hammer union

1. n. [Drilling, Production Testing, Well Completions, Well Testing, Well Workover and Intervention]
A connection common in the oil industry consisting of two joints coupled by a threaded nut. Protrusions on the nut are hit with a sledgehammer to tighten the connection and energize the seals. Hammer unions are commonly used on treating iron because of their ability to be quickly made up or broken down.