Abstract: This invention concerns a device to block at a given torque a rotating machine driven by a hydraulic or gas turbine.The device comprises means of separating a driven disc from a driving disc, consisting of the insertion between the two discs of at least three balls or the like, which roll along three inclined cavities sloping in opposite directions when the longitudinal reaction resulting from the driven-shaft torque exceeds the force of a calibrated spring; these means co-operate with a disc brake fixed to the driven shaft, and which increases the driven-shaft torque.This device is used in particular to block, outside operating periods, driven appliances connected with a hydraulic turbine operated by drilling mud in equipment at the bottom of wells to measure and transmit drilling parameters and characteristics.