BACKGROUND OF THE INVENTION

This invention relates to a device and method for indicating the load in a fastener.

Two methods are widely used for measuring the load in fasteners. Both methods involve forming a hole in the centre of the fastener, fitting a gauge rod in the hole and anchoring the rod at the bottom of the hole.

The first type is generally as shown in principle Fig 3. The strain (the strain of a portion of a member being the elongation of the portion expressed as a fraction of the overall length of the portion) and hence load in the bolt is determined by using a dial gauge or similar to measure the movement of the unstressed centre rod relative to the stressed outer fastener when the load is applied to the fastener. This type is disclosed in US Patent No. 2,413,797.

The second type is shown in Fig 1, and it also has a central gauge pin. It is disclosed in US Patent No. 4,525,114. It can be seen that in this case the movement of the unstressed central rod, rather than being measured with a gauge, actually clamps a rotatabie washer when a given movement of the rod has occurred. The point at which the clamping of the washer occurs indicates that a preset strain has occurred in the body of the fastener and that the correct load in the fastener has been reached. In this case the initial washer gap is very accurately set during manufacturing so that the washer is gripped by the central rod when the correct load is applied to the fastener. Tightening is therefore stopped when the washer cannot be turned. Whether the washer can be turned is usually checked by simple finger pressure.

However both of these systems rely on the nut being in the correct position (the "correct" position being the one assumed when calculating what absolute elongation corresponds to what strain) relative to the end of the fastener. This is because for the strain, and hence load in the fastener to be known correctly, then the length of the portion of the fastener under strain must be known accurately. The above systems, and others relying on the strain measurement to find the load in the fastener, will be in error if the nut is not in the correct position as this changes the strained length of the fastener. This is because the measured length of bolt under strain is the distance between the fixing point of the rod at the bottom of the hole and the position of the nut. Hence a nut near the end of the fastener will result in a longer length of bolt being loaded and strained than if the nut were set further from the end of the fastener. Variation in nut position for a given measured elongation will lead to significant errors in the measurement of the strain and hence the load. In practice it is a difficult to accurately and reliably position the nut so this leads to serious errors in strain and load measurement. This application describes an invention which removes or mitigates the error in load measurement caused by this variable position of the nut .

SUMMARY OF THE INVENTION

In an embodiment of the invention described below, the need to accurately position the nut on the fastener in order to produce an accurate reading of load in the fastener is removed. This is achieved by fitting an annular tube into the centre of the fastener, co-axial with the gauge rod. One end of the tube is anchored in the fastener below the level of the nut face. The other end of the tube provides a datum projecting from the end of the fastener. This datum is not affected by the position of the nut when the fastener is loaded provided the nut is not screwed onto the fastener past the end of the tube anchor point in the fastener. The differential elongation between the end of the tube projecting from the fastener and the end of the central gauge pm now represents the elongation oetween the anchor point of the gauge pxn and the anchor point of the tube. Since these are now two fixed points the reading of the strain, and hence fastener load, is unaffected by the nut position.

Hence an accurate and reliable measurement of the strain and load in the fastener can be made and the installation of the fastener can be completed quicker as the exact positioning of the nut is no longer required.

The incorporation of the tube into the bolt also provides other benefits. As in FIG 4 it can be seen that the tube provides a sound anchoring point to fit and retain the gauge in addition to providing the fixed datum face. Also as in FIG 2 it can be seen that the tube provides a location for a seal to function with a cover fitting over and protecting the complete washer mechanism.

According to a first aspect of the invention there is provided a fastener which includes at one end of its body a threaded portion for receiving a nut, the fastener having a bore extending into the fastener from said one end, an annular tube extending into the bore coaxial with the threaded portion and ancnored to the fastener at a first axial position within the bore, and a rod extending into the bore inside the annular tube and anchored to the fastener at a second axial position within the bore, the second axial position being further from said one end of the fastener body than the first axial position.

The change m spacing between the end of the annular tube opposite the anchored end and the end of the rod opposite the anchored end as the fastener is loaded provides an indication of the load m the fastener. That change in spacing can be unaffected by the position of a nut on the threaded portion of the fastener.

In order to facilitate measurement, the annular tube and the rod preferably project outwardly from their anchored ends to positions in the region of said one end of the fastener body. The annular tube more preferably projects beyond said one end of the fastener body. The rod may also project beyond the end of the fastener body or it may be recessed within the fastener body. End faces on the outer ends of the annular tube and/or of the rod may provide datum faces for measuring the relative axial positions of those parts. While it is preferred for simplicity that the annular tube and the rod are completely fixed by their anchorings, it will be understood that they may be rotatably mounted but axially fixed, if desired.

The fastener may be a bolt having a head at the end opposite the threaded end portion referred to above _r but it is also possible for the fastener to have threaded end portions at both ends, or the threaded portion may extend along the entire length of the fastener; in such cases nuts may be provided at each end of the fastener. The fastener may also have many other shapes: as will be understood, the threaded portion of the fastener may be a projecting portion of a body of any of a wide variety of shapes.

Preferably there is a clearance between the exterior of the annular tube and the wall of the bore along at least a major part of the length of the tube. That assists in ensuring that there is no resistance to relative axial movement between the fastener body and the tube as the fastener is loaded.

The bore, which may be a blind bore, may be of variable cross-section along its length. In embodiments of the invention described below the bore is of a relatively wide circular cross-section at an outer end and a relatively narrow cross-section at an inner end.

The rod may be of circular cross-section.

In one preferred embodiment of the invention, a rotatable element is mounted on the rod and is positioned between the outer end of the annular tube and a head of the rod. In that case the parts may be arranged such that, when a predetermined load in the fastener is reached, the rotatable element becomes sandwiched between the outer end of the annular tube and the head of the rod and ceases to be rotatable. An alternative is simply to provide two elements (which may be the rod and the tube) between which there is a gap which becomes closed when the predetermined load is reached. Other methods of indicating a predetermined displacement may also be adopted.

Preferably the first axial position is further from said one end of the fastener body than the threaded portion of the fastener. In order to obtain the full benefit of the invention the first axial position should be further from said one end than the threaded engagement of a nut on the threaded portion of the fastener and that is ensured by this feature.

The fastener may further include a nut mounted on the threaded portion of the fastener. Preferably the first axial position is further from said one end of the fastener body than the threadingly engaging portions of the nut and the fastener.

The fastener may further include a cap for fitting over at least an outer end of the rod. The cap may also be arranged to be fitted over the annular tube.

The spacing of the first axial position from the second axial position is preferably between 50 per cent and 300 per cent of the diameter of the fastener. More preferably the spacing of the first axial position from the second axial position is between 80 per cent and 250 per cent of the diameter of the fastener. Yet more preferably the spacing of the first axial position from the second axial position is between 100 per cent and 200 per cent of the diameter of the fastener.

The spacing of the first axial position from the second axial position is preferably more than ten per cent of the axial length of the fastener. More preferably the spacing is more than twenty per cent of the axial length of the fastener. In embodiments of the invention described below, the spacing is of the order of one half of the axial length of the fastener.

In the first aspect of the invention reference is made to a first member being an annular member, to the annular member being coaxial with the threaded portion, to a second member being a rod, and to the second axial position being further from said one end of the fastener body than the first axial position. Each of those features are preferred features which may not apply to the broadest aspect of the invention in which there is provided a fastener which includes at one end of its body a threaded portion for receiving a nut, the fastener having a bore extending into the fastener from said one end, a first member extending into the bore coaxial with the threaded portion and anchored to the fastener at a first axial position within the bore, and a second member extending into the bore and anchored to the fastener at a second axial position within the bore, the first and second axial positions being spaced from said one end of the fastener body by different distances .

According to the invention there is provided a method of indicating the load in a fastener, the method comprising the following steps: providing a fastener which includes at one end a threaded portion for receiving a nut, the fastener having a bore extending into the fastener from said one end, an annular tube extending into the bore coaxial with the threaded portion and anchored to the fastener at a first axial position within the bore, and a rod extending into the bore inside the annular tube and anchored to the fastener at a second axial position within the bore, the second axial position being further from said one end of the fastener body than the first axial position, screwing a nut onto the threaded portion of the fastener to generate a load in the fastener, the end of the annular tube opposite the anchored end and the end of the rod opposite the anchored end being displaced relative to one another as the fastener is loaded, to indicate the load in the fastener.

The method of the invention may be carried out using any of the fasteners described above. The step of providing a fastener may be adapted accordingly.

A predetermined displacement between the end of the annular tube and the end of the rod may be detected to indicate a given load in the fastener. Such detection may be achieved using a rotatable element as described above.

Alternatively the relative displacement between the end of the annular tube opposite the anchored end and the end of the rod opposite the anchored end may be measured to indicate the load in the fastener. The relative displacement may be measured using a dial gauge.

It will be appreciated that the fastener and method of the invention as described herein are closely related and that therefore essential or preferred features of one may, unless indicated otherwise or clearly inappropriate, be incorporated into the other. Thus, features described above in respect of the fastener of the invention may be incorporated into the method of the invention and vice versa .

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example certain embodiments of the invention and certain known designs are described with reference to the accompanying drawings, of which:

FIG 1 is a longitudinal section through an existing fastener design with a central gauge pin which indicates a correct load when a washer locks. FIG 2 is a longitudinal section through a first embodiment of the invention which has a tube co-axial with the gauge pin. FIG 3 is a longitudinal section through an existing fastener design with a central gauge pin which uses a dial gauge to measure the fastener elongation . FIG 4 is a longitudinal section through a second embodiment of the invention which has a co- axial tube and is shown with a dial gauge fitted. FIG 5 is a longitudinal section though the second embodiment of the invention showing the gauge removed and a protective cap in place. FIG 6 is a longitudinal section through a third embodiment of the invention which is a modified version of the first embodiment of the invention .

DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

First embodiment of the invention is shown in FIG 2. A screwed fastener 21 has a formed opening (bore) at one end 22 into which is fitted a gauge pin 23 and a co-axial tube 24. The gauge pin is anchored 28 at the bottom of the hole, typically by a screw thread. The tube 24 Is anchored at the bottom of a wider portion of the bore at point 29 typically by an interference fit or adhesive. Note the tube is in a clearance hole for the majority of its length, so that it is free to move axially relative to the fastener 21. The tube protrudes slightly from the end of the fastener providing an abutment for a washer 25 to bear against. The washer 25 is held captive between the projecting end of the tube 24 and a head 26 of the gauge pin 23. When the fastener is in an unloaded condition the washer Is free to rotate in an accurately set gap 27 between the end of the tube 24 and the head 26 of the pin 23. When axial load is applied to the fastener by tightening the nut 31 {the nut bearing against a member being fastened by the fastener but not shown in the drawings), the head 26 of the gauge pin 23 is compelled to move towards the end of the tube 24 so gradually removing the gap 27 in which the washer 25 was originally free to rotate. At a certain predetermined design load the gap 27 disappears altogether and the washer is clamped between the head 26 of the pin 23 and the end of the tube 24. When the washer 25 becomes clamped, which is typically detected by finger pressure checking if the washer will rotate, the load in the fastener 21 is known because the strain is known. This is because when the washer locks the known length of fastener between the anchor point of the gauge pin and the anchor point of the tube has elongated a distance equal to the original washer gap. This provides a direct measure of the strain in the fastener and hence the load. It is a key feature that, provided the loaded face of the nut 31 does not pass the anchor point of the tube 29, the position of the nut on the fastener does not affect the accuracy of locking of the washer and hence the load measured in the fastener

The complete mechanism may be protected from damage by a cap 30 fitted over the washer with a seal 31 around the tube 24.

It is noted that as shown in Fig. 6 a variation on this embodiment is possible where the washer 25 and the tube 24 may be formed as one component and become item 60 as in Fig 6. The complete item 60 is then effectively clamped between the bottom of the portion of the wider hole 22 and the head 26 of the gauge pin when the gap 61 is removed, which occurs when the correct load is reached in the fastener.

In a second embodiment shown in FIG 4 a co-axial tube 42 is anchored at its end 43 in the fastener and its other end provides an external datum face 44. The tube as in the first embodiment, though anchored at its end 43 in the body of the fastener, has small clearance along it sides in the fastener bore. A gauge pin 45 is anchored at one end 46 in the fastener typically by a screw thread. The other end of the gauge pin 45 now provides a moving datum face 47 to press against an indicator rod 48 of a dial gauge 49. The body of the dial gauge is held firmly against the datum face 44 on the tube 42 either by hand pressure by the operator or it may be held onto the end of the tube mechanically by a screw thread formed around the projecting end of the tube 42. Alternatively a retaining clip or other device may be used to ensure the dial gauge remains in close contiguous contact with the end of the tube 42.

The reading from the gauge will be unaffected by the position of the nut 50 provided the nut does not pass the anchor point of the tube 43.

When measurement is complete and the gauge is removed, see FIG 5, it is a feature of this invention that the end face 47 of the gauge pin 45 is protected from damage within the tube 42 and the end of the tube 44 may itself be protected by a cap 51. Typically the protective cap 51 screws into place using the same thread used to retain the dial gauge in position. Alternatively it may fitted by a simple push fit, clip or other device to retain it to the tube .

Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims.