Patent ID: 6543557
Filing Date: 2003-04-08
Classification: E21B

Abstract:
A down the hole drill assembly comprising a backhead fitted into a wear sleeve, a piston forming a hammer reciprocable in the wear sleeve and a drill bit assembly including a foot valve tube and an anvil of smaller surface area than the hammer end of the piston located in the end of the wear sleeve remote from the backhead characterised in that the piston has blind bores extending from each end and ports through the wall of the piston providing for communication between the blind bores, a collar around the piston and the piston having a diameter at the hammer end larger than the backhead end, the backhead including an axial inlet passage and branched outlet passages through the wall of the backhead, longitudinal passages in the inner wall of the wear sleeve to provide selectively flow paths from the inlet to the upwardly exposed surfaces of the piston or alternatively to the upper surface of the collar and the drill bit end of the piston around the anvil with exhaust passages through the piston and communicating externally of the piston between the blind bores in the piston.