| WO/2005/121622 | SCREW COUPLING FOR OIL WELL PIPE |
| WO/2009/020784 | COMMUNICATION CONNECTIONS FOR WIRED DRILL PIPE JOINTS |
| JP06073973 | APPARATUS FOR MOUNTING SCREWTHREADED JOINTS FOR ASSEMBLY OF TUBES |
SUNDIN, Jens (Klasbärsvägen38, Gävle, S-806 36, SE)
LEJON, Susanne (Seljansögatan 13 D, Sandviken, S-811 62, SE)
CARLSTRÖM, Bo (Idrottsvägen 24 C, Sandviken, S-811 32, SE)
SUNDIN, Jens (Klasbärsvägen38, Gävle, S-806 36, SE)
LEJON, Susanne (Seljansögatan 13 D, Sandviken, S-811 62, SE)
| CLAIMS 1. A drill bit for percussive rock drilling, which drill bit (20) comprises: - a drill head (21); - a skirt (24), which has a first end (24a) and an opposite second end (24a), the skirt being connected to the drill head (21) at said first end (24a); - a recess (25) extending axially through the skirt (24) from the second end (24b) of the skirt towards the first end (24a) thereof, the recess being provided with an internal, cylindrical female thread (26) for connection to a corresponding external, cylindrical male thread of a drill rod; and - an impact surface (27) for abutment against an end surface of a drill rod, this impact surface (27) being formed at the bottom of the recess (25) and extending perpendicularly to the longitudinal axis (A) of the drill bit, characterized in, that every cross-section of the recess (25) between the impact surface (27) and the part of the recess where the female thread (26) has its full thread profile has a cross- sectional area which is smaller than the cross-sectional area of a cross-section of the recess (25) at the part of the recess where the female thread has its full thread profile. 2. A drill bit according to claim 1 , characterized in that the impact surface (27) defines the smallest radial cross-sectional area of the recess (25). 3. A drill bit according to claim 1 or 2, characterized in that the radial cross-sectional area of the recess (25) continuously increases along the recess from the impact surface (27) all the way on to the part of the recess where the female thread (26) has its full thread profile. A drill bit according to anyone of claims 1-3, characterized in that an annular material zone (MZ) is formed between a cross- sectional plane (P1) containing the impact surface (27) and the envelope surface of an imaginary cylinder (C) which has such a diameter (Do) that its envelope surface touches the deepest point of the thread bottom of the female thread along the part of the recess where the female thread (26) has its full thread profile. A drill bit according to claim 4, characterized in that the diameter (Do) of said imaginary cylinder (C) is equal to or smaller than 37 mm, while fulfilling the following condition: 1.3 < L/Do < 1.6 where Do is the diameter of the imaginary cylinder (C) and L is the axial distance from the impact surface (27) to the outer end of the part of the recess where the female thread (26) has its full thread profile. A drill bit according to anyone of claims 1-5, characterized in that the axial distance between the impact surface (27) and the inner end of the part of the recess where the female thread (26) has its full thread profile is 8-12 mm. A drill bit according to anyone of claims 1-6, characterized in that the radial distance (Ro) from the longitudinal axis (A) of the drill bit (20) to the deepest point of the thread bottom of the female thread (26) is equal to or smaller than 18.5 mm at the part of the recess where the female thread has its full thread profile. 8. A drill bit according to anyone of claims 1-7, characterized in that the number of thread turns with full thread profile of the female thread (26) is 2.8-3.2. 9. A drill bit according to anyone of claims 1-8, characterized in that the recess (25) is provided with at least one unthreaded cylindrical surface (28) between the female thread (26) and said second end (24b) of the skirt (24). 10. A drill bit according to anyone of claims 1-9, characterized in that rock cutting members in the form of front buttons (22) and peripheral buttons (23) are provided at the front end of the drill head (21). 11. A drill bit according to anyone of claims 1-10, characterized in that the recess (25) is in fluid communication with the front end of the drill head (21) through one or more flush channels (29) extending through the drill head. 12. A drill bit according to anyone of claims 1-11, characterized in that the female thread (26) is a rope thread or a trapezoidal thread. 13. Use of a drill bit according to anyone of claims 1-12 for percussive rock drilling, wherein the drill bit (20) is connected to a top hammer device through one or more drill rods (30). |
FIELD OF THE INVENTION AND PRIOR ART The present invention relates to a drill bit for percussive rock drilling according to the preamble of claim 1.
A previously known drill bit for percussive rock drilling configured in accordance with the preamble of claim 1 is illustrated in Fig 1. A drill bit of this type is for instance disclosed in WO 2004/003334 A1 (and US 2002/074797, US 2008/304904 or WO 01/38685). This known drill bit 1 comprises:
- a drill head 2;
- a skirt 3;
- a recess 4 extending axially through the skirt 3 from the rear end of the skirt towards the drill head 2, the recess being provided with an internal female thread 5 for connection to a corresponding external male thread 6 of a drill rod 7 and; and
- an impact surface 8 for abutment against an end surface 9 of a drill rod, this impact surface 8 being formed at the bottom of the recess 4 and extending perpendicularly to the longitudinal axis of the drill bit. The part of the recess 4 between the inner end of the female thread 5 and the i mpact surface 8 is wider than the female thread so as to provide a thread clearance 1 0 at the inner end of the female thread.
Each of US 3258077A and US 3258284 A discloses a drill bit having a shoulder abutment. J P 8 1 51 885 A, on which the preamble of clai m 1 is based, discloses a drill bit with locally increased diameter of the drill bit in the vicinity of the i mpact surface.
SU M MARY OF TH E I NVENTION
The object of the present invention is to achieve a further development of a drill bit of the above-mentioned type so as to provide a drill bit which is i mproved in at least some aspects.
According to the invention , this object is achieved by a dri ll bit having the features defined in clai m 1 .
The drill bit of the present invention comprises :
- a dri ll head ;
- a skirt, which has a first end and an opposite second end, the skirt being connected to the drill head at said first end ;
- a recess extending axially through the skirt from the second end of the skirt towards the first end thereof, the recess being provided with an internal , cylindrical female thread for connection to a
corresponding external , cylindrical male thread of a drill rod ; and
- an i mpact surface for abutment against an end surface of a dril l rod, this i mpact surface being formed at the bottom of the recess and extending perpendicularly to the longitudinal axis of the drill bit.
According to the invention , every cross-section of the recess between the i mpact surface and the part of the recess where the female thread has its full thread profile has a cross-sectional area wh ich is smaller than the cross-sectional area of a cross-section of the recess at the part of the recess where the female thread has its full thread profile. Thus, the drill bit of the present invention lacks a widened thread clearance between the inner end of the female thread and the i mpact surface. Such a thread clearance wil l give a reduced thickness of the ski rt wal l in the part of the skirt wall surrounding the th read clearance as compared to the part of the skirt wall surroundi ng the female thread. Such a part of the skirt wall with reduced wall thickness, and consequently reduced cross-sectional area of the skirt wall , will constitute a weakened part or fracture zone FZ (see Fig 1 ) at the drill bit. The present invention is based on the realization that the strength of the drill bit can be i mproved by dispensing with a thread clearance between the inner end of the female thread and the i mpact surface. The strength i mprovement has been measured to 1 8% for a drill bit according to the present invention as compared to a corresponding drill bit according the above-mentioned prior art. Without such a thread clearance, the recess of the dril l bit can be formed in such a manner that every cross-section of the recess between the i mpact surface and the part of the recess where the female thread has its full thread profile has a cross-sectional area which is smaller than the cross-sectional area of a cross-section of the recess at the part of the recess where the female thread has its full thread profile. Hereby, no point of reduced strength is introduced in the part of the skirt extending between the i mpact surface and the female thread.
I n this description and the subsequent clai ms, the expression "cross- section of the recess" refers to a section across the recess
perpendicularly to the longitudinal axis of the drill bit.
Further advantages as well as advantageous features of the drill bit according to the invention wil l appear from the following description and the dependent clai ms. BRI EF D ESCRI PTION OF TH E D RAW I NG With reference to the appended drawing, a specific description of preferred embodiments of the invention cited as examples follows below. In the drawings: Fig 1 is a partly cut lateral view of a drill bit according to prior art connected to a drill rod,
Fig 2 is a partly cut lateral view of a drill bit according to
embodiment of the present invention, and
Fig 3 is a partly cut lateral view of the drill bit according to Fig 2 connected to a drill rod.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
A drill bit 20 for percussive rock drilling according to an embodiment of the present invention is illustrated in Figs 2 and 3. This drill bit 20 comprises a drill head 21, which is provided with rock cutting
members 22, 23 at its front end. In the illustrated embodiment, the rock cutting members are constituted by cemented carbide inserts in the form of front buttons 22 and peripheral buttons 23. At its rear end, the drill head 21 is connected to a skirt 24. The drill head 21 and the skirt 24 are formed in one piece of a suitable metallic material, such as for instance steel.
The skirt 24 has a first end 24a, through which the skirt is connected to the drill head 21, and an opposite second end 24b. A recess 25 extends axially through the skirt 24 from the second end 24b of the skirt towards the first end 24a thereof. This recess 25 is provided with an internal, cylindrical female thread 26 for connection to a
corresponding external, cylindrical male thread 36 provided at an end of a drill rod 30. The female and male threads may be of any
conventional thread form , such as rope threads or trapezoidal threads. The thread form shown in the figures is a rope thread. An impact surface 27 for abutment against an end surface 37 of a drill rod 30 is formed at the bottom of the recess 25. This i mpact surface 27 extends perpendicularly to the longitudinal axis A of the dril l bit 20. The recess 25 is suitably provided with one or more unthreaded cylindrical surfaces 28 between the outer end of the female rope thread 26 and the second end 24b of the skirt 24, as il lustrated in Figs 2 and 3. The recess 25 is in fluid com munication with the front end of the drill head 21 through a nu mber of flush channels 29 extending through the dril l head. When the drill bit 20 is connected to a drill rod 30 with the female thread 26 of the drill bit in engagement with the male thread 36 of the dril l rod and with the i mpact surface 27 of the drill bit in abutment against the end surface 37 of the drill rod (as illustrated in Fig 3) , the flush channels 29 of the drill bit are in fluid com munication with a flush channel (not shown) wh ich extends axially through the dril l rod along the centre axis thereof . Every cross-section of the recess 25 along the part of the recess where the thread 26 has its full thread profile has one and the same cross-sectional area. The expression "full thread profile" can be defined as the profile which is present along a majority of the entire thread length .
No thread clearance is provided in the recess 25 between the inner end of the female thread 26 and the i mpact surface 27, and every cross-section of the recess 25 between the i mpact surface 27 and the part of the recess where the female thread 26 has its full thread profile has a cross-sectional area which is smaller than the cross- sectional area of a cross-section of the recess 25 at said part of the recess where the female thread 26 has its full thread profile. The recess 25 has its smallest cross-sectional area at its bottom and the i mpact surface 27 defines the smallest radial cross-sectional area of the recess 25. Thus, the area of the impact surface 27 is smaller than the cross-sectional area of any cross-section of the recess 25 between the i mpact surface 27 and the second end 24b of the skirt 24.
It can also be seen i n Fig 2 that the cross-sectional area of the recess 25 is gradually reduced from the inner end of the female thread 26 to the i mpact surface 27. The radial cross-sectional area of the recess 25 continuously increases along the recess from the i mpact surface 27 all the way on to the part of the recess where the female thread 26 has its full thread profile. The axial distance between the i m pact surface 27 and the inner end of the part of the recess where the female thread 26 has its full thread profi le is preferably 8-1 2 m m , which i mpl ies that the part of the recess where the female thread 26 has its full thread profile begins 8- 1 2 m m from the i mpact surface 27 as seen in the axial direction .
The radial distance Ro from the longitudinal axis A of the drill bit 20 to the deepest point of the thread bottom of the female thread 26, i .e. the largest radius of the female thread, is preferably equal to or smaller than 1 8.5 m m at the part of the recess where the female thread has its full th read profile.
Preferably, the female thread 26 has 2.8-3.2 thread turns with full thread profile. An i maginary cylinder C is i llustrated in broken lines in Fig 2. This imaginary cylinder C has such a diameter Do that its envelope surface touches the deepest point of the thread bottom of the female thread along the part of the recess where the female thread 26 has its full thread profile. Thus, Do=2 - Ro.
The diameter Do of said i maginary cylinder C is preferably equal to or smaller than 37 m m , while fulfilling the following condition :
1 .3 < L/Do < 1 .6 where L is the axial distance from the i mpact surface 27 to the outer end of the part of the recess where the female thread 26 has its full thread profile.
An annular material zone MZ is formed between a cross-sectional plane P 1 extending throug h the i mpact surface 27 and the envelope surface of the above-mentioned i maginary cylinder C. The annular material zone MZ forms an area at a certain axial cross-section that tapers towards where the female thread has its full thread profile.
The drill bit 20 is to be used for percussive rock drill ing with the dril l bit 20 connected to a top ham mer device throug h one or more dri ll rods 30.
With the drill bit 20 according to the present invention , it wi ll be possible to provide a longer female thread 26, and thereby a longer contact surface between the female thread 26 of the drill bit 20 and the corresponding male thread 36 of the drill rod 30, as compared to a drill bit according to the above-mentioned prior art, without increasing the axial distance L from the i mpact surface 27 to the outer end of the part of the recess where the female thread 26 has its full thread profile. Furthermore, there wi ll be better possibil ities to alter the space for cuttings removal while maintaining the skirt sufficiently strong, or to increase the strength of the ski rt 24, due to the fact that the wall of the skirt 24 can be designed without having to take into account any fracture zone at the inner end of the skirt. The presently clai med drill bit i .a. solves the problem of weak ski rts, i .e. by
increasing the wall thickness without affecting the outside of the drill bit.
It should be noted that the female thread 26 of the drill bit according to the present invention is a cylindrical thread and not the type of conical th read used in drill pipes or dril l tubes for rotary oil dril ling .
The invention is of course not in any way restricted to the embodi ment described above. On the contrary, many possibi lities to modifications thereof will be apparent to a person with ordinary skill in the art without departing from the basic idea of the invention such as defined in the appended clai ms.
The disclosures in European patent application No. 1 01 61 383.4, from which this application clai ms priority, are incorporated herein by reference.
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