EP0096620A1 - Method of tightening a bolted joint - Google Patents

Abstract

The present invention relates to a tightening method making it possible to apply a predetermined tightening force. The method is characterized in that the real coefficient of proportionality (K) is determined connecting the tightening force of the assembly to the tightening torque applied to the threaded element after having carried out a preliminary tightening cycle and a cycle preliminary unscrewing. Application in particular to the automation of tightening of prestressed assemblies.

Description

Method for tightening an assembly comprising a threaded assembly element.

The present invention relates to a method of tightening an assembly comprising a threaded assembly element to apply to this assembly a predetermined clamping force.

All the tightening methods currently used to maintain assemblies by means of a threaded assembly element are intended to apply to these assemblies a predetermined clamping force on which the holding of the assembly depends during its use.

• - Le vissage au couple. C'est la méthode de loin la plus utilisée, elle consiste à visser jusqu'à l'obtention d'un couple résistant donné. Elle est facile à mettre en oeuvre : manuellement, il suffit de disposer d'une clé dynamométrique, pour l'automatisation en chaîne de montage, on place un couplemètre dans la chaîne de vissage et l'on arrête le vissage lorsque l'on a mesuré le couple spécifié. Malheureusement, la dispersion au niveau de la force de précontrainte est très importante, le coefficient de frottement étant extrêmement variable. On a tenté de remédier à ce grave inconvénient en utilisant une lubrification solide au niveau des filets grâce à l'application de vernis spéciaux mais le traitement du boulon et de l'écrou augmente fortement le prix de revient de l'assemblage.
• - Le vissage à l'angle. Le principe du vissage à l'angle est simple, il consiste à détecter la mise en contact des pièces de l'assemblage en observant la montée en couple, puis à visser d'un angle déterminé. La détermination par le calcul de l'angle ne donne qu'un angle très approximatif car il est difficile de faire intervenir tous les paramètres (le calcul de la rigidité de l'assemblage est délicat). Pour déterminer l'angle, on peut visser une série de boulons équipés de jauges qui fournissent la force de précontrainte et faire la moyenne des angles obtenus. Une autre méthode consiste à utiliser un assemblage témoin et un comparateur qui mesure l'allongement du boulon qui est proportionnel à la force de précontrainte. Les variations de rigidité sont faibles d'un échantillon à l'autre et cette méthode donne de meilleurs résultats que le contrôle du couple. Le principal problème étant toutefois la détection de la montée en couple qui sert de point de départ pour le comptage de l'angle.
• - Le vissage à la limite d'élasticité. Cette méthode consiste à arrêter le vissage lorsque l'on atteint la limite élastique de l'élément d'assemblage fileté. Pour ce faire, après avoir remarqué sur la courbe donnant le couple de vissage en fonction de l'angle de rotation de l'élément d'assemblage que le vissage dans la plage élastique correspond à une montée linéaire, on considère le gradient de couple par rapport à l'angle ; celui- ci est constant dans la plage élastique et chute brutalement dans le. domaine plastique. Il suffit donc d'utiliser un système relativement simple pour détecter la chute du gradient.

Among the known tightening methods, there are in particular:

• - Torque screwing. It is by far the most used method, it consists of screwing until a given resistant torque is obtained. It is easy to implement: manually, all you need is a torque wrench, for assembly line automation, place a torque meter in the screwing chain and stop screwing when you have measured the specified torque. Unfortunately, the dispersion in the prestressing force is very large, the coefficient of friction being extremely variable. We have tried to remedy this serious drawback by using solid lubrication at the threads through the application of special varnishes, but the treatment of the bolt and the nut greatly increases the cost price of the assembly.
• - Screwing at the corner. The principle of screwing at the angle is simple, it consists in detecting the contacting of the parts of the assembly by observing the rise in torque, then in screwing at a determined angle. The determination by the calculation of the angle gives only a very approximate angle because it is difficult to involve all the parameters (the calculation of the rigidity of the assembly is delicate). To determine the angle, you can screw a series of bolts fitted with gauges that provide the prestressing force and average the angles obtained. Another method is to use a witness assembly and a comparator which measures the elongation of the bolt which is proportional to the prestressing force. Rigidity variations are small from one sample to another and this method gives better results than torque control. The main problem, however, being the detection of the increase in torque which serves as a starting point for counting the angle.
• - Screwing to the elastic limit. This method consists in stopping the screwing when the elastic limit of the threaded connecting element is reached. To do this, after having noticed on the curve giving the screwing torque as a function of the angle of rotation of the connecting element that the screwing in the elastic range corresponds to a linear rise, we consider the torque gradient by relation to the angle; this is constant in the elastic range and drops suddenly in the. plastic field. It is therefore sufficient to use a relatively simple system to detect the fall of the gradient.

The use of screwing at the elastic limit has the disadvantages, however, of perfectly dimensioning the assembly, of only allowing the use of screws and bolts of which the quality is perfectly known, of preventing the interposition of washers because the possible sliding of this one falsifies the detection of the fall of the gradient.

The object of the invention is to propose a tightening method which overcomes the drawbacks of the methods known from the prior art.

• a) appliquer audit élément d'assemblage un cycle de vissage préliminaire ;
• b) mesurer le couple de vissage appliqué à l'élément d'assemblage en fonction du déplacement angulaire de ce dernier, lors de l'application dudit cycle de vissage préliminaire ;
• c) appliquer audit élément un cycle de dévissage préliminaire ;
• d) mesurer le couple de dévissage appliqué à l'élément d'assemblage en fonction du déplacement angulaire de ce dernier, lors de l'application dudit cycle de dévissage préliminaire ;
• e) calculer le coefficient réel de proportionnalité reliant l'effort de serrage de l'assemblage au couple de vissage appliqué audit élément d'assemblage, en fonction des résultats obtenus au cours des étapes de mesure b) et d) ;
• f) calculer le couple de vissage final à appliquer à l'élément d'assemblage pour obtenir ledit effort de serrage prédéterminé, en fonction dudit coefficient réel de proportionnalité ; et
• g) appliquer ledit couple de vissage final audit élément d'assemblage.

To this end, the invention proposes a method characterized in that it comprises the following steps:

• a) applying to said assembly element a preliminary tightening cycle;
• b) measuring the tightening torque applied to the assembly element as a function of the angular displacement of the latter, during the application of said preliminary tightening cycle;
• c) applying to said element a preliminary unscrewing cycle;
• d) measuring the unscrewing torque applied to the assembly element as a function of the angular displacement of the latter, during the application of said preliminary unscrewing cycle;
• e) calculating the real coefficient of proportionality connecting the tightening force of the assembly to the tightening torque applied to said assembly element, as a function of the results obtained during the measurement steps b) and d);
• f) calculating the final tightening torque to be applied to the connecting element to obtain said predetermined tightening force, as a function of said real coefficient of proportionality; and
• g) applying said final tightening torque to said assembly element.

The method according to the invention has the main advantage of identifying for each assembly carried out the real coefficient of proportionality relating the tightening force to the tightening torque.

In addition, the method according to the invention has the advantage of being able to be easily substituted for the method using the torque screwing method, without modification of the assembly or additional study of the latter. Such substitution is not possible, neither in the case of screwing to the elastic limit which requires precise sizing of the screw, nor in the case of angle screwing which requires a preliminary study and numerous and costly tests.

When designing the assembly, it suffices to determine the predetermined clamping force which it is desired to apply to it and it is not necessary to calculate a priori the value of the tightening torque which it will be necessary to apply in order to obtain a predetermined clamping force.

The invention will now be described in detail with reference to the accompanying drawing in which FIG. 1 represents an assembly to which the invention is capable of being applied.

FIG. 1 shows a conventional assembly comprising two flat parts 12 and 14 which it is desired to assemble by means of a threaded assembly element. In the example shown, the threaded element 14 is constituted by a bolt 16 comprising a head 18, a threaded rod 20 and a nut 22. The threaded rod 20 is received in two bores 13 and 15 formed in the parts 12 and 14 A first washer 24 is also provided, disposed between the lower face 19 of the bolt head 18 and the upper face opposite the part 12, and a second washer 26 disposed between the upper face 21 of the nut 22 and the underside facing part 14.

dans laquelle :

• Cv = couple de vissage,
• p = pas du filet de l'élément fileté,
• µ = coefficient de frottement moyen de l'assemblage,
• D = bras de levier équivalent de la force tangentielle due aux frottements,
• F = effort de serrage axial prédéterminé.

In an assembly such as that shown in FIG. 1, the approximate relationship which links the screwing torque Cv to be applied to the assembly to obtain a predetermined axial clamping force F between the parts to be assembled can be given by the following formula:

in which :

• Cv = tightening torque,
• p = not the thread of the threaded element,
• µ = average friction coefficient of the assembly,
• D = lever arm equivalent to the tangential force due to friction,
• F = predetermined axial clamping force.

dans lequel K est un coefficient de proportionnalité constant.We therefore see that for a given assembly to which correspond particular values of and D specific to this assembly, we have a formula of the type:

where K is a constant proportionality coefficient.

Si on considère le couple en fonction de la force on constate donc qu'il existe un hystérésis entre le vissage et le dévissage, et en combinant les formules (1) et (3) on obtient la formule suivante :

When unscrewing formula (1) becomes:

If we consider the torque as a function of the force we therefore see that there is a hysteresis between the screwing and the unscrewing, and by combining formulas (1) and (3) we obtain the following formula:

The principle of the process which is the subject of the present invention is to identify the real coefficient of proportionality K for each assembly by virtue of the existence of said hysteresis.

L'invention propose donc d'utiliser le procédé suivant :

• a) appliquer à l'élément d'assemblage un cycle de vissage préliminaire ;
• b) mesurer le couple de vissage Cv appliqué à l'élément d'assemblage en fonction du déplacement angulaire de ce dernier, lors de l'application du cycle de vissage préliminaire ;
• c) appliquer à l'élément un cycle de dévissage préliminaire ;
• d) mesurer le couple de dévissage Cd appliqué à l'élément d'assemblage en fonction du déplacement angulaire de ce dernier, lors de l'application du cycle de dévissage préliminaire ;
• e) calculer le coefficient réel de proportionnalité K reliant l'effort de serrage de l'assemblage au couple de vissageappli- qué à l'élément d'assemblage, en fonction des résultats obtenus au cours des étapes de mesure b) et d);
• f) calculer le couple de vissage final Cf à appliquer à l'élément d'assemblage pour obtenir l'effort de serrage prédéterminé Ff, en fonction du coefficient réel de proportionnalité K ; et
• g) appliquer le couple de vissage final Cf à l'élément d'assemblage.

By combining formulas (2) and (4) we obtain the following formula:

The invention therefore proposes to use the following method:

• a) applying a preliminary tightening cycle to the assembly element;
• b) measuring the screwing torque Cv applied to the assembly element as a function of the angular displacement of the latter, during the application of the preliminary screwing cycle;
• c) applying a preliminary unscrewing cycle to the element;
• d) measuring the unscrewing torque Cd applied to the assembly element as a function of the angular displacement of the latter, during the application of the preliminary unscrewing cycle;
• e) calculate the real proportionality coefficient K connecting the tightening force of the assembly to the torque applied to the assembly element, as a function of the results obtained during the measurement steps b) and d);
• f) calculating the final tightening torque Cf to be applied to the connecting element to obtain the predetermined tightening force Ff, as a function of the real coefficient of proportionality K; and
• g) apply the final tightening torque Cf to the assembly element.

• e1) relever la valeur du couple de vissage C'v appliqué à l'élé-1 ment d'assemblage à la fin du cycle de vissage préliminaire;
• e2) relever la valeur du couple de dévissage C'd appliqué à l'élément d'assemblage au début du cycle de dévissage préliminaire; et
• e3) calculer le coefficient réel de proportionnalité K en fonction des valeurs des couples de vissage et de dévissage C'v, C'd relevées aux étapes intermédiaires e1) et e2).

According to a first embodiment, step e) comprises the following intermediate steps:

• e1) note the value of the screwing torque C'v applied to the assembly element at the end of the preliminary screwing cycle;
• e2) note the value of the unscrewing torque C'd applied to the assembly element at the start of the preliminary unscrewing cycle; and
• e3) calculate the real proportionality coefficient K as a function of the values of the screwing and unscrewing couples C'v, C'd noted in the intermediate steps e1) and e2).

In this first embodiment, during the intermediate step e3), the real coefficient of proportionality K is calculated using the following formula:

Once K has been calculated, all that remains is to calculate Cf using the formula:

• e'1) calculer le gradient Gv de la courbe, donnant le couple de vissage en fonction du déplacement angulaire de l'élément d'assemblage, établie à partir des résultats obtenus au cours de l'étape de mesure b) ;
• e'2) calculer le gradient Gd de la courbe, donnant le couple de dévissage en fonction du déplacement angulaire de l'élément d'assemblage, établie à partir des résultats de l'étape de mesure d) ; et
• e'3) calculer ledit coefficient réel de proportionnalité K en fonction des valeurs de gradients Gv, Gd calculées aux étapes intermédiaires e'1) et e'2).

According to a second embodiment of the invention, and considering that the rigidity of the assembly is constant or that the law of variation of this rigidity is known, step e) comprises the following intermediate steps:

• e'1) calculating the gradient Gv of the curve, giving the tightening torque as a function of the angular displacement of the assembly element, established from the results obtained during the measurement step b);
• e'2) calculating the gradient Gd of the curve, giving the unscrewing torque as a function of the angular displacement of the assembly element, established from the results of the measurement step d); and
• e'3) calculate said real proportionality coefficient K as a function of the gradient values Gv, Gd calculated in the intermediate steps e'1) and e'2).

dans laquelle 6. est le déplacement angulaire de l'élément de vissage.The gradient Gv is given by the formula:

in which 6. is the angular displacement of the screw element.

Likewise, Gd is given by the formula:

In this second embodiment, during the intermediate step e'3), the real coefficient of proportionality K must be calculated using the following formula:

Although the two embodiments which have just been described give good results, these can be improved by repeating steps a) to e) n times and by calculating the real coefficient of proportionality K using any method. suitable statistics such as, for example, mean method, least squares method, etc.

In order to obtain the greatest possible precision as to the value of K, it is desirable that the final value of the tightening torque, during the preliminary tightening cycle, is equal to approximately 80% of the approximate value of Cf that the we can calculate when designing the assembly.

Similarly, when steps a) to e) are repeated at least twice, it is desirable to apply for the first time a final value of the tightening torque corresponding to the approximate value of Cf which can be calculated during of the design of the assembly taking for µ the estimated minimum value of this coefficient. During the second cycle, it is then desirable to apply a tightening torque, the final value of which at the end of step a) corresponds to approximately 80% of the value Cf which could have been calculated at the end of steps a ) to e) of the first cycle.

Claims (6)

1. Method for tightening an assembly comprising a threaded assembly element for applying a predetermined tightening force to this assembly, characterized in that it comprises the following steps: a) applying to said assembly element a preliminary tightening cycle; b) measuring the tightening torque (Cv) applied to the assembly element as a function of the angular displacement of the latter, during the application of said preliminary tightening cycle; c) applying to said element a preliminary unscrewing cycle; d) measuring the unscrewing torque (Cd) applied to the assembly element as a function of the angular displacement of the latter, during the application of said preliminary unscrewing cycle; e) calculating the real coefficient of proportionality (K) connecting the tightening force of the assembly to the screwing torque applied to said assembly element, as a function of the results obtained during the measurement steps b) and d); f) calculating the final tightening torque (Cf) to be applied to the connecting element to obtain said predetermined tightening force (F), as a function of said real coefficient of proportionality (K); and g) applying said final tightening torque (Cf) to said assembly element. 2. Tightening method according to claim 1, characterized in that steps a) to e) are repeated n times, said real coefficient of proportionality (K) being obtained statistically from the n values of the coefficient (K) calculated during n step). 3. A tightening method according to claim 1 or 2, characterized in that said step e) comprises the following intermediate steps: el) reading the value of the tightening torque (C'v) applied to said assembly element at the end of said preliminary tightening cycle; e2) reading the value of the unscrewing torque (C'd) applied to said assembly element at the start of said preliminary unscrewing cycle; and e3) calculating said real proportionality coefficient (K) as a function of the values of the screwing and unscrewing couples (C'v, C'd) noted in said intermediate steps el) and e2). 4. Tightening method according to claim 3, characterized in that during said intermediate step e3), the real coefficient of proportionality (K) is calculated using the following formula:

in which p is the pitch of the thread of the connecting element. 5. A tightening method according to claim 1 or 2, characterized in that said step e) comprises the following intermediate steps: e'1) calculating the gradient (Gv) of the curve, giving the tightening torque as a function of the angular displacement of the assembly element, established from the results obtained during said measurement step b); e'2) calculate the gradient (Gd) of the curve, giving the unscrewing torque as a function of the angular displacement of the element assembly, established from the results of said measurement step d); and e'3) calculate said real proportionality coefficient (K) as a function of said gradient values (Gv, Gd) calculated in said intermediate steps e'l) and e'2). 6. tightening method according to claim 5, characterized in that during said intermediate step e'3) the real coefficient of proportionality (K) is calculated using the following formula:

in which p is the thread pitch of the connecting element.

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