LOAD-INDICATING DEVICE

Abstract: The device consists of first and second load receiving members 10,14, a breakable member 20 and a part-­ spherical washer 18 received in a seat 32 in the second load-­ receiving member. In use, a load is applied by way of a rod or bar passing centrally through the load-receiving members 10, 14 and the breakable member positioned therebetween, the rod or bar also passing through a central bore 34 in the washer 18 and being angularly movable because of the support provided by the seat 32. When a predetermined load is achieved, annular cutting surfaces 26, 40 on the load-receiving member cut the breakable member with a predominantly shearing action to indicate that the load has been achieved. Fractured parts of fingers 50 of the bearing member are retained between surfaces 24, 38 to be visible at the periphery of the device.

Description: This invention relates to devices for indicating when an applied load has achieved a desired value. A device provided by the invention may be used when a load is applied to a rock bolt or a tie bar used, for example, to stabilise a rock face or strengthen a brick or masonry structure, in a building, tunneling or mining project. In the past applied loads have been measured using various hydraulic, electrical and optical load-­ indicating devices. These devices tend to be delicate and expensive and are therefore not completely satisfactory, particularly when used in building, tunneling, mining or civil engineering projects. One aim of the present invention is to overcome this disadvantage of these known devices. Devices depending upon deformation of an element (e.g. a Belleville washer) are disclosed in US-A-­ 3474701 and US-A-4040734. In FR-A-2044880, a plastics element is extruded to zero thickness as the load is applied to the indicating device. In JP-A-4720289, a washer fitted around the shank of a bolt is fractured upon application of a predetermined load to the bolt. The present invention provides a device for indicating attainment of a predetermined value by a load applied to the device, the device comprising first and second members and a breakable member therebetween, the first and second members having formations which co-operate when a load tending to cause relative movement of the members towards each other is applied to the device, the formations including a first cylindrical surface on the first member and a second cylindrical surface on the second member, which formations co-operate to break the breakable member, with an action which is at least partially shearing, when the applied load attains the predetermined value, characterised in that the co-operating formations also include guiding formations comprising a first cylindrical surface on the first member and a second cylindrical surface on the second member, the guiding formations co-operating to guide the first and second members in their relative movement in response to the applied load, and in that the guiding surface and the breakage surface of the first member are joined by a first annular surface which is inclined to the direction of the relative movement of the first and second members, the guiding surface and the breaking surface of the second member are joined by a second annular surface which is inclined to the said direction, and the first and second annular surfaces define between themselves an annular cavity for reception of a fragment of a said breakable member after the breakage thereof. Usually, the breakable member will be subjected to a shearing and bending action to cause breakage of the member, the spacing of the points of application of the load to the breakable member determining the relative magnitudes of the shearing and bending components. The guiding surfaces are advantageously concentric with the breaking surfaces. Preferably, the guiding surfaces are radially inward of the breaking surfaces. In such a device, the first and second members may be shaped outwardly of the first and second breakage surfaces to define between themselves at least one recess which is open to visual inspection from the exterior of the device and which receives a portion of the breakable member the portion occupying respective first and second positions relative to the first and second members prior to and after breakage, whereby breakage of the breakable member having taken place can be detected visually by inspection as to which of the positions is occupied by the breakable member portion. Advantageously, one of the guiding surfaces provides a means for locating the breakable member on one of the first and second members against transverse movement relative to the direction of movement of the members in response to the applied load. Conveniently, the breakable member is annular and receivable around one of the guiding surfaces in order to locate the breakable member against the said transverse movement. In a preferred arrangement, the first and second members and the breakable member have aligned apertures through which a rod or bar can be passed. If the rod or bar is provided with a means for engaging one of the first and second members (for example a nut screw-­ threaded onto the rod or bar) and movement of the other member is resisted, a load can be applied to the device by tensioning of the rod or bar. In this way, the load-indicating device of the invention can act as an end plate or pattress and can be used for setting a pre-load in a rock bolt or tie bar in a building or civil engineering application. When the breakable member is apertured, it preferably comprises a plate having an annular portion with inwardly- or outwardly-protruding portions which are engaged by formations on the load-receiving members and are fractured when the predetermined load value is reached. By modification of the dimensions of the protruding portions and/or the breaking formations on the load-receiving members, the predetermined load value can be altered. Preferably, this is achieved by changes in the dimensions of the protruding formations or the thickness or material of the plate so that alteration of the load value is achieved without modification of the first and second members. When apertured as mentioned above, one of the first and second members advantageously has a portion through which a portion of the aperture extends and which is movable relative to the remainder of the member. In this way, the orientation of a rod or bar received in the aperture of the movable portion relative to the member can be varied. Conveniently, the movable portion has a part-spherical surface engaging a complementarily-shaped surface on the remaining portion of the member, the aperture in which is preferably flared to allow angular movement of the rod or bar. An embodiment of the invention will now be described by way of example with reference to the drawings, in which: Figure 1 is a vertical sectional view of a load-­ indicating device in the form of a load-indicating end plate, a breakable plate of the device being omitted, Figure 2 is a plan view of the breakable plate omitted from Figure 1, and Figures 3a, 3b and 3c are scrap sectional views showing cutting teeth of the device of Figure 1 and, in Figures 3b and 3c, also the breakable plate of Figure 2. Figure 1 of the drawings show a first, annular load-receiving member 10, having a central circular aperture 12, and a second, load-receiving member 14 which has a central bore 16 and supports a part-­ spherical washer 18. The load-indicating device is completed by a breakable member in the form of an annular plate 20 which is shown in Figure 2. The load-receiving members 10, 14 and the washer 18 are of SG iron, whilst the shear plate 20 is of mild steel. In other embodiments, other materials, such as stainless steel and non-ferrous metals, may be used. The first load-receiving member 10 is circular in plan and has a base area to suit the load bearing properties of the substrate. Typically, the diameter of the load-receiving member is seven times the diameter of the bar or rod to be used with the device. The bar or rod will usually have a diameter within the range of 16 mm to 39 mm, thus giving a range of load-receiving member diameters of 112 mm to 273 mm. The annular surface of the member which is uppermost in Figure 1 has inner and outer ramped annular surfaces 22, 24 which, as shown, are inclined upwardly in the radially-inward direction. A cylindrical cutting surface 26 joins the ramped surfaces 22, 24 and, inwardly of the inner ramped surface 22, a cylindrical alignment surface 28, which provides annular alignment of the first and second load members, joins the inner ramped surfaces to an annular shoulder 30 which is inclined in the opposite direction to the ramped surfaces 22, 24. The second, domed load-receiving member 14 is also circular in plan and has in its upper surface a part-­ spherical seating 32 for support of the part-spherical washer 18. The washer 18 has a circular cylindrical bore 34 in which a screw-threaded rod or bar is a somewhat slack fit. By virtue of movement between the washer and its seating 32 and the frusto-conical shape of the bore 16, the rod or bar can adopt positions in which its axis is inclined to the axis of the frusto-conical bore 16. The surface of the second load-receiving member 14. Which confronts the first load-receiving member 10 has inner and outer ramped annular surfaces 36, 38 which are parallel to the ramped surfaces 22, 24 of the load-receiving member 10. The ramped surfaces 36, 38 are joined by a cylindrical cutting surface 40 which is parallel to the cylindrical cutting surface 26 of the first load-receiving member. A cylindrical alignment surface 42 extends para­ llel to the cylindrical alignment surface 28 to provide alignment of the load-receiving members. A further annular surface 44 extends between the cylindrical surface 40 and the wider end of the bore 16 in the second load-­ receiving member 14 to face the inclined shoulder 30 on the first load-receiving member. The alignment and cutting portions of the load-receiving members are shown in more detail in Figure 3a. The annular plate 20 of this embodiment is about 4 mm in thickness and, in other embodiments, will usually have a thickness in the range of 1 to 10 mm. The plate has an annular portion 46, defining a central circular aperture 48, and eight outwardly extending protruding portions 50 defined by gaps 52. The diameter of the aperture 48 is slightly greater than the diameter of the cylindrical surface 42 on the second load-receiving member 14, on which the plate 20 is positioned by reception of the aperture central boss formed by the surfaces 42 and 44 in the aperture 48 of the plate 20. The portions 50 of the plate 20 extend from the annular portion 46 to a radial distance equal to the radius of the outer edges of the outer ramped surfaces 38 on the second load-receiving member 14. The material and thick­ ness of the shear plate 20 and the number and the sum of the circumferential widths of the protruding portions 50 at the radius of the cutting surfaces 26, 40 can be varied in order to set the predetermined load to be indicated by the device. For use of the load-indicating device the parts shown in Figure 1 are first assembled with the plate 20 positioned between the load-receiving members 10, 14 and the cylindrical surface 42 of the second load-receiving member 14 received in the aperture 48 of the shear plate. The members 10, 14 and plate 20 are shown in more detail in Figure 3b. The tie rod or rock bolt is then passed through the aligned apertures 12, 48, 16, 34 and a nut fitted to a threaded end portion of the rod or bolt exten­ ding beyond the washer 18. With the first load-receiving member 10 supported by a fixed structure and the rod or bolt possibly inclined to the axis of the frusto-conical bore 16, the nut is tightened. When the tension in the rod or bolt corresponding to the breaking stress of the annular plate 20 is exceeded, the plate 20 is fractured by a shearing and bending action between the leading edges of the pair of cutting surfaces 26, 40, the radial spacing of the cutting surfaces determining the relative contributions of shearing and bending to the fracture. The fractured plate is shown in more detail in Figure 3c, it being evident that the protruding portions 50 of the shear plate 20 are fractured along a circular line corresponding to the radius of the cylindrical cutting surfaces 26, 40. The resulting relative movement of the load-receiving members 10, 14 when fracture takes place produces a visual indication that the predetermined load has been achieved and, moreover, a further visual indica­ tion is given by the fact that the peripheral gaps between the ramped surface 24 on the first load-receiving member 10 and the lower surfaces of the protruding portions 50 plate 20 have become closed, the fractured parts of the portions being retained between the two device parts 10, 14. One use of the load-indicating device described with reference to the drawings is in the tensioning of a rock bolt used to stabilise a rock face in, for example, a mine. A blind bore is first drilled in the rock face. The rock bolt inserted into the bore and secured by a suitable cement or grout. To stabilise the rock face, the bolt must be tensioned to a predetermined value. This is achieved by use of the load-indicating device described. Attainment of the predetermined load will serve to indicate that the bolt is properly secured to the rock. After ten­ sioning, the device remains to serve as a load-distributing end plate. After tensioning and fracture of the plate 20, the rock bolt can be re-tensioned to the same or a different predetermined value by removal of the nut, the second load-­ receiving member 14 and the fractured plate 20 and replace­ ment of the plate before re-assembly of the load-receiving member and nut. The load-indicating device according to the present invention has many other applications, for example as overload-indication devices in weighbridges and cables. It will be appreciated that the bending contribution to the fracture will be zero when the clearance between the cutting surfaces is zero and will increase with increasing spacing. Thus, when the clearance is small as shown, the mode of fracture will be predominantly shearing with a small bending component.

Claims: 1. A device for indicating attainment of a predetermined value by a load applied to the device, the device comprising first and second members (10, 14) and a breakable member (20) therebetween, the first and second members having formations which co-operate when a load tending to cause relative movement of the members towards each other is applied to the device, the formations including a first cylindrical surface (26) on the first member and a second cylindrical surface (40) on the second member, which formations co-­ operate to break the breakable member, with an action which is at least partially shearing, when the applied load attains the predetermined value, characterised in that the co-operating formations also include guiding formations comprising a first cylindrical surface (28) on the first member and a second cylindrical surface (42) on the second member, the guiding formations co-­ operating to guide the first and second members in their relative movement in response to the applied load, and in that the guiding surface (28) and the breakage surface (26) of the first member (10) are joined by a first annular surface (22) which is inclined to the direction of the relative movement of the first and second members, the guiding surface (42) and the breaking surface (40) of the second member (14) are joined by a second annular surface (36) which is inclined to the said direction, and the first and second annular surfaces (22, 36) define between themselves an annular cavity for reception of a fragment of a said breakable member (20) after the breakage thereof. 2. A device according to claim 1, in which the guiding surfaces (28, 42) are concentric with the breaking surfaces (26, 40). 3. A device according to claim 1 or 2, in which the first and second annular surfaces (22, 36) are planar and extend parallel to each other. 4. A device according to any preceding claim, in which the guiding surfaces (28, 42) are radially inward of the breaking surfaces (26, 40). 5. A device according to claim 4, in which the first and second members (10, 14) are shaped outwardly of the breaking surfaces (26, 40) to define between themselves at least one recess which is open to visual inspection from the exterior of the device and which receives a portion (50) of the breakable member (20), the portion occupying respective first and second positions relative to the first and second members (10, 14) prior to and after breakage, whereby breakage of the breakable member having taken place can be detected visually by inspection as to which of the positions is occupied by the breakable member portion. 6. A device according to any preceding claim, in which one of the guiding surfaces (28, 42) provides a means (42) for locating the breakable member (20) on one of the first and second members (10, 14) against transverse movement relative to the direction of relative movement of the first and second members. 7. A device according to claim 6, in which the breakable member (20) is annular and receivable around one of the guiding surfaces (28, 42), in order to locate the breakable member (20) against the said transverse movement. 8. A device according to any preceding claim, in which the first and second members (10, 14) and the breakable member (20) are each annular and have respective apertures (12, 16, 48) which overlap to form a passage extending through the device. 9. A device according to claim 8, which the breakable member (20) comprises an annular portion (46) and a plurality of portions (50) extending outwardly therefrom, the breaking surfaces (26, 40) co-operating to cause breakage of the breakable element at locations on the outwardly-extending portions (50). 10. A device according to claim 8 or 9, in which one of the first and second members (10, 14) comprises a first portion (14) having thereon the co-operating formations of the respective member and a second portion (18) which is movable relative to the first portion, the passage (16, 34) through the first or second member extending through the first and second portions thereof and the second portion of the member being engageable by an elongate member, whereby the elongate member can adopt any of a plurality of possible orientations relative to the first portion of the first or second member through movement of the second portion relative to the said first portion. 11. A device according to claim 10, in which the first and second portions have respective part-­ spherical surfaces (32) which co-operate to permit the relative movement of the portions. 12. A device according to claim 10 or 11, in which the passage (16) extending through the first portion is flared to allow angular movement of an elongate member engaging the second portion (18) of the first or second member. 13. A load-indicating end plate or pattress incorporating a device according to any preceding claim.

Inventors: HAYNES, JOHN MICHAEL
Applicants: BIWATER INDUSTRIES LIMITED
Application year: 1986
Main classification: F16B31
IPC codes: E21D21, F16B31