threaded fittings

Hello '
Here's a problem thats been vexing us for a couple years.installed new pressure tank threaded fittings become loose after awhile how can this happen?

Vibration is most likely the reason. What type of fittings? Material?

Yes, sounds like vibration, and as rugged asked, what materials are involved? Most likely the pressure tank is steel with a female threaded fitting to connect to the plumbing system.

This time when you re-plumb the tank, make sure all fittings and threads are dry, use a quality pipe thread compound and allow it to fully cure prior to pressurizing the system.