Patent ID: 7581599

Claim:
Percussion device ( 1 ) for a rock drilling machine including a housing ( 2 ) with a reciprocating percussion piston ( 3 ), the movement of which being controlled by a valve device (V, 4 ′, 4 ″), wherein the housing includes a first chamber ( 5 ) that can be pressurized for forward driving of the percussion piston ( 3 ) and a second chamber ( 8 ) that can be periodically pressurized for back-driving of the percussion piston ( 3 ), characterised in that at least one driving piston ( 7 ) arranged for actuating the back-driving of the percussion piston ( 3 ) is arranged by the percussion piston ( 3 ), that a portion of said driving piston ( 7 ) is arranged to enter into the second chamber ( 8 ) for pressure actuating on a driving surface of the driving piston by pressure medium being present in this chamber in the direction of back-driving, that said driving piston ( 7 ) includes an engagement surface ( 13 ) for back-driving co-operation with an actuating surface ( 14 ) on the percussion piston ( 3 ), and that said driving piston ( 7 ) is free for axial movement with respect to the percussion piston ( 3 ), opposite the direction of back-driving, from a position of engagement between the engagement surface ( 13 ) and the actuating surface ( 14 ).