The invention relates to a screw coupling where a certain amount of prestress is desired in the screw. The pre-tensioning is accomplished by using a pre-tightening washer under the screw head or a nut.

Usually it is attempted by a washer placed under a screw or nut to make it easier to tighten the screw by torsionai moment, to reduce sur- face pressure exerted on the support or to ensure the fixing of the screw by shaping the washer suitably. By means of an appropriately shaped washer, a bending effect on the screw by a sloped support can be avoided, or the washer can be used for sealing the coupling or for gal¬ vanic insulation of the parts of the coupling. The washer can also be so shaped, for example forming it cup-like, that it will be straightened upon tightening and causes tensile stress in 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 applying torsionai moment or by stretching the screw by a predeter¬ mined force corresponding to the desired prestress before the nut or screw is screwed down.

The screw is tightened by means of torsionai moment often 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 tensile stress in the screw, the tool must be provided with a torque restriction or measurement. Because of many factors, such as friction, the pre¬ stress brought about in the screw by means torsionai moment is rather inaccurate. Further, the pre-tensioning taking place through the tor¬ sionai moment causes shear stress in the screw which leads to the dissipation of a part of the pre-tensioning potential of the screw.

The screw can also be warmed up before its assembly, whereby it is stretched by thermal expansion. After the screw has been assembled hot and stretched to the desired extension, it upon cooling tightens the

coupling, and the desired prestress will remain in the screw. 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-tensioning methods based on thermal expansion is often the high temperature called for by a sufficient elongation, making the as¬ sembly 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 often suitable for pre-tensioning of standard screws.

U.S. Patent publication 4,450,616 shows a washer made of shape-me¬ mory metal, used for ensuring the tightness of a screw if the screw tends to slacken. According to the publication, an elastic washer punched out of a shape-memory metal is pressed in an austenitic form at an elevated temperature (200°C) above the A _f point to a cup-like shape. When it is cooled down to the martensitic area below the M _f point (below — 150°C), the screw can be easily screwed down, and the washer deforms to its disc-like shape. As the temperature rises by heat conduction above the A _f point, the washer tends to recover its cup¬ like shape. Because of its cup-like shape and elasticity, the washer cannot stretch the screw in a manner required for the pre-tensioning.

Japanese publication 62-116292 shows a spring washer intended for tightening of a nuclear reactor container cover. The washer is of shape- memory metal and designed to tighten the bolt in order to maintain the sealing effect as the temperature increases and the bolt is extended by thermal expansion. The presented invention is not suitable for pre-ten¬ sioning of a bolt.

A washer according to the invention, defined in the characterizing por- tion of the claim, offers a very simple possibility to produce a prestress of a desired degree without the use of special tools. The screws and nuts can be usual standard screws and nuts. The pre-tensioning meth¬ od according to the invention does not excert torsionai shear stress in

the screw either, and the whole pre-tensioning potential of the screw can be utilized if desired. The screw can also be pre-tensioned 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 crystal structure of the shape-memory metal, effected by temperature. The phenomenon is called martensitic reaction. The shapes and vol¬ umes of pieces made of shape-memory metal alloys are recovered exactly to correspond to the original status at a relatively low tempera¬ ture on heating after a deformation caused by an external force. The re¬ covery is complete typically still after a deformation of — 8%. The martensitic reaction starts and takes place in a quite narrow tansition temperature range of typically ca. 20°C (range A _s to A _f ), which can usually be fitted in a desired area within the temperature range of — 30...+100°C. On certain conditions, the martensitic reaction is crystal- lographically completely reversible. The deformations of shape-memory metals are not based on thermal expansion but on a phase change in the crystalline structure triggered by a temperature change. The trans- formation takes place quickly once the transformation temperature has been attained. Because of the low transformation temperature, the piece to be heated does not require large amounts of heat.

The shape-memory metals exist 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. In the case of the two-way shape-memory effect, the deformation is reversible as the piece is being cooled below a predetermined temperature (range M _s to M _f ). Hereinafter, a shape- memory metal based on the one-way shape-memory effect will be de¬ signated 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, and the temperatures will be designated upper transformation temperature (A _f ) and respectively lower transformation temperature (M _f ).

Several shape-memory metal alloys can withstand very high stresses without plasticizing and losing their shape-memory characteristics, even 800 to 1000 N/mm ² , and consequently a washer made of such metal can produce large forces in connection with the deformation.

In the following, the invention will be described in more detail with re¬ ference to the accompanying drawings, wherein

Fig. 1 shows a screw having a washer made of shape-memory metal placed under its head, and

Fig. 2 shows a screw having a shape-memory metal washer placed under its head and having a clearance between the washer and the screw head for adjusting the prestress.

The operation of a washer 4 according to Fig. 1 made of a one-way shape-memory metal is the following: The shape-memory metal washer 4, the thickness of which has been dimensioned in view of the length of a screw 2 to be pre-tensioned and the pre-tensioning need, is assembled in its place under a head 3 of the screw between it and a piece 1a to be fastened, and the screw is screwed lightly down. The washer 4 is heated to the upper transformation temperature by means of a heating device, which can be e.g. a usual hot-air fan, whereby the washer 4 will expand and will cause the screw 2 to be tensioned to the using state. The screw can be released in normal ways.

When a washer 4 made of a two-way shape-memory metal is used, the pre-tensioning takes place in the same manner as described herein- above. The release of the screw 2 can be accomplished by cooling the washer 4 to the lower transformation temperature e.g. by using dry ice, whereupon the washer 4 recovers its original dimensions, and the screw can be released by rotating it lightly without using strong tools. The two-way shape-memory metal washer can be used again several times.

In the solution according to Fig. 2, a clearance 5 is left between the screw head 3 and the shape-memory metal washer 4. By adjusting the

size of the clearance, the extent of the screw elongation and con¬ sequently the tightening effect of the screw can be influenced.

The clearance 5 can be calculated in normal ways, which are not described here more closely, when the dimensions of the screw and the properties of the shape-memory metal washer are known. The use of the clearance enables the use of e.g. standard-sized shape-memory metal washers together with screws of varying lengths.

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 of the washer accord¬ ing to the invention. The washer can equally be used also in screw/nut couplings both for screws having heads and for tenons.

It is also apparent for a man skilled in the art that the pre-tensioning washer according to the invention can be used also in association with other fastening elements than screws and nuts. It is also clear that washers can be disposed several one on top of the other at the same tightening point, whereby a set of more than one washer (for example having standardized thicknesses) can be utilized for pre-tensioning screws of varying lengths by the effect of washers of identical thick¬ nesses.

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 this case the capability of producing elonga¬ tion, is in the magnitude of 4 — 8%, as mentioned hereinabove. It is thus possible to attain an elongation sufficient for the screw pre-tensioning already using a relatively thin washer.

The elongation of the screw can be measured more accurately, if de¬ sired, in a hole borred in the screw centre or possibly in the washer.

As a practical example, the tightening of a screw M24x3 8.8 SFS 2063 of total length of 315 mm by means of shape-memory metal washer is presented.

It is supposed that the screw is disposed in a threaded hole over the length of 1.5 times the diameter, resulting in the stretchable length of ca. 259 mm. If the screw is tightened 90% of yield point, the prestress equals 576 N/mm ² . The pre-tensioning force is in this case 195 000 N and the screw elongation ca. 0.54 mm (0.2% of the stretchable length).

If it supposed that an elongation of 5% is attained through the shape- memory metal, the required thickness of the shape-memory metal washer will be ca. 11 mm. (On supposing that the elastic modulus of the shape-memory metal and that of steel are the same. A difference in the elastic moduli has an effect on the thickness of the needed washer.)

When a shape-memory metal washer of a 36 mm outer diameter, 26 mm inner diameter and 11 mm thickness is used, the compression stress in the washer as well as the surface pressure under the washer will be 400 N/mm ² .

The use of the enclosed invention is not restricted to any single way to warm up the shape-memory metal washer. Sometimes it may be ad- vantageous to bring the whole machine part in question to the tempera¬ ture triggering the function of the shape-memory metal, especially if there are a lot of fastening points.

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