FR2780457A1 - Fixing screw between synthetic material and metal piece, especially in vehicles - Google Patents

Abstract

The screw with incorporated spacer for the fixing of a synthetic material (A) to a metallic base (B) is made up of a screw proper (50) with an incorporated shouldered spacer (53) where its height is slightly less than the thickness of the piece (A). The shank of the screw contains a non-threaded section (54) of a length (ho) and a threaded section (55). The non-threaded section can deform under load such that the pre-loading of the screw reduces its tendency to become unscrewed when subjected to vibrations. To maintain the screw on the first piece (A), the first piece (A) contains retaining feet (56) that are integral with the first piece.

Description

SCREWS FOR FIXING PIECES, PARTICULARLY ONE PIECE

  SYNTHETIC MATERIAL ON A METAL PART, AND DEVICE

  FIXING SYSTEM COMPRISING SUCH A SCREW.

  The present invention relates to a screw for fixing a first part, for example of synthetic material to a second part, for example of metal, these parts being placed in an environment subjected to mechanical and / or thermal stresses, particularly vibrations, especially in a vehicle, and more

  particularly a motor vehicle.

  The invention also relates to a device for fixing a first part by

  example in synthetic material, on a second piece for example in metal.

  It is imperative, in the automotive or aviation field, or in other fields where the environment subjects parts to very strong mechanical and / or thermal stresses, that the parts fixed by means of screws, cannot free themselves from their support precisely because of these constraints. When two parts are fixed to each other by means of screws, the screws must therefore not be able to loosen during the whole life of the part. It frequently happens that the parts are of a different nature, for example a plastic part to be fixed on a metal support part. Vibrations, changes in

  temperature, result in a tendency for the screws to loosen.

  To avoid loosening of the screws, conventional screws are usually used in combination with a spacer, which makes it possible to meet the requirements imposed

  by the usual specifications of the automotive industry.

  However, this practice has drawbacks. It requires the installation of a spacer then of the screw, that is to say the management of two separate parts where

  risks of forgetting, and a complicated and expensive assembly process.

  The present invention proposes to overcome these drawbacks by offering a new screw which combines the functions of a conventional screw and a conventional spacer. It also offers a fixing device comprising such a screw and making it possible to make

this captive screw.

  For this purpose, the screw according to the invention is characterized in that it comprises a

integrated spacer.

  Advantageously, this integrated spacer has a height slightly

  less than the thickness of said first part.

  According to a preferred embodiment, said integrated spacer is full. In another variant, the integrated spacer is hollowed out. In another embodiment,

  the integrated spacer is attached and held by lugs.

  Advantageously, the screw according to the invention comprises a non-threaded section and a threaded section. In a preferred manner, said non-threaded section is of length h0, said length h0 being deformable and having, according to the variants, the most

great possible.

  In the variant with a hollowed-out integrated spacer, said recess is provided around the non-threaded section. Another recess may be provided in the center of said section not

  threaded so as to reduce its section.

  In a particularly advantageous manner, the screw according to the invention is made of a

single piece.

  The screw according to the invention may also include a washer placed by lugs. The invention also relates to a fixing device, characterized in that it comprises a fixing screw as defined above, said screw further having a straight non-threaded section between its circular base and its integrated spacer, and in that that the first part comprises retaining means arranged to hold the screw mounted in said first part before it is fixed to the second part. The screw may comprise a washer below the integrated spacer, the retaining means being arranged to retain the washer and the screw mounted in said

  first part before fixing it on the second part.

  In a particularly advantageous manner, the retaining means may consist of at least two retaining tabs provided on the inner periphery of the

  first piece or a bead provided inside the first piece.

  Preferably, the retaining means are formed in one piece with the first piece. In order not to hinder the contact of the first part with the second part, the height between the base of the first part and the retaining means is greater than the height measured between the lower end of the threaded section and the upper edge of

  the integrated spacer or washer.

  The present invention and its advantages will be better understood in the description

  following of several embodiments given by way of nonlimiting example and with reference to the appended drawings, in which: - Figure 1 shows a view in axial section illustrating a first embodiment of the screw according to the invention, - the FIG. 2 is a view representing a graph representing the tension force of a screw as a function of the deformable length h0, - FIG. 3 is a view in axial section illustrating a second embodiment of the screw according to the invention, - Figure 4 is a top view of the screw shown in Figure 3, - Figures 5 to 7 are views in axial section illustrating other embodiments of the screw according to the invention, - Figures 8 and 11 represent before fixing a fixing device according to the invention with another embodiment of the screw, - Figures 9 and 12 are views of the device respectively according to Figures 8 and 11 after fixing, - Figure 10 is a v ue from above of the first part, - Figure 13 shows after fixing, another embodiment of the fixing device according to the invention, and

  FIG. 14 represents a top view of the screw according to FIG. 13.

  Referring to Figure 1, the screw 10, called shoulder screw, is intended to fix a part A, for example of synthetic material, on a part B, for example of metal. It comprises a head 11 provided with a circular base 12 whose diameter is greater than that of the head, a full integrated spacer 13, a cylindrical non-threaded section 14 of length h0 and a threaded end section 15. The spacer integrated 13 defines a shoulder 13 'which constitutes a bearing surface on the part B. The height of the solid integrated spacer 13 is preferably slightly less than the thickness of the part A. This dimensional difference is of the order a few tenths of a millimeter, so that, when the screw is tightened, the part A is slightly crushed, without this deformation inducing a break or the initiation of a break. The cylindrical section 14 of length h0 represents the part deformable by elongation of the screw during tightening, that is to say the length under tension. The

  length ho is advantageously the greatest possible.

  I0 Indeed, under a given stress e, determined by the torque exerted on the screw during tightening, the screw lengthens and arrives at a length h, from which a deformation Ah0 which increases when h0 increases. When there is creep of the part B under the pressure exerted by the screw at 13 ', the length h decreases by A (Aho) o a loss of effort. The larger ho is, the larger Aho is and the smaller A (Aho) is in percentage compared to Aho for the same constraint a. This means that the variation Aho remains sufficient and

constitutes an elastic reserve.

  Therefore, the loss of tightening force AF tightening is low, that is to say that the tendency to loosen is lower for a screw whose cylindrical section 14 is not threaded

longer.

  The stress y is equal to the ratio of the tensile force of the screw and its section: er = F / S The stress C is also equal to the product of the module E by the relative deformation s: Or, is equal to the ratio of l ongation A and length 1. Consequently: Now, s is equal to the ratio of elongation AI and length l. Consequently: o = EXl / 1

  o: a = E (h - h0) / ho, o ho is the length of the screw to stretch and o h - h0 = Aho.

  It follows that the tension force F of the screw is expressed by the following formula: F = E S (h-ho) / h0 Under a given tension force F, we generate a stress a, E and S being

  constants, h - h0 / h0 is also a constant.

  Let us study the following functions: Fi (h) = ES (hAl- ho) / ho and F2 (h) = ES (hA2 - ho2) / ho2 o hol> h02 As the graph in Figure 2 shows, for a force FA , The lengths

resulting are hA, and hA2.

  If we consider a decrease in length Ah which occurs over time, the variation in the tensile force AF due to creep is less significant if ho is large.

  That is to say if hol> h02, then AF1 <AF2.

  Consequently, the loss of effort is less and loosening becomes more difficult

when ho is important.

  On the basis of this result, it can be affirmed that the loosening problems which one observes with fully threaded shouldered screws do not come from the creep of the synthetic material but from the "capacity" of elongation of the screw since in this

  case, the length ho is very small because it is the first net which is put under tension.

  The consequence is that it is advantageous to use shouldered screws of the type shown in Figure 1 with a length ho as large as possible. For example, for an M8 screw, the length ho is preferably of the order of a few mm. However, if for reasons of space, it is not possible to plan

  such a long length, it is advisable to provide a thread locker.

  One can also accentuate the phenomenon of deformation of the length ho by changing the material, that is to say by taking a lower modulus E. Considering that F = E S (h - h0) / ho, one then obtains, under a given torque, that is to say at constant F, a greater elongation therefore a tendency to loosen less. For example, an aluminum screw will be less likely to loosen than a steel screw. It is also possible to reduce the section of the non-threaded section 14,

  that is to say to take a smaller diameter.

  The screw 10 represented by FIG. 1 also has the advantage of reducing the contact pressure thanks to a larger surface compared to the use.

  of a classic spacer added.

  With reference to FIGS. 3 and 4, the screw 20 is a so-called shouldered screw intended to fix a

  part A, for example in synthetic material, on a part B, for example in metal.

  It comprises a head 21 provided with a circular base 22 whose diameter is greater than that of the head, an integrated hollowed-out spacer 23, a cylindrical non-threaded section 24 of length ho and a threaded end section 25. The recess of

  the integrated spacer 23 is provided around the cylindrical section 24 not threaded.

  The hollowed-out spacer 23 thus defines an annular rim 23 'which constitutes a bearing surface on the part B. The height of the integrated hollowed-out spacer 23 is preferably slightly less than the thickness of the part A. This dimensional difference is of the order of a few tenths of a millimeter, so that, when the screw is tightened, the part A is slightly crushed, without this deformation inducing a

break or initiation of rupture.

  The screw 20 differs from the screw 10 in that for the first the integrated spacer is full while for the second, the spacer is hollowed out. This solution has the advantage of reducing the length of the screw by using the length of the spacer as part of the length to be tensioned. It is also possible to reduce the cross-section of the non-threaded cylindrical section 24 by making, for example by means of a hole 26 in the head 21, a hole 27 in the circular base 22 and the non-threaded section 24. the same deformation A ho but for a shorter length h0. This solution is particularly suitable for screws whose length h0 is reduced, in particular when the thickness of the first part A in

synthetic material is low.

  With reference to FIGS. 5 and 6, the screw 20 and the screw 10 respectively can be modified by adding a washer 30 which is held by four pins 28 arranged at 90. The washer 30 has the additional advantage of not rotating during tightening and therefore does not tend to damage or "machine" the part B. In addition, the surface being increased, the contact pressure is lower, which also makes it possible to limit the crushing of the part B as well as to reduce the creep of the latter, this

  which has the effect of further reducing loosening.

  Referring to Figure 7, the screw 40 according to the invention does not have a shoulder but comprises around its non-threaded cylindrical section 44 a spacer 31 attached and integrated by means of lugs 32. This embodiment also allows 'avoid machining part B because the integrated spacer 31 is simply supported on part B and does not rotate relative to this part B. The present invention also relates to fixing devices represented by

Figures 8 to 14.

  Referring firstly to Figures 8 to 10, the fixing device 1 comprises a fixing screw 50 having the same characteristics as the screws described above, namely in particular a circular base 52, an integrated spacer 53, a section non-threaded 54 of ho length, and a threaded section 55. The screw 50 further comprises a straight non-threaded section 51 disposed between the circular base 52 and the integrated spacer 53. This section 51 has a length substantially equal or slightly less

  to the difference between the length of part A and the height of the integrated spacer 53.

  On the other hand, the first part A has at the upper end of its inner periphery four retaining tabs 56 forming hooks and arranged at right angles to each other. If the part A is made of synthetic material, the retaining lugs 56 are produced by injection at the same time as the part A. The diameter D of the integrated spacer 53 is slightly greater than the diameter d defined by the end of the lugs retainer 56. The height a between the base of the part A and the end of the retainer lugs 56 is greater than the height b measured between the lower end of the threaded section 55 and the upper edge of the integrated spacer 53. As As shown in FIG. 8, the screw 50 is mounted in the first part A by force insertion of the integrated spacer 53 between the retaining lugs 56. The screw 50 is then retained in the part A by the two ends, on the one hand with the circular base 52 and on the other hand with the retaining lugs 56. The screw 50 can then be transported with the part A without risk of it being lost. When the part A is fixed on the part B, the screw 50 is free in translation and does not interfere with the

  positioning of parts A and B since the height a is greater than the height b.

  As shown in FIG. 9, the screw 50 is then screwed and the circular base 52 abuts against the part A a little before the integrated spacer 53 abuts against the part B. Thanks to its non-threaded section 54, one obtains the same advantages as with screws

described above.

  With reference to FIGS. 11 and 12, the fixing device 1 ′ comprises a fixing screw 60 having a circular base 62, a straight non-threaded section 61, an integrated spacer 63, a non-threaded section 64 and a threaded section 65. From more, a washer 66 is provided between the non-threaded section 64 and the threaded section 65. This washer is put in place before the threading of the section 65. In this variant, the straight section 61 has a length substantially equal to the difference between the length of part A and the height of the integrated spacer 63 plus the thickness of the

washer 66.

  In addition, the part A comprises retaining tabs 56 made in one piece. The diameter d defined by the end of the retaining lugs 56 is slightly less than the outside diameter D 'of the washer 66. The height a' between the base of the part A and the end of the retaining lugs 56 is greater than the height b 'measured between

  the lower end of the threaded section 55 and the upper edge of the washer 66.

  As shown in FIG. 11, the screw 60 is mounted in the first part A by force insertion of the washer 66 between the retaining lugs 56. The screw 60 is then retained in the part A by the two ends, of a on the one hand with the circular base 62 and on the other hand with the retaining tabs 56. The screw 60 can then be transported with the part A without risk of it being lost. When the part A is fixed on the part B, the screw 60 is free in translation and does not interfere with the positioning of the parts A and B since the height a 'is greater than the height b'. As shown in Figure 12, the screw 60 is then screwed and, due to the section 61, when the integrated spacer 63 abuts against the washer 66 in contact with the part B, the circular base 62 abuts against the part A. Thanks to its non-threaded section 64, the same

  advantages than with the screws described above.

  FIGS. 13 and 14 show another embodiment of the retaining tabs according to which the retaining tabs 56 are provided at the end of tongues 57 projecting from the interior surface of the part A. In another variant not shown, the retaining lugs 56 are replaced by a circular bead provided on the inner surface of the part A. The dimensions of the bead can be adapted according to the material used to make the part A. It The present invention is not limited to the examples described but

  extends to any modification and variant obvious to a person skilled in the art.

Claims (19)

Claims   1. Fastening screws (10, 20) of a first part (A), for example of synthetic material, on a second part (B), for example of metal, these parts being arranged in an environment subjected to mechanical stresses and / or thermal, particularly vibrations, in particular in a vehicle, and more particularly a motor vehicle, characterized in that it comprises a integrated spacer (13, 23).   2. Fixing screw (10, 20) according to claim 1, characterized in that said integrated spacer (13, 23) has a height slightly less than the thickness of said first piece (A).   3. Fixing screw (10) according to claim 1, characterized in that said   integrated spacer (13) is full.   4. Fixing screw (20) according to claim 1, characterized in that said   integrated spacer (23) is hollowed out.   5. Fixing screw (40) according to claim 1, characterized in that said   integrated spacer (31) is attached and held by lugs (32).   6. Fixing screw (10, 20) according to claim 1, characterized in that it   comprises a non-threaded section (14, 24) and a threaded section (15, 25).   7. fixing screw (10, 20) according to claim 6, characterized in that said   non-threaded section (14, 24) is of length ho, said length h0 being deformable.   8. Fixing screw (10, 20) according to claim 7, characterized in that said   length h0 has the largest possible value.   9. Fixing screw (20) according to claims 4 and 6, characterized in that said   a recess is provided around the non-threaded section (24).   10. Fixing screw (20) according to claim 6, characterized in that another recess (27) is provided in the center of the non-threaded section (24) to reduce its section.   11. Fixing screw (10, 20) according to any one of the preceding claims,   characterized in that it is made in one piece.   12. Fixing screw (10, 20) according to any one of the preceding claims,   characterized in that it comprises a washer (30) put in place by lugs (28). 13. Fastening device (1) of a first part (A), for example of synthetic material, on a second part (B), for example of metal, characterized in that it   comprises a fixing screw according to any one of the preceding claims,   said screw (50, 60) further having a straight non-threaded section (51, 61) between its circular base (52, 62) and its integrated spacer (53, 63), and in that the first part (A) comprises retaining means (56) arranged to hold the screw (50, 60) mounted in said first part (A) before it is fixed to the second part (B). 14. Device according to claim 13, characterized in that the screw (60) comprises below the integrated spacer (63) a washer (66), the retaining means (56) being arranged to retain the washer (66 ) and the screw (60) mounted in said first   piece (A) before fixing it on the second piece (B).   15. Device according to claims 13 or 14, characterized in that said means   retaining devices consist of at least two retaining tabs (56) provided on the   inner periphery of the first part (A).   16. Device according to claims 13 or 14, characterized in that said means   retainers consist of a bead provided inside the first part (A).   17. Device according to any one of claims 13 to 16, characterized in that   that the retaining means are formed in one piece with the first piece (A). 18. Device according to claim 13, characterized in that the height (a) between the base of the first part (A) and the retaining means (56) is greater than the height (b) measured between the lower end of the threaded section (55) and the edge   upper part of the integrated spacer (53).   19. Device according to claim 14, characterized in that the height (a ') between the base of the first part (A) and the retaining means (56) is greater than the height (b') measured between the end bottom of the threaded section (65) and the edge upper washer (66).