Abstract

A shear sub for installation in the drill string used to rotate a drill bit in the drilling of wells includes a mandrel extending upwardly for connection to the drill string. A housing slidably enclosing the mandrel extends downwardly below the mandrel and is threaded for connection directly, or indirectly, through joints of drill pipe to the drill bit. Rotation and longitudinal movement of the housing relative to the mandrel during drilling is prevented by shear pins which engage the housing and the mandrel to hold the housing in a position in which lugs extending inwardly therefrom are directly above and in alignment with splines extending outwardly from the mandrel. If an obstruction to drilling occurs that results in excessive torque on the drill string, the shear pins are sheared. On lifting and rotating the drill string, the mandrel moves upwardly relative to the housing whereupon the lugs interlock with the splines to allow rotation of the drill bit as it is lifted off bottom.