The invention relates to a screw coupling where certain amount of pre¬ stress is desired in the screw. The pre-tensioning is accomplished by using a pre-tensionable screw as the screw.

In most screw couplings, a sufficient and accurate pre-tensioning of the screw is necessary for the reliable functioning of the coupling. A desired prestress is effected by turning the screw in a tighter position by ap- plying torsional moment, or by stretching the screw by a predetermined force correspondrg to the desired prestress before the nut or screw is screwed down.

The screw is tightened by means of torsional moment in general with the help of a torsion tool which turns the screw or nut, in which event often the screw or nut must be retained also on the opposite side in order to succeed in the tightening. To bring about a certain, predefined tensional stress in the screw, the tool must be provided with a torque restrictor or torque measurement. Because of many factors, such as friction, the prestress brought about in the screw by meεπs of torsional moment is rather inaccurate. Further, the pre-tightening taking place through the torsional moment causes, in addition to the desired prestress, also shear stress in the screw which causes a part of the pre- tensioning potential to be lost.

The screw can also be warmed up before screwing it down, whereby it is stretched by thermal expansion. After the screw has been assembled hot and has the desired longitudinal extension, it will return to the length corresponding to the ambient temperature during its cooling, and at the same time will be pre-tensioned to the desired prestress corresponding to the thermal expansion. The screw can be warmed up also by means of a heating element placed in a hole bored therein, for example using an electric resistance. The drawback of the pre-tightening methods based on thermal expansion is often the high temperature called for by a sufficient elongation, making the assembly difficult and possibly weakening the strength characteristics of the screw metal.

The pre-tensioning can also be performed by stretching the screw by means of pre-tensioning tools, which operate hydraulically or in some other way, as is presented for example in German Publication 3733243 or Swedish Publication 7701049-4. The required tools are often compli- cated and are not always suitable for pre-tightening of standard screws. Some tools are not suitable for use in places where the available space is limited.

Japanese Application Publication 60-188608 shows a screw made of a one-way shape-memory metal having a transformation temperature lower than the use temperature of the screw (claim 7). This leads to the need to store the screws at a temperature which is lower than their deformation temperature, which makes their use very difficult.

A pre-tensionable screw according the invention, defined in the characterizing portion of the claim, offers a very simple possibility to produce a desired prestress in the screw without the use of special tools. The screws can have geometry and sizes according to standards, if so is desired. Also the nuts can be quite usual standardized nuts. The pre-tensioning method according to the invention does not exert torsional shear stress in the screw either, and the whole pre-tensioning potential of the screw can be utilized, if desired. The pre-tensioning of the screw is possible also in a place where the shape of the space or lack of space limits the use of tools.

The invention is based on a phase transformation occurring in the crystalline structure of the shape-memory metal, effected by temperature. The phenomenon is called martensitic reaction. Hence, it is not connected with thermal expansion. The shapes and volumes of pieces made of shape-memory metal alloys are recovered exactly to correspond to the original status on heating to a relatively low temperature after a deformation caused by an external force. The recovery is complete typically still after a deformation of 4 — 8%. The martensitic reaction starts and takes place in quite narrow temperature range typically of ca. 20°C, which can usually be fitted in a desired area within the temperature range of - 30...+100°C. On certain conditions, the martensitic reaction is crystallographically completely reversible. The reversion of the deformation caused by an external force takes

place quickly once the transformation temperature has been attained. Because of the low transformation temperature, large amounts of heat need not be brought to the piece to be heated.

The shape-memory metals exists as two main groups, whereof one shows a one-way shape-memory effect and the other a two-way effect. In those shape-memory metals based on the one-way shape-memory effect, the deformation triggered by a predetermined transformation temperature is irreversible. The term deformation herein is understood to mean the reverting of the c ge of shape and possibly also change of volume caused by an exte force in a piece to the original status by the effect of temperature. .s case of the two-way shape-memory effect, the deformation is reve- όiDle as the piece is being cooled below a predetermined temperature. Hence, the shape and volume of a piece made of a two-way shape-memory metal has two states bound to cer¬ tain transformation temper' tures. The transition from one state to the other can be accomplished only by changing the temperature.

Hereinafter, a shape-memory metal based on the one-way shape- memory effect will be designated one-way shape-memory metal, and a shape-memory metal based on the two-way shape-memory effect will be designated two-way shape-memory metal. The upper limit tempera¬ ture of the transition temperature range where the deformation triggered by the warming-up of the shape-memory metal piece is complete will be called hereinafter upper transformation temperature. Correspondingly, the lower limit temperature of the transition ! perature range where th deformation triggered by the cooling- αuwn of the shape-memory letal piece is complete will be called hereinafter lower transformation temperature, which according to the description above has importance mainly in connection with two-way shape-memory metals.

Several shape-memory metal alloys can with! id very high stresses without plasticizing and losing their shape-memory characteristics, even 800 to 1000 N/mm ² , and consequently, a piece made of such metal, such as a screw, can produce large forces in connection with the de¬ formation.

In the following, the invention will be described in more detail with reference to the accompanying figures:

Fig. 1 shows a pre-tensionable screw made of a shape-memory metal, provided with a head, in a threaded hole coupling where clearance has been left between the screw head and a piece to be fastened in order to adjust the prestress.

Fig. 2 shows a pre-tensionable screw made of a shape-memory metal disposed in a nut - stud bolt coupling.

The operation of a pre-tensionable screw 2 according to Fig. 1 made of a one-way shape-memory metal is the following: An elongation dimensioned according to the pre-tensioning need of the screw has been caused through external force in the pre-tensionable screw of shape-memory metal in the longitudinal direction of the expansion screw within the area between a screw head 12 and a screw threading 4 or within a part thereof in such a manner that the elongation is permanent in the usual temperature range of the surroundings. The pre- tensionable screw 2 is inserted through a hole 5 in an upper piece 1a to be joined and it is lightly screwed down in to a threaded hole 13 in a lower piece 1b to be joined, by leaving a clearance 7 between the screw head 12 and the upper piece 1a to be joined, if needed. By adjusting the size of the clearance 7, the tensioning action on the coupling by the screw 2 can be influenced.

The pre-tensionable screw 2 is heated thereafter by means of a heating device, which can be for instance an electric resistance 10 placed in a centrally bored hole 11 of the pre-tensionable screw 2, to the upper transformation temperature. The pre-tensionable screw 2 is thereby constricted by the amount of elongation caused through the external force to it, and it is at the same time pre-tensioned to the desired amount of prestress and will tighten the coupling. The pre-tensionable screw 2 made of one-way shape-memory metal can be released in usual ways.

If the pre-tensionable screw 2 is made of two-way shape-memory metal, the mounting and pre-tensioning take place in the same way as

explained above. The release of this kind of pre-tensioned screw 2 can be accomplished by cooling the pre-tensionable screw 2 down to the lower transformation temperature for instance using dry ice, the pre- tensionable screw 2 thereby being stretched again to its length pre- ceding the heating, and the pre-tensionable screw can be released lightly by turning without any heavy tools. A pre-tensioned screw made of twt. way shape-memory metal can be used again several times.

In Fig. 2 the upper piece 1a to be joined and the lower piece 1b to be joined are fastened to each other through a pre-tensionable screw 2, which in this case is a stud bolt. The pre-tensionable screw having a longitudinal elongation, like in the screw o Fig. 1 , caused by external force in an area between a threading 3 and another threading 4, is inserted through holes 5, 6 disposed in the pieces 1a, 1b to be joined. One of the nuts, in Fig. 2 the nut 9, can be already present on the threading 3 of the screw 2 or it can be screwed to its place after the insertion of the screw 2 through the holes 5, 6 in the pieces 1a, 1b to be joined. The other of the nuts, in Fig. 2 the nut 8, is screwed onto the threading of the screw 2 not until the screw 2 has been mounted to its place. If needed, a clearance 7 can be left between the nut 8 and the piece 1b to be joined, and by adjusting the size of the clearance the magnitude of the prestress of the screw 2 and the tightening effect of the screw 2 on the coupling can be influenced. The pre-tensionable screw 2 is heated thereafter to the upper transformation temperature by means of a heating device, which can be for instance an electric resistance 10 according to Fig. 2 placed in a hole 11 bored centrally in the pre-tensionable screw 2. The screw 2 will thereby be constricted by the amount of elongation caused to it through the external force, and it will be simultaneously pre-tensioned to the desired prestress, which is possibly adjusted by means of the clearance 7, and the coupling will thus be tightened. The pre-tensionable screw 2 made of one-way shape-memory metal can be released in usual ways. The pre- tensionable screw 2 made of two-way shape-memory metal can be released by cooling and lightly turning, such as the screw 2 of Fig. 1. The release procedure has been explained in detail for the screw of Fig. 1.

It is apparent for a man skilled in the art that the applications shown by Figs. 1 and 2 are only examples of the operation and fields of use of the pre-tensionable screw according to the invention. It is clear that the in¬ vention can be applied in connection with all types of screws and nuts and possibly also with other fastening elements than screws and nuts.

The pre-tensioned screws show a pre-tensioning elongation in a magni¬ tude of typically 0.2% of the stretchable length, if the screw is manu¬ factured of steel. On the other hand, the deformation capability of the shape-memory metals in connection with a phase change based on the martensitic reaction, i.e. in this case the capability of producing pre- tensioning elongation, is in the magnitude of 4 — 8%, as mentioned hereinabove. It is thus possible to obtain an elongation which is sufficient for the screw pre-tensioning and will be reversible on heating by virtue of the shape-memory effect.

When desired, the elongation of the screw can be measured more pre¬ cisely for instance in the hole bored centrally in the screw.

The use of this invention is not restricted to any single way to warm up the pre-tensionable screw made of shape-memory metal. Sometimes it may be advantageous to bring the whole machine part in question to¬ gether with its screws to the temperature triggering the action of the shape-memory metal, especially if there are a lot of screws.

In this text, the word "shape-memory metal" has been used, and it de¬ notes such metal alloy, where shape and/or volume changes occur by virtue of crystalline properties, and not for instance by virtue of thermal expansion, and where the changes are permanent within the limits set by threshold temperatures. As an example can be mentioned well- known nickel-titanium alloys provided with additives, but invention is not limited only to these alloys or presently known alloys.