drill string under normal torque load the spiral spline 20 and the receiver nut 54 are held in an interlocked position as shown in fig2 by the resistance of the compression spring 62 under normal torque conditions the spring 62 is of sufficient strength to resist the screwing action between spiral spline 20 and the receiver nut 54 critical torque of a magnitude greater than the input torque force is created in the propeller shaft drive when the output yoke 88 is restrained from turning to any degree by the rotary table torque requirements upon the occurrence of excessive torque the longitudinal screwing action to the right as it appears in fig3 by the spiral spline 20 into the receiver nut 54 overcomes the resistance of spring 62 in proportion to the torque restraint as a result of this longitudinal rightward motion of the spiral spline toward the output end 14 of the apparatus 10 the input shaft 16 also moves toward the right in relation to the drawings in the direction of the output section 14 along its longitudinal axis thus displacing the trunion collar 42 a distance directly proportional to the amount of increased torque load a motion detector and or recorder indicated diagrammatically by reference numeral 94 can be connected directly to trunion collar 42.