Patent ID: 7757779

Claim:
Pressure-air driven percussion device for a down-the-hole drill, the pressure-air driven percussion device comprising a hammer-piston axially reciprocally movable in a hammer-piston chamber through a driving device, said hammer-piston in operation acting with a hammer-end against an upper end of a drill bit positioned inside a chuck, a sealed air cushion for reducing percussive power, said sealed air cushion being arranged at the hammer-end of the hammer-piston in positions where the drill bit has been moved past a predetermined distance in a percussion direction, the drill bit being sealingly slidingly supported in a drill bit bushing, and the hammer-piston being movable inside the drill bit bushing and the hammer-end of the hammer-piston sealingly cooperating with the drill bit bushing in said positions to form said sealed air-cushion, said sealed air-cushion forming a sealed air volume defined vertically between an upper end of the drill bit and the hammer-end of the hammer-piston, with an outermost diameter of the hammer-end of the hammer-piston being located inside the drill bit bushing, and laterally by the drill bit bushing so that strikes performed by the hammer-piston against the upper end of the drill bit are cushioned by the sealed air-cushion and the drill bit is exposed to percussive energy peaks, prior to contact of the hammer-piston against the upper end of the drill bit.