Prior art A conventional manner to secure the rotary cutter of a rotary drilling bit in the axial direction relative to the journal of the leg is by inserting balls into a path which extends along the circumference of said journal. The balls are mounted via a hole in the journal, whereafter said hole is closed by a locking pin which is welded to the journal. These balls primarily do not have any bearing function but when the axial bearings (Stellite washers) become worn the balls can be exposed to axial strains, which is not desirable since that can to a substantial degree shorten the tool life of the rotary cutter bit.

Through US-A-4,256,193 is previously known a rotary drilling bit of the above-captioned type, i. e. it does not include a number of balls for axial locking of the rotary cutter relative to the journal of the leg. The rotary cutter of said rotary drilling bit has a flange at the area of its open end. Said flange cooperates with the rolling elements of the radial bearing which is situated in connection with said open end of the rotary cutter when the same is mounted on the leg, for securing the rotary cutter in the axial direction relative to the journal.

A filling opening is provided in the leg to enable mounting of the rolling elements of the radial bearing at the open end of the rotary cutter. The filling opening passes beyond the flange, without interfering with it and has a smallest diameter which is somewhat greater than the diameter of the rolling elements that shall be mounted through the filling opening. Since the filling opening is straight, extends parallel to the axial axis of symmetry of the rotary cutter and does not interfere with the flange of the rotary cutter, said filling opening will interfere with the bearing race of the radial bearing situated closest to the open end of the rotary cutter, i. e. the filling opening opens into the bearing race and removes a portion of the same in the circumferential direction. This removed portion must be

recreated with the aid of the locking pin which is placed in the filling opening when all the rolling elements are assembled at the radial bearing closest to the open end of the rotary cutter. The part of the locking pin that shall recreate the bearing race must be formed with a curvature that corresponds to the curvature of the bearing races and said part must be mounted with great exactness. Also if this is made with care the obvious risk is that a difference in level arises at transitions between the locking pin and the bearing race, which naturally impair the life of the actual radial bearing.

Obiects and features of the invention A primary object of the present invention is to provide a rotary drilling bit wherein the rotary cutter is axially secured by a flange on the rotary cutter without cutting into the bearing arrangement of the rotary cutter bit.

Another object of the present invention is to improve the radial bearing arrangement of the rotary cutter bit compared to rotary drilling bits of conventional design.

Still another object of the present invention is to improve the axial bearing arrangement of the rotary cutter bit compared to conventional rotary.

At least the primary object of the present invention is realized by means of a rotary drilling bit that has obtained the features indicated in the independent claim. Preferred embodiments of the invention are defined in the dependent claims.

Short description of the drawings Below two embodiments of the invention are described with reference to the enclose drawings wherein: Fig. 1 shows a section through of a first embodiment of a rotary drilling bit according to the present invention; Fig. 2 shows a section along A-A in Fig. 1; and Fig. 3 shows a section through another embodiment of a rotary drilling bit according to the present invention.

Detailed description of preferred embodiments of the invention The rotary drilling bit shown in Fig. 1 is a so-called open rotary drilling bit, which usually is air-cooled. The rotary drilling bit comprises at least one leg 1 and at least one rotary cutter 3. A bearing journal 5 is formed integrally with the leg 1 and the rotary cutter 3 is mounted on the journal 5. In a usual manner the rotary cutter 3 has a number of rock cutting means, preferably in the shape of cemented carbide inserts 4. A number of bearing arrangements are formed between the rotary cutter 3 and the journal 5, and the rotary cutter 3 is symmetrical and rotatable relative to the journal 5 around an axis of rotation 6.

In this manner a radial base bearing provided in connection with the open end of the rotary cutter 3, and said radial base bearing comprises a number of cylindrical first rolling elements 7, which cooperate partly with a first bearing race 9 on the journal and partly with a second bearing race 10 at the inside of the rotary cutter 3 in connection with the open end.

A radial top bearing is provided closer to the free end of the journal 5 than the base bearing, and said top bearing likewise comprises a number of cylindrical second rolling elements 11, which normally have a smaller diameter than the rolling elements 7 of the base bearings and partly cooperates with a third bearing race 12 on the journal 5, partly with a fourth bearing race 13 at the inside of the rotary cutter 3.

A first and a second support disc 14 and 15 respectively are formed as a first axial bearing provided at the area of the forward free end of the journal 5. The first support disc 14 is received by a first recess in the rotary cutter 3, and said first support disc 14 is striated along its circumference for a safer attachment to said first recess. The material in the first support disc 14 is preferably high-speed steel. The second support disc 15 is received by a second recess at the free end of the journal 5, and the second recess is usually circular while the second support disc 15 is hexagonal or octagonal, whereby the second support disc 15 is prevented from rotating relative to the journal 5. The material in the second support disc 15 is preferably cemented carbide.

A second axial bearing is provided between the base bearing and the top bearing. Since the rotary drilling bit according to the present invention

lacks balls for axial positioning of the rotary cutter 3 relative to the journal 5 there is more space for a bigger second axial bearing. In the shown embodiment said second axial bearing consists of a so-called floating washer 16, which as is shown in Fig. 1 is situated between a first axial bearing surface 17 of the journal 5 and a second axial bearing surface 18 of the inside of the rotary cutter 3. As an example but without limiting purpose can be mentioned that in a rotary drilling bit with a diameter of 12 1/4" (311 mm) the floating washer 16 should have a thickness in the axial direction of at least 4 mm, which is especially important at an air-cooled rotary drilling bit according to the shown embodiment. The material in the floating washer 16 can vary, and as an example but without limiting purpose can be mentioned Stellitet), beryllium copper, aluminum bronzes and special nickel aluminum bronzes.

A cooling channel 19 is provided in the leg 1, which branches into at least one first branch channel 20 and at least one second branch channel 21, and the first branch channel 20 terminates in the first axial bearing while the second branch channel 21 terminates in the second axial bearing.

As is shown in Fig. 1 the rotary cutter 3 at its open end is equipped with a flange 22 extending towards the rotational center of the rotary cutter 3, which flange extends substantially along all the circumference of the opening of the rotary cutter 3, however with a short disruption 23, which is shown in Fig. 2.

The flange 22 cooperates with the rolling elements 7 in the radial base bearing, i. e. it overlaps said rolling elements 7, to prevent axial dislocation of the rotary cutter 3 relative to the journal 5.

The disruption or the recess 23 is necessary to enable mounting of the rolling elements 7 of the radial base bearing. The mounting of said rolling elements 7 is done through a filling opening 24, which extends parallel to the rotational axis 6. The recess 23 substantially constitutes an elongation of the filling opening 24 and the part of the second bearing race 10 which is in line with the recess 23 is likewise at the same level as the lowest point of the filling opening 24 and of the recess 23 in Fig. 1.

Furthermore in Fig. 1 is shown a locking pin 25 which is received in the filling opening 24 and secured thereto by a weld 26. Figs. 1 and 2 also show

that said locking pin 25 has a protruding, semi-cylindrical lip or heel 27, which substantially occupies only the half cross section of the locking pin 25. Thereby a free space is formed for the flange 22 when this moves past the locking pin 25.

The mounting of the rotary cutter 3 on the journal 5 is done such that the support discs 14,15 are fixed in their recesses. Then the floating washer 16 is placed on the journal 5, and the journal 5 is oriented such that the floating washer 16 through gravity rests on the axial bearing surface 17 of the journal 5.

The rolling elements 11 of the top bearing are greased and placed in their positions on the journal 5. The rotary cutter 3 is now pushed over the journal 5, and the axial bearing and the top bearing are in operational mode between the rotary cutter 3 and the journal 5. The rolling elements 7 of the base bearing cannot be mounted in a corresponding manner as the rolling elements 11 of the top bearing due to the flange 22 of the rotary cutter 3. The rolling elements 7 are therefore mounted one by one through the filling opening 24, while keeping the recess 23 in line with said filling opening 24. When all rolling elements 7 of the base bearing have been mounted the locking pin 25 is positioned, and its lip 27 is brought to abutment against the last inserted rolling element 7, to prevent the rolling elements subsequently entered into the radial cylinder bearing from exit said bearing. The locking pin 25 is subsequently fixed in the filling opening 24 by a weld 26. It is obvious through observation of Fig. 1 that despite the provision of the flange 22 any cutting of the first bearing race 9 is not needed, which obvious is of great significance for the life of the base bearing.

The rotary drilling bit according to Figs. 1 and 2 is thereby ready for operation, and in a usual manner cooling medium, i. e. in this case air, is supplied via channels 19,20 and 21.

The alternative embodiment of a rotary drilling bit according to the present invention shown in Fig. 3 differs substantially from the embodiment according to Figs. 1 and 2 by the flange 22'of the rotary cutter 3'instead being provided in connection with the top bearing, i. e. the flange 22'cooperates with the rolling elements 11 of the top bearing to axially secure the rotary cutter 3' relative to the journal 5'. In a corresponding manner as in the embodiment according to Figs. 1 and 2 the flange 22'has a recess 23', which enables

mounting of the rolling elements 11 in the top bearing. The locking pin 25'at the embodiment according to Fig. 3 is longer than the locking pin 25 according to Fig. 1 but otherwise has the same principal design, i. e. it has a lip 27'at its forward end which occupies substantially the half cross-section of the locking pins 25'. In a corresponding manner as at the embodiment according to Figs. 1 and 2 a filling opening 24'is provided partly for mounting of the rolling elements 11 and partly for receiving the locking pin 25'.

Mounting of the rotary cutter 3'on the journal 5'is done substantially as when mounting the rotary cutter 3 on the journal 5, however the rolling elements 7 of the radial base bearing are mounted in advance on the journal 5' while the rolling elements 11 of the top bearing are mounted afterwards through the filling opening 24'. Also in this case the third bearing race 12, i. e. the bearing race of the journal 5'which is situated in connection with the flange 22', is intact, which brings a normal tool life to the radial top bearing.

At the embodiment shown in Fig. 3 only the main cooling channel 19 is shown, however the embodiment according to Fig. 3 is likewise equipped with branch channels for the cooling medium, and said branch channels terminate at the axial bearings.

Common for both the embodiments is the omission of the balls for axial locking of the rotary cutter 3,3'relative to the journal 5,5', such as already discussed above, which gives an added space for the second axial bearing, i. e. the floating washer 16. This increased space makes it possible that the length of the rolling elements 7 and 11 may be longer than at conventional rotary drilling bits having the above-captioned balls. As an example but without limiting purpose can be mentioned that if the rolling elements 7 are made 6 mm longer <BR> <BR> <BR> in a rotary drilling bit which has a diameter of 12 %" (311 mm) a dynamic bearing capacity of Cdyn = 133,8 kN is obtained, which is about 16% more than at a conventional rotary drilling bit, with balls as axial lock, of the corresponding diameter. When it comes to the rolling elements 11 of the exemplified rotary drilling bit these can be made 5 mm longer, and a dynamic bearing capacity of Cdyn = 49,45 kN is obtained. This is about 10% more than at the above- captioned conventional rotary drilling bit of corresponding diameter.

Conceivable modifications of the invention The two above-described rotary drilling bits are so-called open rotary drilling bits, i. e. a pressurized medium, usually air, is blown into the space between the rotary cutter 3,3'and the journal 5,5'such that a certain excess pressure is created in said space. This pressure prevents impurities, for example mud and drill cuttings, from entering into the space to for example damage the bearings. However the teachings of the present invention may also be true for so-called sealed rotary drilling bits, wherein a seal is provided at the connection of the open end of the rotary cutter to the leg. Thereby impurities are prevented from entering into the space between the rotary cutter and the journal.