URL: 

hammer union

1. S. [Perforación, Pruebas de producción, Terminación de pozos, Pruebas de pozos, Remediación e intervención de pozos]
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.