Instrumented assembly screw

Abstract

The invention relates to an assembly screw comprising at least a threaded part (12A) and a non-threaded part (12C), the non-threaded part being provided with a strain gauge (22) that is sensitive to the instantaneous elongation of the non-threaded part and with an RFID tag (24) connected to the strain gauge by a wired link, the assembly allowing the wireless transmission of an electrical resistance value of the strain gauge representative of the instantaneous elongation.

Description

[0001] Description

[0002] Title of the invention: Instrumented assembly screw

[0003] Technical Field

[0004] The present invention relates to the field of controlled tightening and more particularly that using manual tightening wrenches, such as torque wrenches with trigger or direct reading (mechanical or electronic).

[0005] Prior art

[0006] Tightening of screw elements can be checked using different methods, namely by measuring the torque (tightening force), the rotation angle (conversion of the elongation of the screw body with the thread pitch) or the elongation of the screw.

[0007] Angle tightening is widely used in the automotive industry. This can be implemented with a manual tightening wrench that includes means for measuring the tightening angle. Elongation tightening is used for long screws also called tie rod (manual, hydraulic, or thermal elongation method). Torque tightening is on the contrary the most used method, using either trigger wrenches or electronic wrenches and has the advantage of being simple to use. On the other hand, it has the major disadvantage that for a given tightening torque, the force on the screws and therefore the resulting mechanical deformation, varies greatly due to the significant dispersion of the apparent coefficient of friction (thread, head support of the screw, face of the nut, brake, etc.), torsion of the body of the screw, etc.

[0008] For Teflon-based lubrications, for example, such as those used on cryotechnic engines, experience has led to taking into account a dispersion of around 300% on this coefficient when sizing screwed or bolted connections. The importance of this dispersion is the source of many difficulties, or even impossibilities, in sizing the setpoint torque. Indeed, in design and for tightening to the torque, it is necessary to take into account the extreme limits of the variation range of the coefficient of friction, that is to say both the low coefficients which condition the mechanical strength of the assembly, and the highest coefficients which are responsible for the quality of the tightening of these connections (sufficient crushing of the seals, sufficient tightening of the flanges, etc.).

[0009] Such a situation is not satisfactory because it leads to oversizing of the connections which is detrimental both from the point of view of mass and with regard to the mechanical behavior of the screws over time (fatigue, loosening, etc.).

[0010] On the other hand, the mechanical deformation of the screw resulting from the tensile force applied to it must be taken into account. Indeed, during a tightening operation, there is initially an elastic deformation regime (i.e. reversible), where the deformation varies linearly with the force, then, if tightening is continued, a plastic deformation regime (i.e. irreversible), where the deformation varies more and more rapidly with the stress, to end up at rupture. From this behavior, it follows that tightening to the torque, leading to a very dispersed tensile force, must preferably be carried out in the area of ​​elastic deformation of the screw far from the elastic limit.

[0011] There are currently tightening wrenches that allow control of either the tightening torque alone (trigger wrench, direct reading or ultrasound), or simultaneously the tightening torque and the angle of rotation or elongation in order to tighten screws corresponding to a target value for the tightening torque and / or the angle of rotation or elongation. There are also torque tightening devices with sophisticated processing means that allow the tightening precision to be increased in successive stages.

[0012] However, in all cases, the tightening value is guaranteed by the tightening wrench alone, which, during use, can unfortunately lose its precision. In addition, due to human error, incorrect calibration of the wrench can occur or, worse, forgetting to tighten. This sole control via the tightening wrench therefore does not appear satisfactory either and there is therefore a need for an alternative solution that guarantees perfect tightening of these tightened or bolted connections in all circumstances by knowing the instantaneous tensile force exerted on the screws, a parameter determining the quality of the tightening and its durability over time.

[0013] Statement of the invention

[0014] The main aim of the present invention is therefore to solve the aforementioned problem by allowing real-time knowledge of the value of the tightening torque.

[0015] Another goal is to ensure this knowledge once the connection has been made and possibly become inaccessible. Yet another goal is to be able to be informed at any time of a possible loss of tightening.

[0016] These aims are achieved by an assembly element comprising at least one threaded part and one non-threaded part, characterized in that the non-threaded part is provided with a strain gauge sensitive to the instantaneous elongation of the non-threaded part and an RFID tag connected to the strain gauge by a wired link, in order to allow the wireless transmission of an electrical resistance value of the strain gauge representative of the instantaneous elongation.

[0017] Thus, by relaying the instantaneous elongation of the stretched screw section, control of the tightening torque becomes possible at any time, whether the assembly screw is accessible or not.

[0018] The cap screw may have two threaded end portions separated by an unthreaded center portion.

[0019] Depending on the embodiment envisaged, the strain gauge may be glued or printed on the unthreaded portion of the cap screw and the RFID tag may be glued or printed on the unthreaded portion of the cap screw.

[0020] The strain gauge is advantageously a single-direction resistive tensile strain gauge connected by wire to the RFID tag. The RFID tag advantageously includes an active storage chip and a UHF antenna.

[0021] When the strain gauge and RFID tag are printed, the wire bond is also printed on the unthreaded portion of the cap screw.

[0022] The invention also relates to a system for determining the tightening torque of an assembly screw comprising an assembly screw as mentioned above and an RFID reader comprising an RFID antenna and an RFID chip for receiving the electrical resistance value delivered by the strain gauge of the assembly screw, a processing module for determining a tightening torque from this electrical resistance value, and a display for displaying the determined tightening torque.

[0023] Advantageously, the RFID reader includes sound or light means to give an alert in the event of loss of tightening.

[0024] Brief description of the drawings

[0025] Other characteristics and advantages of the present invention will emerge from the description given below, with reference to the appended drawings which illustrate an exemplary embodiment thereof without any limiting character and in which:

[0026] [Fig. 1] Figure 1 is a sectional view of a wrench used for tightening a cap screw, and

[0027] [Fig. 2] Figure 2 shows an instrumented assembly screw according to the invention.

[0028] Description of the embodiments

[0029] The principle of the invention is based on the instrumentation of an assembly element, screw, stud or bolt, in order to know in real time a characteristic representative of the value of the tightening torque in order to transmit it to an external processing module capable of exploiting it.

[0030] Figure 1 illustrates an example of a tightening wrench 10 used for tightening a cap screw 12 to assemble a first part 14 with a second part 16 tapped to receive the threaded portion 12A of the cap screw. A washer 18 is mounted between the head 12B of the assembly screw and the first part 14. The rotation angle is measured by a measuring device 20 which comprises a clamping sleeve 20A cooperating with the head 12B of the assembly screw and which measures the differential rotation between the sleeve 20A and the first part 14 by means of a spring 20B arranged around the outer periphery of the sleeve, a Teflon® tube 20C, provided with a non-slip ring 20D, being interposed between the spring 20B and the upper surface of the first part 14. The Teflon® tube makes it possible to generate a low coefficient of friction between the moving parts so as not to affect the tightening torque.

[0031] This measurement configuration using a spring is of course in no way limiting and an optical, magnetic or electrical measurement (rheostat type) replacing both the spring and the clamping sleeve is entirely possible.

[0032] Whatever the configuration chosen, the tightening wrench also includes input means for defining a setpoint value for tightening and processing means for calculating the tightening torque from the rotation angle delivered by the measuring device 20 and informing the operator (conventionally by an audible and / or light signal) when the initially defined setpoint is reached. Reference may be made to application FR2852879 for further details on the calculations implemented for this determination of the tightening torque from the rotation angle.

[0033] However, as explained in the preamble, once the connection has been made and the tightening key removed, no control over time of the tightening torque is possible, particularly that of any possible loss of tightening.

[0034] Also, according to the invention and as shown in Figure 2, the non-threaded part 12C of the assembly screw 12 is provided with a strain gauge 22 and an RFID (Radio Frequency IDentification) tag 24 connected to each other by a wired connection 26. The RFID tag is intended to cooperate remotely with an RFID reader 28, advantageously portable, comprising an antenna / RFID chip assembly 30 connected to a processing module 32 itself connected to a display 34 and / or a loudspeaker. The display and / or the loudspeaker can constitute an audible and / or luminous alert device for the operator in the event of loss of tightening. The assembly screw and RFID reader assembly forms a system for determining the tightening torque of an assembly screw comprising at least one threaded portion and one non-threaded portion in order to enable the wireless transmission of an electrical resistance value of the strain gauge representative of the stretching of this assembly screw.

[0035] The strain gauge (a resistive wire extensometer conventionally manufactured by a lithographic process from a metal sheet of a few microns and a polyamide-type electrical insulator), is preferably glued to this non-threaded part in order to detect an instantaneous elongation resulting in a change in the electrical resistance of the strain gauge. Resistive strain gauges to be glued, particularly in single-direction traction, are known in the instrumentation industry (characterized by size, resistance to heat / corrosive environment and gluing on different supports). However, the gauge could also be directly made by printing directly on the barrel (non-threaded part) of the screw.

[0036] The RFID tag, which can also be stuck on the unthreaded part of the assembly screw, comprises an antenna 24A connected to a processing module 24B (called RFID storage chip) comprising a memory and transmission / reception means intended to cooperate with corresponding means of the RFID reader 28. The RFID tags to be stuck are known (characterized by the size, the integration / substrate, the detection distance by the reader, the memory capacity, the active or passive nature). However, the tag could also be directly produced by printing directly on the barrel (unthreaded part) of the screw.Depending on the frequency environment and the detection distance (power characterized by the frequency range: low (Low Frequency, LF 125kHz) / high (High Frequency, HF 13.56MHz) / Ultra high (Ultra High Frequency, UHF 433 and 860-960MHz) / Super high (Super High Frequency, SHF 2.45GHz)), different frequency ranges and associated antennas will be usable.

[0037] The strain gauge and the RFID tag are connected by two measuring wires forming the wire connection 26. When the two aforementioned elements are printed, the wire connection is advantageously also printed. The placement of the strain gauge on the barrel (its non-threaded part) of the assembly screw makes it possible to determine an electrical resistance value depending on the instantaneous elongation of this barrel, which value will be stored in the memory of the RFID chip and can therefore be read at any time by the RFID reader. Appropriate processing known per se will allow the processing module 32 of the RFID reader to deduce the tightening torque from this electrical resistance value and to present it to the operator on the display 34 of the RFID reader.

[0038] Depending on the nature of the RFID chip, active or passive, automatic feedback of information to the RFID reader may be considered, in particular with an audible and / or visual alert in the event of loss of tightening at the reader.

[0039] RFID. A code associated with the torque information will precisely designate the position of the screw in the bolted assembly.

[0040] It should be noted that if the preceding description concerned a conventional screw, it is clear that it is also applicable to a headless screw (or stud) with double thread, the unthreaded central part of which between the two threads would be fitted with the aforementioned strain gauge and RFID tag.

Claims

Claims

1. Assembly screw (12) comprising at least one threaded portion (12A) and one non-threaded portion (12C), the non-threaded portion being provided with a strain gauge (22) sensitive to the instantaneous elongation of the non-threaded portion (12C) and an RFID tag connected to the strain gauge (22) by a wired connection (26), in order to allow the wireless transmission of an electrical resistance value of the strain gauge representative of the instantaneous elongation, characterized in that the strain gauge (22) is printed on the non-threaded portion (12C) of the assembly screw and in that the RFID tag (24) is printed on the non-threaded portion (12C) of the assembly screw (12).

2. A cap screw (12) according to claim 1, comprising two threaded end portions separated by a central unthreaded portion.

3. A cap screw (12) according to any one of claims 1 or 2, wherein the threaded bond (26) is printed on the unthreaded portion (12C) of the cap screw.

4. The assembly screw (12) of any one of claims 1 to 3, wherein the strain gauge (22) is a single-direction tensile resistive strain gauge wired to the RFID tag (24).

5. The assembly screw (12) of any one of claims 1 to 3, wherein the RFID tag (24) comprises an active storage chip (24B) and a UHF antenna (24A).

6. System for determining the tightening torque of an assembly screw comprising an assembly screw (12) according to any one of claims 1 to 5 and an RFID reader (28) comprising an RFID antenna and an RFID chip (30) for receiving the electrical resistance value delivered by the strain gauge (22) of the assembly screw (12), a processing module (32) for determining a tightening torque from this electrical resistance value, and a display (34) to show the determined tightening torque.

7. System according to claim 6, in which the RFID reader (28) comprises sound or light means for giving an alert in the event of loss of tightening.

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