Prior art At drilling especially in open-pit mining the rock is frequently cracked due to previous explosions, such as is shown in Fig. 1. Often the ground is made smooth by means of metres and metres of rock masses 6 such that the drilling machine 1 shall be able to travel at the drilling site. It is difficult to drill straightly in cracked rock, and therefor different solutions have been suggested to improve the straightness of the hole to be drilled.

Therefore, a guide tube 2 is provided in direct connection with the drill bit so as to guide the progress of the drill bit and so as to increase the flushing speed outside and above the drill bit. The guide tube is about 4 to 6 meters long and comprises a male thread facing towards the drill bit and a female thread facing towards the machine. The tube has to be extended at holes which are deeper than the length of the guide tube, preferably with rods that have smaller diameters than the tube with the intention to more simply allow passage of the drill dust. A modern machine for drilling comprises a tong 3 (Fig. 2) for seizing by means of friction close to the upper end of the tube 2 at extension or unscrewing. Frequently, the joint is tightened hardly during drilling and therefor the joint must be loosened by use of percussion and then be unscrewn. At said loosening by use of percussion, which may continue several minutes, the tube will slide relative to the shoes or clamping means 4 of the tong 3 in the position

according to Fig. 6 and furthermore, heat is developed in the joint.

Consequently, the tube end is deformed and martensite is formed in the threads.

Objects of the invention One object of the present invention is to provide a drill string and a guide tube for mechanical handling wherein the above-captioned drawbacks are counteracted.

Another object of the present invention is to provide a drill string and a guide tube for mechanical handling to simplify unscrewing of the thread joint in percussive rock tool equipment.

Still another object of the present invention is to provide a guide tube for mechanical handling which improves the life span of thread joints in percussive rock tool equipment.

These and other objects have been achieved by a guide tube and a drill string for mechanical handling such as is defined in the appended claims.

Brief description of the drawings Fig. 1 schematically shows a drilling machine for percussive top hammer drilling, in a side view. Fig. 2 schematically shows a tong for unscrewing of thread joints, in a top view. Fig. 3 shows a drill string according to the present invention at an initial stage of the drilling. Fig. 4 shows the drill string according to the present invention at a later stage of the drilling. Fig. 5A shows a guide tube according to the present invention in a side view. Fig. 5B shows an end view of the guide tube in Fig. 5A. Fig.

5C shows a cross-section according to the line V-V of the guide tube in Fig.

5B. Fig. 6 shows a tong during gripping of an end of a prior art guide tube.

Fig. 7A shows the tong in seizing the end of a guide tube according to the present invention. Fig. 7B shows the tong in gripping a guide tube according to the present invention in position for unscrewing.

Detailed description of the invention An embodiment of a guide tube 10 according to the present invention for mechanical handling in a rig for rock drilling is described hereinafter with reference foremost to Figs. 5A-5C. The guide tube 10 consists of an elongated body with a substantially cylindrical basic shape, with a diameter D. The guide tube has a first end 11 with a sleeve or female portion 12 and a second end 13 with a projecting spigot or male portion 14.

The spigot 14 has a substantially cylindrical external thread 15 and the sleeve 12 has a substantially cylindrical internal female thread 16. The female thread 16 or a recess 32 wherein the female thread is provided has an axial length L1, which substantially extends from an end surface 24 of the sleeve 12 to an abutment surface or bottom 18 in the sleeve.

A flushing channel which is generally depicted 19 extends internally of the guide tube 10, through which a flush medium, usually air or water, is transferred. The through-going flush channel 19 is provided to lead flush medium to a rock drill bit 20 for percussive top hammer drilling. This channel is suitably centrally positioned in the cylindrical body. The flush channel comprises of at least three parts 19A, 19B and 19C. The axial flushing channel 19 comprises an inlet 19A at the first end 11 and terminates in an outlet 19C. the second end 13. The inlet 19A has a diameter d, that is greater than the outlet diameter do.

The guide tube 10 comprises a region of reduced diameter or a cylindrical waist 23 in connection with or in the vicinity of the first the end 12. The waist 23 has an axial extension L2 that is longer than said axial length L1. A first shoulder 25 and a second shoulder 26 border the waist.

The first shoulder is provided in the vicinity of the female thread 16 at a distance L3 from the bottom 18. The second shoulder 26 is provided at the distance L2 from the first shoulder 25. The distance L2 is at least 50% longer than the axial depth L1 of the recess 32, for example in one case L2 is 200 mm and L1 is 120 mm. The first shoulder 25 comprises a planar portion 27, which is provided perpendicularly to the center axis CL of the guide tube such to not force the shoes 4 apart, see below. The portion 27

connects at 90 degrees to the jacket surface 28 of the sleeve. The jacket surface 28 defines the diameter DA of the sleeve 12. The portion 27 connects radially inwardly via a transition 29 with a radius R1 to the substantially cylindrical jacket surface 30 of the waist 23. The second shoulder connects to the jacket surface 30 via a relatively large radius R2, which is greater than the radius R1 of the first shoulder. The diameter D2 of the waist is chosen in the interval 60-80% of the outer diameter D of the guide tube. There exists a diametrical difference on each side of the waist of-10% to +30%, such that the outer diameter D of the guide tube is 0.9*DA to 1.3*DA, where DA is the outer diameter of the sleeve 12. The flush channel 19 is restricted in the area 19A of the waist to retain the thickness of the goods in the tube, i. e. to retain its capacity to transfer impacts and the strength. The diameter D and the thickness of the goods are constant at the mid portion of the tube. The guide tube 10 consists of the mid portion 10A and the two ends 11 and 13 friction welded thereto. The mid portion 10A has a channel 19B which has a diameter dm which is bigger than, preferably at least twice the diameter di of the inlet 19A The parts are made of carbon steel but may possibly be chosen such that the ends 11 and 13 are made in stainless steel while the mid portion 10a is made in carbon steel.

In Fig. 3 is shown the appearance of the drill string according to the present invention carrying the guide tube 10 according to the present invention at an initial stage of the drilling operation. The end 11 of the guide tube 10 with the female thread is threaded firmly onto a conventional shank adapter 22 and a conventional rock drill bit 20 having cemented carbide buttons for percussive drilling is threaded firmly onto the other end 13 via the male thread. Thereby, drilling in metres and metres of rock masses can be done with a maintained hole straightness and maintained drill dust discharge since the travel of the drill bit is guided and the flush speed is increased outside and above the drill bit due to the tube 10.

In Fig. 4 is shown the appearance of the drill string according to the present invention with the guide tube 10 according to the present invention

at a later stage of the drilling operation, i. e. when the drill bit 20 has reached a depth of at least about 4 to 6 m. The threaded joint between the shank adapter 22 and the guide tube 10 is thus unscrewn since the shoes 4 of the tongs 3 seize the sleeve 12, while the components are rotated relative to each other. The shank adapter 22 is instead threaded into an extension rod 21 when the guide tube 10 has been unscrewn. The usual position of the shoes 4 at unscrewing or tightening of the extension rod 21 to the guide tube is shown in Fig. 7A, i. e. the shoes 4 clamps the sleeve 12.

If the threaded joint at unscrewing is stuck the position according to Fig.

7B is used, i. e. the shoes 4 clamp around the waist 23 while the shoulder 27 rests against upper sides 5 of the shoes. In this position the threaded joint can be impacted loose without overheating the threads or deforming the sleeve 12 since the support from the upper sides 5 is stable. The diameter D2 of the waist 23 is chosen substantially as large as the diameter of extension rod 21. The length L2 of the waist is chosen at least large enough to allow positioning of the shoes 4 around the waist without any problems.

Thus the present invention relates to a guide tube and a drill string for mechanical handling in a rig for rock drilling which simplifies unscrewing of thread joints at percussive : rock driiiing equipment and which improves the life-span for these thread joints.