A wrench assembly for making and breaking connections between tubular members, such as a drill string and a top drive sub. The wrench assembly consists of a lower wrench and an upper wrench. The lower wrench has a curved gate arm and a curved latch arm, pivotable between an open position and a closed...http://www.google.es/patents/US4759239?utm_source=gb-gplus-sharePatente US4759239 - Wrench assembly for a top drive sub
