Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02303693 2000-03-28
METHOD AND APPARATUS FOR TOUGHENING
METAL STRAP BUCKLES
Field of the Invention
The present invention relates to the field of metal buckles, and in
particular, to
5. the field of metal buckles made from a single piece of wire that has been
toughened by
heat treatment.
Background of the Invention
Various types of metal buckles for strapping boxes, bales, bundles (i.e., of
cotton), etc., have been employed in the past. One of the most common types is
often
referred to as a Cotton Tie Buckle. These buckles are typically constructed of
a single
piece of metal wire that has beep bent into a predetermined configuration.
Examples of Cotton Tie Buckles are shown in the following patents: U.S. Patent
No. 686,129, issued to Ragsdale et al., U.S. Patent No.3,014,256, issued to
Derrickson
et al., U.S. Patent No. 3,377,666, issued to Sherman, and U.S. Patent No.
3,624,868,
issued to Somann. Each of these buckles is made of a single piece of wire that
has
been bent into a pre-determined configuration, wherein two pairs of strap
engaging
arms that are substantially parallel to each other are provided to engage the
strap. The
buckle's engaging arms are configured such that the strap can be extended and
secured through the arms, one end of the strap in each pair of arms. One end
of the
strap is held within one pair of engaging arms, and the other end of the strap
is held
within the other pair of engaging arms, wherein the intermediate part of the
strap is
extended tightly about the box, bale, bundle, etc., to be held.
CA 02303693 2000-03-28
Typically, when a strap is extended about a box, bale, bundle, etc., and
tightened, the strap is placed under significant tension. This places great
stress on the
buckle that connects the strap ends together. A problem associated with
conventional
buckles of this type is that the orientation of the strap within the buckle's
arms can
5. cause the arms to bend, such that over time, the buckles can become
distorted, and the
arms can fall out of parallel with one another, which can cause the following
to occur:
First, the arms can pinch the strap in a manner that can cause the strap to
fail
prematurely, i.e., they can cause the strap to tear before the strap's maximum
strength
capacity is reached. For example, even if the strap's maximum strength
capacity is
1,200 pounds, the buckle can, if the load is high enough to cause the buckle's
arms to
bend and distort, cause the strap to break at 600 pounds. This significantly
lowers the
maximum capacity of the strap.
Second, bending of the buckle's arms such that they fall out of parallel with
one
another can also result in an uneven application of stress to the strap. That
is, when the
arms are not parallel, greater tensile stresses can be applied to an isolated
location on
the strap, wherein the strap can begin to tear at the over-stressed point.
And, once the
strap begins to tear, it is likely that it will break at loads far less than
the maximum
strength capacity of the strap.
Given these deficiencies, it was believed that standard Cotton Tie Buckles
would
need to be redesigned to prevent excessive bending and distortion, and thereby
maintain the toad capacity that the strap was intended for. This is
particularly true given
that strap materials have improved significantly to the point where much
greater tensile
strength capacities are possible.
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One of the most common ways that the propensity of the buckle to bend was
reduced was to simply increase the thickness of the wire. Although increasing
wire
thickness has been successful in reducing the likelihood of bending, doing so
has had
its disadvantages, including making the buckle heavier and bulkier, and the
cost of
5. manufacturing the buckles higher (which can lead to failure in the market
place).
Other attempts to solve the problem were directed to using different buckle
configurations. For example, in Derrickson et al., an attempt was made to
configure the
wire in a manner that allowed the buckle's arms to rest on other areas of the
buckle to
help prevent the wire from bending excessively. But as the maximum strength
capacities of straps began to increase, the greater loads that can be applied
to straps
caused the arms to bend and distort, and fall out of parallel with each other,
thereby
resulting in premature failure. In Somann, another attempt was made to help
limit
bending and distortion of the load-bearing members and maintain parallelism
between
the arms by using detents. The disadvantage of this design was that additional
time
and expense were required to form the detents at precise locations on the
wire. Also,
the cost of having to position the detents in such manner proved to be
prohibitive.
Different threading configurations have also been attempted, but have not been
successful, insofar as the engaging arms would still bend and lead to
premature strap
failure. While various attempts have been made to overcome the deficiencies
noted
above, none have been very successful. What is needed, therefore, is an
improvement
to standard tie buckle designs which resists bending and distortion, and
results in
parallelism between the engaging arms, such that the buckle does not lead to
premature strap failure at the joint.
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Summary of the Invention
The present invention relates to a buckle made from a single piece of metal
wire
that has been configured such that it has two pairs of strap engaging arms
that are
substantially parallel to one another. The arms are so configured and
constructed such
5.. that they can engage opposite ends of a tie strap to provide a secure
connection for
strapping boxes, bales, bundles, etc., together.
The present invention represents an improvement over previous designs in that
the metal has been hardened by heat treatment. While processes for hardening
the
surface of metal are known, the specific manner in which the process has been
applied
to the present invention, to increase not only strength, but also toughness,
as well as to
decrease the propensity of the metal to achieve plastic deformation, is
believed to be
unique to the present invention. In this respect, the present invention is
able to
overcome the above noted deficiencies without substantially increasing cost,
and
without altering the configuration of the buckle design.
There are various hardening processes that can be employed to provide the
benefits of the present invention. Conventional quenching of hot steel in
water or oil
provides surface hardening, and can harden some or all of the interior as
well. Other
hardening processes can also be used, such as carburizing, carbonitriding,
nitriding,
induction hardening and flame hardening.
Carburizing is a surface hardening process in which steel is held at a high
temperature, i.e., 1,600 degrees F. (is common), for a relatively long time,
i.e., six
hours, in a carbonaceous environment, usually a gas mixture of carbon monoxide
and
carbon dioxide, with or without hydrocarbons such as methane. Carbonitriding
is similar
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to carburizing except that nitrogen as well as carbon is diffused into the
steel. Nitriding
is another diffusion process in which, by using ammonia without a carbonaceous
gas,
nitrogen is diffused into the steel and combines with other elements,
principally
chromium, already contained in the steel to form nitrides that are extremely
hard and
5. wear resistant. Induction hardening is a process where the steel is briefly
heated by
electrical induction so that only the surface has time to get hot. The metal
is then
immediately cooled or quenched and the portion that was heated above the
critical
temperature (about 1,500 degrees F.) becomes hard. Flame hardening involves no
change in the chemical composition of the steel surface, but involves a high
temperature flame that impinges directly on the metal for a short time,
heating the
surface but not the interior of the piece.
The present invention contemplates using one of these methods to harden the
metal after it has been bent into its predetermined configuration. In this
respect, a large
volume of tie buckles can be heat treated at one time, thereby making it
possible to
reduce the unit cost of manufacture. Moreover, the present invention
contemplates
tempering by annealing to reduce brittleness after the metal has been
hardened. The
present invention also contemplates that the buckles can be finished with a
dry
phosphate coating to prevent slippage.
Brief Description of the Drawing~~s
FIGURE 1 is a perspective view of the buckle design of the present invention;
FIGURE 2 is a top view of the buckle design of the present invention;
FIGURE 3 is a front view of the buckle design of the present invention;
FIGURE 4 is a side view of the buckle design of the present invention;
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FIGURE 5 is a perspective view of the buckle in use with a strap;;
FIGURE 6 is a side view of the buckle in use with the strap threaded through
the
buckle;
FIGURE 7 is a side view of a conventional type buckle in use with the strap
. threaded through the buckle and a tensile load applied to the strap, wherein
the buckle
is shown to be deformed;
FIGURE 8 is a top view of a standard untreated buckle that has been used and
where plastic deformation has occurred;
FIGURE 9 is a side view of a standard untreated buckle that has been used and
where plastic deformation has occurred;
FIGURE 10 is a top view of the buckle of the present invention that has been
used and where only a small amount of deformation has occurred; and
FIGURE 11 is a side view of the buckle of the present invention that has been
used and where only a small amount of deformation has occurred;
Detailed Description of the Invention
Figure 1 shows the buckle of the present invention from a perspective view
having two pairs of strap engaging arms formed by a single piece of wire that
has been
bent into a predetermined shape. While the specific configuration shown in the
drawings is described below, it should be noted that the present invention is
not limited
to the specific configuration described below. The present invention is
intended to
include virtually any tie buckle configuration formed by a single piece of
metal wire that
has been treated in the manner specified.
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In general, buckle 1, as seen in Figures 1 and 2, has four engaging arms, 3,
5, 9
and 13, that are substantially parallel to one another, wherein the parallel
portions of the
engaging arms are relatively straight and have widths that are sufficient to
enable a
conventional size strap to be threaded therethrough, as shown in Figure 5. In
this
.. manner, strap 23 is adapted to engage buckle 1 along the parallel portions
of the
engaging arms, wherein a relatively even distribution of stress can be applied
to the
strap when the strap is in tension. As discussed above, it is important that
the engaging
arms are maintained in a relatively parallel manner during use so that uneven
stresses
do not cause premature tearing of the strap, as well as uneven pinching to
occur, which
can result in premature failure of the strap at the joint.
Buckle 1 comprises a lower level and an upper level. On the lower level, first
and
second engaging arms, 3 and 5, are connected together by a relatively
perpendicularly
oriented connecting portion 7. The three members, 3, 5, and 7, are oriented at
about a
90 degree angle in relation to each other, as shown in Figure 2. On one side,
engaging
arm 3 has an extended portion 4 that extends relatively upward (on the side
away from
connecting portion 7) in a curved manner toward engaging arm 9 on the upper
level.
Another connecting portion 11 extends relatively perpendicularly and upward
from
extended portion 4, through curved joint 19, to connect to the strap engaging
arm 9,
which ends at distal end 17. On the other side, extending from strap engaging
arm 5
(on the side away from connecting portion 7) is another connecting portion 15
that
extends substantially perpendicularly in relation to engaging arm 5.
Connecting portion
15 is extended through a bent portion 16 that wraps around the outside of
extended
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portion 4, as shown in Figure 1, and connects to the other strap engaging arm
13, which
ends at distal end 21.
Strap engaging arms 3 and 5, and strap engaging arms 9 and 13, preferably
extend relatively straight and substantially parallel to one another as seen
in Figures 1
5. and 2. Engaging arm 9 is relatively parallel with engaging arm 5, and
engaging arm 13
is relatively parallel with engaging arm 3. Although engaging arm 3 is
slightly inclined
so that it is not exactly parallel with engaging arm 13, the present invention
contemplates that upon the application of tensile stress to the strap,
engaging arm 3 will
bend to some extent, i.e., achieve some plastic deformation, and therefore,
become
substantially parallel to engaging arm 13 after the load is applied, as can be
seen in
Figure 11. Distal end 17 is preferably extended to a point that extends
outside the
perimeters of engaging arm 5 and connecting portion 7 (in plan view) to help
ensure
that strap 23 will not slip free from buckle 1 during use. Likewise, the
distal end 21 of
engaging arm 13 is preferably extended to a point that extends outside the
perimeters
of engaging arm 3 and connecting portion 7 (in plan view) for the same reason.
Figures 5 through 7 show the buckle 1 of the present invention in use with the
strap 23 extending through the engaging arms and used to tie a box 25. Figure
6
shows a specific threading configuration wherein the ends of the strap 23 are
looped
around the top engaging arms 9 and 13, and under the lower engaging arms 3 and
5.
Figure 7 shows how the buckle can be compressed when the strap 23 is tightened
and
tensile loads are applied, wherein it can be seen that some pinching can occur
between
the respective engaging arms, 5 and 9, and arms, 3 and 13. While the tie
buckle of the
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present invention is treated to avoid or reduce the type of bending shown in
Figure 7,
that drawing is provided to show how bending and distortion can occur.
Figures 8 and 9 are representative drawings of an untreated tie buckle of the
kind
mentioned herein, which has been bent and permanently distorted by the
application of
5.. tensile loads on the strap. These are representations of an actual sample
of an
untreated buckle that has been used and which has been permanently bent and
distorted, i.e., substantial permanent deformation has occurred. As can be
seen in
Figures 8 and 9, considerable plastic deformation has occurred as evidenced by
the
following distortions: 1 ) engaging arms 13 and 9 are no longer parallel to
one another,
i.e., distal ends 21 and 17 are closer to each other than before the load was
applied, 2)
both of the lower engaging arms 3 and 5 are considerably bowed outwardly due
to
stress, wherein it can be seen that engaging arms 3 and 13 are no longer
parallel to one
another, 3) as seen in Figure 9, engaging arm 13 is bent relatively downward
in relation
to engaging arm 9, and 4) the space between engaging arm 13 and engaging arm 3
is
quite narrow, wherein engaging arm 3 is bulged such that it can create an
uneven
pinching stress on the strap. In this particular example, it can be seen that
the engaging
arms are no longer parallel to one another, and that there would likely be an
uneven
amount of stress applied to the strap, as well as uneven pinching between the
engaging
arms, thereby making it likely that premature failure will result.
Figures 10 and 11 show the buckle of the present invention which has been heat
treated and then loaded using a similar test. This sample is only slightly
bent and has
much less distortion than the sample shown in Figures 8 and 9. For example, 1)
the
lower engaging arms 3 and 5 are not bowed outwardly due to stress, wherein it
can be
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seen that engaging arms 3 and 13 still appear to be relatively parallel to one
another, 2)
engaging arm 13 is not bent relatively downward in relation to engaging arm 9,
as can
be seen in Figure 11, and 3) the distance between engaging arm 13 and engaging
arm
3 is relatively constant along the lengths of the engaging arms, so that no
uneven
pinching of the strap is likely to occur, i.e., engaging arms 3 and 13 appear
to be
relatively parallel to one another.
While distal ends 17 and 21 are bent toward each other slightly, it can be
seen
that engaging arms 3 and 13 are even more parallel to each other than they
were
before the loads were applied, indicating that the buckle of the present
invention has,
after being used, achieved some plastic deformation, which has in turn
increased its
ability to resist strap breakage. This indicates that while the buckle resists
plastic
deformation, it nevertheless allows some plastic deformation sufficient to
cause the
buckle to bend in a desirable manner, i.e., so that the arms are more parallel
and the
likelihood of uneven stress and pinching is substantially decreased.
Heat Treatment Methods
There are various hardening processes that can be employed to provide the
benefits of the present invention. Conventional quenching of hot steel in
water or oil
provides surface hardening, but it also usually hardens some or all of the
interior as well
(depending on the thickness of the material). Other forms of surface hardening
methods that can be used include carburizing, carbonitriding, nitriding,
induction
hardening and flame hardening.
Carburizing is a surface hardening process in which steel is held at a high
temperature, i.e., 1,600 degrees F. (is common), for a relatively long time,
i.e., six
CA 02303693 2000-03-28
hours, in a carbonaceous environment, usually a gas mixture of carbon monoxide
and
carbon dioxide, with or without hydrocarbons such as methane. This treatment
causes
carbon to be diffused into the steel to a depth of several thousands or
hundreds of an
inch and leaves the carbon content of the surface much higher than that of the
core.
~- Carbon nitriding is similar to carburizing except that nitrogen as well as
carbon is
diffused into the steel.
Nitriding is another diffusion process in which, by using ammonia without a
carbonaceous gas, nitrogen is diffused into the steel and combines with other
elements,
principally chromium, already contained in the steel to form nitrides that are
extremely
hard and wear resistant.
Induction hardening is a process where the steel is briefly heated by
electrical
induction so that only the surface has time to get hot. The metal is then
immediately
cooled or quenched and the portion that was heated above the critical
temperature
(about 1,500 degrees F.) becomes hard while the rest of the steel which was
not heated
by induction current remains unaffected.
Flame hardening involves no change in the chemical composition of the steel
surface, but involves a high temperature flame that impinges directly on the
metal for a
short time, heating the surface but not the interior of the piece.
The present invention contemplates using one of these methods to harden the
surface of the metal after it has been bent into its predetermined
configuration. It has
been found that some of the heat treating processes used in the present
invention, such
as quenching to full hardness, must be controlled by tempering to limit the
extent to
which the metal can become too brittle, in which case the benefits of heat
treatment can
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be defeated. The present invention also contemplates that the buckles can be
finished
with a dry phosphate coating to prevent slippage.
The Toughness of Metal
The ultimate "strength" of a material is the maximum stress that the material
is
._ capable of developing. Strength has to do with the overall amount of stress
which,
when exceeded, ultimately causes the material to fail. Strength can be
measured in
terms of either the yield strength, which relates to the resistance of the
metal to
permanent deformation, or tensile strength, which is the ultimate tensile
strength of the
metal.
The "stiffness" of a material is defined as the relationship between the
amount of
deformation (i.e., strain), and the applied load (i.e., stress), and is
commonly expressed
in terms of the Young's Modulus, i.e., the slope of the stress-strain curve
before the
yield point. Stiffness has to do with the actual deformation that the metal
goes through
when stress is applied prior to yielding.
While "strength" and "stiffness" are relevant factors that must be considered
in
addressing the bending characteristics of metal, they are not necessarily the
only
factors, nor the most important factors, particularly in this case. For
example, a strong
metal, such as high carbon metal, can be very stiff, but at the same time, it
can be very
brittle, in which case complete failure at the yield point can occur. That is,
while a
strong, stiff metal can have a relatively high yield point, if the yield point
is exceeded,
the metal can fail completely, i.e., shatter.
"Toughness," on the other hand, relates to the energy capacity of a particular
material, i.e., the Modulus of Toughness is defined as the amount of energy
required to
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cause failure in unit volume of a material, and is thus represented by the
total area
under the stress-strain curve. In this respect, the term toughness is more
appropriately
used to describe the combination of strength and "ductility," rather than
strength and
"stiffness." That is, stiffness only relates to the amount of deformation that
occurs prior
5" to the yield point, while ductility relates to total deformation, i.e.,
before and after the
yield point, that occurs before the metal fails.
In this respect, a metal that is high in strength and ductility will not only
resist
bending to a higher degree, but will "give" more at higher stress levels so
that complete
failure will not occur until relatively high stress is applied. The
combination of increased
strength and ductility means that the metal will have a greater ability
(particularly at
higher stress levels) to resist "plastic deformation," which is defined as
permanent
deformation, i.e., the distortion and reformation of atomic bonds. "Plastic
deformation"
is different from "elastic deformation," which is temporary deformation, i.e.,
the
stretching of atomic bonds. It is the propensity of the buckle to achieve
plastic
deformation that the present invention is able to resist by the application of
one of
several types of heat treatment processes.
The Test Results
While it has been known to use heat treatment to harden metals in certain
types
of applications, such as to resist certain kinds of surface wear and stress,
i.e., in gears
and ball bearings, such processes previously had not been used to toughen
metal strap
fasteners such as Cotton Tie Buckles. As far as the Applicant knows, the
Cotton Tie
Buckle industry has never considered using heat treatment as a viable solution
to the
problem of bending and distortion, principally because in the past the only
way it was
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thought that stiffness could be increased was by increasing the thickness of
the wire,
i.e., not by changing the grade or quality of the material. This is based on
the principle
that steel (which is composed primarily of iron alloyed with small percentages
of carbon
and different metals like nickel, chromium, manganese, etc.) has a Young's
Modulus
, equal to about 30,000,000 psi, and therefore, the stiffness of any steel
structure could
not normally be significantly increased by changing the material to a higher
grade
(without increasing the overall size of the members). Moreover, the cost of
using higher
grade materials (other than cheaply fabricated and low-priced steels) for
these types of
fasteners was thought to be cost-prohibitive. For these reasons, previous
attempts to
solve the problem have focused on changing the buckle or threading
configuration, or
increasing the thickness of the metal, not changing the grade or quality of
the metal.
The results achieved by applying heat treatment to improve the performance of
the metal fasteners of the present invention were completely unexpected. When
the
buckles of the present invention were treated in the manner discussed above,
and
tested by applying tensile loads to the tie straps (to which the buckles were
attached),
the overall joint efficiency of the strap was improved in almost every
instance by more
than 40%, and in some cases, by more than 80%. In this respect, while many
standard
tie buckles are unable to satisfy the minimum strength capacity for joint
efficiency
provided by ASTM and other testing standards, the tie buckles of the present
invention
significantly exceeded the minimum strength capacity in each test that was
performed.
In general, tests have been performed to determine the efficiency of the strap
at
the joint where it is connected to the buckle, i.e., the load at which the
strap will break
due to stress caused by the buckle at the joint, using various types of metal
tie buckles,
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including those that have not been heat treated, and those that have been heat
treated.
The straps used in the tests were standard type straps, with standard widths,
having a
specified maximum strength capacity. The specific straps used in these tests
were
samples of TEXband ~ straps made of woven and coated polyester cord fiber.
5. The straps were threaded through the buckles before the loads were applied.
In
this respect, various setups were used to perform the tests. One test
consisted of
using a threading configuration consisting of a double wrap (snub-roll)
extended around
the top leg with the free end inserted into the top grip. Another strap was
inserted into
the bottom grip and single wrapped around the bottom leg. The slack was
removed and
a predetermined pretension was applied using a Capstan and Windlass style
strap
tensioner between the buckle and the bottom grip. The loose strap at the
bottom of the
buckle was also split lengthwise and a square knot was tied snugly against the
bottom
leg of the buckle.
Another test consisted of using a threading configuration involving single
wrapping one strap around the top leg and inserting the free end into the top
grip. An
additional strap was inserted into the bottom grip and single wrapped around
the bottom
leg. The slack was removed and a predetermined pretension was applied using a
Capstand and Windlass style strap tensioner between the buckle and the bottom
grip.
No knot was tied at the bottom of the buckle.
The protocol involved in these tests called for two test applications to be
made:
the first with the legs of the buckle pointing to the left of the technician,
and the second
with the legs pointing to the right. In each case, the tensile tester, such as
the Instrom
4204, was activated until the strap broke. The breaking load (the load that
caused the
CA 02303693 2000-03-28
strap to break) and failure mode for each sample was recorded. Three pulls for
each
test were provided. The average breaking load for each test from each of the
three
pulls was then calculated.
Test Number 1
- This test was conducted using a strap with the designation 66XLP.HD, which
has
a strength of 1,625 pounds per foot. The ASTM standard for joint efficiency
for this
strap is 55 percent of the strap's maximum strength capacity. That is, under
the ASTM
standard, the expectation is that the strap will fail (at the joint where the
strap is
connected to the buckle) when a load equal to 55 percent of the strap's
maximum
strength capacity is applied. In this case, 55 percent of the maximum strength
capaity is
about 894 pounds per foot. The tests were conducted to determine whether this
minimum standard was met.
This test was conducted using Formex B-6XHDG (galvanized) buckles with the
legs pointed to the left, with the strap tension from the bottom, with no snub-
roll at the
top. No knot was provided at the bottom of the strap. A 300 pound pretension
was
applied by the tensile tester. The following results were achieved:
A standard buckle without heat treatment was tested first. Wire sizes of 0.135
inch and 0.148 inch, and wire types 1008 and 1018, were used. An average break
point
was determined from three pulls for each test. These are the results, each
indicating a
joint efficiency of less than 55%.
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Untreated
B-6XHDG
Buckle
Buckle Wire Wire Average Break Standard Deviation
Size Type Point (in pounds per
(in pounds er foot)
foot)
B-6XHDG 0.135 1008 642 60
B-6XHDG 0.135 1018 712 24
B-6XHDG 0.148 1018 818 49
The same Formex B-6XHDG type buckle was then heat treated by carburizing
and then tested. Wire sizes of 0.135 inch and 0.148 inch, and wire types 1008
and
1018, were again used. The carburizing heat treatment process that was
employed
was designed to maximize toughness and increase strength, wherein the metal
was
hardened and tempered to about Rc 40-45. An average break point was determined
from three pulls for each test. These are the results, each indicating a joint
efficiency of
greater than 55%.
Heat Treated
(Carburized)
B-6XHDG
Buckle
Buckle Wire SizeWire Type Average Break Percentage of Increase
Point
(in pounds per
foot)
B-6XHDG 0.135 1008 1007 56.85
B-6XHDG 0.135 1018 1043 46.49
B-6XHDG 0.148 1018 1143 39.73
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These tests show that the buckles that have been heat treated according tv the
present invention have a substantially greater strength capardy than untreated
buckles
of comparable size and type. The treated buckles resulted in a strap break
point, i.e_,
the load at which the strap will fait due to stress at the joint, vrhich is
more than about 40
percent to 57 percent over that which was obtained by standard untreated tie
buddes.
Another way to view this information is that none of the standard untreated
buckles
satisfied the ASTM standard, i.e., the load capacities were anywhere from 8.5
percent
to 28.19 percent below the load capacity of 894 pounds per foot_ On the other
hand,
each of the treated buckles of the present Invention easily satisfied the ASTM
standard,
i.e., the load capadties were anywhere from 12.64 percent to 27.85 percent
above the
standard, which was unexpected.
best Number ~
This test was conducted using a strap designated as 60W, which has a strength
of 900 pounds per ficot. The ASTM standard for Joint efhcH~ncy for this strap
is also 55
percent of the strap's maximum strength capacity. That is, under the standard,
the
expectation is that the strap will fail (at the joint where the strap is
connected to the
buckle) when a load equal to 55 percent of the strap's maximum strength
capacity is
applied. In this case, 55 percent of the maximum strength capaity is only
about 405
pounds per foot.
This test was conducted using Formex B-6X, B-SXHDG (galvanized) and B-
6XHD (ungalvanized) buckles with the legs painted to the left, with the strap
tension
from the bottom, with no snub-rots at the top. The strap was also not knotted
at the
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CA 02303693 2000-03-28
bottom. A 250 pound pretension was applied by the tensile tester. The
following results
were achieved:
A standard buckle without heat treatment was tested first. Wire sizes of 0.120
inch, 0.135 inch and 0.148 inch, and wire types 1008 and 1018, were used. An
average
break point was determined from three pulls for each test. These are the
results.
Untreated
Buckle
Buckle Wire SizeWire TypeActual Break PointStandard Deviation
(in pounds per (in pounds per
foot) foot)
B-6XHDG 0.120 1008 342 22
B-6XHDG 0.135 1018 372 62
B-6XHDG 0.148 1018 465 49
The same type of buckles were then heat treated with carbon nitride and
carburizing and then tested. Wire sizes of 0.135 inch and 0.148 inch, and wire
types
1008 and 1018, were used. The carbonitriding and carburizing heat treatment
processes that were employed were designed to maximize toughness and increase
strength, wherein the metal was hardened and tempered to about Rc 40. An
average
break point was determined from three pulls for each test. These are the
results, each
indicating a joint efficiency of far greater than 55%.
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CA 02303693 2000-03-28
Heat Treated
Buckle
Buckle Wire Wire Actual Break PointPercentage of Increase
Size Type (in pounds per
foot)
B-6XHD* 0.135 1008 578 55.37***
B-6XHD* 0.148 1008 685 47.31 ****
B-6X HDG** 0.148 1018 745 60.21
* These buckles were treated by carbonitriding.
** This buckle was treated by carburizing.
*** This increase is in relation to the 372 pounds per foot achieved by the
untreated galvanized buckle having an equal wire size (0.135) but different
wire type
(1008 compared to 1018) shown in the previous chart. The increase was achieved
even though the type of steel used in the heat treated buckle was of a lower
grade.
This indicates that better performance can be achieved, while at the same
time, a cost-
savings can be obtained by using lower grade steel.
**** This increase is in relation to the 465 pounds per foot achieved by the
untreated galvanized buckle having an equal wire size (0.148) but different
wire type
(1008 compared to 1018) shown in the previous chart. Again, the increase was
achieved even though the type of steel used in the heat treated buckle was of
a lower
grade.
These tests show that the buckles that have been heat treated according to the
present invention have a substantially greater strength capacity than
untreated buckles
of comparable size and type. The treated buckles resulted in a strap break
point, i.e.,
CA 02303693 2000-03-28
the load at which the strap will fail due to stress at the joint, which was
more than about
47 percent to 60 percent over that which was obtained by standard untreated
tie
buckles. Another way to view this information is that only one of the standard
untreated
buckles (the one with the largest diameter of .148 inch) was able to satisfy
the ASTM
5. standard, whereas all of the treated buckles easily satisfied the ASTM
standard, some
by as much as 83.95%.
Test Number 3
This test was conducted using a strap designated as 66XLP.HD, which has a
strength of 1,625 pounds per foot. As discussed above, the ASTM standard for
this
strap is 55 percent of the strap's maximum strength capacity. Again, in this
case, 55
percent of the maximum strength capaity is about 894 pounds per foot.
This test was conducted using Formex B-6XHD (ungalvanized) buckles with the
legs pointed to the left, with the strap tension from the bottom, with no snub-
roll at the
top. The strap was also not knotted at the bottom. A 300 pound pretension was
applied
by the tensile tester. The following results were achieved:
A standard buckle without heat treatment was tested first. Wire sizes of 0.135
inch and 0.148 inch, and a wire type of 1038, were used. These are the
results.
Untreated
B-6XHD
Buckle
Buckle Wire Wire Average Break Standard Deviation
Size Type Point (in pounds per foot)
(in pounds per
foot)
B-6XHD 0.135 1038 602 75
B-6XHD 0.148 1038 753 21
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CA 02303693 2000-03-28
The same Formex B-6XHD type buckles were then quenched and fully hardened
and then tempered by annealing. A wire size of 0.135 inch with a wire type of
1038 was
tested with various levels of hardening. An average break point was determined
from
three pulls for each test. These are the results.
Heat
Treated
B-6XHD
Buckle
Buckle Wire Wire Actual Break Percentage of Increase
Size Type Point
(in pounds per
foot)
B-6XHD 0.135 1038 940 54.86*
B-6XHD 0.135 1038 948 56.18**
B-6XHD 0.135 1038 1067 75.78***
* This buckle was quenched to full hardening and then tempered to about Rc 35.
** This buckle was quenched to a hardness of Rc 52, and then tempered by
annealing to about Rc 38-42.
*** This buckle was quenched to full hardening and then tempered to about Rc
38-42.
Next, buckles having a wire size of 0.148 inch with a wire type of 1038 were
tested with various levels of hardening. An average break point was determined
from
three pulls for each test. These are the results.
Heat Treated
B-6XHD
Buckle
Buckle Wire Wire Actual Break Percentage of Increase
Point
Size Type (in pounds per
foot)
B-6XHD 0.148 1038 1018 35.19*
22
CA 02303693 2000-03-28
B-6XHD 0.148 1038 1103 46.48**
B-6XHD 0.148 1038 1170 55.38***
* This buckle was quenched to full hardening and then tempered to about Rc 35.
** This buckle was quenched to a hardness of Rc 52, and then tempered by
annealing to Rc 38-42.
*** This buckle was quenched to full hardening and then tempered to about Rc
38-42.
These tests show again that the buckles that have been heat treated according
to the present invention have a substantially greater strength capacity than
untreated
buckles of comparable size and type. The treated buckles resulted in a strap
break
point, i.e., the load at which the strap will fail due to stress at the joint,
which was more
than about 35 percent to 75 percent over that which was obtained by standard
untreated tie buckles. Another way to view this information is that none of
the standard
untreated buckles satisfied the ASTM standard, i.e., the load capacities were
in the
range of about 602 to 753 pounds per foot compared to the load capacity of 894
pounds
per foot. On the other hand, each of the treated buckles of the present
invention easily
satisfied the ASTM standard, i.e., the load capacities were anywhere from 940
to 1170
pounds per foot which is well above the standard of 894 pounds per foot.
Additional tests were conducted on buckles that were quenched to full
hardening
without tempering by annealing. The untempered buckles in this case were too
brittle
and shattered when loads were applied. The loads at which the buckles
shattered
varied widely, i.e., of three pulls for the same type buckle, with the ASTM
joint efficiency
23
CA 02303693 2000-03-28
standard being 405 pounds per foot for the strap, one buckle shattered at 693
pounds,
another at 460 pounds, and another at 337 pounds. In another sample, one
buckle
shattered at 536 pounds, and another at 251 pounds. This indicates that
quenching
without tempering does not provide a workable product.
The invention and its various embodiments have been discussed above in an
exemplary manner. Accordingly, it should be apparent that the present
invention is not
intended to be limited to the specific embodiments discussed above. The
present
invention is intended to cover many variations of the buckle design, as well
as various
methods of heat treatment, whether or not specifically discussed above.
a~
