Mechanical well jar

Abstract

A mechanical well jar is disclosed having inner and outer tubular members movable longitudinally relative to each other a limited distance for connecting in a pipe string. Annular shoulders on the members engage to limit the relative longitudinal movement of the members. A plurality of laterally spaced, arcuate cam plates are attached to the inner surface of the outer member. Each plate has a laterally extending U-shaped notch opening into the space between the plates and formed with curved outwardly flaring sidewalls. Rollers are mounted on the outer surface of the inner member to extend into the spaces between the cam plates for movement into the U-shaped notches to hold the members from longitudinal movement relative to each other. Resilient means urge the members to rotate in the direction to move the rollers into the notches and allow the rollers to move out of the notches when a longitudinal force is imposed on the jar through the string sufficient for the flared sidewalls of the notches to provide a lateral component of force on the rollers sufficient to overcome the force of the resilient means and move the rollers out of the notches. This allows the tubular members to move longitudinally relative to each other until the annular shoulders stop such movement and transfer the energy stored in the drill pipe to the other member and to the stuck pipe or fish below the jar.