Hydraulically operated impact device

A hydraulically operated impact device comprising a frame and a percussion piston arranged to reciprocate in the frame due to the pressure fluid, the piston comprising at its upper end a cylindrical guide that has a smaller diameter than the greatest diameter of the percussion piston, the upper end of the guide comprising an impact pressure surface, the device further comprising an inlet duct and a return duct for supplying pressure fluid into and out of the impact device, a control pressure space situated near the upper end of the percussion piston, the guide of the percussion piston being arranged to enter said space substantially tightly at the end of the return motion of the piston, and a control valve for controlling the movement of the piston, the control valve closing in the return position the inlet duct for pressure fluid and opening the return duct, the control valve comprising a pressure surface that continuously communicates with the control pressure space, and a pressure surface that acts in the opposite direction and that is continuously connected to a pressure space situated further from said pressure space in the direction of impact of the percussion piston, the percussion piston closing the connection between said pressure surfaces of the control valve as it moves into the control pressure space and pushing the control valve from the return position to the impact position via the pressure fluid in the control pressure space, so that a connection is opened from the inlet duct for pressure fluid to said space, the pressure of the pressure fluid in the inlet duct acting on both the impact pressure surface of the percussion piston and said pressure surface of the control valve, causing the impact motion of the percussion piston, the control valve closing in the impact position the connection to the return duct so that a closed pressure fluid space is formed, the space being partly bordered by the guide of the piston and the control valve, the percussion piston comprising a second section which is situated below the guide and which has a greater cross-sectional area than the guide, said second section in turn bordering said closed pressure fluid space, the percussion piston thus moving in the direction of impact and, correspondingly, the control valve continuing its movement from the return position towards the impact position over a section of the length of the impact motion, the pressure of the pressure fluid acting on the control valve being transmitted, via the control valve and the pressure fluid in the closed space, to act on the percussion piston in the impact direction thereof, the speeds of motion of the percussion piston and the control valve being thus inversely proportional to the ratio of the difference between the cross-sectional areas of the sections of the piston to the area of the total pressure surface of the control valve facing said closed pressure space.