Patent ID: 7784561

Claim:
A pneumatic hammer for rock drilling in which a piston strikes a button bit while being operated by compressed air in a cylindrical casing, the pneumatic hammer comprising: a cylindrical casing; a back head that is screw-engaged with an upper portion of the casing, the back head having a center hole configured to receive compressed air, an inner hollow space that is separated from the center hole by an inner peripheral edge portion, and at least four air passageways arranged circumferentially on a circle having a common center with the inner hollow space, the air passageways including a plurality of first air passageways that connect the center hole and the inner hollow space via an outlet aperture, and a plurality of second air passageways that are open at a lower end thereof, and that communicate with the inner hollow space via an inlet aperture, wherein the first and second air passageways are alternately arranged with each other; a guide installed in the inner hollow space of the back head, the guide including a lower shaft portion, an intermediate anvil portion configured to be inserted into the inner peripheral edge portion of the back head, and a check valve receiving portion formed opposite the lower shaft portion; a check valve configured to be received in the check valve receiving portion of the guide and elastically supported by a coil spring; a piston adapted to ascend and descend in the casing, the piston including a reference axial portion having a reference outer diameter R, a first axial portion having an extended outer diameter R 1 , a second axial portion having an extended outer diameter R 2 , a third axial portion having an extended outer diameter R 3 , a piston center hole extending along a piston central axis, and a compressed air passageway extending from the inlet aperture, which is formed between the extended outer diameters R 1 and R 2 , to the outlet aperture, which is formed in the extended outer diameter R 3 ; a chuck adapted to prevent escape of the piston from the casing and formed integrally with a bushing portion, the chuck having a bit retainer ring mounting recess formed on an inner peripheral surface of an intermediate axial portion thereof and a plurality of chuck guide grooves formed on an inner peripheral surface of a lower end portion thereof; a button bit having a plurality of circumferential protrusions alternately arranged between a plurality of button bit guide grooves formed on an outer peripheral surface thereof so as to slidably engage with the chuck guide grooves, an outer circumferential groove of length L that is formed on the outer peripheral surface, and a retaining step formed at an upper end of the outer circumferential groove thereof so that a travel length of the button bit is limited to L; and a variable compressed air chamber formed between an outer periphery of the piston and an inner periphery of the casing.