What type of locking device involves a second nut run up against the first to create a secure fit?

The correct answer is the lock nut. A lock nut is specifically designed to resist loosening under vibration and torque. It achieves this by using a secondary nut threaded onto a bolt or screw, which is then tightened against the primary nut. This action creates friction and a tighter fit, preventing the two nuts from turning independently and thus ensuring the assembly remains secure.

Understanding the context is important: while lock washers and safety pins contribute to locking mechanisms in fastening applications, they do so in different ways. A lock washer is typically a flat or curved washer that provides tension and resistance to loosening. Safety pins are used for securing items but do not apply the same principle of friction and resistance that a lock nut does. Therefore, the lock nut is the most appropriate choice for a locking device that involves a second nut creating a secure fit.