GB2106023A

GB2106023A - Self-tapping screw

Info

Publication number: GB2106023A
Application number: GB08203818A
Authority: GB
Inventors: Hermann Grossberndt
Original assignee: Jaeger And Co KG Verbindu GmbH
Current assignee: Jaeger And Co KG Verbindu GmbH
Priority date: 1981-02-13
Filing date: 1982-02-10
Publication date: 1983-04-07
Also published as: FR2500090A1; IT8219576A0; ES271929U; DE8103990U1; SE8200709L; ES271929Y; DE3201846A1; IT1149606B

Abstract

A self-tapping screw S1 comprises a single or double helix of threads which have a flank angle of less than 45 DEG , on a core with constrictions (5) between neighbouring threads, the ratio of the diameter over the threads to the minimum diameter of the constrictions being less than 1.5 and greater than 1.2, more particularly about 1.3. The axial spacing (h/2) between neighbouring threads is preferably 0.24 to 0.30 times, more particularly 0.27 times, the outside diameter. The ratio of the diameter at the thread roots to the minimum diameter of the is preferably 1.04 to 1.10, more particularly 1.07. The forward end of the screw may have cutting edges, preferably constituted by milled recesses. The threads may have indentations, preferably V-shaped, to the root of the thread profile and preferably arranged along each of several coarse-pitch helical lines (e.g. three at 120 DEG spacing) crossing the threads with a direction of inclination, with respect to the longitudinal axis of the core, preferably contrary to that of the threads. <IMAGE>

Description

SPECIFICATION Self-tapping screw The invention relates to a self-tapping screw comprising a core provided with at least one helix of projecting threads which have a flank angle of less than 45" (90 being a right-angle) wherein the core has constrictions between neighbouring threads and the narrowest point of such constriction is situated approximately centrally between two neighbouring threads, the core tapering towards that point from the root ends of neighbouring threads.

A screw of this kind is described in German Patent No. 27 54870 and has the advantage that it grips particularly well in synthetic plastics materials. The relatively thin threads can tap particularly easily into the synthetic plastics material. The constriction promotes the flow of the material which is displaced as the threads plough into it. However, special problems occur with many materials when driving screws into them, because of the relationships described hereinafter.

For example in the case of relatively hard synthetic plastics materials, more particularly with some duroplasts, the great load-bearing depth of the screwthreading cannot be fully utilised, since the shaping forces for the necessary deep incision into the material would become too great, which may result in fracture of the screw as early as at the screwing-in stage. The smaller the pitch angle of a screw is, the greater the preloading or forward-pull force in the screw core becomes with a given turning moment or torque at the screw head, for which reason the pitch angle must not be too small since otherwise there is a risk of stripping or breaking the screw or shearing off the synthetic plastics material.The tightening moment or torque of the screw should be substantially greater than the tapping moment, but reliably below the over-turning or over-tightening moment or torque at which the internal or female thread, formed by the self-tapping screw in the material, is sheared off. The screw proposed in the said German patent has a relatively large spacing between neighbouring threads. In the case of very short screws this results in considerable asymmetry in such a way that the number of engaging threads situated diametrically opposite one another differs considerably in terms of percentage; for example, along one generatrix line of the bore there may be three turns or threads engaging the material but only two threads at the diametrically opposite generatrix line. This can result inconsiderable deviations of the screw axis relatively to the screw hole axis.

It is an object of the present invention to provide a self-tapping screw of the kind specified initially herein but improved so that it is well suited even for relatively hard types of material, e.g. duroplasts, and it can be so formed that it does not undergo lateral offset relatively to the screw hole axis even if the depth to which it is screwed in is not great.

According to the invention this object is achieved in that the ratio (outside-diameter/core-diameter ratio) between the outside diameter of the screw, measured over the threads, and the diameter of the core, measured at the narrowest point of the constriction, is less than 1.5 and greater than 1.2. It may be advantageous to make this ratio approximately 1.3.

A screw formed in this way has, with a large screw-thread pitch, a relatively small axial spacing between neighbouring threads and a relatively large core diameter. This arrangement allows, whilst retaining a slender helix profile, a relatively low profile height. The advantages of the known screw are retained as regards the flow behaviour of the material in which the screw hole is situated, but with a relatively small thread penetration depth. The relatively small penetration depth is compensated as far as load-bearing ability is concerned by the larger number of threads per unit of screw-thread length.

This larger number of threads also gives better symmetry regarding the number of engaging threads at mutually opposite generatrix lines in short screw holes.

Advantageously the flank angle is approximately 30 (claim 2). This kind of profile also can easily be achieved with the present invention.

Advantageous values for the ratio between outside diameter and axial spacing of threads are given in claim 4. In the case of a single-start screwthreading this axial spacing coincides with the pitch.

In the case of a two-start or two-helix screwthreading (claim 5) this spacing is only half of the pitch. It can be stated as a general rule that the pitch of the screw-threading is equal to the product of the spacing of neighbouring threads and the number of starts of the screw-threading. To clarify, it may be mentioned here that by a "thread" there is understood one turn of the helix, while "number of starts" means the number of parallel helixes. In the circumstances indicated, the pitch angle is between 3.5 and 3.5 degrees in the case of a single-start screw-thread and between 7 and 11 degrees in the case of a two-start or double-helix screw-thread.

It is particularly advantageous to profile the region between two threads as indicated in claims 6 and 7.

The ratios indicated in claim 6 on the one hand give sufficient space for accommodating material which has been displaced when the female or internal thread is formed, and on the other hand obviate unacceptable weakening of the screw-threading core. The shape indicated in claim 7 is particularly simple and can easily be achieved with a forming process without removal of material.

The screw can also be provided with cutting edges as indicated in claim 8, which is particularly advantageous for driving the screw into relatively brittle material, which is then shaved out by the cutting edges, so that the female (internal) screw-threading is not formed only be displacement of material.

Indentations as indicated in claims 9 and 10 are also advantageous in connection with internal screwthreading in brittle material. These indentations serve as outlets for material which has been cut away. This material can then flow out into the constrictions and fill the space defined on the one hand by the constriction and on the other hand by the wall of the screw hole.

The screw according to the invention can be formed either by cold rolling without removal of material (claim 11) or, more particularly in the case of large screw diameters, by machining or cutting away of material. The screw-thread profile is particularly advantageous for production by cold rolling, since the constriction between neighbouring threads promotes the flow of material in the screw.

Forms of embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 shows a side view of a screw according to a first form of embodiment with a two-start or double-helix screw-thread system; Figure 2 shows a side view of the front or forward end of a screw with cutting edges; Figure 3 shows the end of the screw according to Figure 2; Figure 4 shows a partial side view of a two-helix screw with indented (notched) threads; Figure 5 shows the end of the screw according to Figure 4; Figure 6shows a screw-threading profile; and Figure 7 shows a side view of a screw with a single-start screw-threading.

The screw S1 in Figure 1 has a hexagon head 1 and a screw-threaded portion 2. The screw-threaded portion has two parallel helixes 3,4 both of which have the same profile and form projecting threads.

Each helix has the pitch h which is measured at the outside diameter dA of the threads. A constriction 5 is provided between neighbouring threads. It is also possible to say that there are two helixes of constrictions 6a and 6b, which extend parallel to the helixes 3,4 along the screw-threaded portion of the screw.

The core diameter dK of the screw is measured at the narrowest part, that is to say at the bottom of the constriction 5.

The pitch angle a of the helixes 3,4 (and thus of the constriction helixes 6a, 6b also) is connected with the outside diameter dA and the pitch h in the relationship tan a = h/(2.dA).

The threading profile will be described hereinafter with reference to Figure 6. Two neighbouring threads 7, 8 of course have only the axial spacing h/2 because there are two helixes in the screwthreading. It is a feature of the screw that the flank angle y is small, preferably about 30 , expressed in the usual system of measurement in which a right-angle amounts to 90 . The constriction 5 between neighbouring threads 7, 8, as viewed in the profile, as defined by two straight lines 9, 10 which start at the root points 11, 12 of the threads and together form an obtuse angle which may amount, for example, to 1500.

The core diameter dK of the screw-threading is relatively large in relation to the outside diameter dA, so that the danger of stripping the screw is slight.

The projecting threads 7, 8 are of a relatively low height. The screw is therefore particularly suitable for screwing into relatively hard materials, into which it is not possible in any case to force threads very deeply.

In the case of the illustrated screw-threading the ratio between the diameter dF at the base or root of the projecting threads 7, 8 and the core diameter dK amounts to 1: 0.93. This gives a sufficiently large space 21 (see Figure 6) for the reception of material which is displaced or cut away when the female or internal screw-thread is formed.

The form of embodiment shown in Figures 2 and 3 uses the same screw-thread as has been described in connection with Figures 1 and 6. Additionally to the form of embodiment which has been described, cutting edges 13, 14 are provided in the forward region of the screw S2. These cutting edges are formed by angular milled-out recesses 15, 16. These recesses are so arranged that the cutting edges are situated diametrically opposite one another. When the screw is driven in, it is turned in the direction of the arrow 17, so that the cutting edges 13, 14 lead, shaving off the material and thus cutting threads in the material which forms the wall of the screw hole.

Such a screw is particularly suitable for brittle material, which cannot easily be displaced.

In the form of embodiment shown in Figures 4 and 5, three indentations 18 are arranged in each thread of the screw S3. These indentations are V-shaped.

The bottom of the indentation extends to the root points of the projecting helixes, in other words to the lines 19 shown in the form of broken lines in Figure 6.

As Figure 4 shows, the indentations 18 are situated along helical lines arranged at a very steep angle and crossing the projecting threads. The indentations are situated along three such helixes. One helix is indicated in Figure 4 by the chain-dotted line 20. Its inclination is contrary to the inclination of the screw-thread helixes.

As already mentioned in the introductory part of the description, the indentations 18 act as outlet apertures through which cut-away material, coming from the material bounding the screw hole when the screw is driven in, can pass. This material can accumulate in the space between the bore hole wall and the core of the screw, in other words in the space 21 represented in Figure 6 by the lines 9, 10 on the one hand and by the chain-dotted line 22 on the other hand.

The screw S3 according to Figure 7 has the same screw-thread profile, shown in Figure 6, as the screws S, and S2. However, in the case of the screw S3 this profile is formed by a one-start or single-helix screw-thread. Accordingly, in the case of screw S3, the screw-thread pitch h' between two neighbouring threads 7' and the pitch angle a' are only half as great as the screw-thread pitch h and the pitch angle a obtaining in the case of screws S, and S2.

In the double-helix version (screws S1, S2) high tapping moments or torques are produced since two screw-threads have to be cut or formed at the same time. The over-turning or stripping moment or torque is considerable, since it is necessary to overcome the shearing-off force for the internal or female screw-thread with a large lip or wedge angle (pitch angle). The radial forces and the tangential stresses in a tubular female element are considerable, since two internal screw-threads are being produced at diametrically opposite points at the same time. The preloading orforward pull in the screw shank and the pressure per unit of surface area in the thread flanks are small, since owing to the great lip or wedge angle (pitch angle) only a relatively small preloading force is produced with a given circumferential force. Self-locking ability, i.e.

resistance to automatic loosening, is low because of the large pitch angle. Fitting time is short because a considerable screw-in depth can be achieved with a few revolutions because of the considerable pitch.

In the single-helix version, the tapping moment or torque is small, sincle only one thread has to be cut orformed. On the other hand the over-turning or stripping moment or torque is relatively large since the shearing-off force for the internal or female screw-thread has to be overcome with a small lip or wedge angle (pitch angle). The radial forces and tangential stresses in a tubular female element surrounding the screw are only half as great as in the case of the double-helix version. The preload or forward pull in the screw shank and the pressure per unit or surface area in the thread flanks is high, since owing to the small lip or wedge angle (pitch angle) a considerable preloading force is produced, with a given circumferential force. Self-locking, i.e. resistance to self-loosening, is good because of the small pitch angle.Fitting time, with a given depth of penetration and a given rate of turning, when driving the screw, is twice as great as in the double-helix version.

The consequences for the use of the screws which follow from these properties are as follows: The two-start or double-helix version will be used when the screw diameter is small, in order to arrive at practicable tightening moments or torques. When it is desired to have fast fitting speeds, the two-start version will again be preferred, unless its drawbacks are an obstacle to the use thereof. The two-start version is advantageous with materials of low compressive stress acceptance because of the small preload or forward pull in the screw shank and the small surface pressure at the thread flanks. The two-start version is also advantageous in the case of short screws, since owing to the double helixing a diametral application of force is possible, and thus the screw is driven in centrally.

The single-start or single-helix version is to be preferred when a high self-locking ability is desired, which results from the small pitch angle. With small wall thicknesses, single-helix screws are advantageous since the radial forces and tangential stresses are small. The single-helix version will also be preferable when small tightening moments or torques are desirable. The single-helix version is suitable more particularly in conjunction with materials of high pressure-load acceptance since, as stated hereinbefore, the preload or forward pull in the screw shank and the surface pressure at the thread flanks are high.

Claims

1. A self-tapping screw comprising a core provided with at least one helix of projecting threads and which have a flank angle of less than 45" (expressed in the system of measurement in which a right-angle is 90 ), wherein the core has constrictions between neighbouring threads and the narrowest point of such constriction is situated approximately centrally between two neighbouring threads, the core tapering towards that point from the root ends of neighbouring threads, and wherein the ratio (outside-diameter/core-diameter ratio) between the outside diameter of the screw, measured over the threads, and the diameter of the core, measured at the narrowest point of the constriction, is less than 1.5 and greater than 1.2.

2. A self-tapping screw according to claim 1, wherein the flank angle of the threads amounts to about 30 .

3. A self-tapping screw according to either one of the preceding claims, wherein the outside-diameter/ core-diameter ratio is approximately 1.3.

4. A self-tapping screw according to any one of the preceding claims, wherein the axial spacing between neighbouring threads amounts to 0.24 times to 0.30 times, preferably 0.27 times, the outside diameter.

5. A self-tapping screw according to claim 4, wherein the screw-threading comprises two similarly profiled parallel helixes.

6. A self-tapping screw according to any one of the preceding claims, wherein the ratio (rootdiameter/core-diameter ratio) between the diameter at the root of the projecting threads and the said diameter of the core is in the range 1.04 to 1.10, preferably amounting to approximately 1.07.

7. A self-tapping screw according to any one of the preceding claims, wherein, in the screwthreading profile, the transition from the core diameter to the root points of the projecting threads is substantially rectilinear.

8. A self-tapping screw according to any one of the preceding claims, wherein the forward end of the screw has cutting edges which are preferably constituted by milled recesses, distributed uniformly over the periphery, for example with two such recesses being situated diametrically opposite one another.

9. A self-tapping screw according to any one of the preceding claims, wherein the projecting threads have indentations, preferably of an approximately V-chaped cross-section with extends to the root of the projecting-thread profile.

10. A self-tapping screw according to claim 9, wherein the indentations are arranged along a coarse-pitch helical lines crossing the threads, a plurality of such helixes being disposed on the periphery of the screw, for example three helixes offset by 1200 relatively to one another, the direction of inclination thereof being preferably contrary to the direction of inclination of the screw-threading helixes.

11. A self-tapping screw according to any one of the preceding claims, characterised in that it has been formed without cutting, by cold rolling.

12. Aself-tapping screw substantially as herein described with reference to any one or more of the Figures of the accompanying drawings.