It is previously known to increase the stiffness in wood con¬ structions joined together by holts by inserting metallic elements between the wood parts, which elements on both sides are provided with a plurality of points, which are forced into the wood parts to a considerable depth. Elements of this type are not suitable for demountable constructions, since the surfaces of the construction parts are damaged by repeated demounting. The mentioned elements can neither be used for constructions of material having hard surfaces, which cannot be penetrated by the points, as for instance metals, or materials which should be subjected to cracks or burstings when penetrated by the points, as for instance hard plastic materials and ceramic materials. For such con¬ structions it is previously known to use metallic elements which are bent in such a way at their periphery or at the edge of apertures being punched out that the bent parts form resilient tongues. When the construction parts are for¬ ced against each other, the corners or the edges of the resi¬ lient tongues are pressing against the construction parts with a force dependent on the stiffness of the tongues. The¬ se elements, however, do not function very well for hard construction parts, since said tongues only rest against the construction parts without penetrating the same. Therefore, there has been a strong desire in the market for an element, which shall hold not only construction parts made of soft material, as for instance wood, but especially construction parts made of hard material, as for instance steel.

The present invention is intended to satisfy these desires and hereafter two embodiments of the invention shall be des¬ cribed more closely.

The first embodiment constitutes a frictional element, which is produced by etching thin strips or sheets of material, which is hardened or is capable of being hardened. In this connection the production occurs in the following way

(Figs . 1-5) .

A strip is on both sides covered by "screen-printing", another printing method or in a photographic way with a colour accor¬ ding to a certain pattern, which colour prevents etching, the patterns being so adapted that a covered area on one side always corresponds to an uncovered area on the other side (Fig. 1). The strip is etched on both sides by spraying with or immersion into an etching liquid, for instance fer¬ rous chloride dissolved in water, the surface layer of the strip being removed in the areas that are not covered by co¬ lour. The etching depth is made equal on both sides of the strip by symmetrical arrangement of the etching device or by adapting the conditions of spraying. A certain etching occurs also under the colour on the covered parts, the width of which shall be so large that parts of the original surfa¬ ce of the strip remain after etching, undercut, flat pointed teeth or teeth having the form of flat pointed cones, wedges or pyramids being received. After the etching the colour layer is removed, whereafter frictional elements of desired form can be cut or punched out from the strip (Fig. 2). The¬ se frictional elements, if wished, can be given an increased hardness by hardening, chromium-plating or another hard metallic layer.

When applying an element between two construction parts, which are forced together the thin part of the element will be bent so that the co pressive force against every standing pattern detail is distributed over a substantially larger, smooth surface on the opposite side of the strip (Fig. > ) . Due to that fact considerable shearing forces can be transmitted without giving dangerous concentrations of flexural tensions in the frictional element and with a minimal mutual movement between the construction parts.

The frictional element has the following advantages in rela-

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tion to previously known technics:

1. The points can be very closely located so that the power transmission is distributed on a large number of points, the penetrating depth of which can be made small. Due to that fact the damages on the surfaces of the construction elements can be reduced so that the joint can be demounted and put together repeated times without impaired function.

2. The frictional element can be given a small and well- defined thickness so that the finished construction can be formed with well-defined measures, which are not influenced by the size of the tightening of bolts or other elements for forcing the parts of the construction together.

3. The points of the frictional element can be made so hard and so sharp that with a small penetrating depth they can transmit large forces to the construction parts of metallic materials as for instance unhardened steel.

•4*. The frictional element can be made as large, flexible pieces, which can be formed on the basis of the surfaces of the adjacent construction parts.

The second embodiment comprises a frictional element, which is manufactured from metal strips or metal sheets, which are provided with grits of a very hard material, as for instance cemented carbide, on one or both sides. In this connection a metal strip of a certain length can be provided with the hard grits, whereafter the strip can be cut in suitable si¬ zes. Instead of doing so the metal strip can first be cut in suitable sizes, whereafter the strip pieces are covered by for instance cemented carbide grits.

The hard grits are fastened onto the metal strip or metal sheets by means of gluing or soldering or in another suitab-

ιe way.

The second embodiment of the invention is particularly suitable to use for construction parts made of hard metal¬ lic materials.

The tooth height is not more than 2 mm and preferably less than 1 mm for the two embodiments. It has turned out that a tooth height that is larger or equal to 0,2 mm but less or equal to 0,4 mm is particularly advantageous.

The frictional element is preferably provided with teeth on both surfaces that rest against the parts, which shall be prevented to move relative to each other.

In certain cases, however, it can be suitable to provide on¬ ly one surface of the frictional element with teeth.

The invention is particularly suitable to use separately or together with a screw joint within building industry, vehicle industry and engineering industry.

The construction parts mentioned in the application compri¬ se elements, which shall be prevented to slide relative to each other, as for instance beams of different kinds, parts in vehicles as for instance engine and chassis, and parts in machines as for instance a stand for a crane or another equipment and a plate or another bed in the machine, which stand shall be fastened on to the plate.

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