WO2010095998A1

WO2010095998A1 - A start kit, a casing tube and a starter rod for drilling equipments for percussive rock drilling

Info

Publication number: WO2010095998A1
Application number: PCT/SE2010/050107
Authority: WO
Inventors: Peter Nava; Maria Bergh
Original assignee: Sandvik Intellectual Property Ab
Priority date: 2009-02-19
Filing date: 2010-02-02
Publication date: 2010-08-26
Also published as: SE0900215L; SE532471C2

Abstract

The invention relates to a start kit intended for drilling equipments for percussive rock drilling and of the type that comprises, on one hand, a starter rod (3) having front and rear threads, at least the front one of which is a male thread (5) and which have the purpose of allowing connection of the starter rod (3) with a drill bit as well as an extension rod, and on the other hand an enveloping casing tube (4), the inner diameter of which casing tube is greater than the outer diameter of the rod in order to provide an annular gap (24) through which flushing medium mixed with drill cuttings can be evacuated. In the casing tube (4), a front end wall (16) is included, which comprises, on one hand, a central hole through which a front part of the starter rod (3) provided with a driver (19) extends that has the driver (19) thereof situated behind the end wall (16), and on the other hand a number of peripheral openings (18) in order to allow flushing medium mixed with drill cuttings to pass into the annular gap (24). In addition, the invention relates to a starter rod and a casing tube as such.

Description

A start kit, a casing tube and a starter rod for drilling equipments for percussive rock drilling

Technical Field of the Invention

In a first aspect, this invention relates to a start kit intended for drilling equipments for percussive rock drilling and of the type that comprises, on one hand, a starter rod made with an internal flushing medium duct and having front and rear threads, at least the front one of which is a male thread and which have the purpose of allowing connection of the rod with a drill bit as well as an extension rod, and on the other hand a cylindrical casing tube enveloping the rod, the inner diameter of which casing tube is greater than the outer diameter of the rod in order to provide an annular gap through which flushing medium mixed with drill cuttings can be evacuated.

In additional aspects, the invention also relates to a starter rod and a casing tube as such.

Background of the Invention and Prior Art

In connection with driving tunnels in rock, which is sometimes solid, sometimes unconsolidated, there is a need of consolidating the unconsolidated and not self-supporting rock. This takes most commonly place by drilling holes in the same, into which holes casing tubes are inserted into which a hardenable injection mass, e.g., concrete, can be pressed in and distributed in the unconsolidated rock structure, more precisely via radial holes in the tubes, to subsequently be allowed to harden. Such tubes are distributed as an arch-shaped umbrella over a certain part of the rock, which after consolidation can be dug and/or blasted out to form a part of the tunnel to be made. By those skilled in the art, this consolidated structure is denominated "protection umbrella". The finally applied tubes have most often a total length within the range of 10-20 m and are composed of several shorter tubes, which are spliced as the drilling proceeds.

Among the different components that together are included in the operative drilling equipment, which successively is inserted into the drill hole, some are disposable or expendable components, while others are reusable. Thus, the casing tube and a possibly appurtenant casing shoe form a disposable component that remains in the drill hole, while the other components - the drill bit of which is most expensive - should as far as possible be removed from the hole in a state that allows recycling. For this reason, the drill bit of modern drilling equipments is constructed from two different parts, viz. a pilot bit and a ring bit, the first-mentioned one of which can be pulled out together with the drill string being behind, while the last-mentioned one is allowed to remain in the bottom of the drill hole. Accordingly, as a base for this design philosophy, there is an aim to save as much as possible of the expensive drill bit, viz. the pilot bit.

A disadvantage of previously known drilling equipments (see for instance US 6106200, US 6722454 B2 and WO 02/081856 A1 ), the drill bits of which are composed of a reusable pilot bit as well as a ring bit, which is left in the drill hole, is that particular couplings, usually bayonet couplings, are required to allow loosening of the pilot bit from the ring bit. Such couplings entail that the respective components have to be subjected to expensive and complicated machining operations, and if the different details (male and female elements), which together form a coupling, are clogged by penetrating drill cutting residues during drilling, the disassembly of the bit parts can become time-consuming and cumbersome. An additional complication is that the flushing medium ducts and drill cutting evacuation channels, which have to be included in the drill bit, also require price-rising machining and may be clogged. In addition, the means of the equipment for pulling along the casing tube into the drill hole do not infrequently become unreliable.

Objects and Features of the Invention The present invention aims at obviating the above-mentioned disadvantages of previously known drilling equipments and at providing an improved, inexpensive equipment. Therefore, a primary object of the invention is to provide a start kit intended for the initial drilling of a hole and of the type initially mentioned, and which is structurally simple and inexpensive to manufacture. This should be realizable, among other things, by the use of a drill bit, the service life of which does not need to be essentially more than the time it takes to drill a 10 or 20 m deep hole. Furthermore, the starter rod included in the kit should be attachable to and detachable from the drill bit in a simple and fast way, at the same time as the same should be able to drive the casing tube in an almost foolproof way. In addition, not only an efficient flushing medium supply to the drill bit, but also a reliable evacuation of drill cuttings from the same, should be realizable by means of structurally simple and inexpensive means.

According to the invention, at least the primary object is attained by means of the features defined in the characterizing clause of claim 1. Preferred embodiments of the start kit according to the invention are furthermore defined in the dependent claims 2-12.

In a second aspect, the invention also relates to a casing tube as such. The features of said casing tube are seen in the independent claim 13. Preferred embodiments of the same are furthermore defined in the dependent claims 14 and 15.

In a third aspect, the invention also relates to a starter rod as such intended for drilling equipments for percussive rock drilling. The features of said starter rod are seen in the independent claim 16. In addition, the dependent claims 17-21 define preferred embodiments of the starter rod according to the invention.

Brief Summary of the General Idea of the Invention

The idea of providing a start kit having a starter rod to which a simple drill bit can be coupled, which drill bit without economic detriment can be left in the drill hole, form the basis of the invention. In such a way, the drill bit can be connected to the starter rod via a simple thread coupling, which does not give rise to complications neither in respect of the running of a flushing medium duct to the front surface of the drill bit, nor in respect of the provision of the requisite passages for the evacuation of drill cuttings. In addition, the starter rod may be provided with a simple driver that, in co-operation with a likewise simple end wall of the front end of the casing tube, reliably drives the casing tube during drilling.

Brief Description of the Appended Drawings

In the drawings:

Fig. 1 is a perspective exploded view of a start kit according to the invention composed of a starter rod and a casing tube,

Fig. 2 is a partially cut perspective view of a starter rod included in the kit,

Fig. 3 is a partially cut perspective view of the casing tube of the kit,

Fig. 4 is a longitudinal section through the starter rod and the casing tube,

Fig. 5 is a perspective view of a cup-shaped part included in the casing tube,

Fig. 6 is a front view of the cup part according to Fig. 5,

Fig. 7 is a side view of the same cup part,

Fig. 8 is an exploded view showing how the starter rod is manufactured from three separate components,

Fig. 9 is a perspective exploded view showing a centering device included in the start kit,

Fig. 10 is an enlarged detailed section showing the centering device applied to the starter rod,

Fig. 1 1 is a sectioned perspective view showing an extension rod that is included in a trailing kit and can be coupled together with the starter rod at the same time as an additional casing tube can be connected to the casing tube of the start kit, and

Fig. 12 is a longitudinal section through the kit according to Fig. 1 1.

Detailed Description of a Preferred Embodiment of the Invention In Figs. 1-4, 1 generally designates a start kit according to the invention, and 2 a drill bit. The start kit 1 comprises a starter rod 3 as well as a casing tube in its entirety designated 4. In the starter rod 3, front and rear threads 5, 6 are included, the front one of which is always a male thread. In the example, the rear thread 6 is, on the contrary, a female thread, which is formed inside a sleeve part 7 that opens rearward. Inside the starter rod, a suitably centrally situated flushing medium duct 8 runs, which in the example terminates in a front end surface 9, as well as in the interior of the sleeve part 7. In the shown, preferred embodiment, the starter rod 3 has, along the greater part of the length thereof, a cylindrical envelope surface 10 that is concentric with a centre axis C1. Also the envelope surface 1 1 of the sleeve part 7 is cylindrical and concentric with the centre axis C1.

The casing tube 4, which has a cylindrical basic shape, comprises front and rear ends 12, 13 and outsides and insides 14, 15 that are concentric with the centre axis C2 of the casing tube.

Characteristic of the invention is that the casing tube 1 comprises a front end wall 16, in which a central hole 17 as well as a number of peripheral openings 18 are recessed. A driver 19 formed on the starter rod 3 co-operates with the end wall 16, which driver in this case is in the form of a ring-shaped collar outside the envelope surface 10 of the starter rod. Said driver is situated behind the front male thread 5 and at a certain distance from the same. The term "behind" refers to the normal drilling direction of the drilling equipment. In Fig. 8, it is accordingly seen how the front, ring-shaped end surface 19' of the driver 19 is displaced a distance A axially rearward from a transition 20 between the male thread 5 and a guide surface 10'. When the starter rod is inserted into the casing tube, as is shown in Fig. 4, the casing tube driver 19 is located behind the end wall 16 in order to drive the casing tube 4 during drilling.

Although it per se is feasible to directly connect the front end wall with a plain tube (e.g., by welding), it is preferred, for reasons of manufacturing technique, to compose the casing tube of a front cup-shaped part 4a and a tube or tube part 4b of a simple nature extending rearwardly from the same. In Figs. 5-7, it is seen that the cup-shaped part 4a, in addition to the front end wall 16, comprises a tube wall or skirt 21 extending rearwardly from the same and having a cylindrical shape. The material thickness of the tube wall 21 may advantageously correspond to the material thickness of the tube part 4b, the tube wall at the rear end thereof being formed with a coned surface 22 that, together with a corresponding cone surface on the tube part, forms a fillet for a welded joint 22a (see Fig. 3), which permanently unites the cup part to the tube part. Each individual peripheral opening 18 comprises a recess 18a (see Fig. 6) extending radially into the end wall 16, and/or a recess 18b (see Fig. 7) extending axially into the tube wall 21. In Fig. 5, it is furthermore seen that the material thickness of the end wall 16 is greater than the material thickness of the tube wall 21. In such a way, the end wall becomes sufficiently strong to resist percussive loads from the driver of the starter rod in spite of the fact that three peripheral openings cut in in the same. In the embodiment shown, the number of openings amounts to three. However, this does not prevent another number of openings from being formed in the cup part 4a.

As is shown in Fig. 8, also the starter rod 3 may be manufactured from several different components, viz. a front component 3a in which the male thread 5 and the collar are included, an intermediate component 3b in the form of a plain rod part, as well as a rear component 3c in which the sleeve 7 and the female thread inside the same are included. These three components 3a, 3b, 3c are permanently united to each other, preferably by friction welding. This is carried out by the fact that a pair of components are brought to rotate in relation to each other at high speed and are brought together axially, the material in the end surfaces being heated and melted together while forming a welding seam 23. In this connection, a circumferential welding bulge is formed, which later on is removed by chip removing machining, e.g., turning. This machining operation may advantageously be utilized to countersink shallow grooves 23a (see Figs. 2 and 10) in the envelope surface 10 of the complete starter rod. Fig. 8 also shows that the driver 19, irrespective of embodiment, may have an axially front surface or shoulder 19' that is situated behind the front male thread 5 at a distance A that exceeds a width B of the driver. As an example, it may be mentioned that the distance A may be 35 mm when the width B is 20 mm, i.e., the distance A may be at least 50 % greater than the width B.

In accordance with a preferred embodiment of the invention, the end wall 16 is utilized to form a front or primary centering device for the starter rod 3. This can be realized by the simple measure of forming the hole 17 with an inner diameter that just slightly exceeds the outer diameter of the rod along the envelope surface 10, i.e., at an essentially cylindrical guide surface 10' (see Fig. 8). The guide surface 10' may alternatively be conically shaped, i.e., tapering toward the nearest thread 5. For instance, the hole diameter may be approx. 1 mm greater than the rod diameter, whereby a 0,5 mm wide annular gap is formed between the inside of the hole and the guide surface 10'. Here it should be pointed out that the collar serving as a driver 19 has an outer diameter that is considerably greater (e.g., 10-20 mm) than the diameter of the rest of rod.

Because the casing tube 4 generally has a considerably greater diameter than the rod 3, between the same an annular gap 24 is formed (see Figs. 1 and 4), through which flushing medium mixed with drill cuttings can pass from the openings 18 toward the rear, open end of the casing tube. In the shown, preferred embodiment, in this annular gap 24, two secondary centering devices 25, which center and guide the starter rod together with the front end wall 16, are arranged. As is seen in Fig. 9, each such centering device 25 is composed of two semicircular hub or arch parts 25a, 25b from which radial fingers 26 protrude. In two plane contact surfaces 27, a spigot 28 as well as a hole 29 are formed. In practice, the two hub parts may be identical and made of polyester or another material, the density of which is considerably smaller than the density of the material, viz. steel, from which the other components of the start kit 1 are manufactured. Most suitably, the density of the hub material amounts to maximum 10 % of the density of the steel. Assembling of the two hub parts on the starter rod is carried out by the same being located in one of the grooves 23a and glued with the respective surfaces 27 in contact with each other and with a spigot 28 in one hub part inserted into a hole 29 in the other hub part. In such a way, a light centering device is formed that efficiently guides the starter rod in relation to the casing tube without having any mass that appreciably contributes to the energy consumption during drilling. Here, it should also be pointed out that the inner diameter of the casing tube 4 in the example is approx. 3 times greater than the outer diameter of the starter rod (hence, the radial extension of the annular gap 24 is approximately as great as the last-mentioned one). In its assembled state according to Fig. 1 , the centering device comprises four fingers between which the same number of passages 34 are delimited for flushing medium that is evacuated via the annular gap 24.

In Fig. 3 and 4, it is shown how the casing tube 4 comprises a number of radial holes 30 through which concrete or another injection mass can be pressed out in the surrounding rock, when the drilling and the installation of the casing tube have been completed.

To the starter rod 3 included in the described kit 1 , a drill head 2 of any desired design can be connected. Because the drill head is intended to be left in the drilled hole, and does not need to manage more than a bore depth of 10 to 20 m, the same can, above all when drilling is carried out in particularly unconsolidated rock, be effected without any expensive cemented-carbide buttons or the like. The only essential requirements that are made on the drill head are accordingly that the same should comprise front crushing members 31 , a flushing medium duct 32, as well as passages 33 for return of flushing medium mixed with drill cuttings; all these details may be of simple nature. In addition, in the rear part of the drill head, there should naturally be a female thread (not shown) for connection of the drill head with the male thread 5 of the starter rod.

For the sake of completeness, it should be pointed out that the threads 5, 6 in question are not intended to form butt threaded joints. On the contrary, the threads should be intermittently screwable into and out of each other in order to, in a traditional way, transfer impact motions to the drill head as well as rotate the same so that the crushing members 31 constantly change position in relation to the bottom of the drill hole. In the example, the threads are rope threads, but may also be trapezoidal threads or the like.

It should also be mentioned that the casing tube 4 in the described start kit may have a length of approx. 3 m, while the starter rod 3 is somewhat longer.

When a hole with a depth of 10 to 20 m is to be drilled and lined, the drilling operation is started with the described start kit 1. This start kit is prepared for drilling by mounting the secondary centering devices 25 in the grooves 23a on the starter rod 3, whereupon the same is entirely inserted into the casing tube so that the male thread 5 is located in front of the end wall 16 and the driver 19 behind the same. In this state, the drill head 2 can be screwed onto the male thread 5. Drilling is carried out in a conventional way by the fact that a top hammer unit (not shown) applies combined impact and rotary motions to the starter rod 3, which are transferred to the drill head 2. In this connection, flushing medium is supplied to the flushing medium duct 32 of the drill head via the corresponding duct 8 in the starter rod 3, primarily with the purpose of evacuating the drill cuttings broken off by the drill head. This evacuation is carried out via the passages 33 in the drill head as well as via the openings 18 in the front end of the casing tube, whereupon the medium is flushed out rearward through the annular gap 24 between the starter rod and the casing tube, wherein the flushing medium can pass through the passages 34 between the fingers 26 of the centering devices 25. When the start kit is as deep as possible in the rock, the operation is interrupted, whereupon the starter rod is disengaged from the top hammer unit, more precisely by being rotated in the opposite direction compared to the driving direction of rotation. In the next step, the starter rod 3 is joined with an extension rod 3S, which advantageously may be included in a trailing extension kit 1 S of the type that is shown in Figs. 1 1 and 12. Said extension rod 3S may be of standard type so far that the same lacks the driver 19 of the starter rod 3. Also the casing tube 4S may be of standard type in so far that the same is open at the two opposite ends thereof and lacks end wall. When the male thread of the extension rod 3S has been screwed into the female thread 6 of the starter rod, the casing tube 4S is joined with the casing tube 4 of the start kit. This may in practice be effected either by welding (as is indicated by means of the cone surfaces on the ends of the casing tubes) or via threaded joints. In many practical applications, threaded joints are preferred.

The drill string formed by the starter rod and the extension rod is again coupled together with the top hammer unit, more precisely by the male thread 5 of the extension rod 3S being screwed into the female thread in the starter rod, whereupon the drilling can be continued in the same way. When the drill hole has reached the desired depth, e.g., 15 m comprising 3 χ 5 casing tubes of 3 m, the operation is interrupted by the drill string being fully disengaged from the drill head and pulled out of the permanent, complete casing tube. After this, the starter rod as well as the trailing extension rods can be re-used in connection with drilling of new holes. Also undamaged secondary centering devices can be re-used. On the other hand, the drill head is allowed to remain in the bottom of the drill hole.

After executed drilling, injection mass can be pressed into the remaining casing tube and distributed out into the rock via the radial holes 30 thereof.

An advantage of the start kit according to the invention is that the starter rod thereof as well as the casing tube thereof are structurally simple and therefore inexpensive to manufacture. By means of only a few welding operations and a limited a number of chip removing machining operations, such as turning and drilling, the casing tube as well as the starter rod can be mass-produced at a low cost. Neither the secondary centering devices that advantageously supplement the guiding and centering function of the front end wall require considerable costs. By combining a simple collar on the starter rod with a front end wall in the casing tube, a very reliable, not to say foolproof, axial driver function is obtained for the casing tube, i.e., the casing tube is pulled into the drill hole without any complicating coupling means being required between the casing tube and the drill bit. Accordingly, the starter rod is coupled together with the drill bit only via, for example, a simple threaded joint that is axially entirely separated from the casing tube behind. In addition, the impacts of the top hammer unit are transferred via the starter rod directly to the drill bit, and therefore the casing tube is not subjected to any high stresses that could require extra strong details in the same. By means of the easily made peripheral openings in the front part of the casing tube, a reliable and efficiently acting evacuation of small as well as large drill cuttings is furthermore guaranteed.

The invention is not limited only to the embodiment described above and shown in the drawings. Accordingly, instead of a female thread at the rear end of the starter rod, a male thread may be used that can be screwed on in a coupling sleeve, which in turn is coupled to the extension rod behind. Instead of a circular cross- sectional shape, the starter rod may also have a polygonal, e.g., a hexagonal cross- sectional shape. Furthermore, the front end wall of the casing tube does not necessarily need to be utilized as centering device. Neither does the start kit need to comprise more than a secondary centering device if the end wall is utilized as a primary centering device. The driver of the starter rod may also be realized in another way than in the form of a collar integrated with the rest of the rod. The driver may accordingly also be a separately manufactured ring mounted on the starter rod. Alternatively, the driver may be formed of a plurality of projections or knobs distributed along the envelope surface of the starter rod.

The disclosures in Swedish patent application No. 0900215-5, from which this application claims priority, are incorporated herein by reference.

Claims

1. Start kit for drilling equipments for percussive rock drilling comprising;

a starter rod (3) made with an internal flushing medium duct (8) and having front and rear threads (5, 6), at least the front one of which is a male thread (5) and which have the purpose of allowing connection of the starter rod with a drill bit as well as an extension rod, and

a cylindrical casing tube (4) enveloping the starter rod, the inner diameter of which casing tube is greater than the outer diameter of the rod in order to provide an annular gap (24) through which flushing medium mixed with drill cuttings can be evacuated, characterized in that,

a front part of the starter rod (3) is provided with a driver (19), and in that

the casing tube (1 ) comprises

a front end wall (16), which comprises a central hole (17) through which the front part of the starter rod (3) extends such that the driver (19) thereof is situated behind the end wall (16), and

a number of peripheral openings (18) in order to allow flushing medium mixed with drill cuttings to pass into the annular gap (24).

2. Start kit according to claim 1 , characterized in that the end wall (16) is arranged to serve as a first centering device for the starter rod (3), more precisely by the fact that the hole (17) is circular and has an inner diameter that is only just slightly greater than the outer diameter of the starter rod.

3. Start kit according to claim 1 or 2, characterized in that, behind the end wall (16) and at a distance from the same, one or more secondary centering devices (25) are arranged that individually comprise passages (34) for the flushing medium.

4. Start kit according to claim 3, characterized in that the secondary centering device (25) is detachably mounted on and follows the starter rod (3), as well as comprises two semicircular hub parts (25a, 25b) from which radial fingers (26) protrude, which point toward the inside of the casing tube (4) and between which flushing medium can pass.

5. Start kit according to claim 4, characterized in that the secondary centering device (25) is manufactured from a material having a density that amounts to maximum 10 % of the density of the material from which the starter rod (3) and the casing tube (4) are manufactured.

6. Start kit according to any one of the preceding claims, characterized in that the driver (19) is a ring-shaped collar that protrudes from the envelope surface (10) of the starter rod (3).

7. Start kit according to any one of the preceding claims, characterized in that the starter rod (3) is composed of three components (3a, 3b, 3c) permanently united to each other, viz. a front component (3a) in which the front male thread (5) as well as the driver (19) are included, an intermediate rod component (3b), as well as a rear component (3c) in which the rear thread (6) is included.

8. Start kit according to any one of the preceding claims, characterized in that the rear thread of the starter rod is a female thread (6) inside a sleeve part (7), the outer diameter of which is greater than the outer diameter of the starter rod (3).

9. Start kit according to claim 7, characterized in that, in the area of a joint (23) between two rod components, a ring-shaped groove (23a) is countersunk in the envelope surface (10) of the starter rod for the receipt of a secondary centering device (25).

10. Start kit according to any one of the preceding claims, characterized in that the individual peripheral opening (18) in the end wall (16) of the casing tube comprises a recess (18a) extending radially into the end wall as well as a recess (18b) extending axially into the tube wall.

11. Start kit according to any one of the preceding claims, characterized in that the end wall (16) of the casing tube (4) has a material thickness that is greater than the material thickness of the tube wall behind.

12. Start kit according to any one of the preceding claims, characterized in that the casing tube (4) is composed of, on one hand, a front, cup-shaped part (4a) in which the end wall (16) and a tube wall (21 ) extending rearwardly from the same are included, and on the other hand a tube part (4b) that is longer than the cup-shaped part (4a) and permanently united to the same via a welded joint (22a).

13. Casing tube for drilling equipments for percussive rock drilling, comprising front and rear ends (12, 13) as well as cylindrical outsides and insides (14, 15), which are concentric with a centre axis (C2), characterized in that the front end (12) comprises an end wall (16) having a central hole (17) and a number of peripheral openings (18), each opening comprising a recess (18a) extending radially into the end wall and/or a recess (18b) extending axially into the tube wall.

14. Casing tube according to claim 13, characterized in that the material thickness of the end wall (16) is greater than the material thickness of the tube wall.

15. Casing tube according to claim 13 or 14, characterized in that the same is composed of, on one hand, a front, cup-shaped part (4a) in which the end wall (16) and a tube wall (21 ) extending rearwardly from the same are included, and on the other hand a tube part (4b) that is longer than the cup part and permanently united to the same via a welded joint (22a).

16. Starter rod for drilling equipments for percussive rock drilling, comprising front and rear threads (5, 6), at least the front one of which is a male thread (5), as well as an internal flush duct (8) that extends between the front and rear ends of the rod, characterized in that the starter rod comprises a casing tube driver (19) that is situated behind the front male thread (5) and at a distance (A) from the same.

17. Starter rod according to claim 16, characterized in that the driver (19) is a ring-shaped collar outside the envelope surface (10) of the starter rod.

18. Starter rod according to claim 16 or 17, characterized in that, in the envelope surface (10), a circumferential groove (23a) is countersunk for the receipt of a centering device.

19. Starter rod according to any one of claims 16-18, characterized in that the same is composed of three part components (3a, 3b, 3c) permanently united to each other, viz. a front component (3a) in which the front male thread (5) as well as the driver (19) are included, an intermediate rod component (3b), as well as a rear component (3c) in which the rear thread (6) is included.

20. Starter rod according to any one of claims 16-19, characterized in that the rear thread is a female thread (6) inside a sleeve part (7), the outer diameter of which is greater than the outer diameter of the rest of the starter rod.

21. Starter rod according to claim 19 and 20, characterized in that the groove (23a) surrounds a joint (23) between two individual part components.

22. Starter rod according to claim 16, characterized in that the distance (A) is not less than 10 mm.

23. Starter rod according to claim 16, characterized in that a guide surface (10') is arranged between the driver (19) and the thread (5), the guide surface being essentially cylindrically or conically formed.