GB2261482A - Rod tensioning - Google Patents

Abstract

A rod (R) e.g. of composite material, is tensioned using an apparatus which comprises a first collet assembly (10), a second collet assembly (12), and force applying means (14) operable to move the first collet assembly (10) away from the second collet assembly (12). The collet assemblies (10, 12) are assembled on the rod (R) and the force applying means (14) is operated. The second collet assembly (12) grips the rod (R) so that the rod is stretched, the first collet assembly (10) allowing this movement. The force applying means (14) is then released allowing the rod (R) to contract slightly. This contraction causes the first collet assembly (10) to grip the rod (R) and hold it under tension. The second collet assembly (12) can then be removed leaving the first collet assembly (10) to hold the rod (R) in tension. If required, the first collet (10) can be released and removed to de-tension the rod (R). <IMAGE>

Description

ROD TENSIONING This invention is concerned with tensioning apparatus for use in tensioning a rod or the like and with methods of tensioning rods or the like. The invention finds particular utility for tensioning rods made of composite material but is not limited to that application.

Rods of composite material, e.g. made of plastics material such as polyester or epoxy resin reinforced with fibres such as glass fibres, are strong in tension and are, therefore, desirable in various construction uses, e.g. in the construction of electrical transformers. However, it has hitherto proved to be a problem to apply and maintain the required tension in the rod without substantially damaging the rod and thereby weakening it.

It is an object of the present invention to provide a tensioning apparatus and a method of tensioning whereby said problem may be overcome.

The invention provides tensioning apparatus operable to tension a rod or the like, the tensioning apparatus comprising a first collet assembly, a second collet assembly, and force applying means operable to move the first collet assembly away from the second collet assembly, the first and the second collet assemblies being arranged to be assembled on the rod, the first collet assembly comprising abutment means whereby the apparatus can be braced against a fixed support, and gripping means operable to allow movement of the rod through the first collet assembly in a direction towards the second collet assembly but to grip the rod against movement upon movement of the rod through the first collet assembly in the opposite direction, the second collet assembly comprising gripping means operable to grip the rod when the force applying means operates to move the second collet assembly away from the first collet assembly.

An apparatus according to the invention enables a composite rod to be tensioned without substantial damage thereto provided that the gripping means is selected not to be too fierce in its gripping action. Ideally, where the rod is made of composite material, the gripping means is arranged to break through the matrix of plastics material (which contributes little to the strength of the rod) and grip the fibres without breaking them. The first collet assembly can be assembled on to the rod so that its abutment means engages a fixed wall or side plate through which the rod projects. The second collet assembly can then be assembled on the rod portion which projects beyond the first collet assembly. Operation of the force applying means then moves the second collet assembly away from the first collet assembly and, hence, from the wall or side plate.This moves the rod through the first collet assembly, the first collet assembly allowing this movement, and causes the gripping means of the second collet assembly to grip the rod so that the rod is tensioned by the action of the force applying means. When operation of the force applying means ceases, the rod contracts slightly due to the tension applied thereto. This causes the gripping means of the first collet assembly to grip the rod and prevent further movement of the rod. Thus, the first collet assembly acts to maintain the rod in slightly less tension than the maximum applied by the force applying means. The second collet assembly can now be removed leaving the first collet assembly to maintain tension for as long as required.

In the tensioning apparatus according to the invention, the gripping means of the first collet assembly may comprise a plurality of jaws, e.g. six, distributed around the rod, each jaw being in engagement with a wedging surface which tapers inwardly in said opposite direction and is effective to cause said jaws to move between a contact position in which contact with the rod is effective to move the jaws along the wedging surface upon movement of the rod through the collet assembly, and a gripping position in which the jaws grip the rod against movement.

It is necessary that the tractive forces between the jaws and the rod are greater than those between the jaws and the wedging surface. To ensure that this is so, the coefficient of friction between the jaws and the wedging surface must be lower than that between the jaws and the rod. The first collet assembly may also comprise preloading means operable to urge said jaws into their contact position from a release position in which the jaws allow free movement of the rod through the collet assembly, the pre-loading means being releasable to allow said jaws to return to their release position under the action of circumferential springs acting between the jaws to urge the the jaws apart.

Said jaws may each have a plurality of teeth, preferably extending generally transversely of the rod, arranged to engage said rod. The teeth may extend parallel to one another or be helical at a small angle, e.g. 1 or 2 degrees. Said teeth preferably have flat tops to reduce the possibility of breakage of the fibres when the rod is gripped. To increase the coefficient of friction, the teeth may be coated with a rough, hard, friction-enhancing particulate material to ensure that the co-efficient of friction between the rod and the jaws is higher than that between the jaws and the wedging surface. Said particulate material may be tungsten carbide which is conveniently plasma-sprayed on to said teeth. The teeth may have a depth such that, when they grip the rod, they only reduce its cross-sectional area by less than 10%, e.g. the depth may be approximately lmm.

Preferably, the second collet assembly of tensioning apparatus according to the invention has a similar construction to the first collet assembly thereof, i.e. it has a plurality of jaws of similar or identical construction to those of the first collet assembly and those jaws are in engagement with a wedging surface which tapers inwardly in said opposite direction, i.e. towards the first collet assembly.

Preferably, the force applying means of the tensioning apparatus according to the invention comprises a piston and cylinder assembly comprising a cylinder fixed relative to said second collet assembly, e.g. integral therewith, and a piston arranged to engage said first collet assembly.

The piston and cylinder assembly is preferably annular about said rod so that a substantially even load around the rod can be applied. The piston and cylinder assembly may be hydraulic or pneumatic.

The invention also provides a method of tensioning a rod or the like comprising: applying a first collet assembly to the rod so that the rod passes through the collet assembly and the collet assembly abuts a fixed support associated with the rod; pre-loading jaws of the first collet assembly so that they move from a release position thereof in which they allow free movement of the rod through the collet assembly into a contact position thereof in which they contact the rod for movement therewith; applying a second collet assembly to the rod so that the rod passes through the second collet assembly and the second collet assembly is downstream of the first collet assembly in the direction in which the rod is to be tensioned; pre-loading jaws of the second collet assembly so that they move from a release position thereof in which they allow free movement of the rod through the collet assembly into a contact position in which they contact the rod for movement therewith; moving said second collet assembly away from said first collet assembly while pressing the first collet assembly against said fixed support, thereby causing the jaws of the second collet assembly to grip said rod against movement of the rod through the second collet assembly so that the rod is stretched; releasing said second collet assembly thereby allowing the rod to contract partially, thereby causing the jaws of the first collet assembly to grip said rod against movement of the rod through the first collet assembly; and removing the second collet assembly from the rod leaving the rod held in tension by the first collet assembly.

Preferably, the second collet assembly is removed by removing the pre-loading on its jaws and then moving the second collet assembly towards the first collet assembly to bring the jaws of the second collet assembly to their release position. The second collet assembly can then easily be taken off the rod.

The second collet assembly may be moved away from the first collet assembly by pressurising a piston and cylinder assembly mounted on the second collet assembly and acting against the first collet assembly or the fixed support and the second collet assembly may be released by depressurising said assembly.

There now follows a detailed description, to be read with reference to the accompanying drawings, of a tensioning apparatus and a method of tensioning which are illustrative of the invention.

In the drawings: Figure 1 is a vertical cross-section taken through half of the illustrative apparatus, the omitted half being a mirror image of the half shown; Figures 2 to 5 are views similar to Figure 1, but on a reduced scale, showing different stages of the operation of the illustrative apparatus; Figures 6A and B are vertical cross-sections, on an increased scale, of a first collet assembly of the illustrative apparatus shown in Figure 1, Figure 6A showing a contact position of the assembly and Figure 6B a gripping position thereof; and Figures 7 to 11 are views of a jaw of the illustrative apparatus, Figure 7 being a plan view, Figures 8 to 10 being respectively views in the direction of the arrows VIII, IX and X in Figure 7, and Figure 11 being a side elevational view, on an increased scale of teeth of the jaw.

The illustrative tensioning apparatus is operable to tension a rod R made of composite material. Specifically, the rod R is of circular cross-section 55mm in diameter, and is approximately 8 metres long. The rod R is made of polyester reinforced with glass fibre. A longitudinally extending half of an end portion of the rod R is shown in Figures 1 to 5 and half of a portion of said end portion is shown in Figures 6A and 6B. The rod R projects through a hole in a fixed wall W and it is required that the portion of the rod R projecting beyond the wall W should be pulled to tension the rod R.

The illustrative tensioning apparatus comprises a first collet assembly 10 and a second collet assembly 12.

The first and the second collet assemblies 10 and 12 are both generally annular and are, thus, arranged to be assembled on the rod R with the rod R passing centrally therethrough, the first collet assembly 10 being nearer the wall W than the second collet assembly 12. The illustrative tensioning apparatus also comprises force applying means 14 which, as will appear from the description below, is operable to move the first collet assembly 10 away from the second collet assembly 12 when both are assembled on the rod R in order to tension the rod R. The force applying means 14 operates by pressing the first collet assembly against the 10 against the wall W while pulling the second collet assembly 12 away therefrom.

The first collet assembly 10 comprises a collet ring 20 and a support ring 22 which is bolted to an annular surface 24 of the collet ring 20. The rings 20 and 22 are annular about an axis 23 which axis, when the collet assembly 10 is assembled on the rod R, is also the longitudinal central axis of the rod R. In the interests of clarity, full details of the support ring 22 are not shown in Figures 1 to 5 but are shown in Figure 6. The collet ring 20 provides a cylindrical shoulder 30 bounding the surface 24 thereof on the side nearest the axis 23.

The shoulder 30 serves to locate the support ring 22. The collet ring 20 also provides abutment means whereby the apparatus can be braced against a fixed support provided by the wall W. This abutment means is provided by an annular abutment face 26 at an opposite end of the ring 20.

The first collet assembly 10 also comprises gripping means operable to allow movement of the rod R through the first collet assembly 10 in a direction towards the second collet assembly 12 but to grip the rod R against movement upon movement of the rod R through the first collet assembly 10 in the opposite direction. The gripping means comprises an inner wedging surface 28 of the collet ring 20 which is frusto-conical, six jaws 32, and pre-loading means for said jaws 32. The frusto-conical surface 28 has a highly-polished low-friction finish. The wider end of the surface 28 is formed as a radially-inward surface of the shoulder 30 and the narrower end of the surface 28 terminates at a radially-inwardly-extending flange 31 of the ring 20 adjacent the surface 26. The surface 28, thus, tapers inwardly in said opposite direction, i.e. away from said second collet assembly 12.The wedging surface 28 is, as will appear from the description below, effective to cause the jaws 32 to move between a contact position in which contact with the rod R is effective to move the jaws 32 along the wedging surface 24 upon movement of the rod R through the collet assembly 10, and a gripping position in which the jaws 32 grip the rod R against movement.

The six jaws 32 are evenly distributed in a circle around the rod R and each has a highly-polished lowfriction surface 34 which is complementary to a segment of the surface 28 and is in sliding engagement with the surface 28 (see Figures 7 to 11 for details of the jaws 32). The six jaws 32 are identical to each other so that only the jaw 32 shown in Figures 7 to 11 need be described in detail. This jaw 32 has two adjoining portions, a rear portion 36 and a forward portion 38. The rear portion 36 has the form of a portion (a little less than a sixth) of a hollow cylinder. The rear portion 36 is divided into two by a central axial slot 40, which opens at the rear of the jaw 32. A cylindrical recess 42 is formed in each of the radially-extending side faces of the jaw 32. The recesses 42 open towards adjacent jaws 32 and the recesses 42 of adjacent jaws 32 contain opposite ends of coil compression springs (not shown). There are, thus, six springs associated with the six jaws 32. These springs serve to urge the jaws 32 apart and against the surface 28.

The springs also serve to centralise the jaws 32 and to cause them to move into a release position thereof in which they allow a rod R to move freely through the first collet assembly 10.

The forward portion 38 of each jaw 32 is bounded by the above-mentioned partly-frusto-conical surface 34, which slides on the surface 28, and by a partly cylindrical surface 44 having a radius of curvature slightly less than that of the inner surface of the rear portion 36. The surface 44 is complementary to a portion of the cylindrical surface of the rod R. The surface 44 is provided with inwardly-projecting teeth 46 (omitted from Figure 10) each of which extends generally circumferentially. The form of the teeth 46 is shown in Figure 11. The teeth 46 have flat top surfaces 48 (nearest the central axis of the rod R) and side surfaces 50 inclined to one another at 60 degrees.

The teeth 46 are lmm deep and spaced 2mm apart. The teeth 46 are also coated with rough, hard, friction-enhancing particulate material i.e. tungsten carbide particles (not shown), to a depth of 0.25mm to give a very rough surface.

The tungsten carbide particles are applied by a plasmaspraying operation.

The support ring 22 of the first collet assembly 10 (see Figures 6A and 6B), when bolted to the surface 24 of the collet ring 20, abuts the shoulder 30 which serves to locate the support ring 22. The support ring 22 comprises a base member 52 and a lid member 54. The base member 52 is annular, being centred on the axis 23 as is the lid member 54. A radially-extending surface 56 at one end of the base member 52 abuts the surface 24 of the collet ring 20 and (as afore-mentioned) is bolted thereto. A radiallyextending surface 58 at an opposite end of the base member 52 abuts the lid member 54 which is secured on this surface 58 by a lip member 60 of the support ring 22 but can rotate relative to the base member 52 about the axis 23.

The lip member 60 is generally annular about the axis 23 and is secured to the base member 52 by a screw thread 62. The lip member 60 has an inwardly-extending flange 64 which extends above an outwardly-extending flange 66 of the lid member 54 to secure the lid member 54 on the surface 58.

The lid member 54 has three radially-extending passages 68 therethrough spaced at 120 degree angles about the axis 23. Only one of these passages 68 is visible in Figure 6. Each of these passages 68 is threaded and receives an externally threaded support 70 for an axle 72.

Each of the axles 72 extends radially-inwardly beyond the lid member 54 to support a roller 74 which can rotate about the axle 72. The lid member 54 is also provided with recesses 76 in its radially-outwardly-facing surface.

These recesses 76 can be entered by tools when it is desired to rotate the lid member 54 relative to the base member 52.

The radially- inwardly-facing surfaces of the base member 52 and the lid member 54 of the support ring 22 together define a cylindrical passage 78 centred on the axis 23. The passage 78 has a considerably greater diameter than the rod R and contains the rollers 74. The passage 78 contains a generally-cylindrical pin carrier 80 of the support ring 22 which has an internal diameter sufficient to allow the rod R to pass freely through. The carrier 80 has an external flange 82 located between the shoulder 30 of the collet ring 20 and the rollers 74.

Between the rollers 74 and the flange 82, a cam 84 is located so that it is engaged on one side by the flange 82 and on the other side by the rollers 74.

The cam 84 is generally annular and has a radiallyextending flat surface 86 which is engaged by the flange 82 and an undulating surface 88 which is engaged by the rollers 74. The surface 88 undulates smoothly between three crests (not shown) spaced at 120 degree intervals about the axis 23 and three troughs each located midway between two of the crests. Each crest has a shallow depression therein to retain a roller 74 on the crest. The arrangement is such that rotation of the lid member 54 through 60 degrees about the axis 23 brings all three rollers 74 to troughs on the cam 84 and rotation in the opposite direction through an equal angle brings all three rollers 74 to crests on the cam 84.As will appear from the description below, the pin carrier 80 is spring-urged away from the collet ring 20 and the difference between the effective thickness of the cam 84, depending on whether and the rollers 74 engage the crests or the troughs in the surface 88, causes movement of the carrier 80 axially in the passage 78. In Figure 6A, the pin carrier 80 is shown at its furthest from the collet ring 20 while, in Figure 6B, it is shown at its nearest thereto.

The pin carrier 80 has six axial passages 90 therethrough which are equally distributed about the axis 23.

Each of these passages 90 is threaded at its end which is remote from the collet ring 20 and receives a plug 92.

Partly located in each passage 90 and partly projecting therefrom towards the collet ring 20, is a pin 94 which engages one of the jaws 32 in the slot 40 therein. A spring 96 in each passage 90 acts between the plug 92 and a shoulder of the pin 94. These springs 96 act to urge the pin carrier 80 and the jaws 32 apart so that the jaws 32 are urged towards the shoulder 31 of the collet ring 20 and the pin carrier 80 is urged away from the collet ring 20.

When the rollers 74 are all engaging the troughs of the surface 88 (as shown in Figure 6A), the pin carrier 80 is at its furthest from the collet ring 20 and the jaws 32 are urged by the springs 96 towards the flange 31 with only a small force. The jaws 32 are then in a release position thereof in which they are sufficiently far apart to allow free movement of the rod R through the collet assembly 10.

However, rotation of the lid member 54 through 60 degrees brings the rollers 74 into engagement with the crests of the surface 88. Because the plugs 92 are now nearer to the flange 31, the springs 96 apply greater force to the jaws 32 by means of the pins 94 and the jaws 32 are moved along the surface 28 so that they are brought into a contact position thereof in which they are in engagement with the rod R with sufficient frictional force that movement of the rod R through the collet assembly 10 causes movement of the jaws 32 relative to the surface 28. The jaws 32 do not, however, bite into the rod R in this condition. The collet assembly 10, thus, comprises pre-loading means operable to urge said jaws 32 into their contact position from their release position.This pre-loading means is releasable, by rotation of the lid member 54 to return the rollers 74 to contact with the troughts, to allow the jaws 132 to return to their release position by action of the springs in the rcesses 42. Rotation of the lid member 54 applies a predetermined force to the jaws 32 and rotation of the lid member 54 in the opposite sense removes this force.

The second collet assembly 12 of the illustrative tensioning apparatus is identical in construction to the first collet assembly 10 except for the collet ring 120 thereof. The parts of the second collet assembly 12 are not, therefore, described in detail but are given the same reference numerals as the equivalent parts of the first collet assembly 10 but with the addition of 100. Thus, the second collet assembly 12 has six jaws 132 which are identical to the jaws 32. The jaws 132 of the second collet assembly 12 provide gripping means operable to grip the rod R when the force applying means 14 operates to move the second collet assembly 12 away from the first collet assembly 10.

The collet ring 120 of the second collet assembly 12 differs from the collet ring 20 in that it lacks the annular surface 26 and has an extension 127 instead (see Figure 1). This extension 127 provides a cylinder 200 of the force applying means 14 of the illustrative tensioning apparatus.

The cylinder 200 is annular about the axis 23 and is formed between an inner cylindrical portion 202 of the extension 127 and an outer cylindrical portion 204 thereof.

The cylinder 200 is closed by an annular surface 206 at its end nearest to the surface 124 and extends axially to an open end thereof. The cylinder 200 partly contains an annular piston 208 which projects through the open end of the cylinder 200 and which is slideable axially in the cylinder 200 towards and away from the surface 206. The piston 208 engages the lid member 54 of the first collet assembly 10. The extension 127 contains a passage (not shown) by which hydraulic fluid can be introduced into the space between the piston 208 and the surface 206 to move the piston 208 relative to the cylinder 200 away from the surface 206. Seals 210 are provided in the outer surface of the inner cylindrical portion 202 and in the outer surface of the piston 208.The force applying means 14 of the illustrative tensioning apparatus, thus, comprises a piston and cylinder assembly 200, 208 comprising a cylinder 200 fixed relative to the second collet assembly 12 and a piston 208 which is arranged to engage the first collet assembly 10, specifically on the lid member 54 thereof.

The assembly 200, 208 is annul-ar about the rod R.

The operation of the illustrative apparatus to tension a rod R projecting from a fixed wall W will now be described. The method by which the illustrative apparatus is operated constitutes the illustrative method of tensioning a rod.

Firstly, in the illustrative method, the first collet assembly 12 is applied to the rod R while its rollers 74 are engaging troughs in the surface 88 so that the jaws 32 are in their release position sufficiently far apart to allow the rod R to pass freely between them. The collet assembly 10 is threaded on to the rod R until the surface 26 is brought into abutment with the wall W. Secondly, the jaws 32 of the first collet assembly 10 are pre-loaded so that they move from their release position into their contact position in which the jaws 32 contact the rod R for movement therewith. This is achieved by compressing the springs 96 by rotating the lid member 54 through 60 degrees to bring the rollers 74 into engagement with the crests of the surface 88. This moves the jaws 32 towards one another so that they engage the rod R lightly with their teeth 46.Because of the high friction between the rod R and the teeth 46 and the low friction between the wedging surface 28 of the collet ring 20 and the surfaces 34 of the jaws 32, the first collet assembly 10 will allow the rod R to pass there-through in a direction outwardly of the wall W but movement of the rod R in the opposite direction is resisted by the jaws 32 which are moved by the rod R along the surface 28 towards the flange 31 and grip the rod R with increasing force Thirdly, the second collet assembly 12 and the force applying means 14 are applied to the rod R while the rollers 174 of the second collet assembly 12 are engaging troughs in the surface 188 so that the jaws 132 are sufficiently far apart to allow the rod R to pass freely between them.The rod R then passes through the second collet assembly 12 which is downstream of the first collet assembly 10 in the direction in which the rod R is to be tensioned, i.e. away from the wall W. The piston 208 and the outer cylindrical portion 204 (which extends axially further than the inner cylindrical portion 202 which must clear the pin carrier 80) are brought into engagement with the support ring 22 of the first collet assembly 10. The piston 208 is then in close proximity to the surface 206.

Fourthly, the jaws 132 of the second collet assembly 12 are pre-loaded so that they move from their release position, in which they allow free movement of the rod R through the second collet assembly 12, into their contact position in which they contact the rod R for movement therewith. This is achieved by compressing the springs 196 of the second collet assembly 12 by rotating the lid member 154 through 60 degrees. This moves the jaws 132 towards one another so that they engage the rod R lightly with their teeth 146. This stage in the illustrative method is shown in Figure 2.

Fifthly, the second collet assembly 12 is moved away from the first collet assembly 10 while the first collet assembly 10 is pressed against the wall W. This is achieved by using a pump (not shown) to introduce hydraulic fluid under pressure into the space between the piston 208 and the surface 206. As the piston 208 is engaging the first collet assembly 10 which, in turn is engaging the wall W, the piston 208 cannot move so that the second collet assembly 12 and the cylinder 200 are moved away from the wall W. The first portion of this movement causes the wedging surface 128 to wedge the jaws 132 of the second collet assembly 12 towards one another so that they bit into and firmly grip the rod R (see Figure 3). After the jaws 132 have embedded their teeth 146 in the rod R, continued movement of the second collet assembly 12 causes the rod R to be stretched (see Figures 1 and 4).The first collet assembly 10 allows this movement. The rod R is, thus, tensioned.

Sixthly, the second collet assembly 12 is released thereby allowing the rod R to contract partially. This causes the jaws 32 of the first collet assembly 10 to be moved by the rod R towards the narrow end of the wedging surface 28 so that the jaws 32 are moved towards one another and bite into and grip the rod R against movement of the rod through the first collet assembly 10. Release of the second collet assembly 12 is achieved by connecting the space between the piston 208 and the surface 206 to an exhaust for the hydraulic fluid so that the cylinder 200 is de-pressurised. This removes the tensioning force from the rod R which contracts a little until this contraction is arrested by the first collet assembly 10. Thus, the first collet assembly 10 prevents further contraction of the rod R.

Seventhly, the second collet assembly 12 and the force applying means 14 are removed from the rod R leaving the rod R held in tension by the first collet assembly 10 (see Figure 5). This is achieved by rotating the lid member 154 of the second collet assembly 12 through 60 degrees to relieve the compression of the springs 196 of the second collet assembly 12. This removes the pre-loading on the jaws 132 and then the second collet assembly 12 can be moved towards the first collet assembly 10, with the piston 208 moving towards the surface 206, to bring the jaws 132 of the second collet assembly 12 to their release position.

If, subsequently, it is desired to remove the tension from the rod R, this can be achieved by rotating the lid member 54 of the first collet assembly 10 through 60 degrees to relieve the compression of the springs 96, so that the springs in the recesses 42 move the jaws 32 to their release positions.

It should be noted that the form of the teeth 46 and 146 is very advantageous. The height of the teeth allows them to penetrate the surface layer of the rod R and collect the reinforcing fibres. The flat tops of the teeth prevent them from breaking the fibres and the space between the teeth can be occupied by displaced portions of the plastics material of the rod R which is found to be pulverised.

It is found that the illustrative apparatus allows rods R to be tensioned quickly and effectively with minimal damage to the rods.

The invention can be applied to tensioning rods, pipes, and single or multi-strand wires where it is desired to tension an elongated member without substantially damaging the member.

Collets for use in the invention may have a single jaw member which is slit with the slit being closed as the jaw member grips the rod.

Claims (19)

1 Tensioning apparatus operable to tension a rod or the like, the tensioning apparatus comprising a first collet assembly, a second collet assembly, and force applying means operable to move the second collet assembly away from the first collet assembly, the first and the second collet assemblys being arranged to be assembled on the rod, the first collet assembly comprising abutment means whereby the apparatus can be braced against a fixed support, and gripping means operable to allow movement of the rod through the first collet assembly in a direction towards the second collet assembly but to grip the rod against movement upon movement of the rod through the first collet assembly in the opposite direction, the second collet assembly comprising gripping means operable to grip the rod when the force applying means operates to move the second collet assembly away from the first collet assembly.

2 Tensioning apparatus according to Claim 1, wherein the gripping means of the first collet assembly comprises a plurality of jaws distributed around the rod, each jaw being in engagement with a wedging surface which tapers inwardly in said opposite direction and is effective to cause said jaws to move between a contact position in which contact with the rod is effective to move the jaws along the wedging surface upon movement of the rod through the collet assembly, and a gripping position in which the jaws grip the rod against movement.

3 Tensioning apparatus according to Claim 2, wherein the first collet assembly comprises pre-loading means operable to urge said jaws into their contact position from a release position in which the jaws allow free movement of the rod through the collet assembly, the pre-loading means being releasable to allow said jaws to return to their release position.

4 Tensioning apparatus according to either one of Claims

2 and 3, wherein circumferential springs act between said jaws to urge the jaws apart.

5 Tensioning apparatus according to any one of Claims 2 to 4, wherein said jaws each have a plurality of teeth arranged to engage said rod.

6 Tensioning apparatus according to Claim 5, wherein said teeth have flat tops.

7 Tensioning apparatus according to either one of Claims

5 and 6, wherein said teeth are coated with a rough, hard, friction-enhancing particulate material.

8 Tensioning apparatus according to Claim 7, wherein said material is tungsten carbide.

9 Tensioning apparatus according to either one of Claims '7 or 8, wherein said particles are plasma-sprayed onto said teeth.

10 Tensioning apparatus according to any one of Claims 5 to 9, wherein said teeth have a depth of approximately imam.

11 Tensioning apparatus according to any one of Claims 2 to 9, wherein said collet assembly has a similar construction to said first collet assembly.

12 Tensioning apparatus according to any one of Claims 1 to 11, wherein said force applying means comprises a piston and cylinder assembly comprising a cylinder fixed relative to said second collet assembly and a piston arranged to engage said first collet assembly.

13 Tensioning apparatus according to Claim 12, wherein said piston and cylinder assembly is annular about said rod.

14 Tensioning apparatus substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

15 A method of tensioning a rod or the like comprising: applying a first collet assembly to the rod so that the rod passes through the collet assembly and the collet assembly abuts a fixed support associated with the rod; pre-loading jaws of the first collet assembly so that they move from a release position thereof in which they allow free movement of the rod through the collet assembly into a contact position thereof in which they contact the rod for movement therewith; applying a second collet assembly to the rod so that the rod passes through the second collet assembly and the second collet assembly is downstream of the first collet assembly in the direction in which the rod is to be tensioned;; pre-loading jaws of the second collet assembly so that they move from a release position thereof in which they allow free movement of the rod through the collet assembly into a contact position in which they contact the rod for movement therewith; moving said second collet assembly away from said first collet assembly while pressing the first collet assembly against said fixed support, thereby causing the jaws of the second collet assembly to grip said rod against movement of the rod through the second collet assembly so that the rod is stretched; releasing said second collet assembly thereby allowing the rod to contract partially, thereby causing the jaws of the first collet assembly to grip said rod against movement of the rod through the first collet assembly; and removing the second collet assembly from the rod leaving the rod held in tension by the first collet assembly.

16 A method according to Claim 15, wherein said second collet assembly is removed by removing the pre-loading on its jaws and then moving the second collet assembly towards the first collet assembly to-bring the jaws of the second collet assembly to their release position.

17 A method according to either one of Claims 15 and 16, wherein said second collet assembly is moved away from the first collet assembly by pressurising a piston and cylinder assembly mounted on the second collet assembly and acting against the first collet assembly or the fixed support and the second collet assembly is released byde-pressurising said assembly.

18 A method of tensioning a rod or the like substantially as hereinbefore described with reference to the accompanying drawings.

19 A method according to any one of Claim 15 to 18, wherein an apparatus according to any of Claims 1 to

14 is employed.