Patent ID: 7694747

Claim:
A drop hammer for driving a pile comprising: a housing member defining a housing chamber and a vent port arranged between the lower and upper positions, where the vent port defines a preload position, and allows ambient air to flow into and out of the housing chamber under predetermined conditions; a rigid ram member supported within the housing chamber for movement relative to the housing member between an upper position and a lower position; a rigid helmet member supported by the housing member for movement relative to the housing member between a rest position and an impact position; a first seal arranged to seal a gap between the ram member and the housing member; a second seal arranged to seal a gap between the helmet member and the housing member; and a lifting assembly at least partly disposed within the housing chamber above the ram member; whereby the ram member moves from the lower position to the upper position and back to the lower position to define an operating cycle; the lifting assembly engages and lifts the ram member from the lower position to the upper position once during each operating cycle; when the lifting system raises the ram member above the preload position, ambient air flows into the housing chamber; the ram member free falls as the ram member moves from the upper position to the preload position; when the ram member falls below the preload position, the first and second seals define a preload chamber, where the first and second seals seal the preload chamber such that ambient air within the preload chamber portion of the housing chamber compresses to apply a preload force on the inner portion of the helmet member, and oppose movement of the ram member towards the lower position when the ram member is below the preload position; and when the ram member moves into the lower position, the ram member directly and rigidly impacts the helmet member to force the helmet member from the rest position to the impact position, thereby driving the pile.