Patent ID: 6454026
Filing Date: 2002-09-24
Classification: E21B

Abstract:
A down-the-hole percussive hammer rock drill, comprising:a generally cylindrical casing carrying a drill bit; a piston mounted in said casing for reciprocation in a longitudinal direction to repeatedly impart impacts to the drill bit; a hollow top sub mounted at a rear portion of the casing; the top sub comprising a front face facing towards the piston; a hollow feed tube mounted to the top sub and extending forwardly along a longitudinal center axis of the casing and defining a center passage adapted to conduct pressurized air; the piston including an axial through-hole slidably receiving the feed tube, wherein the piston is slidable in a space disposed in front of the front face; the front face and the feed tube together forming a recess which opens toward the piston, and a volume-changer removably insertable into the recess to vary a volume of the space and a pressure at which the piston operates.