1. Field of the Invention
The present invention relates generally to raise type earth boring drills and, more particularly, to raise drills having flexible connections between the drive stems and the main bit bodies.
2. Description of the Prior Art
A relatively large diameter hole may be provided between two locations in a mine by an operation commonly referred to as raise drilling. A raise drilling operation begins by drilling a small diameter pilot hole through the earth between the locations using a small diameter pilot bit. After the pilot hole is completed, the pilot bit is removed from the drill column and a large diameter raise bit is attached. The raise bit is then rotated and drawn along the pilot hole to enable the drill cutters to contact and disintegrate the earth formations surrounding the pilot hole, thereby enlarging the pilot hole to the desired size. In an exemplary embodiment, the pilot hole may be 11 inches in diameter and the reamed out hole may be six feet in diameter.
During a raise drilling operation, a tremendous amount of wear and stress is imposed upon the raise bit. The drive stem in particular is subjected to considerable wear due to abrasive contact with the surrounding earth formation and is also subject to considerable stress resulting from (a) tension due to the pulling force imparted to the drill, (b) twisting due to the torque applied to the drill, and (c) bending due to uneven loading around the circumference of the drill.
The advantages of having the drive stem removable are that the elements having a relatively short lifespan can be replaced, thereby extending the useful life of the bit, and the low profile of the separated components allows the raise bit to be transported through small drifts or passages.
The disadvantage of the replaceable drive stem is that a certain amount of down time is still required to remove and replace the stem. This non-operating time is costly and it is still preferable to obtain as long a running time as possible for each bit-stem combination.
One area that has not been sufficiently explored in raise drilling is the area relating to improving the operating life of a raise drill drive stem. A co-pending U.S. patent application, Ser. No. 961,038, entitled "An Improved Raise Drill", now Pat. No. 4,202,417 issued May 15, 1980, accomplishes this by providing a flexible shock absorbing connection between the bit body and the drive stem. This flexible connection functions to absorb the uneven bending and impact type loads acting on the bit body in order to prevent these loads from being transferred to the drive stem. The flexible connection consists of an elastomeric annular member bonded to the bit body base plate and a thrust plate which is integrally attached to the drive stem.
One concern with the shock absorbing connection is that a torque overload may break the bonded connections between the base plate and the thrust plate. If this were to occur, no torque would be applied to the bit body since all of the torque is transmitted through the flexible connection.
Another concern with the shock absorbing connection is that when the bit body is lowered in the enlarged hole, which becomes necessary at times, the body could become lodged in the hole while the drill column and drive stem are still being forced downwardly. This downward force could break the connection and the bit body would become separated from the drive stem. Should the drill column be lowered further and the bit body then become dislodged, the bit body would fall freely until it contacted the drive stem once again. In most instances, the drive stem would not be able to take the sudden load and the drill column would also break at some point and the bit body and drill column would fall to the hole bottom and be destroyed.