Deformable locking fastener system and method of use

An improved deformable locking fastener system comprises a fastener, a deformable collar and a dedicated installation tool. The fastener has a threaded exterior defining a plurality of axial flutes which are configured to prevent of the collar from prematurely captivating the fastener during installation. The collar comprises a cylindrical forward portion, a central portion having an elliptical cross-sectional shape and a rearward portion having an elliptical cross-sectional shape. The collar portions have axial heights selected to maximize the strength of the collar and to minimize the weight of the collar. The installation tool contacts the collar along driving ridges which reduces the radial compression of the collar and increases the tangential driving force turning the collar during installation. In addition, the installation tool includes oblique driving ridges and exit vents which prevent premature cam-off of the installation tool when generating a desired preload and swaging the collar onto the fastener to lock the components together.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to fastener systems and, in particular, to 
locking fastener systems. 
2. Description of Related Art 
Various locking fastener systems have been developed to join together 
materials under a desired compressive force and to "lock" in place to 
prevent unintentional loosening primarily due to vibrational forces. Prior 
fastener systems commonly include a collar (i.e., a nut) which fastens to 
a fastener (i.e., a bolt or a pin) to produce a predetermined preload 
between the fastener and the collar. That is, a tension force develops 
between the fastener and the collar as the collar is threaded onto the 
fastener, placing the intervening joint in compression. The collar 
subsequently locks onto the fastener by a variety of means. 
Some prior locking fastener systems rely on swaging the collar onto the 
fastener to lock the components together. Examples of locking fastener 
systems which swag collar material into the fastener are disclosed in U.S. 
Pat. No. 4,383,353, issued to Stencel; U.S. Pat. No. 4,601,623, issued to 
Wallace; and U.S. Pat. No. 5,145,300, issued to Wallace. 
These prior fastener systems, however, suffer from several drawbacks. Prior 
fastener systems which first generate a preload and subsequently swage 
collar material into the fastener tend to produce scattered preload 
values. That is, the designed preload value produced by a particular size 
of fastener could widely ranged between a minimum value and a maximum 
value. For instance, a 1/4"-28 UNF titanium fastener system, of the type 
disclosed in application Ser. No. 07/481,373, typically produces preload 
values ranging between 1500 lbs. and 3000 lbs. Consequently, industry 
commonly used larger fastener systems to ensure a minimum preload value; 
however, larger fastener systems increase the weight of the assembly, 
which the aeronautical and aerospace industries particularly disfavor. 
Additionally, prior installation tools used with these locking fastener 
systems further exacerbate the scattering of preload values. Prior 
installation tools tend to prematurely disengage from the collar (i.e., 
"cam-off"). That is, the forward end of the collar resists being deformed 
and forces the installation tool away from the collar. The resultant 
force, in combination with the continued rotation of installation tool, 
winds the installation tool off of the collar prior to completely 
producing the desired preload, thereby frustrating the installation 
process and generating less compression in the intervening joint than 
desired. 
Scattered preload values also result from a technician's efforts to keep 
the installation tool on the collar (i.e., to compensate for the cam-off 
tendency) by applying end pressure to the installation tool which varies 
from one installation to another. And, although the massive end pressure 
exerted by the technician may prevent the installation tool from camming 
off, a corresponding exertion of equal effort may be required to extract 
the installation tool after installation, further frustrating the 
installation process. 
Moreover, prior installation tools require about 360.degree. of tool 
rotation about the collar, once the desired preload has been produced, in 
order to swage the collar onto the fastener. In confined working quarters, 
this degree of tool rotation is difficult, even using a ratcheting wrench. 
Finally, prior installation tools and fastener systems tend to over-strain 
a flimsy hexagon key which engages a cooperative recess in the tail of the 
fastener and carries the reaction force during blind side installation. 
The hexagon key commonly breaks under excessive force or the fastener tail 
recess commonly erodes. Over straining results when the swaging operation 
commences while the collar is still capable of rotating under the applied 
torque. 
SUMMARY OF THE INVENTION 
The present invention includes a fastener system comprising a duplex 
collar, a fastener and an installation tool used to install the collar 
onto the fastener. The collar comprises a cylindrical forward portion, a 
central portion having an elliptical cross-sectional shape and a rearward 
portion having an elliptical cross-sectional shape. The installation tool 
includes cavities having shapes complementary to the shape of the duplex 
collar and fits over the collar during the installation process. Rotation 
of the installation tool applies a torque to the collar and threads the 
collar onto the fastener. 
The present invention further includes the recognition that by engaging and 
torquing the collar with smooth, arcuate or flat surfaces of the 
installation tool produces a rotational force having a greater compressive 
force vector than a tangential force vector. Consequently, the applied 
torque elastically deforms (i.e., crushes) the collar inwardly into the 
threads of the fastener which increases the friction between the fastener 
and the collar. The increased friction typically causes the fastener to 
rotate with the collar, thereby over-stressing and breaking the hexagon 
key used to restrain the fastener in blind side fastening. 
In addition, the elastic deformation of the collar typically produces 
artificial preload values which decrease after removing the installation 
tool from the collar. The increase in friction between the fastener and 
the collar requires more torque to install the fastener. However, when the 
installation tool is removed, the collar material springs back to its 
pre-deformed configuration and the preload value decreases. 
The installation tool of the present invention minimizes the contact 
surface between the installation tool and the collar by applying a 
tangential force to the collar along driving ridges. The force applied 
along the driving ridges produces a significantly greater tangential force 
vector component than a radial force vector component, thus applying a 
greater driving force and a lesser crushing force than that applied by 
prior installation tools. Consequently, the present fastener system 
enhances the repeatability of preload values over the repeatability 
achieved by prior fastener systems. Additionally, less torque is applied 
to the fastener during installation, thus reducing the torque applied to 
the hexagon key. 
The driving ridges are advantageously spaced apart from each other by a 
distance substantially equal to a diametric distance between camming 
surface of the collar. As a result, rotation of the installation tool 
produces the desired preload valve without a massive amount of torque and 
without over-stressing the threads of the collar and the fastener. 
The concentration of the applied tangential force (i.e., the applied 
torque) along a ridge requires that the force be spread over a sufficient 
axial length to ensure that the collar does not deform prior to applying 
the torque required to produce the desired preload value. However, weight 
considerations limit the axial length of the collar. For use in the 
aeronautical and aerospace industries, the collar is desirably designed to 
be as lightweight as possible and still produce the desired amount of 
compression between the joined materials (i.e., produce the desired 
preload). 
The present collar optimizes the collar's weight and axial height, 
providing sufficient contact surface to disperse the applied torque such 
that the collar resists plastic deformation prior to applying the torque 
required to produce preload while having an acceptable weight. Moreover, 
the collar maximizes the tensile strength of the collar by maximizing the 
mass of the collar forward portion to improve its hoop strength. 
Consequently, the preload sustainable by the fastener system is increased 
over like-size prior fastener systems. 
Through analytical and empirical analysis, it has been determined that a 
collar with the following axial lengths optimizes collar weight and 
strength: a forward portion having an axial height of 2.7 P; a central 
portion having an axial height of 2.9 P; and a rearward section having an 
axial height of 3.4 P, where P equals the pitch of the collar internal 
thread. 
In accordance with another aspect of the present invention, the present 
invention includes the recognition that prior fastener flute designs also 
contributed to over-stressing the hexagon key used in blind side 
installation. Flutes, with concave shapes intersecting the fastener thread 
periphery at shape points, tend to prematurely capture (i.e., 
"precapture") the collar which encourages the fastener to rotate with the 
collar before reaching the required preload. That is, as the collar is 
tightened and the applied torque rises, prior installation tool 
elastically crush the collar inwardly which interferes with the sharp 
intersection between the flute and the fastener thread periphery. As a 
result, the fastener is captivated prematurely and rotates with the 
collar, thereby over-stressing the hexagon key and causing it to fail. 
The present fastener comprises flutes having convex bottoms surfaces 
extending from leading shoulders which chase and blend with the fastener 
periphery (i.e., the crests of the fastener thread). Blending in the 
direction that the collar turns delays capturing the fastener, thereby 
allowing a higher preload to develop without over-stressing the hexagon 
key. 
In accordance with a third aspect of the present invention, the 
installation tool advantageously prevents premature cam-off of the 
installation tool from the collar when swaging the collar. The 
installation tool comprises oblique driving ridges which produce an axial 
force thrusting the installation tool onto the collar when generating the 
desired preload. The installation tool additionally includes exit vent 
reliefs which prevent the installation tool from camming off of the collar 
when swaging the collar into the fastener flutes. Once the installation 
tool has completely swaged the collar, the installation tool freely spins 
off of the collar. 
Additionally, the present invention also includes a preferred method of 
installing a deformable fastener system. The method includes the steps of 
engaging a first pair of driving ridges of the installation tool with the 
collar central portion, and engaging a second pair of driving ridges with 
the collar rearward portion. The installation tool is then rotated, 
treading the collar onto the fastener and compressing the intervening 
materials. Further rotation of the installation tool at a desired torque 
deforms a portion of the collar central portion and a portion of the 
collar rearward portion. Wrenching surfaces of the installation tool are 
then engaged with the collar rearward portion and are rotated to deform 
the rearward portion inwardly into the fastener flutes, thereby locking 
the collar to the fastener.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates in partial cross section a locking fastener system 10 in 
accordance with an embodiment of the present invention. The fastener 
system 10 advantageously is used to join together materials 12, to 
generate a desired compressive force between the joined materials 12 
(i.e., "generates a preload force") and to lock in place to prevent 
substantial degradation of the compressive force due to vibrational 
forces. 
The fastener system 10 comprises a threaded fastener 14, such as a bolt or 
a pin, and a deformable duplex collar 16 which locks onto the fastener 14 
once the desired preload force has been generated. As installed in FIG. 1, 
the fastener 14 passes through the materials 12 from one side and the 
collar 16 threads onto the fastener 14 on the other side. An installation 
tool 18, coupled to a wrench 20, is used to rotate and to tighten the 
collar 16 onto the fastener 14. The individual components of the locking 
fastener system 10 will now be described in detail. 
The Fastener 
As illustrated in FIG. 2, the fastener 14 comprises a head 22 and an 
elongated shank 24 having an externally threaded portion 26 which engages 
the collar 16. The threaded portion 26 comprises a standard thread forming 
a helical series of crests 27 and roots 29 uniformly spaced apart. As 
illustrated in FIG. 2a, the threaded portion 26 has a major diameter 
D.sub.1 defined between the thread crests 27 and has a minor diameter 
D.sub.2 defined between the thread roots 29. 
The fastener 14 can have a variety of lengths and diameters according to 
the particular application of the fastener system 10. For instance, the 
fastener 14 can range in length from 0.40 to 3.0 inches and can range in 
diameter from 0.156 to 0.375 inch. 
Referring to FIGS. 2 and 2a, a rear end 28 of the fastener 14 defines a 
polygonal-shaped cavity 32 extending into the fastener 14 in the 
longitudinal direction which receives a wrenching key 33 during 
installation. As used herein, "in the longitudinal direction" means in a 
direction parallel to the longitudinal axis of the fastener 14 (which is 
also generally parallel to the longitudinal axis of the collar 16). In 
addition, "in the radial direction" means in a direction radiating from 
the longitudinal axis. 
The fastener threaded portion 26 defines a plurality of recesses 34 
positioned proximate to the fastener end 28. Referring to FIG. 2a, each 
recess 34 comprises a convex bottom surface 36 extending from a shoulder 
38. Although FIG. 2a illustrates the bottom surface 36 as being an arcuate 
convex surface, it is understood that the bottom surface 36 could comprise 
a plurality of straight segments connected in such a way as to allow the 
line defining the bottom surface 36 to chase and blend with the thread 
crest 27. 
The shoulder 38 has an arcuate surface 40 defined by a radius r and extends 
between a first point 42 and a second point 44. The first point 42 is 
located along a diameter line 46 of the shank 24, proximate to the thread 
root 29. Thus, the recess 34 has a depth, measured in the radial direction 
from the thread crest 27 towards the fastener longitudinal axis, slightly 
less than the depth of the thread root 29 (i.e., slightly less than 
D.sub.1 -D.sub.2 /2). 
The shoulder 38 also includes a chamfer 48 extending outwardly (i.e., away 
from the longitudinal axis) from the second point 44 at an angle .GAMMA. 
oblique to the diameter line 46. The angle .GAMMA. formed between the 
chamfer 48 and the diameter line 46 ranges between 15.degree. and 
45.degree., and desirably equals 30.degree.. In a preferred embodiment, 
the length C of the chamfer 48, measured in the radial direction, equals 
approximately 0.0075 inch. The curved surface 40 and chamfer 48 forming 
the shoulder 38 generally prevent a roll-over burr from forming at the 
transition of the shoulder 38 and the thread crest 27 when forming the 
recess 34. 
The convex bottom surface 36 extends from the first point 42, chasing and 
blending with the periphery of the thread crest 27. That is, the convex 
bottom surface 36 tapers radially outwardly from the first point 42 to the 
thread crest 27 and smoothly blends into the tread 26. Although FIGS. 2 
and 2a illustrate the convex bottom surface 36 of the recess 34 as chasing 
and blending with the periphery of the thread crest 27 in the clockwise 
direction, it is understood that the convex bottom surface 36 can extend 
in the counter-clockwise direction, where the collar 16 threads onto the 
fastener 14 in the counter-clockwise direction. 
As illustrated in FIG. 2, the recesses 34 form a plurality of axial flutes 
50 which extend across several thread crests 27 and generally lie parallel 
to the longitudinal axis of the fastener 14. During the installation 
process, the flutes 50 receive collar material which is swaged into the 
flutes 50 to lock the collar 16 onto the fastener 14, as discussed infra. 
Desirably, the fastener 14 includes six flutes 50 equally spaced around the 
circumference of the shank 24. However, the fastener 14 can include less 
flutes provided that the aggregate void volume defined by the flutes 50 
remains substantially constant. 
The Duplex Collar 
Referring to FIG. 3, the duplex collar 16 has a forward base 52, a 
deformable central portion 54 and a deformable rearward portion 56. As 
used herein, "rearward" and "forward" are used to indicate the proximity 
of the joined materials 12 when installed. The duplex collar 16 has a 
stepped configuration decreasing in size from the forward base 52 to the 
central portion 54 to the rearward portion 56, the central portion 54 
joining together the forward base 52 and rearward portion 56. 
The collar 16 defines a threaded central axial bore 58 which receives the 
fastener threaded portion 26. The collar 16 also defines a counter-bore 60 
extending into the forward base 52 from a forward end 59 of the collar 16 
to ease inserting the fastener end 28 into the axial bore 58. 
The forward base 52 has a cylindrical shape with a shoulder 57 tapering 
inwardly (i.e., towards the collar longitudinal axis), conforming to the 
shape of the central portion 54. The base shoulder 57 desirably is angled 
from the collar longitudinal axis by about 15.degree. to provide a 
gripping flange for holding the collar 16 during manufacturing and to 
provide a registration surface for the installation tool 18, as is 
discussed in detail infra. This configuration of the forward base 52, in 
combination with the axial length of the forward base 52 specified below, 
maximizes the mass of the forward base 52 to improve its hoop strength. 
Consequently, the tensile strength of the collar 16 is substantially 
greater than prior duplex collar designs. 
As illustrated in FIGS. 4 and 5, the central portion 54 and the rearward 
portion 56 have generally elliptical shapes in cross section. As used 
herein, "elliptical" and "ellipse" are not used in strict definitional 
senses, but instead are used to describe a generally smooth, continuous, 
out-of-round shape which deviates in shape in a cam-like fashion from a 
region of minimum radial dimension to a region of maximum radial 
dimension. 
For example, FIG. 5 illustrates a top plan view of a preferred embodiment 
of the duplex collar 16. The rearward portion 56 is defined by two 
peripheral arcuate surfaces: a first peripheral surface 62 defined by a 
radius A.sub.1 offset from a center 64 of the collar 16 by a distance 
.delta.; and a second peripheral surface 66 defined by a radius A.sub.2 
offset from the collar center 64 by an equal distance .delta., but on the 
opposite side of the collar center 64 along a minor axis of the rearward 
portion 56. The radii A.sub.1, A.sub.2 have equal lengths. Likewise, the 
central portion 54 is defined between two peripheral arcuate surfaces. A 
first peripheral surface 70 is defined by a radius A.sub.3 and a second 
peripheral surface 72 is defined by a radius A.sub.4. Each radius A.sub.3, 
A.sub.4 is offset from the center 64 of the collar 16 by a distance 
.delta., but on opposites sides of the center 64 along a minor axis of the 
central portion 54. The radii A.sub.3, A.sub.4 also have equal lengths. 
The central portion 54 is sized with respect to the rearward portion 56 to 
produce a desired preload before locking the collar 16 onto the fastener 
14. As will be explained in greater detail infra and as illustrated in 
FIG. 4, a moment arm M.sub.1 at a camming surface 76 of the central 
portion 54 is selected to carry the majority of the applied torque up 
until a desired preload is produced. Desirably, the central portion 
carries about two-thirds of the torque required to produce the desired 
preload. A moment arm M.sub.2 at a camming surface 78 of the rearward 
portion 56 is selected to carry the balance of the applied torque up to 
preload. The moment arm M.sub.2 is also selected such that the rearward 
portion 56 plastically deforms at a torque less than the total torque 
required to produce the desired preload but greater than the torque 
carried by the rearward portion 54 up until preload. The collar rearward 
portion 56 plastically deforms at this intermediate torque, swaging into 
the flutes 50 of the fastener 14. Advantageously, the ratio of the major 
axis of the rearward portion 56 to the minor axis of the central portion 
54 ranges from about 1.0 to about 1.2 to provide the desired moment arm 
lengths. In a preferred embodiment, the ratio equals 1.0. 
The ratio of the minor axis to the major axis for both the central portion 
54 and the rearward portion 56 ranges from about 0.83 to 0.93, and 
desirably equals 0.88. In other words, the eccentricity .theta. of the 
elliptical shapes for the central portion 54 and the rearward portion 56 
advantageously ranges between 39.75.degree. and 43.75.degree., and 
desirably equals about 41.75.degree., where the degree of eccentricity 
.theta. equals the arc-tangent of the minor diameter divided by the major 
diameter. If the degree of eccentricity .theta. of the elliptical shapes 
is much greater (i.e., the elliptical shape is "flatter"), too great of a 
moment arm exists and the applied torque will strip the threads of the 
collar 16 and the fastener 14. Conversely, if the degree of eccentricity 
of the elliptical shapes is much less (i.e., the elliptical shape is "more 
round"), too much torque will be required to produce the desired preload. 
The axial lengths for the central portion 54 and the rearward portion 56 
are also designed to generate the desired preload before plastically 
deforming the collar 16. That is, the axial length of the rearward portion 
56 and the central portion 54 are selected to provide sufficient contact 
surface between the collar 16 and the installation tool 18 to disperse the 
applied torque such that collar 16 resists plastic deformation prior to 
applying the torque required to produce preload. The total axial height of 
the collar 16 also must be sufficient to support enough threads such that 
the desired preload is less than the tensile strength of the fastener 
system 10. Desirably, the preload value should be less than 75% of the 
tensile strength of the fastener system 10 for safety. However, the collar 
16 should be as lightweight as possible. 
In an optimal collar design, which optimizes strength and weight, the 
collar 16 has an axial height equal to about 9.0 P, where P equals the 
thread pitch of the axial bore 58. As used herein, "thread pitch" means 
the distance measured parallel with the collar longitudinal axis between 
corresponding points on adjacent thread forms in the same axial plane and 
on the same side of the longitudinal axis. The axial height of the forward 
portion 52, the central portion 54 and the rearward portion 56 desirably 
equal about 2.7 P, 2.9 P and 3.4 P, respectively. Therefore, as 
illustrated in FIG. 3, the rearward portion 56 has an axial length L.sub.1 
of approximately 38% of the total axial length of the collar 16. The 
central portion 54 has an axial length L.sub.2 of approximately 32% of the 
total axial length of the collar 16. 
Thus, for instance, a collar with a 1/4-28 UNF internal thread has a total 
axial height of 0.321 inch. The axial heights of the forward portion 52, 
the central portion 54 and the rearward portion 56 equal 0.096 inch, 0.104 
inch and 0.121 inch, respectively. 
Referring to FIG. 5, the areas of the rearward portion 56 and of the axial 
bore 58 measured in a plane perpendicular to the collar longitudinal axis 
are selected to produce sufficient swaging of the rearward portion 
material to lock the collar 16 onto the fastener 14. The ratio between the 
total cross-sectional area of the rearward portion 56 and the area of the 
axial bore 58 ranges between 1.9 and 2.3, and desirably equals 2.1. 
An exemplary embodiment of the collar 16 includes a 3/8-24 UNF threaded 
axial bore 58. The rearward portion 56 has an axial length of 0.141 inch, 
a major diameter of 0.440 inch and a minor diameter of 0.343 inch. The 
central portion 54 has an axial length of 0.121 inch, a major diameter of 
0.488 inch and a minor diameter of 0.440 inch. The forward base has an 
axial length of 0.108 inch and a diameter equal to 0.488 inch. A fastener 
system 10 using a collar 16 dimensioned accordingly and made of titanium, 
and being installed with the installation tool 18 described below, would 
obtain a preload of about 4000 lbs.+-.400 lbs. Thus, it should be 
understood that one skilled in the art could construct a collar 16 with 
the appropriate dimensions for the desired preload. 
The collar 16 and fastener 14 are advantageously constructed of a 
lightweight, high strength metal alloy, such as, for example, an aluminum 
or a titanium alloy. However, it is contemplated that the collar 16 and 
fastener 14 could be constructed of other materials with an appropriate 
strength and deformability tailored to the particular application for the 
fastener system 10. 
The collar 16 is preferably formed from a single piece of material by 
forging, "heading" or like processes, as known in the art. The fastener 14 
can be fabricated by known processes, such as, for example, forging a 
fastener blank, cold rolling the threads onto the blank and subsequently 
cutting or grinding the flutes 50 into the thread 26. 
The Installation Tool 
Referring to FIG. 6, the installation tool 18 has a polygonal-shaped 
exterior 79 which engages a correspondingly shaped cavity of the box 
wrench 20. Although FIG. 6 illustrates the installation tool 18 as an 
insert for a standard ratcheting box wrench 20, it is understood that an 
installation tool 18a can be formed as a standard socket for a socket 
driver wrench, as illustrated in FIG. 6a. 
As illustrated in FIG. 6, the installation tool 18 defines a stepped axial 
bore 80 extending through the installation tool 18 in the longitudinal 
direction. The installation tool 18 defines a cylindrical counterbore 81 
extending from a forward end 82 into the installation tool 18. The 
installation tool 18 additionally defines a large cavity 84 opening into a 
small cavity 86 in the rearward direction and opening into the counterbore 
81 in the forward direction. 
The large cavity 84 and the small cavity 86 are configured to engage the 
camming surfaces 76, 78 of the collar central portion 54 and the collar 
rearward portion 56, respectively. As illustrated in FIG. 7, the large 
cavity 84 has a shape complimentary to that of the central portion 54, and 
desirably has a generally elliptical shape in plan view. The small cavity 
86 similarly has a generally elliptical shape complimentary to the shape 
of the collar rearward portion 56. The installation tool cavities 84, 86 
are slightly larger than the corresponding collar portions 54, 56 to 
facilitate placement of the installation tool 18 over the collar 16. The 
minor diameter of the small cavity 86 desirably equals the minor diameter 
of the collar rearward portion 56 multiplied by the secant of 10.degree.. 
For instance, the minor diameter of the small cavity 86 is about 0.006 
inch larger than the minor diameter of the rearward portion 56 of a 
1/4"-28 UNF collar 16. The large cavity 84 advantageously is sufficiently 
larger than the collar central portion 54 such that the installation tool 
18 is easily placed over the collar 16. 
Desirably, the major axes of the large cavity 84 and the small cavity 86 
are generally aligned. Likewise, the minor axes of the cavities 84, 86 are 
generally aligned. 
Referring to FIG. 7, the large cavity 84 is defined between a first arcuate 
surface 88 and a second arcuate surface 90. The first surface 88 has a 
radius of curvature R.sub.1 slightly larger than the radius of curvature 
A.sub.4 of the collar central portion 54 (FIG. 5). Likewise, the second 
surface 90 has a radius of curvature R.sub.2 slightly larger than the 
radius of curvature A.sub.3 of the collar central portion 54. Each radius 
R.sub.1, R.sub.2 is offset from a center 92 of the installation tool 18 by 
a distance .pi., but on opposite sides of the center 92 along the minor 
axis of the large cavity 84. 
Similarly, the small cavity 86 is defined between a first arcuate wrenching 
surface 94 and a second arcuate wrenching surface 96. The first wrenching 
surface 94 has a radius of curvature r.sub.1 slightly larger than the 
radius of curvature A.sub.2 of the collar rearward portion 56 (see FIG. 
5). Likewise, the second wrenching surface 96 has a radius of curvature 
r.sub.2 slightly larger than the radius of curvature A.sub.2 of the collar 
rearward portion 56. Each radius r.sub.1, r.sub.2 is off-set from the 
installation tool center 92 by a distance .pi., but on opposite sides of 
the center 92 along the minor axis of the small cavity 86. In a preferred 
embodiment, the distance .pi. approximately equals the collar distance 
.delta. (FIG. 5). 
As best seen in FIG. 7, a pair of driving ridges 98 interrupt the generally 
elliptical shape of the large cavity 84. That is, the driving ridges 98 
are disposed diametrically opposite each other on the arcuate surfaces 88, 
90 of the large cavity 84. As illustrated in FIG. 8, each driving ridge 98 
generally extends along the entire axial height of the large cavity 84 and 
is generally parallel to the longitudinal axis of the installation tool 
18. 
Referring to FIGS. 7 and 8, the intersection of a flat plane 100 and a 
relief plane 102 forms each driving ridge 98. Both planes 100, 102 extend 
in the longitudinal direction parallel to the longitudinal axis of the 
installation tool 18. As seen in FIG. 7, the flat plane 100 lies generally 
along a cord of the arc defined by the arcuate surface radius R.sub.1, 
R.sub.2, which is positioned perpendicularly to the radius R.sub.1, 
R.sub.2 at the driving ridge 98. The relief plane 102 truncates the flat 
plane 100 and angles outwardly from the flat plane 100 by an angle 
.lambda.. The angle .lambda. desirably equals about 15.degree.. The line 
of intersection formed between the intersection of the flat plane 100 and 
the relief plane 102 defines the driving ridge 98 which contacts the 
collar central portion 54 during installation. 
The point of intersection between the flat plane 100 and the relief plane 
102 (i.e., the driving ridge 98) is off-set from the major axis of the 
large cavity 84 by an angle .OMEGA. which advantageously ranges between 
15.degree. and 30.degree., and desirably equals about 20.degree.. The 
distance X between the driving ridges 98 is less than the major diameter 
of the collar central portion 54 and is greater than the minor diameter of 
the central portion 54. The distance X between the driving ridges 98 
desirably equals a diametric distance across the collar central portion 54 
at a point off-set from the central portion major axis by approximately 
15.degree. to 30.degree., and preferably at a point off-set by about 
20.degree.. That is, the distance X between the driving ridges 98 is 
generally equal to the distance between the camming surfaces 76 of the 
collar central portion 54 (see FIG. 5). 
The driving ridges 98 advantageously engage the collar 16 proximate to the 
major axis of the collar central portion 54 to increase leverage. If angle 
.OMEGA. is too small, insufficient torque will be generated by the time 
the driving ridges 98 slip around the ends of the major axis of the collar 
16 during the installation process. Conversely, if angle .OMEGA. is too 
large, too much torque will be generated which may over-stress the threads 
of the collar axial bore 58. 
As best seen in FIG. 8, the installation tool 18 includes an arcuate 
transition section 104 disposed between the large cavity 84 and the 
counter bore 81, forming a curved transition between cavities 81, 84. The 
transition section 104 has a radius of curvature equal to one-half the 
difference between the diametric dimension of large cavity 84 and the 
diameter of the counterbore 81. During the installation process, the 
transition section 104 registers on the collar base shoulder 55, as 
discussed below. 
Referring to FIGS. 8, 9 and 10, the small cavity 86 includes a pair of 
oblique driving ridges 110 and a pair of exit vents 112. The oblique 
driving ridges 110 and the exit vents 112 hold the installation tool 18 on 
the collar 16 and prevent the installation tool 18 from prematurely 
camming-off of the collar 16 during installation, as is discussed in 
greater detail below. 
The oblique driving ridges 110 are disposed diametrically opposite each 
other on the interior wall of the small cavity 86. As illustrated in FIG. 
8, each oblique driving ridge 110 extends generally along the entire axial 
height of the small cavity 86 and is skewed in the longitudinal direction 
at an angled .beta. from the longitudinal axis of the installation tool 
18. 
Referring to FIGS. 7 and 10, each oblique driving ridge 110 has a mid-point 
111 bisecting the oblique driving ridge 110. As illustrated in FIG. 7, the 
mid-point 111 is located along a line extending between the driving ridge 
98 of the large cavity 84 and the installation tool center 92. Thus, the 
mid-point 111 is off-set from the major axis of the small cavity 86 by the 
angle .OMEGA.. 
As illustrated in FIG. 10, the distance Y between the mid-points 111 of the 
oblique driving ridges 110 is less that the major diameter of the collar 
rearward portion 56 and is greater that the minor diameter of the rearward 
portion 56. The distance Y between the oblique driving ridges 110 
advantageously equals the diametric dimension across the collar rearward 
portion 56 at a point off-set from the major axis of the collar rearward 
portion 56 by approximately 15.degree. to 30.degree., and desirably at a 
point off-set from the major axis of the rearward portion 36 by about 
20.degree.. That is, the distance Y between the oblique driving ridges 110 
is generally equal to the distance between the camming surfaces 78 of the 
collar rearward portion 58 (see FIG. 5). 
As best illustrated in FIG. 9, an intersection of a flat plane 114 and an 
oblique relief plane 116 forms each oblique driving ridge 110. The flat 
plane 114 lies generally parallel to the longitudinal axis of the 
installation tool 18 and, as best seen in FIG. 10, lies generally along a 
cord of the arc defined by the wrenching surface radius R.sub.3, R.sub.4, 
which is positioned perpendicularly to the radius R.sub.3, R.sub.4 at the 
mid-point 111 of the oblique driving ridge 110. 
The oblique relief plane 116 truncates the flat plane 114 and angles 
outwardly from the flat plane 114 by an angle .alpha. (FIGS. 9 and 11) 
which equals about 10.degree., and desirably is 8.degree.. In addition, as 
best illustrated in FIGS. 8 and 9, the oblique plane 116 is skewed with 
respect to the longitudinal axis of the installation tool 20 by the angle 
.beta. and slopes negatively (i.e., slopes towards the large cavity 84 
from left to right) for clockwise installation of the collar 16. Angle 
.beta. is on the order of 20.degree., and desirably equals about 
17.degree.. The intersection between the flat plane 114 and the oblique 
relief plane 116 defines the oblique driving ridge 110 which contacts the 
collar rearward portion 56 during installation. 
Referring to FIGS. 8 and 9, each exit vent 112 extends in the forward 
direction from a rearward end 118 of the collar 16 into the small cavity 
86 for a sufficient distance such that when the installation tool 18 is 
placed over the collar 16, the portions of the collar rearward portion 56 
juxtaposing the exit vents 112 have axial heights equal to about 0.75 P. 
The vent 112 also extends into the wrenching surfaces 94, 96 of the small 
cavity 86. 
As best seen in FIG. 9, a leading flat 120, an arcuate back wall 122, a 
terminating flat 124 and a shoulder 126 define each vent 112. The 
terminating flat 124 extends from the minor axis of the small cavity 86 
and, as best illustrated in FIG. 10, lies along a line tangent to the 
small cavity minor axis. The terminating flat 124 extends away from the 
minor axis and intersects with the back wall 122. The back wall 122 has a 
radius of curvature r.sub.3 which is larger than the radius r.sub.2 of the 
wrenching surface 96 by about 0.010 inch. The back wall 122 extends 
through an arc until it intersects with the leading flat 120. In a 
preferred embodiment, as best illustrated in FIG. 9, the leading flat 120 
lies in the same plane as the flat plane 114 forming the oblique driving 
ridge 110. 
Advantageously, the depth of the vents 112 are as small as possible for 
manufacturing considerations; specifically, the design should not produce 
a burr when honing the vents 112 by the manufacturing operation described 
below. However, the vents 112 should be significantly deep to receive 
enough collar material to hold the installation tool onto the collar 
during deformation of the rearward portion 56. 
The leading flat 120, the back wall 122 and the terminating flat 124 
preferably are generally parallel to the longitudinal axis of the 
installation tool 18 and extend between the shoulder 126 and the rearward 
end 118 of the installation tool 18. The shoulder 126 forms a transition 
between the arcuate wrenching surface 94, 96 of the small cavity 86 
defined by the radius r.sub.1, r.sub.2 and the flats 120, 122 and back 
wall 122. As best seen in FIG. 12, the shoulder 126 is desirably angled 
from the longitudinal axis by about 30.degree. to ease fabrication. 
Referring to FIG. 10, the major axis and the minor axis of the small cavity 
86 divide the small cavity 86 into four quadrants. The quadrants trailing 
the major axis as the installation tool 18 is rotated in the clockwise 
direction include the oblique driving ridges 110 and the exit vents 112. 
As a result, the collar locks together with the fastener 14 through a 
180.degree. rotation of the installation tool 18, as discussed in detail 
below. 
The installation tool 18 is preferably made of hardened steel, carbide or 
another suitable high strength metal alloys. The installation tool 18 has 
a yield strength significantly greater than that of the collar material in 
order to deform the collar 16 at the specific preload while substantially 
maintaining the shape of the installation tool axial bore 80 during the 
installation process. 
The installation tool 18 is formed from a round donut-shaped slug extruded 
into the configuration described above. Specifically, the slug is placed 
into a die comprising an extrusion bearing and a mandrel, with the 
donut-shaped slug fitting over the mandrel. 
Desirably, the extrusion bearing has a polygonal shape conforming to a 
standard box wrench configuration. However, the extrusion bearing could 
have other configurations, such as, for example, an elliptical shape, for 
adapting the installation tool 18 to a pneumatically driven wrench. 
The mandrel has an elliptical cross-sectional shape segment configured in 
accordance with the above description of the small cavity's 86 elliptical 
shape. The mandrel additionally includes a larger generally elliptical 
cross-sectional shape portion configured in accordance with the above 
description of the large cavity 84 and includes reliefs and flats which 
form the driving ridges 98. The mandrel also includes a cylindrical 
portion to form the counterbore 81. Under pressure, the slug conforms to 
the shape defined between the extrusion bearing and the mandrel. 
A second mandrel, pushing from the rearward end of the slug, subsequently 
forces the first mandrel out of the formed small cavity 86. During this 
process, the second mandrel hones the small cavity 86, thereby forming the 
oblique driving ridges 110 and exit vents 112 into the interior wall of 
the small cavity 86 of the installation tool 18. 
Although FIGS. 8 through 10 illustrate the oblique relief plane 116 
extending along the entire axial height of small cavity 86, for 
manufacturing purposes the oblique relief plane 116 can extend from a 
rearward end 118 of the installation tool 18 to a point proximate to the 
transition between the large cavity 84 and the small cavity 86, but not 
into the large cavity 84. This configuration of the installation tool 18 
prevents the formation of a burr at the transition between the cavities 
84, 86. 
As an alternative to forming the driving ridges 98, 110 as described above, 
it is also contemplated that the driving ridges 98, 110 can be formed by 
drilling holes into the slug, the hole, in part, break through the walls 
of the cavities 84, 86. Thereafter, pins are press fit into the holes such 
that a portion of the pins peripheries exterior extend into the cavities 
84, 86 to form the driving ridges 98,110 at the positions described above. 
Method of Installing the Fastener System 
Installation of the fastener system 10 will now be described with reference 
to FIGS. 1, 13 and 14. As illustrated in FIG. 1, the threaded fastener 14 
is inserted through the work pieces 12 which are being fastened together, 
and the collar 16 is placed on the threaded portion 26 of the fastener 14. 
With the installation tool 18 press-fit into the wrench 20, the 
installation tool 18 is slid over and onto the duplex collar 16. The 
installation tool transition section 104 abuts against the collar base 
shoulder 57, registering the installation tool 18 with the duplex collar 
16 for axial positioning. 
The small cavity 86 only contacts the upper 80% of the collar rearward 
portion 56 when registered. In other words, a forward segment of the 
rearward portion 56 having an axial height of about 0.75 P remains 
unworked during the installation process described below. If the forward 
segment of the rearward portion 56 is worked, the torque required to swage 
the collar material during the locking process is equal to or greater than 
the preload torque, because the forward segment of the rearward portion 56 
resists deformation during swaging. However, by working only the segment 
of the rearward portion 56 distal of the collar central portion 54, the 
torque required to swage the rearward portion 56 can be designed to be 
less than the torque required to produce preload but greater than the 
torque carried by the rearward portion 56 up until preload. 
The wrenching key 33 extends through the installation tool 18 along the 
longitudinal axis of the fastener 14. The wrenching key 33 closely fits 
inside the fastener cavity 32, holding the fastener 14 stationary while 
the collar 16 is tightened onto the fastener 14. As a result, the collar 
16 can be installed from one side of the joined materials 12 (i.e., from 
the blind side). 
Rotation of the installation tool 18 brings the driving ridges 98, 110 of 
the large and small cavities 84, 86 into contact with the collar central 
portion 54 and the collar rearward portion 56, respectively. Further 
rotation of the installation tool 18 applies a torque to the rearward 
portion 56 and to the central portion 54 along the driving ridges 98, 110 
contacting the collar 16, which threads the collar 16 onto the threaded 
fastener 14. 
Initially, the resistance to rotation is minimal and the installation tool 
18 and collar 16 rotate at the same rate. At this stage of the 
installation process, the central portion 54 has a generally undeformed, 
elliptical cross-sectional shape, as shown in FIG. 13, and the rearward 
portion 56 also has a generally undeformed, elliptical cross-sectional 
shape, as illustrated in FIG. 14. Because the shapes of the central 
portion 54 and rearward portion 56 are unmodified, the driving ridges 
98,110 of the large and small cavities 84, 86 continue engaging the 
central and rearward portions 54, 56, respectively, and applying a torque 
to the duplex collar 16. 
The resistance to threading eventually increases as the collar 16 is 
tightened onto the threaded fastener 14. When the applied torque 
approaches the preload torque, the driving ridges 98, 110 begin digging 
into the central portion 54 and the rearward portion 56, respectively. 
Because the installation tool 18 digs into the collar 16, as opposed to 
pushing on the collar 16, the installation tool 18 generates higher 
preloads than those produced by prior installation tools contacting a 
collar with flats or arcuate surfaces. 
The pure rotational force (i.e., the torque) exerted by the installation 
tool 18 along each oblique driving ridge 110 is split into rectangular 
force components that act perpendicular and parallel to the lay of the 
oblique driving ridge 110, as schematically illustrated in FIG. 8. The 
force component acting in the direction of the oblique driving ridge 110, 
exerts an axial thrust on the installation tool 18, urging the 
installation tool 18 onto the collar 16 to counteract the natural 
reactionary tendency to cam-off. 
Advantageously, at this stage the installation tool 18 only slightly 
compresses the duplex collar 16 into the threads 26 of the fastener 14 
because the installation tool 18 contacts the collar only at its driving 
ridges 98, 110, minimizing the radial compressive force it exerts on the 
collar 16. Importantly, the driving ridges 98, 110 of the installation 
tool 18 have sufficient axial lengths, spreading the applied torque over a 
large enough collar area such that the collar 16 withstands deformation up 
until the torque required to produce the desired preload. 
Additionally, the present flute design of the fastener 14 does not 
exacerbate the effect of elastically compressed collar material because 
the flute design prevents premature captivation of the collar 16. Collar 
material compressed into the flutes 50 travels over the convex surface 36 
of the fastener thread recesses 34 as the collar 16 continues to rotate. 
Blending the bottom of the recesses 34 in the direction in which the 
collar 16 turns delays fastener 14 capture by the rotating collar 16, 
thereby allowing a high preload to develop on the joint 12 without 
over-stressing the hexagon key 33. 
When the applied torque approximately equals the torque required to produce 
preload, the driving ridges 98. 110 begin plastically deforming the collar 
central and rearward portion 54, 56, pushing the collar material around 
their respective major axes. At the point of producing the desired 
preload, the driving ridges 98, 110 have wiped the material of the collar 
camming surface 76, 78 around the circumference of the central portion 54 
and the rearward portion 56 proximate to the major axes of the collar 16. 
FIG. 13 illustrates the deformed configuration of the collar central 
portion 54 in phantom line. 
Although the large cavity driving ridges 98 force some percentage of the 
deformed collar material of the central portion 54 into the fastener 
threads 26, this radial plastic deformation does not lock together the 
collar 16 and the fastener 14. The collar material fills the voids defined 
between the series of thread crests 27, but the deformed material does not 
fill any of the fastener flutes 50. 
At this stage, the driving ridges 98 of the large cavity have slipped 
around the ends of the major axis, completed their function and rotated 
out of engagement with the central portion 54. Consequently, the collar 
rearward portion 56 carries all subsequently applied torque. Additionally, 
the oblique driving ridges 110 of the small cavity 86 have slipped around 
the ends of the major axis, rotated out of engagement with the rearward 
portion 56, and completed their function. The collar 16 will no longer 
turn. 
The arcuate wrenching surfaces 94, 96 trailing the oblique driving ridges 
110 rotates into engagement with the collar rearward portion 56 as the 
installation tool 18 continues its rotation. The arcuate wrenching 
surfaces 94, 96 plastically deform the rearward portion 56 in the radial 
direction towards the fastener 14 and into the fastener flutes 50 because 
the distance between the arcuate wrenching surfaces 94, 96 and the 
installation tool center 92 decreases as the installation tool 18 is 
rotated to sweep its minor diameter past the major axis of the collar 
rearward portion 56. 
The installation tool 18, however, does not contact the collar rearward 
portion 56 at the exit vents 112. Thus, the exit vents 112 are non-working 
reliefs in the arcuate wrenching surfaces 94, 96 of the small cavity 86 
which allow collar material to bypass deformation. The arcuate wrenching 
surface 94, 96 below the exit vent 112, however, deform the rearward 
portion 56 radially inward as previously described. The effect is to delay 
the swaging at the exit vents 112, thus trapping the installation tool 18 
onto the collar 16. The installation tool 18 cannot cam-off at this stage 
because the undeformed rearward portion 56 within the exit vents 112 
prevents the exit vent shoulder 126 from moving axially in the rearward 
direction. 
Continued rotation of the installation tool 18 brings the terminating flats 
124 of the exit vents 112 into contact with the undeformed collar material 
within the exit vents 112. The terminating flats 124, which coincides with 
and are perpendicular to the minor axis of the smaller cavity 86, swage 
the undeformed material from the collar rearward portion 56 into the 
fastener flutes 50. 
Referring to FIG. 14, the rearward portion 56 of the collar 16 is deformed 
radially inwardly from its elliptical, out-of-round shape to a generally 
circular shape (shown in phantom line) which prevents the application of 
further torque. Although some collar material is displaced 
circumferentially, from the major axis towards the minor axis, the 
fastener flutes 50 should have sufficient void volume to receive a 
majority of the radially deformed collar material. 
The smooth surfaces of the arcuate wrenching surfaces 94, 96 and the 
terminating flats 124 finish swaging the collar rearward portion 56 as 
they slip around the major axis of the collar 16. At this stage, the 
collar 16 is completely swaged onto the fastener and the installation tool 
18 rotates to a non-working position (which enables disengagement), 
180.degree. from where the installation tool began rotating about the 
collar 16. Consequently, the installation tool 18 can be used in more 
confined areas than prior installation tool which require 360.degree. of 
tool rotation to complete the swaging operation. 
Once the collar rearward portion 56 completely deforms, the installation 
tool 18 freely cams off of the duplex collar 16. The significant looseness 
provided by the overall diameter of the cavities 84, 86 compared with the 
restrictive distance between the driving ridges 98, 110 allows for ready 
removal of the installation tool 18 at the end of installation. 
The present invention advantageously separates the generation of the 
preload from the formation of the lock, by using the central portion 54 to 
generate a majority of the preload and the rearward portion 56 to form the 
lock. This feature is taught by U.S. patent application Ser. No. 
07/481,373, now issued as U.S. Pat. No. 5,145,300, which is hereby 
incorporated by reference. The subject matter of the present application 
and of patent application Ser. No. 07/481,373 were, at the time the 
inventions were made, subject to an obligation of assignment to the same 
person. 
Moreover, the designs of the duplex collar 16, the fastener flute 50 and 
the installation tool 18 significantly reduce precapture and elastic 
radial compression experienced with prior locking fastener systems. Thus, 
the present fastener system 10 enhances the repeatability and 
predictability of preload values produced at the junction of the joined 
materials 12. 
Modifications and variations of the embodiments described above may be made 
by those skilled in the art while remaining within the true scope and 
spirit of this invention. For instance, oblique driving ridges may be 
disposed in the large cavity of the installation tool. In addition, 
although the tool has been solely described as being formed from a single 
piece of material, the tool could consist of at least two separate rings 
press-fit into the tool in the desired orientation. Finally, it is 
understood that while the installation tool of the present invention is 
shown and described for clockwise rotation, the driving ridges could be 
disposed for counter-clockwise rotation of the installation tool. 
Accordingly, the scope of the invention is intended to be defined only by 
the claims which follow.