Variable timing for front chamber of pneumatic hammer

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 (GL) 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 (SCL) that is less than the axial length of the piston groove, the sleeve being slidably movable within the piston groove between an upstroke position, where the sleeve lower leg is in contact with the lower shoulder of the piston lower end portion, and a downstroke position where the sleeve upper leg is in contact with the upper shoulder of the piston lower end portion;
• the sleeve cavity and the piston lower end portion groove defining a port having an axial length (PLUS) when the sleeve is in the upstroke position and an axial length (PLDS) when the sleeve is in the downstroke position, wherein PLUS>PLDS;
• the at least one upper fluid passage and the at least one lower fluid passage cooperating with the port 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.

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 mounted in the operating chamber, the piston having oppositely disposed impact and non-impact ends, upper and lower end portions, and a circumferential intermediate groove between the upper and lower end portions, 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;
• a sheath disposed within the upper end portion of the cylinder, the sheath and the piston upper end portion cooperating to provide a fluid seal over each upstroke and each downstroke, the sheath and the cylinder defining an axially extending passage in fluid communication with the piston intermediate groove, the sheath defining at least one bore, providing fluid communication between the pressure fluid inlet plenum and the fluid passage, and a plurality of axially extending recesses, bypassing the fluid seal between the piston upper end portion and the sheath over a portion of each upstroke and a portion of each downstroke to provide fluid communication between the pressure fluid inlet plenum and the non-impact end of the operating chamber;
• 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 a plurality of upper fluid passages, a plurality of lower fluid passages, and a land having a sealing surface disposed therebetween, the upper fluid passages being in fluid communication with the piston intermediate groove over a portion of each upstroke and a portion of each downstroke;
• the piston lower end portion defining a circumferential groove having an axial length (GL) 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 lower leg having an axial width (WSLL), the upper and lower legs defining an axially extending cavity therebetween, the sleeve having an axial length (SCL) that is less than the axial length of the piston groove, the sleeve being slidably movable within the piston groove between an upstroke position, where the sleeve lower leg is in contact with the lower shoulder of the piston lower end portion, and a downstroke position where the sleeve upper leg is in contact with the upper shoulder of the piston lower end portion;
• the sleeve cavity and the piston lower end portion groove defining a port having an axial length (PLUS) when the sleeve is in the upstroke position and an axial length (PLDS) when the sleeve is in the downstroke position, wherein PLUS>PLDS, PLUS=GL−WSLL and PLDS=SCL;
• the upper fluid passages and the lower fluid passages cooperating with the port 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.