Compressed air percussive mechanism for a down hole hammer and down hole hammer

1. 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.

2. Percussion device according to claim 1 , wherein the upper end of the drill bit is sealingly, slidingly supported in the drill bit bushing.

3. Percussion device according to claim 1 , wherein the drill bit bushing is arranged to be supported by a housing of the down-the-hole drill.

4. Percussion device according to claim 1 , wherein the hammer-piston chamber is formed by a housing of the down-the-hole drill.

5. Percussion device according to claim 1 , wherein the driving device includes a leakage passage for pressure-air through which a flushing position is established, and the pressure-air is allowed to leak past the driving device in advanced positions in the percussion direction of the hammer-piston.

6. Percussion device according to claim 1 , wherein the hammer-piston is provided with a central axial channel which continues in the drill bit over a foot valve, which is fastened in the drill bit and seals against the hammer-piston.

7. Percussion device according to claim 6 , wherein the air-cushion is limited by an outside surface of the foot valve.

8. Down-the-hole drill comprising

a percussion device with 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,

an air cushion for reducing percussive power 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 air-cushion,

said air-cushion being arranged in 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 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.