Patent ID: 6994175

Claim:
An hydraulically operated percussive in-hole rock drilling machine comprising: a housing ( 11 ); a drill bit ( 15 ) mounted in the front end of the housing and having a through axial flushing fluid channel; a piston hammer ( 50 ) in the housing having a through axial channel ( 51 ) and being arranged to impact on the drill bit; means ( 13 ) for connecting the machine to a tubular drill string; an inlet ( 64 ) for receiving pressurized hydraulic motive fluid from said drill string; a tube ( 35 ) fixed in the housing and extending with a sliding fit into the rear end of the axial channel ( 51 ) in the piston hammer ( 50 ), the piston hammer having a rear annular end forming a first piston surface ( 53 ) in a first annular cylinder chamber ( 54 ) for moving the piston hammer in a forward direction; said piston hammer having a second annular piston surface ( 56 ) in a second annular cylinder chamber ( 55 ) for moving the piston hammer in a rearward direction; a valve ( 62 ) coupled to said inlet ( 64 ) and having a first operative position for pressurizing said first cylinder chamber ( 54 ) and a second position for draining the first cylinder chamber to the tube ( 35 ), for reciprocating the piston hammer and providing flushing fluid to the drill bit; a control conduit ( 40 ) with port means ( 41 , 42 ) controlled by the axial position of the piston hammer for actuating the valve to shift between said first and second positions; wherein: the piston hammer ( 50 ) has first and second annular recesses ( 58 , 59 ) in the surface thereof that is in sliding fit with the tube ( 35 ); passage means ( 46 ) is arranged for pressurizing said first recess ( 58 ); said control conduit ( 40 ) extends in said tube ( 35 ) such that said port means ( 41 , 42 ) are arranged to be alternately open to said first and second recesses ( 58 , 59 ) in response to movement of the piston hammer; and said second recess ( 59 ) is arranged to be in communication with the channel ( 51 ) in the piston hammer at least when said piston is in a rear position.