Percussive device for a rock drilling machine, method for achieving a reciprocating piston movement and rock drilling machine

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).