Patent Publication Number: US-2021187708-A1

Title: Impact wrench with impact mechanism

Description:
1. TECHNICAL FIELD 
     The field of the invention is that of the designing and manufacture of devices for screwing/unscrewing (or tightening/untightening) by impact. 
     The invention relates especially but not exclusively to impact wrenches and more particularly to impact wrenches with impact mechanism. 
     2. PRIOR ART 
     Impact screwing/unscrewing devices such as impact wrenches are commonly used in various fields, especially in motor vehicle garages and for industrial maintenance. 
     These tools are used chiefly for the dismantling of mechanical components on vehicles or on machines. They can also be used for mounting and remounting certain components. 
     The components, which are tightened or loosened by the use of the impact wrench, are varyingly clean and/or oxidized. 
     In addition, the spaces of intervention can be confined, difficult to access and encumbered by other potentially injurious and poorly lit components. 
     Impact wrench users therefore expect their tools to help limit the impact of the difficulties mentioned above. 
     Thus, to reduce the difficulties related to using impact wrenches in confined and encumbered spaces, users wish to use compact tools. 
     More generally, the impact wrench users have expectations especially in terms of ergonomy, efficiency and durability 
     In terms of ergonomy, users seek noiseless, lightweight tools that entail only low levels of vibration. 
     In terms of efficiency, the users wish to be able to carry out swift tightening/loosening operations with an appropriate level of quality (tightening to the desired torque value) while benefiting from high visibility in the intervention area. 
     In terms of durability, users wish to have solid tools that are especially shock-resistant and capable of working well over time. 
     Existing impact wrenches include especially pneumatic impact wrenches with rebound impact mechanism and electrical impact wrenches with rebound impact mechanism. 
     The patent documents U.S. Pat. Nos. 3,661,217 and 4,287,956 describe examples of pneumatic impact wrenches with rebound impact mechanism. 
     Whether these impact wrenches with rebound mechanism are pneumatic or electric, they generally comprise a (pneumatic or electric) motor provided with a rotor and a stator, an impact mechanism driven by the rotor of the motor and a square drive, also called an output shaft, capable of cooperating with a component to be tightened/loosened. 
     The impact mechanism comprises a striking system comprising especially a striker, for example a hammer element, driven in rotation by the stator and capable of coming into collision with the square drive to generate impacts. 
     To perform an impact screwing/unscrewing operation, the striker or hammer system is driven by the motor. During this rotation, the striker element is made to come into collision with the square drive. At each collision, the impact mechanism transmits, in an impact, a torque to the square drive that rotationally drives the element to be screwed/unscrewed. 
     At each impact in the impact mechanism, the kinematic chain between the motor and the square drive gets deformed and thus accumulates potential energy. This potential energy is restored during the relaxation of the kinematic chain, inducing a rebound of the impact mechanism in reverse to the working direction, i.e., in reverse to the sense of the screwing/unscrewing operation. 
     During this rebound, the rotor of the motor rotates in reverse to the sense for which the motor is powered. Then, the motor is re-accelerated in the sense of the screwing/unscrewing operation so as to make the impact mechanism again transmit a torque to the square drive. The cycles get thus repeated to perform the screwing/unscrewing operation up to its end. 
     In other words, the working of an impact wrench with rebound impact mechanism can be sub-divided into three phases, namely:
         a phase of rotation in which the motor, the impact mechanism and the square drive are driven by rotation by a low torque: this is the case when a member to be screwed/unscrewed is subjected to a low external resistance (that can especially arise when approaching and removing the member to be screwed/unscrewed);   a phase of acceleration situated between two impact phases, in which the motor and the impact mechanism re-accelerate so long as the square drive is kept immobile owing to the external resistance to which the member to be screwed/unscrewed is subjected; and   an impact phase in which the motor, the impact mechanism and the square drive work with great variations in speed (due to the rendering of the accumulated kinetic energy) that moves the member that is to be screwed/unscrewed.       

       FIG. 9  schematically represents the layout of the motor, the impact mechanism and the square drive of an impact wrench with rebound impact mechanism 
     The impact wrench  1000 , disposed in an X, Y, Z referential system, comprises a pack  1001 , also called a casing, within which there is disposed a mechanical system composed of a (pneumatic or electric) motor  1002  and an impact mechanism  1003  driven by this motor  1002 . The mechanical system is mobile in rotation relatively to the casing  1001 . More specifically, this mechanical system is coupled to the casing  1001  by pivot links, formed by bearings  1001   1 ,  1001   2 ,  1001   3 , enabling this assembly to be mobile in rotation along the rotational axis AR oriented along the Z axis of the referential system. The impact mechanism  1003  furthermore carries a square drive  1004  oriented along the rotational axis AR. 
     In addition, during use, the motor  1002  generates efforts on the casing  1001 , corresponding to the motor torque, symbolized by the spring  1005 . 
     It can be seen therefore that, during the use of the impact wrench, the vibrations perceived by the user come, on the one hand, from the variations in motor torque and, on the other hand, from the efforts generated by the mechanical system, constituted by the motor  1002  and the impact mechanism  1003 , as a whole, at the level of the bearings  1001   1 ,  1001   2 ,  1001   3 . 
     More specifically,  FIG. 10 , which is a schematic representation of the mechanical system, theoretically illustrates, for reasons of clarity, the distribution of the efforts of the mechanical system on a bearing of the casing during a phase of impact. Naturally, in the presence of a plurality of bearing surfaces, the efforts of the mechanical system are distributed over all the bearing surfaces. 
     As illustrated, during the impact phase, the mechanical system is subjected to:
         a torque {right arrow over (C)} of the motor  1002  oriented along the rotational axis AR of the mechanical system oriented along the axis {right arrow over (Z)};   a torque {right arrow over (T)} coming from the resistance induced by the member to be screwed/unscrewed. The torque values {right arrow over (C)} and {right arrow over (T)} have identical directions and opposite sensors;   a reaction force {right arrow over (F)} on the bearing surface  1001   2  of the casing  1000 ; and   a moment {right arrow over (M)} on the bearing surface  1001   2  of the casing  1000 .       

     Again, in the phase of impact, a moment of rotation appears around the axis of the bearing surface  1001   2 , with angular speed ω{right arrow over (Z)} and angular acceleration 
     
       
         
           
             
               
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     The mechanical system furthermore possesses:
         a mass m;   a center of gravity G with acceleration {right arrow over (a G )}; and   an inertia matrix [J] computed at the center of gravity G and in the Galilean referential system ({right arrow over (X)}, {right arrow over (Y)}, {right arrow over (G)}), presented as follows:       

     
       
         
           
             
               
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     It is important to note that this theoretical representation and the associated discussion consider only the external efforts generated by the impact mechanism. The efforts internal to the impact mechanism, such as the effort coming from the striker system, are not shown. 
     These impact wrenches with impact mechanism include various known impact mechanisms, especially:
         Maurer type single-hammer, jumbo hammer, one hammer, single dog impact mechanisms;   Maurer type twin-hammer, twin-lobe or double-dog impact mechanisms;   Two-jaws mechanisms;   Pin-clutch mechanisms;   Etc.       

     It has also been observed that certain afore-mentioned impact mechanisms cumulatively present the following technical characteristics:
         an impact mechanism fixed in the direction of the rotational axis of the square drive, corresponding to the axis of the bearing surfaces holding the mechanical system, integrating the motor and the impact mechanism within the casing; and   an impact mechanism enabling an acceleration of the striker element on more than one half turn of the square drive before the impact, in doing so without even benefitting from the rebound of a prior strike.       

     It has been observed that the rebound impact mechanism of the Maurer type has these two cumulative technical characteristics. 
     This is why the structures of the Maurer type single-hammer and twin-hammer impact mechanisms shall be described in detail with reference to  FIGS. 11 and 12  respectively. 
       FIGS. 11A to 11B  are perspective (or three-quarter) and schematic views respectively of a Maurer type single-hammer rebound impact mechanism. 
     As illustrated in  FIG. 11A , a Maurer type single-hammer rebound impact mechanism  1100  comprises a cage  1101  to be driven rotationally by a motor, a hammer  1102  disposed in the cage  1101  and driven in rotation by this cage by means of rods  1103 . 
     Such an impact mechanism  1100  further comprises a square drive  1104  having a striker reception element  1104   1 , also called an anvil, having a strike reception surface. The strike reception surface  1104   1  is intended to be situated facing a striker element  1102 , also called a hammer, having a strike surface. The striker element  1102  is furthermore configured to be in sliding contact on the square drive  1104 . Such a layout of the components of the impact mechanism  1100  enables the striker element  1102  to be brought into collision against the strike reception element  1104   1  once per turn of the striker element  1102 . 
     During the phase of impact of a single-hammer rebound impact mechanism  1100 , as illustrated schematically in  FIG. 11B , it can be seen that the striker element  1102  and the strike reception element  1104   1  respectively have a center of gravity (represented by a cross) that is off-centered relatively to the square drive  1104 . It has been observed that such an off-centering, during the impact phase especially, gives rise to a high level of vibrations perceived by the user. 
       FIGS. 12A and 12B  are perspective and schematic views respectively of an impact wrench equipped with a Maurer type twin-hammer rebound impact mechanism. 
     As illustrated in  FIG. 12A , a Maurer type twin-hammer rebound impact mechanism  1200  is distinguished from a Maurer type single-hammer rebound impact mechanism by the implementation, in the cage  1201 , of two striker elements  1202 A,  1202 B that are diametrically opposite and off-centered relatively to each other, i.e., disposed on distinct planes. During a use of such a mechanism, the two striker elements  1202 A,  1202 B are driven in rotation symmetrically relatively to the square drive  1204 . 
     Besides, during the phase of impact of a twin-hammer rebound impact mechanism  1200  as illustrated schematically in  FIG. 12B , it is noted that the diametrically opposite disposition of the striker elements  1202 A,  1202 B enables the cage  1201  to have a center of gravity situated on the square drive  1204 . It has been observed that such a configuration enables the twin-hammer rebound mechanism to reduce the level of vibrations perceived by the user but that this level nevertheless remains very high. 
     It is seen therefore that the prior-art techniques have various drawbacks, including one common drawback relatively to the high level of the vibrations perceived by the user. 
     Indeed, it can be seen that, because of the presence of known impact mechanisms, impact wrenches with rebound impact mechanism, whether of the pneumatic or electric type, give rise to a high level of vibrations perceived by the user. 
     Besides, it is known that such vibrations are harmful and present a risk for the user. Indeed, such vibrations can especially prompt illnesses such as musculoskeletal disorders (tendonitis), vascular disorders (secondary Raynaud&#39;s syndrome for example), disorders of the nervous system (loss of sensitivity and/or dexterity for example), etc. 
     There is therefore a need for technical solutions that at least partly mitigate the problems of the prior-art solutions. 
     3. SUMMARY OF THE INVENTION 
     All or part of these goals as well as others that shall appear clearly here below are obtained by means of a screwing/unscrewing tool comprising:
         a drive motor;   an impact mechanism driven by said motor along a rotational axis and held fixed in translation along said rotational axis, said impact mechanism comprising:   a striker system rotationally driven along said rotational axis, comprising at least one striker element,   a square drive comprising at least one strike reception element,   said at least one striker element being configured to come into contact with said at least one strike reception element to generate an impact torque,   said striker system being configured to be driven in rotation, along said rotational axis, on at least 200° before impact,   characterized in that the impact mechanism comprises anti-vibration means configured so that, at least during an impact:   the center of gravity of the impact mechanism is appreciably positioned on said rotational axis, and   said rotational axis of the impact mechanism is appreciably on its main axes of inertia.       

     The implementing of anti-vibration means with such a configuration makes it possible, for the impact mechanisms fixed in translation along their rotational axis and comprising a striker system giving rise to a maximum of one strike once per turn of the mechanism, to reduce or even eliminate, at least during the phase of impact, the level of vibrations perceived by the user, and to do so at lower cost. This therefore means a reduction of the risk of emergence of illnesses or discomfort caused by such vibrations and therefore an improvement of the comfort of use of the screwing/unscrewing tool. 
     According to one particular embodiment, said at least one striker element comprises a strike surface and said at least one strike reception element comprises a strike reception surface, and 
     said at least one striker element is configured to alternate cyclically between:
         a disengaged position in which at least one striker element is not in contact with said at least one reception element, and   an engaged position, in which said strike surface of said at least one striker element is in contact with said strike reception surface of said at least one strike reception element so as to generate said impact torque.       

     It can thus be seen that the anti-vibration means can be implemented in transversal engagement systems enabling, through the rebound, more than one acceleration turn between two impacts and therefore greater energy per impact, as can be the case especially in Maurer-type impact mechanisms. 
     According to another particular embodiment, said anti-vibration means are configured so that:
         the center of gravity of the impact mechanism is permanently situated appreciably on its rotational axis, and   said rotational axis of the impact mechanism is permanently and appreciably one of its main axes of inertia.       

     In other words, this configuration enables the striker system to be also balanced dynamically during phases of acceleration situated between two successive impact phases, and rotational phases. 
     Thus, the anti-vibration means enable the reduction or even the elimination of the level of vibrations perceived by the user for all the phases of operation of the screwing/unscrewing tool and not solely during the phase of impact. This is expressed by a general improvement of the comfort and experience of use of such a screwing/unscrewing tool. The risk of the emergence of illnesses or discomfort caused by the vibrations is therefore further minimized. 
     According to another particular embodiment, said striker system comprises at least three striker elements having identical cross-sections in a plane essentially perpendicular to said rotational axis and said at least three striker elements constitute said anti-vibration means. 
     The term “cross-section” is understood to mean a section in a plane normal to the rotational axis of the impact mechanism. 
     The implementing of striker elements with identical cross-sections, i.e., radial sections along the rotational axis of the impact mechanism, minimizes or even eliminates the level of vibrations perceived by the user in a simplified way and at lower cost. 
     According to another particular embodiment, said striker system comprises the following, disposed along the rotational axis of said impact mechanism:
         a central striker element having, along the rotational axis of said impact mechanism, a total value of thickness, and   two lateral striker elements disposed on either side of said at least one central striker element, along the rotational axis of the impact mechanism, the lateral striker elements having respectively values of thickness equal to half of the value of the total thickness of said at least one central striker element along said rotational axis,   and said at least one central striker element is disposed so as to be diametrically opposite to said lateral striker elements relatively to the rotational axis of the impact mechanism.       

     According to another particular embodiment, said central striker element comprises at least two juxtaposed elementary central striker elements, the sum of the thicknesses along said axis of said elementary central striker elements being equal to said total value of thickness. 
     According to another particular embodiment, said anti-vibration means comprise at least one counterweight disposed in the impact mechanism, said at least one counterweight being configured to cancel out the efforts radial to said rotational axis induced by said at least one striker element and by said at least one strike reception element. 
     Such a counterweight constitutes a simple solution of implementation enabling the reduction or even the elimination, during the phase of impact, of the level of vibrations perceived by the user, in doing so at lower cost. In addition, such counterweights can be added on by appropriate affixing means to existing impact mechanisms. 
     According to another particular embodiment, said at least one counterweight is disposed in the impact mechanism, said at least one counterweight being positioned at 180°, along the rotational axis of the impact mechanism, to said at least one striker element and of said at least one strike reception element. 
    
    
     
       5. LIST OF FIGURES 
       Other features and advantages of the invention shall appear more clearly from the following description, given by way of a simple illustratory and non-exhaustive example and from the appended figures, of which: 
         FIG. 1  is a schematic representation of a first example of a first embodiment of the proposed technique applied to an impact mechanism of the Maurer single-hammer type; 
         FIG. 2  is a schematic representation of a second example of a first embodiment of the proposed technique applied to a Maurer single-hammer type impact mechanism; 
         FIG. 3  is a schematic representation of a third example of a first embodiment of the proposed technique applied to a Maurer twin-hammer type impact mechanism; 
         FIG. 4  is a schematic representation of a first example of a second embodiment of the proposed technique applied to a Maurer type impact mechanism; 
         FIG. 5  is a view in perspective of a modeling of the schematic representation of  FIG. 4 ; 
         FIG. 6  is a longitudinal sectional view of the modeling of  FIG. 5 ; 
         FIG. 7  is a schematic representation of a second example of the second embodiment of the proposed technique applied to a Maurer type impact mechanism; 
         FIG. 8  is a schematic representation of a third example of the second embodiment of the proposed technique applied to a Maurer type impact mechanism; 
         FIG. 9  is a schematic representation of the mechanical system of a screwing/unscrewing device with impact mechanism according to the prior art; 
         FIG. 10  is a schematic representation of the distribution of the efforts of a mechanical system of a screwing/unscrewing device with impact mechanism according to the prior art; 
         FIGS. 11A and 11B  are different illustrations of a Maurer single-hammer impact mechanism according to the prior art; and 
         FIGS. 12A and 12B  are different illustrations of a Maurer twin-hammer impact mechanism according to the prior art. 
     
    
    
     6. DESCRIPTION 
     For the sake of clarity, the same elements have been designated by the same references in the different figures. 
     6.1. General Principle 
     As already indicated here above, there are known impact mechanism impact wrenches of an electric or pneumatic type implementing various types of impact mechanisms that give rise to high levels of vibrations perceived by the user. 
     Known types of impact mechanisms include mechanisms with the following technical characteristics:
         an impact mechanism fixed in the direction of the rotational axis of the square drive; and   an impact mechanism enabling an acceleration of a striker element, a hammer for example, on more than one half-turn of the square drive before the impact.       

     The proposed technique (described in detail here below) is intended to be implemented in impact wrenches, the impact mechanism of which has two cumulative technical characteristics. In other words, these two technical characteristics constitute prerequisites for proposed technique. 
     In a trade-off for a non-obvious approach described in detail here below, the inventor of the present application has identified a novel and inventive technique making it possible, for the types of impact mechanisms meeting the above-mentioned prerequisites, to minimize or even eliminate the level of vibrations perceived by the user. 
     The general principle of the proposed technique consists in implementing an impact mechanism comprising anti-vibration means configured so that, during the impact:
         the center of gravity of the impact mechanism is appreciably positioned on its rotational axis, and   the rotational axis of the impact mechanism is appreciably one of its main axes of inertia.       

     Such a configuration wholly or in part eliminates undesirable external efforts generated by the impact mechanism. This means a reduction, or even an elimination, of the level of vibrations perceived by the user during the operation of an impact wrench in accordance with the proposed technique. 
     6.2. Detail of the General Principle 
     With reference to the representation of efforts of the mechanical system on a bearing of the casing during a phase of impact, previously described with reference to  FIG. 10 , the inventor of the present invention has identified two cumulative criteria making it possible, when they are met, to minimize or even eliminate the level of vibrations perceived by the user. 
     First of all, the inventor of the present application has observed that the application of the fundamental principle of dynamics on a portion, namely a bearing surface, of the casing holding the mechanical system that incorporates the motor and the impact mechanism, makes it possible to obtain the following equation: 
         {right arrow over (F)}=m ·{right arrow over ( a   G )}
 
     with: 
     {right arrow over (F)} corresponding to the force of reaction of a portion of the casing holding the mechanism system;
 
m corresponding to the mass of the same mechanical system; and
 
{right arrow over (a G )} corresponding to the acceleration of the mechanical system at its center of gravity.
 
     The inventor of the present application has therefore established that, through the positioning of the center of gravity G of the mechanical system on the rotational axis {right arrow over (Z)}, the acceleration {right arrow over (a G )} of the mechanical system at this point is zero when the impact takes place. This results in the elimination of the reaction force {right arrow over (F)} and therefore in a reduction of the level of vibrations perceived by the user. This reasoning can be applied to each bearing holding the mechanical system. 
     Starting from this result, namely that the center of gravity G of the mechanical system is positioned on the rotational axis {right arrow over (Z)}, the inventor of the present invention has furthermore observed that the application of the kinetic moment theorem to the mechanical system makes it possible to obtain the following vector equation (1) at a center of gravity G and in a Galilean referential system (G, {right arrow over (X)}, {right arrow over (Y)}, {right arrow over (Z)}): 
     
       
         
           
             
               
                 
                   
                     
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     with:
 
{right arrow over (C)} corresponding to the torque of the motor;
 
{right arrow over (M)} corresponding to the moment of the portion of the casing holding the mechanical system;
 
{right arrow over (T)} corresponding to the resistive torque of the member to be screwed/unscrewed;
 
[J] corresponding to the inertia matrix of the mechanical system computed at the center of gravity G and in the fixed referential system ({right arrow over (X)}, {right arrow over (Y)}, {right arrow over (Z)}); and
 
     
       
         
           
             
               
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     corresponding to the angular acceleration of the mechanical system along the axis {right arrow over (Z)}. 
     The vector equation (1) is sub-divided into three scalar equations, distributed on the three axes of the referential system ({right arrow over (X)}, {right arrow over (Y)}, {right arrow over (Z)}), namely: 
     
       
         
           
             
               
                 
                   
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     The inventor of the present application has observed that the portion M z  of the moment of the portion of the casing holding the mechanical system {right arrow over (M)} along the axis {right arrow over (Z)} is low, because it corresponds to friction in the bearing and is therefore negligible. 
     The inventor of the present application has therefore established that to reduce, or even eliminate, the level of vibrations perceived by the user, the reactions M x  and M y , corresponding to the reactions of the moment {right arrow over (M)} along the axes {right arrow over (X )} and {right arrow over (Y)} respectively must be deduced or even eliminated. 
     More specifically, the inventor of the present application has therefore established that by defining, during the impact, the rotational axis {right arrow over (Z)} as a principle of inertia of the system, the products of inertia J xz  and J yz , and therefore the associated reactions M x  and M y  are reduced, or even eliminated. 
     Finally, the inventor of the present application is observed that the rotors of the motors, whether electric or pneumatic, have appreciably a symmetry along their respective axes of revolution and therefore along the rotational axis {right arrow over (Z)}. Hence, the center of gravity of the rotor is initially positioned on or in proximity to the rotational axis {right arrow over (Z)} which itself constitutes a main axis of inertia 
     The inventor of the present application has therefore determined that to reduce or even eliminate the level of vibrations perceived by the user, it is appropriate that the impact mechanism should be configured so that, during the impact:
         its center of gravity G is positioned appreciably on its rotational axis {right arrow over (Z)}, and   its rotational axis {right arrow over (Z)} is appreciably one of the main axes of inertia.       

     The inventor of the present application has also observed that each component of the impact mechanism must meet these two criteria, namely the center of gravity of the impact mechanism must be positioned on the rotational axis of this mechanism and the rotational axis must be the main axis of inertia of the impact mechanism. 
     6.3. Description of a First Embodiment 
       FIGS. 1 to 3  are schematic representations of examples of Maurer type rebound impact mechanisms according to a first embodiment of the proposed technique. 
     The principle of this first embodiment consists of the use, in the impact mechanism, of one or more counterweights forming the anti-vibration means and being configured so that, during an impact, the center of gravity of the impact mechanism is positioned appreciably on its rotational axis and so that the rotational of the impact mechanism is appreciably one of its main axes of inertia. 
     More specifically, the impact mechanism comprises at least one counterweight that is diametrically opposite, i.e., positioned at 180°, to the striker element, relatively to the rotational axis of the impact mechanism. 
     The counterweight and the striker element are at least partially disposed in a same plane transversal to the rotational axis of the impact mechanism (the plane normal to this rotational axis). 
     The use of such counterweights is a simple and efficacious solution for reducing or even eliminating the level of vibrations perceived by the user. 
       FIG. 1  illustrates a first example of the first embodiment of the proposed technique applied to a Maurer single-hammer type rebound impact mechanism. 
     Classically, a Maurer single-hammer type rebound impact mechanism  110 , disposed in an X, Y, Z referential system comprises a cage  111  within which a square drive  112  is mounted rotationally mobile, by means of the holding portion  111   1 . 
     The Maurer single-hammer type rebound impact mechanism  110  is furthermore mounted so as to be rotationally mobile, by means of bearings  110   1 , within the casing  100  of the impact wrench. 
     The cage  111  of the Maurer single-hammer type rebound impact mechanism  110  is intended to be driven in rotation by a motor (which is not visible). 
     The motor, the cage  111  and the square drive  112  are coaxial along a rotational axis AR oriented along the Z axis of the referential system. 
     The cage  111  is equipped with a striker system comprising a striker element  111   2  presenting a strike surface intended to be brought into collision against a strike reception surface belonging to a strike reception element  112   1  borne by the square drive  112 . 
     More specifically, the striker element  111   2  is mobile in a plane radial to the rotational axis AR, i.e., in a plane oriented along the axes X, Y (perpendicular to the axis Z), so that it can take the following positions:
         a disengaged position in which the striker element  111   2  can rotate without coming into contact with the square drive  112 , and   an engaged position in which the strike surface of the striker element  111   2  comes into contact with the strike reception surface of the strike reception element  112   1 .       

     In accordance with the first embodiment of the proposed technique, the Maurer single-hammer type rebound impact mechanism  110  comprises several counterweights  111   3 ,  112   2  configured so that, during an impact, the center of gravity G 1  of the Maurer single-hammer type rebound impact mechanism  110  is positioned on its rotational axis AR and so that the rotational axis AR of the impact mechanism is one of its main axes of inertia Z. 
     In this sense, the cage  111  bears a first counterweight  111   3  configured so that, when there is an impact, it brings the center of gravity of the cage  111  and of the striker element  111   2  to the rotational axis AR and so that this rotational axis is one of its main axes of inertia. 
     To this end, the first counterweight  111   3  is configured to cancel out, by counterbalancing, the efforts radial to the rotational axis AR induced by the cage  111  and the striker element  111   2  i.e., the forces oriented along the X, Y axes of the referential system. 
     It must be noted that, in a Maurer single-hammer type rebound impact mechanism  110 , the cage  111  by itself, i.e., without the striker element  111   2 , presents a center of gravity on the rotational axis AR and that this rotational AR is one of its main inertia axes. This observation results in a simplification of the configuration of the first counterweight  111   3 , namely bringing back only the center of gravity of the striker element  111   2  to the rotational axis AR and means that this rotational AR is one of its main axes of inertia. 
     Besides, the square drive  112  bears second counterweights  112   2  configured so that, when there is an impact, they bring back the center of gravity of the strike reception element  112   1  to the rotational axis AR and so that this rotational axis AR is one of its main axes of inertia. 
     To this end, the second counterweight  112   2  is configured to cancel out, by counterbalancing, the efforts radial to the rotational axis AR induced by the strike reception element  112   1 , i.e., the efforts oriented along the X, Y axes of the referential system. 
     In the example illustrated, to cancel out the efforts radial to the rotational axis AR induced by the striker element  111   2 , the first counterweight  111   3  is positioned on the cage  111  opposite the striker element  111   2  and facing it. The sizing of the first counterweight  111   3  is a function of the sizing of the striker element  111   2 , and hence of the efforts radial to the rotation AR induced by of this striker element. 
     In addition, similarly to the first counterweight  111   3 , to cancel out the efforts radial to the rotational axis AR induced by the strike reception element  112   1 , the second counterweights  112   2  are positioned on the square drive  112  diametrically opposite the strike reception element  112   1 . The sizing of the second counterweights  112   2  is a function of the sizing of the strike reception element  112   1 , and therefore of the forces radial to the rotational axis AR induced by this element. 
       FIG. 2  illustrates a second example of the first embodiment of the proposed technique applied to a Maurer single-hammer type rebound impact mechanism. 
     This second example of an embodiment is distinguished from the first one solely by the configuration of the counterweights. 
     In this second example of an embodiment, the Maurer single-hammer type rebound impact mechanism  200  comprises a single counterweight  211   3  configured so that during an impact, the center of gravity G 2  of the Maurer single-hammer type rebound impact mechanism is positioned on its rotational axis AR and so that the rotational axis AR of the impact mechanism is one of its main axes of inertia, the Z axis. 
     The single counterweight  211   3  is therefore configured so that, when there is an impact, it brings back the center of gravity of the striker element  211   2  and of the strike reception element  212   1  to the rotational axis AR and so that this rotational axis AR is one of the main axes of inertia. The single counterweight  211   3  is therefore configured to cancel out, in counterbalancing, the efforts radial to the rotational axis AR induced by the striker element  111   2  and the strike reception element  212   1 , i.e., the efforts oriented along the X, Y axes of the referential system. 
     As illustrated, the single counterweight  211   3  is positioned on the cage  211  at 180 to the striker element  211   2  (and therefore to the strike reception element  212   1  during the impact). The sizing of the single counterweight  211   3  is a function of the sizing of the striker element  211   2  and strike reception element  112   1 , and therefore the efforts radial to the rotational axis AR induced by these elements. 
       FIG. 3  illustrates a second example of the first embodiment of the proposed technique applied to a Maurer twin-hammer type rebound impact mechanism. 
     This third example of the embodiment is distinguished from the first one by the structure of the impact mechanism and by the configuration of the counterweights. 
     Classically, a Maurer twin-hammer type rebound impact mechanism is distinguished from a Maurer single-hammer type rebound impact mechanism by the implementing of two striker elements that are diametrically opposite and off-centered relatively to each other. 
     Thus, the Maurer twin-hammer type rebound impact mechanism  310 , disposed in a X, Y, Z referential system, comprises a cage  311  with which a square drive  312  is mounted rotationally mobile, by means of a holding portion  311   1 . 
     The Maurer twin-hammer type rebound impact mechanism  310  is furthermore mounted so as to be rotationally mobile, by means of the bearings  310   1 , within the casing  300  of the impact wrench. 
     The cage  311  of the Maurer twin-hammer type rebound impact mechanism  310  is intended to be rotationally driven by a motor (not visible). 
     The motor, the cage  311  and the square drive  312  are coaxial along a rotational axis AR oriented along the axis of the reference system. 
     The cage  311  is equipped with a striker system comprising two striker elements  311   2A ,  311   2B  situated at 180° relatively to each other and disposed in different planes radial to the rotational axis AR. Each striker element  311   2A ,  31128  has a strike surface intended to be brought into collision against a strike reception surface belonging to a strike reception element  312   1A ,  312   1B  respectively, borne by the square drive  112 . 
     More specifically, the striker elements  311   2A ,  31128  are mobile in planes radial to the rotational axis AR, i.e., in planes oriented along the X, Y axis (perpendicular to the Z axis), so that they can take the following positions:
         a disengaged position in which the striker element  311   2A ,  311   2B  can rotate without coming into contact with the square drive  312 , and   an engaged position, in which the strike surfaces of the striker elements  311   2A ,  311   2B  come into contact with the strike reception surfaces of the strike reception elements  312   1A ,  312   1B  of the square drive  312 .       

     In accordance with the first embodiment of the proposed technique, the Maurer twin-hammer type rebound impact mechanism  310  comprises several counterweights  311   3A ,  311   3B ,  312   2A ,  312   2B  configured so that, during an impact, the center of gravity G 3  of the Maurer twin-hammer type rebound impact mechanism  310  is positioned on its rotational axis AR and so that the rotational axis AR of the impact mechanism is one of its main axes of inertia, the Z axis. 
     More specifically, the cage  111  carries two first counterweights  311   2A ,  31128  configured, during an impact, to bring the center of gravity of the striker elements  311   2A ,  31128  back to the rotational axis AR and configured so that this rotational axis AR is one of its main axes of inertia. 
     The first counterweights  311   3A ,  311   3B  are configured, in counterbalancing, to cancel out the forces radial to the rotational axis AR induced by the striker elements  311   2A ,  311   2B , i.e., the forces oriented along the X, Y axes of the referential system. 
     To this end, each striker element  311   2A ,  311   2B  is associated with a first counterweights  311   3A ,  311   3B , fixedly attached to the cage  311  which is diametrically opposite to it. The sizing of the first counterweights  311   3A ,  311   3B  is a function of the sizing of the striker elements  311   2A ,  311   2B , and therefore of the efforts radial to the rotational axis AR induced by these elements. 
     In addition, the square drive  312  bears two second counterweights  312   2A ,  312   2B  configured so that, when there is an impact, they bring the center of gravity of the strike reception elements  312   1A ,  312   1B  back to the rotational axis AR and so that this rotational axis AR is one of its main axes of inertia. 
     The second counterweights  312   1A ,  312   1B  are configured, in counterbalancing, to cancel out the efforts radial to the rotational axis AR induced by the strike reception elements  312   1A ,  312   1B , i.e., the forces oriented along the X, Y axes of the referential system. 
     To this end, each strike reception element  312   1A ,  312   1B  is associated with the second counterweights  312   2A ,  312   2B , fixedly attached to the square drive  312  which is diametrically opposite to it. The sizing of the second counterweights  312   2A ,  312   2B  is a function of the sizing of the strike reception elements  312   1A ,  312   1B , and therefore of the efforts radial to the rotational axis AR induced by these elements. 
     6.4. Description of a Second Embodiment 
       FIGS. 4 to 11  are representations of several embodiments of a Maurer type impact mechanism according to a second embodiment of the proposed technique. 
     The principle of this second embodiment of the proposed technique consists in proposing an impact mechanism comprising a striker system equipped with at least three striker elements, having identical cross-sections disposed so that, during the impact in particular, the center of gravity of the impact mechanism is positioned on its rotational axis and so that the rotational axis of the impact mechanism is one of its main axes of inertia. 
     The inventor of the present application has observed that, when striker elements have identical cross-sections, the inertia and the relative mass of each striker element are solely proportional to its thickness. This observation stems from the fact that, when disposed in a cage, the striker elements have the same radial position of their center of gravity at each point in time. 
     This second embodiment is therefore based on obtaining anti-vibration means by means of a special arrangement of at least three striker elements with identical cross-sections. 
     More specifically, this particular layout consists in positioning two lateral striker elements having identical thickness values, on either side, along the rotational axis of the impact mechanism (i.e., along the rotational axis of the impact mechanism), of a central striker element having a total value of thickness equal to twice the value of thickness of a lateral striker element. In addition, the at least one central striker element is diametrically opposite to the two lateral striker elements relatively to the rotational axis of the impact mechanism. 
     This second embodiment further reduces the level of vibrations perceived by the user. Indeed, this second embodiment reduces or even eliminates the level of vibration perceived by the user during the different phases of operation of the impact mechanism. 
     This is because, according to this embodiment, the rotational axis of the impact mechanism is permanently one of the main axes of inertia of the impact mechanism. For example, the central striker element comprises several juxtaposed elementary central striker elements, at least two for example, of which the sum of the thickness along the axis of the elementary central striker elements defines the total value of thickness. 
       FIG. 4  is a schematic representation of a first example of the second embodiment of the proposed technique applied to a Maurer type rebound impact mechanism. 
     Classically, the Maurer type rebound impact mechanism  410  disposed in an X, Y, Z referential comprises a cage  411  within which a square drive  412  is mounted rotationally mobile, by means of a holding portion  411   1 , on a square drive  412 . 
     The Maurer type rebound impact mechanism  410  is further mounted to be mobile in rotation, by means of bearings  410   1 , within the casing  400  of the impact wrench. 
     The cage  411  of the mechanism  410  is intended to be driven in rotation by a motor (not shown). The motor, the cage  411  and the square drive  412  are coaxial along a rotational axis AR oriented along the Z axis of the referential system. 
     In accordance with the second embodiment of the proposed technique, the impact mechanism  410  or more specifically the cage  411  bears three striker elements  411   2A ,  411   2B ,  411   2C , having identical cross-sections, disposed so that the center of gravity G 4  of the impact mechanism  410  is positioned on its rotational axis AR, and so that the rotational axis AR of the impact mechanism is one of its main axes of inertia. 
     To this end, the impact mechanism  410  has two identical lateral striker elements  411   2A ,  411   2C  disposed, along the rotational axis AR of the impact mechanism, on either side of a central striker element  411   2B  constituted by a single elementary central striker element. The three striker elements  411   2A ,  411   2B ,  411   2C  are disposed in three distinct planes radial to the rotational axis AR. 
     In addition, the central striker element  411   2B  is opposite by 180° to the two lateral striker elements  411   2A ,  411   2C  relatively to the rotational axis AR of the impact mechanism. 
     Besides, each lateral striker element  411   2A ,  411   2C  has a first value of thickness e. The central striker element  411   2B  has a second value of thickness  2   e . The second value of thickness  2   e  is equal to twice the first value of thickness e. 
     Similarly, the square drive  412  bears three strike reception elements  412   1A ,  412   1B ,  412   1C  configured to cooperate with the three striker elements  411   2A ,  411   2B ,  411   2C  respectively. 
     More specifically, the striker elements  411   2A ,  411   2B ,  411   2C  are mobile in planes radial to the rotational axis AR, also called transversal and/or normal planes, i.e., planes oriented along the X, Y axes (perpendicular to the Z axis) so that they can take the following positions:
         a disengaged position, in which the striker elements  411   2A ,  411   2B ,  411   2C  can rotate without coming into contact with the square drive  412 , and   an engaged position, in which the strike surface of the striker elements  411   2A ,  411   2B ,  411   2C  come into contact with the strike reception surfaces of the strike reception elements  412   1A ,  412   1B ,  412   1C  of the square drive  412 .       

       FIG. 5  is a view in perspective or three-quarter view of a modelling of a Maurer type rebound impact mechanism according to the first example of the second embodiment described with reference to  FIG. 4 . 
       FIG. 6  is a longitudinal sectional view of the modelling of a Maurer type rebound impact mechanism of  FIG. 5 . 
       FIG. 7  illustrates a second embodiment of the proposed technique applied to a Maurer type rebound impact mechanism. 
     This second embodiment is distinguished from the first one by the number and structure of the striker elements. 
     In this second exemplary embodiment, the impact mechanism  710  comprises four striker elements  711   2A  a  711   2D  having identical cross-sections and values of thickness. 
     Thus, in order that the center of gravity G 7  of the impact mechanism  710  be positioned on its rotational axis AR and in order that the rotational axis AR of the impact mechanism  710  be one of its main axes of inertia, two juxtaposed elementary striker elements  711   2A ,  711   2D  are positioned on either side, along the rotational axis AR of the impact mechanism, of a central striker element constituted by two elementary central striker elements  711   2B ,  711   2C . 
     The four striker elements  711   2A  to  711   2D  have an identical value of thickness, the total value of thickness of the central striker elements  711   2A ,  711   2D  being therefore equal to twice the value of thickness of each lateral striker element  711   2A ,  711   2D . 
     The identical implementation of four striker elements  711   2A  to  711   2D  minimizes the cost of manufacture of an impact wrench according to the proposed technique. 
       FIG. 8  illustrates a third example of the second embodiment of the proposed technique applied to a Maurer type rebound impact mechanism. 
     This third example of an embodiment is distinguished from the first by the number and structure of the striker elements. 
     In this third example of an embodiment, the impact mechanism  810  comprises five striker elements  811   2A  to  811   2E  namely:
         a central striker element constituted by three juxtaposed, elementary central striker elements  811   2B  to  811   2D , respectively having a first value of thickness e, defining a total value of thickness e T  corresponding to the sum of the three values of thickness of the three central elementary central striker elements  811   2B  to  811   2D ,   two identical lateral striker elements  811   2A ,  811   2E  respectively having a second value of thickness e T/2  corresponding to half of the total value of thickness defined by the plurality of central striker elements  811   2B  to  811   2D .       

     Thus, the principle of the second embodiment of the proposed technique is complied with and enables the center of gravity G of the impact mechanism to be positioned on its rotational axis and the rotational axis of the impact mechanism to be one of its main axes of inertia. 
     Thus, it can be seen that the proposed technique is versatile and can be applied to various types of impact mechanisms. 
     6.5. Various Aspects 
     In the light of the prerequisites described here above, it can be seen that the proposed technique is not intended to be applied to impact wrenches comprising impact mechanisms with components mobile in translation along the rotational axis of the square drive, such as “pin clutch” or “two jaws” type mechanisms in particular. Also, it is not the purpose of the proposed technique to be applied to impact wrenches comprising impact mechanisms that generate an impact more than once per turn of the square drive, such as the “double rocking dog” type mechanism for example, which works on a half-turn of the square drive between each impact. 
     The proposed technique is nevertheless intended to be applied to impact wrenches implementing the Maurer single-hammer, Maurer twin-hammer, and single rocking dog mechanisms. 
     Besides, the proposed technique has been described and illustrated in choosing a theoretical model of the different components constituting the impact wrench: non-deformable bodies, non-existence of clearances in the links, etc. These theoretical modifications eliminate the vibrations perceived by the user in proposing an impact mechanism configured so that, at least during the impact, its center of gravity G is positioned on its rotational axis and so that the rotational axis follows one of the main axes of inertia. 
     However, it can be the case that this theoretical modelling is not representative of the real structure of the components. For example, certain components can show deformations according to the level of wear and tear of these components. 
     Thus, it is appropriate to tend towards this theoretical configuration by the implementing of an impact mechanism configured so that, during the impact, its center of gravity G is appreciably positioned on its rotational axis and so that the rotational axis is appreciably one of its main axes of inertia. 
     Tending towards this theoretical configuration makes it possible to reduce the transversal efforts to portions of the casing holding the mechanical system (bearing surfaces of the casing), when the resultant impact, between the striker system and the square drive tends towards a torque directed along the rotational axis. This leads, if not to an elimination, to a major reduction in the level of vibrations perceived by the user. 
     To this end, those skilled in the art could clearly adapt the shapes, sizes and materials of the components of the impact mechanism such as the square drive, the hammers, the counterweights in particular, to make it compliant with the proposed technique. 
     It can be therefore be seen that the proposed technique is not limited to the embodiments described here above and provided purely by way of an example. They encompass different modifications, alternative forms and other variants that could be envisaged by those skilled in the art in the context of the problems and issues posed and in particular they include all combinations of the different embodiments described here above that could be taken separately or in association. 
     An exemplary embodiment of the present disclosure is aimed especially at providing an efficient solution to at least some of the different problems discussed above. 
     At least one embodiment optimizes the impact wrenches with impact mechanism. 
     In particular, at least one embodiment provides an impact wrenches with impact mechanism which, during the phase of impact in particular, reduces or even eliminates the level of vibrations perceived by the user of such a wrench. 
     In particular, at least one embodiment provides such an impact wrench that is simple of design and/or simple to implement. 
     At least one embodiment provides an impact wrench of this kind that is lightweight and/or compact.