Abstract: A uniaxial vibratory force generator especially adapted for use as a pile driver, which comprises a rigid frame structure having a power actuated clamp at its bottom by which the frame structure grips a pile, and attachment means at its top by which the frame structure is suspended from above. In the frame structure are two superimposed pairs of counter-rotating eccentric weights, each of which is driven by a hydraulic motor. Simultaneous rotation of all of the eccentric weights produces a uniaxial force of a magnitude determined by the phase relationship between the superimposed pairs of eccentric weights. An adjustable coupling between the superimposed pairs of eccentric weights enables that phase relationship to be selectively varied.