Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
  • F16B—DEVICES FOR FASTENING OR SECURING CONSTRUCTIONAL ELEMENTS OR MACHINE PARTS TOGETHER, e.g. NAILS, BOLTS, CIRCLIPS, CLAMPS, CLIPS OR WEDGES; JOINTS OR JOINTING
  • F16B31/00—Screwed connections specially modified in view of tensile load; Break-bolts
  • F16B31/02—Screwed connections specially modified in view of tensile load; Break-bolts for indicating the attainment of a particular tensile load or limiting tensile load
  • F16B31/025—Screwed connections specially modified in view of tensile load; Break-bolts for indicating the attainment of a particular tensile load or limiting tensile load with a gauge pin in a longitudinal bore in the body of the bolt
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
  • G01L5/00—Apparatus for, or methods of, measuring force, work, mechanical power, or torque, specially adapted for specific purposes
  • G01L5/24—Apparatus for, or methods of, measuring force, work, mechanical power, or torque, specially adapted for specific purposes for determining value of torque or twisting moment for tightening a nut or other member which is similarly stressed

Definitions

• This invention relates to the field of fastener systems, in particular to those such as nut and bolt systems which are intended for use in a stressed state and are lengthened as a result of applied load.
• PCT publication no WO 99/09327 describes a fastener system which provides an indication of the strain to which it is subjected.
• the system is essentially a bolt with an axial bore hole in the shaft into which a pin-like measuring element is located anchored at the base and ground flush with the top surface of the shaft.
• a portable measuring head can be attached to the top of the bolt in order to measure the size of this displacement. In this way the extension of the bolt, and hence the strain it is experiencing can be deduced.
• US Re. 30,183 also describes a microdisplacement transducer which is suitable for use in, among other things, mine-roof bolts.
• a pin is again located internal to the bolt in such a way that extension of the bolt is not communicated to the pin.
• a mechanical extension of the bolt effects a change in separation of two plates of a capacitor by means of this pin.
• the capacitor is part of an L-C circuit and measurement is made, using an external oscillator, of the resonant frequency of this circuit. From this measurement, capacitance and hence plate separation and finally strain can be deduced.
• the present invention provides a probe attachment with cable arranged to carry electrical signals to and from a sensor with sensing surface, the probe being eatable within a bore hole made within a fastening device, wherein the probe attachment is securely attachable at a region remote from its sensing surface to the fastening device so as to seal a pocket within the bore hole and to leave a gap between its sensing surface and a surface of the bore hole and is adapted to measure a parameter indicative of capacitive reactance of the two surfaces separated by the gap.
• this invention provides a component, such as a bolt, of a fastening system, the component including an internal air gap located intermediate an internal surface of the component and a sensing surface of a probe attachment, the probe attachment being secured to a part of the fastening component remote from the internal surface and adapted to measure a parameter indicative of capacitive reactance of the two surfaces separated by the air gap.
• This invention has numerous advantages over the prior art systems of strain measurement which make it suitable for use in extreme, particularly high temperature, environments. First it avoids the use of coils, moving parts and point contacts. Each of these depends on some mechanical or electrical material property, such as dielectric constant or thermal expansion coefficient, which varies with temperature rendering consistent measurement of strain impossible. Secondly it avoids the necessity of using any hand-held device to perform the measurement. Although such a hand-held device could be used if circumstances permit, it is not necessary to do so.
• the cable carries the output signal away from the locality of the measurement and its length can be adapted to suit the situation. Specifically, the length of the cable merely has to be sufficient for any human observer making an attachment to the cable to take a measurement to be able to do so at a safe distance.
• the measuring surfaces are all enclosed within the fastening system. This significantly reduces the chances of contamination or corrosion of these surfaces, which would upset the accuracy and reproducibility of measurements made. Fourthly, it enables allowance to be made for high-temperature stress-induced permanent displacements. At elevated temperatures an applied load may result in a time-dependent permanent displacement of the fastener. Such displacement is generally associated with the first four threads of a threaded fastener, which are expected to experience the highest stress levels, being tightened. The consequences of this deformation (stress relaxation) with regard to the indicated load must be fully understood. Prior art load-indicating bolts cannot provide for long-term monitoring of this effect as they are unsuitable for use in high-temperature, corrosive or otherwise hostile environments.
• the gap which is most preferably an air gap, may be between the sensing surface of the probe and the surface at an end of the bore hole.
• fasteners such as bolts are strained lengthwise rather than in any other direction when loaded.
• the cable When the probe is secured within a fastening device, the cable may extend only a short distance outside of the device and, in such an embodiment, this cable is attachable either directly or indirectly to a signal processing means.
• the cable is preferably attachable to a second cable which in turn may be attached to the signal processing means. This provides the probe with the advantage of flexibility. Measurements can be taken through the short cable, if circumstances permit, but also through the longer length of cabling when the second cable is attached.
• the probe may include a socket located at the region which is attachable to the fastening device and the cable is internal to the probe and connected to the socket.
• the probe attachment of this invention is suitable for measuring strain regardless of the hostility of the environment, provided the length of cable used is sufficient to extend from a comfortably-housed processor to the fastener location. In room or similar-temperature environments therefore, the cable need not extend outside of the fastener. It can terminate within, leaving no cable length whatsoever to interfere with tightening equipment, nor indeed to offer any more difficulty in loading than is offered by a standard bolt. All that is required in this embodiment therefore is that the processor, which for example may be a hand-held device, is able to connect with the cable termination inside the fastener.
• the shortness of the permanently-attached cable means that it does not provide significant hindrance to fastening the device and the length is sufficient to extend the cable termination to a cooler zone outside of a moderately heated structure. It can therefore also be attached to a hand-held processing means during loading in a slightly- or non-hostile environment. Whenever loading in a hostile environment, or if operating in a hostile environment after loading in a less severe situation, continuous monitoring may be provided by remote measurements taken via attached additional cabling.
• the physical shape of this embodiment of the invention is such that, when not connected to additional cabling, it avoids impediments such as cable connectors which both operate inefficiently at high temperatures and could interfere with torquing equipment as the fastener is loaded.
• the cable has length within the range 150 - 300 mm. In many applications this is sufficient to reach a cooler zone, for example outside thermal lagging placed upon hot pipework. This will generally therefore avoid the need for the connection between the permanently-attached cable and selected attachment (handheld device or the second cable) and the second cable itself to be specifically adapted for high-temperature operation.
• One arrangement to measure capacitive reactance may be provided by current supply means arranged to supply an a.c. current to the sensing surface and the probe adapted to measure potential drop across the gap between the sensing surface and the surface within the bore hole. Capacitive reactance is equal to voltage drop divided by applied current. This therefore provides a straightforward way to measure capacitive reactance by taking only measurements of electrical current and voltage.
• the voltage is preferably measured by means of electrical connections made between one element of a multi-axial cable and the fastener and between a second element and the sensor.
• the current supply means is preferably arranged to supply a constant amplitude, constant frequency current. This avoids even the need to measure the current, as it will already be known, and the voltage drop can be used as a direct measure of the value of the capacitative reactance. Alternatively, changes in strain can be monitored simply by observing changes in the voltage drop.
• the sensing surface is preferably a circular surface of a sensing cylinder.
• a cylindrical sensor reduces potential cost of the system as this shape of hole will be most conveniently bored into the shaft of a fastener.
• the sensing cylinder is part of a guard ring capacitance transducer.
• the capacitance transducer may therefore preferably comprise the cylindrical sensor mounted within a coaxial cylindrical guard ring.
• the guard ring component effectively isolates around the sensor the uniform section of an applied electric field, enabling reactance of a capacitor formed by the sensor and lower surface of the bore hole to be measured with improved linearity.
• the guard ring should be driven at the same electric potential as the sensor, and this is preferably achieved by a connection made by a third element of the multi-axial cable between the guard ring and current supply means.
• the guard ring, sensor and fastening device 12 are preferably fabricated from materials with the same or very similar thermal expansion coefficients. This reduces the possibility of errors being introduced into the measurement of fastener loading by internal displacements arising from differential thermal expansion.
• This probe attachment may be incorporated into a range of fastening systems, or indeed non-fastening systems which operate in a stressed environment, but is particularly suited for placement in the bolt part of a nut and bolt system.
• Figure 1 shows schematically an illustration of a load-indicating bolt implementing one aspect the invention.
• FIG 2 is a detailed illustration of the transducer component of Figure 1 , which embodies a second aspect of the invention.
• Figure 3a is an illustration of a second embodiment of the transducer in accordance with the invention.
• Figure 3b is an illustration of the transducer of Figure 3a located within a bolt.
• Figure 1 illustrates a nut 10 and bolt 12 fastening mechanism, the nut fitting onto a threaded portion 14 of the bolt so as to clamp a joint between its head 16 and the nut 10.
• the bolt 12 In its top portion 18, near the head 16, the bolt 12 contains an axial bore hole 20 with a flat lower surface 22.
• a guard ring capacitance transducer 24 is positioned within the bore hole 20 and held in place by a plug 26 in a top surface of the bolt 12. When in position, an air pocket is trapped in the bore hole 20 by the transducer 24. In particular, an air gap 28 is left between a lower surface 30 of the transducer and the lower surface 22 of the bore hole.
• cabling 32, 34 extends from the transducer 24 through the plug 26 and a short way, typically 150 to 300 mm, outside the bolt.
• cabling 34 is triaxial.
• One connection is made from the triaxial cabling to the bolt 16 in the region of the plug 26 and coaxial cabling 32 connects directly to the transducer 24.
• the triaxial cabling 34 is attachable at its free end either directly to a remote electronic conditioning unit (not shown) or via a second, longer, section of cabling 36.
• the remote electronic unit will be a handset, whereas in the latter case it will be a less-portable signal processing unit. Circumstances in which each option is preferred will be described in more detail below.
• c is a constant characteristic of the bolt, namely Young's modulus of its material multiplied by its cross sectional area (Ex a). Strictly speaking c is not a constant but varies slightly with temperature: Young's modulus, cross sectional area and thermal coefficient of expansion are all temperature dependent. However, as is well known in the field, with an appropriate choice of materials, transducer characteristics may be matched to those of the bolt and any errors introduced by the variation of this constant c with temperature can be limited to around 5%.
• the size of bore hole required to accommodate the transducer is only of the order 4.5% of the cross sectional area of the bolt. This is to be contrasted with threading a standard bolt in which typically 23% of cross sectional area is lost. Thus a small hole on the axis of the bolt, as required to accommodate the transducer, will not induce its permanent failure.
• the transducer 24 consists of a cylindrical sensor 52 contained within a cylindrical guard ring 54. Insulating layers 56a, 56b separate sensor 52 from guard ring 54 and guard ring 54 from the bore hole 20.
• the transducer lower surface 30 thus comprises lower surfaces 58, 60 of the sensor 52 and guard ring 54.
• An electric current from a constant current a.c. source (not shown) is applied to the capacitor formed by the sensor lower surface 58 and the lower surface 22 of the bore hole.
• the guard ring 54 is at the same time driven to the same electric potential as the sensor 52.
• the guard ring 54 serves to ensure that the field generated between the sensor 52 and lower surface 22 is uniform. The required measurements are taken with respect to the capacitor formed between the sensor lower surface 58 and bore hole surface 22 only.
• the guard ring 54 driven at the same electrical potential as the sensor 52, effectively isolates the uniform section of the electric field, improving the linearity of the device. Once the current is applied a potential drop develops between the lower surface 58 of the sensor 52 and the lower surface 22 of the bore hole.
• the output cabling 34 is triaxial enabling three electrical connections to be made to the system.
• a central signal cable 62 is connected to the sensor 52, a coaxial section 32 is connected to the guard ring 54 and an outer sheath of the triaxial section 34 is connected to the bolt 12.
• a constant current is applied via the signal cable 58 to the sensor 52 and the same electric potential is applied via the coaxial section 32 to the guard ring 54.
• the voltage drop over the sensor surface 58 - bore hole surface 22 capacitor is measured over signal cable 32 and outer sheath of the triaxial section 34. This measurement is made by a low capacitance voltage preamplifier.
• ⁇ is the angular frequency of the a.c. current and Cthe capacitance across the air gap.
• the capacitance is given by d
• A is the cross section area of the lower surface 58 of the sensor 52, ⁇ the dielectric constant (of air) and dthe width of the air gap 28.
• the capacitive reactance is directly proportional to the separation gap.
• monitoring of the change in capacitive reactance over time provides an indication of the extension (or contraction) displacement Ad resulting directly from the changing strain experienced by the bolt.
• the device can be pre-calibrated for various known sizes of air gap, and the air gap sizes pre-calibrated for various known loads. This provides a very quick way to take a measurement of capacitive reactance from which the loading of the fastener can be deduced.
• triaxial cable 34 On initially loading the bolt in a moderate or ambient environment, torque is gradually applied and capacitive reactance monitored using a handset directly connected to triaxial cable 34. Sometimes, if the structure being bolted is intended for operation in harsh environments, then initial loading may still be carried out under ambient conditions. Moreover, if the cabling 34 protrudes from the bolt at all, then the preferred length of triaxial cable 34 permanently attached to the bolt 12 is 150 - 300 mm. This is sufficient to ensure that its free end generally lies within a "cool zone" outside of, for example, lagged pipework. Thus loading conditions are generally amenable to load monitoring by means of a hand-held device.
• Loading may be carried out either by tightening the nut 10 or by tightening the bolt 12 with torque equipment adapted for the short length of cabling 34 to pass through the centre.
• applied load can be directly measured locally by means of a hand-held device monitoring capacitive reactance. Ideal loading, under ambient conditions, can thus be ensured.
• the bolted system may be transferred to its operating environment.
• the output triaxial cable 34 of each bolt is attached via respective extension cables 36 to the remote electrical conditioning unit.
• reactance measurements may be regularly taken in order to provide continuous in situ monitoring of the strain experienced by each bolt of the system.
• FIGs 3a and 3b illustrate an alternative embodiment of the transducer of Figures 1 and 2, which is particularly suitable for operation in thermally non-hostile environments.
• an alternative structure of guard ring capacitance transducer 80 is shown connected to a triaxial socket 84 in place of the plug 26 of the first embodiment.
• This transducer 80 consists of the sensor 52 separated from a guard ring 82 by an insulation layer 56a.
• the guard ring 82 is again covered by an insulation layer 56b to separate it from the bolt 12 when the transducer is in place.
• the guard ring 82 in this embodiment extends from the sensor surface to the triaxial socket 84.
• a socket with three such connectors is readily available commercially, for example, as manufactured by Lemo.
• This embodiment of the invention measures capacitive reactance between sensor and bore hole surfaces in much the same way as the first embodiment.
• the difference lies in the fact that the cabling 62 is (see Figure 3b) terminated within the bolt. Electrical connections to supply the constant amplitude constant frequency current and to measure the voltage drop are made via the external plug which can be connected to the triaxial socket. In this way, there is no need for any cabling to extend beyond the bolt either in fastening it or in taking reactance measurements.
• a bolt fitted with this transducer 80 and socket 84 therefore provides no more hindrance to fastening than is provided by a standard bolt. Moreover, the pocket inside the bolt remains sealed to the environment by means of the socket 84.
• this embodiment is particularly suited for use in hostile, but ambient temperature, environments where corrosion or contamination could readily occur if the measuring surfaces were to be exposed.
• environments include, for example, operation in structures such as oil rigs which are exposed to sea water, or parts of nuclear reactors.
• a bolt can be manufactured with one of two permanently-attached lengths of cabling, the longer of these lengths being attachable to a second piece of cabling to create a third, longest, length.
• Three cable lengths can therefore be arranged.
• a very short length may be used which terminates within the fastening device 12. This can be very conveniently used in room- or close to room- temperature applications.
• a short length of cable which extends a short distance outside of the bolt may be fitted. If this second length is attachable either to a processing means or to a further cable extension, then this embodiment may be used in two different temperature regimes. If the temperature is moderately elevated or dropped, then the short length is attached directly to the signal processing means and if it is extreme, or other environmental conditions are extreme, then it is attached to the longer length of cabling which is in turn attached to the signal processing means.

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• Engineering & Computer Science (AREA)
• General Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Measurement Of Length, Angles, Or The Like Using Electric Or Magnetic Means (AREA)

Applications Claiming Priority (3)

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• 2001
  • 2001-09-03 GB GB0121317A patent/GB0121317D0/en not_active Ceased
• 2002
  • 2002-09-03 EP EP02767622A patent/EP1509703A1/de not_active Withdrawn
  • 2002-09-03 WO PCT/GB2002/004040 patent/WO2003021115A1/en not_active Application Discontinuation

Non-Patent Citations (1)

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