Bit retainer system

A bit retainer system comprising a casing, a driver sub adapted to be disposed around a shank of a bit, and a bit retainer. The driver sub comprises a threaded connector, a threaded pin end configured to engage with the hammer case, and a stop. The bit retainer comprises a load carrying threaded connector configured to engage with the threaded connector of the driver sub, and a catch thread configured to retain the head of a bit to the system.

BACKGROUND OF INVENTION

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. 1illustrates a conventional percussion bit assembly design100that does not include a bit retainer. The percussion bit assembly100comprises a hammer case101that connects to a lower end of a drill string (not shown) through a threaded pin connection145. The lower end of the hammer case101is threadedly engaged with driver sub102. A plurality of splines (not shown) disposed on the driver sub102, engage a plurality of splines115disposed on the shank112of the hammer bit110, and rotatatively drive the bit110. The upper end of the hammer bit110includes a piston strike surface148and a foot valve, or blow tube,147. The lower end of the hammer bit includes a head111.

The hammer assembly includes a control tube143and an annular piston chamber146. Pressurized air moves a piston142in a reciprocating motion inside the annular piston chamber146. A check valve144is used to communicate between the control tube143and the drill string (not shown). The lower end of the piston142is adapted to strike the piston strike surface148, thereby imparting kinetic energy to the bit110.

Occasionally, the bit110may fail and crack across the shank112of the bit110during drilling. If this happens, the head111of the bit110is left in the hole and has to be retrieved later through a costly fishing operation. In fact, most conventional hammer bits comprise a fishing thread140formed into the head111of the bit110to facilitate retrieving a broken head111from a drilled hole.

A bit retainer205, as shown inFIG. 2can be used to retain the head211of a bit210to the hammer assembly.FIG. 2shows a conventional bit retainer system200comprising a driver sub202, a bit retainer205, and a bit210. 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 sub202comprises a first outside diameter233and a second outside diameter232, wherein the second outside diameter232is larger than the first outside diameter233. An external shoulder230is formed by the two sections. The driver sub202is disposed around shank212of bit210. A plurality of splines (not shown) on the inside diameter of the driver sub202engage a plurality of splines215on the outside diameter of the shank212and rotatively drive the bit210.

A bit retainer205is disposed around the driver sub202and the bit210. An internal shoulder208engages the bit retainer205with the driver sub202. The pin end203of the driver sub202is threadedly connected with the hammer case201. An upper shoulder234of the bit retainer205abuts the hammer case201. The internal shoulder208between the bit retainer205and the driver sub202supports the axial load generated by tightening the driver sub202to the hammer case201.

The bit retainer205further comprises a catch thread207on its lower end that, in the event that the bit210breaks in the shank212area, a retaining thread213on the bit210engages the catch thread207. This prevents the head211of the bit210from separating from the percussion bit assembly.

The internal shoulder208provides an axial stop function and also serves as the primary load-carrying and torque-carrying mechanism. The inside diameter of the shoulder208must be slightly larger than the first outside diameter233of the driver sub202to allow the bit retainer205to pass over the driver sub202during assembly of the two components. The internal shoulder208must be large enough to support the axial load generated by the makeup of the system. The bit retainer205must have a greater diameter than that of the driver sub external shoulder230to allow the catch threads207at the lower end of the bit retainer205to pass over the shoulder230during assembly. The thickness of the bit retainer205must 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 retainer205is larger than the outside diameter of the drill string casing201.

The drill bit210generates cuttings that are carried by drilling fluid past the bit retainer205and hammer case201. The velocity of the drilling fluid and cuttings is greater around the bit retainer205than around the hammer case201, because the bit retainer205is larger in diameter than the hammer case201. This results in less flow area between the bit retainer205and the hole wall. The cuttings are abrasive and cause erosion of the bit retainer205, limiting its useful life. Further, in sticky formations, the bit retainer205may impede cuttings from being removed from the bit210and 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 retainer205is substantially similar to the outside diameter of the hammer case201.

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 diameter235of the threaded pin end203of the driver sub202is approximately 6.250 in. for a 8¾ in. for a conventional hammer and bit retainer used in an 8¾ in. hole size. The bit retainer205must 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 shoulder208to 6.450 in. The mating shoulder230on the driver sub202is limited by the driver sub outside diameter232, which is about 6.800 in. The shoulder230also has a 45 degree by 0.070 in. chamfer, so the actual driver sub shoulder230outside diameter is 6.660 in. The bit retainer shoulder208inside diameter and the driver sub shoulder230outside diameter define the contact area, which is 2.162 square inches. The yield strength of the bit retainer205is specified as 125,000 psi, and a commonly used design criteria for shoulder loading is 66% of yield strength. Therefore, the shoulder208is 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 shoulder208is likely to crush when high torque is encountered during drilling.

The bit retainer205must pass over the driver sub outside diameter232, so its minor diameter is 6.820 in. The channel209in the bit retainer inside diameter must be large enough to allow the bit retaining thread213to move freely in the channel209without contact. The major diameter236of the retaining thread213on the bit210is 6.984 in., so the channel inside diameter237is 7.00 in. The typical bit retainer outside diameter238is 7.625 in., which leaves a 5/16 in. wall thickness of the bit retainer205in the area of the channel209.

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.

SUMMARY OF INVENTION

In one aspect, the invention comprises a hammer case, a driver sub adapted to be disposed around a shank of a bit, and a bit retainer. The driver sub comprises a threaded connector, a threaded pin end configured to engage with the hammer case, and an axial stop. The axial stop is a shoulder cut into the threaded connector of the driver sub between the threaded pin end and the threaded connector. A bit comprises a head and a shank, the head having a retaining thread. The bit further comprises a plurality of splines disposed axially along the shank that are configured to engage a plurality of splines axially disposed on the inside diameter of the driver sub.

The bit retainer comprises a load carrying threaded connector configured to engage with the threaded connector of the driver sub, and a catch thread configured to retain the head of the bit to the system. The catch thread on the bit retainer is configured to thread past the threaded connector of the driver sub and the retaining thread on the head of the bit.

In another aspect, the invention comprises a hammer case, a driver sub adapted to be disposed around a shank of a bit, and a bit retainer. The driver sub comprises a threaded connector wherein an upper section is configured to engage the driver sub with the hammer case, and an axial stop. The axial stop is a shoulder cut into the threaded connector. A bit comprises a head and a shank, the head having a retaining thread. The bit further comprises a plurality of splines disposed axially along the shank that are configured to engage a plurality of splines axially disposed on the inside diameter of the driver sub.

The bit retainer comprises a load carrying threaded connector configured to engage with a lower section of the threaded connector of the driver sub, and a catch thread configured to retain the head of the bit to the system. The catch thread on the bit retainer is configured to pass over the threaded connector of the driver sub and the retaining thread on the head of the bit.

In another aspect, the invention comprises a hammer case, a driver sub adapted to be disposed around a shank of a bit, and a bit retainer. The driver sub comprises a threaded connector, a threaded pin end configured to engage with the hammer case, and a rotational stop provided by at least one locking device. The at least one locking device may comprise a key. A bit comprises a head and a shank, the head having a retaining thread. The bit further comprises a plurality of splines disposed axially along the shank that are configured to engage a plurality of splines axially disposed on the inside diameter of the driver sub.

The bit retainer comprises a load carrying threaded connector configured to engage with the threaded connector of the driver sub, a rotational stop configured to engage with the at least one locking device, and a catch thread configured to retain the head of the bit to the assembly. The catch thread on the bit retainer is configured to thread pass over threaded connector of the driver sub and the retaining thread on the head of the bit.

The bit retainer system further comprises a sealing device disposed around the driver sub and located axially between the at least one locking device and the load threaded connector. The sealing device may be an o-ring.

In another aspect, the invention comprises a hammer case, a bit having a head and a shank, a driver sub disposed around the shank, a bit retainer, and a sealing device disposed around the driver sub. The driver sub comprises a threaded connector, a threaded end configured to engage with the casing, and a stop. The driver sub comprises a load carrying threaded connector, a threaded end configured to engage with the casing, and an axial stop. The stop is provided by at least one locking device wherein the at least one locking device comprises a locking collar integrally formed with at least one key. The bit further comprises a retainer thread on the head. The bit further comprises a plurality of splines disposed axially along the shank that are configured to engage a plurality of splines axially disposed on the inside diameter of the driver sub.

The bit retainer comprises a load carrying threaded connector configured to engage with the threaded connector of the driver sub, and a catch thread configured to retain the head of the bit to the assembly. The catch thread on the bit retainer is configured to pass over the threaded connector of the driver sub and the retaining thread on the head of the bit.

The sealing device of the bit retainer system is axially disposed between the axial stop and the load carrying threaded connector. The sealing device maybe be an o-ring.

DETAILED DESCRIPTION

When using a hammer drill, the percussion bit occasionally breaks and the bit head may become separated from the assembly. A significant problem may occur if the bit head is lost into the hole. Thus, a bit retainer may be used so that the broken bit can be retrieved from the hole with the assembly. In one aspect, embodiments of the present invention relate to a bit retainer.

Select embodiments of the present invention, as described below, include a threaded connection between a bit retainer and a driver sub in a bit retainer system. The threaded connection between the driver sub and the bit retainer is the primary load carrying mechanism of the axial load of the system generated by the tightening of the driver sub and bit retainer assembly to the hammer case. This bit retainer system design found in select embodiments also provides the bit retainer with an outside diameter substantially similar to the outside diameter of the casing. In select embodiments, a shoulder is cut into the driver sub, the shoulder functioning as an axial stop when threading the driver sub and the bit retainer together. The shoulder size is reduced as compared to the size of conventional shoulders because the shoulder cut into the driver sub provides only an axial stop when threading the bit retainer and the driver sub. In other words, the shoulder is not the primary load carrying mechanism for the axial load of the system, and can therefore be reduced in size. In other embodiments, the shoulder is eliminated entirely and its function is replaced by other stop means, such as keys. Thus, the outside diameter of the bit retainer can be reduced, while maintaining the ability of the bit retainer to sustain axial loads generated by driver sub makeup and by pulling a broken bit head out of the drilled hole.

FIG. 3shows a bit retainer system300, in accordance with an embodiment of the invention. The bit retainer system300includes a hammer bit310inserted inside a driver sub302. A plurality of splines (not shown) on the inside diameter of the driver sub302engage a plurality of splines315disposed axially on the outside diameter of shank312, and rotatively drive the bit310. A bit retainer305is threaded over the driver sub302. A catch thread307on the bit retainer305is configured to thread past a threaded connector304on the driver sub302and a retaining thread313of the bit310.

In this embodiment, an axial stop308on the driver sub302is a shoulder cut into the threaded connector304. A load carrying threaded connector306on the bit retainer305is threaded with the threaded connector304on the driver sub302until the axial stop shoulder308is contacted. The connection between the load carrying threaded connector306on the bit retainer305and the threaded connector304on the driver sub302supports the axial load of the system generated by engaging a threaded pin end303of the driver sub302inside the hammer case301and by engaging the bit retainer305with the driver sub302. A circumferential channel309disposed on the inside diameter of the bit retainer305, between the load carrying threaded connector306and the catch thread307, allows the bit to travel axially as required for normal hammer operation.

The catch thread307of the bit retainer305acts as a “bit catch.” In the event that the drill bit310breaks in the shank312, the retaining thread313will shoulder on the catch thread307as the bit head311falls. The bit retainer305thus retains the bit head311to the bit retainer system300, allowing for easier removal of a broken bit310.

FIG. 4shows another embodiment of the invention. In this embodiment, a bit retainer system400includes a hammer bit410inserted inside a driver sub402. A plurality of splines (not shown) on the inside diameter of the driver sub402engage a plurality of splines415on the outside diameter of shank412, and rotatively drive the bit410. The driver sub402has a threaded connector404, which has an upper section403and a lower section417. A bit retainer has a threaded connector406and a catch thread407. A bit410comprises a head411having a retaining thread413. The catch thread407on the bit retainer405is configured to pass over the outside diameter of the driver sub402and thread past the retaining thread413of the bit410. The upper section403of the threaded connector404is configured to attach to a hammer case401.

In this embodiment, an axial stop408is provided by is a shoulder430formed on the driver sub402adjacent to the threaded connector lower section417. A mating axial stop408is formed into the load carrying threaded connector406of the bit retainer405. The bit retainer405is assembled onto the driver sub by threading the load carrying threaded connector406past the upper section403of the threaded connector404and onto the lower section417until the bit retainer axial stop shoulder408contacts the driver sub axial stop shoulder430. Simultaneously, the bit retainer catch thread407is threaded past the bit retaining thread413. The upper section403of the threaded connector404is engaged with the hammer case401. The threaded connection between the load carrying threaded connector406on the bit retainer405and the lower section417on the driver sub402supports the axial load of the system generated by engaging the upper section403of the threaded connector404of the driver sub402inside the hammer case401. A circumferential channel409disposed on the inside diameter of the bit retainer405, between the load carrying threaded connector406and the catch thread407, allows the bit to travel axially as required for normal hammer operation.

The catch thread407of the bit retainer405acts as a “bit catch.” In the event that the drill bit410breaks in the shank412, the retaining thread413will shoulder on the catch thread407as the bit head411falls. The bit retainer405retains the bit head411to the bit retainer system400, allowing for easier removal of a broken bit410.

FIG. 5shows another embodiment of the bit retainer system of the current invention. The bit retainer system500includes a hammer bit510inserted inside a driver sub502. A plurality of splines (not shown) on the inside diameter of the driver sub502engage a plurality of splines515on the outside diameter of shank512, and rotatively drive the bit510. A bit retainer505is threaded over the driver sub502. A catch thread507on the bit retainer505is configured to pass over a threaded connector504of the driver sub502and a retaining thread513of the bit510.

A load carrying threaded connector506on the bit retainer505is threaded with the threaded connector504on the driver sub502. In this embodiment, a stop508is provided by at least one locking device520, for example at least one key, inserted between the bit retainer505and the driver sub502. Alternatively, other locking devices such as pins or balls could be used. A locking collar519may be disposed around the driver sub502and abut the at least one locking device520. The locking collar519secures the at least one locking device520between the driver sub502and the bit retainer505. The at least one locking device520supports a substantial portion of the torque of the system, allowing the load carrying threaded connector506of the bit retainer505to carry primarily axial load as opposed to additionally carrying torque. The connection between the load carrying threaded connector506on the bit retainer505and the threaded connector504on the driver sub502supports the axial load of the system generated by engaging the threaded pin end503of the driver sub502inside a hammer case501. A circumferential channel509disposed on the inside diameter of the bit retainer505, between the load carrying threaded connector506and the catch thread507, allows the bit to travel axially as required for normal hammer operation.

A sealing device514may be disposed on the outside diameter of the driver sub502in an location axially between the locking devices508and the threaded connector504of the driver sub502. The sealing device514may be any known in the art, such as an o-ring. The sealing device514prevents pressure pulses from blowing out the lubrication between the engaged load carrying threaded connector506on the bit retainer505and the threaded connector504on the driver sub502. The sealing device prevents gouging of the threads that can occur when there is insufficient lubrication between the threads.

The catch thread507of the bit retainer505acts as a “bit catch.” In the event that the drill bit510breaks in the shank512, the retaining thread513will shoulder on the catch thread507as the bit head511falls. The bit retainer505retains the bit head511to the bit retainer system500, allowing for easier removal of a broken bit510.

FIG. 6shows another embodiment of the present invention. The bit retainer system600includes a hammer bit610inserted inside a driver sub602. A plurality of splines (not shown) on the inside diameter of the driver sub602engage a plurality of splines615on the outside diameter of shank612, and rotatively drive the bit610. A bit retainer605is threaded over the driver sub602. A catch thread607on the bit retainer605is configured to pass over a threaded connector604on the driver sub602and a retaining thread613of the bit610.

In this embodiment, a rotational stop608is provided by a locking device that comprises a locking collar619integrally formed with at least one key620. The locking collar619is disposed around the driver sub602, and the at least one key620formed on the locking collar619is inserted between the driver sub602and the bit retainer605. The threaded connector604of the driver sub602is threaded with the load carrying threaded connector606on the bit retainer605. The keys620of the locking collar619transmit a substantial portion of system torque between the driver sub602and the bit retainer605. The connection between the load carrying threaded connector606on the bit retainer605and the threaded connector604on the driver sub602supports the axial load of the system generated by engaging the threaded end603of the driver sub602inside a hammer case601. A circumferential channel609disposed on the inside diameter of the bit retainer605, between the load carrying threaded connector606and the catch thread607, allows the bit to travel axially as required for normal hammer operation.

A sealing device614may be disposed on the outside diameter of the driver sub602in a location axially between the stop608and the threaded connector604. The sealing device614may be any known in the art, such as an o-ring. The sealing device614prevents pressure pulses from blowing out the lubrication between the load carrying threaded connector606and the threaded connector604. The sealing device prevents gouging of the threads that can occur when there is insufficient lubrication between the threads.

The catch thread607of the bit retainer605acts as a “bit catch.” In the event that the drill bit610breaks in the shank612, the retaining thread613will shoulder on the catch thread607as the bit head611falls. The bit retainer605retains the bit head611to the bit retainer system600, allowing for easier removal of the broken bit610.

For illustrative purposes, a detailed example of an embodiment of the invention is provided in contrast to the detailed description of a conventional bit retainer system as described previously. Again, no limitation on the scope of the present invention is intended by referencing particular dimensions. Referring to an embodiment, as depicted inFIG. 6, the outside diameter of the bit retainer605for an 8¾ in. hole size is 7.150 in. The outside diameter of the hammer case601is also 7.150 in. The thread major diameter of the threaded connector604of the driver sub602is 6.555 in., and the mating load carrying threaded connector606on the bit retainer605is 6.395 in. The effective shoulder area of each thread is approximately 1.627 square inches, and approximately 8 threads are in contact. Therefore, the equivalent load carrying shoulder is approximately 13 square inches. This results in a load capacity of the system approximately 6 times the load carrying capacity of a conventional retainer system, which as noted above, has a shoulder area of only 2.162 square inches.

Additionally, in an embodiment of the invention, a rotational stop708is provided by a locking collar719, as detailed inFIG. 7, integrally formed with 36 keys720, which are approximately ¼ in. wide by ¼ in. high. The locking collar is configured to engage the bit retainer705and the driver sub (602inFIG. 6). The keys720alone can carry approximately 18,000 ft.-lb. of torque. When the threaded pin end603, referring back toFIG. 6, of the driver sub602is made up to the hammer case601with a makeup torque of 25,000 ft.-lb., as is desired, the resulting axial load on the mating threaded connectors604and606is approximately 350,000 lb. Using this load value and a thread lubricant friction coefficient of 0.080, the threaded connectors604and606are capable of sustaining approximately 10,685 ft.-lb. of torque. Therefore, the combined torque carrying capacity of the keys620and the mating threaded connectors604and606is over 28,000 ft.-lb, whereas the conventional bit retainer system had a torque carrying capacity of only 11,000 ft.-lb., as calculated above.

While the above description uses “threads” to describe various features (e.g., catch thread and retaining thread) of embodiments of the invention, one of ordinary skill in the art would appreciate that these threads need not be continuous threads. Instead, these threads may comprise, for example, segments of the threads. Likewise, while the above description uses “keys” as an example of a locking device, one of ordinary skill in the art would appreciate that other similar locking devices, such as pins, balls, or splines may be used.

Embodiments of the invention may include one or more of the following advantages:

A bit retainer system with a threaded connection between the bit retainer and the driver sub that supports the axial load generated by the makeup of the system.

A bit retainer system with a locking device between the bit retainer and the driver sub, wherein the locking device transmits torque between the bit retainer and the driver sub.

A system where the outside diameter of the bit retainer is substantially similar to the outside diameter of the casing. This can improve hole cleaning, reduce retainer erosion, and prevent the system from getting stuck or hung up inside the bore hole.

A bit retainer with a thickness sufficient to endure the vibrations of the system due to the reciprocating action of the piston and the repeated impact of the bit on the drilling surface, and with sufficient thickness to sustain the load generated by pulling a broken bit head out of the drilled hole.