BORKRONE FOR UTVIDELSE AV ET STYREHULL.

摘要

A raise bit for enlarging a pilot hole into a larger diameter hole by disintegrating the earth formations surrounding the pilot hole includes a removable drive stem. The removable drive stem allows the raise bit to be transported through small drifts and helps extend the useful lifetime of the raise bit. The raise bit may be assembled without weakening the highly-stressed, heat-treated drive stem by welding. The raise bit body includes a multiplicity of rolling cutters for contacting and disintegrating the earth formations surrounding the pilot hole. The raise bit body includes a fixed collar with a truncated conical inside surface. A two-piece, split, removable collar with an inside surface that matches the outside surface of the drive stem and a conical outer surface that matches the inside surface of the fixed collar is positioned around the drive stem. A flange at the end of the drive stem allows the split collar to be drawn up into the fixed collar thereby developing frictional forces between the mating surfaces of the fixed collar, the split collar, and the drive stem. A preload is induced into the system to insure proper torque transmittal at low bit loads.