Abstract: A safety coupling for preventing backspin at excessive speeds of a torque transmitting drive string which stores reactive torque due to torsional stress. The safety coupling includes a rotatable driving member to be coupled with a rotatable driven member to be coupled with the driving string for axial rotation therewith. The driven member is movable relative to the driving member between an engaged position wherein the driving and driven members are coupled by a torque transfer structure to provide a positive driving connection between the torque transmitting drive and the shaft, and a disengaged position wherein the driving and the driven members are freely rotatable relative to each other so that back-spin of the torque transmitting drive is prevented. The driven member is biased into the disengaged position and forced into the engaged position only during forward rotation of the shaft.