Patent ID: 7681664

Claim:
A percussion drill comprising: a shank movable between a rower position and a position forward of the power position; a valve in fluid communication in with a piston-hammer, wherein the piston-hammer includes a trio section having a forward shoulder, a center area and a back shoulder, the center area having a smaller diameter than the diameter of the forward and back shoulders forming a high pressure fluid communication path from a third port to a second port; and an internal hydraulic dampening system comprising the back shoulder movable at least partially over the second port and configured to decrease the high pressure fluid flow from the third port into the second port for reducing the fluid flow to the valve in response to the shank being forward of the power position relative to the fluid flow to the valve when the shank is in the power position to thereby slow movement of the valve when the piston-hammer travels forward the power position and thereby reduce the frequency of impact blows when the shank is forward of the rower position.