Percussive down-the-hole hammer for rock drilling, a top sub used therein and a method for adjusting air pressure

摘要

A down-the-hole percussive hammer includes a cylindrical casing adapted to carry a drill bit, and a piston mounted in the casing for reciprocal movement to repeatedly strike the drill bit. A top sub is mounted at a rear portion of the casing, the top sub including a front face facing the piston. A feed tube is mounted to the top sub and extends forwardly along a center axis of the casing and defines an air-conducting passage. The piston includes an axial through-hole which slidably receives the feed tube. The front face and the feed tube together define a recess opening toward the piston. A removable volume-changer is insertable into the recess to vary a volume of a space in which the piston slides, and thus control a pressure at which the piston operates.