Patent ID: 8453756

Claim:
A rock drilling device including: a piston housing, a first piston movably mounted in the said piston housing and adapted such that it transfers force while in use, principally continuously, directly or indirectly, in the direction of drilling to a drill steel connected to the rock drilling device, said first piston adapted to receive pressure from a fluid acting thereon to maintain said first piston In contact, directly or indirectly, with said drill steel, a second piston adapted such that it imposes impacts repetitively while in use, either directly or indirectly, onto the connected drill steel, a first changeover control device within or mounted on the piston housing arranged to control, in collaboration with a second changeover control device within the second piston, a change-over of the pressure of a fluid acting on the said second piston, wherein said rock drilling machine includes first displacement control means within said second piston acting on said first piston to counteract displacement of said first and second changeover control devices relative to each other by reducing said fluid pressure acting on said first piston at a predetermined relative position of said first and second changeover control devices, said predetermined relative position corresponding to the relative position of said second piston relative to said piston housing at the instant of initial contact of the second piston onto the drill steel or onto a part connected to the drill steel.