GB2193549A

GB2193549A - Bolt tensioner

Info

Publication number: GB2193549A
Application number: GB08619415A
Authority: GB
Inventors: David John Hughes
Original assignee: Hydra Tight Ltd
Current assignee: Hydra Tight Ltd
Priority date: 1986-08-08
Filing date: 1986-08-08
Publication date: 1988-02-10
Anticipated expiration: 2006-08-08
Also published as: GB8619415D0; GB2193549B

Abstract

The puller used in conjunction with a hydraulic bolt tensioning tool is constituted by a split nut (3) comprising a screw-threaded member divided into two or more portions (4,5) by a plane through and parallel to the axis of the bolt to be tensioned. The portions may be hinged together. Fluid under pressure raises piston head (1) to act against end faces (6,7) of the split nut (3) to generate radially inward forces in the two portions (4,5) whose threads are urged into firm engagement with the bolt threads. <IMAGE>

Description

SPECIFICATION Bolt tensioning apparatus This invention relates to the hydraulic tensioning of bolts and the like. It is well known to use hydraulic means to develop controlled stress in a bolt, usually just prior to using a threaded connector such as a nut to retain at least most of that stress on subsequent removal of the hydraulic means.

Typical hydraulic means is for example illustrated in Patent GB , which shows a hydraulic bolt tensioner. The latter includes a puller which is screwed down over the free end of the bolt to engage with the top of a piston.

Pressure applied to the underside of the piston is thus transmitted directly to the bolt to develop axial stress therein. Inside the bridge piece of the tensioner and accessible from outside through an access slot is a nut which is located on the bolt below the puller. This is the retaining nut which is to be run down against the associated hardware after the axial stress has been developed in the bolt.

It will be understood from the above brief description that conventional bolt tensioning involves use of at least two screw-threaded elements on the bolt to be tensioned, regardiess of whether the retaining nut is to be tightened or released. Unfortunately, it is not always easy to use such conventional techniques, because the bolt threads are not necessarily free from rust, especially after use in damp or wet environments. Underwater bolted connections are especially vulnerable, also on account of the physical difficulty of handling and installing the bolt tensioner.

Many such connections will require a plurality of tensioners to be used to simultaneousiy stress all or at least most of the bolts.

Thus it has been proposed to use bolt tensioners which incorporate radially-displacea- ble grippers which are operable to seize the bolt threads. Such grippers are mechanically complex and difficult to protect from water.

They also greatly add to the buik and weight of the tensioner. Furthermore, there is still the problem of the retaining nut itself. A further consideration is the extremely high stress developed, because any replacement for the conventional screw threaded puller and retaining nut must withstand such stress, reliably and without risk of failure.

Accqrding to the present invention, the puller of a hydraulic bolt tensioner is constituted by a split nut, as hereinafter defined.

Split.nut for present purposes means an internally screw-threaded nut or like member which is divided into at least two portions in a plane extending through and parallel to the axis of the bolt to be tensioned, one axiallydirected face of said nut bering provided with a cam surface engageable with a complementary recess in the piston of the tensioner with which the nut is to be used, the engagement being such as to urge said nut portions towards one another and into engagement with the threads of said bolt.

The cam surface is preferably of a generally convex or domed shape; it may also be a conic section. Surprisingly, it has been found that even a relatively shallow cam surface and complementary shallow recess are adequate to generate sufficient radial force against the bolt threads as to ensure that the split nut remains in place, even under the extremely high axial bolt loads commonly used in bolt tensioning.

The portions of the split nut may be loose, or hinged together, the latter being particularly convenient for use underwater by a diver because it eliminates the risk of one portion being accidentally dropped and thereby lost. Because the split nut need not be run down the length of the free end of a bolt or stud, it can be kept clipped onto a safety line, as necessary or appropriate to the task in hand.

In use, the tensioner is fitted over the bolt/ stud in the usual way, with the retaining nut enclosed inside/under the tensioner bridge piece. The split nut portions are simply clamped around the free end of the bolt, with the cam surface engaged in the complementary recess in the tensioner piston top. It has been found that even if the bolt is rusty, the subsequent energisation of the tensioner will cause the split nut portions to bite into the rusty threads, gripping the bolt at least as firmly as a conventional nut would do. In extreme cases, the nut can be used to cut its own thread on the bolt-provided that the material of the nut portions is sufficiently hard, of course.

This feature may be particularly useful where the objective is to remove or loosen an old, badly corroded bolt down which it would be impossible to screw a conventional nut/puller without completely re-cutting the threads.

The retaining nut used in association with the hydraulic tensioner and puller of this invention can be a conventional nut, although it is normal to use a nut with a set of tommy bar holes in the flats. A ring nut with a number of radially-drirected tommy bar holes can also be used. Nuts modified in this way are preferred because they are easier to tighten or loosen through the limited access slot in the tensioner bridge piece. However, it is also possible to use a split nut (as defined earlier) as a retaining nut, provided that the associated hardware such as a flanged connection also features a complementary recess. Previous comments about ease of fitting a split nut to a bolt, or of subsequently removing it from a rusted bolt apply equally to split retaining nuts as well.It is especially useful to be able to simply stress a rusted bolt enough to relieve the stress (or most of it) from the retaining nut and to than just knock the latter loose from the threads without having to wind it back-by hand-against the rust, before removing the tensioner. In such circumstances, a split nut can be simply lifted away; a conventional retaining nut would have to be removed by unscrewing it down the full length of the free end of the bolt.

In order that the invention be better understood, one embodiment of it will now be described by way of example with reference to the accompanying drawing in which the sole figure is cross-sectional side view of a hydraulic bolt tensioner of the kind shown in GB-A1590131.

The actual construction/operation of this tensioner is fully described in this latter patent, to which attention is hereby directed.

However, it is to be noted that in the present case- the piston head 1 does not react against the conventional screw threaded puller shown in the above patent. Instead, the piston head has a concave (annular) recess 2. A split nut 3 comprising two identical screw-threaded portions 4 and- 5 having matching convex, domed end face portions 6 and 7 is shown engaged with threads of the bolt 9, with the convex portions engaged in the recess 2. It will be appreciated that the nut 3 is split in the plane of the paper, so that the face show in section at 4 is in fact the inner face of the one half of the split nut whilst the external flat 5 is the outer face of the opposed half of the split nut.

On energising the tensioner, by supplying hydraulic fluid under pressure via connector 8 the piston head 1 will move upwardly, causing the annular recess 2 to act as a cam surface against the convex end faces, 6, 7 of the split nut to generate radially-inwardly directed forces in the two nut portions 4 and 5. The threads of the latter are thereby urged into firm engagement with the bolt threads, transmitting the full force of the piston to the bolt 9 so as to stress it. It will be noted that the cam action, plus the interengagement of the threads is all that holds the split nut in place on top of the energised tensioner.

As an illustration qf the effectiveness of the cam action, an approximately 7.5 cm diameter bolt.was stressed to about 220 tonnes, without using anything other than a two-part split nut as a puller, the two halves of the nut being held in place on the bolt by the cam action alone. No external clamp was needed around the nut halves.

Claims

1. A hydraulic bolt tensioner, characterised in that 'the puller thereof: is constituted by a split nut, as defined herein.

2. The tensioner of Claim 1 characterised in that the puller comprises an internally screw-threaded nut divided into at least two portions in e plane extending through and parallel to the axis of the bolt to be tensioned, one axially-directed face of said nut being pro vided with a cam surface engageable with a complementary recess in the piston of the tensioner with which the nut is to be used, the engagement being such as to urge said nut portions towards one another and into en gagement with the threads of said bolt.

3. The tensioner of claim 1 or claim 2 in that the cam surface is of a generally convex or domed shape.

4. The tenisoner of Claim 1 or Claim 2 charaterised by a conic section.

5. A method of using a bolt tensioner in accordance with any of Claim 1-4, character ised by the steps of fitting a bolt tensioner over a bolt or stud to be tensioned, applying the portions of a correspondingly threaded split nut to said bolt, with the cam surface of said nut in engagement with the complemen tary recess of said tensioner, followed by en ergising the tensioner by supplying hydraulic fluid thereto to cause cam action between said nut and tensioner, whereby the nut por tions grip said bolt and progressively stress it.