FR2500090A1 - SELF-TAPPING SCREW - Google Patents

Abstract

THE INVENTION RELATES TO A SELF-TAPPING SCREW. THIS SCREW 1-2 INCLUDES AT LEAST ONE THREAD 3 FORMING protruding coils; BETWEEN TWO NEIGHBORING SPIRES AT EQUAL DISTANCES, A HELICOIDAL ZONE 5 OF STRICTION IS ESTABLISHED; ACCORDING TO THE INVENTION, THE RATIO BETWEEN THE EXTERNAL DIAMETER OF THE THREAD 3, D, AND THE DIAMETER OF THE CORE, D, MEASURED AT THE BOTTOM OF THE STRICTION ZONE 5, IS LESS THAN 1.5 AND GREATER THAN 1.2. THE INVENTION IS APPLICABLE IN PARTICULAR TO SELF-TAPPING SCREWS FOR PLASTIC MATERIALS, IN PARTICULAR THOSE THAT ARE RIGHTLY HARD.

Description

The present invention relates to a self-tapping screw, comprising at

  less a thread forming protruding turns, the flank angle of which is less than forty five degrees (90th part of the right angle), the thread core having, between adjacent turns, restrictions, the narrowest part of which is substantially in the middle between the two adjacent turns, and said core going to

  shrinking from the bases of the two neighboring turns.

  German Patent No. 2,754,870 describes a screw of this type, which has the advantage of having a particularly good anchorage in synthetic materials. The turns, relatively

  thin, can be formed in a particular way

  easy in the plastic. The necking promotes the creep of the material which is pushed back during the digging of the turns. When screws are engaged by screwing in many materials there appear particular problems, which result

following peculiarities.

  In the case of, for example, relatively hard synthetic materials, especially in the case of some thermosetting agents, the fact that the great carrying depth of the threading can not be fully exploited, because the deformation forces necessary to dig deep into the material would be too important, which could lead to a breakage of the screw during its engagement by screwing. The smaller the helix angle of a screw, the more important is, for a given torque, applied to the head of the screw, the initial stress force in the core of said screw, so that the angle The propeller must not be too small, because otherwise there would be a fear of breaking the screw or shearing the synthetic material. The tightening torque of the screw must be significantly greater than the tightening torque, but certainly less than the mismatching torque, for which the thread of the nut is

  shear. The screw described in the above German patent

  mentioned above has a relatively large spacing between neighboring turns. As a result, for very short screws, there appears to be a strong dissymmetry, such that the numbers of diametrically opposite turns which are in engagement are very different; for example, three turns may be engaged along a piercing generator, and two turns only along the diametrically opposite generatrix. This can result in significant differences between the axis of the screw

and that of his hole.

  One of the aims of the present invention is to allow the production of a self-tapping screw of the type indicated initially, which is also well adapted to relatively hard materials, for example to thermosetting, this screw can be shaped so as to avoid any lateral offset with respect to the axis of the screw hole for

  a shallow depth of insertion of said screw.

  The self-tapping screw according to the present invention is of the type indicated initially, and it is characterized in that the ratio between the outer diameter of the thread and the diameter of the core, measured at the narrowest part of the

  necking, is less than 1.5 and greater than 1.2.

  A screw thus formed has, for a large pitch of its turns, a relatively small axial distance between adjacent turns and a relatively large core diameter. This arrangement allows a relatively small height of the profile of the net, keeping it fairly thin. The advantages of the known screw are retained, concerning the creep of the material in which the hole of the screw is arranged, however with a relatively low depth of penetration of its turns. The smallest depth of penetration is compensated, as regards the bearing capacity, by the greatest number of turns per unit length of the thread. This greater number of turns also provides a better symmetry with regard to the number of turns engaged along generatrices, opposite to one another,

  screw holes of short lengths.

  Advantageously, the angle of the flanks of the net (t) is, in known manner, about thirty degrees. Such a profile can also be realized

  without difficulty to implement the present invention.

tion.

  According to another characteristic of the in-

  vention, the ratio of the outer diameter to the

  core diameter is about 1.3, which is

  advantageously. Preferably, the axial spacing between neighboring turns is from 0.24 to 0.50 times,

  and preferably about 0.27 times the outer diameter.

  This axial spacing coincides, in the case of a thread with a single thread, with the pitch of the thread. In the case of a thread with two threads, preferably the thread is constituted by two threads, parallel, of the same profiles; in this case, the axial spacing is only half the pitch of each net. As a general rule, the pitch of the thread is equal to the product of the axial spacing between adjacent turns by the number of threads

  threading. It is specified that in the present description

  as well as in the claims, is designated by

  "turns" a turn of the corresponding helical thread, and by "number of threads", the number of parallel helical threads. Under the conditions indicated, the pitch of the thread is, in the case of a single thread, between 3.5 and 5.5 degrees, and in the case of two threads,

between 7 and 11 degrees.

  According to another particular characteristic-

  advantage of the present invention, the relationship between

  the diameter at the base of the protruding turns and the diameter

  The core is in the range of 1.04 to 1.10, and is preferably about 1.07. These values, on the one hand, ensure that they are sufficient to receive the material which has been repurposed

  formation of the female thread, and. on the other hand-

  they avoid an unacceptable weakening of the core

  threading. According to another particular characteristic

  advantageously, on the profile of the screw, Wls lines connecting the diameter of the core to the points of

  base of the protruding turns are substantially rectilinear.

  This form is particularly simple, and can be easily obtained by shaping without chip removal. According to another provision, particularly advantageous, when the female thread is made of a fragile material, the protruding turns have

  notches, having preferably a sensing section

  V-shaped, and extending to the base of the projecting profile of the nets according to an advantageous possibility, the notches are arranged according to steeply pitched propellers, which intersect the turns of the net, several propellers, for example three propellers offset by one hundred and twenty degrees,

  distributed on the periphery of the screw, their sense of

  It is preferably opposite to that of the threads of the screw. These cuts are used to drain the material cut into chips. This matter then

  flowing into the necking and filling the space

  on one side, by the said necking, and on the other side,

through the wall of the screw hole.

  The screw according to the present invention can be manufactured both by cold rolling, without removing chips, and by removing chips,

  especially in the case of large diameter screws.

  The profile of the threading is particularly favorable to the manufacture by cold rolling, because the necking between neighboring turns promotes the creep of the material

of the screw.

  By way of example, there is described below and illustrated schematically in the accompanying drawing, several

  embodiments of the invention.

  FIG. 1 is an elevational view of a screw according to a first embodiment,

carrying two nets.

  Figure 2 is an elevational view

  of the front end of a screw having cutting edges.

  Figure 3 shows the end of the screw

of Figure 2.

  FIG. 4 is a partial view, in elevation

  tion of a screw with two notched threads.

  Figure 5 shows the end of the screw

of Figure 4.

  Figure 6 shows the contour of the thread

of a screw & a single net.

  Figure 7 is an elevational view of

the screw of Figure 6.

  Sl screw. according to Figure 1, comprises a hexagonal head 1 and a threaded portion 2. The threaded portion has two parallel threads 3, 4, which have

  both have the same profile and form protruding turns.

  Each net has a pitch h, which is measured at the level of externeldà diameter, turns. Between two neighboring turns, there is a zone of necking 5. There are also two "necking threads", 6a and 6b, which extend along the threaded part of the screw, parallel to the threads 3, 4. The diameter dK, the core of the screw is measured in its narrowest area ^,

  that is to say at the bottom of the necking 5.

  The helix angle, a, threads 3, 4 (and hence also necking threads 6a, 6b) is connected to the outer diameter d and to the pitch h by the relation tg cc mh: (ir d) On The profile of the threading will be described below with the help of FIG. 6. Two neighboring turns 7, 8 have of course only an axial spacing h / 2, because of the existence of two threads. It is a characteristic of the screw according to the present invention that the angle of the flanks, *, is small, preferably close to thirty degrees (the degree being defined as the 90th part of a right angle). The necking between two adjacent turns 7, 8, is delimited, when in profile, by two straight lines 9, 10, which pass through the base points, 11, 12, turns and form an obtuse angle, worth by

example 150 degrees.

  The diameter, dK, of the thread core, is relatively large compared to the external diameter dA, so that the risk of breaking the screw is low. The protruding turns 7n 8 have a height

  relatively weak. This screw is therefore particularly suitable

  to be screwed into relatively

  in which it is not possible to drive

  screw threads too deeply.

  In the case of the threading shown, 1, the ratio between the diameter, 4F, & the base of the protruding turns 7, 8, and the diameter dK of the core, is 1 / 0.93. A space 21 is thus obtained which is large enough to serve as a housing for the material which is repelled or cut during the forming of the female thread. In the case of the embodiment according to FIG. 2, it is the same thread as that

  described using Figures 1 and 6 which is used.

  In addition to the embodiment

  described, cutting slices 13, 14 are provided -

  at the front of the screw S2. These cutting boards are formed by sector-shaped milling,, 16. These are arranged in such a way that the edges

  cutting are diametrically opposed. When engaging

  the screw, it is rotated in the direction of the arrow 17, so that the cutting money 13, 14 are forward and scrape the material, so as to cut turns in the material that forms the wall of the hole of the screw. A screw of this type is particularly suitable for fragile materials, which can not be easily pushed back. In the embodiment according to Figures 4 and 5, three notches 18 are arranged in each turn of the screw S These cuts are V-shaped. The bottom of the notch reaches the base points of the projecting threads, it is to the lines

  19, represented in phantom in FIG.

  As shown in Figure 4, the notches 18 are arranged along spirals with a very steep slope, which intersect the protruding turns. The cuts

  are arranged along three such propellers.

  In FIG. 4, a helix is indicated by the dashed line 20. Its slope is opposite to that of the

Thread threads.

  As already mentioned above, the notches play the role of drainage openings, which can pass through the material which, during screwing of the screw, is cut into chips in the material limiting the screw hole. This material can accumulate in the space between the wall of the hole and the core of the screw, that is to say in the space 21 which is repeated in FIG. lines

  9 and 10, and, on the other, by the dotted line 22.

  The screw S according to FIG. 7 has the same thread profile, shown in FIG. 6, as the screws Si and S2. However, in the case of the screw S, this thread comprises a single thread. As a result, in the case of the screw S5, the pitch h 'between two neighboring turns 7' and the helix angle x 'are respectively only half the pitch h and the angle of the helix a screws

Si and S2.

  In the case of two-threaded constructions (screws S1 and S2), large torques are obtained by screwing, because two threads must be cut or formed simultaneously. The mismatching torque is important since the shear force is required for the wide angle screw threading of the screws. Radial forces and the; Tangential tucks in a tubular nut are important, and then two diametrical threads must be simultaneously made. The initial stress in the screw face and the superficial pressure on the flanks of the threads are small because. as a result of the g-angle

  propeller, there is no production, for an effector Tpriph-

  given only a relatively low initial stress effort. Self-locking, ie self-loosening resistance qze, is low due to the large helix angle. The assembly time is short because you get a great depth of

  screwing in a few turns due to the important step.

  In the case of the single-threaded embodiment, the engagement torque by screwing is low,

  for there is nothing to tillage or to form but one fetus.

  On the other hand, the mismatching torque is important, since it is necessary to overcome the shear force for the female thread with a small helix angle. The radial forces and tangential stresses in a tubular nut around the screw are only half of what they are in the case of the two-threaded embodiment. The initial stress in the rod of the screw and the superficial pressure on the flanks with the thread are important, since, because of the small angle of helix, there is production, for a given peripheral force,

  of an important effort of initial constraint. Car-

  blocking, that is, the self-loosening resistance

  is important because of the small helix angle.

  For a given depth of screwing and a given number of turns when the screw Dar screwing is engaged, the assembly time is double what it is in

  the case of the realization with two nets.

  From these particularities, it follows

  following consequences for the use of the screws.

  The two-threaded embodiment will be used when the diameter of the screw is small, in order to obtain achievable tightening torques. When a high speed of assembly is desired, preference will also be given to the embodiment with two threads, insofar as its disadvantages do not preclude its use. The two-threaded embodiment is advantageous with the materials accepting only a small pressure load, as a result of the low initial stress.

  of the screw shaft and the super low pressure

  on the flanks of the nets. The two-threaded embodiment is, on the other hand, advantageous in the case of short screws because, because of the two threads, the force can be applied diametrically so as to

get a centered screwing.

  Single-threading should be preferred when significant self-locking is desired, which results from the small helix angle. In the case of small wall thicknesses, the single-threaded screws are therefore advantageous, since the radial forces and the tangential stresses are small. Preference will be given to the single-threaded embodiment also when low tightening torques are desired. The single-threaded embodiment is particularly suitable for co-operating with high pressure load materials because, as indicated above, the initial air pressure in the screw shaft and the superficial pressure on the

flanks of the net are important.

Claims (11)

  1. Self-tapping screw, comprising at least one thread forming protruding turns, the flank angle is less than 45 degrees (9th part of the right angle), the thread core having, between neighboring turns, restrictions , the narrowest portion of which is substantially in the middle between the two adjacent turns, and the said core tapering from the bases of the two adjacent turns, screw characterized in that the ratio between the outer diameter (d) of the net and the diameter of the core (dK), measured at the narrowest part of the necking (5), is less than 1.5 and greater than i, 2. 2. Self-tapping screw according to claim 1, characterized in that the angle of the flanks of the thread (Y ') is, in a manner known per se, about thirty degrees.   3. Self-tapping screw according to any one   than the preceding claims, characterized by   the fact that the ratio between the outer diameter and the   Core diameter (d / dK) is about 1.5.   4. Self-tapping screw according to any one   of the preceding claims, characterized by the   the axial spacing (h / 2; hl) between adjacent turns (7; 8; 7 ') is 0.24 to 0.30 times, and preferably   about 0.27 times the outer diameter (d).   5. Self-tapping screw according to the claim   4, characterized in that the threading is constituted by two threads (3, 4), parallel, of the same profiles. 6. Self-tapping screw according to any one   of the preceding claims, characterized by the   the ratio of the diameter (cl) at the base (11, 12) of the protruding turns (7, 8; 7 ') to the diameter of the core (dt) is in the range from   1.04 to 1.10, and is preferably about 1.07.   he 7. Self-tapping screw according to any one   of the preceding claims, characterized by the   on the profile of the screw, the lines connecting the diameter of the core (dc) to the base points (11, 12) of the protruding turns (7, 8; 7 ') are substantially rectilinear. 8. Self-tapping screw according to any one   of the preceding claims, characterized by the   its front end has cutting edges (13, 14), which are preferably formed by countersinks (15, 16) distributed evenly around the circumference, by   example by two diametrically opposed milling.   9. Self-tapping screw according to any one   of the preceding claims, characterized by the   the protruding turns have notches (18), preferably having a substantially V-shaped cross-section, and extending to the base (11, 12) of the protruding profile of the net.   10. Self-tapping screw according to the claim   9, characterized in that the notches (18) are arranged in helices (20) with a steep slope, which intersect the turns of the net, several propellers, for example three propellers, offset by one hundred twenty degrees, being distributed on the periphery of the screw, their winding direction being preferably   opposite to that of the threads of the screw.   11. Self-tapping screw according to any one   of the preceding claims, characterized by the   it is formed without chip removal, by cold rolling.