Patent ID: 8739896

Claim:
Percussion device for rock drilling machines including a machine housing, a forwards and backwards movable percussion piston in a cylinder bore in the machine housing, and a reciprocatingly movable valve body in the machine housing, said percussion piston being adapted to subject a tool to impacts, and said percussion piston including a first and a second percussion piston driving surface intended to be subjected to pressure in order to drive the percussion piston forwards and backwards, said valve body including a first valve body driving surface and a second valve body driving surface, wherein pressurizing the first valve body driving surface tends to drive the valve body in a first direction and pressurizing the second valve body driving surface tends to drive the valve body in a second direction, said valve body being arranged, over a first channel arranged in the machine housing, to connect at least the second of the percussion piston driving surfaces alternately to a pressure source or to low pressure, wherein the second valve body driving surface is connected, dependent on the position of the percussion piston, either to the pressure source or, over a valve device for creating a counter pressure, to the low pressure in a tank for reducing the time necessary for the valve body to change direction of movement between said first and second directions, said valve device being positioned in a second channel connecting the cylinder bore with the tank.