the nut can generate a lot of heat, melting the nylon insert. Because of this some people insist on only installing nylocs using hand wrenches, although this is only a practical concern in very rare circumstances.
As far as the application, nylon insert nuts don't perform well in high temperature environments, or in the presence of chemicals that attack the specific plastic used. It's also worth noting that they are not the most robust of thread-locking options, so in very high vibration applications, or applications where the bolt forms a pivot, they may not be advisable. All-steel locknuts, or pinned nuts may be a a better option in such cases.
Nylon insert locking nuts, also called elastic stop nuts or nyloc nuts, are nuts with a plastic insert of an ID smaller than the screw to which it is applied. Nyloc nuts are useful in applications where vibration or insufficient clamping loads may cause the nut to loosen and back off. These nuts are extremely common and fairly reliable in tension, though they excel in shear loaded scenarios. They are not useful in higher temperature applications (>250F), as the elastic material can degrade, and these nuts are typically avoided for repeated use due to wear and hardening of the elastic material.