The present invention relates to an extension coupling for rock drilling equipment, comprising a coupling rod having a screw-threaded portion with an external thread of smaller diameter than the diameter of the non-threaded part of the rod, and a sleeve-like portion having a part provided with an internal thread corresponding to the external thread of said rod. In all rock drilling operations efforts are made to increase the efficiency of said operation, such as an increased drilling rate, i.e. an increase in the number of hole-meters or drilling-meters achieved per unit of time. The increase in efficiency sought for is primarily achieved by increasing the power of the drilling machine. When percussion drilling, for example, the power of the drilling machine is increased by increasing the percussion energy of the hammer of the drilling machine, and also by increas¬ ing the torque of the machine, or by increasing the pressure at which the drilling equipment is fed against the rod, or by means of a coπbination of these measures.

An increase in the pressure at which the drill is fed to the rock causes the drill rod or rods, which transmit percussion and/or torque energy to the rock-crushing or rock-breaking drill bit, to be subjected to powerful bend¬ ing stresses which increase with increasing feed pressure and which become greater with increasing weakness of the drill rod relative to the drill bit .

In combination with other stresses and strains originating from percussion and torque, the bending stresses accelerate the occurrence of fatigue fractures in the drill rods. These fractures primarily occur in weakened or "sensitive" sections of the drill rod, for example at the clearances in screw-thread or coupling connections, at the bottoms of the threads or in other recessed portions of the extension coupling.

In current practice a drill bit or a coupling sleeve is mounted onto the coupling rod via a coupling joint, for example screwed. It is also conventional for the length of the hollowed portion in the drill bit or

Λ

coupling sleeve of the joint to be somewhat shorter than the corresponding part of the rod to be inserted into said bit or sleeve.

Subsequent to assembling the extension coupling or coupling joint, there are regions behind the drill bit or in front of and behind the sleeve which are not supported by the skirt of said bit or said sleeve, and where the smaller outer diameter of the rod merges with a larger surface diameter of said rod. These regions constitute the sensitive or risk sections where fracture normally occurs as a result of the high bending stresses occurring during a drilling operation. An object of the present invention is to provide a supporting action effective to increase the bending stresses in said sensitive or risk sections, by extending the drill bit and coupling sleeve. To this end the extension coupling according to the invention is characterized in that the sleeve-shaped part includes an extension in the form of a non-threaded skirt- shaped portion which extends from the threaded part of the sleeve-shaped portion to its outer end, and which embraces a part of the coupling rod behind the screw- thread, to provide a supporting effect which decreases the bending stresses on the coupling.

In accordance with a particularly preferred enbodi- ent of the invention, the extension coupling is charac¬ terized in that the extension having the form of a non- threaded skirt-like portion of the sleeve-like part has an innter diameter which is slide-fitted to the outer diameter of the non-threaded part of the coupling rod. The extension coupling according to the invention has the advantage of eliminating practically completely the occurrence of bending stresses at said sensitive or risk sections of the drill rods, and also the dangerous fatigue effects of said bending stresses . The extended part of the skirt of the drill bit or coupling sleeve has a εupporτing effect and because it assists in reducing the occurrence of harr.ful bending stresses in operation it is effective in reducing the risk of fracture at the

0

aforementioned risk sections . Tests have shown that the bending stresses can be lowered by up to 80% with the aid of said extension.

The invention will now be described in more detail with reference to the accompanying drawing, in which

Figure 1 is a partial sectional view of a drill bit pro¬ vided with an extended skirt in accordance with the inven¬ tion, and Figure 2 is a partial sectional view through a coupling sleeve provided with an extended skirt in accordance with the invention.

In Figure 1 there is shown a coupling rod 1 which is provided with a screw-thred and which has a drill bit 2 screwed thereonto. The coupling rod has a screw-threaded portion 3 which meshes with a corresponding screw-threaded portion 4 of the drill bit. The drill bit has an extended skirt 5, the end part of which is slide-fitted over the plain part 6 of the coupling rod, i.e. the part thereof not provided with a screw thread. Thus, the inner diameter of the skirt 5 is a slightly greater or substantially equal to the outer diameter of the coupling rod, while the length of said skirt is such that subsequent to assembling the coupling the skirt will extend over the necked portion 7 of the coupling rod, said necked portion forr.ing a weak or sensitive section, and continues up along the plain portion 6 of the coupling rod.

Figure 2 illustrates two coupling rods 8 and 9 which are also provided with a screw-thread and screwed into a coupling sleeve 10 so that the mutually opposing ends of said rods abut each other. The screw-threaded portion 12 of the coupling rod 9 meshes with the screw-threaded portion 13 of the sleeve, and the extended skirt 14 of said sleeve extends over the necked portion 15 of the rod and continues up along the plain part 16 of said rod. The skirt 14 is a slide fit with the plain portion 16 and thus has an inner diameter which is somewhat greater or substantially equal to the outer diameter of τhe plain portion 16.

Although the illustrated embodiments of the coupling

O.Y.PI

VΓ:PO

extension have been described with reference to screw- threads it will be understood that other types of screw- threads can be used, such as different types of sawtooth threads, different types of rope-threads and different types of round threads, and other types of threads . In the embodiments illustrated in Figures 1 and 2, the extended skirt passes beyond the necked portion of the coupling rod and embraces the plain part of said rod. It is also possible, however, to make the skirt shorter, so that it does not reach across the plain portion of the rod, but embraces solely the necked portion or a part of said necked portion, for example by being made a slide- fit with said portion. It is also possible to produce the coupling rod without the necked portion, in which case the plain portions 6, 16 of the illustrated embodiments are connected directly to the screw-threaded portions 3 , 12. Finally, an intermediate sleeve made of a plastics or rubber material, or some other soft and damping material, can be arranged between the extended skirt and the coupling rod.