Rivet installation method

A rivet installation method creates a preload on the workpieces being joined. The rivet shank is urged to resist compression during installation while a ring on the end of the shank is compressed against the workpieces. Withdrawing the compression releases the shank to provide the preload.

BACKGROUND OF THE INVENTION 
This invention relates to a method of installing a rivet in a plurality of 
workpieces, and to the resulting riveted joint, as well as the rivet 
itself. 
One rivet-type fastener commonly used in aircraft constructions includes a 
shank with a manufactured head on one end and a tail on the other end. In 
use, the tail end of the shank is inserted through aligned holes of two or 
more workpieces with the rivet head engaging the outer face of one of the 
workpieces and with the tail extending beyond the outer faces of the other 
workpiece. The tail is then deformed by means of an axial force, 
compressing the rivet axially and upsetting the tail material outwardly to 
form an upset head which is larger in diameter than the hole through the 
workpieces, so that the two workpieces are fastened together. 
In one highly successful rivet, or shear pin, of this general type, the 
shank which extends through the workpieces is made of a strong material 
which is high in shear strength while the tail is made of a more ductile 
material which is easier to deform than the shank. The two metals are 
typically joined together by friction welding. In one example of this 
type, the shank is formed of (95 ksi shear) 6AL-4V titanium alloy while 
the tail is formed of 55 Ti 45 Cb titanium alloy. Such a fastener is sold 
by the assignee of the present invention under the trademark Cherry BUCK. 
U.S. Pat. No. 3,848,389 issued to Gapp, et al. further describes 
bimetallic shear pins of this general type. 
All types of fasteners having a tail to be upset are often installed by 
squeezing, wherein the ductile tail is compressed until the upset head 
formed attains a diameter of about 1.3 to 1.5 times the initial shank 
diameter. When the squeezing force used to form the upset head is 
released, the column of the rivet shank "springs back" or lengthens a 
certain distance. Although the material of the workpiece being fastened 
also springs back, some of the materials in common use do not spring back 
as much as the rivet shank, with the result that a small gap is created 
between portions of the upset head and the workpiece after the 
installation is complete. For example, a sold rivet of 2117T3 aluminum, 
used for many years in large quantities in the aircraft industry and 
commonly referred to as an AD rivet, when installed in 7075T6 aluminum 
material can exhibit a small gap between the upset head and the workpiece. 
However, some gap is acceptable for most practical uses except those 
involving high fatigue loads. 
In aircraft structures, particularly those involving shear fatigue loading 
of the fastener, it is desirable that the gap between the upset head and 
the workpiece be zero. Ideally, the underside of the upset head should 
exert a compression force against the workpiece after the installation. 
When such a loading is achieved, the fastener is said to exert a residual 
tension force against the workpiece after installation. This loading is 
often referred to as a "preload" in the joint. 
While millions of rivets of the type having a ductile tail and a strong 
shank have been used satisfactorily, their use has been limited in areas 
involving shear fatigue loading due to the inability to provide a preload. 
A fastener of this type which provides a preload is usable in a much 
extended range of applications, particularly those involving shear fatigue 
and light tension fatigue or a combination of both. Further, the ability 
to use a bi-metallic fastener in fatigue applications is of great benefit 
to the aircraft industry because it is the lightest shear pin type 
fastener available with a 95 ksi shear rating. In the aircraft industry, 
weight savings are very valuable. Savings of one pound of weight can be 
worth $1,000 to the designer in some critical areas of an aircraft design. 
Another type of shear pin fastener consists of a bolt with a small head on 
one end and a small light nut threaded on the other end. The small head 
and nut are of sufficient strength to develop close to the full shear 
strength of the fastener when it is installed in two or more workpieces, 
but not of sufficient strength to develop the full tensile strength of the 
fastener. In joints involving high axial tension loads, tension type 
fasteners are used. Typically, these have larger and stronger manufactured 
heads and larger and stronger matching nuts. These tension type fasteners 
are, of course, heavier. The size and weight of the squeezed tail of the 
one piece shear pin herein described is much smaller than the nut and 
threaded portion of a threaded pin and nut-type shear pin fastener of the 
same strength. Also, the small height and size of the squeezed tail of the 
shear pin is advantageous with regard to the installation and positioning 
of other components in crowded areas within aircraft or other assemblies. 
A one-piece fastener is particularly desirable as opposed to a two-piece 
fastener in that it is easy to feed and install using automatic equipment. 
Most fasteners of this general type which are capable of providing a 
significant preload are of the two-piece design. Two-piece designs involve 
serious feeding problems when automatic or robot installation is 
attempted. Automated installation of fasteners is advancing rapidly in the 
aircraft industry because it is cheap and produces much more uniform and 
satisfactory joints. In the past, preload in a riveted joint has been 
achieved by using hot rivets which, after being upset, contract upon 
cooling and produce the desired preload in the joint. This hot rivet 
approach of course adds other costs and complexities; and while it is 
still used in large commercial structures, such as bridges, it is not used 
in aircraft structures. 
Accordingly, a substantial need exists for a method of installing a 
one-piece, cold, upset rivet or shear pin fastener in a manner which can 
provide a significant axial preload. 
SUMMARY OF THE INVENTION 
In accordance with the invention, an installation method is provided which 
produces a riveted joint with the desired preload on the workpieces 
through which the rivet extends. This is accomplished by applying a 
deforming force to the rivet tail to form an upset against the portion of 
the workpiece surrounding the hole in the workpiece, while applying force 
to the rivet shank sufficient to resist compression or to slightly stretch 
the shank. The elongating force is then withdrawn allowing the rivet shank 
to "spring back" providing a preload on the workpieces. A manufactured or 
an upset head is, of course, on the other end of the shank to surround the 
hole on the other workpiece. 
In a preferred approach, the installation method includes a two-step, 
compressive approach using a two-part tool assembly. With the rivet tail 
extending beyond the outer face of one of the workpieces, a first axial 
compressive force is applied to the rivet using a suitable pin-like tool, 
causing the tail of the rivet to be deformed axially and deformed radially 
outwardly to form an upset head substantially larger than the hole through 
the adjacent workpiece. This first step is essentially the same as that 
currently being utilized in installing rivets of this type. 
As the second major step of the installation method, the tool which formed 
the upset head is unloaded, and a second tool which surrounds the first 
tool is squeezed onto the upset head. This outer tool preferably has a 
cylindrical opening formed therein so as to be slidably mounted on the 
inner pin-like tool. The circular edge of the cylindrical opening of the 
outer tool engages the outer ring of the upset head; and as the outer tool 
is moved downwardly onto the upset head, it starts to shear the outer ring 
of the upset head. Continued downward movement of the outer tool flattens 
the sheared or swaged ring while it remains integrally connected at its 
inner edge with the upset head. During the forming of this ring, the upset 
head material is, of course, pressed against the adjacent workpiece 
placing the workpieces under compression. 
When the outer tool is withdrawn, the resulting joint construction has a 
preload on the workpieces applied in the vicinity of the axis of the shear 
pin. It is believed that one factor producing this preload is due to the 
nature of the metal movement which occurs during the flattening of the 
sheared ring. As a compression load sufficient to deform the material is 
applied, the ring will attempt to expand radially inwardly as well as 
radially outwardly. The radially inward expansion urges the length of the 
pin to be increased by a small but appreciable increment. When 
installation is complete and the cylindrical tool is withdrawn, the length 
of the fastener springs back to a slightly shorter length and applies 
compression or preload to the workpieces. 
An additional mechanism which contributes to joint preload also operates 
during the downward stroke of the outer mandrel. This mechanism operates 
as follows. The load required to shear material from the upset head of the 
rivet is typically about 2000 lbs. maximum, dropping to about 1000 lbs. 
towards the end of the operation for a 3/16 inch diameter rivet having a 
tail fabricated from 55 TiCB material. This 1000 lb. load compresses the 
rivet shank in column. When the partially sheared ring contacts the 
workpiece a further load is applied, typically 6000 lbs. for the rivet 
before mentioned. All except the 1000 lb. of this load goes to push the 
workpieces together and then compress them to a slightly lesser thickness 
locally in the vicinity of the flattened ring. When the outer tool is 
withdrawn the rivet shank will elongate slightly due to the spring back 
following release of the 1000 lb. load, and the workpieces will spring 
back slightly more due to release from the higher 5000 lb. (6000 lb. minus 
1000 lb.=5000 lb.) load. Thus, the workpieces will be maintained in a 
compressed condition between the flat ring and the preformed head. This 
compressed condition is referred to as preload in the joint. For 
simplicity, loaded areas and material modulii have been omitted. 
While these methods for producing a preload are particularly advantageous 
in the bi-metallic type rivet discussed above, the methods are also useful 
in any cold installed metallic rivet. 
In addition to the above-described method, the invention includes the 
resulting rivet which is formed by the method. Such rivet has a 
cylindrical head with a flattened larger diameter ring on the end of the 
head adjacent the rivet shank. The head and ring thus have somewhat of a 
hat shape. The invention also includes the preloaded joint construction 
provided by the method and includes the apparatus for performing the 
method.

DETAILED DESCRIPTION OF THE SYSTEM ILLUSTRATED IN THE DRAWINGS 
As indicated above, FIG. 1 illustrates a prior art rivet or shear pin 10 
having a cylindrical shank 12 with a preformed or "manufactured" head 14 
on one end of the shank and a tail 16 on the other end of the shank, with 
the diameter of the tail tapering from that of the shank to a reduced 
diameter on its outer tip. Such a rivet may be made of various materials, 
but a commonly used rivet which is particularly useful in connection with 
the method of the invention has a shank formed of 6AL-4V titanium alloy 
and a ductile, easy to form tail made of 55 Ti 45 CB titanium alloy. These 
two metals are joined in suitable manner by friction welding such as that 
disclosed in U.S. Pat. No. 3,848,389. 
FIG. 2 illustrates the rivet of FIG. 1 installed in a conventional manner, 
fastening two workpieces 18 and 20 together. As may be seen, the tail 16 
of the rivet has been upset axially and deformed radially outwardly to 
form an upset head 17 which holds the rivet in place. After the squeezing 
or compressive force on the upset rivet is removed, the column of the 
rivet shank springs back a certain distance. Although the workpieces 18 
and 20 also spring back somewhat, the materials in common use for the 
workpieces do not spring back as much as the rivet shank with the result 
that a gap V is formed between the interface of the upset head 19 and the 
workpiece 20, as seen in FIG. 2a. This gap is acceptable for most 
practical uses except those involving fatigue loads. As shown in FIG. 2 
and FIG. 2a, the shank near the upset head is expanded slightly to prevent 
looseness. This expansion occurs as the upset head is formed. 
In accordance with the method of the invention, this gap V is eliminated, 
and the rivet shank is under tension after installation providing a 
compressive force or preload on the workpieces. A two-part tool assembly 
is provided to install the rivet. As schematically illustrated in FIG. 3, 
this includes an inner tool comprising a piston-like pin 24 having a flat 
end face with a diameter which is larger than the diameter of the rivet 
shank 12 but is smaller than the diameter of the upset head 28 which is to 
be formed, as illustrated in FIG. 4. Surrounding the tool pin 24 and 
slidably mounted thereon is a cylindrical or tubular tool 26. The tool 26 
has a flat annular end face which is shown flush with the end face of pin 
24 in FIG. 3. Note that the inner tool 24 is positioned above and axially 
aligned with a rivet 10 extending through workpieces 18 and 20, with the 
end face of the tool being adjacent the rivet tail 16. 
During the initial step of the installation method, the pin 24 is moved 
downwardly as indicated by the arrows 25 in FIG. 3 to apply a squeezing or 
axially compressive force to the rivet tail. The rivet preformed head 14 
and the workpieces are of course supported by suitable means (not shown) 
to withstand the load. This force axially compresses the tail and deforms 
it outwardly into the flattened barrel-like shape illustrated in FIG. 4, 
forming the upset head 28. The compressive or squeezing force is not 
normally applied by the outer tool 26. Thus it does not normally deform 
the rivet during this first step. 
Preferably, the outer diameter of the upset head is about 1.3 to 1.5 times 
the original shank diameter, which as indicated in FIG. 4 is larger than 
the inner diameter of the outer tool 26. The upset head actually has an 
outwardly curved or bulged outer periphery such that the diameter on the 
flat upper end of the upset head is about equal to the inner diameter of 
the end face of the tool 26, and the outer diameter of the end face of the 
inner tool 24, but the outermost portion of upset head 28 extends 
outwardly beyond the inner diameter of the outer tool 26. 
During the second forming step in the method of the invention, the 
squeezing force on the inner tool 24 is unloaded, and it may be withdrawn 
slightly as illustrated in FIG. 5. The outer tool 26 remains adjacent the 
upper end of the upset head. The diagram of FIG. 6 indicates that a load 
is not yet applied. A squeezing or compressive force is then applied to 
the upset head by means of the outer tool. As the outer tool moves down it 
starts to shear the outer ring or annular portion 30 of the head 28, as 
illustrated in FIG. 5a; and the load increases as illustrated in the 
diagram of FIG. 6a. 
As the outer tool 26 approaches the largest diameter of the upset head as 
shown in FIG. 5b, the load increases to an initial high point as 
illustrated in diagram 6b. With continued downward movement of the outer 
tool 26 to the position illustrated in FIG. 5c, the load decreases 
slightly due to the reduction in diameter of the upset head, such slight 
load reduction being illustrated in FIG. 6c. Continued compressive 
movement of the outer tool 26 deforms the sheared upset head material 30 
into a flatter ring as shown in FIG. 5d. 
As the material is compressed and swaged further into a flattened ring 
increasing its outer diameter, the load increases rapidly from point A to 
point B as illustrated in FIG. 6d. The preload is produced within that 
portion of the loading diagram because the workpieces 18 and 20 are 
pressed together through the action of the compressed ring 30 which at all 
times remains attached to the remaining upset head 28a. As this ring 30 is 
compressed, it is believed that it tends to expand radially inwardly, as 
well as radially outwardly, causing the length of the pin to be increased 
by a small, but appreciable, increment. This tendency toward inward flow 
of material is illustrated schematically by the small arrow 36 shown in 
the enlargement of FIG. 5e. When installation is complete and the 
cylindrical tool 26 is withdrawn, the length of the fastener springs back 
to a slightly shorter length and applies the desired compression or 
preload to the workpieces. 
An additional mechanism which contributes to joint preload, also operates 
during the downward stroke of the outer mandrel, 26. This mechanism 
operates as follows: the load required to shear the ring, 30, from the 
upset head of the rivet, 28, is indicated by the curve in FIG. 6d between 
the beginning of the curve and the point A. Between the point A and point 
B the load on the workpiece exerted by the ring 30 increases to a maximum 
at B, while the load on the pin tending to collapse it in column reduces 
to approximately point C on the dotted extension of the curve in FIG. 6d. 
The load at C felt by the pin is estimated to be about 1/6 of the load at 
B. The load at B of course represents the combined load on the pin and the 
load on the workpieces. When the load on the outer anvil is removed, the 
pin will spring back a small amount due to the removal of the load C, 
while the workpieces will tend to spring back by a larger amount due to 
removal of the larger load B minus C. The pin will restrain the spring 
back of the workpieces and thus, the workpieces will remain in the desired 
preloaded condition. 
In practice, the two mechanisms that tend to provide preload operate 
simultaneously. 
Note that due to the action of the outer tool, the outer diameter of the 
flattened ring 30 has been increased beyond the diameter of the original 
upset head 28, as viewed in FIG. 5. With the flattened ring material 
having been sheared from part of the outer portion of the original upset 
head shown in FIG. 5, a more completely cylindrical head 28a, as shown in 
FIG. 5d is left. This gives the head 28a and ring 30 combination a 
miniature top hat shape with the ring 30 representing the brim. While the 
size of the upset head 28a and the flattened ring 30 in relation to the 
shank of the rivet may be varied to some extent, a successful prototype 
had a maximum upset head diameter as viewed in FIG. 5 of about 1.5 times 
that of the original rivet shank diameter. This was reduced to a diameter 
of 1.3 times the original shank 12 diameter in its sheared cylindrical 
form 28a shown in FIG. 5d, while the flattened ring 30 had an outer 
diameter of 1.75 times that of the original shank 12 diameter. 
The expansion of the ring 30 radially inward sufficiently to produce an 
appreciable lengthening of the rivet shank 12 during the A to B stage of 
the installation, as viewed in FIG. 5d, was surprising in that it was not 
anticipated that lengthening would be of sufficient magnitude to produce a 
useful result. It has been found however that a rather remarkable 
improvement is obtained by this mechanism and the additional mechanism 
also described. 
Based on a number of tests of 3/16 inch diameter CherryBUCK fasteners 
having 6AL-4V titanium for the preformed head and shear portion of the 
shank welded to a soft tail of 55TiCb, the following are the preferred 
loadings to be applied to the rivet tail by the inner anvil 24 at the 
first stage of installation, and by the outer tubular anvil 26 during the 
second stage of installation. The range of resulting preloads is also 
shown. 
______________________________________ 
Preferred Installation Loads 
______________________________________ 
First stage load 
5,800 LB Min/7,800 LB Max 
Second stage load 
4,500 LB Min/6,500 LB Max 
Resultant Preload 
500 LB Min/1,000 LB Max 
______________________________________ 
A chart of lap joint shear fatigue test results is set forth below. For the 
rivets with preload, first and second stage installation loads were in 
accordance with those listed above under Preferred Installation Loads. 
Shown is the number of cycles to failure for the rivet with preload versus 
the same rivet without preload and also versus a competitive fastener sold 
under the trademark HI-LOK, which employs a threaded nut on a threaded 
shear pin. It will be noticed that each of the three different Cherry-BUCK 
part numbers gave consistent cycles to failure with the special upset, as 
compared to lower and less consistent cycles to failure with the standard 
upset. Noticeable also are the favorable cycles to failure of the rivets 
with the special upset compared to the cycles to failure of the Hi Lok. 
Due to its very common use throughout the aircraft industry the Hi Lok 
fastener is widely regarded as an industry standard with the type of 
fastening applications under discussion here. It is apparent from the 
results that a very significant improvement has taken place due to the 
method of installation which produces preload. This dramatic improvement 
for a fastener not employing a threaded nut is particularly useful in that 
a one piece fastener lends itself readily to automated installation of 
rivets. 
______________________________________ 
FATIGUE TESTS 
PER MIL-STD-1312, TEST 21 
Full Load Transfer, No Restraints 
CHERRYBUCK WITH PRELOAD 
In 2024-T3 Clad Sheets, 0.123" Thick. Each 
At 10 KSI Gross, R = +0.1 (1850/185 Lbs.) 
Clearance Holes (0.1900") 
Fastener Installation 
Fatigue Life Failure 
Part No . Condition (Cycles to Failure) 
Code 
______________________________________ 
CSR 926-6-4 
Standard 1.4 D 
427300 C 
Standard 1.4 D 
388260 B 
Special 533910 A 
Special 599790 A 
CSR 924-6-4R 
Standard 1.4 D 
478450 B 
Standard 1.4 D 
438840 B 
Special 495300 A 
Special 627610 A 
CSR 926-6-5 
Standard 1.4 D 
121350 C 
Standard 1.4 D 
97280 C 
Special 540120 A 
Special 490950 B 
Hi-Lok Power Tool 389100 B 
(HL11V6-4 
Pin, 
HL70-6 367100 A 
Collar) 
______________________________________ 
Failure Code A refers to the condition shown in FIG. 8 wherein the top 
workpiece in a riveted joint breaks at a location spaced from the holes in 
the workpieces through which the rivets extend. 
Failure Code B refers to that shown in FIG. 9 wherein the bottom workpiece 
of a riveted joint ruptures at a location spaced from the holes in the 
workpieces through which the rivets extend. 
Failure Code C refers to the condition wherein the top workpiece in a 
riveted joint ruptures on a line through the holes in the top workpiece 
through which two rivets extend. The upper workpiece is slightly weaker 
than the lower workpiece at that location because of the countersink in 
the top workpiece, whereas the bottom workpiece is not countersunk in this 
example. 
It will be noticed that with the rivets in the standard installation 
condition (no preload), the failure Code C indicates that joints failed 
across the rivet holes, while with rivets in the special installation 
condition (with preload), failure Codes B and C indicate that joints 
failed away from the rivet holes. It is widely considered that when 
failures occur remote from the rivet holes, the cycles to failure are 
close to optimum for that particular joint and fastener configuration. 
It has been found that the preload is obtained whether the fastener is 
installed in an interference fit or in a clearance fit hole in the 
workpieces. 
It should be noted that the material sheared from the head 28 does normally 
not contribute much to the strength of the riveted joints. Thus the 
advantageous preload is obtained without appreciable loss of strength of 
the joint formed by the first deforming step, and without the need for 
additional material to be added to the rivet tail. 
While the method described is particularly advantageous with a bimetallic 
rivet, it is useful with any cold formed rivet operation. 
In rare situations, it may be desirable to utilize a shear pin 38 not 
having a preformed head on either end, as shown in FIG. 7. Both tails 40 
of the shear pin 38 can be upset in the manner discussed above to form an 
upset head 28a and a ring 30, as in FIG. 5d, on each end of the shear pin 
38. In theory, the preload could be doubled if both ends were 
simultaneously formed. 
The preload could also possibly be increased by restricting the radially 
outward flow of materials when the ring 30 is being flattened. This could 
force more inward material flow, and thus more axial lengthening, and 
ultimately more "spring back" when the load is removed. This could 
possibly be accomplished by forming a shallow annular recess (not shown) 
in the end face of the tool 26. The outer diameter of the recess would be 
sized to limit outward flow of the ring 30 during the flattening of the 
ring, so as to increase inward flow. 
The top hat-shaped head can, of course, be used on various types of shear 
pins. FIG. 11 illustrates another arrangement wherein the top hat-shaped 
head 52 has been formed with preload on the small end of a tapered pin 50. 
The pin is forced into holes in the workpieces 54 and 56 until the large 
head or end 51 of the pin prevents further movement. The preloaded head on 
the tapered pin is more practical than an upset head of the type shown in 
FIG. 2, wherein the gap between the upset head and the workpiece exists.