Patent ID: 7234537

Claim:
An apparatus for generating sinusoidal pressure waves for application to a drill string, said apparatus including: a cylinder including a chamber which has a bore, an inlet gallery and an exhaust gallery; a work piston having reciprocal movement in the bore of the chamber and having a radial wall for sealing against the wall of the bore of the chamber during the reciprocal movement within the chamber; the work piston having a first land at one end of the work piston and a second land at the second end of the work piston; an apparatus to alternately duct fluid under pressure from the inlet gallery into the bore of the cylinder above the first land of the work piston and be exhausted from the bore of the cylinder below the second land of the piston into the exhaust gallery, and to duct fluid under pressure from the inlet gallery into the bore of the cylinder below the second land of the work piston and be exhausted from the bore above the first land of the piston into the exhaust gallery to reciprocate the piston within the bore; a piston shaft connected to the work piston for transmitting the forces generated by the reciprocatory motion of the piston to the drill string; and a relief bore communicating with the bore of the chamber and having a first end open to the bore of the chamber above the first land of the work piston and a second end open to the bore of the chamber below the second land of the work piston; the relief bore including a reciprocatable relief piston, wherein the movement of said reciprocatable relief piston is determined by the movement of fluid into and out of the relief bore from the bore of the chamber.