Hydraulic drill string jar

Claim 1

A hydraulic jar for use in a drill string including elongated telescopically-arranged mandrel and housing members relatively movable along the longitudinal axis of the jar between contracted and extended positions, said members respectively having a hammer and an anvil, said anvil being spaced from said hammer when said members are in a contracted position and arranged for contact when said members are in an extended position, hydraulic means in said jar for retarding movement of said mandrel member relative to said housing member between a contracted and extended position for a given time during a jarring stroke when the jar is tensioned, said hydraulic means including a chamber which in use is filled with fluid in said housing member with adjoining first and second bores, said first bore having a lesser diameter than said second bore, and a piston on said mandrel member receivable in said first bore, said piston having a slightly smaller diameter than said first bore to restrict the flow of fluid past said piston as it moves relative to said first bore during the jarring stroke until said piston exits from said first bore where-upon said members move rapidly to the relatively extended position so that the hammer delivers a blow to the anvil, the piston being of annular form and being positioned around the mandrel member for movement therealong between first and second axially spaced stops on said mandrel member, said piston having a first end adapted to cooperate with the first stop to permit free flow of the fluid between the piston and mandrel member as aid members move toward the contracted position, and the piston having a further portion adapted to co-operate with said second stop to provide sealing engagement therewith during movement of the piston through the first bore during the jarring stroke, one of said further portion and said second stop including a smooth annular wall portion which is sloped relative to the longitudinal axis of the jar while the other includes a smooth annular surface having a convexly arcuate contour as seen in a longitudinal section, with said annular wall portion and said annular convexly arcuate surface being relatively disposed to come into said sealing engagement during the jarring stroke, and wherein said further portion is located interiorly of said piston intermediate said first end thereof and the end of the piston opposite said first end.