Drill Rig's Top Drive Drops Uncontrollably

摘要

A 65-FT.-HIGH DRILL rig, mounted on a freshwater barge, was allowing its top drive head to drop uncontrollably. The circuit controlling this function shows (see illustration) both a brake (counterbalance) valve and a hydraulically controlled mechanical brake on the hoist function. The systems' power unit was mounted close to the deck, with its A and B line hoses going up to top drive's hydraulic motor. The brake and shuttle valve were plumbed close to the motor, which lifted and lowered the drill drive head.