A method and system for yield similarity of semiconductor devices. The head or neck includes a mechanical motorized vibratory device, including a drive which causes the head to vibrate. The gear drive mechanism comprises a groove formed around an end of at least one of the driven shafts other than the specific shafts, a ring-shaped washer having a slit in a direction of a diameter fit in the groove as a gear removal preventive and a recessed portion provided to a side surface of the gear throughout the washer.