Hydraulic Lock Nut 30LH The Clamping Principal: The actuation of the pressure screws or the addition of the hydraulic medium activates the self-contained hydraulic system with the pressure transformation. The amount of acting clamping pressure can be identified and monitored via an integrated pressure indicator. Hydraulic Retract: Our hydraulic lock nuts are also available with hydraulic retract. Constructions with hydraulic retract reset the hydraulic pressure automatically to the zero/starting position as soon as the pressure breaks down