GB1604502A

GB1604502A - Blind fastener

Info

Publication number: GB1604502A
Application number: GB1972878A
Authority: GB
Original assignee: SPS Technologies LLC
Current assignee: SPS Technologies LLC
Priority date: 1978-05-12
Filing date: 1978-05-16
Publication date: 1981-12-09
Also published as: DE2821356A1; GB1604503A; FR2425573B1; FR2425573A1; GB1604504A

Description

(54) BLIND FASTENER (71) We, SPS TECHNOLOGIES, INC., a Corporation duly organised and existing under the laws of the Commonwealth of Pennsylvania, United States of America, of Jenkintown, Pennsylvania 19046, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to improvements in blind fasteners and to the joints produced therewith.

Advanced composite materials have recently been gaining increasing usage in various aerospace applications because they offer potential weight savings and increased stiffness.

The anisotropic properties of composite materials present a unique opportunity to optimise certain structural designs such as elongated aircraft stringers and ribs. In order to form acceptable joints of relatively thin sheet composite materials, conventional blind fasteners have not proven to be completely acceptable. One reason is the low allowable bearing stress of composite materials, which means that the full tensile strength of the blind fastener may not be realised because of bearing failure of the composite material. Present blind fasteners have a relatively limited blind side bearing area and typically expand to approximately 1.3 times the original sleeve diameter.. Another reason is that composite materials are weaker in shear, thus causing any holes to require greater edge distances and more area build up to develop full efficiency of the composite material.These materials are further constructed as a resin matrix which has a tendency to crack when holes in the material are expanded, such as by cold working or by an interference fit fastener. Rivets which swell when upset may also crack the resin matrix. Composite materials are generally fabricated in layers and can be formed to complex shapes whose surfaces are not flat. Conventional blind fasteners generally cannot form to the irregular contour of the composite material surface and tend to crush the high points of the material, reducing overall material strength.

In order to solve these problems, the present invention is directed to an improved blind fastener which has a large bearing area, does not expand the holes in respective workpieces forming a joint in which the fastener is installed and forms to the contour of the blindside workpiece surface.

Accordingly, it is a general purpose and object of the present invention to provide an improved blind fastener assembly. It is a further object to provide an improved blind fastener assembly for use with composite materials. It is yet another object of the present invention to provide an improved blind fastener having a novel grip-accommodating feature. It is still a further object to provide a blind fastener assembly in which the blind side bearing surface will conform to an irregular contour of the blind side workpiece surface.

And it is an object of the present invention to provide a blind fastener assembly which is expandable to approximately 1.5 times its unexpanded diameter.

These and other objects are accomplished according to the present invention by a blind fastener assembly including a pulling pin, a sleeve and an expander. The expander is formed to be placed through opening in a workpiece, and the pin, with the sleeve carried thereon adjacent a headed, blind side end of the pin, is placed through the expander. One end of the sleeve adjacent a tapered nose surface on the expander has a reduced radial sickness portion over a selected length thereof including a reduced outside diameter and a corresponding increased inside diameter in the portion adjacent the reduced outside diameter portion.Upon application of an axial force on the pin, the reduced thickness sleeve portion advances over the tapered nose surface of the expander forcing the reduced thickness portion radially outwardly, and continuing to advance the sleeve until it contacts the blind side of the workpiece surface. A radially outwardly extending bulb thereupon forms with its apex intermediate the end of the reduced thickness sleeve portion, and bears against the blindside workpiece surface.

A grip-accommodating feature may be included by providing a reduced varying radial thickness portion in a sleeve adjacent the other end in contact with the pulling pin head.

Continued application of axial force to the pin causes a bulb to form at this reduced thickness portion to accommodate variations in workpiece thickness.

The invention provides a blind fastener assembly for installation in a workpiece having an opening therein, said assembly comprising a pin member having an enlarged head at one end thereof, an elongate shank portion and means on said shank portion operative to advance said pin member in a direction along its longitudinal axis; expander means having an enlarged head at one end thereof formed to bear upon an accessible surface of the workpiece, a tapered surface of varying radial thickness in a longitudinal direction adjacent the other end thereof, said tapered surface varying from a minimum radial thickness at said other end to a maximum radial thickness at a locus of points spaced from said other end, and a shank portion intermediate said enlarged head and said tapered surface, said shank portion having an outer surface formed to be placed in the workpiece opening, and an internal bore extending through said expander means carrying said pin member shank portion; and an expandable sleeve carried on said pin member shank portion adjacent said enlarged head, said sleeve including a first portion of greater radial thickness adjacent one end thereof adjacent said pin member enlarged head and a second portion of reduced radial thickness extending from the end of said first portion remote from the enlarged head of said pin member and adjacent said minimum thickness and of said expander means tapered surface, said second portion having a reduced radial distance to the outside surface thereof and an increased radial distance to the inside surface thereof each relative to the radial distance to the respective outside and inside surfaces of said first portion, whereby there is a discontinuity in the outside surface and in the inside surface of said sleeve at the junction of said first and said said second portions thereof, said sleeve second portion being arranged to be advanced along said expander tapered surface and said expander outer surface and expanded thereover upon advancement of said pin member toward a blind side surface of the workpiece, the second portion of said sleeve forming a radially outwardly extending bulb at said junction of said first and second portion of the sleeve, said bulb ultimately bearing against the blind side workpiece surface.

The invention also provides a joint assembly comprising in combination a workpiece having a first surface, a second surface and a hole therebetween a blind fastener assembly as set out in the immediately preceding paragraph.

Several embodiments of blind fastener assemblies in accordance with the invention are now described with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view illustrating a first embodiment of a blind fastener assembly installed in a pair of workpieces prior to any forming operation Figure 2 shows the embodiment of Figure 1 in an intermediate, partially formed condition; Figure 3 shows the embodiment of Figure 1 in a finally formed condition; Figure 3A shows the formed blind fastener assembly in Figure 3, adhering to an irregular contour of the blindside workpiece surface; Figure 4 is a cross-sectional view illustrating a second embodiment of a blind fastener assembly installed in a pair of workpieces prior to any forming operation; Figure 5 shows the embodiment of Figure 4 in an intermediate, partially formed condition;; Figure 6 shows the embodiment of Figure 4 in a finally formed condition; Figure 7 is a cross-sectional view of another embodiment of the sleeve shown in Figure 1 illustrating another feature in accordance with the present invention; Figure 8 is a cross-sectional view illustrating the sleeve shown in Figure 7 in a finally formed condition; Figure 9 is a cross-sectional view of another embodiment of the sleeve/expander shown in Figure 4; Figure 10 is a cross-sectional view of the sleeve and expander of the embodiment of Figure 1 showing certain dimensional relationships; Figure 11 is a cross-sectional view of the sleeve and expander of the embodiment of Figure 1 showing another possible dimensional relationship; Figure 12 shows the bulb formed according to the dimensional relationships in Figure 10; ; Figure 13 shows the bulb formed according to the dimensional relationship of Figure 11; Figure 14 illustrates a grip accommodation dimensional relationships of the sleeve and expander shown in Figure 1, and Referring to Figures 1-3, the first embodiment of a blind fastener assembly in accordance with the present invention is shown in varying stages of installation. Fastener assembly 10 includes a pin member 12, an expander 14 and a sleeve 16. Pin member 12 has an enlarged head 18 at one end thereof and may, in one configuration, include standard external threads 20 which are designed to engage mating threads 22 on expander 14 in order to draw sleeve 16, which is carried on the shank of pin member 12 adjacent head 18, axially toward expander 14.In the particular configuration shown, a wrenching configuration such as a pair of flats 23 are included on pin member 12 at a free end 24 for accepting a power tool driving bit. It should be understood that pin member 12 may include a plurality of circumferential grooves (not shown) adjacent end 24 instead of external threads 20 and mating threads 22 on expander 14. Such grooves are commonly employed on similar pulling pin devices and are formed to be gripped by any one of a number of conventional pull-type guns for exerting the necessary axial force on pin member 12. This arrangement is not illustrated because it is quite conventional and fully known to those skilled in the art. Pin member 12 also includes a breakneck groove 26 of reduced cross-sectional diameter, which is designed to fracture at a predetermined axial or torsional load.Expander 14 includes an enlarged head 28, a shank portion 30 of constant outside diameter and a nose portion having a tapered surface 32 opposite from head end 28. Tapered surface 32 forms an angle of the order of approximately 12" with the longitudinal axis of expander 14. Expander 14 extends through aligned openings in a pair of workpieces 34 and 36, with head 28 designed to bear upon a corresponding countersunk section in workpiece 34, and outer peripheral surface 31 of shank 30 designed to fit through the aligned openings with a relatively close fit, that is something less than an interference fit. Head 28 may include a recess 29 for accepting a portion of the power tool bit (not shown) to keep expander 14 from rotating during the assembly of the joint.It is to be understood that head 28 may be of any configuration while still remaining within the scope of the present invention. Sleeve 16 includes a portion 38 adjacent head 18 of pin member 12, and a portion 40 of reduced radial thickness adjacent tapered surface 32 of expander 14. Portion 40 includes an outer surface 42 and of a diameter smaller than the diameter of the outer surface of the portion 38 of the sleeve a counterbored internal surface 44. A shoulder 46 joins surface 42 and the outside surface of portion 38, and a shoulder 48 joins inside surface 44 and the inside surface of portion 38. Shoulder 48, preferably, but not necessarily, forms an approximately 1200 included angle, while shoulder 46 preferably forms an approximately 20 angle with the longitudinal axis of sleeve 16.

When an axial force is exerted on pin member 12, such as by applying torque to the pin or by pulling on the pin by means of a pull gun (not shown), surface 44 on sleeve 16 contacts tapered surface 32 on expander 14, and portion 40 begins to advance over the tapered surface 32, where it is expanded from its original diameter. Portion 40 continues to advance over intersection 33 of surfaces 31 and 32 along surface 31 of expander 14 until the free end 41 of portion 40 contacts the blind side surface of workpiece 36 as shown in Figure 2. At this point, intersection 33 should be mid-way between the ends of reduced thickness portion 40 in an optimum configuration joint. Optimum dimensional relationships between the length of portion 40 and the distance from the blind-side surface of workpiece 36 to intersection 33 of expander 14 will be discussed more fully hereinafter.As pin member 12 continues to advance in an axial direction, portion 40 begins to bulb radially outwardly at approximately the middle of its length, causing a maximum diameter bulb 49 to form in an optimum configuration adjacent the blind-side surface of workpiece 36. This maximum diameter bulb is typically 1.5 times the original unexpanded diameter of sleeve 16. During the formation of bulb 49, as shoulder 48 contacts the free end of tapered surface 32, a tip portion 47 of the expander 14 deforms shoulder 48 producing an extruding effect on the thicker portion 38.

However, shoulder 46 is not affected during this extruding process, thus insuring that reduced thickness portion 40 always remains at a fixed position forward of shoulder 46. This assures optimum formation of the bulb within the reduced thickness portion. Continued application of torque or pulling of pin member 12 causes the pin to fracture at breakneck groove 26, resulting in the final assembled joint shown in Figure 3. It should be understood that some form of locking configuration (not shown) may be provided between pin member 12 and expander 14 to keep the pin member from moving in an axial direction after the joint is clamped, and to maintain preload in the joint.It should be understood that the leading edge of the reduced thickness portion 40 of sleeve 16 could include a radius or combination of curvatures which would help to prevent the sleeve from marring the blind-side surface of workpiece 36 as the bulb forms. It is also pointed out that reduced thickness portion 40 could be selectively annealed along its length to provide a hardness gradient with the softest point being at approximately the mid-point of portion 40 to ensure that the bulb forms at the optimum point to produce a maximum diameter and optimum shaped bulb. Lubricants can be provided between the contacting surfaces of expander 14 and sleeve 16 to reduce the forces required to expand and bulb the sleeve.

Referring now to Figure 3A, a formed blind fastener assembly is shown wherein the bulb 49a adheres to the irregular contour of the blind-side surface workpiece of 36a. The present blind fastener assembly can accomplish this desirable result because of the control over the formation of the blindside bulb which will be discussed in greater detail hereinafter. Present blind fasteners generally tend to form more rigidly and straighter, thus only contacting the higher irregularities on the blind-side surface of workpiece 36a.

Referring now to Figures 4-6, the second embodiment of a blind fastener assembly in accordance with the present invention is illustrated in successive stages of installation. A fastener assembly 50 includes a pin member 52 and a sleeve/expander member 54. The pin member has an enlarged head 56 and a breakneck groove 58 which function in the same manner as in pin member 12 shown in Figures 1-3. Sleeve/expander member 54 is generally similar to the configuration of expander 14 and sleeve 16 shown in Figures 1-3, except that they are joined at a frangible section 60 to form an integral member. Preferably, but not necessarily, a reverse tapered surface 55 is provided on the inner surface between the sleeve portion and the expander portion of member 54 to aid in the formation of the bulb.Upon application of an axial load to pin member 52, either by torquing or by pulling, a bulb 62 begins to form in a reduced radial thickness sleeve portion 64 (Figure 4) as shown in Figure 5. At some predetermined force, the sleeve portion with the bulb fully formed breaks away from the expander portion at frangible section 60. Continued axial force causes the formed bulb 62 to advance along the tapered surface nose portion of the expander portion where it is increased in diameter, and onto the constant diameter outer surface of the expander portion and thereupon clam against the blind-side surface of workpiece 36, as shown in Figure 6. One advantage of the configuration shown in Figure 4 is that formation of the bulb away from the blind-side surface of workpiece 36 prevents possible marring of the workpiece surface during formation of the bulb.This feature may be desirable for certain types of composite materials.

In Figures 7, 8 and 9, a grip accommodating feature in accordance with the present invention is illustrated in two embodiments of blind fasteners. In Figure 7, a sleeve 66 is shown which is generally similar to sleeve 16 in Figure 1. Sleeve 66 includes a normal radial thickness portion 68 and a reduced thickness portion 70 at one end, having a reduced outside diameter and a corresponding counterbore on its internal surface. Portion 68 includes a section 72 of varying reduced radial thickness adjacent the other end of the sleeve. Section 72 must be of a greater thickness throughout its axial length than portion 70.

Referring to Figure 8, when pin member 12 is moved in an axial direction either by torquing or by pulling, portion 70 of sleeve 66 advances over the tapered surface nose portion 2 of expander 14 until the leading edge contacts the blind-side surface of workpiece 36. Further axial movement of pin member 12 causes the thinner radial portion 70 to form a bulb 49 at the blindside workpiece surface, as previously described with reference to Figures 1-3.

After bulb 49 is formed, further axial movement of pin member 12 causes a second bulb 74 to form at section 72 in sleeve 66. Thus, any variations in the thickness of workpieces 34 and 36 can be accommodated by formation of this second bulb 74 before the break-neck groove 26 in pin member 12 fractures at a predetermined axial position with respect to expander 14 to form the final clamped joint. In order to ensure that pin member 12 stops in the same axial position each time relative to expander 14, threads 20 extend a predetermined axial distance on pin member 12 to a shoulder 75 having a diameter approximately equal to the major thread diameter of threads 20. If the thickness of the workpieces 34 and 36 is closer to the maximum grip range of the blind fastener assembly, a larger bulb 74 will form before the pin member 12 fractures.If the thickness of the workpieces 34 and 36 is closer to the minimum value of the grip range of the blind fastener assembly, a smaller diameter bulb 74 will form. This grip-accommodating feature in sleeve 66 is quite useful in adapting blind fastener assemblies to relatively wide variations of workpiece thicknesses.

Figure 9 illustrates another embodiment of the grip-accommodating feature in conjunction with the sleeve/expander of Figures 4-6. A variable reduced thickness section 76, similar to section 72 in Figure 7, is included in the sleeve portion of a sleeve/expander 78, similar to sleeve/expander member 54 in Figure 4 - 6, and functions in the same manner as previously described. Referring to Figure 6, after the initial bulb 62 is fully formed and clamped against the blind side surface of workpiece 36, further axial movement of the pin member will cause a second bulb to form at section 76, as previously described with reference to Figure 8. The pin member will then break at a predetermined axial position with respect to the expander.

The grip-accommodating feature shown in Figures 7, 8 and 9 is also described and claimed in a blind fastener assembly in the complete specification of our Patent Application No. 80 38897 Serial No. 1604503 which is divided from this Application.

Referring now to Figures 10 and 11, the sleeve 16 and expander 14 of Figures 1-3 are shown. Critical dimensional relationships will be discussed with reference to these Figures.

Dimension "L" represents the axial length of reduced thickness portion 40 of sleeve 16, and dimension "d" represents the distance from the surface of workpiece 36 to the intersection 33 of surfaces 31 and 32 on expander 14. A critical dimensional relationship exists between the position of expander 14 relative to the position of sleeve 16 at the time during installation when the leading edge 41 of the sleeve 16 contacts the blind-side surface of workpiece 36. As mentioned previously with respect to the formation of the joint in Figures 1-3, reduced thickness portion 40 advances along surfaces 32 and 31 of expander 14, and is deformed at intersection point 33 when the leading edge 41 of sleeve 16 contacts the blind-side surface of workpiece 36.This is true, however, only if intersection 33 is located somewhere within dimension "L" of portion 40 on sleeve 16 when edge 41 contacts the blind-side surface of workpiece 36. The optimum location of intersection 33 is when D =4, since with this dimensional configuration, the bulb will start to form in the centre of the column (reduced thickness portion 40), producing an optimum-shaped, maximum diameter bulb 86, as shown in Figure 12. If intersection 33 is not located within dimension "L", as shown in Figure 11, a bulb 88 of smaller diameter will form adjacent the blind-side surface of workpiece 36. As can be seen in Figure 13, bulb 88 is not optimally formed as is bulb 86 in Figure 12.Since intersection 33 is not located within length "L" of reduced thickness portion 40 on sleeve 16, the intersection 33 has no effect on where the bulb forms. In this configuration, the column can be considered as having a free end 41 where it contacts the blind-side surface of workpiece 36, and a fixed restraint at shoulder 46 where it joins the normal thickness portion 38 of sleeve 16. The result of this configuration is nonsymmetrically formed bulb 88, being smaller in diameter than bulb 86 in Figure 12 and having a gap 89 between normal thickness portion 38 and bulb 88.

In order to minimize the overall length of the blind-fastener assembly, dimension "d" should be the minimum length at which an optimum-shaped, maximum diameter bulb will form.

Referring now to Figure 14, if the blind fastener assembly is to accommodate a range of grip lengths, then the following optimum dimensional relationships should be used: L + 0 dMAX = 2 + 2 = L - dMIN 2 2 where G = a predetermined variation in workpiece thickness commonly referred to as a grip-accommodation. A typical grip-accommodation value usable in accordance with the present invention is 1/16 inch. Beyond "dMin" or less, the resultant bulb tends to form to a smaller final diameter due to the location of intersection 33, and, correspondingly, beyond "dMaX" or greater the resultant bulb also tends to form at a smaller diameter, as shown for example in Figure 13. Therefore, exceeding the optimum dimensional range results in a less than optimum bulb diameter and shape.

For a predetermined grip-accommodation value "G", optimum dimensional ranges have been determined for a nominal 3/16 inch diameter, and a nominal 1/4 inch diameter blind fastener assembly. For a 3/16 inch diameter assembly, d = .25 L to .75 L and for a 1/4 inch diameter assembly, d = .33 L to .65 L These dimensional ranges have generally yielded optimum-shaped, maximum diameter bulbs for the two sizes listed.

Having thus described several embodiments of the present invention, some of the advantages should now be readily apparent. Formation of an optimum-shaped, maximum diameter bulb which will repeatably form adjacent the blindside workpiece surface is achieved by the present invention. The bulb will form against an irregularly contoured blind-side surface. There is no radial expansion of the workpiece holes during the forming of the joint. A novel grip-accommodation feature which can be used over a relatively wide variation in workpiece thicknesses can be employed on the blind fastener assembly of the present invention. Because of the controlled size and shape of the bulb which is formed, the blind fastener assembly of the present invention is ideally suited for use with advanced composite materials.It should also be clearly understood that the blind fastener assembly of the present invention is equally useful with conventional aerospace materials.

Two modifications of the sleeve shown in Figure 14 are described and claimed in a blind fastener assembly in the complete specification of our Patent Application No. 8102111 Serial No. 1604504 which is divided from this Application.

WHAT WE CLAIM IS: 1. A blind fastener assembly for installation in a workpiece having an opening therein, said assembly comprising a pin member having an enlarged head at one end thereof, an elongate shank portion and means on said shank portion operative to advance said pin member in a direction along its longitudinal axis; expander means having an enlarged head at one end thereof formed to hear upon an accessible surface of the workpiece, a tapered surface of varying radial thickness in a longitudinal direction adjacent the other end thereof, said tapered surface varying from a minimum radial thickness at said other end to a maximum radial thickness at a locus of points spaced from said other end, and a shank portion intermediate said enlarged head and said tapered surface, said shank portion having an outer surface formed to be placed in the workpiece opening, and an internal bore extending through said expander means carrying said pin member shank portion; and an expandable sleeve carried on said pin member shank portion adjacent said enlarged head, said sleeve including a first portion of greater radial thickness adjacent one end thereof adjacent said pin member enlarged head and a second portion of reduced radial thickness extending from the end of said first portion remote from the enlarged head of said pin member and adjacent said minimum thickness end of said expander means tapered surface, said second portion having a reduced radial distance to the outside surface thereof and an increased radial distance to the inside surface thereof each relative to the radial distance to the respective outside and inside surfaces of said first portion, whereby there is a discontinuity in the outside surface and in the inside surface of said sleeve at the junction of said first and said second portions thereof, said sleeve second portion being arranged to be advanced along said expander tapered surface and said expander outer surface and expanded thereover upon advancement of said pin member toward a blind side surface of the workpiece, the second portion of said sleeve forming a radially outwardly extending bulb at said junction of said first and second portions of the sleeve, said bulb ultimately bearing against the blind side workpiece surface.

2. A blind fastener assembly in accordance with Claim 1 wherein said expander means and said sleeve are an integral unit with a frangible section therebetween.

3. A blind fastener assembly in accordance with Claim 1 or 2 wherein said pin member further includes a portion of reduced cross-section designed to break at a predetermined load greater than the load necessary to form said bulb.

4. A blind fastener assembly in accordance with any preceding claim wherein said means on said shank portion operative to advance said pin member is an external thread, and wherein said expander means further includes a mating internal thread on said internal bore thereof.

5. A blind fastener assembly in accordance with any preceding claim wherein said expander means tapered surface forms an angle of substantially 12C with the longitudinal axis of said expander means.

6. A blind fastener assembly in accordance with any preceding claim wherein said sleeve further includes a section of reduced varying radial thickness in said sleeve first portion intermediate the end thereof, said section being of greater radial thickness than said sleeve second portion, whereby upon formation of said bulb in said sleeve second portion, further advancement of said pin member toward the blind side workpiece surface causes a second radially outwardly extending bulb to form at said section to accommodate variations in the thickness of the work iece.

7. A blind fastener assembly in accordance with any preceding claim wherein a first distance from the blind side workpiece surface to said maximum radial thickness of said expander means tapered surface is less than the axial length of said sleeve second portion.

8. A blind fastener assembly in accordance with Claim 7 wherein said first distance is equal to substantially one half of the axial length of said sleeve second portion.

9. A blind fastener assembly in accordance with Claim 7 wherein said first distance is within a range of between 0.25 and 0.75 times the axial length of said sleeve second portion for a nominal 3/16 inch diameter blind fastener assembly.

10. A blind fastener assembly in accordance with Claim 7 wherein said first distance is within a range of between 0.33 and 0.65 times the axial length of said sleeve second portion for a nominal 1/4 inch diameter blind fastener assembly.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. variation in workpiece thicknesses can be employed on the blind fastener assembly of the present invention. Because of the controlled size and shape of the bulb which is formed, the blind fastener assembly of the present invention is ideally suited for use with advanced composite materials. It should also be clearly understood that the blind fastener assembly of the present invention is equally useful with conventional aerospace materials. Two modifications of the sleeve shown in Figure 14 are described and claimed in a blind fastener assembly in the complete specification of our Patent Application No. 8102111 Serial No. 1604504 which is divided from this Application. WHAT WE CLAIM IS:

1. A blind fastener assembly for installation in a workpiece having an opening therein, said assembly comprising a pin member having an enlarged head at one end thereof, an elongate shank portion and means on said shank portion operative to advance said pin member in a direction along its longitudinal axis; expander means having an enlarged head at one end thereof formed to hear upon an accessible surface of the workpiece, a tapered surface of varying radial thickness in a longitudinal direction adjacent the other end thereof, said tapered surface varying from a minimum radial thickness at said other end to a maximum radial thickness at a locus of points spaced from said other end, and a shank portion intermediate said enlarged head and said tapered surface, said shank portion having an outer surface formed to be placed in the workpiece opening, and an internal bore extending through said expander means carrying said pin member shank portion; and an expandable sleeve carried on said pin member shank portion adjacent said enlarged head, said sleeve including a first portion of greater radial thickness adjacent one end thereof adjacent said pin member enlarged head and a second portion of reduced radial thickness extending from the end of said first portion remote from the enlarged head of said pin member and adjacent said minimum thickness end of said expander means tapered surface, said second portion having a reduced radial distance to the outside surface thereof and an increased radial distance to the inside surface thereof each relative to the radial distance to the respective outside and inside surfaces of said first portion, whereby there is a discontinuity in the outside surface and in the inside surface of said sleeve at the junction of said first and said second portions thereof, said sleeve second portion being arranged to be advanced along said expander tapered surface and said expander outer surface and expanded thereover upon advancement of said pin member toward a blind side surface of the workpiece, the second portion of said sleeve forming a radially outwardly extending bulb at said junction of said first and second portions of the sleeve, said bulb ultimately bearing against the blind side workpiece surface.

11. A blind fastener assembly in accordance with any one of Claims 1 to 6 wherein the

distance from the blind surface of one of the workpieces to said maximum radial thickness of said expander means tapered surface is between substantially one half of the axial length of said sleeve second portion plus one half of a predetermined grip range and substantially one half of the axial length of said sleeve second portion minus one half of a predetermined grip range.

12. A joint assembly comprising in combination a workpiece having a first surface, a second surface and a hole therebetween and a blind fastener assembly according to any preceding claim.

13. A joint assembly in accordance with Claim 12 wherein said workpiece second surface has an irregularly shaped contour and wherein the surface of the first mentioned bulb conforms to the irregular contour of said workpiece second surface.

14. A blind fastener assembly constructed and arranged substantially as described herein with reference to Figures 1 to 3 and 3A; Figures 4 to 6; Figure 7; Figure 8; Figure 9; Figure 10; Figure 11 or Figure 13 of the accompanying drawings.