GB2124789A - Load measurement devices - Google Patents

Abstract

A load measurement device comprises an outer elongate body having a bore in which is embedded an inner elongate body, which may be made of ceramic or glass, carrying strain-sensing means for example in the form of an applied conductive resistive layer. The device is particularly useful for measuring shear loads and can be employed in places which are generally inaccessible such as on the sea bed. There are many applications including cranes, ships, rigs, winches, cables, weighbridges, marine moorings, testing machines, aircraft, helicopters, e.g. attachment of blades to rotor hub. <IMAGE>

Description

SPECIFICATION Load measurement This invention relates to load measurement particularly the measurement of loads in loadbearing structures having structural connections, such as lug and strap joints. Load measurement is widely used in mechanical environments and also scientific and engineering research. It is well known that loads can be measured by employing a strain-sensing transducer such as a strain gauge or strain-sensitive wire e.g. longitudinal strain is measured in terms of the change in electrical resistance caused by the change in length, L, of the wire resulting from the strain.

where Al 1 = strain and R = original electrical resistance of the wire. Other types of strain include bearing strain and shear strain which may also be measured in terms of the change of resistance of the strain-sensing means. Longitudinal strain can be compressive or tensile.

A single, conventional strain gauge measures the strain occuring in one direction only and a plurality of strain gauges at different angular dispositions can be employed to measure the strain in more than one direction at a particular point. For elastic material, Hooke's Law applies: Stress = Strain X E (Young's modulus for the particular material) Known strain transducers are summarised under headings relating to a particular fundamental mode of loading, as follows: 1. Shear Loads These transducers employ electric resistance strain gauges directly attached to holes, pockets, or flats, variously machined in the load carrying member, for example, a bolt or compression load cell.In a bolt or pin the sensing axes of suitable pairs of diametrically opposed gauges are arranged to lie mutually at 90 , and at 45 to the axis of the hole or holes formed in the bolt.

The gauges used in such a configuration are sometimes loosely referred to as 'shear gauges'.

The gauges are generally located at two stations along the length of the bolt to permit the measurement of so-called shear strains arising from a load applied to the bolt, usually in a double shear mode. The direction of the applied load may be determined by multiple arrays of gauges located at the aforementioned stations. The external diameter of the bolt is invariably reduced in the vicinity of the gauge stations to effect a concentration of load, and the bolt is often keyed to an associated structural member in order to inhibit rotation or to ensure rotation, as might be required.

Generally the load monitor displays the total load applied to the joint, but could be connected so as to display the part load applied to each side of the joint.

2. Bending Loads The bending load applied to a beam, shaft or other structural member can be measured by conventional strain gauging technology, for example using a shaft which carries strain sensing means and is insertable in a hole in an object.

3. Tensile/Compressive Loads The measurement of tensile/compressive loads applied to various structural configurations, e.g. bolts, commercial studs, and load cells, may be achieved by: a) Linear electric resistance strain gauges.

b) Electric resistance 'shear' gauges.

c) Fine wire filaments of various configurations.

In one conventional load measurement device, shear and tensile loads are separately measured by means of electrical resistance strain gauges attached by cement to a central bore in a transducer bolt. The strain gauges are disposed in-line with the shear faces of the structural connection. These may be single or double lug joints with one or two shear faces respectively as shown in Figs. 1 a and 1 b, or they may have a plurality of pins/bolts/tines.

According to this invention, I propose a load measurement device for insertion in a loadbearing structure comprising an outer elongate body having an internal, longitudinal bore, an inner elongate body fixed in the bore of the outer elongate body and having a close, sliding fit therein, wherein non-wire strain-sensing means is disposed on the exterior of the rod.

Throughout the whole of this specification by "bolt" is meant the outer elongate body and by "rod" is meant the inner elongate body, wherein elongate is taken to mean of length substantially greater than width.

The bore in the bolt can be symmetrically or asymmetrically disposed to the central, longitudinal axis of the bolt. The direction of an applied load can then be obtained. The bolt may have a plurality of bores parallel to the central axis of the bolt. The term "bolt" is used to include pins, clevis pins and the like for use in single shear lug joints, double shear lug joints or multiply connected joints e.g. found in helicopter rotor heads or shackle assemblies used in marine mooring. Pins or bolts with a head can be used.

The bore in the bolt may be made by drilling and reaming for short bolts or by gun drilling for longer bolts, or by spark erosion or by a combination of these and/or other known machining methods.

The rod can be cylindrical and hollow or solid; also the rod and bolt may be made of any durable material e.g. metal, aluminium alloy, brass, ceramic or plastics.

The rod may have an external collar made of an insulating material e.g. plastics to which the strain sensing means is connected electrically.

A typical load measurement device according to the invention includes a pin, 1 60 mm long and 1 3mm diameter, with a precision-drilled, internal, longitudinal bore which houses a cylindrical rod. The rod, of diameter 3.2 mm, has an external collar near one end and the distance from the collar to the other end of the rod (to be inserted into the pin) is 80mm. The pin is made of plastics or metal and the collar is made of plastics for insulation. The strainsensing means is connected to the collar via stub copper wires (EC).

The rod, is fixed to the bore by any suitable adhesive or cement e.g. an epoxy resin such as Araldite (RTM) cement. During hardening/curing of the embedding adhesive/cement pressure can be applied to the adhesive/cement via a communicating hole at the tail of the bolt in which case if the collar is initially secured by a thread or other means to the bolt head, the ensuing hydrostatic pressure will effect a radial compression between the rod and its host hole in the bolt.

It is not essential for the shank to be undercut but undercutting results in the bearing loads at the or each shear face (SF) being concentrated at discrete distances or "lands" along the shank.

The microstrain will peak at the or each shear face on a graph of local strain versus distance through the joint, as shown in Fig. 3.

Referring to Fig. 3, the graph is of local strain versus the distance through the double shear lug joint. This is obtained by passing a load measurement device through the joint with the applied load normal or substantially so to the central, longitudinal axis of the bolt as shown in Fig 3 (a) and taking strain measurements at regular intervals e.g. every 2 mm. Clearly the maximum strain is suffered at or near the shear faces SF of the lug joint. For a given lug joint and a given device the total area under the graph is related to the total external applied load.

Shear strain can be indirectly obtained.

The direction of the applied load is found by angularly moving in increments e.g. in 10 steps, a testing device about its longitudinal axis and taking strain measurements. By plotting a polar diagram to find the radial strain distribution the direction of applied load is in the direction in which maximum strain is found.

The strain sensing means according to the invention can include any non-wire resistive material, alone or in combination with wire e.g. any number and arrangement of electrical resistance strain gauges alone or with strain-sensitive wires or ribbons. A typical strain gauges is in the form of an etched foil grid or semi-conductive material (e.g. silicon and its alloys) on a plastics carrier having soldering tags and attachable to the rod by adhesive.

Alternatively, the strain sensing means, in the form of an etched foil grid or semi-conductor material might be formed directly on the rod if the latter is made of an insulating material or has an insulating surface. (e.g. plastics, ceramic). The strain sensitive alloy wire can be a copper/nickel alloy or any other suitable strain sensitive alloy. There are commercially available numerous combinations/permutations of foil and carrier. The strain gauges can be of any kind, purpose-made or of standard manufacture. The following are examples.

i) Longitudinal grid i.e. sensing axis parallel to largest dimension of foil, as shown in Fig. 2 (a), ii) Transverse grid i.e. sensing axis normal to largest dimension of foil, as shown in Fig. 2 (b), iii) Shear gauge, inclined grid i.e. sensing axis at 45 to largest dimension of foil, as shown in Fig. 2 (c), and iv) Rosette formation of gauges, see Fig. 2 (o).

The gauge can be piezo-resistive, semi conductive, vibrating wire, capacitive, inductive or magneto-strictive.

Types i to iv are illustrated respectively in Figs. A, B, C and D of the drawings. In Fig. A, line A-A is the se'nsing axis for measurements under tension or compression and B-B is the nonsensing axis. In Fig. B, line A-A is the non-sensing axis and line B-B is the sensing axis for tension or compression. The "output" in the direction of the non-sensing axis is designed to be substantially zero when strained in this direction to minimise cross sensitivity. In Fig. C, a shear gauge is shown; this does not have to lie on the central axis of the bolt, merely parallel thereto.

Axes X, Y and Z are the sensing axes for the 120 rosette shown in Fig. D, which is an example of a composite gauge based on types i and ii of which there are many possibilities.

The magnitude and direction of the applied load can be determined by using a gauge rosette mounted on the end of the rod, that it to say an array of gauges e.g. three gauges at 120 spacings. Stacked rosettes can be used, i.e. rosettes on top of one another.

Longitudinal wires, centrally embedded within or disposed on the rod, can be used to measure shear e.g. in a lug assembly.

The preferred gauges are those having an expansion/contration temperature coefficient matching that of the material to which the gauge is to be attached i.e. self-temperature compensating gauges. Another particularly useful type of strain gauge is the fatigue life gauge which responds to the number of loading cycles applied. For instance, the foil of a standard electric resistance strain guage may be made from self-temperature compensating CONSTAN TAN (Trade Mark), There is available a range of temperature compensations for any given strain gauge configuration to match the expansion/contraction characteristics of steel(s), aluminium alloys, and many other materials such that the "apparent strain" arising from an increase in temperature, and hence expansion of the substrate, is negligible.

According to a second aspect of this invention, I propose a load measurement device for insertion in a load-bearing structure, comprising an outer elongate body having an internal, longitudinal bore, an inner elongate ceramic or glass body fixed in the bore of the outer elongate body and having a close, sliding fit therein, wherein strain sensing means is disposed on the exterior of the inner elongate body.

The strain sensing may be achieved by using any resistive means such as composition resistors or resistive layers or films applied to the rod alternatively to or in combination with strain gauges, and optionally with wires/ribbons. The resistive layers or films may be applied to any desired region of the rod. For example the film may be continuous and cover the whole surface of the rod. For rods made of conductive material, the film should be insulated therefrom.

The film may be of any suitable thickness and is typically (1.2-2.6) X 10-3 cm.

There are many conventional techniques for applying resistive films to substrates, as described in "Thick-Film Microelectronics" by Morton L. Topfer published by Van Nostrand Reinhold Company.

The substrate should be smooth, insulating and resistant to high temperatures. Examples of suitable materials are high-temperature resistant glasses, and ceramics e.g. alumina and sillimanite porcelain, zircon porcelain, quartz, beryllia, zirconia, magnesia, thoria, boron carbide, various titanates, steatite and fosterite.

Ceramic substrates have several advantages: their coefficient of linear expansion approximates to that of steel. Their Young's modulus is greater than that for steel. Their Poisson ratio is about the same as that for steel, Their insulative ability is excellent. They can be employed at temperatures higher than 1 000 C. They are virtually non-hygroscopic. They will accept metallic (and other) films (resistive and conductive). They can be produced in a wide variety of shapes in.

large numbers and with close dimensional tolerances. Finally, they are relatively inexpensive.

There are some limitations to ceramics in that they are quite brittle, having a critical fracture strain of about 0.10 percent. They have some limited ability to withstand thermal shock. Their compressive strength is approximately ten times that in tension: therefore designs should be based on compression whenever possible. Also, the maximum dimensions of size which can be easily manufactured are limited.

The resisitve material should have a stable or constant resistive value with age and use, a relatively hard surface resistant to mechanical abrasion. Also, the material should be adherant to the substrate and have a high bulk resistivity and low temperature coefficient. The resistive material could be for example any precious metal, base metal, semiconductive material or a plastics material. Specific examples of suitable resisitive materials are the transition metals in Groups IVA, VA, VIA and VIIA of the periodic table, preferably tungsten, tantalum, molybdenum and vanadium. Others include gold/platinum/rhodium alloys which have a low range of ohmic resistance and low positive temperature coefficient or resistance, gold/palladium/rhodium alloys which have a higher resistance (increasing with the amount of palladium), and bismuth oxide and chromium oxide.

Furthermore, the resistive material should be non-reactive, non-oxidizable and resisitve to moisture, high humidity and funghus. Other specific examples are palladium/palladium oxide/ silver/ glass composition and thallium oxide/glass/ruthenium oxide/niobium oxide/ composition and tin oxide.

In addition to the resistive materials already described the resistive material may comprise any of the metallic alloys normally used for strain gauge work, such as CONSTANTAN (Trade Mark), ISOELASTIC (Trade Mark), NICHROME (Trade Mark), or any other metallic/etc alloy the resistance of which changes with applied strain, and cermets.

Typically, the resistive material is applied to the substrate as a paste which also includes a binder, such as ceramic glass or lead borosilicate glass, and an organic carrier. The resistive material may be in compound form such as oxide or halide and upon heating the compound is decomposed to leave the metal behind and the organic material is driven off. The metal compound e.g. a resinate or abieate, used may be soluble in a solvent which is mixed with the binder.

The resistive material might be in the form of granules, particles, powder, flakes or platelets.

A suitable paste comprises the resistive metal in powder form plus a fusible glass frit with an organic vehicle.

Suitable metals include gold, silver, platinum, palladium, rhodium iridium, ruthenium, iron, cobalt, nickel and copper. Platinum is the most preferrred and more so in the form of small flakes or platelets.

Resistive paste or "ink" is typically deposited similarly to silk-screen printing in that a fine mesh screen may be used to define the required pattern (comprising the resistive path) of the film, for example in the form of a helix along the length of the rod substrate or at ends of the rods on flats or recesses. A helical track to receive the winding may be formed in the rod by grinding with e.g. diamonds, or a grinding wheel or disc. Thus the film can be applied in any required pattern e.g. a grid or a continuous layer. The pattern is photographically made and the holes in the mesh through which paste is not to be passed are blocked by emulsion. The structure is fired to sinter the paste as earlier described.

Alternative methods of applying a resistive material include vapour deposition, pyrolitic deposition, cracking, electro-deposition/plating, chemical reduction, spraying, sputtering, dipping, brushing and stencilling.

The resistive material may need to be trimmed to the required size and shape. This can be done with a laser, an air abrasive machine, spark erosion, chemical etching, lapping down or grinding. The first two are higher accurate and preferred. Wire windings can be attached to ceramic substrates by any suitable adhesive or cement e.g. ceramic cement or epoxy resin.

Resistor terminations and the like can be applied similarly to the resistive material.

An encapsulant glaze may be applied to the coated rod in order to protect and insulate the coating. The glaze is also a barrier to moisture and obnoxious gas to improve stability. The glaze may comprise for example a glass/ceramic compound.

In a further device a layer of the resistive material may be applied over some or all of the substrate and by making lesions to this layer, the required resistive quantity can be obtained.

With helical grooves, for instance, the pitch of the grooves determines the value of the resistance of the device.

The use of resistive films is advantageous especially because the rods can be mass-produced at low cost. Also, the reliability is good. Therefore, to reiterate the load measurement device of this invention can have any required combination of the strain measuring means abovementioned. Also, load measurement devices according to this invention may be employed in any desired number and combination of orientations. For example, two devices may be employed with mutually perpendicular sensing means so as to give directional sensitivity.

The device according to this invention is advantageous because it can be mass-produced rather than purpose-made and it can be employed in diverse load-bearing applications. They are especially useful for testing structural joints in structures such as aircraft, helicopters, cranes, ships, rigs, winches, cables, weighbridges, marine moorings, testing machines and the like.

Furthermore it is straightforward and relatively inexpensive to install. It would in many cases be feasible to drill bolts in existing structures to accommodate the rod and strain sensing means.

The fit between the rod and bolt is not greatly critical with respect to output level and angularlity of bolt insertion in a strucutre. This is shown in the examples. Typical spacings are in the range 0.0001-0.001" . The accuracy of fit between the rod and bolt can be obtained using known techniques. Preferably, the bore has a non-smooth finish to improve the keying of the emedding adhesive. These comments also apply to the fit of the device in a load-bearing structure. Replacement of rods in bolts can be achieved, if necessary, by drilling out the old rod.

Tensile, compressive, bearing and shear loads can be measured with the load measuring device of this invention. By bearing stress is meant the force acting on the test pin/bolt per unit area of pin/bolt i.e. the localised load.

The total load can be calculated from a graph of the load applied versus the distance along the structural connection. The shear strain can be indirectly obtained.

As mentioned previously, the direction in which the applied load acts can be determined.

According to a third aspect of this invention, I propose a method of determining the direction in which an applied load acts in a strained structural connection, comprising incrementally, angularly moving a load measurement device, including a plurality of transverse or longitudinal gauges or a longitudinal wire, about its longitudinal axis, measuring the strain in the incremental positions and taking the direction of application of load to be parallel to the direction in which maximum strain takes place.

Also, the load due to shear forces can be measured by locating the load measurement device so that the forces are applied to the longest dimension of the device. By shear forces are meant tangential forces which tend to produce angular deformation of a body without change in volume.

According to a fourth aspect of the invention, I propose a method of measuring loads applied in a load-bearing structure using a calibrated load measurement device comprising an outer elongate body having an internal, longitudinal bore, an inner elongate body fixed in the bore of the outer elongate body and having a close, sliding fit therein, wherein strain-sensing means is disposed on the exterior of the inner elongate body comprising locating the device in a loadbearing structure so that shear forces are capable of acting in a plane normal or substantially normal to the central, longitudinal axis of the outer, elongate body and obtaining one or more measurements of load representative of shear forces acting in the load-bearing structure by taking one or more readings from the strain-sensing means.

Thus, it is possible to measure the shear load applied through single shear, double shear or multiply connected lug joints. The load due to shear forces acting in lug or strap joints i.e.

structural connections consisting of a plurality of bolt/pin joints can be determined. These are found in for example aircraft and helicopters, in particular the attachement of the blades to the rotor hub.

It has been found that unexpectedly extremely linear shear load characteristics are obtained using load measurement devices according to this invention. This is significant since accurate prediction can be obtained of the effect on the structure bearing the shear load.

It is important to point out that by the method of this invention a complex distribution of strains representative of the load due to shear forces is measured, which may be a combination of one or more of compressive and tensile strains and strain proportional to the shear load.

Helical windings of narrow pitch substantially less than the length of the rod are especially useful in measuring shear, as illustrated later.

Embodiments of the invention, in relation to which further features of the invention are disclosed, are described by way of example with reference to the drawings, in which: Figure 1 (a) shows diagrammatically a single shear lug joint; Figure 1(b) shows diagrammatically a double shear lug joint; Figures 2 (a) to (d) show diagrammatically strain gauges employable in the invention; Figures 3 (a) and (b) show the strain across a double shear lug joint; Figures 4(a) (side view) and (b) (end view) show a load measurement rod according to this invention having a strain gauge on one end; Figures 5 (a) (side view) and (b) (plan view) show rods carrying strain gauges in flats on the rod;; Figure 6 (a) shows a ceramic rod according to this invention carrying a single start helical winding and having a central, longitudinal bore and Figure 6 (b) shows a ceramic rod as shown in Fig. 6 (a) but with no bore; Figure 7 (a) is a side view of a ceramic rod, with no central hole, carrying a conductive film connecting a strain gauge at one end of the rod and Figure 7 (b) is the end view of that end; Figure 8 (a) is a side view of another ceramic rod using solder pads in flats connected by conductive film and Figure 8 (b) is a plan view of the same; Figure 9 (a) illustrates a rod under "full-clamp"; Figure 9 (b) illustrates the strain curve of a rod under "full-clamp" using strain gauge measurement; Figure 9 (c) illustrates the strain curve for "half-clamp" using strain gauge measurement;; Figure 9 (d) illustrates the strain curve for "full-clamp" using helical winding measurement; Figure 10 is a graph of strain against fit of rod in bolt; Figure.11 shows diagrammatically the lug assemblies used in a series of tests in measuring shear loads; Figure 12 is a graph of applied load versus measured strain for lug assemblies 1, 2 and 3 and bolt number 2; Figure 13 is a polar plot of strain for a given angle of disposition of bolt for bolts 1, 2 and 3 in lug assembly 1; Figure 14 is a graph of applied load versus strain for bolts 1 to 4 in lug assembly 1; Figure 15 is a graph of applied load versus strain for given angles of bolt 4 in lug asembly 1; Figure 16 is a polar plot of strain for a given angle of disposition for bolt 4 in lug assembly 1;; Figure 1 7 is a graph of applied load versus measured strain for bolt 4 in lug assemblies 1 and 2; Figure 18 is a graph of applied load versus measured strain for a series of ceramic rods 1 to 4; Figures 19 (a) and (b) show diagrammatically strain gauge bridges in single and double configurations respectively employed in measuring strains; Figure 20 is a graph of measured strain versus applied load using gauge y in double and single configuration bridges; Figure 21 is a graph of measured strain against distance of load measurement device using strain gauge x through a clamping system under full (FC) and half (HC) at a load of 1 50 LBF; Figure 22 is a graph of measured strain against distance of device using strain gauge y through clamping system under full (FC) and half clamp (HC) at a load of 1 50 LBF;; Figure 23 is a graph of strain versus applied load at a series of distances through the load using gauge x under half clamp; Figure 24 is a graph of strain using gauge y under full clamp; Figure 25 is a polar plot for gauge x; Figure 26 is a polar plot for gauge y; Figure 27 is a graph of measured strain against distance along pin for a device employing a single, longitudinal grid gauge under a load of 300 LBF; and Figure 28 is a graph of measured strain against distance along pin for a device employing a single, longitudinal grid gauge in a double shear lug assembly under a load of 1.97KN.

In a load measuring rod 1 (bolt not shown) shown in Figs. 4 (a) and (b) there are two longitudinal, diametrically opposed grooves 2, made by cutting. A strain gauge 3, herein referred to as the master gauge, is attached to the end 4 of the rod. Two copper wires 0.002" in diameter (47 S.W.G.) are attached to the gauge stub wires 5 at the collar 6 by soldering and each is cemented into one of the grooves. More than two wires and grooves can be used accordingly. The electrical junctions at the collar are strengthed and protected with a bead of Araldite (RTM) Cement. The plastics collar 6 can be made of Araldite CT 200 and is also cemented in position.

Although it has been stated that the rod can be of any material, plastics rods are insulating and they facilitate alternatively soldering non-insulated 47 S.W.G. (standard wire gauge) copper wire to the strain gauge. However, plastics rods can take up moisture over a period of time.

Thus, the rod should be sealed in to minimise the entry of moisture into the bore and this is easily achieved. Metal, e.g. steel, rods require insulated wire. The above construction is similar to that of the next six rods In another type of rod there is a transverse groove machined in the end of the rod in which the master gauge is cemented and the groove is filled in. A slave gauge e.g. of either of type and ii previously mentioned, is fitted on the end of a third rod. The sensing axis of the slave gauge is at 90t to that of the master gauge. These gauges do not have to be of the same type.

Four longitudinal grooves 8 are required for this rod, two for the wires for each gauge.

In a fourth rod, a portion of the rod has a reduced diameter; this can assist in obtaining a good fit in the housing bore of the pin or bolt. A fifth rod, comprises metal and plastics portions joined together. A sixth rod, has a threaded portion at the end to be inserted in the pin or bolt first, in which load transfer over the flanks of the thread can be obtained.

All six of the configuration types just described, may utilise gauges of types i and ii either alone or in combination.

Strain gauges may be mounted on flats on the rod. In a seventh rod, shown in Figs. 5 (a), and (b) (plan view), strain gauges (7) are cemented to shallow recesses (flats) 8 made in the surface of the rod e.g. by machining. Standard or purpose-made gauges can be used. The connecting wires (not shown) can be taken down a central hole in the rod if desired.

This seventh rod may be used to measure tensile strains, and hence tensile loads, as well as bearing loads and shear loads, that is to say tensile loads parallel to the direction of the longitudinal axis of the device and bearing loads and shear loads in a direction 90 to the longitudinal axis. If the device can measure tensile loads parallel to the direction of the longitudinal axis, then it will measure compressive loads in the opposite direction. The tensile strain can be measured with the sensing axis parallel to the longitudinal axis of the rod. The stresses and loads related to the strains may also be determined.

Load measurement devices employing strain sensitive wire can be used in the methods of the second and third aspects of the invention. The wire may be wound along the whole length of the rod or over any part of the rod according to where measurement is required. These may be described as integrating transducers since they measure strain over a length of the rod, according to the effective length of the rod occupied by the wire winding. The strain sensitive wire may be wound round the trough of a continuous, circumferential V-shaped thread cut in the rod e.g. 0.003" deep and 56 TPI (turns per inch) pitch. The windings are, for instance, single start or double start. An earth is made via copper wire (e.g. 47 S.W.G.) cast generally centrally within the rod. If the wire is insulated a central earth is not required. The wire may be located in longitudinal grooves.Strain-sensing wire can be alternatively embedded in the rod.

Devices of the types mentioned, can be used to measure the direction of the applied load, (for example should the direction of the applied load change during use of the device), without the need to rotate the pin or bolt. The change in the direction of the applied load can be determined by the magnitude of the strain signal measured, The directional change could occur in a device which is inaccessible e.g. an anchor shackle on the sea bed and the invention is particuarly useful in such an environment.

Ceramic rods are illustrated in Figs. 6 (a) and (b) and resistive means may be applied to the surface of the rod by any means. The rod may be insulated by a glaze, or other surface treatment, from the hole into which it has to be inserted in the metal pin or bolt. A ceramic rod carrying a single start helical winding and having a central, longitudinal bore 9 is shown in Fig. 6 (a). A similar rod without a central bore is shown in Fig. 6 (b). In a particular rod the length of the rod, is 2 < "; The diameter is 3.2 mm to tightly fit into a "pin" or "bolt" having a central bore. The central bore of the rod is 0.01-0.02" in diameter. The pitch of the winding (p) is 20 to 40 TPI and nearer 40 is preferable. The width of the winding, w, is optional. The resistance wire often used is 0.001" diameter copper-nickel or nickel-chrome.The deposited material can be self-temperature compensating. The prime consideration is that the resistance of the material changes with the applied load. A strain gauge bridge may employ the load measuring device according to the invention in 4, T or full bridge modes. The ceramic rod can have a double start helical winding, wherein suitable pitches are 10-20 TPI and nearer 20 TPI is preferable.

A rod (without a central bore) with an elongate conductive/resistive film 10 applied along the side of the rod is shown in Figs. 7 (a) and (b). The rod has a strain gauge 11 at one end thereof as shown in Fig. 1 5 connected by solder pads 1 2 and the conductive film to the lead-out wires e.g. in a strain gauge bridge.

A ceramic rod having conductive/resistive film applied discretely thereto is shown in Figs. 8 (a) and (b). The electrical connections are made via solder pads 1 to 8 in two sets of flats comprised by portions of reduced diameter in the surface of the rod, and one end of the rod.

The load measurement device according to this invention must be calibrated for use i.e. the change in the resistance of the resistive material must be measured over a range of known applied loads so that unknown applied loads can be determined for known changes in resistance. This can be done as follows. Known applied loads may be applied for instance around the entire periphery of the pin/bolt housing the rod ("full clamp") or around one half of the periphery of the rod ("Half-clamp") using one or two loading plates A and B, as shown by Fig. 9 (a) and passing the device therebetween.

For example for a given load under full clamp and a given contact width of the clamping means with the rod, a strain distribution for strain gauges as shown in Fig. 9 (b) is obtained. For the same under half-clamp when the second load plate may be replaced by a cantilever or simple beam support, the strain distribution is shown in Fig. 9 (c). These strain distributions may be obtained using a special test rig. The values of the maximum ordinate of strain (Fig. 9 (b), (c)) gives an indication of the sensitivity and efficiency of the transducer without recourse to the testing of the same in a double shear lug joint.

The clamping plates employed may suitably be 2mm thick for reasonable accuracy. The strain distribution is measured over the length of the rod. The maximum ordinate of strain under full clamp is twice that under half clamp. Similarly, for helical windings under full clamp a strain distribution according to Fig. 9 (d) is obtained.

The effect of pin/bolt to hole fit (clearance) can be examined by machining in increments the width Wof the clamping/loading plates and taking measurements of strain for each width value. A typical result is shown in Fig. 1 0. A is a possible range of close tolerance fits and B is that of wide tolerance fits. At 1.0 there is full contact of pin/bolt and hole and at 0.0 only point contact.

The loading plates represent locally and incrementally the load circumstances obtaining if one considers discrete widths of loaded members in a structural connection, that is to say full clamp exists at or near the shear faces in a double shear lug assembly.

In real-life load-bearing structures e.g. a crane jib, multiple gauges or a combination of different types of strain-sensing means will be employed to measure both applied load and angle of application simultaneously.

Tests were carried out applying shear forces according to this invention to load measurement devices as follows.

Load transducers manufactured from a filament of fine electric resistance strain gauge wire were embedded in a series of four steel bolts, of constant shank diameter, 1 3 mm, and constant shank length, 50 mm. The transducer assembly essentially comprised a plastics rod of diameter 1/8 in, and effective length, approximately 50 mm, to which the wire filament was suitably attached. Each bolt was centrally drilled and reamed throughout its entire length to accept the transducer. The transducers were embedded in the bolts in a matrix of Epoxy resin. The bolts were tested in a double shear lug joint.

Headed bolts were used in these experiments solely for accommodating electrical connections via an electrical socket cemented within a recess machined in the bolt head. A nut, assembled to the bolt(s) for safety reasons, was not tightened to an extent which 'clamped' the lug assembly.

Details of the bolts, rods, and transducers investigated are summarised in the Table 1.

The transducers embedded in Bolts Nos. 1, 2 and 3, take the form of a rod, helically grooved, to accept a winding (single layer solenoid), of strain gauge wire in copper/nickel, 0.001 in diameter. The transducer rod assembly relating to Bolt No. 1 was an easy push fit in the hole, but still, a close fit.

The central holes drilled in Bolts Nos. 2 and 3 were 1/32 in larger in diameter than the transducer rods embedded therein, the gap being filled by Epoxy resin. Deliberately, no attempt was made, using suitable spacers, to centralise these rods throughout the length of the embedment.

Table 1 All Bolts Diameter 1 3 mm Ground Material Steel Finish, Central Hole Drill/Ream All Rods (Transducers) Diameter 1/8 in Nominal Material Araldite Casting resin.

Earth return wire cast in-situ (copper) Bolt Rod (Transducer) Groove Wire Bolt Colour Nom. Hole No. Code Dia. Type Form Type Dia.

5.B.A.

Blue Thread 1 Red 1/8" Helical 43.10 Cu.Ni or'001 Blue G.P.I.

"V" Depth Blue 01'003 2 Blue 5/32" Helical 56 Cu.Ni 0''001 Blue G.P.i.

"V" Depth Red 0''003 3 Red 5.32" Helical 56 Cu.Ni. Or'001 Red G.P.I.

Red , ''V'' 4 Blue 1/8" Longi- Depth Ni.Cr 0''0005 Red tudinal 0''003 N.B.

G.P.I. = Grooves per inch Cu.Ni = Copper/Nickel Ni.Cr = Nickel/Chrome Since the tranducers embedded in Bolt Nos. 1, 2 and 3 are, on the face of it, axially symmetric, the output obtained under load from these bolts should be substatially insensitive to the angular orientation of the bolt(s) in a lug assembly. This was found, subsequent to test, to be very nearly true for Bolt No. 1, but not so for Bolt Nos. 2 and 3, as might be anticipated bearing in mind the method of embedment, a point discussed later.

An entirely different geometry of transducer was embedded in Bolt No. 4. This consisted of a single wire filament in Ni.Cr., 0.0005 in diameter, located in a shallow longitudinal "V" groove machined in the surface of the rod. The transducer assembly was embedded in the bolt via an easy push fit comparable to that employed for Bolt No. 1. Since the transducer embedded in Bolt No. 4 is non-axially symmetric, the output under load would be expected to depend upon its angular orientation in a lug assembly. Such a characteristic is useful, insofar as it permits the approximate direction of the applied load to be ascertained.

The rods used for the transducer embedded in all the four bolts incorporated a central cast 'in situ' copper earth return wire. Thus, electrical connections could be made at the head of the bolt(s) via a subminiature multi-way plug and socket.

All transducers were, in turn, connected into the same arm of the strain gauge bridge, in T bridge configuration.

The geometries of the three types of double shear lug assemblies investigated, in steel, are shown in Fig. 11, all dimensions being in mm. The width of all the assemblies, normal to the axis of the hole, was 36 mm. The thickness of the tines of the fork end and of the eye end of assemblies, Nos. 1 and 2, was 8 mm and 16mm respectively, i.e. in the ratios 1:2:1 to give approximately equal tensile and bearing stresses in each element. The tines of the fork ends and eye ends of both assemblies had separately assembled within them, by means of an interference fit, standard commercial jig bushes of external diameter 1 8mm.The internal diameter of the jig bushes in the case of assembly No. 1 was 1 3 mm, providing a close push fit to the bolt(s), and in assembly No. 2,14 mm, giving a dimatrical clearance to the bolt(s) of 1 mm, or about 7.7% based on 1 3 mm. The jig bushes afforded a means of achieving a hole surface finish difficult to obtain with drill and reamer, thereby affecting some control over one of the relevant variables, surface roughness. Lug assembly No. 3 had tines and an eye in the ratios 1:1:1, with unbushed holes, diameter 1 3 mm, drilled and reamed, to give a close push fit to the bolt(s).

If the transducers embedded in all four bolts are connected into the same arm of the strain gauge bridge, as indeed was the case, then for Bolt Nos. 1, 2 and 3, the scales of shown in the accompanying Figures should take a - VE sign, contrary to the + VE sign indicated. This has been used solely for convenience of presentation and to facilitate comparison of the results obtained from all four bolts. The se scales appertaining to Bolt No. 4 are, in all Figures, accorded the correct + VE and - VE signs.

A series of loads was therefore, applied using the double shear lug assemblies and the corresponding, resulting shear strain measured. A 'holding' load of 1 kN was applied in all tests to prevent movement of the lug assembly from its initial set position in the jaws of the testing machine.

A summary of the test results for Bolt No. 1 in each lug assembly is given in Fig. 12, (the results for Lug No, 2 being extrapolated from 10 kN to 20 kN as indicated). The maximum load applied in these initial test runs was generally restricted to 20 kN in order to avoid indentation and permanent damage to the bolt. In the case of Lug No. 2, the maximum load applied was restricted to 10 kN for the same reason.

The output characteristics for Bolts Nos. 1, 2 and 3, are summarised, for an applied load of 30 kN, in the polar plot given in Fig. 1 3.

A summary of tests on all four bolts, using Lug No. 1, is given in Fig. 14.

The results of tests on Bolt No. 4, assembled in Lug No. 1, are given in Figs. 1 5 and 16, which illustrate the anticipated angular dependence of this bolt. Comparative results are shown in Fig. 7, for Bolt No. 4 in Lugs Nos. 1 and 2.

The results indicate: (a) The axially symmetric transducer embedded in Bolt No. 1 is virtually insensitive to the angular position of the bolt in Lug No. 1 (Fig. 13).

(b) The transducers embedded in Bolts Nos. 2 and 3 should also be angularly insensitive since the transducers themselves comprise axially symmetric helical windings. However, owing to the (deliberate) method of 'loose' embedment adopted, angular polarisation occurs (Fig. 13), although the polar plots for these bolts are geometrically similar, if not in magnitude.

(c) Notwithstanding the comments made in (b) the load/output characteristics for Bolts Nos.

1, 2 and 3, were found to be surprisingly linear, irrespective of the angular position of the bolt(s) in the lug assembly.

(d) The fit of the bolt(s) within the holes in the lug assemblies appears not to be critical in respect of the linearity obtained, although maximum outputs differ in magnitude. Bolt No. 1 exhibited good linearity of output more or less irrespective of lug geometry, although unexcepectedly the 1 4 mm clearance hole, Lug No. 2, gave the best results, (f) Maximum output readings for all four bolts investigated, using Lug No. 1, are given in Fig.

1 4 in which the sign conventions previously discussed should be borne in mind. The scale of in this Figure should be read as - VE for Bolts Nos. 1, 2 and 3 and + VE as indicated for Bolt No. 4.

(g) The plots for Bolt No. 4 (Fig. 15) are markedly non-linear apart from those relating to B= 1 20, 1 50 and 1 80 degrees. The output obtained from this transducer could be increased and modified in shape by the embedment of a number of longitudinal wires in lieu of the single wire configuration considered here.

The polar plot for the same bolt (Fig. 1 6) suggest that this characteristic might be used to determine the approximate direction of an applied load if a modification of this transducer were to be embedded together with another transducer of different type, the latter being dedicated to the measurement of load magnitude.

(h) From tests on Bolt No, 4, loaded in Lug Nos. 1 and 2 (Fig. 1 7) such a transducer system could be 'tuned' by adjustment of the bolt to hole clearance, a consideration which might also apply to Bolt No, 1.

The output from the helical windings embedded in Bolt Nos. 1, 2 and 3 should be virtually insensitive to the angular position of the bolt(s) in the lug joint, provided certain rod insertion conditions are met, since the windings are axially symmetrical. This is so for Bolt No. 1, see Fig.

13.

The output from Bolt No. 4 (single longitudinal wire) is dependent upin the angular postion of the bolt in the lug joint (Figs. 1 5, 1 6 and 17) because the embedded wire does not lie on the central axis of the bolt. A maximum output is obtained (Fig. 16) when the embedded wire is located at 180 degrees towards the lug eye end.

Beariny in mind the sign convention previously discuseed, it can be seen from Fig. 1 4, Bolt Nos. 1 and 4 (maximum output) that the sign of the output for Bolt No 1 should be negative, and that for Bolt No. 4 positive, as shown. Thus the test results indicate that a change in the sign of the output occurs somewhere between the configurations of a helical winding of relatively small pitch, i.e. 43.10 T.P.I. for Bolt No. 1, and a helical winding of infinite pitch, i.e.

the single straight wire embedded in Bolt No. 4.

The results (applied load versus measured strain) of mechanical tests in which a bolt having a ceramic rod embedded, which bears a helical winding, is employed in two similar, separate lug assemblies A and B are given in Fig. 1 8. Each lug assembly was cycled three times between the 'holding-load', 1 KN and the maximum load applied, 20 KN, prior to the recording of load/strain readings. Lines 1, and 2, relate to lug assembly A, and lines 3, and 4. to lug assembly B. The curves, 1 to 4 each represent single sequential test runs and not the average of several runs per test.

It can be inferred that the fit between a custom designed transducer (rod), and the hole in the bolts is not so critical.

Tests were also conducted on load measurement devices employing strain gauges. The devices had steel pins and the gauge or gauges were attached to the end of a constant diameter Araldite (Trade Mark) rod, about 0.32 mm in diameter and the length of the rod was approximately half that of the pin so that the gauges were located approximately mid-way along the pin.

Two types of gauges were tested: (x) Single longitudinal grid (y) Two transverse grids, sensing axis mutually perpendicular in strain gauge bridges having single and double configurations (see Fig. 1 9A) which shows single configuration and 1 9B) which shows double configuration and 1 and 2 denote master gauges, 3 denotes a compensating master gauge, 4 and 6 denotes a compensating slave gauge and 5 denotes a slave gauge.

Readings were taken of strain against applied load with gauge y, in single and double configurations, in the way of the centre line of the load measurement device (i.e. line through centre of device perpendicular to central longitudinal axis) under full clamp loading. The results are shown in Fig. 20.

Readings were also taken of strain at predetermined distances through a loading ring for gauges types x and y, single configuration, full and half clamp and the results are shown in Figs. 21 and 22 respectively.

Fig. 23 shows strain versus load for gauge x device under half clamp taken at predetermined distances through the load.

Fig. 24 shows the same as Figs. 2 and 3 but for a gauge y device under full clamp.

Fig. 25 shows a polar plot of the strain at incremental angles of orientation with a gauge x device.

Fig. 26 shows the same as Fig. 25 but for a gauge y device.

In further tests, a single longitudinal grid gauge was cemented to the end of a rod, which was about half the length of the pin so that the gauge was about mid-length of the pin on insertion of the rod. The holes in the test lugs were not bushed.

Readings were taken of strain versus distance along the pin and the results are shown in Fig.

27. Results are shown in Fig. 28 for the strain versus distance through assembly of the pin in double shear lug assembly No. 1 (referred to earlier).

Obvious changes and variations in the devices and methods described herein will be apparent to persons skilled in the art. Accordingly, the scope of the invention is not to be limited except as defined in the claims.

Claims (32)

1. A load measurement device for insertion in a load-bearing strucutre, comprising an outer elongate body having an internal, longitudinal bore, an inner elongate body fixed in the bore of the outer elongate body and having a close, sliding fit therein, wherein non-wire strain-sensing means is disposed on the exterior of the inner elongate body.

2. A device according to claim 1, wherein the bore in the outer elongate body is symmetrically disposed about the central, longitudinal axis of the outer elongate body.

3. A device according to claim 1 or 2, wherein the inner elongate body has an external collar adjacent its end to be inserted last into the bore of the outer elongate body, made of insulating material and to which the strain-sensing means in connectable.

4. A device according to claim 1, 2 or 3, wherein the inner elongate body is attached to the outer elongate body by an adhesive or cement.

5. A device according to any one of claims 1 to 4, wherein the adhesive is prepressurised during hardening to effect a radial compression between the inner elongate body and the bore of the outer elongate body.

6. A device according to any preceding claim, wherein the strain-sensing means includes one or more strain gauges located in one or more recesses in the surface of the inner elongate body.

7. A device according to claim 6, wherein the or each strain gauge is self-temperature compensating.

8. A device according to any preceding claim, wherein one or both of the elongate bodies is cylindrical.

9. A device according to any preceding claim, wherein one or both of the bore and the exterior of the outer elongate body is non-smooth.

10. A device according to any preceding claim, wherein the open end of the bore of the outer elongate body is sealed.

11. A device according to any preceding claim, wherein the inner elongate body is made of glass or ceramic.

1 2. A device according to claim 11, wherein the strain-sensing means includes a layer of conductive or resistive film applied to exterior of the inner elongate body.

1 3. A device according to claim 12, wherein the film is continuous.

14. A device according to claim 12, wherein the film is helical, which produces a predetermined resistance.

1 5. A device according to any one of claims 1 2 to 1 4, wherein the applied material comprises one or more of metals from Groups IVA, VA, VIA, and VIIA of the Periodic Table.

1 6. A device according to any one of claims 1 2 to 15, wherein the applied material is trimmed to a predetermined size by laser or air abrasive trimming.

17. A device according to any one of claims 12 to 16, wherein an encapsulant glaze is carried on the inner elongate body.

1 8. A method employing a device according to any preceding claim, comprising locating the device on a load-bearing structive, incrementally, angularly moving the device about its longitudinal axis and taking measurements of strain at the incremental positions, whereby the direction of the applied load is the direction in which maximum strain is found.

1 9. A method employing a device according to any one of claims 1 to 17, comprising locating the device in a load-bearing structure and measuring strain at predetermined times to determine change in strain with time corresponding to change in direction of applied load.

20. A method of calibrating a device according to any one of claims 1 to 17, comprising applying a series of known loads to the device and measuring the strain corresponding thereto.

21. A load measurement device for insertion in a load-bearing structure, comprising an outer elongate body having an internal, longitudinal bore, an inner elongate ceramic or glass body fixed in the bore of the outer elongate body and having a close, sliding fit therein, wherein strain-sensing means is disposed on the exterior of the inner elongate body.

22. A device according to claim 21, wherein the strain-sensing means is in the form of wire or a layer of resistive film.

23. A method of determining the direction in which an applied load acts in a structural connection, comprising incrementally angularly moving a load measurement device, including a plurality of transverse or longitudinal gauge or a longitudinal wire, about its longitudinal axis, measuring the strain in the incremental positions and taking the direction of application of load to be parallel to the direction in which maximum strain takes place.

24. A method of measuring loads applied in a load-bearing structure using a calibrated strain measurement device comprising an outer elongate body having an internal, longitudinal bore, an inner elongate body fixed in the bore of the outer elongate body and having a close, sliding fit therein, wherein strain-sensing means is disposed on the exterior of the inner elongate body, comprising locating the device in a load-bearing structure so that the shear forces are capable of acting in a plane normal or substantially normal to the central, longitudinal axis of the outer elongate body and obtaining one or more measurements of the load representative of shear forces by taking one or more readings from the strain-sensing means.

25. A method according to claim 24, wherein the strain-sensing means includes a helical winding of pitch substantially less than the length of the inner elongate body.

26. A method according to claim 25, wherein the pitch is the range 20 to 40 turns per inch.

27. A method according to claims 25 or 26, wherein the winding is single start or double start.

28. A method according to claims 24, wherein the strain-sensing means includes one or more strain gauges mounted on the end of the inner elongate body or in recesses along the length of the inner elongate body.

29. A load measurement device constructed and arranged substantially as herein described with reference to any of the Figs. 4 and 5 of the drawings.

30. A load measurement device including a glass or ceramic inner elongate body constructed and arranged substantially as herein described with reference to any of the Figs. 6 to 8 of the drawings.

31. A method of determining the direction in which an applied load acts in a structural connection substantially as herein described.

32. A method of measuring loads applied in a load-bearing structure including locating a load-bearing device so that the shear forces are capable of acting in a plane normal or substantially normal to the central, longitudinal axis of the device, substantially as herein described with reference to any of Figs. 3, 9 and 1 2 to 28 of the drawings.