WO2006042368A1

WO2006042368A1 - Shear force measurement in a structural member

Info

Publication number: WO2006042368A1
Application number: PCT/AU2005/001612
Authority: WO
Inventors: Gregory Boswell Beecroft
Original assignee: Meridian Engineers Pty Ltd
Priority date: 2004-10-19
Filing date: 2005-10-18
Publication date: 2006-04-27

Abstract

A strain measuring device (11) for a structural member (13), comprising at least one plate member (21) having at least one strain responsive element (23) affixed thereto. The plate member(s) (21) are secured to the structural member (13) to allow shear stress in the structural member to be transmitted to the plate member(s) (21). The plate member(s) are secured by at least two nuts (27) and bolts (25), with the strain responsive element(s) (23) located between axial extents of opposed bolts (25). Each nut (27) and bolt (25) have located thereon a structural member contacting washer (29) located on one side of each plate member (21), and a plate member securing washer (31) located on the other side of each plate member (21). The structural member contacting washers (29) have an annular surface (37) to contact the structural member (13), in a peripheral extent around and spaced from the bolt shaft circumference.

Description

"Shear Force Measurement in a Structural Member"

Field of the Invention

This invention relates to measurement using strain gauges of shear stresses in structural members, and in particular but not limited to measurement of such stresses indicative of weight or load. A particular application of the invention is the measurement of axle loads in railway applications.

Background Art

Hitherto, strain gauges have been used in situ, in order to measure shear strain in structural members. Such strain gauges have been bonded directly to the structural member in the factory, and then the structural member is taken to the field and fitted in-situ. In a railway application, parallel lengths of rail are removed from a track, and two lengths of rail incorporating strain gauges fitted in the factory are shipped and are welded into the railway in parallel configuration, in place of the removed lengths of rail.

Problems with many strain gauges which are prefabricated in a structural member such as a rail include difficulties of procuring structural members which match the profile of the structural member at the location of the intended installation. This is particularly problematic in export applications. Other problems include handling and shipping difficulties associated with the prefabricated structural members that can often be 5 metres in length. In railway weighbridge applications, the installation of a prefabricated rail having shear strain gauges can require closure of the railway for a day while the prefabricated rails comprising the weighbridge are installed. There is also the difficulty associated with replacing the weighbridge in the event that the rail becomes worn, or the strain measurement gauges are otherwise damaged, for example due to train derailments. Furthermore, for a company specialising in manufacture and export sales of prefabricated rails incorporating strain measuring devices to make weighbridges, there is the problem with maintaining stock, due to the large number of different cross- sectional profiles of rail in use around the world. Notwithstanding these problems, in rail weighbridge applications, prefabrication of rails incorporating strain measuring devices is the norm, due to the extreme difficulty in fabricating these arrangements in-field.

It is an object of this invention at least to ameliorate some of these difficulties.

Cognisant of these difficulties, the applicant has devised many arrangements over a number of years to attempt to solve the problem. After conceiving several arrangements that appeared promising but ultimately proved to be unsuitable, the applicant has finally met with success.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "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.

Disclosure of the Invention

In accordance with the invention there is provided a strain measuring device for a structural member, said strain measuring device comprising at least one plate member having at least one strain responsive element affixed thereto, said at least one plate member being secured to said structural member to allow shear strain in said structural member to be transmitted to said at least one plate member, said at least one plate member being secured by at least two nuts and bolts, with said strain responsive element located between axial extents of opposed bolts, each said nut and bolt having located thereon a structural member contacting washer located on one side of each said plate member, and a plate member securing washer located on the other side of each said plate member, said structural member contacting washer having an annular surface to contact said structural member, said annular surface contacting said structural member in a peripheral extent around and spaced from said bolt shaft circumference.

Preferably annular surface is raised so as to be spaced from the main body of the structural member contacting washer. Preferably said annular surface is located atop a frustro-conical portion of said structural member contacting washer.

Preferably said annular surface is located atop a tapering portion of said structural member contacting washer.

Preferably said tapering portion extends axi-symmetrically about the bolt centre¬ line.

Preferably said tapering portion exhibits asymmetry in cross-section to maximise the diameter of the annular surface while providing structural support for the annular surface.

Preferably said annular surface of said structural member contacting washer is arranged to completely and continuously contact said structural member.

Preferably said structural member contacting washer has an annular face opposed to said annular surface for contacting said at least one plate member.

Preferably said annular face of said structural member contacting washer is arranged to completely and continuously contact said at least one plate member.

Preferably said plate member securing washer has opposed annular surfaces, at least one of which is commensurate with the shape and size of said annular face for contacting said plate member. In this manner the compressive forces exerted on said plate member provide a well defined contact area and contact geometry, so as not to distort the plate member. As the annular surface and annular face of the structural member contacting washer, and at least one of the opposed annular surfaces of the plate member securing washer are spaced from the bolt shaft circumference, it is necessary to ensure that the inner extent of these are concentric with the bolt shaft when being fitted. This may be achieved in a number of ways, one of which would be to employ a jig when fitting the strain measuring device to the structural member, so as to ensure that the washers are spaced from the bolt shaft when being fitted. - A -

Another way to space the annular surface and annular face of the structural member contacting washer, and at least one of the opposed annular surfaces of the plate member securing washer from the bolt shaft circumference could be to utilise a sleeve formed of a soft material between the structural member contacting washer and the bolt shaft, and between the plate member securing washer and the bolt shaft. The sleeve can be formed of a material that is softer than the material that the structural member contacting washer and the plate member securing washer are made from. As the sleeve functions as a guide, and does not in itself provide structural integrity, the sleeve could be formed of nylon or other plastic material.

Yet a further way, and the most preferred way to space the annular surface and annular face of the structural member contacting washer, and at least one of the opposed annular surfaces of the plate member securing washer from the bolt shaft circumference is to provide at least one internal annular step adjacent an inner periphery of each of the structural member contacting washer and the plate member securing washer, the internal annular steps having a minimum internal diameter commensurate with the bolt shaft diameter.

Preferably both opposed annular surfaces of said plate member securing washer are commensurate with the shape and size of said annular face for contacting said plate member. In this manner, it is not critical which way around that the plate member securing washers are fitted.

Preferably said at least one plate member has two said strain responsive elements affixed thereto, said strain responsive elements measuring strain in the vertical plane in two directions normal to each other, and in use at 45° to the longitudinal extent of said structural member.

Preferably said strain measuring device includes two said plate members for securing one on each side of said structural member, collectively said strain responsive elements measuring strain in four different vertical directions extending in use at 45° to the longitudinal extent of said structural member. Alternatively said at least one plate member has four said strain responsive elements affixed thereto, two on each side thereof; said strain responsive elements measuring strain in four different vertical directions extending in use at 45° to the longitudinal extent of said structural member.

Preferably each said nut and bolt assembly also includes a slip washer located adjacent to the bolt head.

Preferably each said nut and bolt assembly also includes a slip washer located adjacent to the nut.

Preferably said strain measuring device has two said plate members.

Preferably said strain measuring device has two said nuts and bolts.

Preferably said strain measuring device includes plate portions located on said bolt between each said plate member securing washer and the head of each bolt, and between each said plate member securing washer and each said nut, said plate portions being part of an assembly guide plate utilised when fitting said strain measuring device.

These plate portions may be provided in addition to or in lieu of slip washers. Preferably adjacent said plate portions include means to secure against rotation while said strain measuring device is secured in place to said structure member.

Preferably adjacent said plate portions are joined by at least two peripheral tags which are severable once said strain measuring device is secured in place to said structure member.

Alternatively said plate portions incorporate fastener portions to allow adjacent said plate portions to be secured to an assembly jig while said strain measuring device is secured in place to said structural member. Preferably said fastener portions comprise at least two threaded holes into which complementary threaded fasteners may be located to rigidly secure said plate portions to said assembly jig.

Either arrangement secures the plate portions and parts secured therebetween against rotation while the nuts and bolts are tightened.

In accordance with a preferred feature of the present invention there is provided a guide plate for use in fitting said strain measuring device, said guide plate having apertures for receiving each said bolt arranged in a bolt hole pattern corresponding with said plate member, said guide plate being divided into separate portions joined at the peripheral extent of said guide plate each portion being associated with a bolt hole, in use when fitting the strain measuring device said guide plate being located in contact with said plate member securing washer. In use a guide plate will be located between each said plate member securing washer and bolt head or nut or slip washer. The guide plate assists in alignment as the strain measuring device is fitted, and reduces torque being transferred to the plate members and strain responsive elements during tightening of the nuts and bolts. Once the nuts and bolts have been tightened, the joins at the peripheral extent can be cut through or ground off, so that the shear strain in the structural member is transmitted through the plate member(s).

In accordance with an alternative preferred feature there is provided an assembly jig for use in fitting said strain measuring device, said assembly jig comprising a rigid plate having clearance apertures through which the head of a said bolt or a said nut may be received, said clearance apertures being arranged in a pattern corresponding with said plate member, said clearance apertures having arranged thereabout at least two screw holes for securing plate portions by fastener portions thereof, said plate portions having apertures for receiving a said bolt, said plate portions when secured to said assembly jig having their apertures aligned with the bolt hole pattern of said plate member.

Preferably said fastener portions comprise threaded holes and said plate portions are secured to said assembly jig by screws extending through said screw holes. In use a strain measuring device will be fitted to drilled holes in a structural member by passing two bolts through the assembly of assembly jig and plate portions, then each bolt passing through a plate member securing washer, opposed ends of a plate member having the strain responsive elements, a structural member contacting washer, the structural member, and then a structural member contacting washer, opposed ends of a plate member having the strain responsive elements, a plate member securing washer, and then a further assembly of assembly jig and plate portions, and two nuts which are subsequently tightened to secure the strain gauge to the structural member. Once secured, the screws securing the assembly jigs to the plate portions are removed, allowing the assembly jigs to be removed, and leaving the plate portions in place.

Also in accordance with the invention there is provided a strain measuring device kit for assembly to form a strain measuring device to measure shear strain in a structural member comprising a pair of nuts and bolts, a pair of plate members each having two bolt holes and at least one strain responsive element affixed thereto between said bolt holes, four structural member contacting washers having an annular surface to contact said structural member in a peripheral extent spaced from said bolt shaft circumference, four plate member securing washers to secure said plate members to said structural member contacting washers.

Preferably said kit includes two guide plates for use in fitting said strain measuring device, said guide plates each having apertures for receiving each said bolt arranged in a bolt hole pattern corresponding with said plate member, said guide plate being divided into separate portions joined at the peripheral extent of said guide plate each portion being associated with a bolt hole, in use when fitting the strain measuring device said guide plate being located in contact with said plate member securing washer.

Alternatively said kit includes an assembly jig comprising a rigid plate having clearance apertures through which the head of a said bolt or a said nut may be received, said clearance apertures being arranged in a pattern corresponding with said plate member, said clearance apertures having arranged thereabout at least two screw holes for securing plate portions by fastener portions thereof, said plate portions having apertures for receiving a said bolt, said plate portions when secured to said assembly jig having their apertures aligned with the bolt hole pattern of said plate member.

Preferably said fastener portions comprise threaded holes and said plate portions are secured to said assembly jig by screws extending through said screw holes.

In use a strain measuring device will be fitted to drilled holes in a structural member by passing two bolts through the assembly of assembly jig and plate portions, then each bolt passing through a plate member securing washer, opposed ends of a plate member having the strain responsive elements, a structural member contacting washer, the structural member, and then a structural member contacting washer, opposed ends of a plate member having the strain responsive elements, a plate member securing washer, and then a further assembly of assembly jig and plate portions, and two nuts which are subsequently tightened to secure the strain gauge to the structural member. Once secured, the screws securing the assembly jigs to the plate portions are removed, allowing the assembly jigs to be removed, and leaving the plate portions in place.

Also in accordance with the invention there is provided an axle load measuring apparatus for a railway comprising two pairs of strain measuring devices located in predetermined spaced configuration along the railway, each strain measuring device of each pair being located together side by side, one on each rail.

Preferably each strain measuring device is located on the rail neutral axis.

Also in accordance with the invention there is provided a method of installing a shear strain measuring device on a structural member, said method including steps of providing a strain measuring device as hereinbefore described for said structural element, said at least one plate member having at least two apertures for securing said at least one plate member to said structural member by said at least two nuts and bolts; forming apertures through said structural member commensurate with the spacing of apertures in said at least one plate member, and securing at least one plate member using said nuts and bolts, said structural member contacting washers and said plate member securing washers to said structural member as hereinbefore described to allow shear strain in said structural member to be transmitted to said at least one plate member.

Preferably said strain measuring device includes two said plate members which are secured one on each side of said structural member, and collectively said strain responsive elements measure strain in four different vertical directions extending in use at 45° to the longitudinal extent of said structural member.

Preferably said method includes fitting two guide plates having apertures for receiving each said bolt arranged in a bolt hole pattern corresponding with each said plate member, said guide plate being divided into separate portions joined at the peripheral extent of said guide plate by peripheral joining portions, with each portion being associated with a bolt hole, one said guide plate being located between a said plate member securing washer and the head of a said bolt, and the other said guide plate being located between a said plate member securing washer and a said nut; and severing said peripheral joining portions after having tightened said nuts and bolts. The peripheral joining portions may be severed with a nibbling tool, or by use of an angle grinder. The guide plate assists in alignment as the strain measuring device is fitted, and reduces torque being transferred to the plate members and strain responsive elements during tightening of the nuts and bolts. Once the nuts and bolts have been tightened, the joins at the peripheral extent can be cut through or ground off, so that the shear strain in the structural member is transmitted through the plate members.

Alternatively said method includes fitting plate portions having apertures for receiving each said bolt, said plate portions being supported on an assembly jig comprising a rigid plate having clearance apertures through which the head of a said bolt or a said nut may be received, said clearance apertures being arranged in a pattern corresponding with said plate member, said clearance apertures having arranged thereabout at least two screw holes for securing plate portions by fastener portions thereof, said plate portions having apertures for receiving a said bolt, said plate portions when secured to said assembly jig having their apertures aligned with the bolt hole pattern of said plate member. Preferably said fastener portions comprise threaded holes and said plate portions are secured to said assembly jig by screws extending through said screw holes.

In use a strain gauge will be fitted to drilled holes in a structural member by passing two bolts through the assembly of assembly jig and plate portions, then each bolt passing through a plate member securing washer, opposed ends of a plate member having the strain responsive elements, a structural member contacting washer, the structural member, and then a structural member contacting washer, opposed ends of a plate member having the strain responsive elements, a plate member, and then a further assembly of assembly jig and plate portions, and two nuts which are subsequently tightened to secure the strain gauge to the structural member. Once secured, the screws securing the assembly jigs to the plate portions are removed, allowing the assembly jigs to be removed, and leaving the plate portions in place.

Preferably each said at least one strain responsive element is located along or near the axial center of gravity of said structural member.

Brief Description of the Drawings

Two preferred embodiments of the invention, being part of a weighbridge for railfreight, will now be described, with reference to the drawings in which:

Figure 1 is an exploded orthographic projection of part of a strain measurement device of the first embodiment showing fitting to a rail to form part of the weighbridge;

Figure 2 is a horizontal cross-section through the longitudinal extent of the rail and through the strain measuring device shown in figure 1 , as fitted to the rail;

Figure 3 is a vertical cross-section through the rail and through the longitudinal extent of one of the bolts securing the strain measuring device shown in figure 1 , as fitted to the rail;

Figure 4 and Figure 4a are a third angle orthographic projection of a plate member for carrying two strain measuring elements according to the first embodiment, Figure 4 being a plan view and Figure 4a being an end view; Figure 5 and Figure 5a are a third angle orthographic projection of a structural member contacting washer according to both of the embodiments, Figure 5 being a plan view and Figure 5a being a cross-section through B-B;

Figure 6 and Figure 6a are a third angle orthographic projection of a plate member securing washer according to both of the embodiments, Figure 6 being a plan view and Figure 6a being a cross-section through A-A;

Figure 7 and Figure 7a are a third angle orthographic projection of a guide plate according to the first embodiment, Figure 7 being a plan view and Figure 7a being a cross-section through the centre of the guide plate;

Figure 8 is a schematic showing circuitry of the strain measurement device according to both of the embodiments;

Figure 9 is a perspective view showing the spatial arrangement of strain measuring elements utilised in the strain measurement device according to the first embodiment;

Figure 10 is an exploded orthographic projection of part of a strain measurement device of the second embodiment showing fitting to a rail to form part of the weighbridge;

Figure 11 is a horizontal cross-section through the longitudinal extent of the rail and through the strain measuring device shown in Figure 10, as fitted to the rail; Figure 12 is a vertical cross-section through the rail and through the longitudinal extent of one of the bolts securing the strain measuring device shown in Figure 10, as fitted to the rail;

Figure 13 and Figure 13a are a third angle orthographic projection of a plate member for carrying two strain measuring elements according to the second embodiment, Figure 13 being a plan view and Figure 13a being an end view;

Figure 14 and Figure 14a are a third angle orthographic projection of a plate portion used in the second embodiment, Figure 14 being a plan view and Figure 14a being an end view thereof;

Figure 15 is a perspective view showing the spatial arrangement of strain measuring elements utilised in the strain measurement device according to the second embodiment; and

Figures 16 to 21 are a sequence of orthographic projections showing fitting of two assemblies each of two strain measurement devices according to the second embodiment forming part of a railway weighbridge; and

Figure 22 is an exploded orthographic projection of two assemblies each of two strain measurement devices of the second embodiment showing fitting to a rail to form half of the weighbridge.

Best Mode(s) for Carrying Out the Invention

Both embodiments are directed toward a strain measurement device for use in a weighbridge for a railway track, for measuring shear stress in the rail generated by axle loads as they pass, and thereby allowing determination of the load weight of freight wagons. In use, four strain measuring devices 11 as shown in figures 2 and 3 or Figures 11 and 12, are attached to structural members in the form of rail 13. To form the weighbridge, four such strain measuring devices 11 are attached to the web 15 of the rails 13, two per rail at a spacing less than the distance between adjacent sleepers supporting the rail, and side by side so that the strain gauges in each rail are traversed simultaneously or near simultaneously by a passing axle. To fit the strain measuring devices, two holes 19 of 18mm diameter are drilled through the web 15 of the rail 13 at the required spacing of 50mm apart and in the required locations. Two strain measuring devices 11 fitted to a rail in this manner are shown in the second embodiment illustrated in Figures 19 to 21.

In this description where both embodiments utilise identical or similar parts, identical numbers will be used. Where the second embodiment differs from the first, the points of difference are discussed later in this description.

Each strain measuring device comprises two plate members 21 , each having two strain responsive elements 23 bonded thereto. The orientation of the strain responsive elements 23 is such that they are sensitive to strain in two vertical directions (in the plane of the plate members 21) at 90° to each other, and when fitted at 45° to the longitudinal extent of the rail 13. The plate members 21 are constructed identically, and by orienting the plate members 21 one up and one down, strain is measured in four directions at 90° apart, in the same vertical plane as the rail. This is best seen in Figures 1 and 9 illustrating the first embodiment and Figures 10 and 15 illustrating the second embodiment. The two plate members 21 are secured, one on each side of the web 15 of the rail 13 by two bolts 25 and two nuts 27. The bolts 25 and nuts 27 are formed of high tensile steel with tensile strength of at least 400Mpa, and preferably 800Mpa. A typical steel is bolt grade 8.8. While M16 nuts and bolts are exemplified, it is possible to vary the diameter. For example, a smaller diameter could be utilised, but it may be necessary to increase the tensile strength. Structural member contacting washers 29 are located between the plate members 21 and rail 13 on each bolt 25. In addition, adjacent each plate members 21 opposite the structural member contacting washers 29 are provided plate member securing washers 31 on each bolt 25. Finally, a guide plate 33 is provided between the bolt 25 heads and the plate member securing washers 31 and a guide plate 33 is provided between the nuts 27 and the plate member securing washers 31. It should be noted that the guide plate 33 is not provided in the same form in the second embodiment.

The plate members 21 are shown in more detail in figure 4. As can be seen, they are formed with 16.1mm diameter apertures 35 spaced with their centres 50 mm apart, in a plate 1.6mm thick formed in the shape shown, and of mild steel. In practice the plate member 21 can be formed of any material that is within its elastic range in the application. The strain responsive elements are located between axial extents of the apertures 35 through which the bolts 25 pass.

The structural member contacting washer 29 illustrated in figure 5 has a flat annular surface 37 to contact the rail 13 web 15, in an inner peripheral extent spaced from the bolt 25 shaft circumference. The annular surface 37 is located atop a tapering portion 39 of frustro-conical cross-section. The structural member contacting washer 29 has two annular steps 41 , adjacent an inner periphery 43 of the structural member contacting washer, the annular steps having a minimum internal diameter commensurate with the bolt shaft diameter.

The structural member contacting washer 29 has a flat annular face 45 opposed to the annular surface 37 for contacting the plate member 21. The flat annular face 45 aligns with equivalent flat annular faces 47 (see figure 6) formed on the plate member securing washer 31 located on the other side of the plate member 21. The plate member securing washers 31 are also formed with two annular steps 49, adjacent an inner periphery 51 of each of the plate member securing washer 31 , the annular steps 49 having a minimum internal diameter commensurate with the bolt shaft diameter. The purpose of these features in both washers is to align the washers on the bolts with respect to each other, when tightened. With the flat annular faces 47 being identical on either side of the plate member securing washers 31 , it is not critical which way around that the plate member securing washers 31 are fitted. It should be noted that all annular faces are flat, so that there is continuous contact with surfaces against which they abut.

Referring to figure 7, the guide plate 33 is illustrated. The guide plate 33 has apertures 53 of 18mm diameter and 50mm centre spacing for receiving each bolt 15. This spacing corresponds with the aperture spacings in the plate members 35. The guide plate 33 is divided into two portions 55 separated by a bisecting slot 57, the two portions being joined at their peripheral extent by joins 59.

The guide plate 33 assists in alignment as the strain measuring device 11 is fitted, and prevents torque being transferred to the plate members 21 and strain responsive elements 23 during tightening of the nuts 27 and bolts 25. Once the nuts and bolts have been tightened, the joins 59 at the peripheral extent can be cut through or ground off, so that the all of the shear stress in the structural member is transmitted through the plate members 21.

The plate members 21 and washers 29 and 31 , and guide plate 33 are secured to the web 15 of the rail 13 with nuts 27 and bolts 25. The bolts 31 are tensioned using the "turn of the nut" method defined in AS4100 (the contents of which are incorporated by cross-reference) to provide a residual tension force in the bolts 31. This entails an initial tightening of the nuts to snug tight, and then further tightening by applying a nut rotation of 120° after the initial tightening. If the structural member has a curved surface, the contacting washer and/or the structural member will deform sufficiently to allow continuous contact with high contact stresses over the full contact area such that friction is sufficient to resist any shear stress applied to the contact area. Tension forces in bolts 25 provides compressive contact stresses between the guide plate 33 (which functions as a slip washer), plate member securing washers 31 , plate members 21 , structural member contacting washers 29, and the web 15 of the rail 13. These compressive contact stresses allow rail 13 shear deformations to be transferred to structural member contacting washers 29 and then in turn to plate members 21 by friction.

The structural member contacting washers 29 are tapered on the rail side to provide a high compressive contact stress over the annular contact area formed by the flat annular surface 37. The high contact stress may cause some deformation in the flat annular surface 37 or the rail 13, to accommodate any rail 13 web curvature or surface irregularity, and the tapering assists in allowing this deformation to occur. The tapered annular shape was selected to minimise slippage due to the shear and torsion forces resulting from the transfer of rail 13 shear deformations to the plate members 21.

The structural member contacting washers 29 have an annular contact area with plate members 21. The annular shape was selected to minimise slippage due to the shear and torsion forces resulting from the transfer of rail 13 shear deformations. The width of the annulus was selected to provide a high contact stress over the full contact area with the plate members 21 .

The plate member securing washers 33 have the same annular contact areas as structural member contacting washers 29 have with plate members 21. The annular contact areas of structural member contacting washers 29 and plate member securing washers 33 with plate member 21 are common to ensure a uniform contact stress between structural member contacting washers 29 and plate member 21.

This uniform and high contact stress between contacting surfaces is designed to avoid any localised slipping between the contacting surfaces. Localised slipping could result in hysteresis. It is necessary for the bolts 25 and holes in the mating components to be concentric. It is preferred to make the structural member contacting washers 29 and load application washers 22 concentric with the bolts 31 by providing an internal thin annulus formed between the annular steps 41 , 49 which does not effect the geometry of the contact surfaces.

Once the strain measurement devices have been fitted to the rails, covers (not shown) made from stainless steel sheet are fitted over the devices and secured to the rail by a flexible bonding agent. The covers prevent ingress of dust and moisture. The bonding agent forms a layer approximately 3mm thick between the edge of the cover and the rail. A suitable bonding agent is Sicaflex™ 292, a marine grade bonding and sealing agent.

Referring to figures 7 and 8, wiring and instrumentation shall now be discussed. The two plate members 21 have their strain gauges 23 (which in the embodiment are Micro-Measurements Group 187UV two element 90° CEA-Series types) wired to form a Wheatstone bridge 61 , the circuitry of which is shown in figures 7 and 8. The rail 13 has an aperture 63 to allow wiring to pass through.

The strain gauges 23 on each plate member 21 measure strains in two directions, at right angles to each other and at 45° to horizontal.. Thus when two plate members 23 are placed opposite each other, strain in four directions and in two dimensions are measured, due to the strain gauges 23 having identical alignment on the plate members 21.

The strain gauges are wired to form the Wheatstone bridge 61 which is in turn wired into a strain amplifier 65, which provides power to the Wheatstone bridge 61 , and receives the voltage output indicative of shear strain from the Wheatstone bridge 61. The Wheatstone bridge 61 is set up to measure shear strain and ignore the effects of torsion, bending moment, axial load and temperature change. The output from the strain amplifier is multiplied by a calibration factor to give the shear force incident on the rail 13, between the holes 19. The resistors labelled by the circled numerals 1 , 2, 3, and 4 of the strain gauges 21 are identified for reference in figures 7 and 8. The plate members 21 of the second embodiment shown in figures 10, 13, 13a and 15 differ from those of the first embodiment in that four tags 69 with cable tie apertures 71 are provided around the periphery thereof, above and below the apertures 35. These tags are used to secure cabling 73 that is connected to the strain responsive elements 23.

In the second embodiment the guide plate 33 is replaced by two separate plate portions 75. The plate portions 75 each have fastener portions in the form of two threaded screw holes 77 to receive screws 79 which secure an elongate plate 81 (best seen in figures 17 to 19) forming an assembly jig. The plate portions 75 also each include a central aperture 83 through which the shafts of a bolt 25 can pass. The elongate plate 81 and plate portions 75 are manufactured with predetermined configuration, to ensure accuracy in the positioning of the components of the strain measuring devices 11. Once the nuts 27 and bolts 25 are tightened, the screws 79 can be loosened and the elongate plate 81 removed, leaving the plate portions 75 in place, and disconnecting them from each other so that all strain in the rail is transmitted to the plate members 21 and strain responsive elements 23. The plate portions 75 effectively also function as slip washers, ensuring that excessive torsional stresses are not transmitted to the plate members 21 when the nuts 27 and bolts 25 are tightened.

Figures 16 to 21 show the sequence of fitting a pair of strain measuring devices 11 to a rail 13, as part of a railway weighbridge to measure axle weights of passing freight wagons. As described in the preceding discussion, there are two such devices 11 secured to each rail 13 side by side, so that the devices 11 are traversed simultaneously by the axles of the wagons.

A suitable position is selected for installation of the weighbridge between two adjacent sleepers so that the rail underneath the weighbridge is unsupported by any sleepers or ballast on which the railway may be supported. There should be a centre to centre spacing between adjacent sleepers of about one metre and the weighbridge should be installed centrally therebetween. Any embossing or pitting in the web 15 between the adjacent sleepers should be ground flush, prior to assembling the weighbridge. Referring to figure 16, a drilling template 85 is positioned on the web 15 of the rail 13 using a neutral axis height gauge 87 which lines up with axial centreline tags 89 on the template 85. The template 85 is positioned with centre tags 91 positioned at the predetermined centreline 93 of the weighbridge. A centrepunch is used to mark the position for the bolt holes 19 and the cabling apertures 63, and these are then drilled, and the cabling apertures 63 are chamfered.

The plate portions 75 are supplied pre-assembled to the elongate plate 81 using screws 79, with the central apertures 83 aligned, conforming with the drilling template. As can be seen the elongate plate 81 has clearance apertures in the form of large holes 95, sufficiently large to allow the head of bolt 25 and the nuts 27 to pass through, when the elongate plate 81 is detached, as will be described later.

Each bolt 25 is passed through the central aperture 83 of a plate portion 75 (with the elongate plate 81 mounted outermost, so the heads of screws 79 are externally accessable), then through a plate member securing washer 31 , one aperture 35 of a plate member 21, a structural member contacting washer 29, then through a hole 19 in the web 15 of the rail, before passing through a further structural member contacting washer 29, one aperture 35 of a further plate member 21 , a further plate member securing washer 31 and the central aperture 83 of a further plate portion 75 (again with the elongated plate 81 mounted outermost, so the heads of screws 79 are externally accessable) before being secured to a nut 27. The relative alignment of the plate members 21 is shown in figure 15.

The nuts 27 are tightened on the bolts 25 as shown in figure 18 using normal spanners, and then using a socket with an extension bar, are tightened 1/3 of a turn.

The screws 79 are removed, allowing both elongate plates 81 to be removed from the strain measuring devices 11 , as shown in figure 19 (although note only the near side elongate plate 81 is shown in figure 19) and a plate 97 carrying a cable termination printed circuit board 99 is fitted using one (or two) screw 101 fitted into a screw hole 77 of the plate portion 75. Cables are terminated and tied, as necessary and run via conduit 103 and covers 105 are secured to the rail using Sicaflex™ flexible bonding agent. The flexible bonding agent prevents ingress moisture and dust to ensure long-term reliability of the weighbridge.

Figure 22 shows an exploded view of the componentry referred to in the preceding discussion.

The embodiments provide a shear force measurement method and system that features use of prefabricated plates incorporating strain gauges, allowing strain measurement to be made in a structural member. In a retrofitting situation the only in-situ fabrication work required is formation of the bolt holes and aperture in the structural member, and preparation of the structural member surface, with all other fabrication work being undertaken in the factory.

The strain measuring devices 11 are especially suitable for forming a shear force measurement system designed to be installed in structural members such as I beams and rails. The system so formed is designed to be rugged and durable and is prefabricated to maximise quality and repeatability. Immediate applications are weighbridges for trains and trucks and bin or equipment weight measurement where load measurements can be made by totalling the measured shear forces in beams.

It should be appreciated that the scope of the invention is not limited to the particular embodiment described herein.

Claims

The Claims Defining the Invention are as Follows

1. A strain measuring device for a structural member, said strain measuring device comprising at least one plate member having at least one strain responsive element affixed thereto, said at least one plate member being secured to said structural member to allow shear strain in said structural member to be transmitted to said at least one plate member, said at least one plate member being secured by at least two nuts and bolts, with said strain responsive element located between axial extents of opposed bolts, each said nut and bolt having located thereon a structural member contacting washer located on one side of each said plate member, and a plate member securing washer located on the other side of each said plate member, said structural member contacting washer having an annular surface to contact said structural member, said annular surface contacting said structural member in a peripheral extent around and spaced from said bolt shaft circumference.

2. A strain measuring device as claimed in claim 1 wherein said annular surface is raised so as to be spaced from the main body of the structural member contacting washer.

3. A strain measuring device as claimed in claim 2 wherein said annular surface is located atop a frustro-conical portion of said structural member contacting washer.

4. A strain measuring device as claimed in claim 2 wherein said annular surface is located atop a tapering portion of said structural member contacting washer.

5. A strain measuring device as claimed in claim 4 wherein said tapering portion extends axi-symmetrically about the bolt centre-line.

6. A strain measuring device as claimed in claim 4 or 5 wherein said tapering portion exhibits asymmetry in cross-section to maximise the diameter of the annular surface while providing structural support for the annular surface.

7. A strain measuring device as claimed in any one of the preceding claims wherein said annular surface of said structural member contacting washer is arranged to completely and continuously contact said structural member.

8. A strain measuring device as claimed in any one of the preceding claims wherein said structural member contacting washer has an annular face opposed to said annular surface for contacting said at least one plate member.

9. A strain measuring device as claimed in claim 8 wherein said annular face of said structural member contacting washer is arranged to completely and continuously contact said at least one plate member.

10. A strain measuring device as claimed in claim 8 or 9 wherein said plate member securing washer has opposed annular surfaces, at least one of which is commensurate with the shape and size of said annular face for contacting said plate member.

11. A strain measuring device as claimed in any one of claims 8 to 10 wherein said structural member contacting washer has at least one internal annular step adjacent an inner periphery thereof, the internal annular step having a minimum internal diameter commensurate with the bolt shaft diameter, to space the annular surface and annular face of the structural member contacting washer from the bolt shaft.

12. A strain measuring device as claimed in claim 10 or 11 wherein said plate member securing washer has at least one internal annular step adjacent an inner periphery thereof, the internal annular step having a minimum internal diameter commensurate with the bolt shaft diameter, to space the opposed annular surfaces of the plate member securing washer from the bolt shaft.

13. A strain measuring device as claimed in any one of claims 10 to 12 wherein both opposed annular surfaces of said plate member securing washer are commensurate with the shape and size of said annular face for contacting said plate member.

14. A strain measuring device as claimed in any one of the preceding claims wherein said at least one plate member has two said strain responsive elements affixed thereto, said strain responsive elements measuring strain in the vertical plane in two directions normal to each other, and in use at 45° to the longitudinal extent of said structural member.

15. A strain measuring device as claimed in claim 14 wherein said strain measuring device includes two said plate members for securing one on each side of said structural member, collectively said strain responsive elements measuring strain in four different vertical directions extending in use at 45° to the longitudinal extent of said structural member.

16. A strain measuring device as claimed in any one of the preceding claims wherein said strain measuring device has two said plate members.

17. A strain measuring device as claimed in any one of the preceding claims wherein said strain measuring device has two said nuts and bolts.

18. A strain measuring device as claimed in any one of the preceding claims wherein said strain measuring device includes plate portions located on said bolt between each said plate member securing washer and the head of each bolt, and between each said plate member securing washer and each said nut, said plate portions being part of an assembly guide plate utilised when fitting said strain measuring device.

19. A strain measuring device as claimed in claim 18 wherein adjacent said plate portions include means to secure against rotation while said strain measuring device is secured in place to said structure member.

20. A strain measuring device as claimed in claim 19 wherein adjacent said plate portions are joined by at least two peripheral tags which are severable once said strain measuring device is secured in place to said structure member.

21. A strain measuring device as claimed in claim 19 wherein said plate portions incorporate fastener portions to allow adjacent said plate portions to be secured to an assembly jig while said strain measuring device is secured in place to said structural member.

22. A strain measuring device as claimed in claim 21 wherein said fastener portions comprise at least two threaded holes into which complementary threaded fasteners may be located to rigidly secure said plate portions to said assembly jig.

23. A guide plate to fit a strain measuring device as claimed in any one of claims 1 to 17 to said structural member, said guide plate having apertures for receiving each said bolt arranged in a bolt hole pattern corresponding with said plate member, said guide plate being divided into separate portions joined at the peripheral extent of said guide plate, each portion being associated with a said guide plate aperture, in use when fitting the strain measuring device said guide plate being located in contact with said plate member securing washer.

24. An assembly jig to fit a strain measuring device as claimed in any one of claims 1 to 17 to said structural member, said assembly jig comprising a rigid plate having clearance apertures through which the head of a said bolt or a said nut may be received, said clearance apertures being arranged in a pattern corresponding with said plate member, said clearance apertures having arranged thereabout at least two screw holes for securing plate portions by fastener portions thereof, said plate portions having apertures for receiving a said bolt, said plate portions when secured to said assembly jig having their apertures aligned with the bolt hole pattern of said plate member.

25. An assembly jig as claimed in claim 24 wherein said fastener portions comprise threaded holes and said plate portions are secured to said assembly jig by screws extending through said screw holes.

26. A strain measuring device kit for assembly to form a strain measuring device to measure shear strain in a structural member comprising a pair of nuts and bolts, a pair of plate members each having two bolt holes and at least one strain responsive element affixed thereto between said bolt holes, four structural member contacting washers having an annular surface to contact said structural member in a peripheral extent spaced from said bolt shaft circumference, four plate member securing washers to secure said plate members to said structural member contacting washers.

27. A strain measuring device kit as claimed in claim 26 including two guide plates for use in fitting said strain measuring device, said guide plates each having apertures for receiving each said bolt arranged in a bolt hole pattern corresponding with said plate member, said guide plate being divided into separate portions joined at the peripheral extent of said guide plate each portion being associated with a bolt hole, in use when fitting the strain measuring device said guide plate being located in contact with said plate member securing washer.

28. A strain measuring device kit as claimed in claim 26 including an assembly jig comprising a rigid plate having clearance apertures through which the head of a said bolt or a said nut may be received, said clearance apertures being arranged in a pattern corresponding with said plate member, said clearance apertures having arranged thereabout at least two screw holes for securing plate portions by fastener portions thereof, said plate portions having apertures for receiving a said bolt, said plate portions when secured to said assembly jig having their apertures aligned with the bolt hole pattern of said plate member.

29. A strain measuring device kit as claimed in claim 28 wherein said fastener portions comprise threaded holes and said plate portions are secured to said assembly jig by screws extending through said screw holes.

30. An axle load measuring apparatus for a railway comprising two pairs of strain measuring devices as claimed in any one of claims 1 to 22 located in predetermined spaced configuration along the railway, each strain measuring device of each pair being located together side by side, one on each rail secured to the web thereof.

31. A method of installing a shear strain measuring device on a structural member, said method including steps of providing a strain measuring device as claimed in any one of claims 1 to 22 for said structural member, said at least one plate member having at least two apertures for securing said at least one plate member to said structural member by said at least two nuts and bolts; forming apertures through said structural member commensurate with the spacing of apertures in said at least one plate member, and securing said at least one plate member using said nuts and bolts, with said structural member contacting washers being located between said structural member and said plate member, and plate member securing washers being located adjacent said plate member opposed from said structural member contacting washers to allow shear stress in said structural member to be transmitted to said at least one plate member.

32. A method of installing a shear strain measuring device on a structural member as claimed in claim 31 wherein said strain measuring device includes two said plate members which are secured one on each side of said structural member, and collectively said strain responsive elements measure strain in four different vertical directions extending in use at 45° to the longitudinal extent of said structural member.

33. A method of installing a shear strain measuring device on a structural member as claimed in claim 31 wherein said method includes fitting two guide plates having apertures for receiving each said bolt arranged in a bolt hole pattern corresponding with each said plate member, said guide plate being divided into separate portions joined at the peripheral extent of said guide plate by peripheral joining portions, with each portion being associated with a bolt hole, one said guide plate being located between a said plate member securing washer and the head of a said bolt, and the other said guide plate being located between a said plate member securing washer and a said nut; and severing said peripheral joining portions after having tightened said nuts and bolts, strain in the structural member is transmitted through the plate members.

34. A method of installing a shear strain measuring device on a structural member as claimed in claim 31 wherein said method includes fitting plate portions having apertures for receiving each said bolt, said plate portions being supported on an assembly jig comprising a rigid plate having clearance apertures through which the head of a said bolt or a said nut may be received, said clearance apertures being arranged in a pattern corresponding with said plate member, said clearance apertures having arranged thereabout at least two screw holes for securing plate portions by fastener portions thereof, said plate portions having apertures for receiving a said bolt, said plate portions when secured to said assembly jig having their apertures aligned with the bolt hole pattern of said plate member.

35. A method of installing a shear strain measuring device on a structural member as claimed in claim 34 wherein said fastener portions comprise threaded holes and said plate portions are secured to said assembly jig by screws extending through said screw holes.

36. A strain measuring device for a structural member, substantially as herein described with reference to the drawings.