Patent ID: 7607490

Claim:
A pneumatic impact tool, comprising: a cylindrical tube having a hollow interior extending between first and second oppositely disposed ends; an impact plate slidably mounted in said interior of said tube for reciprocation therein and sealingly closing said first end of said tube interior; a cover connected with said tube and sealingly closing an outer portion of said tube interior disposed adjacent to said second end thereof; an impact piston slidably mounted in said interior of said tube for reciprocation therein and having first and second circularly-shaped, oppositely disposed end surfaces defining a first pressure chamber between said impact plate and said first end surface of said impact piston in which compressed air is communicated to shift said impact piston to a fully retracted position, and a second pressure chamber between said cover and said second end surface of said impact piston in which compressed air is communicated to shift said impact piston to a fully extended position; an opening disposed through said covert, communicating with said second pressure chamber, and defining the sole inlet for said second pressure chamber; said opening having an exterior side thereof communicating with a source of pressurized air, and an interior side thereof with a plan shape and size that is substantially smaller than the plan shape and size of said second end surface of said impact piston; and an annularly-shaped seal mounted on one of said second end surface of said impact piston and said interior side of said opening in said cover in a generally concentric relationship with said opening, whereby in said fully retracted position, said second end surface of said impact piston and said interior side of said opening in said cover axially abut said seal to sealingly close said opening; said seal being shaped geometrically similar to and slightly larger than said interior side of said opening, such that in said fully retracted position, said opening communicates pressurized air to only a relatively small portion of the total area of said second end surface of said impact piston, whereby when the air pressure differential acting on said first and second end surfaces of said impact piston exceeds a predetermined amount, said piston shifts out of axially abutting contact with said seal, thereby communicating the pressurized air with the entire area of said second end surface of said impact piston, and causing the impact piston to shift abruptly and quickly against said impact plate to said fully extended position.