EP0096620B1 - Method of tightening a bolted joint - Google Patents

Description

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).

Among the tightening processes known by MACHINE DESIGN (vol 47 N ° 5 March 1975), 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).

• - Le vissage à la limite d'élasticité. Cette méthode connue par la publication FR 2371679 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.

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 known from the publication FR 2371679 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 area. 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) relever la valeur du couple de vissage (C v) appliqué audit élément d'assemblage à la fin dudit cycle de vissage préliminaire;
• c) appliquer audit élément un cycle de dévissage préliminaire;
• d) relever la valeur du couple de dévissage (C d) appliqué audit élément d'assemblage au début dudit cycle de dévissage préliminaire;
• e) calculer un coefficient réel de proportionnalité (K) en fonction des valeurs des couples de vissage et de dévissage (C v, C d) mesurés et calculer le couple de vissage final (Cf) à appliquer à l'élément d'assemblage pour obtenir ledit effort de serrage prédéterminé (F), en fonction dudit coefficient réel de proportionnalité (K); et en fonction de la formule F = K x Cf;
• f) appliquer ledit couple de vissage final (Cf) 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) reading the value of the tightening torque (C v) applied to said assembly element at the end of said preliminary tightening cycle;
• c) applying to said element a preliminary unscrewing cycle;
• d) reading the value of the unscrewing torque (C d) applied to said assembly element at the start of said preliminary unscrewing cycle;
• e) calculate a real proportionality coefficient (K) as a function of the values of the screwing and unscrewing couples (C v, C d) measured and calculate the final screwing torque (Cf) to be applied to the connecting element for obtaining said predetermined clamping force (F), as a function of said real coefficient of proportionality (K); and according to the formula F = K x Cf;
• f) applying said final tightening torque (Cf) to said assembly element.

dans laquelle p est le pas du filetage.The coefficient K is given by the formula

in which p is the thread pitch.

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 a substitution is not possible, neither in the case of screwing at the elastic limit which requires precise sizing of the screw, nor in the case of angle screwing which requires a preliminary study and numerous tests and expensive.

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 also relates to a method according to Claim 3. The invention will now be described in detail with reference to the appended 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 observe that for a given assembly to which correspond particular values of ¡.t and D specific to this assembly we have a formula of the type:

where K is a constant proportionality coefficient.

When unscrewing formula (1) becomes:

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

The principle of the method 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.

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

• 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 vissage appliqué 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.

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) calculating the real coefficient of proportionality K connecting the tightening force of the assembly to the tightening 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'élément 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 tightening torque C'v applied to the connecting element at the end of the preliminary tightening 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:

Selon un deuxième mode de réalisation de l'invention, et en considérant que la rigidité de l'assemblage est constante ou bien que la loi de variation de cette rigidité est connue, l'étape e) comprend les étapes intermédiaires suivantes:

• 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).

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

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) calculate the gradient Gv of the curve, don nant the screwing 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 O est le déplacement angulaire de l'élément de vissage. The gradient Gv is given by the formula:

in which O 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 (5)

1. A process for clamping an assembly comprising a screwthreaded assembly element with a pitch p for applying to said assembly a predetermined clamping force F, which comprises the following steps:

a) applying to said assembly element a preliminary screwing cycle;

b) detecting the value of the screwing torque (C v) applied to said assembly element at the end of said preliminary screwing cycle;

c) applying to said element a preliminary unscrewing cycle;

d) detecting the value of the unscrewing torque (C d) applied to said assembly element at the beginning of said preliminary unscrewing cycle;

e) calculating a real proportionality coefficient (K) in dependence on the values of the screwing and unscrewing torques (Cv, Cd) and calculating the final screwing torque (C f) to be applied to the assembly element to obtain said predetermined clamping force (F), in dependence on said real proportionality coefficient (K) and by applying the formula F = K x Cf;

f) applying said final screwing torque (Cf) to said assembly element.

2. A process according to claim 1 characterised in that the coefficient K is given by the formula

3. A process for clamping an assembly comprising a screwthreaded assembly element with a pitch p for applying to said assembly a predetermined clamping force F, which comprises the following steps:

a) applying to said assembly element a preliminary screwing cycle;

b) measuring the screwing torque (Cv) applied to the assembly element in dependence on the angular displacement of the latter on applying said preliminary screwing cycle;

c) applying to said element a preliminary unscrewing cycle;

d) measuring the unscrewing torque (Cd) applied to the assembly element in dependence on the angular displacement of the latter on application of said preliminary unscrewing cycle;

e) calculating the gradient (Gv) of the curve, giving the screwing torque in dependence on the angular displacement of the assembly element, which is established on the basis of the results obtained in the course of said measuring step b);

f) calculating the gradient (Gd) of the curve, giving the unscrewing torque in dependence on the angular displacement of the assembly element, which is established on the basis of the results of said measuring step d); and

g) applying a real proportionality coefficient (K) in dependence on said gradient values (Gv, Gd) and calculating the final screwing torque (Cf) to be applied to the assembly element to obtain said predetermined clamping force (F), in dependence on said real proportionality coefficient (K) and by applying the formula F = K x Cf; and

i) applying said final screwing torque (Cf) to said assembly element.

4. A clamping process according to claim 3 characterised in that the coefficient K is given by the following formula:

5. A clamping process according to any one of the preceding claims characterised in that the steps a) to f) are repeated n times, said real proportionality coefficient (K) being obtained statistically from the n values of the coefficient (K) which are calculated in the n steps.

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