In order to achieve an elastic pretension in a screw-joint there is a need for a certain so-called pre-tension distance between the contact surface of the screw head against an object and the contact point of the threads of the screw. When the object is of a large thickness it is often sufficient to the let the shaft of the screw be provided with an unthreaded portion immediately adjoining the screw head. In screw-joints with a bolt, the clamping distance in thinner objects can be increased by choosing a relatively thick bolt with a partially unthreaded portion. In screw-unions with materials with relatively thin waits and without a nut, self-threading screws are used in certain cases with thread pressing or thread cutting screw threads which in conventional screws results in a short clamping distance and small possibilities of an elastic pre- tension. This makes the screw union relatively un-elastic, with a risk of mate- rial fractures or loosening.

SUMMARY OF THE INVENTION The purpose of the present invention is to obtain a screw in which the draw- backs stated above are eliminated.

The said object is obtained by means of a screw according to the invention, the characteristics of which will become apparent from the appended claim 1.

FIGURES The invention will in the following be described more closely using an exam- ple of an embodiment with reference to the appended drawings in which

Fig. 1 in a partial cut view shows the screw according to the invention screwed into an object while Fig. 2 shows an end view of the screw.

PREFERRED EMBODIMENT The screw according to the invention consists in a known manner of two in- tegrated main parts, i. e. a screw head 1 and a screw shaft 2 which protrudes from the screw head. The screw shaft exhibits an inner end 3 and an outer end 4, between which there extends a threaded portion 5. In the example shown, the threaded portion 5 thus extends more or less from the level of the contact surface 6 of the screw head against the surface 7 of the object 8 in which the screw is intended to be screwed into. The object is hinted at in Fig.

1, and can either have thin walls or have a thickness which exceeds the length of the screw shaft 2. The screw is primarily of the self-threading kind, i. e. thread pressing or thread cutting in the object 8, i. e. an unthreaded hole 20 is drilled in the object with a diameter which corresponds to the core di- ameter of the screw shaft 2. Since the screw is made of a considerably much harder material than the object, the screw thread of the screw itself can be used to form a corresponding thread in the object 8. Alternatively the hole 20 (and the screw also) can be machine threaded. The screw is usually made in metal, suitably steel. For the assembly/disassembly of the screw, the screw head exhibits a contact organ to be brought along by a screwing tool, either hand-tools or a power screwdriver. In the example shown, the contact organs are formed by a cornered contact surface 9 in the perimeter of the screw, for example a hexagonal head. Alternatively, the head can be provided with a recess in the outer side of the head 10, for example a track, a straight track, star track, Torx, Philips or a hexagonal socket head. The screw is according to the invention provided with an internal track 11, which in more detail is ar- ranged in the screw head 1 on the inside of the head, i. e. at the contact area 6, in more detail between the area and the inner end 3 of the shaft 2. The track 11 is ring-shaped and relatively deep, i. e. exhibits a relatively great ex- tension in the axial direction, i. e. the direction of the longitudinal axis 12.

Furthermore, the track 11 in the example shown has a width b, i. e. a radial extension or measurement in the radial plane of the screw which is substan- tially smaller than the depth of the track a, in more detail the width is smaller than 1/3 of the depth of the track. Furthermore, the width b of the track is smaller than the smallest radial outer extension c of the screw head 1 which varies according to how the cut is made, depending on the edged perimeter.

In more detail, the width of the track is smaller than half of the width c of the contact surface 6, and in the order of size of 1/3 of the smallest width c of the contact surface 6. The depth of the track is in the example shown larger than half the height d of the screw head 1. In embodiments with an internal recess in the screw head for the contact organs of the screw, their dimensions should be chosen so that a sufficient material thickness is maintained be- tween the contact organs and the track 11. The depth of the track is chosen to achieve an optimal balance between the demands of a large elastic pre- tension force and a high structural strength. Depending on dimensions and choices of material, the depth of the track 11 should however be within the interval 25-75% of the height of the screw head 1. For example, the depth of the track 11 in a hexagonal edged screw M8 should be in the interval 2-3 mm, while the corresponding depth in an M20 should be in the interval 3-9 mm, for example 7 mm at a total height of the head of 12,5 mm.

The screw shaft is essentially cylindrical with a surface profile in the form of the threaded portion. The shaft can alternatively have another cross-sectional form than circular. The cross-section can also vary over the length of the shaft, for example be threaded towards the free end, such as entirely or par- tial conical, for example pointed. In a corresponding manner, the track 11 forms two cylindrical surface areas which face each other, an inner radially outwards facing track surface 13, and an outer radially inwards facing track surface 14. These surfaces extend co-axially with the screw along its longitu- dinal axis 12. The inner track surface 13 in the example shown exhibits the same parameter as the thread ridges of the screw shaft 2, but can alterna- tively have another diameter, for example equal to the core diameter of the

shaft. The track 11 additionally forms a ring-shaped opening 15 between the inner end of the screw shaft and the contact surface 6 which is delimited by a sharp ring-shaped edge 16 which constitutes a transition from the track sur- face 14 and the contact surface 6. This is entirely plane and is situated in a radial plane of the longitudinal axis 12.

The track 11 furthermore advantageously exhibits a rounded bottom 17, i. e. an inner portion which forms a rounded concave surface between the cylin- der shaped track surfaces 13,14. The rounded surface 17 in a cross-section exhibits the shape of a semicircle, and eliminates local tension concentra- tions in the material which easily arise in sharp inner corners.

The purpose of the relatively deep ring-shaped track 11 in the screw head 1 is thus to obtain an extra clamping distance as seen from the inner end of the threaded portion to the contact surface 6 of the screw head, by means of which a good elastic pretension can be obtained in the screw union in ques- tion. From the point of view of material stress, it can be said that the track 11 divides the screw head into a compression portion 18, see the arrows, posi- tioned radially outside of the track 11, and an expansion portion 19, see the arrows, positioned radially inside of the track 11, which in practise constitutes an extension of the screw shaft 2, so that the elasticity in the screw material is utilized maximally in order to create an elastic pretension with a reduced risk of material fractures or loosening.

The invention is not limited to the examples of embodiments which have been described above and shown in the drawings, but can be varied within the scope of the appended claims. For example, track 11 can be of another form and other dimensions in order to still achieve the same effect. Addition- ally, the threaded portion can end at a level which is below the screw head by means of which additionally increased clamping length is obtained.