Patent ID: 7165478

Claim:
A pneumatically operated screw driver comprising: an outer frame; a pneumatic motor disposed in the outer frame and rotatable about its axis; a cylinder fixedly disposed in the outer frame and formed with at least one compressed air introduction hole and at least one compressed air flowage hole, a return chamber being defined between the outer frame and the cylinder so that a compressed air is flowed from the cylinder to the return chamber through the air flowage hole and is flowed from the return chamber into the cylinder through the air introduction hole; a piston reciprocally movable with respect to the cylinder and comprising a main piston and an auxiliary piston; a driver bit having one end connected to the piston and another end engageable with a head of a fastener; the main piston slidably disposed in the cylinder and movable in an axial direction of the cylinder between its top dead center and a bottom dead center, the main piston being in a form of a sleeve like configuration defining an inner space and an outer space and being formed with a first communication hole permitting fluid communication between the inner space and the outer space, the main piston having an abutment end; a bumper disposed at the cylinder, the abutment end of the main piston being abuttable on the bumper; the auxiliary piston movable in the axial direction between its top dead center and a bottom dead center and rotatable about its axis by the rotation of the pneumatic motor, the auxiliary piston comprising: a hollow section; an intermediate section connected to the hollow section; and another end portion connected to the intermediate section and provided with a piston section and a flange section, the piston section being slidably movable with respect to the main piston, and the flange section being positioned within the inner space and seated on the bumper upon completion of a screw driving operation, the flange section being positioned axially spaced away from the piston section.