GB2168122A - Blind fastener - Google Patents

Abstract

In a blind fastener which is set by expanding the leading end 62 of a sleeve 16 by forcing it over a tapered nose 22 as the bolt 14 is screwed into the tubular nut 12, the leading end 62 is formed with a greater internal diameter and a lesser wall thickness than the rest of the sleeve 16. The nose 22 is shaped to engage the thicker-walled part 70 of the sleeve 16 and expand it to a diameter greater than that of the shank 18 without engaging the thin-walled portion 62, so as to cause the thin-walled portion 62 to be reorientated to form an annular disc 72. The clamping force is applied mostly to a region of the workpiece surface 52 spaced from the hole 44, and is distributed over an area greater than the cross-sectional area of the thick-walled portion 70 of the sleeve 16. <IMAGE>

Description

SPECIFICATION Blind fastener The invention relates to a blind fastener which is particularly, although not exclusively, suitable for fastening together apertured members of a workpiece of which the member on the blind side is formed of a soft or frangible material which is unable to withstand high local stresses.

The invention is particularly concerned with a fastener of a generally conventional kind and which comprises a headed body having a tapering nose and a tubular sleeve and which can be set by forcing the sleeve over the nose of the body so as to radially expand the sleeve, the expanded sleeve being secured to the body and providing a blind head which can cooperate with the headed body to fasten together apertured members of a workpiece.

Such fasteners usually comprise a screwthreaded, headed bolt which is threadedly engaged in a screw-threaded axial bore of the body and which, on rotation relative to the body, is caused to move along its axis so that the head of the bolt moves towards the body and forces the tubular sleeve over the tapering nose, thereby expanding the sleeve and thus providing the blind head and securing the expanded sleeve to the body.

In some fasteners of this conventional kind, a leading portion of the sleeve which passes over the nose of the body is expanded so that its internal diameter becomes the same as, or not appreciably greater than, the maximum external diameter of the part of the body which has entered the sleeve. The external diameter of the blind head thus formed is therefore generally no greater than the maximum diameter of the part of the body within the sleeve plus twice the thickness of the expanded wall of the sleeve. Consequently, such a blind head inevitably engages the workpiece in the edge region immediately bordering the aperture through which the fastener extends and therefore subjects the blindside member of the workpiece to localised stress, which in soft or frangible materials, may cause damage to the blindside member .It has therefore been proposed to increase the area over which the blind head engages the blindside member of the workpiece so that the force applied by the sleeve to the blindside member is distributed over a greater area of the workpiece, and thus avoid excessive local stress in the workpiece in the region of the aperture.

Nevertheless, conventional fasteners which attempt to overcome this problem are so contructed that they still apply a substantial load to the workpiece in the area immediately bordering the aperture through which the fastener extends, and, although the total load is applied over a substantial area, that which is applied to the region immediately adjacent the aperture can cause damage to the workpiece.

In our co-pending British Patent Application No. 8400345, we have disclosed a three piece blind fastener of the general kind referred to, and in which the nose is so shaped as to cause the sleeve to be expanded to an internal diameter substantially greater than the external diameter of the part of the body over which it passes so that the leading end of the sleeve will engage the blindside member of the workpiece in a region spaced radially from the aperture and not in a region immediately adjacent the aperture. Nevertheless, in that fastener, the sleeve engages the blindside member of the work-piece over an area not greater than the cross-sectional area of the wall of the sleeve so that the blindside member is still subjected to a fairly high localised stress but in a region spaced radially from the edge region bordering the aperture.

We have now devised a fastener of the general kind referred to, in which the sleeve and the nose of the body are so shaped as to cause the formation of a blind head which will apply most of the clamping force to the blindside member of the workpiece in a region substantially spaced from the body of the fastener and by which the clamping force may be distributed over an area substantially greater than the cross-sectional area of the wall of the sleeve.

According to the present invention there is provided a three piece blind fastener of the kind referred to, in which an end portion of the tubular sleeve adjacent to the leading end of the sleeve which will first be entered by the nose of the body during setting of the fastener, has a thinner wall, and a greater internal diameter, than the wall, and internal diameter respectively, of a thick-walled portion of the sleeve spaced from the leading end by the thin-walled portion, the sleeve having a shoulder between the thick-walled and thin-walled portions, and the angle of taper of that external surface of the nose of the body which operates to cause expansion of the sleeve increases towards the head of the body to an angle such that, when the sleeve is forced over the nose so that part of the thick walled portion of the sleeve passes beyond the nose, the thick-walled portion is expanded progressively from its leading end to an internal diameter substantially greater than the maximum diameter of the nose, whereby the thin-walled portion is re-orientated to a substantially flat annular form in which its external surface comes to lie transversely and substantially at right angles to the axis of the fastener.

The sleeve may have a substantially cylindrical external surface.

The shoulder between the thick-walled portion and the thin-walled portion may be substantially square.

The axial length of the thin-walled portion may be such that, after expansion of the sleeve, the thin-walled portion can be accommodated within the radial distance between its point of attachement to the thick-walled portion, and the shank.

The axial length of the thin-walled portion may be between one and three times the wall thickness of the thick-walled portion. We prefer that the length of the thin-walled portion be greater than the wall thickness of the thick-walled portion, and most preferably about twice the wall-thickness of the thickwalled portion.

The wall thickness of the thin-walled portion may be between one eighth and one quarter of the wall thickness of the thick-walled portion.

The thin-walled portion may have a constant diameter throughout its length. The angle of taper of the nose may increase towards the head of the body through a continuous concave curve. The curve may form an arc of a circle. The diameter of the nose may reduce progressively to a diameter slighty greater than the diameter of the bore of the nut.

The longitudinal curvature of the surface of the nose may be such that a tangent at the curve at the point at which the nose meets the cylindrical shank forms an angle of from 30 to 90" with the axis of the nut. The said angle may be 35 .

An embodiment of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is an axial section through a fastener according to the invention and shows, in cross-section, a workpiece in which the fastener is to be installed, and Figures 2, 3, 4, 5, 6 and 7 are fragmentary views, similar to Fig. 1 showing successive stages, respectively, in the setting of the fastener during installation in the workpiece.

Referring first to Fig. 1 of the drawings, a blind fastener 10 comprises three pieces and is adapted to set by rotation of one piece relative to another. The fastener 10 comprises a tubular body 12 which constitutes a nut, a bolt 14, and an expansible sleeve 16.

The nut 12 comprises an elongate cylindrical shank 18, an enlarged hexagonal head 20 at one end of the shank, and a nose 22 at the end of the shank remote from the head 20.

The nose 22 tapers to a smaller diameter away from the head 20. The nut has a screwthreaded bore 24 which extends axially through the head, shank and nose.

The bolt 14 comprises an elongate stem 30 and an enlarged head 32 at one end of the stem. The stem 30 is threaded for engagement with the thread of the bore 24 of the nut 12, and is longer than the combined lengths of the nut 12 and the sleeve 16. The head 32 of the bolt has substantially the same diameter as the shank 18 of the nut. The stem 30 of the bolt has a tail portion 36, remote from the head 32. The tail portion 36 is provided with a pair diametrically opposed flats 38 to facilitate gripping and rotating of the bolt and is connected to the remainder of the stem by a breackneck 34 at which the stem will break when subjected to a torque of a predetermined value, thus enabling the tail portion 36 to be detached from the remainder of the bolt which remains as an essential part of the installed fastener.

The -sleeve 16 is tubular, and has a cylindrical external surface having substantially the same diameter as that of the shank 18 of the nut 12, and a stepped bore 40 which extends axially throughout its length.

Prior to use, the nut 12, bolt 14, and sleeve 16 are assembled together, the stem of the bolt being inserted through the sleeve and screwed into the end of the nut remote from the hexagonal head 20 until the tail porton 36 projects from the head end of the nut 12, and the sleeve 16 is held adjacent to the tapered nose 22 of the nut. The assembled fastener can then be inserted into aligned apertures 42, 44 in members 46, 48 respectively, of a workpiece which are to be fastened together, so that the shank 18 extends through the members with the head 20 abutting the near face 50 of the workpiece, and the bolt head 32, sleeve 16, nose 22, and part of the shank 18 projecting beyond the opposite, or blind, face 52 of the workpiece, which may be inaccessible to the operator.

The fastener can then be set by gripping and turning the projecting tail portion 36 so as to rotate the bolt relative to the nut and cause the bolt head 32 to move axially towards the nut, and thereby forcing the sleeve 16 on to and over the nose 22 of the nut until the sleeve clamps the member of the workpiece tightly against the head 20 of the nut. At this stage, the torque required to turn the bolt further rises abruptly, and continued rotating therefore results in the bolt breaking at the breakneck 34, which allows the tail portion 36 to be discarded, leaving the remainder of the bolt as an integral part of the set fastener. In accordance with the invention, the nose 22 of the nut and the sleeve 16 are so shaped that, when the sleeve is forced over the nose 22 in the direction towards the head 20 of the nut so that part of the sleeve passes beyond the nose, part of the sleeve which passes beyond the nose will be expanded to form a blind head which can engage the blindside member 48 of the workpiece in a region having a diameter substantially greater than that of the shank of the nut.

This movement of the sleeve gives rise to the concept of an end which "leads" in the direction of movement.

Thus, referring more particularly to Fig. 1 of the drawings, the sleeve 16 has a leading end 60 which, in the assembled fastener, is adjacent to the tapered nose 22 of the nut, while the other end of the sleeve is adjacent to the head 32 of the bolt. As previously indicated, the bore 40 of the sleeve is stepped so as to have a region 62, adjacent to the leading end 60, in which the diameter of the bore 40 is greater than in a region 64 of the bore adjacent to the other end of the sleeve. The stepping of the bore between the regions of larger and smaller diameter provides a substantially square shoulder 66 facing axially towards the leading end 60 of the sleeve.It will be appreciated that, because the sleeve is externally cylindrical, a portion 68 of the sleeve surrounding the region 62 of the bore has a thinner wall than the remaining portion 70 surrounding the smaller diameter region 64 of the bore, in which the wall is relatively thick.

During setting of the fastener, the sleeve is expanded over part of its length extending from the leading end 60 so that the thickwalled portion 70 becomes bell-shaped, as illustrated in Figs. 2 to 7, and, as particularly shown in Fig. 6, the thin-walled portion 68 becomes re-orientated so as to extend transversely to the axis of the fastener and substantially at right angles thereto while remaining attached to the leading end of the thick-walled portion 70 peripherally of the shoulder 66. It will also be noticed, particularly in Figs. 6 and 7, that the internal diameter of the leading end of the thick-walled portion 70 has been expanded to an internal diameter substantially greater than the external diameter of the shank 18.The axial length of the thin-walled portion 68 is therefore made of a length such that it can be accommodated within the radial distance between the external surface of the shank 18 and its point of attachment to the thick-walled portion 70 after expansion of the sleeve. Since the distance available to accommodate the re-orientated thin-walled portion 68 will depend upon the extent to which the leading end of the thickwalled portion 70 is expanded by the nose of the nut, we prefer to arrange that the axial length of the thin-walled portion 68 is between one and three times the wall thickness of the thick-walled portion 70, and that the nose of the nut is so shaped as to expand the sleeve by an amount sufficient to permit the re-orientated thin-walled portion 68 to be accommodated, preferably with a slight clearance between its leading end 60 and the external surface of the shank 18.As illustrated, the axial length of the thin-walled portion 68 is slightly greater than the wall thickness of the thick-walled portion 70 of the sleeve, so that, during setting and after the thin-walled portion has been re-orientated, its leading end 60 lies radially inwardly of the internal surface of the leading end of the thick-walled portion 70. However, we generally prefer that the axial length of the thin-walled portion be approximately twice the wall thickness of the thickwalled portion of the shank.The length of the thin-walled portion is also related to its own thickness and the thickness of the thick-walled portion and also to the taper angle of the nose 22 of the nut in such a way that the nose of the nut will engage the radially inner surface of the thick-walled portion at the shoulder 66 so as to cause expansion of the leading end of the thick-walled portion 70 without at the same time engaging the inner surface at the leading end 60 of the thinwalled portion 68. In addition, the wall thickness of the thin-walled portion should not exceed half the wall thickness of the thickwalled portion and preferably is substantially less.We generally prefer that the wall thickness of the thin-walled portion be between one eighth and one quarter of that of the thick-walled portion, and in this embodiment the wall thickness of the thin-walled portion 68 is two ninths of the wall thickness of the thick-walled portion 70.

The nose 22 has an external surface which forms, in the direction longitudinally of the nut, a continuous concave curve which forms an arc of a circle which is arranged so that the diameter of the nose reduces progressively throughout its length to a diameter slightly greater than the diameter of the bore 24 of the nut so that the nut has an appreciable wall thickness at the end of the nose remote from the shank, and is therefore able to withstand compressive forces which act upon it when the sleeve 16 is forced onto the nose. The radius of curvature of the longitudinally curved surface of the nose 22 is such that, at the point at which it meets the cylindrical shank 18, a tangent to the curved surface forms an angle of at least 30 to 90 to the axis of the nut and, in this embodiment, the angle is 35 .

The fastener is set by gripping and rotating the projecting tail portion 36 of the bolt and rotating it relative to the body while restraining rotation of the body by gripping the hexagonal head 20 with a suitable tool, the bolt being rotated in a sense appropriate to move the head 32 of the bolt towards the nose 22 of the nut whereby the sleeve 16 is forced over the nose. The sleeve is first engaged by the small diameter end of the nose which engages the internal surface of the thick-walled portion 70 at the radially inner edge of the shoulder 66 without engaging any part of the thin-walled portion 68.The axially directed force exerted on the sleeve by the head 32 of the bolt is, in part, translated by the tapering external surface of the nose 22 into a radially outwardly directed force which acts first upon the thick-walled portion of the sleeve at the radially inner edge of the shoulder 66 and, as the sleeve moves axially towards the head 20 of the nut, acts progressively on a greater axial length of the thick-walled portion 70, causing a progressively greater length of the thick-walled portion to be progressively ex panded in a radially outward direction. This causes part of the thick-walled portion adjacent its leading end to increase in both internal and external diameter and to acquire a flared shape.As neither the leading end 60 of the thin-walled nor any other part of the thinwalled portion is engaged by the nose of the nut, none of the radially outward force generated by the nose is applied directly to the thin-walled portion 68. However, due to the attachment of the thin-walled portion to the radially-outwardly moving leading end of the thick-walled portion 70, part of the thin-walled portion 68 adjacent to the shoulder 66 is carried radially outwardly with the adjacent part of the thick-walled portion 70 so that its diameter is also increased, while hoop stress in the thin-walled portion 68 tends to resist the expansion of the thin-walled portion to an extent which increases towards the leading end 60 of the thin-walled portion, so that the radial expansion undergone by the leading end 60 of the thin-walled portion 68 is less than that undergone by the thin-walled portion 68 in the region of its attachment to the thickwalled portion 70. Consequently the thinwalled portion 68 is progressively re-orientated until its leading end 60 comes to lie radially inwardly of the region in which the thin-walled portion 68 is attached to the thickwalled portion 70. This is first shown clearly in Fig. 3, and the stages successive thereto are illustrated in the following Figures of the accompanying drawings.As the sleeve advances further along the nose, with the shoulder encountering progressively more steeply tapered regions of the nose, the leading end of the thick-walled portion 70 is expanded radially at a progressively increasing rate, with the expansion extending progressively along the length of the thick-walled portion 70.

Eventually, the shoulder 66 reaches and passes the point of intersection between the tapered surface of the nose 22 and the cylindrical surface of the shank, at which point the taper angle of the nose reaches a maximum.

Due to a certain inherent stiffness in the material of the sleeve, the leading part of the thick-walled portion continues to move substantially in line with the tangent to the nose at the point of its intersection with the shank for a short distance, and thus continues further to expand radially until, eventually, hoop stress in the material overcomes the tendency for the leading end of the thick-walled portion 70 to continue expanding. Its rate of expansion is therefore then reduced abruptly so that it begins to move again along a path more nearly parallel to the axis of the bolt. The commencement of this stage is to be seen in Fig. 5 and, in Fig. 6, it can be seen that, at a later stage, the leading end of the thick-walled portion 70 has assumed a substantially cylindrical shape with an internal diameter substantially greater than the external diameter of the shank.It will also be seen in Fig. 6 that the re-alignment of the leading part of the thickwalled portion 70 has brought the re-orientated thin-walled portion 68 to lie in a substantially flat plane at right angles to the longitudinal axis of the fastener and substantially parallel to the blind face 52 of the workpiece, in which condition it has assumed the form of a disc 72 of which the radially outer part is attached to the thick-walled portion 70, and which disc is spaced by a short distance from the shoulder 66.

Continued rotation of the bolt with respect of the nut causes the sleeve to advance further until eventually the disc 72 abuts the blind face 52 of the blindside member 48 of the workpiece and urges it towards the head 20 of the nut, thereby clamping the workpiece between the head 20 of the nut and the expanded sleeve which thus constitutes a blind head. As the sleeve continues to be urged to compress the workpiece, the gap between the disc 72 and the shoulder 66 closes progressively, and when this is complete and the workpiece has been adequately compressed, the torque required to turn the bolt further rises abruptly and when the predetermined torque required to break the breakneck 34 is reached, the bolt breaks at the breakneck 34 allowing the tail portion 36 to be removed, leaving the fastener fully set in the workpiece.

By providing the sleeve 16 with a thinwalled portion 68 of adequate length initially, the disc 72 formed by re-orientation of the thin-walled portion 68 can present a surface into engagement with the blind face 52 of the workpiece which is of sufficient area to distribute the load applied to the blind face by the blind head so that the stress developed in the workpiece is less localised than would be the case if the workpiece were engaged merely over an area corresponding to the cross-sectional area either the thick- or thin-walled portions of the expanded sleeve.Moreover, due to the fact that the leading part of the thickwalled portion 70 has been expanded to a diameter substantially greater than the external diameter of the shank, most of the clamping force which is applied to the workpiece through the sleeve is transmitted through the radially outer region of the disc 72 and is therefore applied to the blind face 52 mainly in a region spaced substantially from the shank of the nut. Consequently, provided the fastener is installed through an aperture which is not substantially greater in diameter than that of the shank, the entire clamping force applied to the blind face 52 by the blind head can be applied in a region spaced radially outwardly away from the edge of the material of the workpiece bordering the aperture. Thus, as shown in Fig. 7, the radially inner edge of the disc 72 which was formerly constituted by the leading end 60 of the sleeve is spaced radially outwardly of the edge of the aperture 44 in the blindside member 48. Furthermore, as will be evident from Fig. 7, the finally expanded thick-walled portion 70 acts as a strut which applies the clamping force derived from the bolt primarily in a region spaced from the shank of the nut and consequently also from the edge of the aperture through which the fastener is installed.

Although we have disclosed one specific embodiment it will be appreciated that the invention is not restricted thereto, as it is possible to make variations in the details of shape and relative dimensions. For instance instead of being longitudinally curved the tapering nose may be formed of a plurality of frustoconical portions, of which one or more frustoconical portions may, if desired, merge into the next by means of a portion of suitable concavely curved profile.

Claims (16)

1. A blind fastener, comprising: a headed body having a nose which tapers away from the head of the body, an expansible tubular sleeve, and means for setting the fastener comprising means for forcing the sleeve over the nose with one end of the sleeve leading, whereby the nose can be caused to enter the one leading end and to expand the sleeve to form a blind head, and means for securing the expanded sleeve to the body so as to cooperate with the headed body to fasten members of a workpiece together, wherein the sleeve has a thin-walled portion adjacent to the said leading end which will be entered by the nose, a thick-walled portion spaced from the leading end by the thinwalled portion, and a shoulder between the thin-walled and thick-walled portions, said thin-walled portion having a thinner wall and a greater internal diameter than the wall and internal diameter respectively of the thick-walled portion, and wherein the angle of taper, of the external surface of the nose which operates to cause expansion of the sleeve when the sleeve is forced over the nose so that part of the thick-walled portion passes beyond the nose, increases towards the head of the body to an angle sufficient to expand the thickwalled portion of the sleeve progressively from its leading end to a diameter substantially greater than the maximum diameter of the nose, so as to cause reorientation of the thin-walled portion to a substantially flat annular form in which its external surface comes to lie transversely and substantially at right angles to the axis of the fastener.

2. A blind fastener as claimed in Claim 1, wherein the sleeve has a substantially cylindrical external surface.

3. A blind fastener as claimed in either Claims 1 or 2, wherein the shoulder between the thick-walled portion and the thin-walled portion is substantially square.

4. A blind fastener as claimed in any preceding claim, wherein the external length of the thin-walled portion is such that, after expansion of the sleeve, the thin-walled portion can be accommodated within the radial distance between its point of attachment to the thick-walled portion, and the shank.

5. A blind fastener as claimed in any preceding claim, wherein the axial length of the thin-walled portion is between one and three times the wall thickness of the thick-walled portion.

6. A blind fastener as claimed in any preceding claim, wherein the axial length of the thin-walled portion is greater than the thickness of the thick-walled portion.

7. A blind fastener as claimed in any preceding claim, wherein the axial length of the thin-walled portion is about twice the wallthickness of the thick-walled portion.

8. A blind fastener as claimed in any preceding claim, wherein the wall thickness of the thin-walled portion is between one eighth and one quarter of the wall thicknesss of the thick-walled portion.

9. A blind fastener as claimed in any preceding claim, wherein the thin-walled portion has a constant diameter throughout its length.

10. A blind fastener as claimed in any preceding claim, wherein the angle of taper of the nose increases towards the head of the body through a continuous concave curve.

11. A blind fastener as claimed in Claim 10, wherein the curve forms an arc of a circle.

12. A blind fastener as claimed in either of Claims 10 or 11, wherein the longitudinal curvature of the surface of the nose is such that a tangent to the curve, at the point at which the nose meets the cylindrical shank, forms an angle of from 30 degrees to 90 degrees with the axis of the body.

13. A blind fastener as claimed in Claim 12, wherein the said angle is 35 degrees.

14. A blind fastener as claimed in any preceding claim, wherein the diameter of the nose reduces progressively to a diameter slightly greater than the diameter of the bore of the body.

15. A blind fastener as claimed in any preceding claim, wherein the headed body has an axial screw-threaded bore, and the means for forcing the sleeve over the nose and securing the expanded sleeve to the body comprises a bolt having a screw-threaded stem in threaded engagement with the body, and a head at one end of the stem, the other end of the stem projecting from the head of the body, and the sleeve being disposed on the stem between the nose and the head of the bolt.

16. A blind fastener as claimed in Claim 1, and substantially as hereinbefore described with reference to the accompanying drawings.