Patent ID: 8091649

Claim:
A pneumatic screw driver comprising: a tool body; a nose portion provided on a leading end of the tool body; a main air chamber which stores compressed air; a hammering cylinder provided in the tool body; a driver bit integrally combined with a hammering piston in the hammering cylinder; an air motor operated by the compressed air; a valve housing provided in an air passage between the main air chamber and the air motor; a stop valve piston provided in the valve housing, the stop valve piston which opens and closes the air passage; a stop valve stem coaxially provided with the stop valve piston, the stop valve stem which causes the stop valve piston to open and close; a timer chamber connected to the hammering cylinder; a first pipeline and a second pipeline respectively formed at upper and lower positions in a lower portion of the stop valve stem; and an upper end proximal portion of a contact member provided slidably in a driving direction in such a manner as to project from the nose portion, the upper end proximal portion provided engageably with the stop valve stem, wherein the first pipeline and the second pipeline are arranged so as to close the first pipeline after closing the second pipeline by moving the stop valve stem upward from a bottom dead center, the compressed air from the hammering cylinder is supplied to the first pipeline and the second pipeline through the timer chamber when the hammering piston reaches the bottom dead center during a hammering operation, the tool body is lowered together with a drive screw in consequence of a tightening operation by the air motor after the hammering operation, the contact member is moved upward relative to the tool body to push the stop valve stem upward, thereby closing the second pipeline, and when the stop valve stem is moved upward by the compressed air which cannot flow out fully from the first pipeline, the stop valve piston effects a shutoff operation of the air passage, thereby closing the first pipeline by the compressed air in the air passage.