PILE HAMMER

A double-acting hydraulic impact hammer includes a drop weight and an inner hydraulic piston. The drop weight is arranged to be driven in an upward and downward direction and the drop weight acts on a pile during its downward motion. The piston has a voided internal area defining a piston volume which is arranged to receive a piston rod and along which the piston extends and retracts axially. The piston volume is in sealed hydraulic communication with the hollow rod volume to form a first volume which changes size as the piston moves along the piston rod whereby the application of hydraulic pressure to the first volume biases the piston towards its extended position. The hammer includes a bore and a collar. The collar forms an outer piston having a working surface which when exposed to hydraulic fluid has sufficient surface area to lift the piston and the drop weight.