EP0077336B1

EP0077336B1 - An extension coupling for rock drilling equipment

Info

Publication number: EP0077336B1
Application number: EP82901041A
Authority: EP
Inventors: Gösta Christer GUSTAFSSON
Original assignee: Fagersta AB; SECOROC AB
Current assignee: SECOROC AB
Priority date: 1981-04-24
Filing date: 1982-03-26
Publication date: 1985-06-19
Also published as: AU8275682A; FI77303C; SE426089B; WO1982003653A1; IE52613B1; FI77303B; AU547091B2; SE8102626L; CA1178944A; EP0077336A1; FI824324A0; IE820955L; ZA822203B; DE3264233D1; FI824324L

Abstract

An extension coupling for rock drilling equipment comprises a coupling rod (1) having a screw-threaded portion (3) with an outer thread whose diameter is smaller than the diameter of the non-threaded portion (6) of the rod, and a sleeve-like part (2) with a portion (4) having an internal screw-thread which corresponds to the outer screw-thread of the rod. The screw-like part (2) includes an extension (5) in the form of a plain skirt-like part which extends from the screw-threaded portion (4) of the sleeve-like part to the outer end thereof and which embraces a part (6) of the plain portion of the coupling rod. The skirt (5) according to the invention provides a supporting effect which reduces the occurrence of bending stresses on the coupling.

Description

The present invention relates to an extension coupling for rock drilling equipment, comprising a coupling rod having an externally screw-threaded part and a smooth non-threaded part, the diameter of which is greater than the diameter of the external screw thread on the threaded part of the rod, and further comprising a sleeve-like part having a first part which is provided with an internal thread corresponding to the external thread of said rod, and the second part which has the form of a plain skirt and extends from the first, screw threaded part of the sleeve to the outer end thereof, and encloses part of the coupling rod behind the screw thread thereon.
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 increasing 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 combination of these measures.
An increase of 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 bending 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 coupling sleeve is mounted onto the coupling rod via a coupling joint, for example screwed. It also conventional for the length of the hollowed portion in the drill bit or coupling sleeve of the joint to be somewhat shorter than the corresponding part of the rod to be inserted into said bit or sleeve.
Drill couplings of the kind described in the introduction are known in all essential details from the U.S. specification No. 2,122,757 and the Swedish specification No. 7006062-9.
The U.S specification describes a coupling for joining two drill stem sections together in a manner to obtain a seal between mutually abutting ends of two pipe sections. The coupling comprises a socket having a central shouldered portion which forms a seat for respective ends of pipe sections to be joined together. The inner surfaces of the socket have screw-threads provided thereon, and one end of the socket has a smooth, tapered portion which provides a wedging fit for the respective pipe sections fitted thereinto.
Swedish specification No. 7006062-9 illustrates a coupling sleeve for joining two drill rods together, in which the screw-threads on the coupling sleeve are somewhat shorter in height than the corresponding screw-threads on the rods, this to avoid fracture of the drill rods at the screw-threaded regions thereof, when the rods are subjected to undue strain.
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 a region of a rod of smaller outer diameter merges with a region of larger diameter, to form a step between the two regions. These stepped regions constitute the sensitive or risk sections where fracture normally occurs as a result of the high bending stresses occurring during the drilling operation. An object of the present invention is therefore to provide a supporting action effective to decrease the bending stresses in said sensitive or risk sections, by extending the drill bit and the coupling sleeve.
To this end in accordance with the invention an extension coupling of the kind described in the introduction is characterized in that the second, plain part of the sleeve like part is a slide-fit with the smooth rod part of larger diameter, and encircles said smooth rod part in a manner to reduce the occurrence of bending stresses.
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 supporting effect and because it assists in reducing the occurrence of harmful bending stresses in operation it is effective in reducing the risk of fracture at the 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 provided with an extended skirt in accordance with the invention, 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 couping rod 1 which is provided with a screw-thread and which has a drill bit 2 screwed thereonto. The coupling rod has a screw-threaded portion 3 which co-acts 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 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 stepped area 7 of the coupling rod, said stepped area forming 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 engages the screw-threaded portion 13 of the sleeve, and the extended skirt 14 of the said sleeve extends over the stepped area 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 the plain portion 16.
Although the illustrated embodiments of the coupling 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 stepped area 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 stepped area or a part of said stepped area, for example being made a slide-fit with said portion. It is also possible to produce the coupling rod without the stepped area, 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.

Claims

1. An extension coupling for rock drilling equipment comprising a coupling rod (1;9) having an externally screw-threaded part (3;12) and a smooth non-threaded part (6; 16), the diameter of which is greater than the diameter of the external screw-thread on said threaded rod part (3;12) thereby to form a step (7;15); and further comprising a sleeve-like part (2;10) having a first part (4;13) which is provided with an internal screw-thread corresponding to the external screw-thread on the rod part (3;12), and a second part (5;14) which has the form of a plain skirt (5;14) and extends from the first screw-threaded part (4;13) to the outer end of the sleeve-like part of the coupling, and encloses part of the coupling rod behind the screw-thread (3; 12), characterized in that said second, plain part (5;14) of the sleeve-like part (2;10) is a slide-fit with the smooth rod part (6;16) of larger diameter, and encircles said part and the region of the step (7;15) in a manner to reduce the occurrence of bending stresses on the coupling.

2. A coupling according to claim 1, characterized in that the plain skirt-like part (5;14) embraces a stepped part (7; 15) on the coupling rod, between the screw-threaded portion and the plain portion.