A vibratory drive system comprises a rotatable drive shaft and an out-of-balance wheel that is coaxial with and rotatable relative to the drive shaft. A rotary transmission system couples the drive shaft to the wheel in order to transmit rotational movement of the drive shaft to the wheel. The rotary transmission system comprises a shaft gear, a wheel gear, and intermediate gears coupling the shaft gear to the wheel gear. The intermediate gears are carried by a gear carrier that is rotatable relative to the drive shaft, and are rotatable with respect to the gear carrier. The arrangement allows the phase angle of the out-of-balance wheel to be adjusted with respect to the drive shaft.