A class of fasteners commonly known as load indicating fasteners is widely known to engineers. These are used in applications where it is important to know that a fastener has been tightened sufficiently, or that a fastener has not been inadvertently loosened by human error, movement of a structure, mechanical failure of some component or by some other means. The most common applications of such fasteners are as bolts in large steel structures such as the frames of buildings, bridges, electrical transmission towers and other structures, on pressure vessels and chemical reactors, on mining machinery and earthmoving equipment and in aircraft.

Prior art load indicating fasteners include those which require the some measurement of the length of the bolt. However for this type of fastener access is needed to both ends of the bolt and this is not always convenient or even possible. Other types include those where the depth of a hole drilled longitudinally into the bolt is measured by means of a probe extending down the hole. Others have a rod fixed down such a hole and some form of indicator carried on the end of the bolt and linked to the rod in order to indicate the state of tension in the bolt.

While existing load-indicating bolts perform generally reliably, they tend to fall into two categories. The first category encompasses fasteners which are less expensive and relatively robust, but these are not very accurate or reliable. However it will be appreciated that reliability and accuracy are important features of load indicating bolts. The category includes fasteners which indicate a simple yes/no condition, ie whether the bolt has above a given load or not.

The second category of prior art bolts encompasses fasteners which are more accurate and reliable, but they are relatively expensive to manufacture due to more complex

fabrication and calibration and are less robust due to the delicate and complex nature of fitments to the bolt. Strain gauged bolts are one example. These can be used for continuous monitoring but suffer from delicate wires extending from the bolts and associated bulky monitoring equipment. It is common for such bolts to sell for hundreds of dollars each where an equivalent sized bolt without the load indication might cost less than ten dollars. Bolts having a fine resolution of tension indication are extremely expensive and thus can be justified in only mission critical or extreme safety conditions such as for nuclear power equipment.

For both the aforementioned categories a major cost in the manufacture of the bolts is the provision, on the bolt, of a means of indicating the tensile load clearly to an observer.

Modem engineering emphasis on reduced size, weight and cost of structures, leads to constructions which are less tolerant of fasteners being insufficiently tightened. This means there is a need for a system for indicating the tensile load on a fastener which has the desired combination of being more reliable, more accurate, more robust, and less expensive to manufacture than existing systems and also provides a reading of the actual load on the fastener either on demand or continuously.

An object of the present invention is to alleviate the disadvantages of the prior art.

The present invention addresses this by removing the need to provide an accurate load indication means on the fastener. In preferred embodiments a load indicating means is incorporated instead into a separate instrument which may quickly and easily be moved from fastener to fastener.

Accordingly, in one aspect the invention provides a method of indicating the tensile load on a fastener including providing on said fastener an observation face surrounded by a reference face, the planes of said faces being parallel to each other and adapted to move relative to each other as said tensile load changes, placing a reference face of a measurement instrument against said fastener reference face, determining the deviation of the observation face from the plane of the instrument reference face,

comparing this deviation with the known deviation characteristics for the fastener and indicating therefrom the tensile load on the fastener.

Preferably the observation face lies in the same plane as the reference plane when there is no tensile load on the fastener.

Alternatively the observation face may lie on one side of the plane of the reference face when the fastener has a tensile load less than a pre-selected load, lie on the other side of the plane of the reference face when the fastener has a tensile load greater than said pre-selected load, and lie in the plane of the reference face when the fastener has a tensile load equal to said pre-selected load.

The fastener may be a bolt, screw or stud tensioned by screwed engagement with a mating component.

Examples of the invention will now be described with reference to the attached drawings where: Figure 1 is a side view of a bolt in accordance with a preferred embodiment of the present invention; Figure 2 is a partial cross section through the bolt shown in Figure 1; Figure 3 is an end view of the bolt shown in Figure 1; Figures 4 and 5 are respectively end and cross section side views of a cap used with the bolt shown in Figure 1; Figure 6 is a diagrammatic illustration of measurement equipment being used in accordance with one embodiment of the present invention; Figure 7 is a block diagram showing functional components of the equipment shown in Figure 6; Figure 8 is a cutaway view through an alternative fastener which may be used in accordance with the present invention; and Figure 9 is an enlarged view of part of the head of the bolt in Figure 8 showing a step in the process of its manufacture;

Referring to Figure 1 to 3, a bolt 2 according to a preferred embodiment of the invention has a head portion 4, shank portion 6 and threaded portion 8. In use the bolt is tensioned in the normal manner by tightening it into a hole having a mating thread or tightening onto it a mating nut. The top of the head 4 of the bolt carries a raised boss 5 which carries a smoothly machined reference face 12. The bolt illustrated is of 20mm diameter and 100mm nominal length.

Drilled into the bolt perpendicular to, and at the centre of, the reference face 12 is a blind bore 14 which penetrates down the axis of the bolt, beyond the plane defined by the back face 8 of the head 4, for part of the length of the shank portion 6. The bore 14 thus has one end as an aperture 15 in the reference face 12 and a blind distal end 16 deep within the shank of the bolt. However the bore 14 is dimensioned in such a way that it does not downgrade the strength of the bolt. This is achieved by making the effective stress area of the drilled shank portion greater than the tensile stress area of the threaded portion of the fastener. Accordingly the bore 14 does not extend as far as the threaded portion 8 of the bolt.

Fitted into the bore 14 is a rod 22 which has a slightly smaller diameter than that of the bore. The rod 22 has a distal end 23 affixed to the distal end 16 of the bore. This affixation may be by threading, adhesive or an interference fit but is most preferably by capacitor discharge projection welding the distal end 23 of the rod to the distal end 16 of the bore.

Between the cylindrical face of the rod 22 and the wall of the bore 14 is a layer 28 of soft elastomer. For a bolt of, for example, 20 mm diameter, the rod may be 4.0 mm diameter and the bore 5.0 mm diameter.

To manufacture the bolt 2, the overall body of the bolt is first machined or forged from steel stock using conventional bolt making procedures. The bore 14 is then drilled to its full depth with a 5mm drill and then to only part of its depth with an 8mm drill to form an opened out or relieved bore portion 18. The rod is selected to be of compatible material to the body of the bolt. The rod is inserted into the bore after

the rod has been coated on its cylindrical face with an electrically insulating material such as paper or a thin polymer coating. The welding action thus occurs only at the distal end of the rod and bore. The welding operation is performed using a rod somewhat longer than the final length necessary and, following the welding, the rod is chopped close to its final length.

The assembly is then heat treated to relieve stresses and the gap between the bore and rod is partly filled with a low viscosity reacting mixture which cures to form a low strength elastomer. The reaction mixture is added to the bore until it completely fills the smaller diameter portion of the bore and just encroaches into the relieved portion 18. A sleeve 30 of low surface friction polymer such as PTFE is then slid over the free end of the rod and press fitted with an interference fit into the relieved portion of the bore. The elastomer is allowed to fully cure and the end face of the bolt head is machined, ground and polished to create the reference face 12 on the boss and the observation face 24 on the rod as exactly coplanar faces. The degree to which faces 12 and 24 are coplanar is important to the correct working of this embodiment The end face of the sleeve 30 is almost also coplanar but its different rate of abrasion during the grinding operation may cause the surface to be slightly recessed from the others.

The bolt is then sandblasted, plated, and the appropriate identification codes are chemically etched onto the reference face 12. The code includes in particular the class of steel used for the bolt, such as"Class 8.8" to indicate steel with 800MPa UTS and 0.8 x 800MPa Yield Stress. The plating process is preferably electroless nickel plating because of its superior retention of the flatness of the substrate.

When the bolt is tensioned in use, the shank of the bolt elastically elongates under its tensile load. However the rod does not similarly elongate because it is attached at its distal end 23 only and the elastomer packing and PTFE sleeve allows the rod to slide within the bore. The firm fit of the sleeve onto the rod and into the bore prevents ingress of foreign material which might corrode the rod assembly and so affect the accuracy of the bolt.

The present invention uses an instrument to provide a measurement of the distance the observation face of the rod is protruding from or recessed into the reference face.

Such an instrument may be constructed from adapting a micrometer mechanism, vernier caliper or a dial gauge referenced to the reference face of the bolt. However these means are cumbersome, less reliable and do not provide a means for continuous monitoring. Figure 6 illustrates a preferred embodiment of a measuring instrument which utilises a linear variable differential transformer (LVDT) to provide the measurement.

The observation end of the rod is machined level with the plane of the reference face on the fastener before the fastener is tensioned. The rod therefore never protrudes from the reference face. As a tensile load is applied to the bolt, the observation face on the rod moves from its initial position in the plane of the reference face to positions increasingly recessed into the fastener.

By keeping the depth of the bore under the head constant for all sized bolts, and the indication on the display as a percentage of yield load, the measurement instrument can be used on any bolt of the same material without any additional adjustment or calibration, so making it more user friendly and less complicated.

Referring to Figures 4 and 5, these show a cap 32 made from plastics material which is adapted to clip via its peripheral flange 34 onto the boss 5 when the bolt is not being monitored. The cap 32 serves to keep contaminants, including moisture, away from the faces 12 and 24. Preferably the plastic material on the inside of the cap has a strong affinity for water vapour and accordingly provides a drying action on the atmosphere trapped inside the cap when sealed onto the boss 5.

Referring to Figure 6, the measurement instrument indicated generally as 40 has two main components, a sensor body 42 and a display case 44, connected by wiring.

Instead of wires, the body 42 and case 44 may be connected by radio linkage. The base 48 of the sensor body 42 is generally cylindrical with an annular reference face

50 which in use is brought into contact with the reference face 12 on the head of the bolt 2. At the centre of the annular reference face 50 is a hole from which protrudes the tip of a spring loaded plunger shown diagrammatically as 52 which contacts the observation face 24 on the rod 22. The top of the plunger 52 is connected to a miniature displacement tranducer (MDT) incorporating a linear variable differential transformer (LVDT) which provides an electrical output relative to its armature position. A suitable MDT for the purpose is a model DFgl transducer manufactured by Solartron.

The base 48 is fitted with strong permanent rare earth magnets (not shown) adjacent the reference face 50 so that it clamps firmly to the steel bolt head. In addition to facilitating good close contact between reference faces 12 and 50, the magnetised base also permits easy measurement by allowing an operator to have both hands free while making a measurement. As an alternative to a flat reference base 48, the sensor body may contact the bolt by way of three protruding rounded point contacts spaced in a triangle so as to form a tripod which rests upon the reference face 12. The tips of these protrusions would define a plane which acts in the manner of the reference plane for the purpose of calculating loads.

In order to correctly align the reference faces 12 and 50, a collar 45 is slipped onto the bolt head to locate around the boss 5. The sensor body 42 has an outside diameter at its base such that it slides neatly into the collar 45. The collar thus serves as a centring device for the sensor onto the reference face. The sensor body diameter at its base and the boss 5 are the same diameter so that the collar 45 is reversible.

Referring to Figure 7, the measurement instrument is seen to comprise of nine functional blocks, namely sensor 60, signal conditioner 61 amplifier 62, A/D converter 63, microcontroller 64, power supply 65, display 66, input switches 67, serial communications 68 and a feedback 69 to the tightening tool.

The sensor has been discussed above in relation to Figure 6. The signal conditioner 61 cleans up the signal from the sensor by conventional means. The amplifier 62

amplifies the output signal from the sensor 60 by a factor of 10 as the signal is initially in the millivolt range. The A/D converter 63 receives the amplified analogue signal from the amplifier and digitizes it for further processing by the microcontroller.

The converter 63 is an 8 bit converter so there are 256 levels between the minimum and maximum values in the range. If better resolution is required, a 10 bit or 12 bit A/D converter could be incorporated. The microcontroller processes input data received from the sensor 61 and switches 67 and sends the results to the display 66 which provides a visual reading to the user. The power supply 65 supplies electric power to all the components of the device. It preferably has a rechargeable battery pack. The input switches 67 are used to select various functions and modes for the device. The serial communications module 68 allows communication between the device and a computer. This permits updating software in the controller or uploading and downloading data from the controller. The module 68 also allows feedback, via the feedback module 69, to a tool tightening the fastener so that tensioning may be automatically slowed down and stopped as the load on the fastener respectively approaches and reaches the set limit.

The option of the feedback module would only be required on some embodiments of the invention.

This measurement instrument provides a low-cost, hand held, battery operated bolt tension monitoring device with a convenient LCD readout. One such device can be used on many bolts, so removing the costly requirement to have a separate tension readout means on each bolt. Alternatively, in highly critical applications requiring on-line monitoring of bolt tension, one detector body can be permanently allocated to each bolt with each detector connected to a common recording/readout means. Also several recording devices may be networked together and then monitored individually through addressable electronic links.

To use the apparatus to control the tension or load in a bolt of the form described, the bolt would first be inserted in its intended hole and the desired washer (s) and nut put on but not tightened. Before the nut is tightened, the collar 45 is placed over the boss

5 and the sensor body 42 located into the collar with the plunger 52 resting against the observation face 24 of the rod. The appropriate class of steel is selected from a menu shown on the display 43, the display altered to select the readout derived from the sensor body, and the readout is zeroed. The nut is then turned to tension the bolt and the readout on the display is monitored. The readout is directly of the percentage of yield load imparted to the bolt. When the required percentage yield stress is indicated, further turning of the nut is stopped.

The computations undertaken by the apparatus are relatively straightforward once the principle of the intent is understood. This will now be described.

When the bolt is tensioned, the relationship E = (F/A)/ (8/L) applies, where E = Young's Modulus F = force A = stress area 8 = elongation L = underhead length of hole and rod This transforms to F/A = (E/L). 8 So if L is kept constant for all bolts used by the apparatus, the relationship (E/L), remains constant for the bolts and when the stress required is presented as a percentage of yield, then the required displacement 6 can be calculated as the same percentage of the displacement at yield. Displacement at yield is governed by length L and bolt material class.

The apparatus is thus able to present a direct output of percentage yield stress from inputs of only two pieces of data. These are the class of steel and the required percentage yield load, both of which are input manually.

Furthermore, the measuring instrument (also called a monitoring device) can also be used to measure the existing tension of a previously installed fastener. In such a circumstance an initial measurement is taken on the reference face 12 of the head away from the rod 22 and sleeve 30 in order to establish a datum. Then a second

measurement is taken at the observation face 24 on the rod and its difference from the datum is interpreted to give the tension in the fastener. This technique provides a simple, straightforward and user-friendly way of determining tension without sacrificing the robustness of the system. Here again the only input the user needs to provide is the type of material, such as class 10.8 or 9.9 for example, which may be chosen from an electronic menu on the display 43.

Referring to Figures 8 and 9, a bolt 102 according to an embodiment of the invention which is less preferred than that described with reference to Figure 2 has a head portion 104, shank portion 106 and threaded portion 108. In use the bolt is tensioned in the normal manner by tightening it into a hole having a mating thread or tightening onto it a mating nut. The top of the head 104 of the bolt carries a smoothly machined reference face 112.

Drilled into the bolt perpendicular to, and at the centre of, the reference face 112 is a blind bore 114 which penetrates down the axis of the bolt for almost the full length of the shank portion 106. The bore 114 thus has one end at an aperture 115 in the reference face 112 and a blind distal end deep within the shank of the bolt.

Fitted into the bore 114 is a rod 122 which has a slightly smaller diameter than that of the bore. The rod 122 has a distal end 123 affixed to the distal end 116 of the bore.

This affixation may be by threading, adhesive or an interference fit but is most preferably by capacitor discharge projection welding the distal end 123 of the rod to the distal end 116 of the bore.

Between the cylindrical face of the rod 122 and the wall of the bore 114 is a layer of PTFE 128. This may be produced by coating the cylindrical face of rod 122 with PTFE before its insertion into the bore or it may be provided by sliding the rod into a PTFE sleeve before inserting the assembly into the bore. For a bolt of, for example, 16 mm diameter, the rod may be 4.0 mm diameter and the PTFE sleeve of 0.5 mm thick. The bore 114 would thus be approximately 5.1 mm diameter to allow a sliding fit.

The PTFE provides an electrically insulating medium to allow the welding of the rod to the distal end of the bore without any welding action elsewhere between the rod and bore. As an alternative to PTFE, the sleeve may be of other material, for example paper, which also provides the appropriate electrically insulative properties. The PTFE sleeve may be removed after the welding operation or it may be retained in order to provide a slipping surface and to provide a barrier to entry of foreign material into the annular gap between the rod 122 and the wall of the bore 114. Removal of the sleeve may be desired if it is required to heat treat the welded assembly.

The welding operation is performed using a rod somewhat longer than the final length necessary. As shown in Figure 9, a hardened steel washer 132 of specified thickness is placed over the protruding rod in order to act as a spacer gauge so that the protruding end of the steel rod may be ground down to the stage when the gauge washer 132 just commences to be touched by the grinder. The gauge washer 132 is then removed and the rod is left protruding from the reference face 112 by the thickness of the gauge. The observation face 124 so produced on the rod 122 is parallel to and displaced from the plane of the reference face 112 by the thickness of the gauge washer.

The bolt elongation under selected maximum or preferred tensile loads can be readily calculated and the gauge thickness chosen to be the same. By this means, as the bolt is tensioned, the displacement between the observation face 124 on the rod and the plane of the reference face on the bolt head decreases until, at the load selected to be indicated, the observation face of the rod is coplanar with the reference face. At higher tensile loads on the bolt, the observation face on the rod will be below the plane of the reference face. It is therefore possible to roughly determine whether the bolt is sufficiently tensioned by gently wiping a finger across the reference face to feel if the rod is protruding. So used, a finger is capable of detecting a step as small as about 5 um, which may correspond to a resolution of 3 kN when measuring a load of 147 kN on a class 8.8,20 mm diameter bolt, 100 mm long that might use the present invention.

However such a measurement is not fully reliable and does not allow convenient determination of over-tensioning as a finger is not as good at detecting whether the rod is recessed below the reference face. Also a finger cannot determine the extent of any under-tensioning or over-tensioning.

Alternatively, a tool comprising a sharp blade drawn across the reference face is capable of detecting a step of 2 um, which would correspond to a resolution of 1.2 kN for the above 20 mm diameter class 8.8 bolt. But this cannot detect between the observation face of the rod being coplanar with the reference face or the observation face being below the reference face (ie with the bolt over-tensioned).

The bolt described with reference to Figure 8 may also be used with the sensor body 42 and associated apparatus providing the tripod support configuration as described earlier provides sufficient clearance for the protruding rod 122.

The fasteners described above are more expensive to produce than conventional equivalent fasteners which do not indicate their tension. It would be desirable to reduce the cost of fastening systems having the advantages of the bolts described above. This may be achieved by means of a further aspect of the present invention.

Structural applications most commonly involve two or more bolts at each joint.

Accordingly bolts may be supplied for such a joint in matched sets containing the number of bolts required for the joint. One of the bolts in the set would be a tension indicating bolt as described above and the remaining bolts in the set would have the same mechanical, thread and surface characteristics as the first bolt so that it gave identical performance.

The method of installation follows the following steps: (a) The first bolt to be installed is the tension indicating bolt which is tightened up to the required tension as indicated by the load measurement apparatus. At the same time the required torque would be measured by use of a conventional torque wrench.

(b) The remaining bolts are installed to the joint with the same torque as that used for the bolt in step (a).

(c) The load indicating bolt is then checked for tension by using the load indicating meter and is adjusted (usually tightened) to take the bolt to the required load. Again the required tightening torque is measured.

(d) The remaining bolts are then torqued to the same degree as measured in step (c).

(e) Steps (c) and (d) are repeated until there is no further adjustment in the torquing required.

Benefits to the user include the ability to accurately recheck tension and tighten as appropriate at any time in the future using just the load indication meter and a torque wrench without needing to refer to anything other than the fasteners at the joint.

Some embodiments of the present invention require the rod to be carefully positioned relative to the reference face on the bolt during manufacture. But in other embodiments of the invention the initial relative positions of the two reference faces need not be so carefully controlled. Instead measurement of the relative positions before and after tensioning of the fastener gives the change in relative position and this change can be used to determine the tension on the bolt. To facilitate this the display case may carry a button which zeros the reading prior to commencement of tensioning.

Such an arrangement would be suitable for initial installation of the fasteners but would be difficult to use where it is desired to return to previously installed fasteners to see if their tension had changed. One embodiment of the present invention incorporates a means for overcoming this difficulty. The bolt head has embedded into a small recess on the reference face an electronic memory chip which carries an individual identification number unique to each fastener plus a record of the initial deviation of the rod end from the reference plane on the bolt head. This information may be incorporated into the memory chip when the bolt is manufactured.

The base of the sensor body described above would then have appropriate electrical terminals to make contact with corresponding terminals from the memory chip. In this way the measurement instrument could, when placed on the reference face, measure the difference between the two reference faces and, at the same time, by electrical contact with terminals on the bolt head determine from the memory chip what the initial position was before tensioning. This would allow calculation, by the instrument, of the tension on the fastener.

The instrument described in relation to Figure 6 has two main components separated for convenient use and reference. The present invention anticipates that those components could be manufactured in a single housing which could attach to the bolt head much as the sensor body 42 is shown in Figure 6. Such an instrument would be powered by internal batteries, could be easily carried in a person's pocket and thus be especially convenient to use on a construction site.

The term rod, when used in this specification, is intended to encompass rod-like materials such as fine tube. Although such rod is preferably of circular cross section, it may if wished be of other shape such as for example elliptical, square or rectangular cross section.

Throughout this specification, unless the context requires otherwise, the word "comprise", and variations such as"comprises"and"comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.