BACKGROUND OF THE INVENTION Drive wheels of the type referred to above are known e. g. through US-A-4,472, 880 and have greatly contributed to the technical and commercial breakthrough of so called ring saws, which are cutting machines having annular saw blades. This, however, does not mean that the drive of the annular saw blades has been problem-free. In order that the drive between the drive wheel and the saw blade shall be efficient, the bevelled surfaces, which define the wedge-shaped drive-groove, must bear with an adequate pressure on the wedge-shaped inner edge of the saw blade. According to said US-A- 4,472, 880, this pressure is provided by causing the said first annular surfaces of the annular elements, which in the foregoing preamble are referred to as first and second annular elements and which between them form the wedge-shaped drive-groove, to be pressed against the wedge-shaped edge of the saw blade. In practice it has turned out that the pressure force which is required for providing a contact pressure sufficient for an efficient drive must be so great that it has caused severe problems, including a great wear of the saw blade as well as of the drive wheel, which in turn has reduced the service life of these machine elements.

It is suggested in US-A-4,793, 065 that the wedge-shaped edge of the saw blade shall be pressed in the radial direction into the drive-groove of the drive-wheel. Those parts of the drive wheel which between them define the wedge-shaped drive wheel, in that case may be designed to be stationary relative to each other. This principle has turned out to give a better drive than that one which is based on pressing in the axial direction according to said US-A-4,472, 880. However, also in this case the walls of the wedge- shaped drive-groove are subjected to heavy wear. This problem can be addressed by manufacturing the drive wheel of a more wear resistant material, but that is expesive.

According to US-A-6,243, 956 only a minor part of the drive wheel is made of a wear resistant material, while the rest of the drive wheel is made of a conventional structural

steel. But also an annular drive ring of the kind which is shown in the said US-A- 6,243, 956 is difficult to manufacture, independent of the used material. The drive ring can for example, because of its geometry, not be manufactured through cold forging, even if the material is a conventional structural steel, something which would be desirable, since cold forging is a simple manufacturing technique which could make the product cheaper. Even more difficult would the manufacturing be if the drive ring shall be made of a more qualified material. This is particularly true if it shall be made of a very wear resistant material, such as a sintered carbide material or a wear resistant ceramic material, because of the difficulties in machining such materials by means of cutting tools. It is particularly difficult to establish the drive-groove and especially the annular recess which should exist in the bottom of the wedge-shaped groove in order to prevent the wedge-shaped edge of the saw blade from reaching the bottom of the wedge-shaped groove, since that would severely impair the driving efficiency of the drive wheel and also increase the wear of the drive ring as well as of the saw blade.

BRIEF DISCLOSURE OF THE INVENTION It is the purpose of the invention to address the above complex of problems. More particularly, the invention aims at providing a drive wheel which has a good drive performance and which can be manufactured in a way which is favourable from an economic point of view.

The invention is characterised in that said second, annular surfaces are in snug contact with each other, and that the drive wheel also comprises a third and a fourth annular element, said third and fourth annular elements being concentric with the first and second annular elements and provided on each side of said first and second annular elements, and provided to be pressed against them. Preferably, said second annular surfaces are flat.

According to an aspect of the invention, it is a purpose of the invention to provide a drive wheel, those parts of which, which are subjected to wear, having a good wear resistance. According to this aspect of the invention, said first and second annular elements consist of a material which has a better wear resistance than said third and fourth annular elements. For example, said first and second annular elements may consist of any of those wear resistant materials which include cemented carbide materials, ceramic materials, and high speed steels.

According to another aspect of the invention, it is a purpose to provide a drive wheel which can be produced in a way which is favourable from an economic point of view.

The invention, according to this aspect of the invention, is characterised in that said first and second annular elements consist of a material which can be cold forged, for example consist of a structural steel.

According to still another aspect of the invention, the first and second annular elements have, between said first and second annular surfaces, a third annular surface, such that the third annular surfaces between them define a narrow recess, which extends in the radial direction, inwards into the drive wheel. Preferably, said narrow recess has flat, or at least essentially flat side walls, and is 0.5-1. 5 times deeper than the wedge-shaped groove. According to one more aspect of the invention, means are provided for preventing any rotation of said first, second, third, and fourth annular elements relatively to each other about the centre of rotation of the drive wheel. These means can be designed in different ways, e. g. as splines, non-round form locks, and other geometric locks preventing rotation, one or more wedge locks and the like. According to a conceivable embodiment, however, they comprise at least one element, here referred to as anti-relative-rotation member, which extends in the axial direction through apertures or recesses in said first and second annular elements, inside of the drive- groove with reference to the radial direction. Preferably they also, according to said embodiment, extend into or through apertures or recesses in said third and fourth annular elements. Said anti-relative-rotation member or members may consist of one or more elements of the type of elements which include pins, tubular pins, and rods.

According to a further aspect of the invention, said first and second annular elements are designed as drive ring halves having identically equal shape, which promotes an easy manufacturing, but they are invertedly mounted in the drive wheel. However, also not identically equal shapes of the first and second annular elements can be conceived.

Further aspects of the invention will be apparent from the patent claims and from the detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWING In the detailed description of an embodiment of the invention, reference will be made to the accompanying drawing, which shows an axial section through the drive wheel, mounted on a drive shaft.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION In the drawing, a drive wheel is designated 1 and a drive shaft is designated 2. The drive shaft 2 can be rotated about its centre of rotation 3 by means of a hydraulic, pneumatic or electric motor or by a combustion engine, via a suitable power transmission, e. g. a belt-transmission. In this connection, reference may be made to US-A-6,374, 501, which shows a belt-transmission from a combustion engine. In this case a belt pulley is mounted on the drive shaft. The drive shaft 2 is tubular according to the embodiment and is internally provided with threads, which co-operate with the threads of a screw, which has a screw head 5 for clamping the drive wheel 1 on the shaft 2. As an alternative, the drive wheel 1, however, may be mounted directly on the driving shaft of a hydraulic motor, as is shown in said US-A-6,243, 956. In this case, the drive shaft is suitably homogeneous and may be provided with a threaded hole for a clamping screw in its front end, as is shown in US-A-6,243, 956, or be provided with external threads in its front end for co-operation with a clamping nut. Although the drive of the drive shaft does not form part of the present invention, reference is made to said US-A-6,374, 501 and US-A-6,243, 956 as far as the drive of the drive shaft 2 and thence of the drive wheel 1 are concerned, said US patents being incorporated in the present description through reference.

The drive wheel 1 comprises four annular elements, namely a first annular element 11, a second annular element 12, a third annular element 13, and a fourth annular element 14.

In the following, these annular elements will be referred to as inner drive ring half 11, outer drive ring half 12, follower plate 13, and clamping plate 14, respectively, in this description. Any restrictive meaning shall not be attributed to the chosen terminology for these four elements. The drive ring halves 11 and 12, according to a mentioned aspect of the invention, consist of a very wear resistant material, preferably a sintered carbide material or a ceramic material, while the follower plate 13 and the clamping plate 14 are made of a conventional structural steel. According to an other mentioned aspect of the invention, also the drive ring halves 11 and 12 consist of a conventional structural steel. Because of the geometric design of the drive ring halves it is possible to manufacture them through the employment of cold forging to ready or near ready shape, when the material consists of a conventional structural steel. In this case, the drive ring will be a comparatively cheap replacement part of the drive wheel.

According to the preferred embodiment, the two drive ring halves have identically equal design but are invertedly mounted, as is shown in the drawing. The sides of the drive ring halves 11 and 12 facing each other have a first annular surface 21, which is

bevelled in the way shown in the drawing, so that the bevelled surfaces 21 between them form a circumferential, wedge-shaped groove 20 matching the inner, wedge- shaped edge of an annular saw blade having a wedge-shaped inner edge. Inside the first, bevelled surface 21, with reference to the radial direction, and at a distance from said surface, each drive ring half 11,12 has a second, annular surface 22, which is flat.

These second annular surfaces 22 are in snug and unresilient contact with each other, so that the two drive ring halves 11 and 12 in combination form a drive ring 15 having a wedge-shaped groove 20 with a width which does not change, except from the wear of the groove which may occur during use.

Between said first and second annular surfaces 21,22, each drive ring half 11,12 has a third annular surface 23 which is flat except in the region adjacent to the outer border line of the second annular surface, where it is rounded as is shown in the drawing.

Between said third annular surfaces 23, an annular recess 24 is formed, having flat side walls and a rounded bottom. This recess 24 extends from the first annular surfaces 21, and thence from the wedge-shaped groove 20, inwards in the radial direction to a depth which approximately corresponds to the depth of the wedge-shaped groove 20. The purpose of this inner recess 24 is to prevent the wedge-shaped edge of the saw blade from touching the bottom of the wedge-shaped groove 20, as the edge of the saw blade and/or the wedge-shaped groove successively is/are being worn. The width of the annular recess 24 is about 1.5 mm, while the depth is substantially larger; about 4 mm.

A recess having these dimensions is very difficult or impossible to establish through the employment of conventional technique in an object made of a sintered carbide or of a ceramic material. Also the wedge-shaped groove 20 would be difficult to establish, if the drive ring 15. were homogenous. However, because the drive ring 15 according to the invention consists of two identical halves, the desired contour of the drive ring can be established by forming and possibly machining the drive ring halves before they are brought together to define an integrated drive ring 15, regardless the drive ring halves are made of a conventional structural steel or other suitable material which can be cold forged, or are made of a more qualified, more wear resistant material. Also drive ring halves made for example of a structural steel can be conceived, where the surfaces of drive ring halves which are subjected to wear, i. e. said annular, bevelled surfaces, have been provided with a coating of a more wear resistant material, for example titanium carbide or titanium nitride.

The other surfaces of the drive ring halves 11,12 are less complex and comprise an outer cylindrical surface 25, an annular, flat outer side 26, and an inner cylindrical

surface 27. According to the embodiment, each drive ring half 11,12 also comprises two cylindrical, through apertures 28, which are located diametrically opposite each other and extend in the axial direction between said second annular surface 22 and the flat outer side 26 at a short distance from the inner cylindrical surface 27.

The follower plate 13 is provided, in the region of its inner part, with a flange shaped portion 30 having a flat surface which fits a correspondingly designed surface 31 on the drive shaft. The surface 31 on the drive shaft 2 works as a follower member in co- operation with the flange shaped portion 30 for the rotation of the follower plate 13, when the drive shaft 2 is rotated about its centre of rotation 3. The follower plate 13 extends from the flange shaped portion 30 outwards in the radial direction to near the outer cylindrical surface 25 of the inner drive ring half 11. The side 32, which faces the inner drive ring half 11, is flat and contacts the flat outer side 26 of the drive ring half 11. An annular recess on the outer side of the follower plate for a not shown sealing collar is designated 33. The follower plate 13 also is provided with two diametrically located holes 34 on the side facing the drive ring half 11, which holes 34 extend from the flat side 32 of the follower plate in the axial direction to a depth corresponding to appr. 2/3 of the thickness of the follower plate 13. The holes 34 are located just in line with the through apertures 28 of the drive ring halves 11 and 12. The clamping plate 14 has a central, cylindrical through hole 35, corresponding to the shape of the outermost portion 36 of the drive shaft 2. The clamping plate 14 is mounted snug fit on said portion 36 of the drive shaft 2. The outer side 37 of the clamping plate 14 is completely flat. A peripheral portion 38 of the clamping plate extends to near the outer, cylindrical surface 25 of the outer drive ring half 12. The inner side 39 of this portion also is flat and contacts the flat outer side 26 of the outer drive ring half. Said peripheral portion may be termed wheel disc and the inner part of the clamping plate 14 may be termed hub portion. These terms shall not be given any restrictive meaning. The hub portion has an outer, cylindrical surface 41, which the inner, cylindrical surfaces 27 of the two drive ring halves contact snug fit. Two cylindrical through holes 42 are provided in the clamping plate 14, said holes having the same diameter as the holes 28 in the drive ring halves and as the not through hole 34 in the follower plate 13. The holes 42 are located in line with the other holes. A tubular pin 45 extends through each of the holes 42 and 28, which are in line with each other, and it also extends into the not through hole 34 in the follower plate 13. The tubular pins 45 have two functions. Firstly, they shall, mounted in the holes 42,28, and 34 in the clamping plate 14, the drive ring halves 12 and 11, and the follower plate 13, prevent relative rotation between said elements.

Therefor"anti-relative-rotation members an adequate term for the tubular pins 45 in

co-operation with said holes. Secondly, the tubular pins 45 mounted in said holes also function as follower members for transmitting the rotational movement of the follower plate to the drive ring halves 11 and 12 and to the clamping plate 14. The tubular pins 45 are retained by means of a spring washer 46, which by means of the screw head 5 is pressed against the outer side 37 of the clamping plate 14, when the screw 4 is tightened, when also the flat surfaces 22 of the drive ring halves 11 and 12 are pressed against each other and the thus integrated drive ring 15 is pressed against the follower plate 13.