Method of controlled installation of a threaded fastener

Abstract

Method for controlled installation of a threaded fastener The invention relates to a method for installing a threaded fastener (10) in a structure, the threaded fastener comprising a screw (20) and a nut (28), the method comprising the steps of: a) inserting the screw into the structure, and screwing the nut onto the screw, b) simultaneously transmitting / receiving ultrasonic waves in the fastener, and measuring a parameter representative of a variation of said waves, said first parameter providing an indirect measurement of an axial tension in the screw, c) stopping the screwing when a target value of the first parameter is reached. The method further comprises determining a thickness of structure to be tightened, and determining the target value of the first parameter as a function of the thickness of structure to be tightened and geometric characteristics of the fastener (10). Figure for abstract: Figure 5

Description

Title of the invention: Method for controlled installation of a threaded fastener

[0001] The present invention relates to a method of installing a threaded fastener in a bore of a structure, the threaded fastener comprising a screw and a nut, the method comprising the steps of: a) inserting the screw into the bore, and screwing the nut onto the screw; b) simultaneously emitting ultrasonic waves into the fastener, receiving reflected ultrasonic waves and measuring a first parameter representative of a variation of said waves, said first parameter providing an indirect measurement of an axial tension in the screw; c) stopping the screwing when a target value of the first parameter is reached.

[0002] Such a method, notably described in document EP1570956, makes it possible to control the installation of a threaded fastener to a target tension, for example by measuring the variation in the time of flight of ultrasonic waves emitted and received in the screw, the time of flight providing an indirect measurement of the axial tension in the screw, the latter being elongated under the effect of the tension.

[0003] When tightening the structure between the screw head and the nut, the screw is subjected to elongation. Two screws of the same length can be used to assemble structures of different thicknesses, and the tension actually installed in the screw depends on the tightened thickness. However, a target tension for a given screw does not take into account the thickness actually tightened.

[0004] Furthermore, tightening the nut on the screw heats the screw, the increase in temperature amplifying the elongation of the screw and therefore reducing its axial tension. The time of flight of an ultrasonic wave injected into the screw during screwing is therefore modified, which alters the estimation of the tension actually installed in the screw.

[0005] The aim of the present invention is to propose a method for the controlled installation of a threaded fastener which corrects the variations in measurement of the installed tension due to the thicknesses actually tightened, and to heating.

[0006] For this purpose, the invention relates to a method of the aforementioned type further comprising the following steps: d) determining a thickness of structure to be tightened; and e) determining the target value of the first parameter as a function of the thickness of structure to be tightened and geometric characteristics of the fastener.

[0007] According to other advantageous aspects of the invention, the method comprises one or more of the following characteristics, taken in isolation or in all technically possible combinations:

[0008] - the first parameter is chosen from: a time of flight of an ultrasonic wave Ion- longitudinal; a resonant frequency shift of the screw; and a ratio of the time of flight of a longitudinal or transverse ultrasonic wave to the thickness of the structure to be tightened;

[0009] - the geometric characteristics of the fixing are chosen from: a length of the screw, a length of a threaded portion of the screw, a thread pitch of the screw, a nut length, a nut counterbore length, a nut tapping length and a distance between an end surface of the nut and at least one locking point;

[0010] - determining the target value of the first parameter includes comparing from the structural thickness to a set of thickness values, said set of thickness values ​​being associated with a set of target values ​​of the first parameter for a given screw, corresponding to a set of target tensions for a given screw;

[0011] - the determination of the structure thickness comprises a step of detecting a start of tightening of the structure, corresponding to the appearance of axial tension in the screw;

[0012] - the screwing step is carried out with a setting device equipped with a motor electric, the method further comprising: measuring a second parameter representative of a variation in energy of the electric motor as a function of time; and detecting the start of tightening, said start of tightening corresponding to a predetermined variation of the second parameter;

[0013] - the second parameter is chosen from an intensity of the electric motor and a torque of a motor shaft;

[0014] - the determination of the structure thickness comprises a step of detecting the initial position of the nut on the screw;

[0015] - the step of detecting the initial position of the nut comprises the following steps: bringing an annular metal element into contact with an annular surface of the nut, an inner diameter of the annular metal element being smaller than an inner diameter of the annular surface of the nut; detecting contact between the annular metal element and the annular surface of the nut; rotating the nut on the threaded portion of the screw, and applying pressure to the annular metal element so as to maintain contact with the annular surface of the nut; detecting loss of contact between the annular metal element and the annular surface of the nut as soon as one end of the threaded portion crosses the inner diameter of the annular surface of the nut, the detection of the loss of contact corresponding to the initial position of the nut;

[0016] - the method further comprises monitoring the temperature in the screw and adjusting the target value of the first parameter as a function of temperature;

[0017] - at least one of the screw and the nut comprises a marking containing at least one identifier or a geometric characteristic of said screw or said nut, the installation method further comprising a step of reading the marking and identifying at least the identifier or the geometric characteristic of the screw and / or the nut, said reading step being carried out before step e).

[0018] The invention will be better understood on reading the description which follows, given solely by way of non-limiting example and made with reference to the drawings in which:

[0019] [Fig-1] [Fig.l] is a longitudinal sectional view of a fastener and a part of a laying device, the fastener being capable of being installed by an installation method according to an embodiment of the invention, the fastener being in an initial configuration,

[0020] [Fig.2] [Fig.2] is a longitudinal sectional view of the fastener and a portion of the installation device in an intermediate configuration,

[0021] [Fig.3] [Fig.3] is a longitudinal sectional view of the attachment of [Fig.l] and of a part of the installation device, in an installed configuration,

[0022] [Fig.4] [Fig.4] is a diagram of a laying device suitable for use in a installation method according to one embodiment of the invention,

[0023] [Fig.5] [Fig.5] is a block diagram illustrating steps of an installation process according to one embodiment of the invention,

[0024] [Fig.6] [Fig.6] is a graphical representation of data for the implementation of the process of [Fig.5].

[0025] To facilitate reading of the drawings, only the elements necessary for understanding the invention have been shown. The same elements bear the same references from one drawing to another.

[0026] [Fig.l] shows a threaded fastener 10 used to assemble a structure 12 shown schematically as a block, which may comprise several separate elements to be assembled. The structure comprises in this example a front face 14, a rear face 16 and a through bore 18. The fastener comprises a screw 20 provided with a head 22 of length Lt, a body 24 of which at least one portion 26 is threaded. The fastener also comprises a nut 28, of length Le, partially screwed onto the threaded portion 26. In a known manner, either the screw head 22 or the nut 28 comprise drive surfaces for driving the screw or the nut respectively in rotation.

[0027] The screw and nut may optionally include a unidirectional or bidirectional identification marking 60 applied to a portion of the screw, such as the end surface of the screw head 22, or the screw end 27, or to a surface of the nut. In the figures, the marking 60 is shown as an example on the head 22 of the screw, without this example being limiting.

[0028] In the position of the fastener 10 visible in [Fig.l], the fastener 10 is considered in an initial state: no tension is installed in the screw, the nut being at a distance from the rear face 16 of the structure 12. The length of the screw in the initial state, measured between the end of the head 22 to the end 27 is indicated by the reference Lo. The structure thickness is indicated by the reference Lh

[0029] [Fig. 3] shows the threaded fastener 10 in an installed configuration, tightening the structure 12. The underside of the screw head contacts the front face 14 of the structure 12, while the nut 28 contacts the rear face 16 of the structure 12. In a tightening position, an axial tension F is installed in the screw, while a compressive force is installed in the structure 12.

[0030] [Fig.4] shows a laying device 30, suitable for installing the fixing 10 in the structure 12.

[0031] The installation device 30 comprises a rotating sleeve 32, capable of contacting drive surfaces provided on the head 22 of the screw or on the nut 28 and an electric motor 34 provided with a shaft 36, capable of driving the sleeve 32 in rotation.

[0032] The laying device 30 may be provided with an ultrasound generating device 38 such as a transducer comprising a piezoelectric component configured to convert electrical signals into ultrasonic waves and vice versa, or an EMAT (“ElectroMagnetic Acoustic Transducer”).

[0033] In one embodiment, shown in Figures 1 to 3, the delivery device 30 comprises a metallic annular element 39 such as a ring, inside which the transducer 38 is disposed. The ring 39 and the transducer 38 are connected so as to form a unit, and are capable of being translated together inside the socket 32, without however rotating with the socket 32.

[0034] In this example, the ring 39 has an inner diameter and an outer diameter. The rear surface of the nut forms an annular surface, comprising an inner diameter, corresponding to the diameter of the thread. The inner diameter of the ring is designed to be less than the diameter of the thread of the nut 28, so that the ring and the ultrasonic generating device 38 cannot translate inside the thread of the nut. A holding device, such as a compression spring (not shown) holds the ring 39 and the nut 29 in abutment on the rear annular surface 29 of the nut.

[0035] The outer diameter of the ring is less than the inner diameter of the sleeve 32. The outer diameter of the transducer 38 is chosen so as to correspond to an outer diameter of the threaded portion 26 of the screw. A contact detection device (not shown) makes it possible to detect contact between the ring 39 and the rear surface 29 of the nut. This device is for example an open circuit, a closed circuit, an electromagnetic device or switch.

[0036] The transducer 38 may however not be arranged in the installation device 30. For example, the ultrasound generating device may be affixed to the end 27 of the screw.

[0037] The motor 34 is preferably provided with an ammeter 40 to measure the intensity of the current used by the motor to screw the nut. Alternatively, the motor is provided with a torque sensor 40 capable of measuring the torque of the motor shaft. A screw temperature sensor 42 can be used in addition to the tool, as will be described later.

[0038] The signals from the transducer 38, the sensor 40 and possibly the temperature sensor 42 are sent to a controller 50, capable of controlling the activation or stopping of the motor, in particular when the target value of a first parameter representative of the target axial tension in the screw is reached.

[0039] The controller 50 is preferably integrated into the installation device 30, but it can be remoted into a laptop or an automaton connected to the installation device, in particular to benefit from greater computing power, the display of signals and the measured or calculated values ​​in real time.

[0040] Alternatively, the laying device 30 is provided with a display capable of displaying one or more values ​​selected by an operator from a pre-selection of possible values, such as the measured voltage, the measured motor current or torque, the calculated voltage, or the temperature.

[0041] The controller 50 comprises a processor 44, which receives, amplifies, filters the signals emitted by the sensors, a computer 46 and a memory 48.

[0042] The processor 44 may comprise a converter of analog signals from the sensors into digital signals.

[0043] The computer 46 is configured to calculate a variation in energy of the electric motor and a thickness Li of the structure 12 to be tightened, in particular based on certain values ​​from the processor 44 and geometric characteristics of the screw and the nut. The computer can thus comprise one or more algorithms for processing the digital signals from the processor 44, and extracting specific values ​​therefrom. The computer can also be configured to correct target values ​​in particular as a function of the temperature in the screw, if a temperature sensor 42 is used during the installation of the fastener 10.

[0044] The calculator 46 is also configured to compare measured or calculated values ​​to predefined values ​​or ranges of values ​​of torque, intensity, temperature, voltages and geometric characteristics of the screw 20 or the nut 28, stored in the memory 48. The result of the comparison is used by the calculator to determine a target value of the first necessary parameter, and to control consequently engine 34.

[0045] The calculator 46 is also configured to calculate the distance traveled by the nut 28 on the threaded portion 26 of the screw from an initial position to a tightening start position, in particular as a function of geometric characteristics of the screw and the nut.

[0046] A method 100 for installing the fastener 10 in the structure 12 by means of the installation device 30 will now be described. The method is shown diagrammatically in [Fig.5].

[0047] Prior to implementing the method 100, the fastener is placed in the initial configuration shown in [Fig.l]. Thus, the screw 20 is inserted into the bore 18 of the structure 12 until one end 27 of the threaded portion 26 emerges from the rear face 16. The nut 28 is placed on said end, engaging at least one thread, for example by screwing it in by hand until these possible braking points are engaged. In this example, the installation device 30 comprises a transducer 38 housed in the ring 39 inside the sleeve 32. The sleeve is adapted to drive the nut 28 in rotation. The detail of this part of the device 30 is shown schematically in Figures 1 to 3.

[0048] The screw 20 is kept fixed in rotation by inserting a key of non-circular section into an imprint of the same section arranged in the end 27 of the fixing (not shown).

[0049] The installation method begins with the screwing step 101, during which the installation device 30 is brought close to the fastener 10 so that the sleeve 32 engages the driving surfaces of the nut 28. The ring 39 and the transducer 38 are in contact with the rear surface 29 of the nut 28, but not with the end 27 of the screw. The contact between the ring 39 and the nut 28 is detected by the contact detection device. The motor of the device 30 is then activated in order to rotate the nut. The nut then moves over the threaded portion 26 of the screw. The assembly formed by the ring 39 and the transducer 38 are pressed in the longitudinal direction towards the head of the screw by the holding device, throughout the installation method.

[0050] When the annular surface 29 of the nut is coplanar with the end 27 of the screw, as shown in [Fig.2], the ring 39 and the transducer 38 come into contact with the end 27 of the screw.

[0051] The motor continues to drive the sleeve 32 in rotation, and therefore continues to drive the nut 28 onto the threaded portion 26 of the screw. The ring 39 and the transducer 38 remain in abutment against the end 27 of the screw, held by the holding device, since the internal diameter of the ring is less than the diameter of the thread of the nut 28. The loss of contact between the ring 39 and the annular surface 29 of the nut is detected by the contact detection device. The loss of contact indicates the initial position of nut 28 on screw 20.

[0052] The distance Lp traveled by the nut 28 from this initial position is then calculated in real time by the computer 46 and recorded in the memory 48. For example, the distance Lp is equal to the thread pitch multiplied by the number of turns made by the nut from its initial position. The number of turns is for example measured by means of a rotation angle sensor of the shaft 36 of the motor.

[0053] In a second step 102, following the first step, the transducer 38 is activated so as to emit ultrasonic waves into the end 27 of the screw, and receive the ultrasonic waves reflected by the end surface of the head 20.

[0054] The processor 44 is configured to measure a first parameter, representative of the axial tension in the screw.

[0055] In a first embodiment, the transducer 38 is configured to emit longitudinal or transverse ultrasonic waves. The first parameter is the acoustic time of flight of a longitudinal or transverse ultrasonic wave measured at defined time intervals. The computer 46 is thus configured to calculate the variation over time of the time of flight, said variation providing an indirect measurement of the tension in the screw.

[0056] In a second embodiment, the transducer 38 is configured to emit longitudinal ultrasonic waves. The first parameter is the time shift of the resonant frequency induced in the screw by the ultrasonic waves. The processor 44 is thus configured to measure a resonant frequency of the screw, and the computer 46 is configured to calculate the shift of the resonant frequency of the screw, said shift providing an indirect measurement of the tension in the screw.

[0057] In a third embodiment, the transducer 38 is configured to emit longitudinal and / or transverse ultrasonic waves. The first parameter is the ratio of the time of flight of the ultrasonic wave over a thickness to be tightened, said ratio providing an indirect measurement of the tension in the screw. The processor 44 is thus configured to measure the time of flight of a longitudinal or transverse wave, and the computer 46 is configured to calculate said ratio.

[0058] In a third step 103, simultaneous with the second step 102, the signals from the motor intensity or torque sensor 40 are sent to the processor 44, which processes the signals and sends them to the computer 46. The computer 46 is further configured to calculate a second parameter representative of a variation in the torque or intensity of the electric motor as a function of time.

[0059] In a fourth step 104, following the first three steps, the computer detects a start of tightening corresponding to a generation of tension in the screw. This start of tightening corresponds to the moment when the nut 28 contacts the rear surface 16 of the structure 12. The detection of the start of tightening can be carried out by various methods and means.

[0060] In a first method, the computer 46 can be configured to detect a variation in torque over time AC / At, or a variation in motor intensity over time AI / At, above a predefined threshold. The computer identifies the instant ts of start of tightening as being the moment when the variation in torque or intensity reaches the predefined threshold.

[0061] In a second method, the computer 46 can be configured to detect a change in slope of the curve of the torque measured in time AC / At, or of the intensity measured in time AI / At, for example by identifying the inflection point of one of said curves. The computer is capable of linearizing the slope beyond the inflection point, the intersection of the linear line and the time abscissa indicating the instant ts of start of tightening.

[0062] Whatever the method used, the calculator 46 determines the distance Lp traveled by the nut 28 from an instant t0 corresponding to the initial position of the nut on the screw ([Fig.2]), up to an instant ts of start of tightening, corresponding to the moment when the nut 28 comes into contact with the rear face 16 of the structure 12 ([Fig.3]).

[0063] In a fifth step 105, the calculator 46 determines the thickness of the structure to be tightened as a function of the distance traveled by the nut defined in the previous step, and as a function of geometric characteristics of the fastener. These characteristics include in particular the length Lo of the screw 20, the length Lt of the screw head, the length of the threaded portion 26 of the screw, the thread pitch of the screw, the length Le of the nut 28, the length of the tapping and the distance between an end surface of the nut and a braking point if the nut is braked, the standardized reference of the screw and the nut, the materials of the screw and the nut, the standardized references of the threads of the screw and the nut,...

[0064] These geometric characteristics may already be contained in the memory 48 of the controller.

[0065] In the optional case where these characteristics are integrated into the marking(s) 60 on the screw and / or nut, a suitable reader such as a camera, capable of reading said markings and sending the characteristics to the computer 46, must be used beforehand, so that the information is known to the computer to perform the calculation. The corresponding optional step 108, comprising reading the marking and identifying at least one geometric characteristic of the screw and nut, is shown in dotted lines in [Fig.5].

[0066] In one embodiment, the calculation of the thickness of the structure to be tightened Li is carried out by subtracting from the length of the screw Lo the distance Lp traveled by the nut 28 from its initial position to the tightening start position, the thickness Lt of the head 22 of the screw and the thickness Le of the nut.

[0067] In a sixth step 106, the calculator 46 determines a target value of the first parameter to be reached as a function of the thickness to be tightened Li calculated in the previous step.

[0068] In a first embodiment, the calculator 46 is configured to compare the thickness to be tightened Li to a set of thickness values ​​L; stored in the memory 48. Each thickness of the set of values ​​is associated with a target value of the first parameter.

[0069] [Fig.6] represents in the form of an abacus the target TOF flight time and tension values ​​of four identical fasteners 10 inserted into four structures of different thicknesses, denoted respectively Lm, Lx, Ly and Lz. For example, if the computer determines that the clamped thickness is Ly, then it deduces that the target flight time is “target TOF (Ly)”, corresponding to a target tension “target T (Ly)”.

[0070] In a second embodiment, the calculator 46 is configured to use the thickness of the structure to be tightened by means of a mathematical equation stored in the memory 48, the thickness of which is a variable and one or more geometric characteristics of the screw are represented by one or more constants, in order to calculate the target value of the first parameter.

[0071] The mathematical equation may incorporate other variables, such as the screw temperature from the temperature sensor 42, in order to adjust the target value of the first parameter.

[0072] The discrete values ​​of the first parameter or the constants of the mathematical equation are preferably determined by one or more test plans carried out with one or more fasteners 10 inserted into at least two structures of different thicknesses, prior to the installation of the fastener 10, the time of flight and the axial tension generated in the screw are measured.

[0073] In a seventh step 107, the computer 50 stops the motor 34 when the first measured parameter is equal to the target value determined in step 106. The measured value of the first parameter can be recorded in the memory 48 in conjunction with a reference of the screw 20, for a subsequent measurement of the installed tension and a possible subsequent tightening of the nut 28 if the tension in the screw has decreased over time.

[0074] The method is not limited to the example described above. For example, in an alternative embodiment, the head 22 of the screw has a non-circular internal or external shape, such as a hexagon, allowing a tool (not shown) to hold the screw fixed in rotation. The screw head can of course be countersunk.

[0075] Any other means than the ring device 39, transducer 38 and device for detecting the contact between the ring 39 and the nut 28 can be used to identify the initial position of the nut on the screw.

Claims

Claims

1. A method of installing a threaded fastener (10) in a bore (18) of a structure (12), the threaded fastener comprising a screw (20) and a nut (28), the method comprising the steps of: a) inserting the screw into the bore, and screwing the nut onto the screw, b) simultaneously emitting ultrasonic waves into the fastener, receiving reflected ultrasonic waves and measuring a first parameter representative of a variation of said waves, said first parameter providing an indirect measurement of an axial tension in the screw, c) stopping the screwing when a target value of the first parameter is reached, characterized in that the method further comprises the following steps: d) determining a thickness (LJ) of the structure to be tightened, said determination comprising a calculation of said thickness (LJ) using a calculator (46);and e) determining the target value of the first parameter as a function of the thickness (LJ) of the structure to be tightened and of the geometric characteristics of the fastener (10).;

2. Method according to claim 1, in which the first parameter is chosen from: a time of flight of a longitudinal ultrasonic wave; a resonance frequency shift of the screw; and a ratio of the time of flight of a longitudinal or transverse ultrasonic wave to the thickness (Li) of structure to be tightened.

3. Method according to claim 1, in which the geometric characteristics of the fastener are chosen from: a length (Lo) of the screw (20), a length of a threaded portion of the screw, a thread pitch of the screw, a nut length (Le), a nut counterbore length, a nut tapping length and a distance between an end surface of the nut and at least one braking point.

4. The method of claim 1, wherein determining the target value of the first parameter comprises comparing the structure thickness to a set of thickness values, said set of thickness values ​​being associated with a set of target values ​​of the first parameter for a given screw, corresponding to a set of target tensions for a given screw.

5. The method of claim 1 or 2, wherein determining the thickness (Li) of the structure comprises a step of detecting the start of tightening of the structure (12), corresponding to the appearance of axial tension in the screw (20).

6. Method according to claim 5, in which the screwing step is carried out with a fitting device (30) provided with an electric motor (34), the method further comprising: measuring a second parameter representative of a variation in energy of the electric motor as a function of time; and detecting the start of tightening, said start of tightening corresponding to a predetermined variation of the second parameter.

7. A method according to claim 6, wherein the second parameter is selected from an intensity of the electric motor and a torque of a motor shaft.

8. Method according to claim 1 or 2, wherein the determination of the thickness (Li) of structure comprises a step of detecting the initial position of the nut (28) on the screw (20).

9. The method of claim 8, wherein the step of detecting the initial position of the nut comprises the following steps: - Contacting an annular metal element (39) with an annular surface (29) of the nut (28), an inner diameter of the annular metal element being smaller than an inner diameter of the annular surface of the nut, - Detecting contact between the annular metal element and the annular surface of the nut, - Rotating the nut on the threaded portion (26) of the screw, and applying pressure to the annular metal element so as to maintain contact with the annular surface of the nut, - Detecting loss of contact between the annular metal element and the annular surface of the nut as soon as one end (27) of the threaded portion (26) passes through the inner diameter of the annular surface (29) of the nut,detection of loss of contact corresponding to the initial position of the nut.,

10. The method of claim 1, further comprising monitoring the temperature in the screw (20), and adjusting the target value of the first parameter as a function of the temperature.

11. The method of claim 1, wherein at least one of the screw (20) and the nut (28) comprises a marking containing at least one identifier or geometric characteristic of said screw or said nut, the installation method further comprising a step (108) of reading the marking and identifying at least the identifier or geometric characteristic of the screw and / or the nut, said reading step being carried out before step e).

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