Instrumented nut, clamping sleeve, clamping device and method for tightening such a nut

Abstract

Instrumented nut, clamping sleeve, clamping device and method for tightening such a nut. The invention relates to an instrumented metal nut (10b) comprising a tubular body having a polygonal drive surface (14). The nut comprises, on at least one annular portion (34) of the outer surface: an electrically insulating layer (36) applied to the annular portion; a piezoresistive conductive polymer (38) whose ohmic resistance varies according to a stress exerted during tightening of the nut on a threaded fastener, said conductive polymer being applied to the electrically insulating layer; and at least two electrodes (40). Figure for the abstract: Figure 2

Description

Title of the invention: Instrumented nut, clamping sleeve, clamping device and method for clamping such a nut

[0001] The present invention relates to the field of screw tightening and, in particular, to an instrumented nut for determining the stresses present in the screw during and / or after tightening. Specifically, the invention relates to an instrumented metal nut comprising a tubular body extending along an axis between a first face and a second face, the body having a polygonal drive surface.

[0002] The present application also relates to a clamping sleeve suitable for tightening said nut and to a device for tightening said nut by means of said sleeve.

[0003] To achieve optimum tightening of an assembly, it is necessary to optimize the preload of the fasteners used to tighten the assembly. The preload of a fastener is the initial tension created in the fastener when it is installed in the assembly. Precise knowledge of the tension applied to the fastener is essential to ensure the long-term durability of the assembly under external stresses. Indeed, excessive tightening can damage the screw or the part into which it is screwed, and insufficient tightening can lead to the screw becoming detached from the part.

[0004] Tightening can be carried out using a torque wrench which indicates a tightening torque. However, the tightening torque is not an accurate measure of the preload in the screw, as the latter also depends on the coefficients of friction between the threads of the screw and the nut, as well as between the nut and the contact face, which are difficult to control.

[0005] It is also known to indirectly determine the preload of a screw by equipping the nut with strain gauges. For example, document FR3106634A1 describes a nut having a polygonal drive surface and a base, the base being provided with strain gauges configured to measure a deformation of the nut in the axial direction. The deformation of the nut is indicative of the preload in the screw. However, the gauges are glued either directly onto the body of the nut or onto a film which is then glued onto said body. Such a system requires a durable adhesive to ensure that the gauges do not detach over time.

[0006] The present invention aims to remedy all or part of the drawbacks of the prior art.

[0007] To this end, the invention relates to a nut of the aforementioned type, comprising, on at least an annular portion of the outer surface: an electrically charged layer insulating, applied to the annular portion; a piezoresistive conductive polymer, whose ohmic resistance varies according to a stress exerted during tightening of the nut on a threaded fastener, said conductive polymer being applied to the electrically insulating layer; and at least two electrodes.

[0008] The invention makes it possible to precisely adjust a preload in a screw, exerted by a tightened threaded fastener, with limited additional cost, reduced additional mass and increased reliability.

[0009] According to other advantageous aspects of the invention, the nut comprises one or more of the following characteristics, taken individually or in all technically possible combinations:

[0010] - the annular portion of the outer surface is a groove formed on the surface exterior;

[0011] - the annular portion is a portion of a frustoconical surface adjacent to the surface training;

[0012] - the annular portion is a portion of the polygonal training surface;

[0013] - each electrode is arranged on the conductive polymer;

[0014] - at least one electrode is located at a distance from the conductive polymer and is isolated electrically connected to the outer surface, each electrode being electrically connected to a contact point arranged on the conductive polymer

[0015] The invention also relates to a sleeve adapted to drive the nut in rotation, the sleeve comprising as many electrical pins as the nut has electrodes, each electrical pin being able to connect electrically to an electrode of the nut.

[0016] According to an advantageous aspect of the invention, each electrical pin is a spring-loaded electrical pin.

[0017] The invention also relates to a clamping device for a fastener, said fastener comprising: a nut as described above; and a screw comprising a threaded end adapted to cooperate with the nut; the clamping device comprising a clamping sleeve as described above, said clamping device further comprising a measuring device configured to: send an electric current into at least one electrode of the nut; measure at least one potential difference between the other electrodes of said nut; and calculate a preload value in the screw as a function of the potential differences measured.

[0018] According to an advantageous aspect of the invention, the device is capable of implementing a method for reconstructing the resistivity value in the nut by electrical impedance tomography.

[0019] The invention will become clearer upon reading the following description, given solely by way of non-limiting example, and made with reference to the drawings in which:

[0020] [Fig. 1] [Fig. 1] is an isometric view of a nut illustrating a first embodiment of the invention,

[0021] [Fig.2] Fig.2 is an isometric cross-sectional view of a nut illustrating a second embodiment of the invention,

[0022] [Fig.3] Fig.3 is an isometric view of a nut illustrating a third embodiment of the invention,

[0023] [Fig.4] Fig.4 is an isometric cross-sectional view of a nut illustrating a fourth embodiment of the invention,

[0024] [Fig.5] Fig.5 is an isometric view of a clamping sleeve suitable for tightening the nut of Figures 2 and 3.

[0025] [Fig. 6] Fig. 6 is a cross-section of an assembly comprising a nut of Fig. 1 coupled to a clamping sleeve according to a second embodiment, and a device for measuring the tension in the nut, and

[0026] [Fig.7] The [Fig.7] is a diagram of a nut, a nut clamping device and a device for measuring deformation in the nut illustrating an embodiment of the invention.

[0027] In the following description, nuts 10a, 10b, 10c and 10d will be described simultaneously, similar elements of the nuts being designated by the same reference numbers.

[0028] The nut 10a, 10b, 10c, lOd is made of metal, for example made of TA6V titanium alloy.

[0029] The nut 10a, 10b, 10c, 10d extends along an axis XX and comprises: a tubular body 12, having a hexagonal drive surface 14; a base 16a, 16b, 16c, 16d; first 18 and second 20 end faces; and a through hole 22, opening at said first and second end faces. The through hole includes a tapped portion 24 configured to cooperate with the threaded portion of a threaded element such as a screw. The through hole also includes a counterbore 25, unthreaded and with a diameter larger than the largest diameter of the tapped portion 24.

[0030] The base 16a, 16b, 16c, 16d includes a frustoconical external surface 26, adjacent to the hexagonal drive surface 14. Preferably, the base 16a, 16b, 16c, 16d further includes a cylindrical external surface 28, adjacent to the frustoconical external surface 26 opposite the hexagonal drive surface 14.

[0031] In the second embodiment of the nut 10b, the cylindrical external surface 28 of the base 16b includes an annular groove 34.

[0032] In the third embodiment of the nut 10c, the base 16c further comprises six facets 30 cutting the frustoconical external surface 26, each facet 30 being aligned with a flat surface of the hexagonal drive surface 14.

[0033] In the fourth embodiment of the nut lOd, the frustoconical external surface 26 of the base 16d comprises a flat surface, preferably a shoulder 35, intersecting the frustoconical external surface 26. The shoulder 35 is substantially normal to the axis XX of the nut lOd. In an alternative not shown, the frustoconical external surface 26 of the base 16d comprises an annular groove 34.

[0034] The nut 10a, 10b, 10c, lOd has an electrically insulating layer 36 applied to at least a portion of the outer surface of the nut, either directly onto the bare material or onto an aluminum pigment coating of the HLKOTE™ type, described for example in document EP2406336. In the examples in Figures 1 to 4, the electrically insulating layer 36 is applied: to the cylindrical outer surface 28 of the nut 10a; to the frustoconical surface and to a portion of the cylindrical outer surface of the nut 10b; to the cut frustoconical surface of the nut 10c, lOd.

[0035] Alternatively, the base 16a, 16b, 16c, 16d could be completely covered with the insulating layer 36, for example by masking the hexagonal drive surface 14 and spraying a solution comprising a polymer and a solvent onto the base, then allowing the solvent to evaporate. An alternative would be to immerse the nut completely (without masking) in such a solution. Alternatively, the nut 10a, 10b, 10c, 10d could be anodized to create an insulating oxidation layer on its outer surface.

[0036] A conductive polymer 38 is disposed at least partially on the electrically insulating layer 36. In the examples of nuts 10a, 10c, the conductive polymer completely covers the electrically insulating layer 36. In the examples of nuts 10b, 10d, the conductive polymer partially covers the electrically insulating layer 36. In the examples of nuts 10b, 10d, the conductive polymer only covers the annular groove 34 and the shoulder 35, respectively.

[0037] The conductive polymer 38 is piezoresistive, meaning that its ohmic resistance varies according to the stress applied when tightening the nut onto a threaded fastener. The conductive polymer 38 is sufficiently conductive to allow the passage of an electric current. The conductive polymer is also sufficiently flexible to deform with the nut 10a, 10b, 10c, lOd without cracking when the nut is subjected to deformation, as long as the deformation remains within the elastic range.

[0038] The conductive polymer 38 is suitable for electrical impedance tomography. Such a polymer comprises conductive nanoparticles, for example Carbon, silver, copper, iron, nickel, etc., nanofibers, and a polymer resin selected from the group consisting of epoxy resins, polyimides, bismaleimides, cyanate esters, polyesters, vinyl esters, and methanes. Examples of conductive polymers suitable for electrical impedance tomography are described in US5989700A and EP1425166B1. The selection of a suitable polymer resin can be based on the environmental conditions the nut will experience during its service life once the aircraft is in operation. The nature and quantity of conductive particles are preferably chosen to provide a balance between electrical resistance (fewer particles mean higher resistance) and viscosity (more particles mean higher viscosity).

[0039] Electrical impedance tomography, abbreviated as EIT, is a technique used primarily in medical imaging to perform non-invasive measurements of electrical conductivity, or to identify pressure zones in tissue. For example, electrodes are placed on the edges of a tissue, an electric current is injected between two electrodes, one of which is grounded, and the potential in the tissue is measured by all the other electrodes. The current injection is applied successively to the other electrodes to obtain a set of measurements. From this set of measurements, an image of the electrical conductivity is reconstructed, and using a suitable reconstruction algorithm, pressure zones in the tissue can be extrapolated. An example of the use of the EIT technique is described in the article "EIT-based fabric Pressure Sensing" by A. Yao, CL Yang, JK Seo, and M.Soleimani, accessible at http: / / dx.doi.org / 10.1155 / 2013 / 405325. .

[0040] The conductive polymer 38 is disposed on an outer annular surface of the nut.

[0041] In the embodiment of [Fig. 1], the annular surface comprises a cylindrical external surface 28 of the base. This embodiment allows monitoring of the radial deformation of a counterbored nut, the base being less rigid than the rest of the nut.

[0042] In the embodiment of [Fig.2], the annular surface includes the annular groove 34 of the base 26 of the nut 10b. This embodiment preserves the integrity of the conductive polymer 38.

[0043] In the embodiment of [Fig. 3], the annular surface comprises the frustoconical external surface 26. This embodiment allows for monitoring the bending stresses experienced by the nut. This arrangement is advantageous for nuts whose frustoconical surface is larger than their cylindrical surface.

[0044] In the embodiment of [Fig. 4], the annular surface includes the shoulder 35 of the base 26 of the nut lOd. This embodiment preserves the integrity of the conductive polymer 38.

[0045] In another embodiment not shown, an annular groove is disposed on the hexagonal drive surface 14. This embodiment, advantageous for nuts without a base, also allows the integrity of the conductive polymer 38 to be preserved.

[0046] The nut 10a, 10b, 10c, lOd also comprises a plurality of electrodes 40. An electrode is understood to be the end of an electrical conductor through which an electric current enters or exits. The electrodes 40 are, for example, deposited by printing with silver-based ink, or by bonding.

[0047] Each electrode 40 is brought into contact with the polymer conductor 38 to capture the variations in the electric current produced by the deformation of the nut when tightening the nut onto a threaded fastener.

[0048] The nut 10a, 10b, 10c, lOd comprises at least two electrodes 40 spaced apart from each other, advantageously diametrically opposed. It should be noted that the more electrodes the nut has, the better the reconstruction of the deformation undergone by the nut.

[0049] According to a first embodiment, the electrodes are arranged on the conductive polymer 38. In the examples shown, the nut 10a of [Fig.1] comprises six electrodes 40 arranged opposite the edges between two sides of the hexagonal drive surface 14; the nut 10b of [Fig.2] comprises six electrodes 40 arranged opposite the middle of a side of the hexagonal drive surface 14; the nut 10c of [Fig.3] comprises six electrodes 40 arranged opposite the middle of a side 30 of the base 16c; and the nut 10d of [Fig.4] comprises twelve electrodes 40 distributed uniformly on the flat shoulder 35. Other configurations with a different number of electrodes 40 are possible and / or with a different placement of the electrodes 40.

[0050] The electrodes 40 can be deposited at equidistant intervals along the axis XX across the width of the annular surface. In variants not shown, the electrodes can be distributed homogeneously along one edge of the annular surface, or even along both edges of the annular surface. In this case, the electrodes 40 can be arranged opposite each other or in a staggered pattern.

[0051] The mesh formed by the set of electrodes 40 is configured to generate electrical signals representative of a deformation of the nut, either in a radial measurement direction if the set of electrodes is placed in the same plane perpendicular to the axis XX of the nut, or in two directions if the electrodes are placed in two planes perpendicular to the XX axis of the nut, spaced along the XX direction.

[0052] According to another embodiment, when the conductive polymer 38 covers a portion of the cylindrical outer surface 28 of the base or the annular groove 34, the electrodes 40 are advantageously positioned outside said cylindrical outer surface or annular groove to facilitate electrical contact with a clamping sleeve, as will be described later. Each electrode 40 is thus offset from the conductive polymer 38 and electrically insulated from the outer surface of the nut, by being positioned, for example, on a portion of the electrically insulating layer 36. Each electrode 40 is electrically connected by a track 42 to a contact point 44 disposed on the conductive polymer 38 ([Fig. 2]).

[0053] The electrodes 40 are retained on the nut 10a, 10b, 10c, lOd throughout the nut's lifetime. Thus, the preload can be controlled, even after the initial tightening of the nut, as long as the nut 10a, 10b, 10c, lOd is installed on a bolt.

[0054] A clamping sleeve 50a adapted to drive the nut 10a in rotation is shown schematically in [Fig. 6]. A clamping sleeve 50bc, adapted to drive the nut 10b, 10c, lOd in rotation, is shown in [Fig. 5].

[0055] The sleeve 50a, 50bc includes an inner surface 52 with a hexagonal cross-section adapted to interface with the hexagonal drive surface 14 of the nut to rotate the nut 10a, 10b, 10c, lOd. Obviously, if the nut has a four-, eight-, ten-, or twelve-sided drive surface, the inner surface 52 will be modified accordingly. The sleeve 50a, 50bc includes pins 54 adapted to connect electrically to the electrodes 40 of the nut 10a, 10b, 10c, lOd. Preferably, the sleeve 50a, 50bc includes at least as many pins 54 as the nut 10a, 10b, 10c, lOd has electrodes 40.

[0056] Advantageously, each pin 54 is a spring pin, also known as a "Pogo pin", in order to compensate for any misalignment between the sleeve 50a, 50bc and the nut 10a, 10b, 10c, 1Od.

[0057] The pins 54 of the sleeve 50a, adapted to drive the nut 10a in rotation, are arranged on a surface coaxial with the cylindrical external surface 28 of the nut 10a. The pins 54 of the sleeve 50bc, adapted to drive the nut 10b, 10c in rotation, are arranged on a frustoconical front surface 55, preferably of complementary shape to the frustoconical external surface 26 of the nut 10b, 10c.

[0058] The sleeve adapted for driving the nut lOd in rotation is identical to the sleeve 50bc, except that it has twelve pins 54 suitable for connecting to the twelve electrodes 40 of the nut lOd. The flat shoulder 35 of the nut lOd is not in contact with the frustoconical front surface 55 of the sleeve, which prevents deformation or damage to the conductive polymer 38.

[0059] A cable 56 groups together the wires (not shown) connecting each pin 54 to a measuring device 62.

[0060] A fastener 94 is considered suitable for being formed by the nut 10a, 10b, 10c, lOd and by a screw 96. An embodiment of the fastener 94, including the nut 10a of [Fig.1], is shown in [Fig.7].

[0061] In the embodiment shown, the screw 96 comprises: a shank 98, having a first threaded end 99, and a head 100 integral with a second end of the shank 98. In a variant not shown, the shank 98 is a stud and the head 100 is formed by a nut screwed to one end of the stud.

[0062] The first threaded end 99 of the rod 98 is suitable for cooperating with the threaded portion 24 of the nut 10a, 10b, 10c.

[0063] Consider a clamping device 60 for a nut 10a, 10b, 10c, 10d. Said device 60 comprises the clamping sleeve 50a, 50bc described above and a measuring device 62. An embodiment of the device 60, comprising the sleeve 50a described above, is shown in [Fig.7].

[0064] The measuring device 62 is configured to control the rotation of the clamping sleeve, send an electric current to at least one electrode 40, and measure at least one potential difference between the other electrodes. The measuring device 62 is also configured to determine a preload value in the screw by reconstructing the local change in resistivity in the nut due to nut deformation, for example, by applying inverse problem analysis.

[0065] The measuring device 62 can thus include a controller 64, at least one demultiplexer 66, at least one multiplexer 68, a current source 70, a calculator 72, a memory 74 and optionally a display 76.

[0066] The controller 64 may include an analog-to-digital converter for sampling the signals it receives.

[0067] The controller 64 has the role of controlling the demultiplexers 66 and the multiplexers 68 in order, on the one hand, to sequentially excite the electrodes 40 in pairs by moving the mass each time, and on the other hand to measure a potential difference across the terminals of the other electrodes.

[0068] The demultiplexers 66 and multiplexers 68 function respectively to demultiplex the excitation signals emitted by the controller 64, and to multiplex the measurement signals at the electrodes 40 as described above. The controller 64 is arranged to perform the excitation according to a TIE excitation scheme in order to induce a change in electrical resistance in the nut 10a, 10b, 10c, lOd at the electrodes 40, said change being characteristic of the deformation undergone by the nut. The current source 70 supplies the demultiplexers 66 with the current which is multiplied for the excitation currents. The current source 70 can be DC, in which case the measurement voltage will be measured simultaneously at the 40 electrodes, or AC, in which case the amplitude and offset of the voltage relative to the AC current will be measured. Alternatively, the demultiplexers and multiplexers could be omitted by using a controller connected directly or indirectly to the electrodes.

[0069] By TIE excitation scheme is meant a scheme chosen from:

[0070] - a neighborhood scheme according to which the excitation current is introduced into adjacent electrodes, and the potential difference is measured successively at the other electrodes, each pair of electrodes being used successively to achieve an excitation,

[0071] - an opposition scheme according to which the excitation current is introduced in diametrically opposed electrodes, and the potential difference is measured successively in the other electrodes, each pair of electrodes being successively used to achieve an excitation, and

[0072] - a transverse arrangement according to which the excitation current is introduced into electrodes opposite each other with respect to a fixed axis, and the potential difference is measured successively in the other electrodes, each pair of electrodes being successively used to achieve an excitation.

[0073] Regardless of the TIE excitation scheme chosen, the current passing through the nut creates a volumetric distribution of the electrical potential. The potential decreases along the current line as a function of the distance from the active electrodes 40 between which the current is injected. The potential drop per unit length is proportional to the current intensity and the resistance of the nut according to Ohm's law. By measuring the potential drop and knowing the current value, the resistance value of the nut can then be calculated. The calculator 72 can thus include a tomographic reconstruction algorithm that uses the potential differences measured on the surface of the nut to calculate the spatial distribution of the resistivity within it, and converts this resistivity into a voltage in the screw. The calculator 73 can also include a filtering algorithm to improve the calculated resistivity.

[0074] Memory 74 includes one or more TIE excitation schemes, as well as target voltage values ​​associated with different nut references.

[0075] Display 76 is typically a screen, which displays either a value or a curve. Display 76 is optional.

[0076] A method for installing the fastener 94 in an assembly will now be described in relation to [Fig.7].

[0077] First, the screw 96 is inserted into a hole in an assembly comprising two structural elements 90, 92. The assembly is considered It has a front face 90.1 and a rear face 92.1. The rod 98 passes through the hole, and the head 100 rests against the front face 90.1 of the assembly. The first threaded end 99 of the rod 98 protrudes from the rear face 92.1 of the assembly.

[0078] Next, the nut 10a is positioned on the first threaded end 99 of the rod 98.

[0079] Next, the measuring device 62 is connected to the nut by assembling the clamping sleeve 50a to the nut 10a until the pins 54 make contact with the electrodes 40.

[0080] The controller 64 then activates the rotation of the clamping sleeve 50a, causing the nut 10a to screw onto the first threaded end 99 of the screw. When the end face 18 of the nut 10a contacts the rear face 92.1 of the assembly, the tension in the screw, and therefore in the nut, increases.

[0081] During tightening, the controller 64 sends a current to the electrodes 40 according to the selected TIE excitation scheme. The computer calculates the voltage in the nut in real time and compares the calculated value to a reference value for the nut 10a stored in memory 74. The calculated voltage value, or a curve representing the calculated voltage, can be displayed on the screen 76 of the measuring device if it is equipped with a screen.

[0082] The controller 64 stops the rotation of the nut when the calculated voltage value reaches the reference value. The measuring device 62 is disconnected from the nut by removing the nut clamping sleeve 10a.

Claims

Demands

1. Instrumented metal nut (10a, 10b, 10c, 10d) comprising a tubular body (12) extending along an axis (XX) between a first face (18) and a second face (20), the body having a polygonal drive surface (14), characterized in that the nut comprises, on at least one annular portion (26, 28, 34) of the outer surface: an electrically insulating layer (36), applied to the annular portion; a piezoresistive conductive polymer (38), the ohmic resistance of which varies as a function of a stress exerted during tightening of the nut on a threaded fastener, said conductive polymer being applied to the electrically insulating layer; and at least two electrodes (40).

2. Nut according to claim 1, wherein the annular portion of the outer surface is a groove (34) formed on the outer surface.

3. Nut according to claim 1 or 2, wherein the annular portion is a frustoconical surface portion (26) adjacent to the polygonal drive surface (14).

4. Nut according to claim 1 or 2, wherein the annular portion is a portion of the polygonal drive surface (14).

5. Nut according to claim 1, wherein each electrode (40) is disposed on the conductive polymer (36).

6. Nut according to claim 1, wherein at least one electrode (40) is offset away from the conductive polymer (38) and electrically insulated from the outer surface, each electrode being electrically connected to a contact point (44) disposed on the conductive polymer (38).

7. A socket (50a, 50bc) for clamping a nut (10a, 10b, 10c, 10d) according to any one of claims 1 to 6, said socket being adapted to drive the nut in rotation, the socket comprising at least as many electrical pins (54) as the nut has electrodes (40), each electrical pin (54) being able to connect electrically to an electrode (40) of the nut.

8. Clamping socket (50a, 50bc) according to claim 7, wherein each electrical pin (54) is a spring-loaded electrical pin.

9. A clamping device (60) for a fastener (94), said fastener comprising: a nut (10a, 10, 10c, 10d) according to any one of claims 1 to 6; and a screw (98) comprising a threaded end (99) adapted to cooperate with the nut; the clamping device comprising a clamping sleeve (50a, 50bc) according to claim 7 or 8, said clamping device further comprising a measuring device (62) configured to: send an electric current into at least one electrode (40) of the nut; measure at least one potential difference between the other electrodes (40) of said nut; and calculate a preload value in the screw as a function of said measured potential differences.

10. Clamping device (60) according to claim 9, capable of implementing a method for reconstructing the resistivity value in the nut (10a, 10, 10c, lOd) by electrical impedance tomography.

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