GB2566575A

GB2566575A - Load indicating fastener

Info

Publication number: GB2566575A
Application number: GB1809310.4A
Authority: GB
Inventors: Hirst John; Metzger David
Original assignee: Pronut Ltd
Current assignee: Pronut Ltd
Priority date: 2017-10-18
Filing date: 2018-06-06
Publication date: 2019-03-20
Anticipated expiration: 2038-06-06
Also published as: GB201717079D0; WO2019077314A1; GB2566575B; GB201809310D0

Abstract

A female fastener 100 for indicating the preload in a threaded connection between male and female fasteners comprises a threaded portion 102 with a first end face 110, the threaded portion 102 further comprising a thread 108 for engaging with a threaded male fastener. An elongate pin 116 extends through the threaded portion 102 and controls a load indicator 122 located at the first end face 110. A set level of compression of the threaded portion 102 relative to the elongate pin 116 moves the load indicator 122 from a first state to a second state.

Description

Load Indicating Fastener

Field of the Invention

The present invention concerns a female fastener for indicating the preload in a threaded joint, such as a connection between male and female fasteners. More particularly, but not exclusively, this invention concerns a female fastener able to indicate when the fastener has been tightened so as to apply a threshold preload.

Background of the Invention

A threaded joint is formed by a female fastener having an internal thread engaging with a male fastener having an external thread. An example of a female fastener is a nut, and an example of a male fastener is a bolt.

In order to produce a reliable threaded joint it is important for the fasteners to be tightened to the correct preload. The preload is the tension force (or compression force) created in the fasteners when they are tightened. Typically the preload is equal and opposite to the clamping force applied by the fasteners. If the fasteners are not tightened to the correct preload it can lead to the joint coming undone or failing.

Some prior art methods of tightening fasteners to the correct preload involve the use of torque wrenches or hydraulic tensioners. There are however disadvantages to the use of such devices. Torque wrenches may fail to apply the correct preload because of the load variation caused by variable friction on the threads. Hydraulic tensioners may fail to apply the correct preload because

- 2 of the unknown preload loss as the tensioner is removed and the preload transfers to the nut.

US4525114 discloses a device which overcomes the above mentioned problems by incorporating a load indicator directly into a bolt. The device relies on the bolt being stretched when a preload is applied to the joint. When a threshold preload is applied, and the bolt is stretched a certain amount, a rotatable indicator element is clamped in place. The device may only correctly indicate the applied preload if the corresponding nut is tightened at the correct position along the bolt. Therefore the bolt typically needs to be tailored to the specific dimensions of the joint it will be used with in order to provide a correct indication of the preload. Thus these bolts are in general made to special order for a specific application.

One way around the above mentioned problem is to instead use the compression of the female fastener to indicate the preload in the joint. Various attempts have been made (for example see GB1370792) to devise a way to measure the elastic compressive load in a nut. However in general these methods have been of limited commercial or practical use. A difficulty is that to find the compressive load in a nut it is necessary to measure the elastic compression of the nut, which is typically less than 0.020 microns, and measuring such a small distance to an accuracy of even just +/- 10% requires precision eguipment which is typically impractical to use in the field. In an attempt to circumvent these problems, some devices employ strain magnifying mechanisms. Typically these mechanisms use a mixture of springs and levers in some format which introduce new sources of inaccuracy, weakness and/or unreliability.

- 3 US2830486A discloses a female load indicating device having a flexible body comprising an upper part and a lower part separated by a slot. When a threshold preload is applied to the device, the upper part flexes towards the lower part. In an embodiment, the device comprises an indicator pin which is fixed to the lower part, but moves freely through a bore in the upper part. The indicator pin has a length such that, when a threshold load is applied, it protrudes above the upper surface of the upper part. The device of US2830486A is disadvantageous as the slot introduces an area of weakness in the fastener. Also the indicator pin requires close visual inspection to confirm the correct preload has been applied.

WO2014106752 discloses an example of a female load indicating device in the form of a nut. The device has an adjustment screw which extends through the body parallel to the axis of compressed due screw pushes a the threaded bore. When the body is to an applied preload, the adjustment spring clip which in turn pushes a gauge pin out of the side of the body in a direction orthogonal to the axis of the bore. The gauge pin can operate a load indicating device which can detect the displacement of the pin. A disadvantage of the device of WO2014106752 is that the load indicator is located at the side of the nut and therefore the load indicator may be difficult to view or use whilst the nut is being tightened; for example because the gauge workpiece and the pin could be disposed between the tightening apparatus, and the tightening apparatus may need to be removed before the gauge pin can be viewed or accessed. Further, the device of WO2014106752 comprises several moving parts which all must be assembled within the nut, thus adding to the difficulty of manufacture. Having more moving parts can

- 4 also degrade the accuracy of the device, particular over time if the parts are subject to movement and wear.

The present invention seeks to mitigate the abovementioned problems. Alternatively or additionally, the present invention seeks to provide an improved female fastener for indicating the preload in a threaded joint.

Summary of the Invention

The present invention provides, according to a first aspect, a female fastener for receiving and engaging with a threaded male fastener, the female fastener comprising: a threaded portion with a first end face, the threaded portion further comprising a thread for engaging with the threaded male fastener;

a second end face; and an elongate pin extending through the threaded portion, the elongate pin controlling a load indicator located at the first end face, whereby a set level of compression of the threaded portion relative to the elongate pin moves the load indicator from a first state to a second state.

The female fastener according to the present invention may provide a more convenient and accurate way of obtaining an indication of the preload applied to a threaded joint. In particular, providing the load indicator at an end face of the female fastener may make the load indicator more easily accessible during tightening of the female fastener. Therefore during certain situations it may advantageously be made easier, or be made possible, to obtain an indication of the applied preload whilst the female fastener is being tightened. During certain situations it may also be made easier, or be made possible, to obtain an indication of

- 5 the applied preload without having to remove the tightening apparatus. A notable advantage over the use of, for example, torque wrenches, is that the present invention may allow the preload in the female fastener to be checked a significant period of time after it is first tightened without the need to use specialist equipment.

Further, with the load indicator provided at the first end face, the load indicator may stay oriented in generally the same direction whilst the female fastener is tightened. This may allow for the preload indication to be obtained from an approximately consistent position during tightening. With a female fastener where the load indicator is positioned on the side, the orientation of the load indicator may change significantly if the female fastener needs to be rotated during tightening. In this case the final orientation of the load indicator may depend on how tightly the female fastener is fastened and the orientation of threads of the female and male fastener.

Providing a load indicator which changes state when a set preload is reached may advantageously provide a more recognisable indication that a certain the preload is reached and/or exceeded. For instance the provision of such a load indicator may remove the need to rely on a visual indication of the position of the elongate pin itself in order to obtain an indication of the preload.

It will be understood that the thread of the threaded portion is an internal thread. The thread being provided on an internal surface of a bore extending through the female fastener. It will be understood that the thread of the female fastener is for threadedly engaging with the thread of the male fastener through mating of their respective threads.

- 6 It will also be understood that, in use, the female fastener may be tightened by rotating the female fastener relative to the male fastener so as to increase the preload in the joint. In the following, references to tightening of the female fastener may refer to any such relative movement of the female fastener and male fastener which effect tightening, including, for example, holding the male fastener still and rotating the female fastener, or holding the female fastener still and rotating the male fastener.

In some applications the nut may be tightened using a tensioner, for example a hydraulic tensioner. A preload may be applied to the male fastener, and the female fastener may be turned in a direction that would tighten it until it abuts the object to be joined. The hydraulic tensioner may then be released so as to transfer the load to the female fastener. The load applied using the hydraulic tensioner can then be checked using the load indicator belonging to the female fastener.

The female fastener may be in the form of a nut, for example a hex nut, a bi-hex nut, a square nut, a wing nut, a nut comprising tommy-bar holes, and other such nuts. The threaded portion may comprise a hexagonal outer surface .

The present invention may have particular application on large diameter female fasteners, for example female fasteners having an internal diameter of 36mm or larger, for example having a size of M36 or larger. In some cases the female fastener may have an internal diameter of up to about 200mm or 250mm, for example having a size of M200 or M250. The female fastener may have an internal diameter of 64mm or smaller, for example have a size of M64 or smaller.

- 7 The female fastener may further comprise a flange portion. The flange portion may provide the second end face. The flange portion may comprise a bore (which may be an extension of the bore running through the threaded portion) for receiving, but not engaging with, the threaded male fastener. The elongate pin may also extend through the flange portion. A set level of compression of the threaded portion and the flange portion relative to the elongate pin may move the load indicator from the first state to the second state.

The flange portion may increase the amount that the female fastener is compressed at a particular preload (as compared to a fastener which comprises only the threaded portion). The presence of the flange portion may thus increase how easily and/or accurately the compression of the female fastener can be detected and/or measured. The flange portion may thus provide an easier, or more accurate, way to obtain an indication of the preload in the joint from the female fastener.

The distance by which the female fastener is compressed (the compression of the female fastener) may be taken as the change in distance between the first end face and the second end face as measured in a direction parallel to the longitudinal axis of the bore.

The flange portion may be in the form of an annular ring. The flange portion may be integrally formed with the threaded portion. The flange portion may be a unitary part completely integrally formed with the threaded portion. Alternatively, the flange portion may comprise a first part integrally formed with the threaded portion, and a second part movable relative to the first part, the interfacing surface between the first and second parts being curved. The second part moving relative to the first part may allow for greater misalignment of the

- 8 female fastener and male fastener when attempting to engage the female fastener with the male fastener.

The flange portion may be unthreaded. The flange portion may be configured such that a male fastener that would threadedly/threadably engage with the internal thread of the threaded portion, can pass freely past the flange portion.

As the flange portion does not engage with the male fastener, a particular lateral slice of the flange portion may move freely past the threads of the male fastener. The flange portion may be arranged such that none of the compressive load experienced by the flange portion is transferred directly to the male fastener. Therefore the flange portion may be, in use, compressed by the entire compressive load applied between the threaded portion of the female fastener and the male fastener.

The flange portion may extend beyond the threaded portion in a direction parallel to the longitudinal axis of the bore. The flange portion may add to the length of the female fastener (as measured in a direction parallel to the longitudinal axis of the bore) as compared to a fastener which comprises only the threaded portion. The flange portion and the threaded portion may together form a body, the body comprising both the threaded portion and the flange portion.

The flange portion may have a length (as measured in a direction parallel to the axis of the bore), the length may be approximately equal to the diameter of the threaded male fastener to which the female fastener is to receive and engage.

The flange portion may comprise one or more apertures, for example three apertures. The apertures may

- 9 be equispaced around the circumference of the flange portion.

The apertures may increase the distance by which the female fastener is compressed at a particular preload. This may be because the compressive load is being transferred through less material (the material between the apertures) than would be the case with a solid flange portion. Therefore the compressive load per unit area (the compressive stress) is higher between the apertures.

The further the female fastener is compressed at a particular preload, the easier and/or more accurate it may be to detect and/or measure the compression of the female fastener, and thus the easier and/or more accurate it may be to obtain an indication of the preload in the joint.

The first end face of the female fastener may, in use, be the face which faces away from the joint. The second end face of the female fastener may, in use, be the face through which the clamping force of the female fastener is imparted. The second end face may, in use, abut an object to be joined. The second end face may be referred to as the pressure face.

The second end face may be flat. Alternatively, the second end face may comprise one or more raised areas, for example three raised areas, arranged to abut an object to be joined. The raise areas may be regularly distributed around the second end face, i.e. distributed at regular intervals around the bore. The three raised areas may be arranged in a tripod configuration. The use of three raised areas may be particularly beneficial when second end face as abutted against an uneven surface as the three raised areas may assist in transferring the load more evenly through the nut. The raised areas may

- 10 form a series of castellations. The second end face may comprise a castellated surface.

The raised areas may increase the distance by which the female fastener is compressed at a particular preload. This may be because the compressive load is being transferred through less material (the raised areas) than would be the case with a solid end face. Therefore the compressive load per unit area (the compressive stress) is higher in the raised areas.

By way of example, the compression of a standard M36 nut under a typical load is typically 18 microns. By including a flange portion in the form of an integral annular ring having a second end comprising raised areas, the compression of the nut can be increased to typically 50 microns at the same load. This increase in compression may help make it possible to measure the nut compression directly by multiple methods, described later in this application, to a sufficient degree of accuracy to make practical industrial application of the invention possible .

The internal thread of the threaded portion may be tapered towards the first end face (i.e. the internal diameter of the bore is smallest at the first end face and increases in diameter towards the second end face). The tapered thread may advantageously make it easier to remove the female fastener from the male fastener. The tapered thread may also allow for greater misalignment when attempting to engage the female fastener with the

- 11 male fastener, without adding bending stress to the female fastener. Another benefit of the taper thread is that it may have the effect of moving the centre of load towards the first end face of the female fastener. This may result in more nut compression for a given applied preload.

The elongate pin may extend through an opening in the threaded portion and/or flange portion. The elongate pin may extend in a direction parallel to the longitudinal axis of the bore. A first end of the elongate pin may be fixedly attached to the threaded portion and/or flange portion. The first end may be fixedly attached to the threaded portion and/or flange portion by, for example, an interference fit, a screw thread attachment, welding and/or by adhesive. A second end of the elongate pin may be movable relative to the threaded portion and/or flange portion.

The second end of the elongate pin may extend beyond the first end face of the female fastener. The second end of the elongate pin may extend beyond the first end face even when there is no preload applied to the joint (i.e. when there is no compression of the threaded portion and/or flange portion). The second end of the elongate pin may extend further beyond the first end face when a preload is applied to the joint (i.e. when there is compression of the threaded portion and/or flange portion).

The elongate pin may extend from the first end face of the female fastener to, or close to, the second end face of the female fastener. The further the elongate pin extends through the female fastener, the greater the change in the relative positions of the second end of the elongate pin and the first end face of the female fastener. Therefore the further the elongate pin extends

- 12 through the female fastener, the easier it may be to detect and/or measure the compression of the female fastener.

The material of the elongate pin may have the same thermal expansion characteristics as the material of the threaded portion and/or flange portion. This may advantageously mean that should the female fastener expand with heat, the elongate pin and the threaded portion and/or flange portion expand and contract at substantially the same rate such that a correctly indication of preload is maintained.

The load indicator may be an indicator element, wherein in the first state the indicator element is not movable, and in the second state the indicator element is movable. The indicator element may be a rotatable body, wherein in the first state the rotatable body is not rotatable, and in the second state the rotatable body is rotatable. The rotatable element may be rotatable around the elongate pin. The rotatable body may, for example, be a washer.

Whether the rotatable body can be rotated or not may thus provide a tactile indication of whether the set level of compression, and thus the threshold level of applied preload, has been reached.

The elongate pin may have a clamping element thereon. The clamping element may be arranged to clamp the indicator element below the set level of compression. The clamping element may be arranged such that the indicator element is released when the set level of compression is reached. The indicator element may be considered to be released when the clamping force on the indicator element is negligible or zero, such that the indicator element may be free to move. The indicator element may be considered to be released when the

- 13 clamping force on the indicator element is reduced to an extent that the indicator element can be moved under finger pressure. The clamping element may be provided at the second end of the elongate pin. The clamping element may be in the form of a projection extending laterally from the elongate pin. The laterally extending projection may be the head of the pin. The indicator element may be clamped between the first end face and the projection below the set level of compression. The indicator element may be released because, as the preload is applied the threaded portion and/or flange portion are compressed, the first end face moves away from the clamping element on the elongate pin.

The aforementioned elongate pin may be a first elongate pin, and the aforementioned load indicator may be a first load indicator, and the aforementioned level of compression may be a first level of compression. The female fastener may further comprise a second elongate pin. The second elongate pin may control a second load indicator, whereby a second level of compression of the threaded portion and flange portion relative to the second elongate pin moves the second load indicator from a first state to a second state.

The second level of compression may be different to the first level of compression. The first load indicator and second load indicator may be used together to indicate a minimum and maximum acceptable preloads in a joint.

The second elongate pin and/or second load indicator may be take the same form as the first elongate pin and/or first load indicator (except that the second load indicator is configured to change state at a different level of compression to the first load indicator). By way of example, the first and second load indicators could

- 14 both be in the form of rotatable bodies arranged around respective first and second elongate pins, wherein the elongate pins are arranged such that the rotatable bodies are released so as to be rotatable at different levels of compression (which correspond to different threshold preloads).

Further elongate pins and load indicators may be provided. The female fastener may comprise three or more load indicators. Each load indicator may be configured so as to move from the first state to the second state at different predetermined preload. The load indicators could change state at regularly spaced intervals of load. For example, there could be a first, a second and a third load indicator, the load indicators being configured to change state at 280kN, 300kN and 320kN of applied preload, respectively. Each load indicator may be provided with a different colour and/or be provided with a numbered indication to assist a user in identifying the applied preload.

The load indicator may be a measurement gauge. The measurement gauge may comprise a linear voltage displacement transducer. The measurement gauge may provide a measurement of position and/or displacement. The measurement gauge may be arranged to measure the relative position of an end of the elongate pin relative to the first end face. In the first state the end of the elongate pin may be in a first position (relative to the first end face), and in the second state the elongate pin may be in a second position (relative to the first end face) .

The measurement gauge may provide a distance measurement in dependence on the displacement of the elongate pin relative to the first end face. The measurement gauge may provide an actual measurement of

- 15 the distance between the end of the elongate pin and the first end face. The measurement gauge may provide a measurement of the change in the distance between the end of the elongate pin and the first end face when a preload is applied, compared to when there is no applied preload. In other words, the measurement gauge may be calibrated such that the reading at zero applied preload corresponds to a distance reading of zero. The measurement gauge may provide a measurement of the compressive strain of the female fastener (i.e. the compression of the female fastener divided by the length at zero applied preload).

Alternatively or additionally, the measurement gauge may provide a reading of the preload applied to the joint. The measurement gauge may be configured to calculate the applied preload for a given distance so measured.

The load indicator may comprise a diaphragm. The load indicator may be a strain-gauged diaphragm connected to an end of the elongate pin. The diaphragm may be actuated by the elongate pin in response to compression of the threaded portion and/or flange portion. The diaphragm may be displaceable in a direction parallel to the longitudinal axis of the bore. The diaphragm comprising a sensor being arranged to detect the amount of displacement of the diaphragm. The strain-gauged diaphragm may be configured to provide an indication of the preload applied to the joint.

The load indicator may be removably mounted to the first end face. The measurement gauge may be removably mounted to the first end face by magnets.

The load indicator may comprise a wireless communications unit, the wireless communications unit arranged to send measurement data to a remote receiving unit.

- 16 A load indicator which comprises a measurement gauge and/or sensors may be able to provide a continuous indication of the applied preload, rather than just providing an indication that the preload has crossed one or more thresholds. Such a load indicator may not just undergo a binary change of state, the first and/or second state may effectively fall within a range of possible states .

The present invention provides, according to a second aspect, a kit comprising a female fastener for receiving and engaging with a threaded male fastener, the female fastener comprising:

a threaded portion with a first end face, the threaded portion further comprising a thread for engaging with the threaded male fastener;

a second end face; and an elongate pin extending through the threaded portion, the elongate pin for controlling a load indicator located at the first end face;

the kit further comprising a load indicator;

wherein, when the load indicator is located at the first end face, a set level of compression of the threaded portion relative to the elongate pin moves the load indicator from a first state to a second state.

The present invention provides, according to a third aspect, a female fastener for receiving and engaging with a threaded male fastener, the female fastener comprising:

a second end face; and an ultrasound transducer mounted to the first end face .

- 17 The female fastener may further comprise a flange portion with the second end face, the flange portion comprising a bore for receiving, but not engaging with, the threaded male fastener.

The ultrasonic transducer may be configured to emit ultrasonic waves at the first end face, and to receive reflections of the ultrasonic waves so emitted. The reflections may be from the second end face of the fastener. The ultrasonic waves may be in the form of an ultrasonic pulse. The pulse may travel in a direction parallel to the longitudinal axis of the bore.

A control unit may be configured to determine the compression of the female fastener in dependence on the timings of the reflections so received. The control unit may be configured to determine the preload in the joint.

The ultrasound transducer may be removably mounted to the first end face. The ultrasound transducer may be removably mounted to the first end face by magnets. The ultrasound transducer may be permanently mounted to the first end face.

The ultrasound transducer may comprise a wireless communications unit, the wireless communications unit arranged to send measurement data to a remote receiving unit.

The present invention provides, according to a fourth aspect, a threaded joint comprising: a female fastener according to the first, second or second aspect of the invention; and a male fastener engaged with the female fastener.

The male fastener may be a bolt. The threaded joint may therefore be a bolted joint.

The present invention provides, according to a fifth aspect, a method of forming a threaded joint, the method comprising the steps of: providing a female fastener

- 18 according to an aspect of the invention, engaging the female fastener with a male fastener, increasing the preload in the joint by tightening the female fastener, confirming whether a threshold preload has been met or exceeded on the basis of the load indicator being in the first state or the second state.

The load indicator may be an indicator element, and in the first state the indicator element is not movable, and in the second state the indicator element is movable. The method may comprise a step of confirming that the threshold preload has been met or exceeded by attempting to move the indicator element. Where the indicator element is a rotatable body, the method may comprise a step of attempting to rotate the rotatable body.

The female fastener may comprise two load indicators, the second load indicator adopting a second state at a second threshold preload, the second threshold preload being different to the threshold preload of the first load indicator. The method may comprise a step of confirming whether the second threshold preload has been met or exceeded.

The threshold preload of the first load indicator and the threshold preload of the second load indicator may respectively define a minimum acceptable preload and a maximum acceptable preload. The method may comprise a step of confirming whether the applied preload is within an acceptable range.

The present invention provides, according to a sixth aspect, a method of manufacturing a female fastener according to the first aspect of the invention, wherein the load indicator is an indicator element, and in the first state the indicator element is not movable, and in the second state the indicator element is movable. The method comprising: loading the female fastener on a load

- 19 cell to a threshold load and then tightening the elongate pin so that the indicator element is on the threshold of being clamped so as to prohibit movement, and then removing the female fastener from the load cell. This method may enable the level of compression / threshold preload to be set to the desired level for a particular fastener during manufacture.

It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the female fasteners of the second, third and fourth aspects of the invention may have any of the features described in relation to the female fastener of the first aspect of the invention. The method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa.

Description of the Drawings

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

Figure 1 shows a cross-sectional side view of a female fastener according to a first embodiment of the invention;

Figure 2 shows a plan view of a female fastener according to a first embodiment of the invention;

Figure 3 shows an underneath view of a female fastener according to a first embodiment of the invention;

- 20 Figure 4 shows a cross-sectional side view of a female fastener according to a second embodiment of the invention;

Figure 5 shows a plan view of a female fastener according to a second embodiment of the invention;

Figure 6 shows a cross-sectional side view of a female fastener according to a third embodiment of the invention;

Figure 7 shows a plan view of a female fastener according to a third embodiment of the invention;

Figure 8 shows a cross-sectional side view of a female fastener according to a fourth embodiment of the invention;

Figure 9 shows a plan view of a female fastener according to a fourth embodiment of the invention;

Figure 10 shows a cross-sectional side view of a female fastener according to a fifth embodiment of the invention;

Figure	11	shows	a plan view of a	female fastener
according to	a	fifth	embodiment of the	invention;
Figure	12	shows	a side view of a	female fastener
according to	a	sixth	embodiment of the	invention; and
Figure	13	shows	a cross-sectional	side view of a

female fastener according to a seventh embodiment of the invention.

Detailed Description

Figure 1 to 3 show a female fastener 100 according to a first embodiment of the invention. The female fastener comprises a body having a threaded portion 102 and a flange portion 104. The threaded portion 102 comprises a hexagonal side wall for being gripped by a tightening apparatus such as a spanner (wrench). In

- 21 alternative embodiments the side wall may take another form, for example the side wall may be bi-hexagonal, sguare, cylindrical or knurled.

The threaded portion 102 comprises an internal thread 108 for receiving and engaging a correspondingly threaded male fastener. The flange portion 104 comprises an annular ring without an internal thread. The aforementioned threaded male fastener could be received within the flange portion 104, but would pass freely through (not engage with) the flange portion 104. The internal thread 108 and annular ring together define a bore which runs through the threaded portion 102 and the flange portion 104. The central longitudinal axis 106 of the bore is shown.

The threaded portion 102 has a first end face 110. The flange portion 104 has a second end face 112. The first end face 110 and the second end face 112 are on opposing sides of the female fastener 100. The first end face 110 is substantially flat. The second end face 112 comprises a castellated surface comprising three raised areas 114. Each raised area 114 spans 60 degrees, and is separated by 60 degrees.

An elongate pin 116 extends through an opening 118 in the threaded portion 102 and the flange portion 104, the elongate pin extending from the first end face 110 to the second end face 112 in a direction substantially parallel to the longitudinal axis 106 of the bore.

A first end of the elongate pin 116 comprises a male thread which can screw into the side walls of the opening 118 near to the second end face 112, the opening 118 having a reduced internal diameter 120 near to the second end face 112. The first end of the elongate pin 116 is thereby fixedly attached to the flange portion 104 at the second end face 112. The side walls of the opening 118 do

- 22 not otherwise grip the elongate pin 116 and a second end of the elongate pin 116 is free to move relative to the threaded portion 102 and flange portion 104.

The second end of the elongate pin 116 extends beyond the first end face 110. The second end of the elongate pin 116 projects out laterally to form a pin head 124. The pin head 124 overlaps the first end face 110 .

A load indicator in the form of a washer 122 (a rotatable body) is provided around the second end of the elongate pin 116. The washer is posited between the first end face 110 and the pin head 124.

The female fastener 100 is configured such that, when the threaded portion 102 and flange portion 104 are uncompressed (no preload is applied), the washer 122 is clamped between the first end face 110 and the pin head 124. When the washer 122 is clamped it is not able to rotate about the elongate pin 116 under finger pressure. This non rotating state is a first state of the load indicator .

In use, the female fastener 100 engages a male fastener, the male fastener first passing through the flange portion 104, then engaging with the internal thread 108 of the threaded portion 102. The female fastener 100 is then rotated relative to the male fastener until the second end face 112 abuts an object to be joined. As the female fastener 100 is tightened the threaded portion 102 and the flange portion 104 are compressed. In particular the flange portion 104 is subject to the entire compressive load between the threaded portion 102 and the object to be joined. The raised areas 114 on the second end face also act to increase the overall amount of compression.

- 23 As mentioned above, the elongate pin 116 can move freely relative to the threaded portion 102 and flange portion 104 within the opening 116. Therefore the elongate pin 116 will not be subject to compressive loading during tightening (except perhaps a small amount where it fixedly attaches to the flange portion). Therefore as the female fastener 100 is tightened, the distance between the pin head 124 and the first end face 110 increases. When the threaded portion 102 and flange portion 104 are compressed by a certain amount (because a threshold preload is applied), the washer 122 is released such that it is able to rotate about the elongate pin 116. This rotatable state is a second state of the load indicator. In Figure 1 the washer 122 is shown in the rotatable state.

In use, a user can therefore check whether a threshold preload has been reached and/or exceeded by checking whether the rotatable body is in the first state (clamped and non-rotatable) or the second state (rotatable). The preload can also be checked long after the joint has been assembled.

A removable covering cap 126 can be provided over the second end of the elongate pin 116 and the washer 122 to prevent mechanical damage and/or the ingress of dirt.

According to an embodiment of the invention, there is provided a method of manufacturing the female fastener

100. The method comprises compression loading the female fastener on a load cell to the threshold load and then tightening the elongate pin 116 so that the washer 122 is on the threshold of being clamped so as to prohibit rotation. The female fastener 100 is then removed from the load cell with the pin head 124 clamping the washer 122 at the required load. With this method it is possible to set level of compression / threshold preload at which

- 24 the load indicator will change from the first state to the second state.

Figures 4 and 5 show a female fastener 200 according to a second embodiment of the invention. The female fastener 200 comprises two elongate pins 216A and 216B each having a respective load indicator in the form of washers 222A and 222B (which are rotatable bodies). The female fastener 200 is configured such that the two washers 222A and 222B are released so as to be able to rotate at different set levels of compression. Therefore the load indicators will each change from a first clamped and non-rotatable state to a second rotatable state at a different threshold preload.

The female fastener 200 can therefore be used to indicate whether the preload applied to a joint is within an acceptable range. For example the female fastener 200 can be configured such that the washer 222A begins to rotate (changes state) at a minimum acceptable preload, and the washer 222B begins to rotate (changes state) at a maximum acceptable preload.

In use, when the female fastener is tightened, when the washer 222A begins to rotate it therefore indicates to the user that a minimum acceptable preload has been reached, and if the washer 222B becomes free to turn then it indicates to the user that a maximum acceptable preload has been exceeded. In Figure 4 the washer 222A is shown as in a rotatable state, and the washer 222B is shown in a clamped and non-rotatable state.

In alternative embodiments, further elongate pins and load indicators can be incorporated so as to provide further information when threshold preloads have been met or exceeded. In embodiments each load indicator and pin is colour coded and provided with a numerical indication

- 25 of the threshold preload associated with the load indicator .

Figures 6 and 7 show a female fastener 300 according to a third embodiment of the invention. The female fastener 300 comprises an elongate pin 316, however unlike the first embodiment, the second end of the elongate pin does not have a pin head. Instead, the elongate pin 316 terminates level with the first end face 310 when no preload is applied.

A load indicator in the form of a measurement gauge 326 is mounted on the first end face 310. The measurement gauge 326 is configured to measure the relative position of the second end of the elongate pin 316 relative to the first end face 310.

In alternative embodiments the elongate pin projects above or below the first end face at zero applied preload, and the measurement gauge is zeroed at that relative position.

The measurement is acquired using a linear voltage displacement transducer. When the elongate pin 316 is in a first position the transducer is in a first state in which it is providing a first output, and when the elongate pin 316 is in a second position the transducer is in a second state in which it is providing a second output. The measurement gauge 326 comprises a control unit configured to provide a reading of the applied preload for a given transducer output by referencing load compression data for the particular female fastener. The female fastener 300 is thereby also able to provide a continuous reading of the applied preload.

The measurement gauge 326 is removably attached to the first end face 310 by magnets 328. The female fastener 300 therefore lends itself to high temperature applications as the gauge can be removed as soon as a

- 26 reading is taken and so may not be damaged by heat experienced during use. The elongate pin 316 also has the same thermal expansion as the material of the threaded portion and flange portion.

In alternative embodiments, the female fastener 300 may additionally include minimum and/or maximum threshold load indicators as described above in relation to the first and/or second embodiments.

Figures 8 and 9 show a female fastener 400 according to a fourth embodiment of the invention. The female fastener 400 is similar to the female fastener 300 according to the third embodiment, however in the fourth embodiment the load indicator is in the form of a straingauged diaphragm 430.

The diaphragm 430 is mounted to the first end face

410 of the female fastener 400 over the opening in which the elongate pin 416 is provided. When the threaded portion and the flange portion are compressed the second end of the elongate pin 416 projects out of the opening and displaces the diaphragm 430. A sensor in the diaphragm, an electronic strain gauge, detects the displacement of the diaphragm. When the elongate pin 416 is in a first position the sensor is in a first state in which it is providing a first output, and when the elongate pin 316 is in a second position the sensor is in a second state in which it is providing a second output.

The load indicator further comprises a wireless communications unit 432. The wireless communications unit is arranged to send measurement data received from the sensor to a remote receiving unit.

Figures 10 and 11 show a female fastener 500 according to a fifth embodiment of the invention. Unlike the first to fourth embodiments, the female fastener 500 does not comprise an elongate pin. The female fastener

- 27 500 instead comprises an ultrasonic transducer 534 for detecting the compression of the threaded portion and the flange portion.

The ultrasonic transducer 534 is configured to measure the distance between the first end face 510 and the second end face 512 of the female fastener. The change in the distance between the first end face 510 and the second end face 512 provides a measurement of the compression, which can in turn can be used to provide an indication of the applied preload.

In the fifth embodiment, the electronic transducer 534 is removably mountable to the first end face 510. In alternative embodiments the ultrasonic transducer is permanently fitted to the first end face 510 of the female fastener 500.

Figure 12 shows a body of a female fastener 600 according to a sixth embodiment of the invention. The female fastener comprises a flange portion 604 comprising three apertures in the form of slots 636. When the flange portion 604 is under compression due to an applied preload the compressive stress in the three zones in between the slots 636 is higher than the rest of the flange portion 604 which causes those zones, and thus the flange portion 604 as a whole to compress by a greater distance than without the slots 636.

Figure 13 shows a body of a female fastener 700 according to a seventh embodiment of the invention. The female fastener comprises a threaded portion 702 comprising an internal thread 708 tapered towards the first end face 710.

No load indicating means is shown in Figures 12 and 13, however the bodies of the female fasteners 600 and 700 could be used with any of the first to fifth embodiments of the invention.

- 28 Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein.

For example, in alternative embodiments, the flange portion may be omitted all together. In such cases the threaded portion provides the second end face of the female fastener, and the elongate pin extends through the threaded portion alone.

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 eguivalents. 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. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.

Claims

1. A female fastener for receiving and engaging with a threaded male fastener, the female fastener comprising:

2. A female fastener as claimed in any preceding claim, wherein the elongate pin extends from the first end face to, or close to, the second end face.

3. A female fastener as claimed in claim 1 or claim 2, wherein the load indicator is a rotatable body, and in the first state the rotatable body is not rotatable, and in the second state the rotatable body is rotatable.

4. A female fastener as claimed in claim 1 or claim 2, wherein the load indicator is a measurement gauge arranged to measure the relative position of an end of the elongate pin relative to the first end face, and in the first state the end of the elongate pin is a first position, and in the second state the elongate pin is in a second position.

5. A female fastener as claimed in claim 4, wherein the measurement gauge comprises a linear voltage displacement transducer .

6. A female fastener as claimed in claim 1 or claim 2, wherein the load indicator is a strain-gauged diaphragm connected to an end of the elongate pin.

7. A female fastener as claimed in any of claims 4 to

6, wherein the load indicator is removably mounted to the first end face.

8. A female fastener as claimed in claim 7, wherein the measurement gauge is removably mounted to the first end face by magnets.

9. A female fastener as claimed in any of claims 4 to

8, wherein the load indicator comprises a wireless communications unit, the wireless communications unit arranged to send measurement data to a remote receiving unit.

10. A female fastener as claimed in any preceding claim, wherein the second end comprises a castellated surface.

11. A female fastener as claimed in claim 10, wherein the castellated surface comprises three regularly spaced raised areas.

12. A female fastener as claimed in any preceding claim, further comprising a flange portion providing the second end face; wherein the flange portion comprises a bore for receiving, but not engaging with, the threaded male

- 31 fastener; and the elongate pin extends through the flange portion.

13. A female fastener as claimed in claim 12, wherein the flange portion comprises one or more apertures.

14. A female fastener as claimed in claim 12 or 13, wherein the flange portion has a length, the length being approximately equal to the diameter of the threaded male fastener to which the female fastener is to receive and engage .

15. A female fastener as claimed in any preceding claim, wherein the elongate pin is a first elongate pin, the load indicator is a first load indicator, and the level of compression is a first level of compression, and the fastener further comprises a second elongate pin, the second elongate pin controlling a second load indicator, whereby a second level of compression of the threaded portion relative to the second elongate pin moves the second load indicator from a first state to a second state .

16. A female fastener as claimed in claim 15, wherein the first level of compression is different to the second level of compression.

17. A female fastener as claimed in any preceding claim, wherein the threaded portion is tapered towards the first end face.

18. A kit comprising a female fastener for receiving and engaging with a threaded male fastener, the female fastener comprising:

- 32 a threaded portion with a first end face, the threaded portion further comprising a thread for engaging with the threaded male fastener;

the kit further comprising a load indicator;

19. A method of forming a threaded joint, the method comprising the steps of:

providing a female fastener according to any preceding claim, engaging the female fastener with a male fastener, increasing the preload in the joint by tightening the female fastener, and confirming whether a threshold preload has been met or exceeded on the basis of the load indicator being in the first state or the second state.

20. A female fastener for receiving and engaging with a threaded male fastener, the female fastener comprising:

a second end face; and an ultrasound transducer mounted to the first end face .

21. A female fastener as claimed in claim 20, further comprising a flange portion with a second end face, the flange portion comprising a bore for receiving, but not engaging with, the threaded male fastener.

22 . A female fastener as claimed in claim 20 or 21, whe rein the ultrasound transducer is removably mounted to the first end face .

23 . A female fastener as claimed in claim 20 or 21, whe rein the ultrasound transducer is permanently mounted to the first end face.

24. A threaded joint comprising: a female fastener according to any preceding claim; and a male fastener engaged with the female fastener.