WO2000019115A1

WO2000019115A1 - Tensioning nut

Info

Publication number: WO2000019115A1
Application number: PCT/GB1999/003038
Authority: WO
Inventors: Michael Shaun Brian Gethings; Paul Raymond Egginton
Original assignee: Tentec Limited
Priority date: 1998-09-29
Filing date: 1999-09-29
Publication date: 2000-04-06
Also published as: GB9821018D0; AU6210699A; GB2342135B; GB2342135A; GB9922982D0

Abstract

A tensioning nut comprising a body (1, 31) which screws onto a threaded part of an article, has one or more stacks of compressible components (3, 33) located with regard to the longitudinal axis of the nut. The or each stack acts in use of the nut on an item connected to the screw-threaded part. The compressible components (3, 33), preferably Belville washers, are compressed and held releasably compressed by load retaining means. The nut may be a hydraulic tensioning nut with a piston (2) which acts on the item, the body and piston being urged apart by hydraulic pressure to tension the screw-threaded part connected to the item. In this case the compressible components compress under the hydraulic pressure applied between the body (1) and the piston (2). This allows enhanced relative movement between the body and piston under the hydraulic pressure before retention means (4) is operated to retain the body and piston in positions to which they have been hydraulically urged.

Description

TENSIONING NUT

This invention relates to a tensioning nut for application to an externally screw threaded part of an article, such as a bolt or stud, to enhance the security of a joint made with the article.

Hydraulic tensioning nuts are known which comprise an internally screw-threaded body which screws onto a complementary externally screw-threaded part of an article, an annular piston co-axial with the body which is adapted to fit freely around the screw-threaded part, is connected to the body for relative axial movement therebetween and between which and the body hydraulic pressure is arranged to be applied to urge the body and piston apart, and retention means operable to retain the body and piston in the relative positions to which they have been urged when the hydraulic pressure is removed, the arrangement being such that in use of the nut to secure the article to another item to which the screw-threaded part of the article is connected, the body is screwed onto the screw-threaded part adjacent to the item, with the piston towards the item, hydraulic pressure is then applied between the body and piston which urges the body away from the item and thereby tensions the screw-threaded part, and the retention means is operated to hold a proportion of the tension in the screw-threaded part when the hydraulic pressure is subsequently removed, some tension loss occurring. Such a hydraulic tensioning nut is hereinafter referred to as "of the kind described" .

The hydraulic pressure applied to the nut induces a load in the screw-threaded part of the article, i.e. tensions that part. This causes elongation of the screw-threaded part. For a given load, the elongation of the screw-threaded part will be directly proportional to the tension length of that part. Therefore for a given load the elongation produced in a short screw-threaded part will be less than that produced in a longer screw-threaded part.

When the hydraulic pressure in the nut is released the load induced in the screw-threaded part is maintained by the retention means, which acts on the body and piston of the nut to retain the relative positions thereof. However, as the hydraulic pressure is released, there is some relative movement between the retaining means and the body and/or piston as the retaining means beds onto the body and/or piston, or, when the retaining means comprises, for example, a threaded collar screwed on the body, thread deflection occurs between the collar and body. These actions result in a loss in the load or tension in the screw-threaded part. This load or tension loss is particularly severe in a short screw-threaded part where initial elongation is small. Indeed, for a threaded part of very short length the load or tension loss can be so large that little, or even no, residual load or tension is retained in the threaded part. An aim of the present invention is to overcome, or at least reduce, this problem.

According to a first aspect of the present invention there is provided a tensioning nut comprising an internally screw-threaded body adapted to screw onto a complementary externally screw-threaded part of an article connected to another item by the screw-threaded part, one or more stacks of compressible components located with respect to the longitudinal axis of the nut and adapted to act on the item when the body is screwed onto the screw-threaded part, the compressible components of the or each stack being compressible lengthwise of the longitudinal axis, and load retention means whereby load applied to the compressible components to compress them relative to the body is retained, the arrangement being such that when the body has been screwed onto the screw-threaded part and tightened against the item and the compressible components, having been compressed, the load retention means is released, the compressive loading of the compressive components is exerted on the item thereby tensioning the nut on the screw-threaded part.

There may be two or more of the stacks substantially equally spaced around the longitudinal axis.

The compressible components may be partially compressed under maximum load applied to them for use. The compressive loading may be applied by hydraulic pressure, mechanically or by other suitable means.

The compressive loading may be applied before or after the body of the nut has been applied to a screw-threaded part for use. For some uses, it may be more convenient to apply the compressive loading to the compressible components before the body is applied to a screw-threaded part, so that the components are pre-compressed when the body is applied to the part and the compressive loading retained by the load retention means. Therefore once the body has been tightened onto the part against the item to which the screw-threaded part is connected, the load retention means can simply be released for the compressive loading of the components to be exerted on the item.

The pre-compressing may be done in the factory before the nut is dispatched to a customer for use. This minimises the equipment needed by the customer to apply the nut for use. The customer will not need to have hydraulic or other load applying means to compress the compressible components in the or each stack.

The tensioning of the nut, when it has been applied for use, resulting from the compressive loading of the compressible components enables the nut to be particularly effective when applied to a screw-threaded part of short length.

The compressible components may take various forms. In a preferred form they comprise disc springs, for example of the conical kind known as Belville washers, which deflect under compressive loads. Usually more than two such disc springs will be provided in the or each stack. In another form the compressible components may be made of a compressible material, such as a rubber, synthetic rubber or plastics. The or each stack of compressible components may be received in a pocket or pockets in the body from which the stack projects at least when the components are in an unloaded state. In such an arrangement the or each stack may abut at one end against an inner end of the, or the respective, pocket and at the other end against a bearer on which load is caused to be applied to compress the compressible components. If there are two or more of the stacks there may be a separate bearer for each stack, or a single bearer, for example of annular form, may be provided which serves all of the stacks .

The load retention means may comprise one or more screw-threaded elements which attach to the body and to the or each stack and are operable by rotation relative to the body to release compressive loading on the compressible components. The or each screw-threaded element may comprise a headed bolt which is rotatably retained by its head to the body at one end of the, or the respective stack, extends in the direction of the longitudinal axis through the, or the respective, stack and is screw-threadedly engaged with a bearer at the opposite end of the, or the respective, stack. By rotation of the screw-threaded element the bearer is movable relative to the body in the direction of the compressive loading on the compressible components. The bearer may serve one or more stacks, as mentioned above. The or each bolt may have a socket head for rotating it by a key or other tool to tighten and release its engagement with the bearer.

In another form the or each screw-threaded element may comprise a collar, cup or comparable part which is screwed onto the body and engages with the or each stack for release of compressive loading on the compressible components.

The or each screw-threaded element may provide a means for applying compressive loading to the compressible components.

The tensioning nut may be a hydraulic tensioning nut.

According to a second aspect of the present invention a hydraulic tensioning nut is provided of the kind described including one or more stacks of compressible components located with respect to the longitudinal axis of the piston, the compressible components of the or each stack being compressible lengthways of the longitudinal axis of the piston, the or each stack being arranged to be able to act on the item connected to the screw-threaded part of the article to which the body is applied for use of the nut, and the compressible components being compressible under hydraulic pressure applied between the body and the piston for tensioning the screw-threaded part when the nut is in use, thereby allowing enhanced relative movement between the body and piston under the applied hydraulic pressure before the retention means is operated, which movement results in reduced tension loss in the screw-threaded part when the hydraulic pressure is released and the retention means is operated. The tensioning load produced by the hydraulic pressure causes compression of the compressible components and elongation of the screw-threaded part. The effective elongation the nut generates is the combined elongation of the screw-threaded part and the compression of the extent of the compressible components of the stacks. Thus, when the load is transferred to the retention means the proportion of elongation loss is reduced, thereby enabling a much higher residual load or tension to be maintained.

In consequence load or tension loss on the screw-threaded part is reduced when the hydraulic pressure is removed from the nut and enhanced security of the nut on the screw-threaded part is achieved. It has been found that improved security can be obtained when the screw-threaded part is of short length.

The proportion of the tensioning load, expressed as a percentage, which is maintained when the retention means has been operated can be calculated from the following equation: x - y x 100% x

For example, when under a tensioning load x = 1.5mm and y = 0.3mm the proportion of the tensioning load which is maintained is:

1.5 - 0.3 x 100 = 80% 1.5

There may be two or more of the stacks of compressible components substantially equally spaced around the longitudinal axis of the piston.

The or each stack may be located in the piston and projects from the piston when the compressible components are in an unloaded state to be able to act on the item connected to the screw-threaded part of the article to which the body is applied for use of the nut. Alternatively the or each stack may be located externally of the piston so as to be disposed between the piston and the item when the nut is fitted for use.

The compressible components are preferably only partially compressed when the maximum hydraulic pressure is applied to the nut.

The compressible components may again take various forms. As before, in a preferred form they comprise disc springs, for example Belville washers. The or each stack of compressible components may be received in a pocket or pockets in the piston of the nut. At least one bearer element such as a pad, washer or the like may be received in the or each pocket. The or each pocket is of shorter length than the stack of compressible components it receives in the unloaded state, or than the compressible components and the or each bearer element. As load from the hydraulic pressure is exerted on the or each stack it becomes compressed. When maximum hydraulic pressure is applied to the nut, the compressible components may be partially compressed and/or the stack or stacks and bearer element or elements, partially received within the pocket or pockets. Alternatively the components may be fully compressed and/or the stack, or stacks and bearer element or elements fully received within the pocket or pockets. The compressible components may be held in a rigid sleeve or sleeves in the piston of shorter length than the stack or stacks when the compressible components are un-loaded, or than the compressible components and the bearer element or elements.

In another form the compressible components may comprise components of a compressible material, such as a suitable rubber, synthetic rubber or plastics material. The compressible components of the or each stack may be suitably contained in a pocket in the piston or in a rigid sleeve in the piston, or may contain one or more rigid components, by which the load is taken when full compression of the compressible components has occurred.

The compressible components may be selected or made to allow a pre-determined extent of relative movement between the body and piston under the maximum tensioning load produced by the hydraulic pressure at the nut. It is desirable to have as much relative movement as practicable.

The or each stack of compressible components may act directly on the item engaged by the screw-threaded part or a bearing element or elements may be interposed between the stack or stacks and the item.

The retention means of the nut may comprise one or more shims which insert between the body and the piston. An alternative is for the retention means to comprise a collar which may be provided on the body and may be movable, as by screw-threaded interengagement with the body, relative to the body to engage with the piston, the collar being held securely in any position to which it is moved for engagement with the piston.

A hydraulic tensioning nut in accordance with the second aspect of the present invention may be used with a stud or bolt for various applications. It may be used with other articles having an externally screw-threaded part which is to be connected to an item, or to have an item secured to it such as, for example, tie or connection rods. The nut may be used to provide added security of connections of, or associated with, moving parts of mechanisms so as to resist loosening of the connections under the loads and stresses of the movement. The invention is particularly advantageous when applied to a relatively short screw-threaded part of an article. The hydraulic nut may be fitted with a quick release hydraulic connection, enabling quick and simple connection of a hose for hydraulic fluid to the nut. The nut may be fitted with a device to direct oil flow away from the operator, should the piston be over-stroked.

The hydraulic nut may be fitted with self-energising seals which require no adjustment.

The nut may be coated for enhanced corrosion protection.

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawing in which:

Figure 1 is an axial section through a first embodiment of a tensioning nut in accordance with the present invention, and

Figure 2 is an axial section through a second embodiment of a tensioning nut in accordance with the present invention

Referring to Figure 1 of the drawings, the tensioning nut in this embodiment is a hydraulic tensioning nut which comprises a cylindrical body 1 , an annular piston 2 co-axially retained, and sealingly connected, to the body for relative axial movement between the two, stacks of compressible components 3 retained to the piston, and retention means 4.

The body 1 and piston 2 are of a substantially known form, the body having an internally screw-threaded axial bore 5 and being adapted for connection at an inlet 6 to a hydraulic fluid supply for admission of hydraulic fluid into a chamber 7, defined between the body and piston, to urge the body and piston apart. When there is no fluid pressure in the chamber 7 opposed annular faces 8, 9 of the body and piston abut. Seals 30 are provided between the piston and the body.

The stacks of compressible components 3 each comprise several groups of nested Belville washers 12. Adjacent groups of the washers are arranged in the stack so that the conical shapes of the washers in one group are facing in the opposite axial direction of the stack to those in the next group. The stacks are held inside pockets 13. The pockets are formed by machining into the bearing face 11 of the piston 2. Hardened outer washers 14 seat on one end of the stacks of Belville washers and are a free fit inside the pockets. Further hardened, inner washers 15 seat on the other end of the stacks of Belville washers 12, and are a free fit inside the pockets. The Belville washers 12 and washers 14, 15 are stacked around bolts 16. The bolts have heads 17 at one end and screw-threaded portions 18 at the other end. To assemble each stack, an outer washer 14 is first located on a bolt 16 such that an internal shoulder 19 of the washer abuts the inner end of the head of the bolt. The Belville washers 12 are located next on the bolt such that they seat on the outer washer 14, and an inner washer 15 is located on the bolt such that it seats on the innermost Belville washer. The stacks are then inserted into the pockets 13 and the screw-threaded portion of each bolt engaged in a complementary screw-threaded hole 20 in the piston 2 such that the inner washers 15 abut the inner end of the pockets 13. In the totally free conical state of the Belville washers 12 the stacks of washers are longer than their pockets so that the outer washers 14 project a short distance out of the pockets. The outer ends of the outer washers 15 seat on a bearing ring 21.

Under axial compressive loading, the Belville washers 12, which present a high reactive force against the loading, will be urged towards a flattened state from the free conical form so that the stack shortens in length and the outer washers 15 move into the pockets, at which point the bearing ring 21 abuts the outer ends of the pockets 13 and further compression is resisted. Typically the Belville Washers compress by about 75% under the maximum applied load.

As illustrated in Figure 1 there are twelve Belville washers in each stack. There may be more, or fewer. A hydraulic nut having two stacks of compressible components is shown. There may be just one such stack or there may be more stacks substantially equally spaced around the nut.

The retention means 4 comprises an internally screw-threaded collar which is threadedly engaged with an external screw thread on the body 1 of the hydraulic tensioning nut. Rotation of the collar moves it axially relative to the piston and body.

For use on an externally screw-threaded part 22 of an article which is threadedly engaged in an internally screw-threaded bore of an item 23, the nut is fitted to the screw-threaded part 22 adjacent a bearing surface 24 of the item 23, the stacks of compressible components 3 towards the bearing surface. The body 1 screws onto the screw-threaded part 22. Initially the body 1 is screwed along the screw-threaded part 22, which may be done by hand, until the stacks of compressible components bear on bearing ring 21 and urge it against the bearing surface 24. Hydraulic pressure is then applied in the chamber 7 which urges the piston away from the body towards the item 23, causing the Belville washers 12 of the stacks to begin to be compressed in the pockets 13 and separating the annular faces 8, 9 of the body and piston. This compression continues until the maximum hydraulic pressure is applied. During this operation, the hydraulic pressure acting on the body 1 urges the body away from the piston and hence away from the bearing surface 24, thereby tensioning the screw-threaded part 22. The tensioning causes some elongation of the screw-threaded part.

The hydraulic pressure induced in the chamber 7 is regulated as required according to the tensioning of the screw-threaded part desired. Suitable indicator means associated with the hydraulic fluid supply shows tensioning readings.

When a desired level of tensioning is achieved, the collar 4 is turned on the body 1 and tightened onto the piston. This locks the piston and body in the relative axial positions they have attained under the applied hydraulic pressure. The hydraulic pressure is then released.

Some loss in the tensioning load on the screw-threaded part occurs as the hydraulic pressure is released and the collar 4 beds on the piston, and some thread deflection occurs between the collar and the body. Due to the operation of the stacks of compressible components 3, which enhances the movement of the piston relative to the body under the applied tensioning load, the load loss is however small, as can be ascertained from the equation given above, and the screw-threaded part can be very securely retained to the item 23 to which it is connected.

Referring now to the embodiment shown in Figure 2 of the drawings, the tensioning nut comprises a cylindrical body 31 with an internally screw threaded bore 32, stacks of compressible components 33 located in pockets 34 in a bearing end face 35 of the body 1 , bearers 36 at the stacks and bolts 37.

There are several of the stacks of compressible components 33 equally spaced around the bore 32 of the body 31 in the pockets 34. The pockets 34 are formed by cylindrical blind bores machined into the bearing end face 35 parallel with the central longitudinal axis of the body. Co-axial with each pocket 34 is a smaller diameter bore 36 which extend from the opposite remote end face 38 of the body 1 into the inner end of the pocket.

Each stack of compressible components 33 comprises two groups of nested Belville washers 39, the groups being disposed so that the conical shapes of the washers of the one are facing in the opposite direction, axially of the stack, to those of the other group. There may be more than two groups of the Belville washers 39 in each stack. When the Belville washers are in an unloaded state each stack has a height corresponding to at least the depth of the pocket 34 in which it is located, as shown by the stack at the right had side of Figure 2.

Each stack has its own bearer 36 which is a hardened disc of corresponding external diameter to the Belville washers 39 and with a central internally screw-threaded bore 40. The bearer seats on the outer end of the stack.

Into the smaller diameter bore 36 at each pocket 34 is inserted, from the remote end face 38, the shank 41 of one of the bolts 37. The shank 41 of each bolt 37 is of similar length to the axial length of the body and has an externally screw-threaded end portion 42. The shank extends through the Belville washers of the stack in the respective pocket 34 and screws into the threaded bore 40 of the bearer 36 on the stack. The head 43 of the bolt is able to bear on the remote end face 38 of the body.

The stacks of Belville washers are compressed for use of the nut by any suitable pressure applying means. They may be compressed, for example, by hydraulic pressure which is applied to urge the washers into the pockets. When the stacks have been compressed as desired the bolts 37 are tightened into the bearers 36 so as to hold the stacks compressed when the hydraulic pressure is withdrawn. As the bolts are tightened the bearers 36 are drawn into the pockets, as shown in respect of the stack at the left-hand side of Figure 2. The washers in each stack may be selected to suit any particular intended use of the nut, and the compressive load exerted on stacks can be set to suit the intended use of the nut as well. These requirements can be determined in accordance with the level of tensioning required of the externally screw-threaded part to which the nut is applied for use.

The stack may conveniently be compressed in the factory before the nut is dispatched to a customer for use. The customer may stipulate the tensioning load required of the nut in use and the Belville washers and their compressive loading are determined accordingly. The nut with the pre-compressed stacks is delivered to the customer for fitting directly to the screw-threaded part of the article which is to be tensioned.

For use of the nut, the threaded bore 32 of the body 31 is engaged with the screw-threaded part 45 of the article to be tensioned and the body is screwed onto the part to tighten it so that the bearing end face 35 bears on the item 46 to which the screw-threaded part is connected. The bolts 37 are then unscrewed to release them from engagement with the bearers 36. Once they are released the re-action of the compressed Belville washers 39urges the bearers to bear on the item 46 thereby tensioning the screw-threaded part 45 and retaining it securely by the nut to the item to which it is connected. The loading exerted on the screw-threaded part 45 provides very secure retention by the nut. The effectiveness is particularly advantageous when the screw-threaded part is short in length.

Claims

1. A tensioning nut comprising an internally screw-threaded body (1 , 31) adapted to screw onto a complementary externally screw-threaded part of an article connected to another item by the screw-threaded part, and characterised in that it includes one or more stacks of compressible components (3,33) located with respect to the longitudinal axis of the nut and adapted to act on the item when the body is screwed onto the screw-threaded part, the compressible components (3,33) of the or each stack being compressible lengthwise of the longitudinal axis, and load retention means (4,37) whereby load is applied to the compressible components to compress them relative to the body (1 ,31) , the arrangement being such that when the body has been screwed onto the screw-threaded part and tightened against the item and the load retention means is released, the compressive loading of the compressible components (3,33) is exerted on the item thereby tensioning the nut on the screw-threaded part.

2. A tensioning nut according to claim 1 characterised in that there are two or more of the stacks of compressible components (3,33) substantially equally spaced around the longitudinal axis.

3. A tensioning nut according to claim 1 characterised in that the compressible components (3,33) partially compress under maximum load applied thereto in use.

4. A tensioning nut according to any preceding claim characterised in that the compressible components (3,33) comprise disc springs (12,40) which deflect under applied compressive loads.

5. A tensioning nut according to claim 4 characterised in that the disc springs are Belville washers (12,40).

6. A tensioning nut according to claim 5 characterised in that the Belville washers (12,40) in the stack are stacked around a bolt (16,37) located with respect to the body.

7. A tensioning nut according to any of claims 1 to 3 characterised in that the compressible components (3,33) are of a compressible material.

8. A tensioning nut according to any preceding claim characterised in that the or each stack of compressible components (3,33) locates in a pocket (13,34) in the body (31) and projects therefrom at least when in an unloaded state.

9. A tensioning nut according to any preceding claim characterised in that the load retention means (37) comprises one or more screw-threaded elements which attach to the body (31) and to the or each stack of compressible components (33) and are operable by rotation relative to the body to apply and release compressive loading on the compressible components.

10. A tensioning nut according to claim 9 characterised in that the or each screw-threaded element (37) comprises at the or each stack of compressible components (33) a bolt which is rotatably retained by its head (44) to the body (31) at one end of the, or the respective, stack, extends in the direction of the longitudinal axis through the, or the respective, stack and is screw-threadedly engaged with a bearer (36) at the opposite end of the, or the respective, stack, the arrangement being such that by rotation of the screw-treaded element (37) the bearer (36) is movable relative to the body (31) in the direction of the longitudinal axis to apply and release compressive loading on the compressible components.

11. A tensioning nut according to any preceding claim characterised in that the load retention means (37) is operable to apply load t the compressible components (33) prior to screwing the body (31) onto the screw-threaded part of an article.

12. A hydraulic tensioning nut comprising an internally screw-threaded body (1) which screws onto a complementary externally screw-threaded part of an article, an annular piston (2) co-axial with the body which is adapted to fit freely around the screw-threaded part, is connected to the body for relative axial movement therebetween and between which and the body hydraulic pressure is arranged to be applied to urge the body and piston apart, and retention means (4) operable to retain the body and piston in the relative positions to which they have been urged when the hydraulic pressure is removed, characterised in that one or more stacks of compressible components (3) are located with respect to the longitudinal axis of the piston (2) , the or each stack (3) being arranged to be able to act on an item connected to the screw-threaded part of the article to which the body (1) is applied for use of the nut, and the compressible components being compressible under hydraulic pressure between the body and the piston for tensioning the screw-threaded part when the nut is in use, thereby allowing enhanced relative movement between the body and piston under applied hydraulic pressure before the retention means (4) is operated, which movement results in reduced tension loss in the screw-threaded part when the hydraulic pressure is released and the retention means is operated.

13. A hydraulic tensioning nut according to claim 12 characterised in that there are two or more of the stacks of compressible components (3) substantially equally spaced around the longitudinal axis of the piston (2) .

14. A hydraulic tensioning nut according to claim 12 characterised in that the compressible components (3) partially compress under maximum hydraulic pressure applied to the nut in use.

15. A hydraulic tensioning nut according to any of claims 12 to 14 characterised in that the compressible components (3) comprise disc springs (12) which deflect under applied compressive loads.

16. A hydraulic tensioning nut according to claim 15 characterised in that the disc springs are Belville washers (12) .

17. A hydraulic tensioning nut according to claim 16 characterised in that the Belville washers (12) in the or each stack are stacked around a bolt (16) secured to the piston (2) .

18. A hydraulic tensioning nut according to any of claims 12 to 14 characterised in that the compressible components (3) are of a compressible material.

19. A hydraulic tensioning nut according to any of claims 12 to 18 characterised in that the compressible components (3) allow a pre-determined extent of relative movement between the body (1) and piston (2) under the maximum tensioning load produced by the hydraulic pressure at the nut.

20. A hydraulic tensioning nut according to any of claims 12 to 19 characterised in that the or each stack of compressible components (3) contains one or more rigid components (14, 15) by which the load is taken when full compression of the compressible components (3) has occurred.

21. A hydraulic tensioning nut according to any of claims 12 to 20 characterised in that the or each stack of compressible components (3) is located in the piston (2) and projects from the piston when the compressible components (3) are in an unloaded state, so as to be able to act on the item engaged by the screw-threaded part of the article to which the body (1) is applied for use of the nut.

22. A hydraulic tensioning nut according to claim 21 characterised in that the or each stack of compressible components (3) is located in a pocket (13) in the piston (2) and projects therefrom at least when in an unloaded state.

23. A hydraulic tensioning nut according to claims 20 and 22 characterised in that the or each rigid component (14, 15) is received in the, or the respective, pocket (13) .

24. A hydraulic tensioning nut according to claim 20, claim 21 or 23 as dependent from claim 20, or claim 23 characterised in that the or each rigid component is a hardened washer.

25. A hydraulic tensioning nut according to any of claims 12 to 24 characterised in that the or each stack of compressible components (3) is held in a sleeve in the piston (2) of shorter length than the stack when the compressible components are unloaded.

26. A hydraulic tensioning nut according to any of claims 12 to 25 characterised in that the retention means (4) comprises one or more shims which insert between the body (1) and the piston (2) .

27. A hydraulic tensioning nut according to any of claims 12 to 25 characterised in that the retention means (4) comprises a collar provided on the body (1) which is movable relative to the body to engage with the piston (2) and, when the hydraulic pressure is removed, holds the body and piston in the relative positions to which they have been urged.

28. A hydraulic tensioning nut according to claim 27 characterised in that the collar is screw-threadedly engaged with the body (1) , the screw-threaded engagement allowing the collar to be moved relative to the body and holding the collar in any position to which it is moved for engagement with the piston (2) .

29. A hydraulic tensioning nut according to any of claims 12 to 28 including a quick release hydraulic connection for a hose for hydraulic fluid to the nut.

30. A hydraulic tensioning nut according to any of claims 12 to 29 including a device which directs oil flow away from the operator, which the piston be over-stoked.

31. A hydraulic tensioning nut according to any of claims 12 to 30 including self -energising seals which require no adjustment.