Pre-tensionable bolts are previously known, which can be provided with a strain by means of a pressurized medium chamber arranged at one end of the bolt, after which it is possible to tighten a tightening member at the opposite end of the bolt. Such arrangements, for exam- ple, in connection with bolts intended for joining two elements, are pre- sented e. g. in GB patent 1 382 192 and in DE publication 24 58 810.

This latter publication also presents a traction means which is screwed around one end of a fixing means and comprises two parts, between which there is a space where pressurized medium is introduced and which traction means is, in the final joint, replaced with an ordinary nut.

Pre-tensioning arrangements of prior art have required a particular chamber structure which is screwed or joined by a quick coupling at the end of an elongated fixing means, such as a bolt. To perform the pre- tensioning, an axial bore is also needed inside the bolt as well as a power-transmitting rod placed therein, whose end forms a piston of the pressurized medium chamber which is acted upon to provide the pre- tensioning.

A simpler arrangement for providing the strain required by the pre-ten- sioning in an elongated fixing means for joining two elements is pre- sented in the publication GB 2267944. This document discloses a tightening member, a"hydraulic nut", which, on one hand, is fixed at the end of the elongated fixing means, provided with an outer thread- ing, by means of a corresponding inner threading, and, on the other hand, abuts at its front surface on the surface of the first element to be joined. The tightening member is formed of three parts in such a way that the first part is fixed to the fixing means in a way transmitting power in the longitudinal direction, and the second part abuts at its front surface on one of the elements to be joined. The pressure of the pressurized medium can be introduced between the first and the sec- ond parts. The pressure is used to make the parts move farther away

from each other, wherein the fixing means is stretched. The parts can be locked in this position by means of a third part which is effective therebetween and which is transferred to a locking position, after which the pressure can be removed.

In the arrangement presented in the publication, the second part which abuts the element to be joined, is locked to be axially immobile in rela- tion to the first part by means of an annular locking part which is screwed by screw threads around the first part and which abuts, by its front surface facing the element to be joined, on the second part. The second part is thus placed underneath the first part and the locking part, joined together. So that the diameter of the tightening member could be reduced from the arrangement of prior art as presented in Fig. 1 of the GB document, the inner wall of the pressurized medium chamber in the first part is made thin and flexible elastically by the effect of the pressure in the chamber against the outer surface of the elongated fixing means.

Although the outer surface of the fixing means, according to the docu- ment, supports the inner wall against any further deflection caused by problems of tightness, this thinner inner wall is, however, the weak point in the structure and a potential cause of problems in the tightness for the pressurized medium.

It is an aim of the invention to present a novel arrangement in which the diameter of the hydraulic nut can be reduced without potential weak points in view of the tightness of the pressurized medium chamber. To attain this purpose, the tightening member according to the invention is primarily characterized in what will be presented in the characterizing part of the appended claim 1.

In the invention, it is possible to achieve, by the method known from the publication GB 2267944, a strain in the fixing means with the same tightening member by which the joint between the two parts is tight- ened. There is no need for separate pre-tensioning devices or parts to be removed after pre-tensioning. Because the locking part is placed around the second part and underneath the first part (when the first

end of the tightening member is considered to be"up"), the whole diameter of the tightening member can still be kept small, without a need to form the inner wall of the pressurized medium chamber thin, which is the result if the recess forming the pressurized medium cham- ber were transferred close to the fixing means in the radial direction so that the locking part with a reasonable diameter could be placed around the first part.

As for the preferred embodiments of the invention and the advantages entailed by them, reference is made to the appended dependent claims and to the description hereinbelow.

In the following, the invention will be described in more detail with ref- erence to the appended drawings, in which Fig. 1 shows a first embodiment of the invention in a longitudinal sectional view of the fixing means, and Fig. 2 shows a second embodiment of the invention in a longitudi- nal sectional view of the fixing means.

Figure 1 shows a fixing means 1, which is inserted through two ele- ments to be joined together, for example through bores made in the elements. The fixing means can be an ordinary stud bolt equipped with an outer thread, or the like. Figure 1 shows the first element 6 to be joined and the second element 7 to be joined, and a joint plane 8 therebetween, through which the fixing means 1 is inserted. The ele- ments 6,7 can be joined together, for example, at their flange-like pro- trusions, through which the bores have been made. The elements may be some machine parts. At the opposite end of the fixing means 1, there is a part which abuts the second element to be joined and which at that point fixes the fixing means to be immobile in its longitudinal direction. This part, which may be an ordinary nut or a protrusion inte- grated in the fixing means, is not shown in more detail.

The tensioning means 2 screwed around the fixing means 1, close to its outermost end, comprises a first part A which is connected around

the first end of the fixing means 1. This part can be connected to the outer thread 15 of the fixing means 1 by means of a suitable inner thread 16, and it can be moved by screwing in the axial direction.

When the first part has been moved to a given position, it is capable of transmitting tensions in the axial direction to the fixing means 1, being immovable in relation to it in the axial direction, when subjected to a force effective in the longitudinal direction of the fixing means. The position of the first part A in the axial direction of the fixing means 1 can be changed only by rotating it on the outer thread 15 of the fixing means, i. e. by"screwing".

In addition, the tightening member 2 comprises a second part B which is separated from the fixing means 1, in an annular manner around it, between the first part A and the element 6. In this part B, there is an annular front surface 3 facing towards the second end of the fixing means 1, this surface abutting the first element 6 to be joined at a pressure determined by the tightening force. The front surface 3 is not necessarily directly laid against the first element 6 to be joined, but a washer or a corresponding piece can be provided between the front surface and the element. The front surface 3 is also in this case against the element 6 in the functional sense, because the washer, or the like, can be considered to be part of the element 6.

The first part A and the second part B may move in relation to each other in the axial (longitudinal) direction of the fixing means 1. The first part A and the second part B are shaped and placed in connection with each other in such a way that a pressurized medium chamber D is formed therebetween, having a volume dependent on said movement.

Figure 1 shows, on the left hand side, a situation in which the tighten- ing member 2 is untightened, that is, the joint is"open". On the right hand side, the figure shows a situation in which pressure has been introduced into the pressurized medium chamber D via a channel 4 extending through the first part A into the pressurized medium chamber D in the axial direction; and as a result, the first part A is moved farther away from the second part B and, correspondingly, from the element 6 to be joined, on whose surface the second part B abuts at its front sur- face 3 either directly or indirectly. Because the first part A is screwed to

be axially immovable around the end of the fixing means 1, the fixing means 1 is stretched. The substance to be introduced in the pressur- ized medium chamber D along the channel 4 may be a liquid, whose pressure can be used to make the first part and the second part move in the axial direction in relation to each other.

The first part A and the second part B are locked in the position shown on the right hand side in the figure by means of a third part, locking part C, effective therebetween. The locking part is placed axially immovably on the second part B, and it abuts, at its front surface 9 transverse to the axial direction, on the first part A, obstructing the movement of the part A in the axial direction by this front surface. In Fig. 1, this is imple- mented in such a way that the outer surface of the second part B is provided with an outer thread 10, on which the annular locking part C is screwed by its inner thread 11. Thanks to the matching screw threads 10,11, the locking part can be screwed in the axial direction so that it abuts the front surface 12 of the first part A. After the locking part C has been screwed in contact with the front surface 12 of the first part A, the pressure can be removed from the pressurized medium chamber D, because the locking part C will remain in its position into which it has been transferred by screwing and will prevent the return movement of the first part A. The fixing means 1 will thus remain in a permanently stretched state and the joint will remain permanently tightened.

In Fig. 1, the first part A is closer to the outermost end of the fixing means 1, and the end of the second part B facing this outermost end of the fixing means is placed in an annular recess formed in the first part A, opening in the opposite direction towards the element 6 to be joined and towards the second end of the fixing means 1. The above-men- tioned pressurized medium chamber D is formed between the bottom 14 of this annular recess and the front surface 13 of the end of the second part B. The axially outermost rim face of the collar-like outer wall of the recess forms the front surface 12 to come into abutment with the front surface 9 of the locking part C in the first part A. The locking part C is thus placed, in the axial direction, i. e. in the longitudi- nal direction of the fixing means 1, in the area between the first part A and the element 6 to be joined. The outer diameter of the periphery of

the second part B, to which the locking part C is fixed, is dimensioned smaller than the outer diameter of the first part A, so that the locking part C, which comes around the second part, would not increase the outer diameter of the tightening member too much. The cylindrical outer surface of the locking part C may form an extension of the cylin- drical outer surface of the part A in the direction of the element 6; that is, the diameters are equal. Futhermore, Fig. 1 shows that the second part B is narrowed inwards at its butt end, i. e. in the part on the side of the front surface 3, in which recess the annular locking part C is placed in such a way that the outer periphery of the locking part C will be approximately flush with the outer periphery of the first part A.

The pressurized medium chamber D can be sealed at its outer periph- ery and inner periphery (at the outer wall and inner wall of the recess) by means of annularly placed seals indicated with references 5 in Fig. 1. The seals 5 are placed in the second part B far from the opening of the recess of the first part A and closer to that front surface 13 of the end of the second part B which limits the pressurized medium chamber D. In general, the seals 5 can be placed in such a way that in the minimum volume position of the chamber D they are on the sliding surfaces farther away in the sliding direction from that end surface which would, if the chamber expanded too much, move past the seal, and closer to the end surface of that part to which they are connected.

The distance from the seal 5 is greater to the end surface of the part moving in relation to the same than to the end surface of the part comprising the seal. If the seals 5 are fixed to the first part A, to the walls of the recess, they are thus closer to the front surface 12 of the part A.

As seen from Fig. 1, in the recess which forms the pressurized medium space D and to which the second part B is fitted in an axially slidable manner, the wall on the side of the fixing means 1, i. e. the inner wall, is placed against the outer surface of the fixing means 1; that is, it cannot bend inwards in the radial direction. The first part A is, on that section which is between the bottom of the recess and the opening of the recess at the level of the front surface 9, provided with an inner thread in the inner wall, that is, the inner thread 16 of the first part A extends,

in the axial direction, also to the area of the recess, in which area the first part is screwed onto the outer thread 15 of the fixing means 1.

Figure 2 shows an embodiment modified from the embodiment of Fig. 1, wherein the first part A is a part integrated in the fixing means 1, that is, it cannot be moved in relation to the fixing means in its axial direction. Compared with Fig. 1, the threaded joint between the fixing means 1 and the first part A is now replaced with a permanent connec- tion, and the part A can be of the same piece as the fixing means 1, i. e. , in a way, it forms the head of the bolt. In other respects, the shape and position of the part A in relation to the parts B and C, the shapes of the parts B and C, and the operating principle of all the three parts in the tightening and locking are quite the same as in Fig. 1, only with the exception that the position of the first part A before the tightening and locking is determined by the position of the whole fixing means 1.