1. Field of the Invention
The invention relates generally to percussion bits. More specifically, the invention relates to a bit retention system that, in the event of bit breakage, retains the head of the bit for easier removal from the drilled hole.
2. Background Art
Percussion bit systems are often used in drilling or boring through the earth's surface. In a percussion bit system, a percussion hammer is used to drive the percussion bit into the ground using the reciprocating action of a piston to energize the bit.
FIG. 1 illustrates a conventional percussion bit assembly design 100 that does not include a bit retainer. The percussion bit assembly 100 comprises a hammer case 101 that connects to a lower end of a drill string (not shown) through a threaded pin connection 145. The lower end of the hammer case 101 is threadedly engaged with driver sub 102. A plurality of splines (not shown) disposed on the driver sub 102, engage a plurality of splines 115 disposed on the shank 112 of the hammer bit 110, and rotatatively drive the bit 110. The upper end of the hammer bit 110 includes a piston strike surface 148 and a foot valve, or blow tube, 147. The lower end of the hammer bit includes a head 111.
The hammer assembly includes a control tube 143 and an annular piston chamber 146. Pressurized air moves a piston 142 in a reciprocating motion inside the annular piston chamber 146. A check valve 144 is used to communicate between the control tube 143 and the drill string (not shown). The lower end of the piston 142 is adapted to strike the piston strike surface 148, thereby imparting kinetic energy to the bit 110.
Occasionally, the bit 110 may fail and crack across the shank 112 of the bit 110 during drilling. If this happens, the head 111 of the bit 110 is left in the hole and has to be retrieved later through a costly fishing operation. In fact, most conventional hammer bits comprise a fishing thread 140 formed into the head 111 of the bit 110 to facilitate retrieving a broken head 111 from a drilled hole.
A bit retainer 205, as shown in FIG. 2 can be used to retain the head 211 of a bit 210 to the hammer assembly. FIG. 2 shows a conventional bit retainer system 200 comprising a driver sub 202, a bit retainer 205, and a bit 210. U.S. Pat. No. 5,065,827 assigned to the assignee of the current invention, is an example of such a conventional bit retainer.
The driver sub 202 comprises a first outside diameter 233 and a second outside diameter 232, wherein the second outside diameter 232 is larger than the first outside diameter 233. An external shoulder 230 is formed by the two sections. The driver sub 202 is disposed around shank 212 of bit 210. A plurality of splines (not shown) on the inside diameter of the driver sub 202 engage a plurality of splines 215 on the outside diameter of the shank 212 and rotatively drive the bit 210.
A bit retainer 205 is disposed around the driver sub 202 and the bit 210. An internal shoulder 208 engages the bit retainer 205 with the driver sub 202. The pin end 203 of the driver sub 202 is threadedly connected with the hammer case 201. An upper shoulder 234 of the bit retainer 205 abuts the hammer case 201. The internal shoulder 208 between the bit retainer 205 and the driver sub 202 supports the axial load generated by tightening the driver sub 202 to the hammer case 201.
The bit retainer 205 further comprises a catch thread 207 on its lower end that, in the event that the bit 210 breaks in the shank 212 area, a retaining thread 213 on the bit 210 engages the catch thread 207. This prevents the head 211 of the bit 210 from separating from the percussion bit assembly.
The internal shoulder 208 provides an axial stop function and also serves as the primary load-carrying and torque-carrying mechanism. The inside diameter of the shoulder 208 must be slightly larger than the first outside diameter 233 of the driver sub 202 to allow the bit retainer 205 to pass over the driver sub 202 during assembly of the two components. The internal shoulder 208 must be large enough to support the axial load generated by the makeup of the system. The bit retainer 205 must have a greater diameter than that of the driver sub external shoulder 230 to allow the catch threads 207 at the lower end of the bit retainer 205 to pass over the shoulder 230 during assembly. The thickness of the bit retainer 205 must also be sufficient to support the axial load sustained when pulling a broken bit head out of the hole. In a conventional bit retainer, these considerations result in a system where the outside diameter of the bit retainer 205 is larger than the outside diameter of the drill string casing 201.
The drill bit 210 generates cuttings that are carried by drilling fluid past the bit retainer 205 and hammer case 201. The velocity of the drilling fluid and cuttings is greater around the bit retainer 205 than around the hammer case 201, because the bit retainer 205 is larger in diameter than the hammer case 201. This results in less flow area between the bit retainer 205 and the hole wall. The cuttings are abrasive and cause erosion of the bit retainer 205, limiting its useful life. Further, in sticky formations, the bit retainer 205 may impede cuttings from being removed from the bit 210 and carried further up the hole due to its larger diameter. It is therefore desirable to construct a system wherein the outside diameter of the bit retainer 205 is substantially similar to the outside diameter of the hammer case 201.
Typical dimensions for a conventional hammer and bit retainer are now provided to illustrate exemplary bits. No limitation on the scope of the invention is intended by any reference to any specific dimension. The major diameter 235 of the threaded pin end 203 of the driver sub 202 is approximately 6.250 in. for a 8¾ in. for a conventional hammer and bit retainer used in an 8¾ in. hole size. The bit retainer 205 must pass over this diameter as a clearance fit, so the designed inside diameter is approximately 6.310 in. It is customary to bevel sharp corners to prevent handling damage to parts and personnel, so a 45 degree by 0.070 in. chamfer is used. This brings the inside diameter of the shoulder 208 to 6.450 in. The mating shoulder 230 on the driver sub 202 is limited by the driver sub outside diameter 232, which is about 6.800 in. The shoulder 230 also has a 45 degree by 0.070 in. chamfer, so the actual driver sub shoulder 230 outside diameter is 6.660 in. The bit retainer shoulder 208 inside diameter and the driver sub shoulder 230 outside diameter define the contact area, which is 2.162 square inches. The yield strength of the bit retainer 205 is specified as 125,000 psi, and a commonly used design criteria for shoulder loading is 66% of yield strength. Therefore, the shoulder 208 is capable of sustaining about 178,365 lb. of load. Based on this load, and a thread lubricant friction coefficient of 0.080, the connection is limited to a makeup torque of about 11,000 ft.-lb. of torque. This value is about half that of the nearby connections in the hammer, and less that half the makeup torque of the connections in the drill string. As a result, the shoulder 208 is likely to crush when high torque is encountered during drilling.
The bit retainer 205 must pass over the driver sub outside diameter 232, so its minor diameter is 6.820 in. The channel 209 in the bit retainer inside diameter must be large enough to allow the bit retaining thread 213 to move freely in the channel 209 without contact. The major diameter 236 of the retaining thread 213 on the bit 210 is 6.984 in., so the channel inside diameter 237 is 7.00 in. The typical bit retainer outside diameter 238 is 7.625 in., which leaves a 5/16 in. wall thickness of the bit retainer 205 in the area of the channel 209.
The clearance between the bit retainer outside diameter and the wall of the 8¾ in. drilled hole is 0.562 in. per side, resulting in about 14.5 square inches of annular flow area. In deep hole drilling with this hole size, between 2400 and 4000 standard cubic feet per minute (SCFM) of fluid flow is typically used to operate the hammer and remove cuttings from the hole. Depending on the air flow rate and the abrasivity of the drilled cuttings, the life of a bit retainer may be limited to roughly fifty to two hundred operating hours due to the erosive action of the cuttings on the bit retainer. The outside diameter of the mating hammer case is 7.150 in., resulting in a hole wall clearance of 0.800 in. and an annular flow area of about 20 square inches. The air velocity past the hammer case is about 27% lower than air velocity past the bit retainer. As a result, hammer cases erode much less than bit retainers and have a longer useful life. It is, therefore, desirable to reduce the outside diameter of the bit retainer to thereby reduce air velocity and erosion. However, in reducing the outside diameter of a conventional bit retainer, either wall thickness or shoulder diameter must also be reduced. This reduction in either wall thickness or shoulder diameter reduces the strength and torque capability of the bit retainer.