Abstract:
The invention concerns a blind rivet (R) of the type consisting of a deformable bushing ( 300 ) and a cleavable mandrel ( 400 ), the thus formed assembly being urged to be positioned and pass through the orifices ( 110  and  120 ) made substantially coaxial and provided in at least two elements ( 100  and  200 ) to be assembled. Said rivet is characterized in that the bushing ( 300 ) consists of one single piece but preformed to have a second end ( 320 ) in two parts ( 330  and  340 ) which, under the action of an axial load (arrow F), are cleaved apart so that one part of the bushing ( 330 ) moved by the tensile load of the mandrel ( 400 ) is urged to be opened around the fixed part of the bushing ( 340 ), the expansion of the mobile part ( 330 ) called sleeve being produced without folding. The invention also concerns a method for inserting such a rivet. The invention is applicable to rivet fixing.

Description:
FIELD OF APPLICATION OF THE INVENTION 
       [0001]    The present invention relates to the field of blind rivet type fixings and in particular to the adaptations making it possible to distribute the clamping forces of these rivets under the best conditions. 
       DESCRIPTION OF THE PRIOR ART 
       [0002]    Conventionally, a blind rivet comprises a tubular sleeve and a pulling mandrel passing through said sleeve. The rivet is placed in a hole passing through the elements to be assembled. The sleeve comprises on a first end a head leaning on a first element. The sleeve comprises a second, non-shouldered, end that extends beyond the second element. 
         [0003]    Pulling the mandrel provides deformation of the non-shouldered end of the sleeve so as to clamp the elements between the shoulder of the sleeve and its deformed end (bulb). The surface area of the bulb in contact with the second element determines the distribution of the clamping forces of the rivet on the elements to be assembled. 
         [0004]    Enlarging this contact surface area is a constant concern of rivet designers in order to optimize the distribution of the clamping forces. 
         [0005]    The prior art offers sleeves in two parts, that is to say whereof the non-shouldered end is divided into two separate parts, namely:
       a first stationary part; and   a second movable part that moves and deforms under the pulling force over and around the stationary part in order to finally offer, by opening out over the stationary part, an enlarged surface area for the distribution of forces on the second element.       
 
         [0008]    The main drawback of this type of rivet is that it requires the manufacture and management of a rivet comprising three elements. 
         [0009]    The prior art also offers sleeves in a single part but whereof the non-shouldered end has bending grooves in order that, once subjected to the pulling force of the mandrel, the wall of the sleeve bends in order to double the support surface created by the bulb. 
         [0010]    The main drawback of the bending technique is not avoiding the final breaking of the bulb into two parts and requiring more material. 
         [0011]    The document GB 1 604 502 describes a blind rivet of the type of that consisting of a deformable sleeve and a divisible mandrel, the assembly thus formed being positioned and passing through the apertures made substantially coaxial and formed in at least two elements to be assembled, the sleeve comprising two ends: 
         [0012]    a first end comprising a preformed head for defining a first support surface on the external surface of a first element, and 
         [0013]    a second end which, under the action of deformation by pulling on the mandrel, comes to define a second support surface on the external surface of the second element, 
         [0014]    prolonging the force having the effect of bringing the two support surfaces closer together and consequently bringing closer together the two elements to be assembled. 
         [0015]    The sleeve consists of a single piece, but is preformed in order to have a second end in two parts which, under the action of an axial force, whereof a first part opens out over the sleeve part that remained stationary and then separates from the latter by breaking, the expansion of the movable part referred to as a collar being done with internal bending of its end onto itself in order to form a flange. 
       DESCRIPTION OF THE INVENTION 
       [0016]    Starting from this established fact, the applicants carried out research aimed at proposing a blind rivet capable of solving the problems of distribution of the clamping forces by equipping the blind rivet with an improved sleeve which, under the action of the pulling force generated by the mandrel, deforms to offer a larger contact surface of the bulb with the metal sheet. 
         [0017]    The blind rivet of the invention is of the type of that consisting of a deformable sleeve accommodating a divisible mandrel, 
         [0018]    the assembly thus formed being positioned and passing through the apertures made substantially coaxial and formed in at least two elements to be assembled, 
         [0019]    the sleeve comprising two ends: 
         [0020]    a first end comprising a preformed head for defining a first support surface on the external surface of a first element, and 
         [0021]    a second end which, under the action of deformation by pulling on the mandrel, comes to define a second support surface on the external surface of the second element, 
         [0022]    prolonging the force having the effect of bringing the two support surfaces closer together and consequently bringing closer together the two elements to be assembled. 
         [0023]    In accordance with the invention, the rivet is remarkable in that the sleeve consists of a single piece, but is preformed in order to have a second end divided into two parts connected by an easily broken part and which, under the action of an axial force, separate by breaking of the easily broken part so that a sleeve part set in motion by the pulling force of the mandrel opens out around the sleeve part that remained stationary, the expansion of the movable part referred to as a collar being done without bending. The rivet of the invention therefore changes from a configuration where it consists of two distinct pieces (a mandrel and a sleeve) to, once fitted, a configuration where it consists of three pieces (mandrel, crimping sleeve, and clamping sleeve). 
         [0024]    This rivet is particularly advantageous in that it offers an enlarged support surface, as a result of the opening out, for transmission of the pulling force which contributes towards better clamping by the mandrel. This mandrel is preformed with a plurality of serrations onto which the head of the sleeve is crimped which contributes towards maintenance of the clamping by the mandrel. 
         [0025]    The rivet of the invention provides this function without pre-cutting its sleeve into two parts. This is because the sleeve of the rivet of the invention is in a single part and it is only under the pulling force produced on the mandrel that the two parts of the non-shouldered end of the sleeve separate. Packaging and use of this rivet are greatly facilitated thereby since only two elements have to be managed. 
         [0026]    The phases of fitting of the rivet of the invention are as follows:
       pulling exerted on the mandrel   breaking of the sleeve into two pieces   opening out and leaning of the clamping sleeve on one element   crimping of the crimping sleeve in the grooves or serrations of the mandrel   breaking of the mandrel at its fracture groove.       
 
         [0032]    In order to carry out this opening out, the sleeve is formed internally and externally with fracture areas and conical areas. Similarly, the head of the mandrel and the non-shouldered end of the sleeve with which it is brought into contact adopt an inclined contact surface so as to distribute the pulling force well. 
         [0033]    To do this, the end of the movable part of the sleeve opening out over the stationary sleeve part is advantageously preformed internally with a conical shape which slides over the stationary part. Similarly, the end of the stationary sleeve part over which the collar opens out adopts a conical external shape. 
         [0034]    Although the document GB 1 604 502 effectively describes a sleeve in a single piece, it does not describe breaking prior to the opening out. Moreover, this document describes a bulb, that is to say that the movable part of the sleeve will form a bulge. This bulge consists of a consequent radial extension towards the outside of the movable part of the sleeve with a bend area distant from the end thereof in order to finally form a support flange. 
         [0035]    Apart from the fact that the creation of a bulge-based bulb is not the effect sought in the rivet of the invention, it seems important to recall the technical effect sought in the type of rivet of the invention, namely that the blind part must have substantially the same height from one rivet to another, and from one thickness to be clamped to another. Thus, the opening-out part is not deformed in order to avoid the creation of a bulge whereof the variation is liable to make the height of the fitted blind part vary. In the invention, this requirement is satisfied by the fact that the sleeve is initially in a single piece and (consequently) in one and the same material. Locking is achieved by crimping the sleeve head onto the serrations provided for that purpose in the mandrel. A constant height of the blind part guarantees good clamping irrespective of the thickness to be clamped. 
         [0036]    In the invention, the displacement of the mandrel depends on the thickness to be clamped, which makes it possible to obtain good crimping irrespective of where located in the clamping range of the fixing. The breaking before opening out defined by the wording of the first claim plays a part in maintaining the height of the movable part of the sleeve by avoiding any deformation before breaking. Moreover, in the invention, the deformation following breaking is only an opening out around the stationary part and does not require a consequent bending liable to modify the height of the movable part or make the limit of resistance to elastic deformation thereof be exceeded. Crimping is carried out before the pulling force exceeds the limit of resistance to elastic deformation of the opened-out movable part and therefore before this part varies in height. 
         [0037]    The breaking before opening out also avoids too great an amplitude of deformation before clamping, an amplitude which would have as a consequence the formation of cracks in the metal. Thus, another object of the invention relates to a fitting method which is remarkable in that it consists of providing the breaking of the sleeve into two parts before starting the expansion of the movable part. 
         [0038]    Moreover, in order to guarantee a constant height for the blind part of the fixing, the method is remarkable in that it consists of crimping the head of the sleeve on the serrations of the mandrel once the movable part of the sleeve is leaning on the element to be fixed and before the exceeding of a pulling force corresponding to the maximum limit of resistance to elastic deformation of said movable part once opened out. 
         [0039]    With the fundamental concepts of the invention having just been described above in their most elementary form, other details and characteristics will emerge more clearly from a reading of the following description and with reference to the accompanying drawings, giving by way of a non-limiting example one embodiment of a rivet in accordance with the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]      FIG. 1  is a schematic drawing of a sectional view of one embodiment of a rivet in accordance with the invention and installed in a hole passing through two elements; 
           [0041]      FIGS. 2   a ,  2   b ,  2   c  illustrate the deformation of the rivet illustrated in  FIG. 1 . 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0042]    As illustrated in the drawing of  FIG. 1 , the blind rivet referenced R in its entirety provides the assembling of at least two elements  100  and  200 . This rivet R is an assembly consisting of a deformable sleeve  300  and a divisible mandrel  400 . As illustrated, the assembly thus formed is positioned and passes through two apertures  110  and  210  made substantially coaxial and formed in the elements  100  and  200  to be assembled. 
         [0043]    The sleeve  300  comprises two ends:
       a first end  310  comprising a preformed protruding head for defining a first support surface  311  on the external surface  120  of a first element  100 , and   a second end  320  which, under the action of deformation by pulling on the mandrel  400 , comes to define a second support surface on the external surface  220  of said second element  200 .       
 
         [0046]    Prolonging the force has the effects of bringing the two support surfaces closer together and as a result bringing closer together and fixing the two elements  100  and  200  to be assembled. 
         [0047]    As illustrated, the mandrel  400  has two ends:
       a first end comprising a head  410  which comes to lean on the sleeve  300  during the pulling force in order to deform it to create a bulb, and   a second end with which the pulling module of the fitting tool comes to be associated.       
 
         [0050]    In accordance with the invention, the sleeve  300  consists of a single piece, but is preformed to have a second end in two parts which, under the action of an axial force, separate by breaking so that a sleeve part  330  set in motion by the pulling force of the mandrel  400  opens out around the sleeve part  340  that remained stationary, the expansion of the movable part  330  referred to as a collar being done without bending. 
         [0051]    As illustrated by the drawing of  FIG. 1 , the collar  330  is connected to the rest of the sleeve by means of an easily broken part or fracture area  331 . 
         [0052]    Moreover, the end of the collar  330  that comes to open out over the stationary sleeve part  340  is advantageously preformed internally with a conical shape  332  that slides over the external conical shape  341  of the stationary part  340 . 
         [0053]    According to a preferred but non-limiting embodiment, the angle formed by the surfaces  332  and  341  belonging respectively to the collar  330  and the stationary part of the sleeve  340  and coming into contact during the opening out was the subject of a study so as to optimize the opening out of the sleeve in order to avoid in particular the creation of cracks. Thus, according to a preferred embodiment, the angle A 1  formed by the conical surface  341  belonging to the stationary part of the sleeve lies between 14 and 19 degrees, and the angle A 2  formed by the conical surface  341  belonging to the movable part or collar  330  lies between 5 and 10 degrees. According to another embodiment, the angle A 1  lies between 10 and 19 degrees. 
         [0054]    The correct determination of these angles allows the movable part forming the clamping collar to slide over the sleeve once breaking has occurred. Moreover, it makes it possible to reduce the frictional force between the two parts of the sleeve. There is therefore no obstacle to the sliding of the movable part which will only get bigger without its edges turning over. 
         [0055]    Moreover, in order to optimize the pulling forces, the end of the collar  330  coming into contact with the head  410  of the mandrel  400  is preformed with a conical surface  333  coming into correspondence with a conical surface  411  made on the head  410  of the mandrel  400 . 
         [0056]    According to one embodiment, the angle of the tapered surfaces ( 333  and  411 ), belonging respectively to the collar ( 330 ) and the mandrel head ( 410 ) and remaining in contact during the riveting, is equal for the two tapered surfaces to approximately 25 degrees. 
         [0057]    According to another preferred but non-limiting embodiment, the angle A 3  of the tapered surface  411  belonging to the collar is equal to approximately 25 degrees. The angle A 4  of the tapered surface  333  belonging to the mandrel and remaining in contact during riveting with the tapered surface  411  is equal to approximately 18 degrees. 
         [0058]    This angle improves the positioning of the sleeve  300  on the mandrel  400  and avoids any slipping out of the sleeve during fitting. The difference in angle makes it possible to carry out auto-centring of the pieces coming into contact. 
         [0059]    According to another particularly advantageous characteristic of the invention, the sleeve  300  is made from an aluminium alloy such as for example the alloy known by the designation 2017 T4 whereof the mechanical strength lies between 420 and 520 MPa. The sleeve can of course be envisaged in other materials such as those known by the following designations:
       2024T4   304 stainless steel   titanium/niobium   etc.       
 
         [0064]    According to another particularly advantageous characteristic of the invention, the mandrel  400  has a mechanical strength lying between 1500 and 2000 MPa. According to another characteristic, the mandrel  400  has a mechanical strength lying between 1200 and 2000 MPa. 
         [0065]    According to a preferred embodiment, this mandrel is made from stainless steel known by the stainless steel designation AISI 302. According to another preferred embodiment, this mandrel is made from stainless steel known by the stainless steel designation AISI 304. According to another embodiment, the mandrel is made from a strain-hardened stainless steel. This material has a high modulus of elasticity which minimizes the elastic relaxation after breaking of said mandrel. 
         [0066]    This mandrel can of course be made from other materials such as titanium known by the designation TA6V. 
         [0067]    Moreover, for certain material, an annealing heat treatment is also provided in order to increase the stiffness of the material and thus reduce its elastic relaxation. This is because the applicants have the objective of reducing the elastic relaxation of the mandrel in order to reduce the stress relief in the elements to be assembled after breaking which has the effect of increasing the clamping. Thus, the holding force of the fixing once fitted lies between 50 and 70% of the pulling force exerted by the mandrel. 
         [0068]    The association of the two materials used respectively for the sleeve and for the mandrel contributes towards the setting up of an installed tension. 
         [0069]      FIGS. 2   a ,  2   b  and  2   c  show, in sectional view, the progressive deformation of the collar  330  after the sleeve  300  has split into two parts. 
         [0070]    The first force F to which the sleeve  300  is subjected breaks the fracture area  331  (see  FIG. 2   a ). Continuation of the force F ensures the opening out of the collar  330  around the stationary part  340  of the sleeve  300  (see  FIG. 2   b ). Moreover, continuation of the pulling force F continues the opening out until the collar covers the stationary part  340  and comes into contact with the element to be assembled  200  (see  FIG. 2   c ). The sleeve head  310  will then be crimped onto the serrations  420  of the mandrel  400 . The disposition of these serrations  420  is particularly important in that they guarantee the crimping of the head  310  irrespective of the thickness to be clamped. This is because, in the invention, the height of the blind part remains constant on account of the fact that, once opened out and in contact with the element to be fixed  200 , the movable part  330  of the sleeve  300  does not deform any more. 
         [0071]    As illustrated, the support surface of the bulb formed by the deformation of the non-shouldered end of the sleeve  300  of the rivet R in accordance with the invention is greatly enlarged thus meeting the requirements of the users of such rivets. 
         [0072]    The forces taking part in the method for fitting this rivet can be broken down as follows:
       F 1 =pulling force on the mandrel   F 2 =resistance-to-tearing-off force of the fixing   F 3 =installed tension of the fixing   F 4 =force for breaking the sleeve into two parts   F 5 =force for opening out the movable sleeve part over the stationary part of the sleeve   F 6 =extrusion start force of the crimping grooves       
 
         [0079]    The method of the invention is remarkable in that it consists of crimping the head of the sleeve onto the serrations of the mandrel once the movable part of the sleeve is leaning on the element to be fixed and before exceeding of a pulling force corresponding to the maximum limit of resistance to elastic deformation of said movable part once opened out. According to the breakdown of the forces made above, the method is summarized by the following inequalities: F 4 &lt;F 5 &lt;F 6 &lt;F 1 . 
         [0080]    Continuation of the pulling force causes breaking of the mandrel at the fracture groove provided for that purpose. 
         [0081]    It should be understood that the rivet R and its fitting method which have just been described and depicted above, have been so with a view to a disclosure rather than a limitation. Of course, various arrangements, modifications and improvements can be made to the example above, without for all that departing from the scope of the invention. 
         [0082]    Thus, for example, geometrical modifications are capable of being made, most particularly if a version of this rivet is developed with swelling (the mandrel makes the sleeve swell by a few hundredths of a millimetre in order to introduce radial and tangential stresses into the hole for increasing fatigue resistance). 
         [0083]    Also, depending on the materials used, a surface treatment is performed for the purposes of protecting against corrosion. Moreover, the sleeves can be lubricated in order to reduce the forces due to friction in particular when the collar slides over the stationary part of the sleeve and during extrusion of the sleeve into the fixing nose of the tool.