Blind rivet with recessed expanding head

A blind rivet comprising a sleeve with a preformed rivet head at one end, and a setting pin telescoped into the sleeve and having a pulling section projecting beyond the headed end of the sleeve and an expanding head projecting beyond the other end of the sleeve, to be pulled into the sleeve from the headed end thereof to expand the other end of the sleeve into a bulb-shaped rivet head against one side of two workpieces that are to be riveted together, the pulling section being broken off after the rivet is set, in response to engagement of the expanding head with stop means in the sleeve. The expanding head has an expanding shoulder for initiating expansion of the sleeve, and a taper beyond this shoulder which progressively increases the diameter of the head toward its free end. The setting pin is composed of material of relatively high hardness for optimum strength in the completed joint, and the larger end portion of the taper is structurally weakened by a recess extending into the free end of the head; but stopping short of the expanding shoulder and the portion of the head that extends across the shear plane of the completed joint, so that the full strength of the material is available in the shear plane and the strength in the taper is substantially reduced. The weakening recess enhances the ability of the expanding head to form an optimum blind side rivet head in workpieces of varying grip length.

BACKGROUND OF INVENTION 
The present invention relates to rivets and, more particularly, to a 
so-called blind rivet especially adapted for the riveting of side-by-side 
workpieces where access is convenient from only one side of the 
workpieces. The side of the work from which the rivet is applied is 
referred to as the front or "access" side and will be so referred to 
herein. The other side is called the rear or "blind" side. 
A blind rivet of the general type to which the present invention relates is 
described in U.S. Pat. No. 3,285,121 wherein it will be seen that a headed 
sleeve is telescoped over, and positioned on, an elongated and specially 
formed setting pin having a pulling section projecting beyond the headed 
end of the sleeve and an expanding and locking head projecting beyond the 
opposite end of the sleeve. This head has an expanding shoulder adjacent 
the end of the sleeve and a taper which progressively increases the size 
of the head beyond the expanding shoulder. 
With its two basic parts in telescoped relation, the rivet customarily is 
inserted into aligned holes in two workpieces that are to be riveted, from 
the access side thereof, and is positioned with the head of the sleeve 
adjacent this side of the work and with the expanding head of the setting 
pin on the blind side. Then the pin is pulled from the access side while 
the sleeve is held against movement. This draws the expanding head into 
the blind-side end of the sleeve to clinch the workpieces together, 
expands the sleeve to fill the holes in the workpieces, and form a bulged, 
bulb-shaped head on the blind-side end of the sleeve. 
As the setting operation is completed, a locking crown disposed around the 
setting pin within the sleeve head on the access side is swagged into a 
locking groove in the pin, and the pulling section of the pin is broken 
off substantially flush with the sleeve head. In this manner, the 
workpieces are clinched together and are held between the two heads on the 
opposite ends of the sleeve. 
The tightness and fatigue life of each joint depend upon both the tensile 
and shear strengths of the rivet and the amount of bearing area that can 
be developed between the expanded, blind-side head of the blind-side 
workpiece without splitting or tearing the latter. Tensile and shear 
strengths, of course, depend primarily upon the strength of the materials 
used in the rivet, and increase as the hardness of the materials is 
increased. On the other hand, the amount of blind-side bearing area that 
can be developed depends upon the deformability of the sleeve, and the 
ductility of the expanding head, that is, the ability of the expanding 
head to wire-draw during the setting operation. This wire-drawing ability 
is particularly important to accommodate variations in the overall 
thickness of the work, or grip lenght, within permissible tolerances, and 
to avoid danger of splitting or tearing of the blind-side end of the 
sleeve or the blind-side workpiece, which sometimes has relatively low 
bearing strength. 
The aforesaid patent discloses a rivet in which the material used 
represents a compromise between high strength and high ductility. The 
aforesaid application discloses an improved rivet in which the expanding 
head is formed with a central core of high strength and has a softer, more 
ductile peripheral layer of annealed metal which is readily deformable 
during setting of the rivet. Thus, the core imparts high strength, while 
the peripheral layer provides the desired ductility. 
While it is possible to obtain such improved strength and head-forming 
characteristics by peripherally annealing the head of the setting pin, a 
need also has existed for a rivet with similar characteristics obtained 
without peripheral annealing, either because such annealing is impractical 
with certain materials, or because of the added manufacturing expense 
involved in the annealing operation. For example, it is desirable to use 
aluminum rivets in aircraft structures because aluminum is relatively 
strong and light in weight, but aluminum does not respond to annealing as 
rapidly as steel, and thus is difficult to anneal in the manner described 
in the aforesaid patent. 
SUMMARY OF THE INVENTION 
Accordingly, a primary objective of the present invention is to provide a 
blind rivet of the foregoing general character in which the optimum 
hardness of a given material may be used for the setting pin to provide 
high strength in the completed joint, and in which the formation of the 
blind-side head may be accomplished with virtually the same effectiveness 
as with a peripherally annealed expanding head, but without need for a 
peripheral annealing operation. For these purposes, a weakening recess is 
formed in the free end of the head to extend into the larger end portion 
of the taper and to reduce the cross-sectional thickness of the head 
sufficiently to permit controlled collapsing of the head during setting of 
the rivet. The weakening recess terminates short of the expanding shoulder 
and of the shear plane of the completed joint, so that the full 
cross-sectional thickness of the setting pin is disposed in the shear 
plane for relatively high shear strength in the completed joint. Thus, the 
hardness of the setting pin may be higher than would be satisfactory for 
the expanding head, which is structurally weakened by the recess to deform 
as needed during the setting operation. 
The weakening recess preferably is cylindrical in initial shape, but may be 
tapered inwardly to a rounded end, and also may have an inturned, 
reinforcing flange around its outer end. The taper of the expanding head 
typically is conical and constant, but alternatively may have a leading 
section of relatively steep taper and a trailing section of lesser taper, 
to enhance its bulge-forming capability. 
Other aspects and advantages of the invention will be apparent from the 
following detailed description, taken in conjunction with the accompanying 
drawings.

DETAILED DESCRIPTION 
As shown in the drawings for purposes of illustration, the invention is 
embodied in a blind rivet, indicated generally by the reference number 10, 
for joining together two workpieces, such as flat plates 11 and 12, when 
access is convenient from only one side of the work, the right side as 
viewed in FIGS. 2 through 5. In general, this type of rivet comprises two 
main parts, a tubular sleeve 13 sized to fit freely through two aligned 
holes 14 and 15 in the plates, and a setting pin 17 extending through the 
sleeve and positioned therein with one end portion 18 of the pin 
projecting through and beyond the right end of the sleeve, and with an 
expanding and locking head 19 on the opposite end of the pin, spaced 
beyond the left end 20 of the sleeve. The sleeve has a preformed head 21 
on its right end, so that the rivet 10 may be inserted into the holes 14 
and 15 from the access side and positioned in the plates with the sleeve 
head 21 against the right plate and the expanding head 19 projecting to 
the left beyond the left plate. With the parts in this condition, the 
setting pin is pulled to the right by a suitable tool head 22 while the 
sleeve and the right plate are restrained against movement, thus drawing 
the expanding head into the blind-side end of the sleeve to a bulb-shaped 
head 23 on the sleeve on the blind-side of the work. To complete the 
joint, a locking crown 24 integral with the sleeve head 21 is pressed into 
an annular locking groove 25 in the setting pin, and the pin is snapped 
off substantially flush with the access side of the joint. 
More specifically, the illustrative setting pin 17 comprises a central 
section 27 disposed primarily within the sleeve 13 and having an outside 
diameter fitting loosely therein, a right end section 18 formed with a 
series of closely spaced peripheral grooves 18a facilitating gripping of 
the pin by the tool head 22, and a weakening "breakneck" groove 28 around 
the central section adjacent the locking groove 25 for determining the 
eventual break-off plane of the pin. The locking groove separates the 
central section from the expanding head 19, the right end of which is 
formed by the left sidewall of the groove. The outer portion 29 of this 
sidewall extends radially outwardly beyond the outside diameter of the 
central section 27 and forms an expanding shoulder, which preferably is 
harder than the remainder of the expanding head. 
Beyond the shoulder 29 is a relatively short cylindrical portion 30 which, 
with the shoulder, makes up the expansion section of the head. The axial 
length of this section varies according to the thickness of the workpieces 
for which the rivet is designed, and may be limited to the axial length of 
the shoulder alone, as illustrated in FIGS. 7 and 8, or may be 
substantially longer, as illustrated at 30a in FIG. 6. The form shown in 
FIGS. 1 and 2 has an expanding section of intermediate length. 
Formation of the blind-side head 23 on the sleeve 13 during setting of the 
rivet 10 is accomplished primarily by a conical taper 31 on the head, 
which increases the size of the head progressively from the expansion 
section thereof toward the free, blind-side end 32. The taper may be 
formed at various angles, (for example, six degrees) and typically has a 
constant slope from the expansion section to a cylindrical free and end 
portion 33, as shown in FIGS. 1, 2, 7 and 8. 
Herein, the head 21 on the access-side end of the sleeve 13 is 
frustro-conical in shape and is fitted into a countersunk recess 34 in the 
access side of the plate 11 so as to be substantially flush with the 
access side of the work. The locking crown 24 is partially sheared out of 
the central portion of the head 21 and remains integrally joined thereto 
at 35 around the interior of the sleeve. The inside diameter of the sleeve 
is reduced inside the head 21 to form an annular internal stop shoulder 37 
facing toward the left and opposing the expanding shoulder 29 to abut 
against the latter and stop rightward movement of the expanding head 19 as 
setting of the rivet is completed, thereby assisting the tool head 22 in 
snapping off the pulling section 18 at the breakneck groove 28. 
Preparatory to the setting operation, the pulling section 18 is gripped 
between a plurality of jaws (not shown) in the tool head 22, and the end 
38 of the tool head is pressed against the locking crown 24 to seat the 
sleeve head 21 firmly in the countersunk recess 34. Then the setting pin 
17 is pulled to the right while the sleeve 13 is restrained by the tool 
head against movement to the right, thus drawing the expanding head 19 
into the blind end of the sleeve, which may be counterbored or internally 
relieved at 39 to facilitate the initial expansion of the blind end of the 
sleeve. The expanding shoulder 29 preferably is positioned in the 
counterbore adjacent the inner end thereof as the setting operation 
begins. 
As the shoulder 29 passes the end of the counterbore 39 and moves along the 
sleeve 13, an expansion wave 40 (FIG. 3) is set up in the sleeve and 
progresses along the sleeve with the shoulder. Upon engagement of this 
wave with the blind-side plate 12, it shifts the plate to the right and 
cooperates with the tool head 22 in clinching the plates tightly together. 
Then, when further movement of the blind-side plate 12 is prevented by the 
access-side plate 11, the expanding shoulder 29 is pulled into the portion 
of the sleeve within the plates (FIGS. 4 and 5), and expands the sleeve to 
fill the holes 14 and 15. The shoulder continues through the sleeve and 
across the shear plane of the joint (the plane between the two plates) 
until it abuts against the stop shoulder 37. 
As the expansion shoulder 29 is moving through the sleeve 13, the taper 31 
of the expanding head 19 follows the expansion wave into the blind-side 
end of the sleeve and further expands the sleeve to begin forming the 
blind-side head 23 thereon. As shown in FIG. 3, the entry of the taper 
into the sleeve forces the latter toward the blind-side plate 15 and 
begins to produce a pronounced bulge against the plate. Such bulging 
increases as the larger end portion of the taper enters the sleeve (FIG. 
4), and is accompanied by yielding deformation of the taper, as 
illustrated in FIGS. 4 and 5. 
As previously suggested, such deformation enables the setting pin 17 to 
accommodate variations in grip length without splitting the sleeve or the 
blind-side workpiece, and it therefore is highly desirable to provide a 
relatively soft and ductile expanding head for yielding during the setting 
operation. On the other hand, the overall strength of the completed joint 
depends upon the strength of the material of the rivet 10, and thus 
increases with the hardness of the material. Accordingly, the practice in 
the past has been either to compromise in selecting the hardness of the 
material to be used, or to provide an annealed peripheral layer in the 
manner described in the aforesaid application. 
In accordance with the present invention, the setting pin 17 is composed of 
material having optimum hardness for the desired strength that can be 
obtained with a given material, and the larger portion of the taper 31 is 
structurally weakened sufficiently to obtain the desired deformability of 
the expanding head 19 for formation of the blind-side sleeve head 23, 
thereby satisfying both requirements without need for peripheral annealing 
or the like. Such structural weakening is achieved by means of a recess 41 
which extends into the expanding head from the free end thereof, and into 
the taper, but terminates short of the expanding shoulder 29 and the 
adjacent portion of the head which will extend across the shear plane in 
the completed joint. Accordingly, the rivet 10 forms a joint of high 
overall strength, and also is capable of deforming to a relatively high 
degree in the area of the taper to form the blind-side head with high 
bearing area against the blind-side plate, while accommodating variations 
in grip length. 
As shown in FIGS. 1 through 3, the weakening recess 41 may be simply a 
cylindrical indentation in the free end of the head, of a type that can be 
formed as an incident to the formation of the head in a header. The 
diameter of the recess is shown as somewhat less than one-half the 
diameter of the head, and the depth of the recess is such that the inner 
end 42 thereof is spaced a substantial distance to the left of the 
expanding shoulder 29. This spacing insures that, in the completed joint, 
the recess will be spaced far enough from the shear plane to avoid any 
reduction of the shear strength of the joint. 
It can be seen in FIG. 3 that the recess 41 initially has no effect on the 
deformation of the expanding head during the pin-setting process. As the 
unweakened leading portion of the head enters the sleeve 13, the only 
deformation of the head that occurs is the slight amount resulting from 
the resistance exerted by the sleeve to entry of the head. 
In this respect, it is to be noted that the blind-side end of the sleeve is 
to be ductile enough to be expanded and bulged into the blind-side head 
23, without splitting in the process, and that the pin typically is 
composed of metal of a substantially greater hardness and lower ductility 
than the sleeve. For example, the sleeve may be 2017 (T-4) aluminum, and 
the pin may be 7075 (T-6) aluminum, achieved by heat treating the pin to a 
substantially uniform hardness through out after the heading operation in 
which the expanding head 19 is formed. The expanding shoulder 29 
preferably is hardened subsequent to such heat treating by rolling and 
cold-working, which raise its hardness substantially above that of the 
remainder of the expanding head. 
As the larger portion of the taper 31 enters the sleeve 13, as shown in 
FIG. 4, the taper begins to yield inwardly as the annular wall 43 around 
the inner end portion of the recess begins to collapse inwardly, as 
indicated at 44. Accordingly, this yielding, combined with a limited 
amount of wire-drawing deformation of the metal around the periphery of 
the head, produces the deformation necessary for proper formation of the 
head 23 on the blind-side end of the sleeve 13. 
It will be evident that the thickness of the wall 43 determines its 
strength, and that this strength may be varied by enlarging the recess. 
Moreover, by increasing its depth, the wall 43 will be weakened, and the 
point along the taper where deformation begins will be moved closer to the 
expanding shoulder. In general, the spacing of the inner end 42 from this 
shoulder should be about the same as the spacing of the internal stop 
shoulder 37 from the blind side of the work when the plates are of minimum 
thickness or grip length, so that the inner end will be about even with, 
or slightly outside, the blind side with such workpieces, and will be 
drawn into the work in maximum grip. 
As the setting operation is completed, as shown in FIG. 5, the blind-side 
sleeve head 23 is bulged further into tight bearing contact with the blind 
side of the work. Then, when the expanding shoulder 29 reaches the opposed 
stop shoulder 37 in the sleeve 13, the markedly increased resistance to 
further movement of the expanding head first results in forcing of the 
locking crown 24 to the left and into the locking groove 25, and then in 
the snapping off of the pulling section 18 at the breakneck groove 28. 
This sets the rivet securely in the workpieces with the latter clinched 
tightly together. 
The condition shown in FIG. 5 is illustrative of the condition in maximum 
grip, that is, with workpieces having a combined thickness near the higher 
end of the range for which the rivet 10 is designed. In maximum grip, the 
degree to which the expanding head 19 is drawn into the work is the 
greatest, and the capability of the head to deform during the setting 
operation is the most important. In minimum grip (not shown), on the other 
hand, the inner end 42 of the weakening recess 41 need not enter the work, 
but instead will stop outside the blind side thereof. Thus, the capability 
of the head to deform during the setting operation in minimum grip is less 
important, although a minor amount of collapsing of the head into the 
weakening recess may occur. 
Shown in FIG. 6 is a modified form of the expanding head, indicated 
generally by the reference number 45, for increasing the bearing area that 
is obtained between the blind-side plate and the blind-side head formed on 
the sleeve. Instead of the illustrative six-degree taper 31 of the first 
form shown in FIGS. 1 through 3, this head has a smaller, end portion 47 
tapered with a steeper slope (for example, nine degrees), and a larger end 
portion 48 tapered with a lesser slope (on the order of three degrees, for 
example). 
With this head configuration, the bulging of the sleeve that will be 
accomplished before the weakening recess 41 becomes effective is greater, 
due to the greater initial resistance to drawing of the taper 41 into the 
sleeve. It will be seen in FIG. 6 that the inner end 42 of the weakening 
recess is close to the juncture of the two tapers, so that the recess will 
become effective as the larger end portion of the steeper taper 47 
approaches the blind side of the work. 
Two additional modifications are shown in FIGS. 7 and 8, to illustrate 
possible variations in the configuration of the weakening recess. In FIG. 
7, the head 49 has a recess 50 that is generally conical in shape, with a 
rounded inner end 51. Since it is the cross-sectional thickness of the 
annular wall 52 around the inner end of the recess that determines the 
deformability of the taper 53, the shape of the outer portion of the 
recess is of little significance in this respect. 
If a less yieldable free end portion of the head is desired, the 
configuration of FIG. 8 may be used. This head 54 has a recess 55 with an 
inner end 58 that is similar to the inner end 51 of the recess 50 in FIG. 
7, and in addition, has an inturned annular flange 58 around the outer end 
of the recess for reinforcing the free end portion of the head. This 
flange will not affect the ability of the taper 59 to collapse during 
formation of the blind-side head on the sleeve, but reduces the 
deformability of the free end portion, around the reinforcing flange. 
From the foregoing, it will be apparent that the present invention provides 
a blind rivet 10 which permits the use of a setting pin 17 having the 
optimum hardness for a given material, for optimum strength in the 
completed joint, and also having optimum deformability in the expanding 
head 19 for proper formation of the blind-side head 23 on the sleeve 13. 
Moreover, these important advantages are achieved without need for 
peripheral annealing, and thus may be achieved in rivets in which 
peripheral annealing is impractical or economically undesirable. 
Although the present invention is particularly well suited for use with 
aluminum rivets, it should be evident that it is not so limited. It also 
should be evident that, while several specific embodiments have been 
illustrated and described, various modifications and changes may be made 
without departing from the spirit and scope of the invention.