EP3670014A1 - Method for straightening a shaft by applying a radial variable strain hardening force on the shaft in rotation - Google Patents

Abstract

The invention relates to a method for straightening a metal shaft (10) with a longitudinal axis which comprises: - an initial step (E1) of locating the angular position of an arrow (12) of the shaft (10) on at least one determined section (14); - a straightening step (E3) which consists in plastically deforming the shaft (10) under the action of a radial strain hardening force (Br) to reduce the deflection (12 ); characterized in that the straightening step (E3) consists in rotating the shaft (10) relative to a fixed frame and in continuously applying a radial work hardening force (Br), in a direction of fixed application with respect to the fixed frame, on the determined section (14), the intensity of the radial work hardening force (Br) varying progressively between a maximum value (Bmax) and a minimum value (Bmin).

Description

Technical field of the invention

The invention relates to a method for straightening a metal shaft with a longitudinal axis, in particular a gearbox shaft of a motor vehicle or an electric motor shaft of a motor vehicle, which comprises a straightening step which consists of to plastically deform the shaft under the action of a radial work hardening force to reduce the deflection.

Technical background

The shafts of a motor vehicle gearbox are highly stressed, in particular in friction and fatigue. To make them more resistant, they are subjected, during their production, to a heat treatment to harden their surface. This heat treatment is generally carried out on the tree when it generally has its final shape.

It is, for example, a heat treatment by carbonitriding or by case hardening. During such a heat treatment, the tree is subjected to a very high temperature, for example around 900 ° C., in an atmosphere rich in carbon, in propane, in nitrogen. Then, the tree thus heated is suddenly cooled by a quenching operation.

Such a heat treatment makes it possible to obtain a shaft whose metallic material has a hard surface layer and a soft and ductile core.

However, it has been found that the thermal amplitudes to which the tree was subjected tended to create residual internal mechanical stresses. These last cause significant deformations of the tree, especially in bending. The tree thus treated has an arrow which, very often, does not respect the manufacturing tolerances. The tree thus has a concave side and a convex side.

It is therefore recommended to carry out a step of straightening the shaft following this step of heat treatment. Traditionally, straightening is carried out by placing the shaft on support anvils in a determined angular position in which the convex side of the shaft is oriented towards actuators, for example hydraulic cylinders. The shaft is locked in this position and the actuators apply a punctual bending force at one or more fixed points on the convex side of the shaft to cause its bending, in a direction opposite to the arrow, in the plastic domain in order to restore to the tree a main axis generally rectilinear permanently.

The actuators are controlled to move over a sufficient stroke to allow plastic deformation of the shaft. In doing so, the actuator is capable of applying to the shaft an extremely high bending force, for example of the order of several tens of thousands of Newton, over a prolonged period.

It was observed that for certain shaft geometries, the intensity of the bending force applied during the straightening operation created, on certain sections of the shaft, concentrations of mechanical stresses greater than the tensile strength of the material of the tree. This results in the formation of cracks, or even a total rupture of the tree at these sections promoting stress concentrations.

The rate of trees discarded for this cause can be quite high, which has significant financial consequences for the manufacturer.

Summary of the invention

• une étape initiale de repérage de la position angulaire d'une flèche de l'arbre sur au moins un tronçon déterminé, la flèche étant déterminée comme étant la distance radiale de décalage de l'axe réel de l'arbre par rapport à un axe rectiligne nominal ;
• une étape de redressage qui consiste à déformer plastiquement l'arbre sous l'action d'un effort radial d'écrouissage pour réduire la flèche ;

caractérisé en ce que l'étape de redressage consiste à mettre en rotation l'arbre autour de son axe nominal par rapport à un bâti fixe et à appliquer continûment un effort radial d'écrouissage, selon une direction d'application fixe par rapport au bâti fixe, sur le tronçon déterminé, l'intensité de l'effort radial d'écrouissage variant progressivement entre une valeur maximale, appliquée en coïncidence avec l'amplitude maximale de la flèche, et une valeur minimale, appliquée en coïncidence avec l'amplitude minimale de la flèche.The invention provides a method for straightening a metal shaft with a longitudinal axis, in particular a gearbox shaft of a motor vehicle or an electric motor shaft of a motor vehicle, which comprises:

• an initial step of locating the angular position of a shaft arrow on at least one determined section, the arrow being determined as being the radial offset distance of the real axis of the shaft relative to a rectilinear axis nominal;
• a straightening step which consists in plastically deforming the shaft under the action of a radial hardening force to reduce the deflection;

characterized in that the straightening step consists in rotating the shaft around its nominal axis relative to a fixed frame and in continuously applying a radial hardening force, in a direction of application fixed relative to the frame fixed, on the determined section, the intensity of the radial work hardening varying progressively between a maximum value, applied in coincidence with the maximum amplitude of the deflection, and a minimum value, applied in coincidence with the minimum amplitude of the arrow.

• l'effort radial est appliqué avec l'intensité de valeur minimale sur un secteur angulaire de la surface du tronçon déterminé s'étendant symétriquement de part et d'autre de la direction de la flèche, ledit secteur angulaire déterminé s'étendant par exemple sur un angle total d'environ 180°;
• l'effort radial d'écrouissage est appliqué lors de l'étape de redressage par l'intermédiaire d'un galet d'action roulant sur la surface cylindrique du tronçon déterminé de l'arbre sans interruption pendant toute la durée de l'étape de redressage ;
• l'étape de redressage est précédée d'une étape de galetage au cours de laquelle l'arbre est mis en rotation autour de son axe nominal avec application d'un effort radial de galetage sensiblement constant sur le tronçon déterminé pendant plusieurs tours afin de durcir la surface cylindrique du tronçon déterminé ;
• l'intensité de l'effort radial de galetage est inférieure ou égale à la valeur maximale d'intensité de l'effort radial d'écrouissage ;
• durant l'étape de redressage, l'arbre fait plusieurs tours, par exemple 4 tours, la valeur de l'intensité de l'effort radial d'écrouissage variant cycliquement entre sa valeur minimale et sa valeur maximale ;
• la valeur minimale d'intensité de l'effort radial d'écrouissage appliqué pendant l'étape de redressage est comprise entre 0 et 2000 N, par exemple 1500 N ;
• la valeur maximale d'intensité de l'effort radial d'écrouissage appliqué pendant l'étape de redressage est comprise entre 5000 N et 20000 N, par exemple 9000 N ;
• le tronçon déterminé présente une face cylindrique non fonctionnelle, notamment un rayon de raccordement ou un fond de gorge.

According to other characteristics of the process carried out according to the teachings of the invention:

• the radial force is applied with the intensity of minimum value to an angular sector of the surface of the determined section extending symmetrically on either side of the direction of the arrow, said determined angular sector extending for example over a total angle of about 180 °;
• the radial work hardening force is applied during the straightening step by means of an action roller rolling on the cylindrical surface of the determined section of the shaft without interruption for the entire duration of the straightening step;
• the straightening step is preceded by a burnishing step during which the shaft is rotated about its nominal axis with the application of a substantially constant radial thrusting stress on the determined section for several turns in order to harden the cylindrical surface of the determined section;
• the intensity of the radial burnishing force is less than or equal to the maximum value of intensity of the radial hardening force;
• during the straightening step, the shaft makes several turns, for example 4 turns, the value of the intensity of the radial work hardening effort varying cyclically between its minimum value and its maximum value;
• the minimum value of intensity of the radial work hardening force applied during the straightening step is between 0 and 2000 N, for example 1500 N;
• the maximum value of intensity of the radial work hardening force applied during the straightening step is between 5000 N and 20000 N, for example 9000 N;
• the determined section has a non-functional cylindrical face, in particular a connection radius or a groove bottom.

• un support de réception de l'arbre, le support comportant des moyens de guidage permettant la rotation de l'arbre autour de son axe nominal ;
• des moyens de mise en rotation de l'arbre autour de son axe nominal ;
• un dispositif hydraulique d'application d'effort comportant une tige dont une extrémité libre est destinée à appliquer un effort d'écrouissage sur l'arbre, le dispositif hydraulique d'application d'effort étant dimensionné pour appliquer un effort de plusieurs milliers de Newton, par exemple jusqu'à 10000 N ;

caractérisée en ce que le dispositif hydraulique d'application d'effort comporte un galet d'action monté rotatif à l'extrémité libre de la tige.The invention also relates to a straightening station of a motor vehicle shaft with a longitudinal axis, in particular of a motor vehicle gearbox shaft or of an electric motor shaft of a motor vehicle, which comprises:

• a support for receiving the shaft, the support comprising guide means allowing the rotation of the shaft around its nominal axis;
• means for rotating the shaft around its nominal axis;
• a hydraulic force application device comprising a rod whose free end is intended to apply a work hardening force on the shaft, the hydraulic force application device being dimensioned to apply a force of several thousands of Newton , for example up to 10,000 N;

characterized in that the hydraulic force application device comprises an action roller mounted to rotate at the free end of the rod.

Brief description of the figures

• [Fig. 1A] La figure 1A est une vue de côté qui représente un arbre avant traitement thermique ;
• [Fig. 1B] La figure 1B est une vue en section de l'arbre de la figure 1A selon le plan de coupe 1B-1B de la figure 1A ;
• [Fig. 2A] La figure 2A est une vue de côté de l'arbre de la figure 1A après traitement thermique, l'arbre étant courbé par rapport à celui de la figure 1A et présentant une flèche ;
• [Fig. 2B] La figure 2B est une vue en section de l'arbre de la figure 2A selon le plan de coupe 2B-2B de la figure 2A ;
• [Fig. 3A] La figure 3A est une vue de côté qui représente l'arbre de la figure 2A monté sur une station de redressage et subissant une étape de repérage de la flèche ;
• [Fig. 3B] La figure 3B est une vue en section de l'arbre de la figure 3A selon le plan de coupe 3B-3B de la figure 3A ;
• [Fig. 4A] La figure 4A est une vue similaire à celle de la figure 3A qui représente l'arbre subissant une étape de redressage sur la station de redressage ;
• [Fig. 4B] La figure 4B est une vue en section de l'arbre de la figure 4A selon le plan de coupe 4B-4B de la figure 4A, les flèches représentant l'intensité de l'effort radial d'écrouissage exercé par le galet d'action lors du passage de chacun des points au droit du galet d'action ;
• [Fig. 4C] La figure 4C est une vue en section de l'arbre de la figure 4A selon le plan de coupe 4C-4C de la figure 4A qui représente un tronçon déterminé de l'arbre sur lequel roulent un galet d'action et un galet de réaction opposé ;
• [Fig. 5] La figure 5 est un diagramme représentant l'intensité de l'effort radial d'écrouissage, en kilo Newton, exercé par le galet d'action en fonction du temps pendant l'étape de de redressage ;
• [Fig. 6A] La figure 6A est une vue similaire à celle de la figure 4A qui représente l'arbre de la figure 4A à l'issue de l'étape de redressage ;
• [Fig. 6B] La figure 6B est une vue en section de l'arbre de la figure 6A selon le plan de coupe 6B-6B de la figure 6A ;
• [Fig. 7] La figure 7 est une vue similaire à celle de la figure 4A qui représente l'arbre subissant une opération préalable de galetage.

Other characteristics and advantages of the invention will appear during the reading of the detailed description which will follow for the understanding of which reference will be made to the appended drawings in which:

• [ Fig. 1A ] The figure 1A is a side view which represents a tree before heat treatment;
• [ Fig. 1B ] The figure 1B is a section view of the tree of the figure 1A according to the section plane 1B-1B of the figure 1A ;
• [ Fig. 2A ] The figure 2A is a side view of the tree of the figure 1A after heat treatment, the shaft being bent relative to that of the figure 1A and having an arrow;
• [ Fig. 2B ] The figure 2B is a section view of the tree of the figure 2A according to the section plane 2B-2B of the figure 2A ;
• [ Fig. 3A ] The figure 3A is a side view that represents the tree of the figure 2A mounted on a straightening station and undergoing a step of locating the jib;
• [ Fig. 3B ] The figure 3B is a section view of the tree of the figure 3A according to the section plane 3B-3B of the figure 3A ;
• [ Fig. 4A ] The figure 4A is a view similar to that of the figure 3A which represents the tree undergoing a straightening step on the straightening station;
• [ Fig. 4B ] The figure 4B is a section view of the tree of the figure 4A according to the section plane 4B-4B of the figure 4A , the arrows representing the intensity of the radial work hardening force exerted by the action roller during the passage of each of the points to the right of the action roller;
• [ Fig. 4C ] The figure 4C is a section view of the tree of the figure 4A according to the section plan 4C-4C of the figure 4A which represents a determined section of the shaft on which an action roller and an opposite reaction roller roll;
• [ Fig. 5 ] The figure 5 is a diagram representing the intensity of the radial work hardening force, in kilo Newton, exerted by the action roller as a function of time during the straightening step;
• [ Fig. 6A ] The figure 6A is a view similar to that of the figure 4A which represents the tree of the figure 4A at the end of the straightening step;
• [ Fig. 6B ] The figure 6B is a section view of the tree of the figure 6A according to the section plane 6B-6B of the figure 6A ;
• [ Fig. 7 ] The figure 7 is a view similar to that of the figure 4A which represents the tree undergoing a prior burnishing operation.

Detailed description of the invention

In the following description, elements having an identical structure or analogous functions will be designated by the same references.

In the following description, a longitudinal direction, directed along the nominal axis of the shaft, vertical and transverse, which is indicated by the arrow "L" in the figures, will be adopted without limitation.

Without further specification, the term "radial" is used in reference to the nominal axis of the shaft.

We have represented schematically in the figure 1 a gearbox shaft 10 of nominal longitudinal “X0” axis. This shaft 10 is intended to receive various elements such as fixed gears and / or mobile. The shaft 10 is made of a metallic material such as steel.

Alternatively, it is an electric motor shaft for a motor vehicle, for example an electric motor traction motor.

The tree 10 represented in Figures 1A and 1B has a rectilinear main axis oriented longitudinally which is substantially coaxial with a nominal axis "X0" of the shaft. For the remainder of the description, the axis "X0" is defined as being a straight line which connects the center of the two end faces of the shaft 10.

This shaft 10 is intended to undergo a heat treatment to harden the surface. Such a heat treatment includes a heating operation during which the shaft 10 is exposed to a high treatment temperature "Tt" followed by a quenching operation. It is for example a treatment by carbonitriding or a treatment by carburizing.

During such a treatment, the treatment temperature "Tt" is very high, for example around 900 ° C. To finish the treatment, the shaft 10 directly undergoes a quenching operation which suddenly drops its temperature, for example to a temperature of about 140 ° C.

During the heat treatment, the significant variation of temperatures in such a short period of time causes a bending deformation of the shaft 10. As shown in Figures 2A and 2B , on at least one determined section 14, the shaft 10 thus has a permanent arrow 12.

The deflection 12 is determined as being the radial offset distance from the real axis "X1" of the shaft 10 relative to the nominal rectilinear axis "X0" of the shaft 10 in an axial bending plane "P" passing through these two axes "X1" and "X0". To simplify the rest of the description, a distal longitudinal straight line 16 is defined formed by the intersection of the plane "P" of flexion with the external cylindrical surface of the determined section 14, the straight line 16 distal being furthest from the nominal "X0" axis, as shown at the top of the figure 2B . There is also defined a proximal longitudinal straight line 18 formed by the intersection of the plane "P" of flexion with the external surface of the determined section 14, the proximal straight line 18 being closer to the nominal axis "X0" than the distal straight line 16, as shown at the bottom of the figure 2B .

To straighten the shaft 10, the latter is installed in a straightening station 20 which includes a fixed frame 22 carrying a support for receiving the shaft 10. As shown in FIG. figure 3A , the support comprises a tip 24A and a tailstock 24B which are aligned along the nominal axis "X0" of the shaft 10.

The tip 24A and the tailstock 24B can be applied to the center of each of the ends of the shaft 10. The tip 24A and the tailstock 24B more particularly take the shaft 10 longitudinally in a vice, the tip 24A and the tailstock 24B being aligned in coincidence with the nominal axis "X0" of the shaft 10. For this purpose, the tip 24A is mounted adjustable longitudinally in position to allow the longitudinal tightening of the shaft 10. After tightening , the tip 24A remains fixed longitudinally throughout the process.

The tip 24A and the tailstock 24B are rotatably mounted around their axis "X0". The tip 24A is rotated by means not shown, such as an electric motor, the rotational movement being transmitted without sliding to the shaft 10, as indicated by the arrow "F" of the figure 3A .

When the deflection of the shaft 10 decreases, the length between its two ends tends to increase. To allow this elongation movement of the shaft 10 to be accompanied, the tailstock 24B is slidably mounted axially relative to the frame 22.

To allow sufficient axial tightening of the shaft 10 between the tip 24A and the tailstock 24B while allowing the elongation of the shaft 10, the tailstock 24B is here elastically constrained in the direction of the shaft 10, for example by means of an elastic member 25 which is mounted prestressed in compression between an element axially fixed relative to the frame 22 and the tailstock 24B.

As shown in Figures 3A and 3B , during an initial step "E1" of marking, the angular position of the arrow 12 of the shaft 10 is marked on at least one determined section 14 of the shaft 10. This marking is for example carried out by means of a radial feeler 26 which is arranged in contact with the external cylindrical wall of the determined section 14 of the shaft 10.

During the rotation of the shaft 10 around its nominal axis "X0", the probe makes it possible to identify the angular position of the arrow 12 on the shaft 10. The angular position and the amplitude of the arrow 12 are stored in an electronic control unit 28. It is thus possible to know at any time the angular position of the arrow 12 relative to the fixed frame 22.

After the step "E1" of tracking, a step "E3" of straightening is triggered when the arrow 12 is greater than a predetermined tolerance threshold. The purpose of step "E3" is to plastically deform the shaft 10 under the action of a radial hardening force "Br" to reduce the deflection 12 of the section 14 determined below the tolerance threshold, ideally for match the real "X1" axis with the nominal "X0" axis.

The straightening step "E3" is carried out on the straightening station 20. It consists in rotating the shaft 10 around the nominal "X0" axis relative to the fixed frame 22. During the rotation of the shaft 10, a radial strain hardening force "Br" is applied continuously to the determined section 14, in a direction of radial application which is fixed relative to the fixed frame 22.

In the embodiment shown in figure 4A , the radial “hardening” force “Br” is applied by the free end of the rod 30 of a hydraulic force application device 31 sliding in a fixed direction relative to the frame 22. The rod 31 is mounted sliding in a body 33 of the hydraulic force application device 31. This is, without limitation, a hydraulic cylinder. The rod 31 is slidably mounted in a body 33 of the hydraulic force application device 31. The hydraulic force application device 31 is dimensioned to apply a force of several thousands of Newton, for example up to 20,000 N.

To prevent the free end of the hydraulic force application device 31 from rubbing against the shaft 10, the free end of the hydraulic force application device 31 comprises an action roller 32 rotatably mounted around 'an axis "Y" which can be longitudinal or inclined with respect to the longitudinal direction. Thus, the radial force “Br” of work hardening is applied by means of the action roller 32. The action roller 32 thus rolls without interruption on the external cylindrical surface of the section 14 determined during its rotation over the entire duration of the straightening step "E3".

In the embodiment shown in figure 4A , the force "B" applied by the hydraulic force application device 31 is slightly inclined in the axial plane relative to a purely radial direction because the force is applied at an angle formed between a cylindrical face of the shaft and a shoulder face of the shaft. In this case, the radial strain hardening force "Br" is determined as being the radial component of the strain hardening force applied by the hydraulic force application device 31.

In a variant not shown of the invention, the hydraulic force application device is arranged so as to exert a purely radial force on the shaft.

The straightening station 22 advantageously comprises a reaction roller 34 which is arranged diametrically opposite the action roller 32. This reaction roller 34 thus rolls on the cylindrical face of the section 14 determined opposite the action roller 32, as is particularly visible on the figure 4C . The reaction roller 34 makes it possible in particular to guarantee that the shaft 10 remains in position between the tip 24A and the tailstock 24B and it also makes it possible to limit the bending deformation of the shaft 10 under the effect of the force. "Br" of hardening.

The reaction roller 34 is for example mounted to slide radially relative to the frame 22 and it is constrained against the determined section 14 (not shown) so as to be able to be in permanent contact with the shaft 10. The reaction roller 35 applies a reaction " Rb "on the shaft 10 which is opposite to the straining effort" Br ".

As shown in the figure 4C , the reaction roller 34 is for example mounted on a gantry 35 which is integral with the body 33 of the hydraulic device 31 for applying force. The gantry 35 is mounted to slide freely parallel to the effort "Br" of work hardening on the frame 22 over a limited stroke. This thus makes it possible to pinch the section 14 determined between the action roller 32 and the reaction roller 34 when the force "Br" of work hardening is applied.

• une valeur maximale "Bmax", appliquée en coïncidence avec le passage de la droite 16 distale au droit de la direction d'application, c'est-à-dire lorsque le galet 32 d'action roule sur ladite droite 16 distale ; et
• une valeur minimale "Bmin", appliquée en coïncidence avec le passage de la droite 18 proximale, au droit de la direction d'application, c'est-à-dire lorsque le galet 32 d'action roule sur ladite droite 18 proximale.

The intensity of the radial hardening effort "Br" is controlled so as to alternate progressively between:

• a maximum value "Bmax", applied in coincidence with the passage from the distal straight line 16 to the right of the direction of application, that is to say when the action roller 32 rolls on said distal straight line 16; and
• a minimum value "Bmin", applied in coincidence with the passage of the proximal straight line 18, to the right of the direction of application, that is to say when the action roller 32 rolls on said proximal straight line 18.

In other words, the maximum value "Bmax" is applied in coincidence with the maximum amplitude of the arrow, while the minimum amplitude "Bmin" is applied in coincidence with the minimum amplitude of the arrow.

The intensity of the radial hardening effort "Br" is thus controlled by the electronic control unit 28 as a function of the angular position of the arrow 12 and of the angular speed of the shaft 10. The intensity varies cyclically at each shaft revolution 10 as shown in the figure 5 .

The minimum value "Bmin" of the radial strain hardening force "Br" applied during the straightening step "E3" is for example between 0 and 2000 N, for example 1500 N.

The maximum value "Bmax" of the radial strain hardening force "Br" applied during the straightening step "E3" is for example between 5000 N and 20000 N, for example 9000 N.

In the example shown in figure 4B , the intensity of the radial "strain" of work hardening undergone by each point of the contour of the section 14 determined during their passage to the right of the direction of application, that is to say during their contact with the action roller 32.

The minimum value "Bmin" of the intensity of the radial work hardening "Br" is here applied to a first angular sector "A1" determined from the surface of the determined section 14, extending symmetrically on either side other of the proximal line 18, that is to say on either side of the direction of the arrow. The first angular sector "A1" here extends over a total angle of approximately 180 °, for example slightly greater than 180 °.

Likewise, the maximum value "Bmax" of the intensity of the effort "Br" radial work hardening is here applied to a second angular sector "A2" determined from the surface of the determined section 14, extending symmetrically from one side and across the distal line 16. The second angular sector "A2" extends here over a very small angle relative to that of the first angular sector "A1", for example around 20 °.

The first angular sector "A1" and the second angular sector "A2" are separated on both sides by two intermediate angular sectors "A3" which make it possible to gradually increase the value of the intensity of the force "Br" radial of work hardening from its minimum value "Bmin" to its maximum value "Bmax" after the passage of the first angular sector "A1" to the right of the direction of application, then gradually decreasing the value of the intensity of the effort "Br" radial work hardening from its maximum value "Bmax" to its minimum value "Bmin" after the passage of the second angular sector "A2" to the right of the direction of application.

We represented at the figure 5 a diagram representing the value of the intensity of the effort "Br" radial hardening as a function of time. It is observed that during the straightening step "E3", the shaft here performs several turns, for example 4 turns indicated respectively by the intervals T1, T2, T3 and T4. The first and the last turns "T1, T4" respectively constitute steps of rise and fall in intensity of the force "Br" radial hardening applied by the hydraulic device 31 for applying force. Thus, during these two turns "T1, T4", the hydraulic force application device 31 applies a force "Br" of maximum radial work hardening which is here substantially half of the maximum value "Bmax" during the passage of the second angular sector "A2". The process is only fully applied during the two intermediate "T2, T3" turns.

At the end of the process, the shaft 10 has an actual axis "X1" substantially coaxial with its nominal axis "X0", the deflection 12 then being reduced below the tolerance threshold, as shown in FIGS. Figures 6A and 6B .

The straightening step thus carried out makes it possible to straighten the shaft 10 by plastic deformation without damaging the shaft 10. This is due to the fact that, contrary to what is practiced in the state of the art, the force "Br" of radial hardening is applied to the contour of the moving shaft. Thus, the stresses caused by the application of the maximum value "Bmax" of the force appear transiently and they disappear cyclically before having been able to cause damage to the structure of the material constituting the tree 10. In addition, the fact of gradually increasing the value of the intensity makes it possible to avoid the sudden appearance of high stresses, thus protecting the structure of the material.

Unlike the prior art methods which involve bending the tree in the plastic field against its arrow, the method of the invention proposes to straighten the material which tends by strain hardening. to reduce the deflection by extending the shaft 10 in its longitudinal axis. The value "Al1 + Al2" of elongation of the shaft 10 is, for example a value of approximately 0.09 mm, which is distributed longitudinally in both directions from the determined section 14 on which the straightening force is applied, as indicated by the references "Al1" and "Al2" of the figure 6A . The value of this elongation depends on the shape, on the size and on the material of the shaft 10, as well as on the radial hardening forces "Br".

In addition, for certain trees 10, this method makes it possible to reduce the maximum value "Bmax" of the intensity of the force applied relative to the bending force applied in the methods of the prior art.

For certain shafts 10, it has been observed that the rolling of the action roller 32 on the contour of the determined section 14 of the shaft 10 could cause the formation of a hollow path on the path of the action roller 32 by local crushing of the material. To prevent this hollow path from interfering with certain functional faces of the shaft 10, for example receiving faces pinions, the determined section 14 is selected so as to present a non-functional cylindrical surface, that is to say a surface which does not require a specific surface finish or a geometry shape of low tolerance.

This is for example a connecting radius with a shoulder, as shown in the figures, or an annular groove bottom (not shown). In addition, the choice of such a determined section 14 ensures guiding of the rolling of the action roller 32 by contact with the shoulder face or with one of the lateral faces of the groove.

When the cylindrical surface of the shaft 10 is likely to be started by the rolling of the action roller 32, the method advantageously has a burnishing step "E2" which precedes the straightening step "E3". This is for example the case when the shaft 10 is made of a soft material.

During the burnishing step "E2", the shaft 10 is rotated about its nominal axis "X0" without leaving the straightening station 20. A substantially constant radial force "E" of burnishing is applied to the section 14 determined for several turns, for example 4 turns, in order to harden the cylindrical surface of the section 14 determined by burnishing.

The intensity of the radial burnishing force "E" here is less than or equal to the maximum value "Bmax" of the intensity of the radial work hardening force "Br".

The roller burnishing force "E" is applied by the hydraulic force application device 31 via the action roller 32. To prevent the application of the roller burnishing force "E" from causing an increase in the deflection 12 of the shaft 10, the roller burnishing force "E" is constant during the rotation of the shaft 10.

The straightening process carried out according to the teachings of the invention makes it possible to straighten shafts 10 by work hardening causing an elongation by plastic deformation in greatly reducing the risk of damage to the shaft 10, even when the shaft is short and / or has complex geometry.

In addition, this method increases the resistance to fatigue of the determined section 14.

Claims (10)

Method for straightening a metal shaft (10) with a longitudinal axis, in particular a motor vehicle gearbox shaft or an electric motor shaft of a motor vehicle, which comprises: an initial step (E1) of locating the angular position of an arrow (12) of the shaft (10) on at least one determined section (14), the arrow (12) being determined as being the radial distance from offset of the real axis (X1) of the shaft (10) relative to a nominal rectilinear axis (X0); - A straightening step (E3) which consists in plastically deforming the shaft (10) under the action of a radial strain hardening force (Br) to reduce the deflection (12); characterized in that the straightening step (E3) consists in rotating the shaft (10) around its nominal axis (X0) relative to a fixed frame (22) and in continuously applying a radial force (Br) work hardening, in a direction of application fixed relative to the fixed frame (22), on the determined section (14), the intensity of the radial work hardening force (Br) varying progressively between a maximum value (Bmax ), applied in coincidence with the maximum amplitude of the arrow, and a minimum value (Bmin), applied in coincidence with the minimum amplitude of the arrow. Method according to the preceding claim, characterized in that the radial force (Br) is applied with the minimum value intensity (Bmin) to an angular sector (A1) of the surface of the determined section (14) extending symmetrically from on either side of the direction of the arrow, said determined angular sector (A1) extending for example over a total angle of approximately 180 °. Method according to any one of the preceding claims, characterized in that the radial force (Br) work hardening is applied during the straightening step (E3) by means of an action roller (32) rolling on the cylindrical surface of the determined section (14) of the shaft (10) without interruption during the entire duration of the straightening step (E3). Method according to the preceding claim, characterized in that the straightening step (E3) is preceded by a burnishing step (E2) during which the shaft (10) is rotated about its axis (X0) nominal with application of a substantially constant radial rolling force (E) on the section (14) determined for several turns in order to harden the cylindrical surface of the section (14) determined. Method according to the preceding claim, characterized in that the intensity of the radial burnishing force (E) is less than or equal to the maximum value (Bmax) of the intensity of the radial work hardening (Br). Method according to any one of the preceding claims, characterized in that during the straightening step (E3), the shaft (10) makes several turns, for example 4 turns, the value of the intensity of the effort ( Br) radial work hardening varying cyclically between its minimum value (Bmin) and its maximum value (Bmax). Method according to any one of the preceding claims, characterized in that the minimum value (Bmin) of the intensity of the radial work hardening force (Br) applied during the straightening step is between 0 and 2000 N, by example 1500 N. Method according to any one of the preceding claims, characterized in that the maximum value (Bmax) of intensity of the radial strain hardening force (Br) applied during the straightening step is between 5000 N and 20000 N, for example 9000 N. Method according to any one of the preceding claims, characterized in that the determined section (14) has a non-functional cylindrical face, in particular a connection radius or a groove bottom. Station (20) for straightening a shaft (10) of a motor vehicle with a longitudinal axis, in particular a gearbox shaft of a motor vehicle or an electric motor shaft of a motor vehicle, which comprises: - a support for receiving the shaft (10), the support comprising guide means allowing the rotation of the shaft (10) about its nominal axis (X0); - means for rotating the shaft (10) around its nominal axis (X0); - A hydraulic device (31) for applying force comprising a rod (30), one free end of which is intended to apply a straining force (B) on the shaft, the hydraulic device (31) for applying 'force being dimensioned to apply a force of several thousand Newton, for example up to 20,000 N; characterized in that the hydraulic force application device (31) comprises an action roller (32) rotatably mounted at the free end of the rod (30).

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