Patent ID: 7987930

Claim:
A drill bit assembly for fluid-operated percussion drill tools comprising: a percussion bit having a head portion formed with an axially extending stub shank; axially extending splines on the stub shank slideably engageable with complementary splines formed on a drive chuck whereby rotational drive from the chuck may be transmitted to the stub shank; bit retaining means at the chuck adapted for engagement with complementary retaining means at a spline portion of the stub shank to retain the stub shank in the drill bit assembly such that splined support for transfer of rotational torque is provided both above and below the bit retaining means; and engagement means on the chuck adapted for connecting the chuck to a drive means of the fluid-operated percussion drill tool, wherein the drive chuck is formed with a screw thread which is adapted to engage with a complementary screw thread on the lower end of a wear sleeve of the drill tool such that the bit retaining means are retained in the chuck; wherein the bit retaining means comprises at least one but preferably a plurality of apertures through the wall of the drive chuck and a corresponding plurality of bit retaining pieces; such that when the bit assembly is assembled, the bit retaining pieces are received in the apertures and engage with the complementary retaining means at the spline portion of the stub shank to retain the stub shank in the drill bit assembly and wherein the apertures are formed as part-annular slots through the wall of the drive chuck and the bit retaining pieces are part-annularly shaped and the complementary retaining means comprises annular shoulder; such that when the bit assembly is assembled, the part-annular bit retaining pieces are received in the part-annular slots and engage with the underside of the annular shoulder to retain the stub shank in the drill bit assembly.