Image Number 6 for United States Patent #7237470.
A fluid power unit has a cylinder in a housing, a piston slidable in the cylinder and dividing the cylinder into a retract side and an extend side, and a two-position control valve. The control valve has a spool and first and second pilots. The position of the spool is responsive to the first and second pilots. There is a retract poppet valve in the retract side of the cylinder, and an extend poppet valve in the extend side. The retract and extend poppet valves are biased to a closed position. The retract poppet valve is in a fluid path between the retract side and the control valve; the extend poppet valve is in a fluid path between the extend side and the control valve. The alternate movement of the piston in the cylinder alternately opens and closes the poppet valves, The circuit connects the poppet valves to the pilots and alternately applies and releases pressure to the pilots, causing the spool of the control valve to shift and apply inlet pressure to the piston on alternate sides. Thus the piston oscillates as long as inlet pressure is applied. All the fluid paths and the poppet valves are fluidly sealed inside the housing, so no gas is vented to the atmosphere.