Safety mechanism for a raise drill

Abstract

A raise bit is disclosed for enlarging a pilot hole into a larger diameter hole by disintegrating the earth formations surrounding the pilot hole. The raise bit includes a removable drive stem for enabling the raise bit to be transported through small drifts. The drive stem is attached to a thrust bearing plate which, in turn, is interconnected to a main bit body. The bit body extends around the drive stem and includes a plurality of rolling cutters for contacting and disintegrating the earth formations surrounding the pilot hole. The interconnection between the thrust bearing plate and the bit body is accomplished by means of a flexible shock absorbing member which is bonded to the plate and bit body. This flexible interconnection functions to absorb the bending and impact type loads acting on the bit body. A safety mechanism is provided to limit the amount of relative axial movement of the flexibly interconnected members should the bonded interconnections separate. The safety mechanism includes a lug connected to and extending downwardly from the main bit body. The lug extends around the thrust bearing plate with a small clearance therebetween in order to retain the interconnected members should the flexible bonded connection break. The safety mechanism further includes a stop located on the thrust bearing plate adjacent the lug. Should the bonded interconnections break, the stop is adapted to engage the lug as the thrust plate rotates to impart a torque force through the lug in order to drivingly rotate the bit body.