I. Field of the Invention
This invention relates generally to earth boring percussion bits, and more specifically to retention systems for ensuring that the bit remains coupled to the hammer at all times during the earth boring operation.
II. Description of the Prior Art
FIG. 1 of the drawings illustrates a standard hammer and percussion bit assembly, generally indicated by arrow 10. The assembly comprises a hammer housing 11 threadedly connected at its upper end to a pin connection 12. The pin is utilized for connection to the lower end of the drill string (not shown). The lower end of the housing 11 is threadedly connected to a driver sub 13, which includes a plurality of splines located on the interior thereof for matingly receiving the splines 14 located on the shank 15 of the percussion bit 16 and rotatively driving the same.
A pair of lock rings 17 are fitted to be located within the interior of the housing 11 with the upper end thereof abutting against an annular shoulder ring 18 fixedly mounted within the housing 11. The other end of the lock rings 17 are adapted to abut the upper end of the driver sub 13 to be held in place therewith. The upper end of the percussion bit 16 includes a piston strike 19 and a foot valve 20. The lower end of the bit 16 includes a head section 21 having a plurality of inserts 22 mounted thereon. A fishing thread 23 is also formed on the upper periphery of the head section 21.
The purpose of the fishing thread 23 is to enable the head section 21 to be successfully fished or threadedly grabbed by a fishing tool should the head section 21 fracture or break away from the bit shank 15 during operation.
Finally, the percussion bit 16 includes an upper bearing section 24 which is adapted to bear against the lock ring 17, and a lower bearing section 25, which is adapted to bear against the interior of the driver sub 13.
The hammer assembly further includes a control rod 26 which, with the interior of the housing 11, forms an annular piston chamber 27. A hollow cylindrical piston 28 is positioned within the chamber 27 to be reciprocated therein. The piston 28 includes a plurality of channels 29 which cooperate with windows 30 of the control rod 26 to enable pressurized air to drive the piston upwardly and downwardly in a conventional manner. The interior of the control rod 26 is adapted to communicate with the interior of the pin connection 12 and the drill string via a back flow valve assembly 31.
The lower end of the piston 28 is adapted to impact against the strike face 19 which imparts a percussive force to the bit 16.
As noted above, the standard bit 16 is retained within a conventional hammer assembly by the ring locks 17, located near the top of the bit shank 15.
Frequently however, in approximately five percent of all bits, the bit will fail due to cyclic fatigue by fracturing below the retaining rings and above the bit head. When this occurs during the drilling operation, the lower section of the bit is left at the bottom of the hole, thereby necessitating a costly and cumbersome fishing operation.