EP0674566B1 - Method for monitoring and controlling stress in a threaded member - Google Patents

Abstract

The tensile or compressive stress (Ft) in a threaded member (18) is determined by subtracting an unscrewing torque from a screwing torque, and dividing the resulting torque difference by a coefficient proportional to the screw pitch (19) of said member. The torque values are recorded either statically at the screwing and unscrewing rest limit, or dynamically in a series of corresponding screwing and unscrewing positions. The method is particularly useful for measuring, monitoring and controlling stress in threaded fastening members used in screwed joints.

Description

The invention relates to a method for controlling and controlling the tension of a threaded member, such as a screw, bolt, nut, or other threaded fastening elements used in particular but not exclusively in screwed assemblies.

The automotive, aeronautics, space, nuclear and all mechanical manufacturing industries make general use of screw connections, in extremely varied and complex uses.

The purpose of the calculation notes for screwed connections is to determine the preload to be applied to the connection according to the payload (or service load) to which the connection will be subjected in its use and a dependent tightening factor the precision of the means for applying this preload.

These calculation notes produced by the study engineers of these industries being very precise according to the defined needs and the operating conditions of the screwed assemblies being more and more severe, it becomes essential to increase the reliability of these assemblies by respecting the calculation notes in their execution in order to increase the performance and safety of the equipment thus produced.

The tension to be applied in a threaded fastener is generally achieved by transforming a rotational force into a tensile force generated by the helical size of the element's thread. This transformation has an imperfect yield mainly due to the friction losses of the surfaces in contact; therefore it will be necessary to apply a rotational force (torque) greater than the rotational force useful for this transformation.

The efficiency of this transformation is extremely dispersive due to the variation of the friction coefficients, the variation of the distance to which the resultant of the friction forces applies with respect to the axis of rotation and the geometric variation of the parts. especially contacting surfaces. Practical observation gives a variation of 50% of the tension induced when applying a constant torque to the same batch of fasteners. It is verified that the improvement in the precision of the applied torque does not lead to a significant improvement in the precision on the observed voltage.

A first improvement in the variation of the tension generated by the torque was to associate the angle of rotation from a certain torque. Indeed the elongation of the fastening element which causes the tension is proportional to the angle of rotation. This proportion must be determined beforehand because it is the result of the assembly and not of the fixing element. Voltage dispersion is improved under special conditions due to preparatory testing and geometric specifications of the engaging parts which can be costly. In addition, the risk of penetration into a plastic zone is not excluded due to the variability of the starting point of the angle measurement, in principle called the pre-tightening torque. It should be noted that the pre-tightening torque can be an accidental torque, for example kneading the threads; in this case, the tension will be far from being achieved.

From the combination of these two torque and angle measurements, a tightening principle was developed: permanent observation during the application of the torque, of the variation of the torque gradient as a function of the angular advance allows the determination of a remarkable point of the torque / angle function, called the elastic limit point. This method brings the screw into a state of stress (or preload) which exploits all of the possible preload of the assembly to the detriment of the service load. A preliminary determination of the tension is carried out in order to integrate the variations in cross-section and in tensile strength of the materials constituting the fastening elements. This method is very sensitive to the reaction efforts of the means which apply the torque; for example, bending of the machine frame, sliding of the clamping of the assembled element. The elastic limit point can be that of the first element which flares up in the chain of action and mechanical reaction. To be applied with maximum security, this method requires additional precautions generating costs. This method offers an accuracy of ± 10% on the tension of a single point exploiting the full capacity of the fastening element for preload. The major drawback of this method is the penetration, even minimal, in the plastic area of the fastening element. Attempts to control and / or control the slope (directing coefficient of the tangent to the curve) at any point in the elastic zone have proved to be imprecise and dispersive. In this case, the dispersion of the torque due to friction is cumulated and the dispersion of the angle due to the flexibility of the assembly.

To eliminate voltage variations due to various variations cited above, a new approach to tensioning is carried out using the ultrasonic method.

This method is based on the variation of a travel time of the sound propagating only inside the fixing element and it is freed from all the other parameters not linked to the element as well as variations of the thickness of the engaging parts. The variation in travel time is in theory the direct representation of the elongation. The tension / elongation relationship is linked to the resistance of the material, to its section and to its initial length. This purely comparative method requires a modeling (specimen element) of the fixing element which will be tested for the determination of a variation of a specific travel time at a given tension. The determined elongation is only a small part of the actual length of the fastener. Indeed, when it comes to the propagation of a sound, only the stretched part of the fixing element participates in the elongation. This stretched part depends on the thickness of the parts in engagement. The machining tolerances have a direct impact on the elongation observed. The fixing element intervenes in its geometrical dimensions in particular for the total variation of the travel time which depends on the length of the element relative to its stretched part. This variation has repercussions on the journey time and on its variation in the ratio of stretched length / total length. Furthermore, the parallelism of the reflection planes of the ultrasonic waves (essentially for a screw, the head and the bottom of the rod) intervenes directly in the effective position of the member for measuring the travel time. The elongation measurement must be carried out during the rise in tension of the element because the repositioning introduces errors which can double the degree of inaccuracy of the ultrasonic method. All of these tolerances being perfectly controlled by a significant additional cost, the precision of the effective measurement of the tension is similar to that of the elastic limit with the advantage of being able to be located at any point of the admissible load of the fastening element, with however the disadvantage of being only a probability of tension taking into account the reference to a specimen (theoretical model) resulting from the average treatment of a batch of fastening elements.

The conditions of application of this method require important precautions such as coupling of the measurement device, absence of marking on the head of the fixing element, effective temperature of the measurement, cleanliness of the measurement surfaces, etc., precautions which result in high operating costs and limited production rates. The cost of the ultrasonic measurement means being itself very high, this method only applies in special cases where the costs and the rates are not essential. This method is often used as a means of controlling or determining conventional tightening parameters (torque and angle).

• Méthode du couple : variation extrême de la tension, tension inconnue ;
• Méthode impliquant l'angle : variation limitée de la tension, tension estimée avec préalable ;
• Méthode utilisant la limite élastique : Point unique de tension au maximum de la capacité de la vis,
  tension estimée avec préalable,
  environnement à maîtriser ;
• Méthode ultra-sonore : Point unique de tension en un point quelconque sur la capacité de l'élément de fixation,
  tension estimée avec préalable,
  géométrie et tolérances d'usinage à maîtriser,
  exploitation limitée par le coût.

In summary, all of the methods mentioned above do not provide a direct measurement of the tension, they are either comparative or theoretical and must require prior tests of adjustment of the parameters and knowledge of the behavior of the assembly. . There is always at least one parameter which varies from one threaded fastener to another and which therefore gives uncertainty about the tension obtained:

• Torque method: extreme variation in voltage, unknown voltage;
• Method involving the angle: limited variation of the voltage, voltage estimated beforehand;
• Method using the elastic limit: Single point of tension at the maximum of the capacity of the screw,
  estimated voltage beforehand,
  environment to master;
• Ultrasonic method: Single point of tension at any point on the capacity of the fixing element,
  estimated voltage beforehand,
  machining geometry and tolerances to master,
  operation limited by cost.

The more precisely an estimate of the voltage is sought, the more the costs of the means, their implementation and operation increase. The fact of arriving at an estimate rather than a direct measurement generates additional control costs. The demands of manufacturers regarding new methods clearly show their dissatisfaction with existing ones.

Knowing that the methods set out above all have one or more deficiencies, we have already explored a new path, following the observation over several hundred screw connections of all kinds of fasteners, of a difference between the value of the applied torque to the screwing and the value of the torque applied to the unscrewing, which reduced by the simplest means of applying the tension by the torque in order to characterize the energy relations between the couple and the energy in the screw connection and lead to a new process.

où : δL est la différence entre la longueur finale et la longueur initiale de l'élément de fixation.Indeed, the screw connection is made of elastic materials. Therefore, it behaves like a spring. By extending this spring in its elastic range using a force Ft, an energy E is accumulated in this screwed assembly, which is expressed as follows: $$\text{E = (Ft.δL) / 2}$$

where: δL is the difference between the final length and the initial length of the fastener.

This accumulated energy (1) is returned to the unscrewing due to the reversibility of the fastening element due to its pitch with helical size and to the potential energy residing in the screwed assembly.

• P étant le pas de filetage

soit : $$\text{E = Cu.δL.π/p}$$

If this elongation is carried out by transformation of a torque Cu into tensile force, by means of a helical ramp of angle α developed on a circumference of diameter 2.r, this expression becomes: $$\text{E = (Cu.cotgα / r). (ΔL / 2) with cotgα = 2πr / P}$$

• P being the thread pitch

is : $$\text{E = Cu.δL.π / p}$$

This accumulated energy is not apparent and represents only a fraction of the energy applied due to the energy consumed by friction which in particular prevents spontaneous unscrewing of the fixing element. These frictions exist in an equivalent manner to screwing and unscrewing and are caused by the parts of the surfaces in contact with the various elements making up the assembly. Therefore there is a difference in energy applied to the screwing and unscrewing due to the energy which is accumulated at the screwing and returned to the unscrewing while the energy consumed by friction is constant.

avec :

• Eapp : énergie appliquée au vissage de l'élément de fixation
• Eapp' : énergie appliquée au dévissage de l'élément de fixation
• Etfr : énergie transformée en tension de l'élément de fixation
• Ett : énergie consommée au frottement tête ou écrou de l'élément de fixation
• Eflt : énergie consommée au frottement filet de l'élément de fixation
• Etg : énergie consommée au frottement tige de l'élément de fixation.

The general distribution of the different energies, for an applied energy is described as follows: $$\begin{array}{c}\begin{array}{c}\text{for screwing: Eapp = Et fr + Ett + Eflt + Etg}\\ \text{in unscrewing: -Eapp '= Et fr-Ett-Eflt-Etg}\\ \text{hence: Etfr = (Eapp-Eapp ') / 2}\end{array}\end{array}$$

with:

• Eapp: energy applied to the screwing of the fastening element
• Eapp ': energy applied to unscrewing the fastening element
• Etfr: energy transformed into tension of the fixing element
• Ett: energy consumed by the head or nut friction of the element fixation
• Effect: energy consumed by thread friction of the fastening element
• Etg: energy consumed by rod friction of the fastening element.

   soit à un instant t : $$\text{Ptfr = (Papp-Papp')/2}$$

   avec :

• Papp : puissance appliquée au vissage
• Papp' : puissance appliquée au dévissage
• Ptfr : puissance transformée en tension

et compte tenu de la relation générale Pu = C. (Puissance = couple X vitesse de rotation)
(4) se simplifie en : $$\text{Cu = (Capp - Capp')/2}$$

Deriving equation (3), we get: $$\text{d (Etfr) / dt = [d (Eapp) / dt - d (Eapp ') / dt] / 2}$$

either at an instant t: $$\text{Ptfr = (Papp-Papp ') / 2}$$

with:

• Papp: power applied to screwing
• Papp ': power applied to unscrewing
• Ptfr: power transformed into voltage

and taking into account the general relation Pu = C. (Power = torque X speed of rotation)
(4) is simplified by: $$\text{Cu = (Capp - Capp ') / 2}$$

en faisant intervenir (1), on obtient : $$\text{Cu = Ft.P/(2.π)}$$

en faisant intervenir (5) et (6), on obtient : $$\text{Capp - Capp' = Ft.P/π}$$

   avec :

• Capp : couple appliqué au vissage
• Capp' : couple appliqué au dévissage

et réciproquement : $$\text{Ft = (Capp-Capp')/(P/π)}$$

The useful torque or torque transformed into voltage Cu can be expressed according to (2) as a function of the energy transformed as follows: $$\text{Cu = Etfr.P / (π.δL)}$$

by using (1), we obtain: $$\text{Cu = Ft.P / (2.π)}$$

by involving (5) and (6), we obtain: $$\text{Capp - Capp '= Ft.P / π}$$

with:

• Capp: torque applied to screwing
• Capp ': torque applied to unscrewing

and reciprocally : $$\text{Ft = (Capp-Capp ') / (P / π)}$$

In other words, the tension force (in tension or in compression) can be controlled as a function proportional to the difference between the torque applied to the screwing, in particular the maximum torque at the stopping point, and the torque applied to the unscrewing, more particularly the maximum unscrewing torque, difference divided by (P / π), quantity proportional to the thread pitch. The quantity (P / π) constitutes a constant which allows the immediate transformation of the difference of the couples of the action of screwing, then of unscrewing a threaded fixing element, expressed in newton-meter (Nm) in a tension expressed in Newton (N), whose precision directly follows the precision of the torques.

From formula (7), follows the general definition of a process for controlling and controlling the tension or compression of a threaded member, in which the tension or compression force (Ft) of the member thread is determined by the action of subtracting a torque applied to the unscrewing (Capp ') from a torque applied to the screwing (Capp), and dividing this difference in torque by a coefficient proportional to the pitch (P) of the thread of said member. Such a process is disclosed, in its general and theoretical aspect, by document EP-A-0096620.

However, this document does not reveal a rational and reliable operating mode for the industrial implementation of this process; the teaching of document EP-A-0096620 only allows an approximation of the clamping force exerted by the threaded element, and thus the drawbacks of the other prior techniques are not eliminated.

More precisely, an analysis of document EP-A-0096620 makes it possible to discern, for the process which it describes, on the one hand theoretical insufficiencies and on the other hand technical insufficiencies.

• 1) Spéculation abusive sur la constance du frottement (qui varie en réalité à chaque point d'avancement en fonction de la surface en contact, de l'état de surface et de la pression de contact) pour la proportionnalité (K). Ceci n'est jamais le cas, donc il est impossible de prédire le couple de vissage final en un point différent de celui de l'observation, et encore moins la tension existante à ce couple.
• 2) Les couples de vissage/dévissage doivent être mesurés dans des conditions particulières qui ne sont pas précisées dans le document cité, et en l'absence desquelles les mesures sont aberrantes, pouvant aller jusqu'à une tension négative ! (ex : un couple de dévissage supérieur à un couple de vissage dû à un grippage ou un collage). Il y a obligation de mesurer dès le début de la mise en mouvement de l'organe fileté et/ou de ne prélever les couples qu'à des instants précis, comme le propose la présente invention.
  Les points précédents sont confirmés par le contenu du document EP-A-0096620 ou il est admis que le coefficient K est différent sur plusieurs tentatives, d'où l'obligation de l'évaluer statistiquement, mais qui ne résoud pas le problème de la proportionnalité du coefficient K sur l'étendue du vissage. Au mieux, on obtient une approximation de K en un point particulier, repéré par une position angulaire.
• 3) Absence de notion de positions concordantes aux prélèvements des couples de vissage/dévissage, ce qui pour un effort progressif comme la montée en couple et en tension introduit un écart de couple supplémentaire qui nuit gravement au procédé (détorsion de l'organe fileté).
• 4) A propos des modalités de mise en oeuvre selon les revendications 5 et 6 du document EP-A-0096620, introduisant la notion de "gradient", on notera que ce mode de mise en oeuvre souffre des mêmes défauts que ceux précédemment indiqués, et de plus le fait de dériver amplifie exagérément les anomalies de mesure et rend impossibles tous calculs. Cette formulation est impropre, et à l'inverse il aurait fallu intégrer.

The mention of a proportionality coefficient relating the tightening force to the difference of the screwing / unscrewing couple seen at a particular point, basis of document EP-A-0096620, contains several aberrations:

• 1) Abusive speculation on the constancy of friction (which actually varies at each point of advancement depending on the surface in contact, the surface condition and the contact pressure) for proportionality (K). This is never the case, so it is impossible to predict the final tightening torque at a point different from that of the observation, and even less the existing tension at this torque.
• 2) The screwing / unscrewing torques must be measured under special conditions which are not specified in the cited document, and in the absence of which the measurements are aberrant, possibly reaching a negative voltage! (ex: a unscrewing torque greater than a screwing torque due to seizing or sticking). There is an obligation to measure from the start of the setting in motion of the threaded member and / or to take the torques only at specific times, as proposed by the present invention.
  The above points are confirmed by the content of document EP-A-0096620 or it is accepted that the coefficient K is different on several attempts, hence the obligation to evaluate it statistically, but which does not solve the problem of proportionality of the coefficient K over the extent of the screwing. At best, we get an approximation of K at a particular point, marked by an angular position.
• 3) Lack of notion of positions consistent with the removal of the screwing / unscrewing couples, which for a progressive effort such as the increase in torque and in tension introduces an additional torque difference which seriously harms the process (distortion of the threaded member) .
• 4) With regard to the methods of implementation according to claims 5 and 6 of document EP-A-0096620, introducing the concept of "gradient", it will be noted that this mode of implementation suffers from the same defects as those previously indicated, and moreover, the fact of deriving exaggeratedly amplifies the measurement anomalies and makes all calculations impossible. This formulation is inappropriate, and conversely it should have been integrated.

In addition to the theoretical shortcomings developed above, document EP-A-0096620 does not contain any practical description of means which would make it possible to apply the theory developed in this document, and its only figure is limited to the representation of a conventional screwed assembly .

The technical insufficiency appears in particular with regard to the concordance of positions for the sampling of torque values, which for any means are to be taken into account, in particular the control of the twists of said means, which without this brings back to the theoretical insufficiency 3) (twisting of the screw, with in addition the twisting of the means).

Document DE-A-4024577 reveals a process which is based on the same theoretical basis as the aforementioned document EP-A-0096620, and which essentially has the same shortcomings, explained above in points 1), 2) and 3), even if it remedies the disadvantage of point 4) by replacing the notion of derivation (gradient) with a proposal for integration. In particular, this document DE-A-4024577 is based on incorrect proportionality formulas, and it involves an empirical correction factor, resulting in an imprecision of the results.

Faced with this state of the art, the present invention aims to avoid the shortcomings of known methods, by providing a new method based on an improved principle and specific operating procedures, allowing good reproducibility and improved precision, by excluding any arbitrary correction. , empirical or statistical.

In this method for controlling and controlling the tension or compression of a threaded member, where the tension or compression force of the threaded member is determined by the action of subtracting a torque applied to the unscrewing of a torque applied to the screwing, and where this torque difference is divided by a coefficient proportional to the pitch of the thread of said member, the invention provides that the sampling of these torque values is carried out either statically at the rest limit for screwing and unscrewing, or dynamically, in a succession of concordant screwing positions and unscrewing, in the context of a screwing-unscrewing action carried out only once, or of a screwing-unscrewing action repeated iteratively, or of a screwing-unscrewing-screwing action of said threaded member.

Simple means make it possible to implement the process, the general definition of which has just been indicated, for knowing, controlling or controlling screwed assemblies in an economical and more efficient than all those existing, and thus the invention allows the use of threaded fasteners of lower cost or lower quality, while having certainty about the tightening of these fasteners.

A first embodiment of the method according to the invention consists in applying this method to a voltage measurement, using manual, mechanical, pneumatic, hydraulic or electric motor means capable, by means of a device for coupling on the threaded fixing element of an assembly, to ensure a controlled effort of rotation of this fixing element.

• soit la force de traction subie par l'élément de fixation
• soit la force de compression exercée par l'élément de fixation
• soit la force de tension résidante dans l'élément de fixation.

In the case of negligible mechanical inertia, the method is characterized by the action of screwing in order to take, at the rest limit of the sliding of the movable part of the fastening element on the fixed part of the assembly, the value of the rotational force, then by the action of unscrewing for the purpose of removing, at the rest limit during the sliding of the movable part of the fixing element on the fixed part of the assembly , the value of the rotational force on passing through its maximum, then of differentiating these two force values in order to divide them by a coefficient notably proportional to the pitch of the thread of the fixing element in order to obtain a value representing, within these precise limits:

• either the tensile force undergone by the fastening element
• either the compressive force exerted by the fastening element
• or the tension force residing in the fastening element.

avec :

• Ft force de tension en Newton
• Cv couple à la limite de vissage en Newton-mètre
• Cd couple maximum de dévissage en Newton-mètre
• π constante du cercle
• P pas du filetage en mètre

When the drive means is instrumented by a torque meter, the direct relationship is expressed as follows: $$\text{Ft = (Cv-Cd) / K where K = P / π}$$

with:

• Ft tension force in Newton
• CV torque at the screwing limit in Newton meters
• Cd maximum unscrewing torque in Newton meters
• π constant of the circle
• P thread pitch in meters

où S = 12

, si Cv est Cd sont exprimés en Newton.foot (N.ft)
et P en inch (in)

où S = 1,6584

, si Cv et Cd sont exprimés en poundal.foot(pdl.ft)
et P en inch (in)

The indirect relation is established for the expression of the tension force in another measurement system, by an additional coefficient restoring the coherence between the units of force and the units of length, for example:
For Ft in Newton $$\text{K = (P / π) .S}$$

where S = 12

, if Cv is Cd are expressed in Newton.foot (N.ft)
and P in inch (in)

where S = 1.6584

, if Cv and Cd are expressed in poundal.foot (pdl.ft)
and P in inch (in)

avec :

• Ft force de tension en Newton
• Ev' élément représentatif de l'énergie à la limite de vissage
• Ed' élément représentatif de l'énergie maximum de dévissage
• τ constante incluant π constante du cercle par le rendement moteur par le transformateur du type d'énergie
• P pas du filetage en mètre.

When the drive means is instrumented on its energy the direct relationship is expressed as follows: $$\text{Ft = (Ev'-Ed ') / K or K = P / τ}$$

with:

• Ft tension force in Newton
• Ev 'element representative of the energy at the screwing limit
• Ed 'element representative of the maximum unscrewing energy
• τ constant including π constant of the circle by the motor efficiency by the transformer of the energy type
• P thread pitch in meters.

The element representative of the energy can be any element giving the image of the torque of the motor means (intensity, voltage, pressure, etc.) by a constant or a function f (x), but also the moment of a mechanical means using the flexion or torsion of an organ for which the transformer is not necessarily a constant, but can also be a function (x).

• soit la force de traction subie par l'élément de fixation
• soit la force de compression exercée par l'élément de fixation
• soit la force de tension résidante dans l'élément de fixation au départ de l'action.

In the case of significant mechanical inertia, the method is characterized by the action of screwing in order to take off, at the start of the sliding of the movable part of the fixing element on the fixed part of the assembly, value of the rotational force, then take, at the rest limit of the sliding of the movable part of the fastening element on the fixed part of the assembly, the value of the rotational force, then by l action of unscrewing for the purpose of taking, at the rest limit during the sliding of the mobile part of the fastening element on the fixed part of the assembly, the value of the rotational force when passing by its maximum , then to differentiate these two force values to divide this result by a coefficient proportional to the pitch of the thread of the fixing element in order to obtain a value representing, for these particular values:

• either the tensile force undergone by the fastening element
• either the compressive force exerted by the fastening element
• or the tension force residing in the fastening element at the start of the action.

avec :

• Ft force de tension en Newton
• Cg couple au début de vissage en Newton.mètre
• Cv couple à la limite de vissage en Newton.mètre
• Cd couple maximum de dévissage en Newton.mètre
• π constante du cercle
• P pas du filetage en mètre

When the drive means is instrumented by a torque meter, the direct relationship is expressed as follows: $$\text{Ft = (Cv-Cd) .I) / K where K = P / π where I = Cg / Cv}$$

with:

• Ft tension force in Newton
• Cg torque at the start of tightening in Newton meters
• CV torque at the screwing limit in Newton meter
• Cd maximum unscrewing torque in Newton meters
• π constant of the circle
• P thread pitch in meters

The indirect relation is established for the expression of the tension force in another measurement system, by an additional coefficient restoring the coherence between the units of force and the units of length.

avec :

• Ft force de tension en Newton
• Eg' élément représentatif de l'énergie au début de vissage
• Ev' élément représentatif de l'énergie à la limite de vissage
• Ed' élément représentatif de l'énergie maximum de dévissage
• τ constante incluant π constante du cercle par le rendement moteur par le transformateur du type d'énergie
• P pas du filetage en mètre

When the drive means is instrumented on its energy, the direct relationship is expressed as follows: $$\text{Ft = (Ev'-Ed '). I) / K where K = P / τ where I = Eg' / Ev '}$$

with:

• Ft tension force in Newton
• Eg 'element representative of the energy at the start of screwing
• Ev 'element representative of the energy at the screwing limit
• Ed 'element representative of the maximum unscrewing energy
• τ constant including π constant of the circle by the motor efficiency by the transformer of the energy type
• P thread pitch in meters

The element representative of the energy can be any element giving the image of the torque of the motor means (intensity, voltage, pressure, etc.) by a constant or a function f (x), but also the moment of a mechanical means using the flexion or torsion of an organ for which the transformer is not necessarily a constant, but can also be a function f (x).

The indirect relation is established for the expression of the tension force in another measurement system, by an additional coefficient restoring the coherence between the units of force, and the units of distance.

• soit celui de la limite du repos s'il n'y a pas inertie
• soit celui de début de glissement s'il y a inertie

afin de restituer les conditions initiales de l'assemblage en ayant contrôlé la force de tension existante dans l'élément de fixation, ou la force de traction subie par l'élément de fixation, ou encore la force de compression exercée par l'élément de fixation.Non-destructive testing of the assembly is achievable by action previously defined tension measurement, followed by the action of re-tightening until the tightening effort taken first, namely:

• either that of the limit of rest if there is no inertia
• that of the beginning of sliding if there is inertia

in order to restore the initial conditions of the assembly having controlled the tension force existing in the fixing element, or the tensile force undergone by the fixing element, or even the compression force exerted by the element fixation.

According to an embodiment of the method according to the invention, a simple servo-control is carried out in successive approximations (iterative method). For this purpose, on a fastening element already tightened in any way, the previously defined action is exercised, repeated as many times as necessary until the ratio of the necessary tension force to the existing tension force is equal to 1. In order to limit the amplitude of the screwing / unscrewing action, the value of the screwing / unscrewing force can be moderated by a coefficient (of reduction) suitable for ensuring the convergence of the action towards the tension required in a number of strokes defined by the precision sought in the control of the action.

It is also possible to produce according to the invention a "dynamic control" allowing, by the use of a manual, mechanical, pneumatic, hydraulic or electric motor means capable, by means of a coupling member on the element. of threaded fastening of an assembly, of ensuring a controlled rotational force and a measurement of the effective position of rotation of the fastening element.

• soit la force de traction subie par l'élément de fixation
• soit la force de compression exercée par l'élément de fixation
• soit la force de tension résidante dans l'élément de fixation suivant les relations directes et indirectes de la définition générale du procédé selon l'invention, puis suivie par :
  • soit l'action de re-visser l'élément de fixation jusqu'à une position déterminée dont la valeur de la force observée est égale à la force nécessaire à l'assemblage,
  • soit l'action de re-visser l'élément de fixation jusqu'à l'effort de rotation au vissage correspondant à la position où l'on a observé une valeur de force égale à la force nécessaire à l'assemblage.

For this purpose, an action is not tightened on a loose fastening element in order to take, at regular position intervals determined by the desired precision, the values of the rotational force, then the action of unscrewing with the aim of taking, at regular intervals for these same positions, the values of the rotational force, then position by position, for effective positions, to differentiate the value of rotational force at screwing from the value of force when unscrewing, to divide them by a coefficient proportional to the pitch of the thread of the fixing element in order to obtain a list of values representing, for each of the positions:

• either the tensile force undergone by the fastening element
• either the compressive force exerted by the fastening element
• either the tension force residing in the fastening element according to the direct and indirect relationships of the general definition of process according to the invention, then followed by:
  • either the action of re-screwing the fixing element to a determined position, the value of the observed force being equal to the force necessary for assembly,
  • or the action of re-screwing the fastening element until the rotational force when screwing corresponds to the position where a force value equal to the force necessary for assembly has been observed.

This re-screwing action can be moderated both in the chosen position and in the value of the force in order to anticipate the inertia of the motor means used. This moderation could be a stored constant corresponding to a given rotation force, or deduced from the stopping phases of said means when the different values of rotation force are sampled on the fixing element.

This re-screwing action can be corrected both in the chosen position and in the value of the force in order to integrate the torsion of the motor means, in particular when the position sensor is not integral with the fixing element or of the coupling member. This correction may come from a memorized correction table deduced from the torsional slope, that is to say from the ratio of the rotational force to its effective position.

The concordance of the screwing / unscrewing positions can be achieved by correlation of the position at the maximum screwing effort with the position at the maximum unscrewing force, by disregarding in this case the notion of torsion slope.

The actions of screwing in order to take, at regular intervals of value of rotational force determined by the desired precision, the successive positions of the fastening element, then to unscrew in order to take, at regular intervals for these same values, the successive positions of the fixing element, then to interpolate the list between the rank and the content, either by permutation or by calculation, relate to the same mode of implementation of the method.

Establishing a relative function of the rotational force with respect to the position of the fixing element, instead of establishing a list of values, in particular when the control is carried out by a computer, a processor or a micro-controller, in order to solve by calculation the position or the rotational force necessary for assembly, is related to the same mode of implementation.

• soit la force de traction subie par l'élément de fixation
• soit la force de compression exercée par l'élément de fixation
• soit la force de tension résidante dans l'élément de fixation suivant les relations directes du principe de l'invention.

Non-destructive testing of the assembly can be carried out using manual, mechanical, pneumatic, hydraulic or electric motor means capable, via a coupling member on the threaded fastening element of an assembly, d '' ensure a controlled rotational force and a measurement of the effective position of rotation of the fastening element: On a fastening element already screwed in any way, we exert the action of unscrewing in order to remove, at a determined position, the maximum value of the rotational force then the action of re-screwing in the same position, for an effective position, to differentiate the value of rotational force upon re-screwing from the maximum force value at unscrewing, to divide them by a coefficient proportional to the pitch of the thread of the fixing element in order to obtain for this position, a value representing:

• either the tensile force undergone by the fastening element
• either the compressive force exerted by the fastening element
• or the tension force residing in the fastening element according to the direct relationships of the principle of the invention.

This re-screwing action may be moderated both in the chosen position and in the value of the force in order to anticipate the inertia of the motor means used. This moderation could be a stored constant corresponding to a given rotational force, or deduced from the stopping phase of said means when unscrewing.

This re-screwing action can be corrected both in the chosen position and in the value of the force in order to integrate the torsion of the motor means, in particular when the position sensor is not integral with the fixing element. or the coupling member. This correction may come from a memorized correction table deduced from the torsional slope, that is to say from the ratio of the rotational force to its effective position.

When in the above, the correspondence of the positions in the action of screwing / unscrewing, or unscrewing / screwing, is not effective due to the torsion of the motor means used and the various deformations in the reaction chain motor motor / frame / assembly, in particular when the position sensor is not integral with the fixing element or the coupling member with respect to the assembly, and with the stack of clearances in the transmission of the force of rotation, the concordance of the screwing / unscrewing positions is carried out from the "torsional slope".

The position sensor sees the position of the fastening element through an assembly which deforms in proportion to the force of rotation applied by the motor means. As long as the resistive torque is not exceeded by the motor torque, the fastening element has not started its rotation while the position sensor registers a displacement proportional to the applied torque. The effective position is masked by an apparent position due to the deformation of the reaction chain. The torsional slope is established from the ratio of the value of the rotational force to the value of the position observed. The tangent to this slope is established by the variation of the value of the rotational force on the variation of the position value. The sudden change in the directing coefficient of the tangent to this slope, during the effective rotation of the fastening element, gives the precise position where this effective rotation occurs. The resistant torque of the fixing element results from the different contact surfaces, head or nut, rod, thread. The element is permanently subjected (when stretched) to a torsion due to the tension force on the helicoid of its net. The modification of the tension force will be carried out essentially when the resistance torque due to the thread is overcome. The total rotation of the fixing element is only carried out from the moment when the thread advances. There is therefore a delay in action between the rotation of the head or the nut and the actual rotation of the fixing element. This delay in action results in an additional twist or partial de-twist of the fastening element depending on whether one is screwing or unscrewing. This variation in torsion of the fastening element intervenes in the value of the position observed, in particular when the length of the fastening element is large relative to its section. This variation creates a difference in slope between screwing or unscrewing.

An analysis of the slope by the servo and / or measurement system allows significant finesse, depending on the desired precision, of the correlation of effective positions between screwing and unscrewing in order to synchronize them with the applied rotational forces, namely :

• pour fixer le point de départ de la montée de l'effort de rotation sur la position observée (rattrappage des jeux), par l'accroissement brusque de la valeur du coefficient directeur ;
• pour annuler le cumul des torsions de vissage/dévissage en soustrayant, pour chacune des actions, la valeur de position effective par rapport à la valeur de l'effort de rotation, de la valeur de la position observée ayant que l'élément de fixation n'entre en rotation, la position effective étant donnée par la régression brusque de la valeur du coefficient directeur ;
• pour annuler le cumul des torsions de dévissage/re-vissage en soustrayant, pour chacune des actions, la valeur de position effective par rapport à la valeur de l'effort de rotation, de la valeur de la position observée avant que l'élément de fixation n'entre en rotation. Cette valeur de position est calculée pour le re-vissage dans la proportion de l'effort de rotation appliqué, la position effective étant donnée par la régression brusque de la valeur du coefficient directeur ;
• pour évaluer à tout instant dans l'action de visser/dévisser ou dévisser/re-visser, la valeur d'une torsion par rapport à l'effort de rotation appliqué.

The action defined above "dynamic servoing" establishes a list of values of rotation force according to position intervals. The ratio of the difference between a V value and a V-1 value over the corresponding position interval gives the directing coefficient of the slope at a given interval. The succession of differentiation of values such as V + 1 and V, V + 2 and V + 1 ... V (n) and V (n-1) where n represents the rank of the value in the list, provides a list of guiding coefficients which are used in the variation of their value:

• to fix the starting point for the rise of the rotational force on the observed position (catching up of the clearances), by the sudden increase in the value of the directing coefficient;
• to cancel the cumulation of the screwing / unscrewing twists by subtracting, for each of the actions, the value of effective position relative to the value of the rotational force, from the value of the position observed having that the fixing element n 'enters into rotation, the effective position being given by the sudden regression of the value of the directing coefficient;
• to cancel the cumulation of the twists of unscrewing / re-screwing by subtracting, for each of the actions, the value of effective position relative to the value of the rotational force, from the value of the position observed before the element of attachment does not rotate. This position value is calculated for the re-screwing in the proportion of the rotational force applied, the effective position being given by the sudden regression of the value of the directing coefficient;
• to evaluate at any time in the action of screwing / unscrewing or unscrewing / re-screwing, the value of a twist with respect to the applied rotational force.

• visser d'une valeur d'effort de rotation constituée d'une valeur variable et d'une valeur fixe "delta" ;
• puis dévisser d'une valeur au maximum égale à cette valeur variable, le dévissage pouvant être partiel ;
• poursuivre l'action de vissage/dévissage, en faisant progresser cette valeur variable tant que le dévissage est possible, la progression de cette variable pouvant être égale à la différence entre cette valeur variable et la valeur réalisée au dévissage ou une fraction de cette différence afin de modérer l'action en assurant une convergence rapide de cette action vers l'impossibilité de dévisser ;
  la valeur d'effort "delta" multipliée par un coefficient proportionnel au pas du filetage de l'élément de fixation représentant :
• soit la force de traction exercée par l'élément de fixation
• soit la force de compression exercée par l'élément de fixation
• soit la force de tension résidante dans l'élément de fixation où, lorsque le moyen moteur est instrumenté par un couplemètre, la relation directe de cette valeur d'effort de rotation est exprimée comme suit : $$\text{δC = Ft.K\hspace{1em}\hspace{1em}\hspace{1em}où K = P/π}$$
  
  avec :
  • δC couple "delta" en Newton.mètre
  • Ft force de tension en Newton
  • π constante du cercle
  • P pas du filetage en mètre
  où, lorsque le moyen moteur est instrumenté sur son énergie la relation directe de cette valeur d'effort de rotation est exprimée comme suit : $$\text{δE' = Ft.K\hspace{1em}\hspace{1em}\hspace{1em}où K = P/τ}$$
  
  avec :
  • δE' élément représentatif de l'énergie
  • Ft force de tension en Newton
  • τ constante incluant la constante π du cercle par le rendement moteur par le transformateur du type d'énergie
  • P pas du filetage en mètre

It is also possible, according to the invention, to produce a "floating servo" allowing, by manual, mechanical, pneumatic, hydraulic or electric motor means capable, and by means of a coupling member on the threaded fixing element d '' an assembly, to ensure a controlled rotation effort with the following sequence of screwing / unscrewing actions:

• screw a rotation force value consisting of a variable value and a fixed "delta"value;
• then unscrew by a value at most equal to this variable value, the unscrewing being able to be partial;
• continue the screwing / unscrewing action, advancing this variable value as long as unscrewing is possible, the progression of this variable may be equal to the difference between this variable value and the value achieved at unscrewing or a fraction of this difference so to moderate the action by ensuring a rapid convergence of this action towards the impossibility of unscrewing;
  the force value "delta" multiplied by a coefficient proportional to the pitch of the thread of the fixing element representing:
• either the tensile force exerted by the fastening element
• either the compressive force exerted by the fastening element
• either the tension force residing in the fastening element where, when the drive means is instrumented by a torque meter, the direct relationship of this value of rotational force is expressed as follows: $$\text{δC = Ft.K where K = P / π}$$
  
  with:
  • δC "delta" couple in Newton.meter
  • Ft tension force in Newton
  • π constant of the circle
  • P thread pitch in meters
  where, when the motive means is instrumented on its energy the direct relation of this value of rotational force is expressed as follows: $$\text{δE '= Ft.K where K = P / τ}$$
  
  with:
  • δE 'representative element of energy
  • Ft tension force in Newton
  • τ constant including the constant π of the circle by the motor efficiency by the transformer of the energy type
  • P thread pitch in meters

The element representative of the energy can be any element giving the image of the torque of the motor means (intensity, voltage, pressure, etc.) by a constant or a function f (x), but also the moment of a mechanical means using the flexion, torsion or shock of an organ for which the transformer is not necessarily a constant, but can also be a function f (x).

The drive means can only be controlled in a differential measurement or action. The control of the absolute rotation force is not essential.

Finally, the process which is the subject of the invention makes it possible to study the coefficients of friction and the general efficiency of the fastening element on the assembly:

Ctfr = (Ft.π)/2P

couple transformé

µ = Ctfr/Capp

rendement général au vissage

µ' = Ctfr/Capp'

rendement général au dévissage

Cs = 1/ µ'

coefficient de sécurité

By the principle of the invention and the dynamic control, a precise relationship is established between the torque applied to the screwing / unscrewing and the tension existing in the fastening element. This tension makes it possible at any time to calculate the torque actually transformed to produce this same tension. The ratio of this transformed torque to the applied, fixed torque the performance of the fastener.

Ctfr = (Ft.π) / 2P

transformed couple

µ = Ctfr / Capp

general screwing performance

µ '= Ctfr / Capp'

general efficiency at unscrewing

Cs = 1 / µ '

safety coeficient

Capp

= Ctfr+Ctt+Cflt+Ctg   distribution générale de vissage

CappO

= Ctfr+Ctt   vissage (limite de repos)

-CappO'

= Ctfr-Ctt   dévissage (limite de repos)

Ctt

= (CappO+CappO')/2-Ctfr   Couple frottement tête

Cappl

= Ctfr+Ctt+Cflt+Ctg   vissage (rotation filet)

-Cappl'

= Ctfr-Ctt-Cflt-Ctg   dévissage (rotation filet)

Cflt

= (Cappl+Cappl')/2-Ctt   Couple frottement filet avec frottement tige négligeable

Cflt

= (Cappl+Cappl')/2-Ctt-Ctg   Couple frottement filet avec couple frottement tige relevé hors tension

The performance of the fixing element is the result of the friction of the different surfaces in contact, head or nut, rod, thread. The distribution is unknown. The use of the "torsional slope" makes it possible to isolate each of the couples due to the friction of each part in contact, by the delayed rotation of each of the parts of the fixing element, observed by the torsional slope.

Capp

= Ctfr + Ctt + Cflt + Ctg general screwing distribution

CappO

= Ctfr + Ctt screwing (rest limit)

-CappO '

= Ctfr-Ctt unscrewing (rest limit)

Ctt

= (CappO + CappO ') / 2-Ctfr Torque friction head

Cappl

= Ctfr + Ctt + Cflt + Ctg screwing (thread rotation)

-Cappl '

= Ctfr-Ctt-Cflt-Ctg unscrewing (thread rotation)

Cflt

= (Cappl + Cappl ') / 2-Ctt Thread friction torque with negligible rod friction

Cflt

= (Cappl + Cappl ') / 2-Ctt-Ctg Thread friction torque with rod friction torque raised off

In the study of the friction coefficients, when the measurement of the delayed position is not sufficiently demarcated (case of short screws), in the friction equation, the torque due to the friction of the thread is preferred, without changing its value, by the introduction under the head or the nut of an intermediate element, such as a thrust bearing or any other element with a low or known coefficient.

pour des pièces de révolution, on déduit le coefficient de frottement à partir de la distribution des couples, pour chaque couple Cx, en fonction de la tension Ft et de la géométrie de l'élément de fixation, avec dans cette relation :

• µn coefficient de frottement
• Cx couple de frottement (Ctt,Cflt, etc...)
• Ft force de tension du procédé
• d1 diamètre intérieur de la surface en contact
• d2 diamètre extérieur de la surface en contact

From the current relationship: $\text{µn = Cx / [Ft. (d1 + d2)]}$

for parts of revolution, the coefficient of friction is deduced from the distribution of the couples, for each couple Cx, as a function of the tension Ft and of the geometry of the fixing element, with in this relation:

• µn coefficient of friction
• Cx friction torque (Ctt, Cflt, etc ...)
• Ft process tension force
• d1 inside diameter of the contact surface
• d2 outside diameter of the contact surface

• Figure 1 montre une clé dynamométrique pour vissage avec contrôle de la tension par le procédé selon l'invention ;
• Figure 2 est un schéma des circuits électriques et électroniques de la clé dynamométrique de figure 1 ;
• Figure 3 montre une broche de vissage asservie, mettant en oeuvre le procédé selon la présente invention ;
• Figure 4 est une vue de détail d'un assemblage vissé réalisable au moyen de la broche de figure 3 ;
• Figure 5 est un diagramme de l'asservissement de la broche de vissage de figure 3.

To more concretely illustrate the process which is the subject of the invention, two practical examples of implementation of this process are described below by controlling and controlling the tension or compression of a threaded member, with reference to the attached schematic drawing in which:

• Figure 1 shows a torque wrench for screwing with tension control by the method according to the invention;
• Figure 2 is a diagram of the electrical and electronic circuits of the torque wrench of Figure 1;
• Figure 3 shows a slave screw spindle, implementing the method according to the present invention;
• Figure 4 is a detail view of a screw connection achievable by means of the pin of Figure 3;
• Figure 5 is a diagram of the servo-control of the screw spindle of Figure 3.

Figure 1 shows the external appearance of a torque wrench for screwing with tension control, the internal electrical and electronic circuits of the key being illustrated by the diagram of Figure 2 in which, for clarity of the drawing, are not not shown some discrete components not essential to understanding, as well as power supplies from batteries suitable for ensuring the portability and autonomy of this torque wrench.

In the example considered here, it is a torque wrench with a capacity of 300 Nm, in which a torque gauge 1 with strain gauges, receiving a supply voltage denoted VA with a value of 10 volts, delivers a voltage proportional to the applied torque. A first amplifier 2 with a gain of 1000 produces a relationship on its output such that for each 1Nm of torque a signal - / + 33.333 mV is ensured depending on the direction of the screwing performed. The output of amplifier 2 is connected to the inputs of two blocking samplers 3,4, as well as to the inputs of two comparators 5,6. The first grouping of sampler-blocker 3 and comparator 5 stores the screwing torque on the right, negative in signal, by the fact that comparator 5 compares the output of the sampler-blocker 3 with its input and only when the input is greater than the output, this comparator 5 causes the blocker mode of the sampler-blocker through an OR logic gate 7, thus memorizing the highest value of the torque measured in the direction of screwing to the right. The second sampler-blocker group 4 and comparator 6 memorizes the screwing torque on the left, positive in signal, by the fact that comparator 6 compares the output of the sampler-blocker 4 with its input and that when the input is greater than the output, this comparator 6 causes the blocker mode of the sampler-blocker 5 through an OR logic gate 8 thus memorizing the highest value of the torque measured in the direction of screwing to the left. Each comparator output 5.6 is connected to an OR logic gate 7.8 having in common an input connected to a push button 9 enabling the memories of the torque values acquired during right-hand tightening and tightening to be reset to zero. left. The outputs of each blocker sampler 3,4 are connected to the input of an amplifier 10, wired in summing mode so that the gain only applies to the difference of the outputs of the blocker samplers 3,4 by the fact that one is negative and the other positive, the algebraic sum producing a subtraction. The amplifier 10 produces a gain proportional to the pitch of the screw chosen by a selector 11 which allows a choice of different screw pitches in the range of couples applicable by the key. The selector 11 orients a gain range by assigning a resistance suitably calculated in the gain loop of the amplifier 10. The output of the amplifier 10 is connected to the input of an analog-to-digital converter driving a digital display in a voltmeter function with display 12. The digital display is established for from 0 to - / + 199.99, scale which directly translates KN (Kilo-newtons) for voltages which go from 0 to - / + 5 volts.

Referring now to Figures 3 to 5, we will describe a screw spindle controlled by a processor, for the implementation of the invention. A brushless electric motor 13 is powered by a power control module 14 which regulates the phase frequency, the voltage and the electric current, and therefore makes it possible to control the rotor of the motor 13 in speed, in direction of rotation and power. The position information of the rotor is provided by a synchro-resolver 15 integral with the motor shaft supporting the rotor. The position of the synchro-resolver 15 determined by a sine / cosine phase shift is converted into digital data by a converter 16. The converter 16 which allows different precisions (10,12,14,16 bits) is adjusted to provide 12 bits of precision on a turn of the synchro-resolver either a definition 1/4096 of turn or 5.27 minutes of arc. A strain gauge coupler 17, taken in reaction to the fixing of the screw spindle, measures the effective torque applied to a fixing element 18, such as a screw of a assembly, threaded in 19, the engaging part 20 is compressed between the head 21 of the screw and a part 22 serving as a nut. The fastener 18 is rotated by a coupling member 23 such as a socket which covers the head 21 of the fastener 18 and which is driven by the square 24 of the axis 25 of the spindle constituting the motor shaft. The compression forces Fc of the part 20 are balanced by the tension force Ft of the fixing element 18. The signal supplied by the torque meter 17 is amplified by an instrumentation amplifier 26, the output of which is connected to the input of an analog / digital converter 27, which provides 11 bits of resolution in bi-polar mode, that is for the torque meter 17 at the nominal of 500 Newtons.meter, a definition of 500/2048 or 0.244 Nm / bit. The digital data of the converters 27 and 16 pass over a bus 28 and are processed by a processor 29 which acts according to a program in memory at 30 and stores its digital data in a memory 31. The processor 29 manages the power control module 14 for the cycles required for screwing / unscrewing / re-screwing. The memory 31 contains in particular the tightening parameters such as desired tension, maximum torque applicable to the fixing element, not of the fixing element, speed of rotation of the motor 13, etc. These parameters are entered on a module communication interface 32 by means of a terminal or a network. The memory 31 also contains the measurements carried out at the screwing and unscrewing in the form of tables (lists of values) as well as the calculation results allowing the decisions of motor control for the tightening in tension and the final results to be produced by the means of the communication interface 32 intended for the terminal and / or the printer. An input / output interface 33 provides the decision-making environment so that an operator or a PLC can, on the one hand by the inputs, start the cycle, stop in an emergency, etc., and on the other hand by the outputs view controls on indicator lights, etc. During the screwing phase, the processor 29 takes the rotor position and assigns to this position the numerical value of the torque exerted by the spindle in an array stored in memory 31, dimensioned by example at 16384 values (4 rotations of the rotor), then starts the operation each time the position changes to a torque value determined by the setting. The processor thus creates in the table a first list of a succession of torque values in tightening. The processor repeats the same operations for the unscrewing phase, for the same positions in opposite directions, creating a second list of a succession of torque values. unscrewing. Then position by position, for the same positions in each list, the processor subtracts the torque value from the second list from the torque value from the first list, thus creating a third list made up of the differences in screwing and unscrewing torque for identical positions. By applying the above formulation as a function of the pitch of the screw 18 the processor 29 calculates, position by position, the tension existing in the screw. Then by comparing the calculated tension with the desired tension, it determines the position to which it re-screws. The algorithm of the processing is greatly accelerated, by a pre-calculation of the desired tension translated in the form of the desired torque difference, leaving only the comparison to be made with the third list, and by limiting the number of comparisons to the first match thus reducing the excursion of the program. The processing speed of modern processors (several million instructions per second) makes electronic times insignificant compared to mechanical times. Indeed, the processing time of the voltage control is simply masked by the mechanical time of the actual reversal of the direction of the motor.

The method of the invention can be extended to a threaded member allowing the transformation of a rotational movement into a linear movement, or the reciprocal transformation of a linear movement into a rotational movement, with force transmission, this in particular in weighing, lifting or pressing devices.

Claims (9)

1. A method of monitoring and controlling the tension or compression of a threaded element and more particularly of a threaded mounting element, wherein the tensive or compressive force (Ft) of the threaded element (18) is determined by subtracting a screwing-out torque (Capp') from a screwing-in torque (Capp) and by dividing the difference between these torques by a factor proportional to the pitch (P) of the screw-thread (19) of said element, characterized in that these torque values are recorded either statically at the rest limit during screwing-in and screwing-out or dynamically at a succession of corresponding positions during screwing-in and screwing-out, in the course of an action of screwing-in/screwing out which is performed only one time, or an action of screwing-in/screwing-out which is performed repeatedly, or an action of screwing-in/screwing-out/rescrewing-in said threaded element (18).
2. The method according to claim 1, applied to measuring tension with use of a motor means (13,24,25) capable via a coupling member (23) on the threaded mounting element (18) of applying a controlled force for rotating this mounting element, characterized by the action of taking at the rest limit of sliding of the movable part of the mounting element on the fixed part of the assembly the value of the force of rotation during screwing-out then taking at the rest limit of sliding of the movable part of the mounting element on the fixed part of the assembly the value of the rotation force as it passes its maximum during screwing-out, then finding the difference between these two force values for dividing them by a factor mainly proportional to the pitch of the screwthread (19) of the mounting element (18) so as to obtain a value representing in these precise limits either:

   - the tractive force applied to the mounting element,

   - the compressive force exercised by the mounting element, or

   - the tensile force inside the mounting element.
3. The method according to claim 1, applied to measuring tension with use of a motor means (13,24,25) capable via a coupling member (23) on the already screwed-in threaded mounting element (18) of applying a controlled force for rotating this mounting element, characterized by the action of taking at the start of sliding of the movable part of the mounting element on the fixed part of the assembly the value of the force of rotation during screwing-out then taking at the rest limit of sliding of the movable part of the mounting element on the fixed part of the assembly the value of the rotation force as it passes its maximum during screwing-out, then finding the difference between these two force values for dividing them by a factor proportional to the pitch of the screwthread (19) of the mounting element (18) so as to obtain a value representing for these precise values either:

   - the tractive force applied to the mounting element,

   - the compressive force exercised by the mounting element, or

   - the tensile force inside the mounting element at the start of the operation.
4. The method according to claim 2 or 3, applied for nondestructively monitoring a screwed-together assembly characterized by the action described above of measuring tension followed by the action of rescrewing-in up to the screwing force first taken, more specifically either

   - to the rest limit, or

   - to the start of sliding

   in order to restore the initial conditions of the assembly while having monitored the existing tensile force in the mounting element (18) or the tractive force to which the mounting element is subjected or even the compressive force exerted by the mounting element.
5. The method according to claim 4, characterized in that a simple control is done by successive approximations on a mounting element (18) that is already stressed in the known manner by exerting the above-defined action as many times as necessary until the ratio of the necessary tensile force to the existing tensile force is equal to 1, according to a repeated method, the value of the screwing-in/screwing-out force being able to be modified by a factor able to assure the convergence of the action toward the tension necessary in a number of strokes defined by the precision desired in the control of the action.
6. The method according to claim 1 doing a dynamic control by using a motor means (13,24,25) capable via a coupling member (23) connected to the threaded mounting element (18) of insuring a controlled rotation force and detection of the actual angular position of the mounting element, characterized in that with the mounting element not tightened one tightens the mounting element while at regular intervals spaced according to the desired precision and corresponding to respective angular positions one ascertains the force applied and then one loosens the mounting element while at the same positions one determines the force applied for screwing-out, one takes the difference between the screwing-in and screwing-out force for each position and divides it by a factor proportional to the pitch of the screwthread (19) of the mounting element (18) so as to obtain a list of values representing for each of the positions either

   - the traction force to which the mounting element is subjected,

   - the compression force exerted by the mounting element, or

   - the tension force in the mounting element, these steps being followed by either

   - the action of rescrewing-in the mounting element to a position whose measured force value is equal to the force necessary for the assembly, or

   - the action of rescrewing-in the mounting element to a screwing-in force corresponding to the position at which one measured a force value equal to the force necessary for the assembly.
7. The method according to claim 1 applied to nondestructive monitoring by using a motor means (13,24,25) capable via a coupling member (23) connected to the threaded mounting element (18) of insuring a controlled rotation force and detection of the actual angular position of the mounting element, characterized in that with the mounting element that is already tightened at any position one screws out the mounting element while, at a defined position, one ascertains the maximum rotation force applied and then one retightens the mounting element to the same position, one takes the difference between the screwing-out maximum force and rescrewing-in force for an actual position and divides it by a factor proportional to the pitch of the screwthread (19) of the mounting element (18) so as to obtain a list of values representing for each of the positions either

   - the traction force to which the mounting element is subjected,

   - the compression force exerted by the mounting element, or

   - the tension force in the mounting element.
8. The method according to claim 6 or 7, characterized in that in order to account for torsion of the motor means (13,24,25) and other deformations, one sets up a list of values of rotation forces as functions of position intervals which form a list of directing factors that are used in the variation of their value:

   - to fix the starting point of the increase of rotation force on the measured position by the sudden increase in the value of the directing factor;

   - to cancel out the sum of torsions for screwing-in/screwing-out while subtracting for each of these actions the value of the effective position relative to the value of the rotation force from the value of the measured position before the mounting element started rotation, the effective position being obtained by the sudden decrease in the value of the directing factor;

   - to cancel out the sum of torsions of screwing-out/rescrewing-in while subtracting for each of these actions the value of the effective position relative to the value of the rotation force from the value of the measured position before the mounting element started rotation, this position value being calculated during rescrewing-in in the proportion of the applied rotation force, the effective position being obtained by the sudden decrease in the value of the directing factor;

   - for evaluating at any instant during the action of screwing-in/screwing-out or screwing-out/rescrewing-in the value of torque relative to the applied rotation force.
9. The method according to claim 1, for carrying out "floating control" by using a motor means (13,24,25) capable via a coupling member (23) connected to the threaded mounting element (18) of insuring a controlled rotation force, characterized by the following succession of actions:

   - screwing-in with a rotation force formed by a variable value and a fixed "delta" value;

   - subsequently screwing-out by a value equal at most to the variable value, wherein the screwing-out may be partial;

   - repeating the action of screwing-in/screwing-out while increasing the variable value as long as screwing-out is possible, wherein the progression of this variable value being may be equal to the difference between this variable value and the value realized during screwing-out or a fraction of this difference so as to moderate the action while insuring a rapid convergence of this action toward a condition of it being impossible to screw out;
   the "delta" force value multiplied by a factor proportional to the pitch of the screwthread (19) of the mounting element (18) representing either:

   - the traction force exerted by the mounting element,

   - the compression force exerted by the mounting element,
   or

   - the tension force in the mounting element.

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