Patent ID: 8181713

Claim:
A drop hammer for driving a pile comprising: a housing member defining a housing chamber and a vent port arranged between lower and upper positions, where the vent port defines a preload position; a ram member supported within the housing chamber for movement relative to the housing member between the lower position and the upper position; a helmet member supported by the housing member for movement relative to the housing member between a rest position and an impact position; and a lifting system for moving the ram member from the lower position to the upper position during each cycle; whereby when the lifting system raises the ram member above the preload position, ambient air substantially freely flows into and out of the housing chamber through the vent port; when the ram member falls below the preload position, fluid is prevented from flowing through the vent port such that ambient air within a preload chamber portion of the housing chamber compresses to apply a preload force on the inner portion of the helmet member; and when the ram member moves into the lower position, the ram member impacts the helmet member to force the helmet member from the rest position to the impact position, thereby driving the pile.