Means for retaining roller cutters on rotary drill bit

Claim 1

A rotary drill bit comprising: a bit body having a depending leg and a generally cylindrical journal extending therefrom with an annular groove therein of a substantially semicircular shape in cross section to define relative to the outer end of said journal spaced outer and inner edges at the entrance to the groove, said groove having a root at its deepest point and defining an outer quadrant on one side of said root beginning at said outer edge and an inner quadrant on the opposite side of said root beginning at said inner edge; a conically shaped roller cutter having a body with a central bore therein for receiving said journal and including internal screw threads defining a portion of said central bore, said journal having a thrust bearing surface thereon contacting said roller cutter for limiting axial movement of said roller cutter toward said leg; a retainer ring mounted within said annular groove and having external screw threads along its outer periphery in threaded engagement with said internal screw threads on said roller cutter for rotation with said roller cutter relative to said journal; and said retainer ring having a smooth arcuate surface along its inner periphery in bearing contact with said outer quadrant segment generally along a line spaced at least 10° from said outer edge of said groove with said smooth arcuate surface being formed of a radius less than the radius forming the adjacent semicircular shape of said groove for minimizing bending stresses exerted at the root of said groove, said retainer ring being spaced from said inner quadrant segment by contact of said roller cutter against said thrust bearing surface.