Patent ID: 6883618
Filing Date: 2005-04-26
Classification: E21B

Abstract:
1. In a downhole impact drill comprising an elongated cylinder; a distributor fixedly mounted in the cylinder and dividing the cylinder into a pressure fluid inlet plenum and an operating chamber; an impact piston with oppositely disposed impact and non-impact ends and upper and lower end portions mounted in the operating chamber, the piston being movable with a downstroke between a recovery position and an impact position and with an upstroke between the impact position and the recovery position, the piston dividing the operating chamber into non-impact and impact ends and dividing the cylinder into upper and lower end portions; and exhaust means for selectively connecting either the impact end of the operating chamber or the non-impact end of the operating chamber to exhaust; the cylinder lower end portion defining at least one upper fluid passage, at least one lower fluid passage, and a land having a sealing surface disposed therebetween; the piston lower end portion defining a circumferential groove having an axial length (G L ) and forming circumferential upper and lower shoulders, each of the shoulders having an outer surface; the drill further comprising a U-shaped sleeve mounted within the piston groove, the sleeve having a base and upper and lower legs extending radially outward from the base to an outer surface, the upper and lower legs defining an axially extending cavity therebetween, the sleeve having an axial length (SC L ) that is less than the axial length of the piston groove, the sleeve being slidably movable within the piston groove between an upstroke position and a downstroke position; the at least one upper fluid passage and the at least one lower fluid passage cooperating with the cavity of the sleeve as the piston reciprocates between the impact position and the recovery position whereby the impact end of the operating chamber is supplied with pressure fluid from the pressure fluid inlet plenum for a portion of each upstroke and a portion of each downstroke; wherein the portion of each downstroke during which pressure fluid is supplied to the impact end of the operating chamber is less than the portion of each upstroke during which pressure fluid is supplied to the impact end of the operating chamber.