GB2152614A

GB2152614A - Blind fastener

Info

Publication number: GB2152614A
Application number: GB08432125A
Authority: GB
Inventors: Harvey Philip Jeal
Original assignee: Avdel Ltd
Current assignee: Avdel UK Ltd
Priority date: 1984-01-06
Filing date: 1984-12-19
Publication date: 1985-08-07
Also published as: IT8547519A1; IT1181835B; GB8432125D0; DE3446186A1; IT8547519A0; FR2557936B1; GB2152614B; JPS60159409A; AU3719384A; GB8400345D0; FR2557936A1; CA1251664A

Abstract

A blind fastener for fastening an apertured workpiece (37) of frangible composite material comprises a tubular shank (18) having a head (20) at one end and a concavely tapering nose (22) at the other end, and and a ductile sleeve (16) which can be forced over the nose (22) to form a blind head by which the workpiece can be clampingly engaged at a position spaced radially away from the shank (18). The sleeve (16) is forced over the nose (22) by screwing a headed bolt (14) into the tubular shank (18). The nose (22) may be continuously curved, or comprise steeply and shallowly tapering frusto-conical regions (46, 44), to cause the leading end of the sleeve (16) to be expanded to an internal diameter substantially greater than the external diameter of the shank. <IMAGE>

Description

SPECIFICATION Blind fastener This invention relates to a blind fastener and particularly to a blind fastener of the kind comprising a headed body having a tapering nose, and a tubular sleeve, and which is 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.

The invention is concerned particularly, although not exclusively, with a fastener of the kind stated and which has a screw-threaded, headed bolt threadedly engaged in a screwthreaded bore in the body, and which bolt can be caused to move along its axis by rotation relative to the body, whereby the head of the bolt can be caused to move towards the body and to force the tubular sleeve over the tapering nose so as to provide a blind head.

Fasteners of the latter kind are well known and commercially available under the Registered Trade Mark "JO-BOLT", and are widely used in the aerospace industry, particularly for fastening metallic sheets, such as sheets of aluminium alloy, to each other or to sheets, such as sheets of aluminium alloy, to each other or to other members, such as structural supporting members. Generally such fasteners are eminently satisfactory when the materials to be fastened are at least moderately hard and not particularly frangible or ductile, but they are less satisfactory when the work member which has to be engaged by the blind head is frangible.

In particular, some composite materials, such as carbon- and glass-fibre reinforced plastics materials which are now often employed in sheet form where formerly metals were used, have desirable characteristics of low density and high tensile strength, but are frangible in that they have low compressive and shear strengths, and are sometimes unable to withstand the stresses to which they are subjected when fastened by conventional fasteners of the kind referred to.

The main reason for the unsatisfactory performance of the conventional fasteners of the kinds referred to in frangible materials is that the blind head provided by the expanded sleeve engages only a small area of the workpiece peripherally of the aperture through which the fastener extends and immediately adjacent to the edge of the material bordering the aperture. This results in the frangible material being subject to localised stresses which it is unable to withstand.

This disadvantage of the conventional fasteners can be attributed, at least in part, to the fact that, while the external diameters of both the sleeve and the part of the body which extends through the aperture in the workpiece are necessarily limited by the diameter of the aperture, the nose of the body is of simple frusto-conical shape, and the body is so shaped that, during setting of the fastener, the expanding sleeve tends to follow closely the contour of the parts of the body over which it is forced. Consequently, the maximum internal diameter of the expanded sleeve is not appreciably greater than the diameter of the part of the body which has been passed through the aperture in the workpiece, and the maximum external diameter of the expanded sleeve is only greater than this by twice the wall thickness of the expanded sleeve.It is therefore virtually inevitable that the blind head thus provided by conventional fasteners of the kind referred to will engage the workpiece in the edge region closely bordering the aperture and will lead to stressing of the material in this region.

We have now devised a fastener of the general kind referred to in which the body is so shaped as to expand part of the sleeve to form a blind head having an internal diameter substantially greater than the external diameter of the part of the body which extends through the workpiece.

According to the present invention, there is provided, a blind fastener comprising a tubular body having a preformed head for abutting one face of an apertured workpiece, an elongate shank joined to the preformed head and able to extend through the workpiece so as to project beyond the opposite face (hereinafter referred to as the blind face) of the workpiece, and a nose at the end of the shank remote from the preformed head, which nose tapers externally from a diameter the same as that of the shank to a smaller diameter at the end remote from the shank, a ductile tubular sleeve, and means for forcing the sleeve over the tapering nose end towards the preformed head so as to cause the sleeve to be progressively expanded radially to form a blind head, wherein that external surface of the nose which operates to cause expansion of the sleeve is tapered at a greater angle to the longitudinal axis of the body in a region adjacent to the shank than in a region nearer the smaller diameter end of the nose, said greater angle being sufficiently great that, when the sleeve is forced over the nose so that its leading end passes beyond the nose, the leading end of the sleeve continues to expand radially to a diameter substantially greater than the external diameter of the shank, and is thereby permanently deformed to an extent such that its internal diameter remains substantially greater than the external diameter of the shank.

The nose may be tapered through a plurality of frusto-conical portions.

The nose may have two frusto-conical portions of which the portion nearer the shank has a greater apical angle than that remote from the shank.

The nose may have a frusto-conical portion immediately adjacent the shank, and two or more further portions of which one may have an apical angle as great as that of the portion adjacent the shank provided that the nose has a further portion of smaller apical angle between the said one further portion and the portion adjacent the shank.

The nose may be tapered to a smaller diameter through a plurality of portions of which each one nearer the shank is more steeply tapered to a greater apical angle than its predecessor nearer the end of the nose remote from the shank.

The nose may have an external surface which forms, in the direction longitudinally of the body, a concave curve providing increasingly steeply tapered portions nearer the shank.

The curve may be continuous.

The curve may be an arc of a circle.

The curve may be an arc of an ellipse.

Specific embodiments 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 first fastener, inserted through a workpiece, before installation therein; Figure 2 is similar to Figure 1 but shows the fastener installed; Figure 3 illustrates the shape of the nose of the fastener shank; and Figure 4 is similar to Figure 3 but illustrates the shape of the nose of a second fastener.

Referring first to Figure 1 of the drawings, a blind fastener 10 comprises three pieces, namely a tubular nut 1 2 which constitutes the body of the fastener, a bolt 14, and an expansible sleeve 1 6.

The nut 1 2 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 of the nut, and is longer than the combined lengths of the nut 12 and the sleeve 16. The head 32 has substantially the same diameter as the shank of the nut. The stem of the bolt has a tail portion 40, remote from the head 32, which is provided with a pair of diametrically opposed flats 42 to facilitate gripping and rotating of the bolt. The tail portion 40 is connected to the remainder of the stem by a breakneck 34 at which the stem will break when subjected to a predetermined torque, thus enabling the tail portion 40 to be detached from the remainder of the bolt which remains as an essential part of the installed fastener.

The sleeve 1 6 of this embodiment is a tubular cylinder having an external diameter substantially the same as that of the shank 18 of the nut, and has a smooth axial bore which is internally chamferd at the leading end to enable the smaller diameter end of the nose 22 to enter easily thereinto. The diameter of the bore of the sleeve 1 6 is sufficiently great to enable the stem 30 of the bolt to extend through the bore with slight clearance.

Prior to use, the nut 12, bolt 1 4 and sleeve 1 6 are assembled together, the stem of the bolt being inserted through the sleeve and then screwed into the end of the nut remote from the hexagonal head 20 until the tail portion 40 projects from the head end of the nut. The assembled fastener can then be inserted into aligned apertures 35. 36 in members of a workpiece 37 which are to be fastened together, so that the shank 1 8 extends through the members with the hexagonal head 20 abutting the near face 38 of the workpiece, and the bolt head, sleeve. nose.

and part of the shank project beyond the opposite or blind face 39 of the workpiece.

The fastener can then be set by gripping and rotating the projecting tail of the bolt relative to the nut so as to cause the bolt head to move axially towards the nut and force the sleeve on to and over the nose of the nut until the leading end of the sleeve clamps the members of the workpiece tightly against the head 20. 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, allowing the tail portion to be discarded.

In accordance with the invention, the nose 22 of the nut of this embodiment is so shaped that, when the sleeve is forced towards the head so that the leading end of the sleeve passes over and beyond the nose, the leading end of the sleeve becomes expanded to a diameter substantially greater than that of the shank of the nut.

Thus, referring more particularly to Figure 1 of the drawings, the nose 22 has a terminal frusto-conical portion 44 adjacent the end of the nut remote from the head 20, and a second frusto-conical portion 46 between the terminal portion 44 and the cylindrical shank 18. The terminal portion 44 extends over about two thirds of the axial length of the nose, and the second portion 46 extends over the remaining one third.

As illustrated in Figure 3, in this embodiment the external surface of the terminal frusto-conical portion 44 tapers towards an apical angle of 40 , while the external surface of the second frusto-conical portion 46 tapers more steeply, towards an apical angle of 70 .

Thus, the apical angle of the portion 43 is substantially less than that of the portion 46, and the apical angle of the portion 46 is substantially greater than the apical angles of single-tapered noses of conventional fasteners of this general type, which are generally in the range of 30"-42". The angle between the surfaces of the two portions should be greater than 90 , and this embodiment is 165 .

It can therefore be appreciated that, when the sleeve is forced over the nose during setting of the fastener, the leading end 48 of the sleeve first encounters the gently tapering terminal portion 44 and, with comparative ease, is progressively expanded radially, with the expansion progressing along the length of the sleeve from its leading end, so that the sleeve becomes flared. Subsequently, the already flared leading end of the sleeve encounters the more steeply tapering second portion 46 by which it is then expanded at a greater rate per unit of longitudinal travel through which it is moved by the bolt head. Eventually, the leading end of the sleeve reaches and passes the point of intersection between the second portion 46 of the nose and the cylindrical shank.As the shank 18 is cylindrical and has a constant diameter the same as the maximum diameter of the nose, the shank provides no internal support for the sleeve.

Due to the taper of the second portion 46 of the nose and notwithstanding the increasing hoop stress generated in the expanding sleeve, the leading end of the sleeve continues to move substantially in line with the surface of the portion 46 for a short distance and thus continues further to expand radially until the hoop stress in the sleeve overcomes the tendency for the leading end to continue expanding. The leading end then ceases to expand further.

We prefer that the leading portion of the sleeve which passes beyond the nose should assume a substantially cylindrical form in the region adjacent to the leading end, while also having a diameter, internally, substantially greater than that of the shank, rather than that it should flare or taper towards the leading end. We therefore make the angle at which the second portion 46 meets the cylindrical shank 1 8 such that eventually, and before the leading end 48 will meet the workpiece, the tendency of the sleeve to continue to expand at the same rate as when passing over the surface of the second portion 46 will be substantially balanced by the hoop stress in the sleeve which simultaneousiy tends to cause the leading end of the sleeve to turn radially inward towards the external surface of the shank.The precise angle at which the second portion of the nose should meet the shank in order to balance the forces acting on the leading portion so that it will become cylindrical must be determined with regard to the length of the second portion and the physical properties, particularly the ductility, elasticity and hardness, of the leading end of the sleeve. In practice, we determine the angle empirically.

Continued rotation of the bolt with respect to the nut causes the substantially cylindrical leading part of the sleeve 1 6 to advance, under the urging of the bolt head at the other end of the sleeve, until it abuts the blind face 39 of the workpiece and applies a force to it.

This it does over an annular zone which is spaced apart radially from the edge of the aperture 36, so that stress is not applied to the workpiece immediately adjacent this edge.

The expanded sleeve provides a blind head which, together with the preformed head 20 of the nut, clamps the two members of the workpiece tightly together. Resistance to rotation of the bolt increases, and the tail portion 40 breaks off, leaving the fastener installed in the workpiece, as illustrated in Figure 2. The nut, bolt and expanded sleeve are effectively secured together by frictional engagement.

The distinct internal chamfer on the leeding end of the sleeve is no longer apparent, having- been smoothed out, by the passage of the sleeve along the nut nose.

The angles chosen for this embodiment and stated above have been found appropriate for a sleeve formed of a stainless steel specified as A.1.5.1.304 (i.e. American Iron and Steel Institute designation No.304). Generally, we have found that the apical angle of the portion 46 should be greater than is usual for the single taper used in conventional fasteners of this kind, and preferably should not be less than about 60' It is to be noted however, that if the apical angle of the portion 46 is too great, not only will excessive force be required to force the sleeve over this portion of the nose, with the attendant risk of buckling the sleeve or causing premature breaking of the breakneck, but also the hoop stresses in the leading end of the sleeve will not be sufficient to prevent continued expansion of the leading end of the sleeve within a reasonable distance of longitudinal travel, with the result that the leading end portion of the sleeve will have a flared shape rather than a cylindrical shape. This is not desirable.

Instead of only two differently tapered portions, the nose may have a plurality of portions of which each one nearer the shank is more steeply tapered than its predecessor nearer the end remote from the head 20.

Thus, in a further embodiment by way of an extreme but practical example of such a construction, a fastener generally similar to that of Figure 1 and illustrated fragmentarily in Figure 4 has a nose 22 of which the external surface forms, in the direction longitudinally of the nut, a continuous concave curve which can be regarded as being constituted by an infinite number of such increasingly steeply tapered portions.

In this embodiment, the curve is formed as an arc of a circle of radius r of which the centre C is spaced forwardly of the end of the nose remote from the head of the nut by a short distance d1, and radially from the axis A of the nut by a distance which is greater than the sum of the radii of the bore of the nut and the circle itself by a small distance d,.

The distance d, is chosen preferably to be such that, at the point where the curved external surface of the nose meets the end face of the nose, a tangent to the curve meets the end face at an angle greater than 90 , so that there is an immediate increase in diameter of the nose away from the end face.

The distance d, is chosen preferably to be such that there is a finite distance radially across the end face between the internal surface of the bore of the nut and the external surface of the nose, so that the end face is not a knife-edge.

The length of the radius of curvature r of the curved external surface of the nose is chosen so that the rate of change of diameter of the external surface of the nose is acceptable in relation to the rate of change in diameter which the sleeve can be forced to undergo, having regard to the size of the fastener, the ductility of the sleeve, and the force avaiiable for deforming the sleeve.

In a specific example of this embodiment, the shank of the nut has an external diameter of 7.9 mum, the major diameter of the threaded bore is 5.6mm, and the minimum diameter of the nose is 5.9mm. The radial width of the end face of the nose is therefore, (5.9 - 5.6) . 2mum = 0.1 5mm. The curvature of the external surface of the nose, in the longitudinal direction, is such that it forms part of a circle having a radius of 5.7mm, and the distance d1, which is determined by the angle at which the external surface of the nose meets the end face, is 1 .0mum, and the distance d,, which is determined by the radial width of the end face and the value of d1, is slightly less than 0.1 5mm.

The effect of the curvingly tapered nose upon the sleeve, when the sleeve is forced on to and beyond the nose, is substantially the same as in the previous embodiment in that the leading end of the sleeve is expanded progressively and at a rate which increases as the leading end progresses from the tip of the nose towards the shank. As in the previous embodiment, the angle at which the leading end of the sleeve eventually leaves the nose, at the intersection of the nose and shank, is sufficient to cause the leading end to continue to expand radially before the increasing hoop stress in the leading end of the sleeve appreciably restricts the radial expansion.As in the previous embodiment, the hoop stress eventually restricts the expansion sufficiently to cause the part of the sleeve following the leading end to assume a substantially cylindrical shape, surrounding but radially spaced from the external surface of the shank. The cylindrical leading part of the sleeve continues to advance, under the urging of the bolt head at the other end of the sleeve, until it abuts the workpiece and thus constitutes a blind head which, in cooperation with the head of the nut, serves to fasten the members of the workpiece.

In contrast to the nose of the first embodiment, in which the taper is provided by two distinct frusto-conical portions of which the second has an appreciably greater apex angle than the first, or terminal, portion, the rate of change from small diameter to maximum diameter of the nose of this embodiment is smooth throughout the nose, and consequently the amount of force required to force the sleeve over the nose does not increase suddenly. There is therefore less risk with this embodiment of a shock reaction causing the breakneck to break prematurely.

In this embodiment, the concave curvature corresponds in function and effect to the internally-angled relationship of the two frustoconical portions of the first embodiment.

Instead of the concave curvature of the nose being in the form of an arc of a circle, it could be in the form of an arc of an ellipse without substantially altering its performance.

provided the criterion as to the angle at which a line or tangent to the curve of taper meets the external surface of the shank, and, desirably, the end face or tip of the nose, is met.

Although, in the foregoing embodiments.

the construction of the fastener is such that it can be set by rotation of the bolt to urge the sleeve on to and over the nose by reason of the lead-screw effect achieved by the threaded interengagement between the nut and bolt.

other constructions whereby the sleeve can be forced over the nose and, once it has been expanded and urged to fasten the members of a workpiece together, be secured to the body of the fastener, are possible.

Thus, it is not necessary for the stem of the bolt and the bore of the nut to have interengaging screw-threads, but instead, the nut could be a headed body having a shank, a tapered nose, and a smooth bore instead of a threaded bore, and the stem of the bolt could then be pulled axially along the bore to cause the bolt head to urge the sleeve over the nose and to engage the workpiece, the stem of the bolt being secured subsequently to the body so as to retain the sleeve in position in any suitable manner such as, for example, by being deformed to form an abutment able to cooperate with the body to prevent retraction of the bolt and loss of the expanded sleeve.

In embodiments of the invention in which the tapering nose is formed of a plurality of frusto-conical portions, as for example in the embodiment of Figure 1, it is to be noted that the length of the frusto-conical portion nearest the shank has an infiuence on the extent to which the leading end of the sleeve continues to expand radially after leaving the supporting surface of the nose. Thus, it has been found that, within limits, the greater the length of the frusto-conical portion nearest the shank, the greater the extent by which the diameter of the expanded sleeve will exceed the diameter of the shank.

Similarly, in embodiments in which the nose presents a concavely curved surface which meets the external surface of the shank, the radius of curvature of the curved surface will have an influence on the extent to which the sleeve continues to expand.

In a still further embodiment, the body of the fastener has a nose having a frusto conical portion adjacent to the shank, and nearer the tip of the nose a further frusto-conical portion of which the apical angle may be as great as, or possibly even greater than, that of the portion adjacent the shank, the two frustoconical portions being separated from each other in the axial direction by an intermediate portion of smaller or zero apical angle.

In considering the magnitude of the angle formed in any embodiment between the longitudinal axis of the bolt and the surface of the terminal portion or the tangent to the curve as it approaches the end face or tip of the nose, any angle formed by any bevelling or radiussing of the tip of the nose for avoiding sharpness of the nose-tip or assisting its location in the bore of the sleeve, can be ignored.

Although we have disclosed some specific embodiments and dimensions, it will be appreciated that the invention is not restricted thereto, as it is possible to make variations in the details of shape and dimensions. For instance when the tapering nose is formed of a plurality of frusto-conical portions, one frustoconical portion may merge into the next by means of a portion of suitably concavely curved profile.

Claims

1. A blind fastener comprising a tubular body having a preformed head for abutting one face of an apertured workpiece, an elongate shank joined to the preformed head and able to extend through the workpiece so as to project beyond the opposite face of the workpiece, and a nose at the end of the shank remote from the preformed head, which nose tapers externally from a diameter the same as that of the shank to a smaller diameter at the end remote from the shank, a ductile tubular sleeve, and means for forcing the sleeve over the tapering nose and towards the preformed head so as to cause the sleeve to be progressively expanded radially to form a blind head, wherein that external surface of the nose which operates to cause expansion of the sleeve is tapered at a greater angle to the longitudinal axis of the body in a region adjacent to the shank than in a region nearer the smaller diameter end of the nose, said greater angle being sufficiently great that, when the sleeve is forced over the nose so that its leading end passes beyond the nose, the leading end of the sleeve continues to expand radially to a diameter substantially greater than the external diameter of the shank, and is thereby permanently deformed to an extent such that its internal diameter remains substantially greater than the external diameter of the shank.

2. A blind fastener according to Claim 1, wherein the nose is tapered through a plurality of frusto-conical portions.

3. A blind fastener according to either of Claims 1 or 2, wherein the nose has two frusto-conical portions of which the portion nearer the shank has a greater apical angle than the portion remote from the shank.

4. A blind fastener according to either of Claims 1 or 2, wherein the nose has a frustoconical portion immediately adjacent the shank, and two or more further portions of which one may have an apical angle as great as that of the portion adjacent the shank provided that the nose has a further portion of smaller apical angle between the said one further portion and the portion adjacent the shank.

5. A blind fastener according to Claim wherein the nose is tapered to a smaller diameter through a plurality of portions which may form a curve, of which each one nearer the shank is more steeply tapered to a greater apical angle than its predecessor near the end of the nose remote from the shank.

6. A blind fastener according to either of Claims I or 5, wherein the nose has an external surface which forms, in the direction longitudinally of the body, a continuous concave curve providing increasingly steeply tapered portion nearer the shank.

7. A blind fastener according to Claim 6, wherein the curve is an arc of a circle.

8. A blind fastener according to Claim 6, wherein the curve is an arc of an ellipse.

9. A blind fastener according to any preceding claim, wherein the nose is tapered in the region immediately adjacent to the shank towards an apical angle of not less than 60

1 0. A blind fastener according to any preceding claim, wherein the region of the nose immediately adjacent to the shank is tapered towards an apical angle of about 70 .

11. A blind fastener according to any preceding claim, wherein the tubular body is a screw-threaded nut, and the means for forcing the sleeve over the tapering nose of the nut comprises a headed bolt having a screwthreaded stem which extends through the sleeve and into threaded engagement with the nut, and the sleeve is disposed on the stem between the head of the bolt and the nose of the nut.

1 2. A blind fastener substantially as hereinbefore described with reference to the accompanying drawings.