GB2130326A

GB2130326A - Fastening insert

Info

Publication number: GB2130326A
Application number: GB08232046A
Authority: GB
Inventors: Andre Nivet; Daniel Binet
Original assignee: ITW de France SAS
Current assignee: ITW de France SAS
Priority date: 1981-10-05
Filing date: 1982-11-10
Publication date: 1984-05-31
Also published as: DE3242059A1; FR2514086B1; FR2514086A1; DE3242059C2

Abstract

A fastening insert for use in fixing a member by means of a bolt to a support 12 made of a fragile plastics material which is subject to deterioration as the insert is fitted into it, comprises an internally screw threaded cylindrical sleeve 2 having an external self-tapping screw thread 5. The screw thread 5 has a very acute angle between its flanks and a pitch which is substantially greater than its radial height. The thread thus cuts a narrow furrow 13 in the wall 14 of the hole 10 in the support 12 which avoids damage to the support. The sleeve is further provided with a helical rib 7 (or a series of protrusions) between the turns of the thread 5. The rib 7 does not cut into the wall 14, but holds the insert centred in the hole 10 as it is screwed in. Further there is a substantial clearance 15 between the sleeve 2 and the wall 14 to avoid any tendency for the insert to split the support as it is screwed in. <IMAGE>

Description

SPECIFICATION Fastening insert This invention relates to fastening inserts which may be made, for example, of steel, brass or other metallic material or even hard plastics material, for fixing a member to a support of plastics material which has the shortcoming of being fragile and accordingly subject to deterioration upon the fixing of inserts therein.

It is known that frequently nuts of the insert type are implanted in one member when two members are to be fixed together by means of a screw and nut device. The one member is a support and is formed with a blind or openended hole to receive the insert and is made of plastics material by injection or compression moulding.

It is also known that such inserts are not entirely satisfactory where the support is of a fragile plastics material which is rigid or has a fibrous structure which degenerates into powder if it is machined, or which splits easily.

The object of this invention is to provide a fastening insert formed in such a way that it can be introduced into a hole in a support of fragile plastics material without the above mentioned drawbacks occuring in said type of material.

According to this invention, such an insert comprises a body having an external cylindrical surface on which are formed firstly a screw thread the pitch of which is greater than the height to an extent such that tearing off or destruction of the support does not occur as the body is screwed into a hole in the support, and secondly a rib or a series of protrusions which lie between the turns of the screw thread, but project from the surface less far than the screw thread, so that the rib or protrusions act to centre the body in the hole as it is screwed in but do not penetrate into the wall of the hole.

An example of a fastening insert in accordance with the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a side elevational view of the insert; Figure 2 is a longitudinal sectional view to a larger scale of part of the insert to show a detail of a screw thread and of a guiding rib on the insert; Figure 3 is a longitudinal partly sectional view showing the insert implanted into a support; and, Figure 4 is a view similar to Fig. 3 but showing also an element secured to the support by a bolt screwed into the insert.

With reference to the drawings, an insert comprises a cylindrical body in the form of a sleeve 2 having an application head 3. In this example the insert is formed therethrough with a cylindrical tapped bore, and on the outer periphery of the sleeve there are a screw thread 5 having two flanks 6 and 6a and a helical rib 7 of a specific profile 8.

The insert is intended for insertion into a hole of a polygonal or circular cross-section as shown in the example. The insert is introduced into a hole 10 in a support 1 2 by means of the screw thread 5 thereon, the height H of which is very pronounced and the flanks 6 and 6a of which make a very acute angle with one another. Together these characteristics cause the screw thread to form a kind of knife blade, the pitch 11 of the screw thread 5 being such that its dimension Y is at least twice or three times as large as the height H of the screw thread 5. This dimensional ratio is intended to prevent tearing out or weakening of the wall of the support 1 2 if the support is very fragile.If the height of the thread 5 were excessive and its pitch too small the support 1 2 would be likely to split or break up as the insert 1 was screwed into it.

The helical rib 7 in the example shown follows the thread 5, being of the same helix angle, and in cross-section has the rounded profile 8. The screw thread 5 and the helical rib 7 may be interrupted by a groove 9 which provides a self-tapping function.

When the insert 1 penetrates into the support 12, the screw thread 5 cuts a furrow 1 3 therein with a very acute included angle between its sides 5 due to the knife blade shape of the screw thread 5 with a self-tapping action because of the groove 9 working as a tap. Under these conditions, the screw thread 5 produces minimum deformations and stresses between the flanks 6 and 6a and the material of the support 1 2.

The insert 1 must have a well-defined clearance X between the sleeve 2 and the wall 1 4 of the hole 10 for two reasons: First, elimination of stresses between the wall of the sleeve 2 and the wall 14 of the hole 10, thereby also eliminating the risk of splitting the support 12; Secondly, the clearance X provides a reservoir for fragments or chips formed in the selftapping operation, thereby eliminating any risk of stuffing, and hence, also preventing splitting of the support 1 2.

When the support is of a delicate material, such as in the illustrated example, the helical rib 7 guides and centres the insert 1 in the hole 10 as it penetrates into the hole and forces the screw thread 5 to remain centred with respect to the wall 14. If the screw thread 5 tended to cut out its furrow more deeply on one side or the other of the periphery of the wall 14, supplementary and asymmetric stresses would occur which would increase the risk of causing breakage or splitting of the material of the support 1 2.

The helical rib 7 with its circular arcuate profile 8 is adapted to slide along the wall 14 so as not to produce any tearing out in the wall 14 between two adjacent turns of the furrow cut by the screw thread 5 of the insert 1.

On the other hand, depending on the technical requirements, it is advantageous to be able to control the pressure between the wall 14 and the profile 8 of the helical rib 7, thereby increasing the area of contact and consequently improving the resistance against loosening of the insert by vibration.

In certain cases, such as in the illustrated example, the material used for the support 1 2 is covered with a superficial skin during curing thereof in compression moulding. Due to this, it is important to not tear off this skin, because otherwise the technical characteristics of the material to form the support 1 2 would be affected.

The invention presents a new type of fastening insert based on the association of a particular form of screw thread with a helical centring and guiding rib, applied to an insert, but the same concept can also be utilized for any other system of screwing assembly, such as screws or bolts.

According to other forms of embodiment, which are not represented, the rib 7 may have discontinuities and consist of sections of the same height.

In certain cases, these sections may simply amount to a series centring and guiding elements or protrusions of the same height as each other.

Preferably, the sections or elements are formed with rounded profiles to facilitate the guiding and centring contact with the wall 14 of the hole 10 in the support 12.

Claims

1. A fastening insert for fixing a member to a support of fragile plastics material, the insert comprising a body having an external cylindrical surface on which are formed firstly a screw thread the pitch of which is greater than the height to an extent such that tearing off or destruction of the support does not occur as the body is screwed into a hole in the support, and secondly a rib or a series of protrusions which lie between the turns of the screw thread, but project from the surface less far than the screw thread, so that the rib or protrusions act to centre the body in the hole as it is screwed in but does not penetrate into the wall of the hole.

2. An insert according to Claim 1, in which the rib or protrusions is in the form of a helical rib which has a rounded profile and has the same helix angle as the screw thread with its turns lying between those of the screw thread.

3. An insert according to Claim 2, in which the rib is formed with discontinuities but is of constant height.

4. An insert according to Claim 1, in which the series of protrusions are of the same heights as each other and lie on a helix which is of the same helix angle as the screw thread with its turns lying between those of the screw thread.

5. An insert according to any one of Claims 1 to 4, further comprising tapping means such that the screw thread can cut a furrow into the wall of the hole in the support and simultaneously progress into the hole by self-tapping.

6. An insert according to Claim 5, in which the tapping means is formed by a groove cut in the leading end of the body and the screw thread.

7. An insert according to Claim 5 or Claim 6, in combination with a support of fragile plastics material formed with a hole, wherein a clearance is provided between the cylindrical surface of the body and the wall of the hole to eliminate stresses between the cylindrical surface and the wall of the hole and to act as a reservoir to hold fragments or chips formed during self-tapping as the insert is screwed into the hole.

8. An insert according to any one of the preceding Claims, in which the body is a sleeve having an internal screw thread for receiving a bolt.

9. An insert according to Claim 1, substantially as described with reference to the accompanying drawings.

10. The combination according to Claim 7, substantially as described with reference to Figs. 3 and 4 of the accompanying drawings.