Note: Descriptions are shown in the official language in which they were submitted.
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APPARATUS AND METHOD FOR ATTACHING A LOAD INDICATING
DEVICE TO A FASTENER
RELATED APPLICATIONS
This application claims the benefit of Provisional Application Serial Number
60/079,460, filed March 26, 1998.
TECHNICAL FIELD OF THE INVENTION
This invention relates generally to load indicating apparatuses, and more
particularly, to apparatus and methods of attaching load-indicating
apparatuses to a
fastener.
BACKGROUND OF THE INVENTION
For safety considerations, it is often important that bolts and studs maintain
a
precise clamping load on specific locations of an assembly. Sufficiently
attaining and
maintaining this correct clamping load are problems which industry has tried
to
overcome with limited success.
Several common methods are used to control the initial clamping load during
assembly. Of these methods, torque control is the simplest and most common.
This
method relies on the complex relationship between torque and tension. The
method is
typically not accurate because there are many variables that effect the
coefficient of
friction in the threads and under the head of the bolts. These variables
include the
type of lubrication (if any), type of thread, condition of the threads,
plating, dirt,
hardness of parts, finishes, and speed of tightening. In general, most of the
energy
input through torque is lost to friction. Since so many variables influence
the
coefficient of friction, in most cases it is very difficult to accurately know
how much
torque energy is lost to friction, and thus it is very difficult to accurately
know the
tension produced from a particular torque value.
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Another problem faced with torque and other conventional tightening methods
is that they provide little information on the loss of tension after the
assembly process.
For example, fasteners typically experience a loss of tension when put into
service for
a variety of reasons including local yielding and relaxation, vibration
loosening,
gasket creeping out of the joint, and thermal expansion. This unmonitored loss
of
tension can lead to serious problems such as joint slippage, premature wear,
and joint
failure.
Thus, several types of apparatuses have been developed in attempts to solve
both the problems of accurate assembly tension and in-service monitoring of
tension.
For example, many of these apparatuses measure the strain (elongation) of the
fastener
in order to determine the clamping load. Various methods are used to measure
the
strain including electrical, mechanical, optical-mechanical, ultrasonic and
the like.
Since the relationship between stress and strain in most bolt materials is
well known,
strain measuring methods can potentially achieve better clamping force
accuracy than
torque methods.
Some strain measuring apparatuses are substantially external to the fastener
while other apparatuses are substantially or entirely internal to the
fastener. For
example strain measuring apparatuses which are external to the fastener are
disclosed
in U.S. Pat. No. 3,954,004 issued May 4, 1976 to Orner, U.S. Pat. No.
4,676,109
issued June 30, 1987 to Wallace, U.S. Pat. No. 4,899,591 issued Feb. 13, 1990
to
Kibblewhite, U.S. Pat. No. 4,823,606 issued Apr. 25, 1989 to Malicki, U.S.
Pat. No.
4,114,428 issued Sep. 19, 1978 to Popenoe, and U.S. Pat. No. 2,600,029 issued
June
10, 1952 to Stone, all of which are hereby incorporated by reference.
Generally, these
are delicate instruments which are attached to the bolt during measurement and
removed when the bolt is put into service.
While there are various types of strain measuring apparatuses which are
substantially or entirely located internally within the fastener, an important
group
includes those strain measuring apparatuses that have the ability to
continually
respond to changes in fastener elongation and continuously display the strain
or
tension. In most cases, the apparatus fits into a bore drilled partially into
the axis of
the fastener. In addition to containing the apparatus, the hole establishes
the section
of the fastener in which the strain measurement takes place. Examples of such
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apparatus are found in U.S. Pat. No. 3,964,299 issued June 22, 1976 to
Johnson, U.S.
Pat. No. 3,987,668 issued Oct. 26,1976 to Popenoe, U.S. Pat. No. 5,668,323
issued
Sep. 16,1997 to Waxman, and UK Pat. No. GB 2-265-954-B Published May 31, 1995
to Ceney, all of which are hereby incorporated by reference.
These devices have unique advantages over many other apparatuses in that
typically no external equipment is needed, no physical interaction with the
device is
needed, and an inexperienced operator may be able to monitor the tension by
simply
making a visual observation of the apparatus. Additionally, in some cases,
remote
sensing technology can continually record the in-service fastener tension.
These devices are typically attached to the fastener at two points which are
used
as reference points in the strain measuring process. The distance between
these two
points is called the gauge length. As shown in Figures 1 a,b the reference
point A is
approximately at the end of the fastener. Typically the strain gauge is
attached to the
fastener at or near this point A. The second reference point, B, is located
near the
bottom of the hole drilled through the axis.
The process of making a strain measurement involves comparing the changing
distance between the two reference points to the original distance, i.e., the
original
gauge length. As shown in Figure 1 (the effect is exaggerated), when the bolt
is
tightened, the reaction forces from the joint and nut cause the bolt to
stretch elastically
(in most applications the body of the bolt will experience stresses in the
elastic region,
safely below the yield point). This measured elongation can be converted to
stress
levels and bolt clamping force by using the known elastic modulus of the
material and
basic engineering equations. The amount of the elongation is typically very
small, for
example, a 1" grade 5 bolt tightened to the ASTM specified maximum safe load
(proof load) will experience only a few thousandths of an inch elongation per
inch.
This elongation causes several potential problems, including yielding
(permanent deformation) of bolt heads and fastener ends, non-uniform
stretching
which occurs at the end sections of the fastener and where the nut engages a
stud, and
the changing position of the nut for applications involving studs.
Strain gauged bolts often become inaccurate after an initial assembly or while
in
service. Often, after the bolt is removed and inspected, the strain gauge will
indicate
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tension in the fastener even though no tension is present. The false
indications are
likely related to a slight yielding in the head or end of the fastener.
Yielding commonly occurs in bolt heads even when the clamping force is well
below the specified capability of the bolt. That is, the head often
experiences
permanent deformation even when the body of the bolt is at stress levels well
below
the yield point of the material.
One common condition that leads to such yielding is when the head of the
fastener sits on a surface that is not perpendicular to the axis of the bolt.
As shown in
Figures 2a-c, as the bolt is tightened, one part of the head experiences a
high reaction
force, creating a prying effect (also known as a wedge effect). These uneven
forces
can cause yielding and permanent deformation.
Another condition which can produce yielding in the head is shown in Figures
3a-c and occurs when the hole upon which the bolt head lies is too large. In
this
situation, the contact force between the head of the bolt and the mating
surface is
concentrated in a small area. Again, this produces high stress levels and
local yielding
in the head.
Additionally, because they typically contain a bore, fasteners with internal
strain
gages tend to have weakened ends. Since they are weakened, they have an
increased
susceptibility to both of these types of yielding.
Although yielding in the head is generally not a problem in non-strain-gauged-
bolts, in strain-gauged-bolts, it can cause many problems and lead to
significant
inaccuracies. These problems arise because of the nature of the strain
measuring
process. As described earlier, the strain measurement is made by comparing the
gauge length of the unstrained bolt to the gauge length of the strained bolt.
The
difference between these two lengths is the elongation. Elongation is
generally very
small, i.e., on the order of only a few thousands of an inch per inch.
As shown in Figures 2 and 3, the gauge length of the unstressed bolt can be
changed due to the yielding. Any change in the gauge length of the unstressed
bolt
must be accounted for when determining the elongation. Since one of the
reference
points is at the end of the fastener, yielding in this end can result in a
changed
unloaded distance between the two reference points that will change the gauge
length
of the unstressed bolt.
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If the change in the gauge length of the unstressed bolt is not accounted for,
errors in the displayed tension can occur. For example, the bolt could self
loosen to
zero tension, while the strain gauge still measures the residual strain
(permanent
elongation due to yielding) and falsely convert this to a stress or tension.
In general,
if the changed reference length is not accounted for, then all of the
calculated stress
levels will be inaccurate.
Many commonly used strain measuring devices offer no method to account for
a changed reference length and thus can not be relied upon after the initial
assembly.
Further, in cases where there is a method to determine or adjust for this
problem, the
procedure for doing so is generally inconvenient. In order to make this
adjustment, it
is typically necessary to remove the tension from the fastener. However,
removing
the tension is typically either not possible or prohibitively burdensome as it
usually
involves taking the machinery out of service.
Another common fastener problem arises when attaching a strain gauge to the
1 S end of a stud (a non-headed fastener). In particular, the final position
of the nut on the
stud after installation will directly affect the length of the strained
region. To
understand this, consider Figures 4a and 4b. The Figures show a stud with nuts
in two
positions. Since the region between point A and B is unstrained, the strained
region
must be located between points B and D. However, the exact region subjected to
strain is very unclear and relies upon at least two variables: ( 1 ) the
respective
distances between A and D and between B and D, and (2) the cumulative effect
of
non-uniform forces on the stud by the nut.
The distance between A, the upper surface of the stud, and B, the upper
surface of the nut, is called the nut standoff. In most bolting applications,
little
attention is paid to precisely controlling and/or maintaining this distance.
Many
variables will effect the nut standoff as shown in Figures Sa and Sb, such as,
for
example, the overall length of the fastener, the amount of thread engagement
of the
nuts, the height of the nuts, variances in the dimensions of the joint and
gasket, and
the dimensions of any washers.
A second variable effecting the length of the strained region involves the non-
uniform forces exerted on the stud by the nut. In general, the greatest
contact forces
occur in the lower threads, and the contact forces decrease to approximately
zero at
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the uppermost thread (point B). The exact distribution varies for each
individual stud
and nut combination and depends upon the exact dimensions, the exact thread
pitch,
the hardness of the parts, and the tension in the fastener.
Furthermore, for an individual nut and stud, the force distribution will
change
over time. As the first few threads experience the highest load, in service
they will
often yield. Thus, nearby threads may exert greater contact forces on the stud
over
time or upon re-assembly. Such unknown and changing force distributions makes
it .
impossible to know exactly where the yield should be located between points B'
and
C' in Figures 4 and 5. Since this end of the stretched section is not known,
the exact
length of the stretched region is not known.
The combined effect of these two variables, and particularly the variations in
the standoff can lead to significant problems in the strain monitoring
process.
Although elongation can still be measured, in order to convert this elongation
to a
stress level it is necessary to know the length of the strained region. To
address this
problem, the user can attempt to control the standoff in the field. However,
this can
be a prohibitively time consuming process that may require the removal of the
nut and
the insertion or removal of spacing washers, or the adjusting of thread
engagement of
the nuts which may involve at least a partial dismantling of the joint.
Further, the
retraining of already experienced operators to pay close attention to nut
standoff is a
problem. In most bolt assembly operations, the emphasis is on speed, and there
is a
great resistance to more complicated assembly procedures.
Sensitivity to shock, external forces and extended cyclic loading are
additional
problems with current fastener and load indicating device designs. For
example, bolts
and studs are often dropped, pounded out of fixtures, subjected to vibration
and
generally exposed to rugged conditions. When the fastener is subjected to such
shocks and external forces, inaccuracy in the calibration of the load
indicating device
can be introduced. Thus, re-zeroing or re-calibration of the load indicating
device is
necessary.
Some load indicating devices have incorporated an "on-off ' mechanism in
attempts to allow re-calibration of the load indicator. Generally, the
mechanism
consists of two dowels, the first pressed into the load indicating device, the
second
pressed into the fastener. When the fastener and load indicating device are
assembled,
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another dowel is placed in the fastener end such that when the load indicating
device
is rotated, the dowels engage one another such that the ring is prevented from
further
rotation in that direction. The dowels are generally accurately positioned in
order that
the Load indicating device can be rotated into an "on" orientation which will
engage
the load indicating device to a specific position for making the strain
measurement.
After the measurement is performed, the load indicating device may be rotated
to an
"off " position. In the "off position, the load indicating device is
disengaged from the
fastener (or gauge pin).
However, with present calibration designs, there is typically no accurate way
to adjust the dowel placement if the calibration of the load indicating
devices changes.
Generally, this is because the dowels are permanently affixed to the ring and
fastener.
Should error in the "zero" of the load indicating device occur, the dowel
placement
cannot easily be adjusted to re-zero or re-calibrate the load indicating
device. The
fastener and load indicating device must typically be returned to the
manufacturer in
order to re-adjust the dowel placement. Additionally, the machining involved
with
properly placing and inserting the towels increases the time and costs
associated with
manufacturing the load indicating and calibrations devices.
The device disclosed in U.S. Patent No. 5,668,323, attempted to solve many of
the aforementioned problems and one embodiment is shown in Figures 6-9.
However,
the technology of the '323 patent still leaves problems relating to
ruggedness,
manufacturability, performance at elevated temperatures, and performance under
cyclic loading applications. To date there has been only one available
embodiment of
the device on the market. As shown in Figure 8, the body of the device
comprises
two main sections, a head 1 and a body 2, which are spot welded together. The
head 1
is a turned and threaded piece with a rectangular slot punched through the
bottom.
The body 2 is a sheet metal bracket. The manufacturing of the bracket is
generally
very time consuming and involves many operations, including wire EDM cutting,
a
series of bending, spot welding, fine tuning and straightening, drilling,
pressing, and
hand chamfering.
Unfortunately, long assembly times are another problem of the design. There
are different methods for locking the device into position. With reference to
Figure 7,
one of these methods includes inserting a series of shims 5 or a locking nut 6
so that
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the device is properly tightened when the indicating lever points to zero
(this process
is called zeroing the device.) However, when doing this, the assembler must be
careful not to over-tighten the device as it will tend to pull the bushing 7
from the
bolt.
S Once the device is assembled into the bolt and zeroed, the bolt is usually
loaded to its proof load at least three times. This lengthy loading process is
in part to
calibrate the device and to work harden the head of the bolt to reduce the
effects of
yielding. The complexities of manufacturing the base of the device and the
lengthy
process of installation and preloading the bolts three times are merely
exemplary of
several manufacturing complications which lead to prohibitively high retail
prices for .
most applications. For example, currently, the insertion of the load indicator
will
usually add $90 to $200 or more to the price of the fastener.
Aside from the cumbersome and complicated manufacturing procedures, there
are other inherent problems with the '323 patent design. First, for most
applications,
the '323 patent device will not adequately perform at higher temperatures.
This is
because the coefficient of thermal expansion of the bracket will usually be
different
from the coefficient of expansion of the alloys used for high temperature
fasteners. In
order for the device to work properly at elevated temperatures, the device
would have
to expand similarly to the material of the fastener. That is, the device and
the bolt
would need to have the same coefficient of thermal expansion.
However, matching these coefficients is usually not possible due to other
constraints of the bracket material. One of these constraints is that the '323
patent
device is formed from a material that can withstand the strain of the bending
without
fracturing, particularly at point E in Figure 9. Another drawback is that the
material is
generally only available in sheets of thickness approximately 0.020 inch. Yet
another
aspect of this problem is that the material must be weldable to the material
of the head
2 as shown in Figure 9. Finally, there must not be a corrosion problem that
can occur
when dissimilar metals are connected.
To date, generally, all of these constraints have only been found to be
satisfied
with one type of material, 1095 sheet metal which has a coefficient of thermal
expansion different than that of most fasteners. Although other materials
having
different coefficients of thermal expansion may exist, they are typically
rare, and may
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involve raw stock that would need to be specially manufactured. Thus, in
practice, it
is only possible to match the coefficient of thermal expansion for a select
few
materials.
Another inherent problem of the current embodiment of the '323 patent device
S involves the interface between the body 2 and a pin 4 as shown in Figure l0a
and l Ob.
The pin 4 has a round hole 9 drilled in it. However, the cross section of the
body 2 is
generally rectangular. Thus, a precise fit of the two pieces is typically
impossible and
the device may slip positions, thereby leading to inaccuracies when the device
is
subjected to shock, vibration, or cyclic loading. Additionally, the
inaccuracies lead to
wearing of the corners of the body 2, further increasing accuracy problems.
A third and significant problem with the disclosed design is the inherently
weak nature of the design. Since the strain in forming the bend E in Figure 9
is
significant, a relatively soft material must be used. However, under shock
this
material readily yields, and even a few tenths of a thousandth of an inch can
significantly affect a measurement which may only be a few thousandths of an
inch.
Further, the rectangular nature of the cross-section renders the bend weaker
in some
directions than in other directions.
The final problem described here regarding the current design is related to a
general inability to properly lock the cartridge into the indicating position
without
affecting the accuracy of the device. It has been noted that when the device
is rotated
from the "on" position to the "off ' position, and then rotated and locked
back to the
"on" position, the device will often lose accuracy. This loss of accuracy is
related to
the nature of the locking methods. Since thread tolerances of the cartridge
can allow
play between the cartridge and the bolt, it is difficult to repeat an exact
distance
between the cartridge top and the gauge pin 4. Thus, shims S and locking nut 6
are
used to tighten the device.
However, for both of these methods, the actual "tightness" of the locked
position is typically dependent upon the person tightening the fastener. Since
different people will have different opinions of what is properly "tight", the
device
will not be locked into the "on" position in the same manner each time. This
inconsistency combined with the thread tolerances in the cartridge again can
cause
significant inaccuracies.
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Accordingly, methods and apparatus for attaching a load-indicating device to a
fastener which do not suffer from these problems is desirable.
SUMMARY OF THE INVENTION
An apparatus for attaching an internal strain gauge to a fastener according to
various aspects of the present invention presents a housing which secures the
strain
gauge to the fastener in such a manner that the strain gauge readings are less
affected
by strains at the end of the fastener as well as external factors on the
fastener such as
non-uniform elastic deformation, yielding, shock, cyclic loading, temperature
changes
and the like.
In accordance with the present invention, the housing is secured internally to
the fastener sufficiently far enough from a region susceptible to yielding and
non-
uniform deformation that the gauge length measured by the strain gauge is
unaffected
by such yielding and non-uniform elastic deformation of the fastener.
Additionally, the housing is provided with a calibration ring for "zeroing"
the
1 S strain gage when no load is applied to the fastener and for re-calibrating
or removing
the load on the strain gauge as desired.
BRIEF DESCRIPTION OF THE DRAWINGS
Additional aspects of the present invention will become evident upon
reviewing the non-limiting embodiments described in the specification and the
claims
taken in conjunction with the accompanying figures, wherein like numerals
designate
like elements, and:
Figure la is a side view of an untightened bolt with an internal strain gauge
bore;
Figure lb is a side view of a tightened bolt with an internal strain gauge
bore;
Figure 2a is a side view of an unyielded bolt on a surface which is not
perpendicular to the bolt's axis;
Figure 2b is a side view of a loaded bolt on a surface which is not
perpendicular to the bolt's axis;
Figure 2c is a side view of a yielded bolt after removal from a surface which
is
not perpendicular to the bolt's axis;
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Figure 3a is a side view of an unloaded bolt in a bore too large for the bolt
head;
Figure 3b is a side view of a loaded bolt in a bore too large for the bolt
head;
Figure 3c is a side view of a yielded bolt after removal from a bore too large
for the bolt head;
Figure 4a is a side view of a stud with a first distance between two nuts;
Figure 4b is a side view of a stud with a second distance between two nuts;
Figure Sa is a side view of a stud with an internal strain gauge using
washers;
Figure Sb is a side view of a stud with an internal strain gauge without
washers;
Figure 6 is a perspective view showing insertion of an internal strain gauge
into a bolt head;
Figure 7 is a cross-sectional side view of a bolt with an internal strain
gauge;
Figure 8 is a cross-sectional side view of an internal strain gauge;
Figure 9 is a front view of the body of the Waxman device;
Figure l0a is a side view of the interface between the head and the pin;
Figure 1 Ob is a top view of the interface between the head and the pin;
Figure 11 a is a cross-sectional side view of a preferred embodiment of an
internal strain gauge and housing in a bolt;
Figure 1 lb is a top view of a preferred ernbodirnent of an internal strain
gauge
and housing;
Figure 12 is an isometric drawing of a bolt head containing internal strain
gauge showing an indicator arrow pointed to a calibrated scale;
Figure 13 is a cross-sectional side view of an internal strain gauge and
housing
retrofitted into a stud;
Figure 14 is a cross-sectional side view of a housing used with the strain
gauge
of U.S. Patent No. 4,571,133;
Figure 15 is a cross-sectional view of one embodiment of housing used with
the strain gauge of U.K. Patent No. GB 2-265-954-B;
Figure 16 is a cross-sectional view of an alternative embodiment of housing
used with the strain gauge of U.K. Patent No. GB 2-265-954-B;
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Figure 17 is a cross-sectional view of yet another embodiment of housing used
with the strain gauge of U.K. Patent No. GB 2-265-954-B;
Figure 18 is a cross-sectional side view of a simplified embodiment of the
housing used with the strain gauge of U.K. Patent No. GB 2-265-954-B;
Figure 19 is a cross-sectional side view of an alternative embodiment of a
housing according to the present invention.
Figure 20a is a cross-sectional side view of the calibration device attached
to
the load indicator;
Figure 20b is a top view of the calibration device attached to the load
indicator;
Figure 21 is a top view of an alternative embodiment of the calibration device
attached to the load indicator; and
Figure 22 is a top view of yet another alternative embodiment of the
calibration device attached to the load indicator
1 S DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The following descriptions are of preferred exemplary embodiments only, and
are not intended to limit the scope, applicability, or configuration of the
invention in
any way. Rather, the following description provides a convenient illustration
for
implementing a preferred embodiment of the invention. Various changes may be
made in the function and arrangement of elements described in the preferred
embodiments without departing from the spirit and scope of the invention as
set forth
in the appended claims.
In general, according to various aspects of the present invention provides a
method and apparatus for attaching a load-indicating device to a fastener such
that the
load-indicating device is capable of continually monitoring the load on the
fastener,
and such that the load indicating device is not subject to external factors
such as non-
uniform elastic deformation, yielding, shock, cyclic loading, temperature and
the like.
Further, though the various embodiments described below will be directed to a
bolt or
stud fastener, various other fasteners subject to external loads including
pins; dowels,
screws and the like may similarly incorporate the present invention.
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Thus, with reference to Figures 11-14, in accordance with the present
invention, a bolt 10 suitably includes a housing 20, a load indicator 30 for
displaying
the load status of bolt 10 and a calibration ring 50.
In accordance with the present exemplary embodiment of the present
invention, bolt 10 is suitably configured with an internal bore 12 for
accommodating
housing 20. Internal bore 12 is suitably configured with a set of internal
threads 14.
As described in more detail herein, when housing 20 is inserted into internal
bore 12
and securely fastened to bolt 10, a gauge length 40 is measured in a section
of bolt 10
which does not yield nor elastically deform in a non-uniform manner.
In accordance with various embodiments of the present invention, load
indicator 30 is suitably comprised of any internal device for indicating a
load, such as,
for example, devices disclosed in U.S. Patent No. 4,571,133, U.K. Patent No.
GB 2-
265-954-B, U.S. Patent No. 4,525,114, and the like. In a preferred embodiment
of the
present invention, load indicator 30 is of a type disclosed in the U.S. Patent
No.
5,668,323. Thus, in general, load indicator 25 is suitably comprised of a
slotted plug
31, a return spring 32, a conical washer 33, a gauge pin 34, O-rings 35, a
transparent
disk 36, snap rings 29 and lever 38.
In accordance with a preferred embodiment of the present invention, gauge pin
34 suitably establishes a reference point B. Slotted plug 31 is suitably
pressed into a
lower end 21 of housing 20. Plug 31 suitably incorporates pivot pin 39 about
which
lever 38 rotates. As bolt 10 is loaded, the strain in bolt 10 causes gauge pin
34 to
move away from housing 20. As gauge pin 34 moves away from housing 20, slotted
plug 31 suitably moves a corresponding amount, thus changing gauge length 40.
As
gauge length 40 changes, lever 39 rotates about pivot pin 34. As shown in
Figure 12,
indicator end 39 opposite pivot pin 39 moves across a display 41, thereby
displaying
the load on bolt 10.
In accordance with one aspect of the present exemplary embodiment, conical
washer 33 sits between housing 20 and fastener 10 and applies continual
pressure at
both an "off ' and an "on" position.
In accordance with another aspect of the present exemplary embodiment, O-
rings 35, along with transparent disk 10 and snap ring 11 suitably secure a
head 45 of
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load indicator 30 and prevent moisture and dirt penetration into load
indicator 30 and
bolt 10.
In accordance with still another aspect of the present exemplary embodiment,
load indicator 30 can be set in an "on" and an "off" position. With reference
to
S Figures 11 a,b, housing 20 is suitably rotated so that a dowel 42 is aligned
with either
the "on" or "off' markings. The maneuver of rotating housing 20 to the off
position
through a counter clockwise revolution suitably lifts lever 38 off pin 34,
thereby
taking the load off indicator 30, placing housing 20 and load indicator 30 in
the "off'
position.
Now, with reference to Figure I I, in accordance with a preferred embodiment
of the present invention, housing 20 is suitably comprised of a hollow
cylinder of the
same material as bolt 10, though housing 20 may suitably be comprised of
varying
other shapes and sizes and materials. Preferably housing 20 is configured of a
material with the same coefficient of thermal expansion as bolt I 0 such as
low carbon
4140 steel, 4340 steel, B-7 steel and the like. Housing 20 is suitably
configured to
contain load indicator 30 with the hollow of housing 20.
As mentioned briefly above, in the case where fastener I O is a bolt, the
influence of the end yielding and non-uniform elastic deformation decreases as
the
upper attachment point of housing 20 is moved away from an upper surface I S
of a
head I I of bolt 10. In accordance with the present invention, if housing 20
is suitably
secured beneath head 11 or, alternatively, closer to a bottom surface 17 of
head 11
(i.e., where head 11 meets body 1 b) the adverse effect of such yielding and
elastic
deformation on the load monitoring process can be entirely eliminated.
Thus, housing 20 is suitably configured to be rigidly secured to bolt 10 in an
area on bolt 10 outside the region susceptible to yielding and non-uniform
elastic
deformation. In the present exemplary embodiment, housing 20 is configured
with a
set of external threads 31 designed to engage a set of internal threads 22 in
bore 12.
However, in accordance with various alternative embodiments of the present
invention, housing 20 may be secured in bore 12 by press fit (see Figure 17),
set
screws, shear pins and the like. Additionally, in accordance with an
alternative
embodiment of the present invention, housing 20 and load indicator 30 may
suitably
be integrated as one unit. That is, rather than assembling housing 20 and load
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indicator 30 in separate steps, housing 20 and load indicator 30 are
manufactured as a
self contained assembly.
Thus, in accordance with a preferred embodiment of the present invention, and
with reference to Figures 11 and 13, securing housing 20 to bolt 10
establishes a
reference point A. For bolt 10, the proper positioning is suitably somewhere
below a
head 14 of bolt 10 or where head 14 meets a body 16 of bolt 10.
In accordance with one aspect of the preferred embodiment of the present
invention, the preferable position of reference point A is found by first
determining
the region of bolt 10 most susceptible to yielding. For example, determination
of the
region is suitably done through finite element analysis, physical experiment,
mathematical calculation and the like.
Thus, reference points A and B, and corresponding gauge length 40, are
suitably outside the region subject to yielding or non-uniform elastic
deformation. In
this manner, though yielding and non-uniform elastic deformation at one end of
stud
10 or bolt 10 can still occur, such yielding and deformation will not
influence gauge
length 40.
Now, with reference to Figure 13, in accordance with another embodiment of
the present invention, housing 20 can be retrofitted into stud 10. However,
the proper
position of reference point A may vary depending upon the particular stud 10
and the
particular application of stud 10. For example, in accordance with the present
exemplary embodiment, in the case where fastener I O is a stud, it should
first be
determined what the maximum nut standoff will be after assembly when stud 10
is in
service. Once the standoff is determined, the proper position for reference
point A
will suitably be either below a nut 50 or toward the bottom of nut 50 when at
this
maximum standoff position.
In accordance with a preferred embodiment of the present invention, housing
20 may be suitably provided with calibration device 60. With reference to
Figures
20a,b, calibration device 60 is suitably adjustable. Calibration device 60 is
suitably
configured with a clip 61 and a set screw bore 62 suitably configured to
accept a set
screw 63.
In the present exemplary embodiment, clip 61 is suitably configured as a ring
which surrounds load indicating device head 45. However, with momentary
reference
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to Figure 21, according to various alternative aspects of the present
exemplary
embodiment, clip 61 may suitably configured as a arcuate segment which does
not
entirely surround load indicating device head 45.
In accordance with the present exemplary embodiment and with reference to
Figures 20 and 21, set screw bore 62 and set screw 63 are suitably oriented
perpendicular to a tangent of ring 61. However, in accordance with various
alternative aspects of the present embodiment and with reference to Figure 22,
set
screw bore 62 and set screw 63 may be suitably oriented tangentially to ring
61,
through a set of brackets 64 surrounding an opening 65, such that set screws
closes
opening 65, thereby tightening ring 61. Additionally, in accordance with
various other
alternative aspects of the present exemplary embodiment, set screw 63 may be
replaced with other suitable locking means, such as, for example, shear pins
and the
Like.
During assembly of load indicating device 30 and fastener 10, load indicating
device 30 is suitably secured to fastener 10 such that load indicating device
30 is at a
"zero" load position. Ring 61 is rotated about load indicating head 45 until
set screw
63 suitably engages dowel 42, thus setting the zero load point of load
indicating
device 30: Set screw 63 is then suitably tightened to rigidly secure ring 61
to load
indicating device head 45.
Thus, in accordance with the present exemplary embodiment, when ring 61 is
rotated, load indicating device 30 is similarly rotated, thus turning load
indicating
device 30 "on" and "off '. Should the calibration of load indicating device 30
become
inaccurate, set screw 63 is simply loosened, unloaded load indicator 30 is
rotated until
display 41 indicates "zero" load, ring 61 is rotated until set screw 63
engages dowel
42 and set screw 63 is re-tightened, thus re-calibrating load indicating
device 30.
The various embodiments described above therefore have several distinct
advantages. First of all, the embodiments suitably allow reference points A
and B,
and thus gauge length 40, to be established outside the region where problems
can
arise from end yielding, non-uniform elastic deformation and nut locations.
Housing
20 can easily be lengthened or shortened depending upon the design of fastener
10 and
the application of fastener 10. In terms of manufacturability, housing 20 and
pivot pin
39 can be made on conventional lathes or screw machines, and no welding or
complicated forming processes are necessary. However, alternatively, housing
20
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may also be manufactured from sheet metal forming processes or other suitable
manufacturing or machining processes. Additionally, the machining of bolt 10
is
simplified, and internal threads 14 can be easily tapped rather than using an
internal
single point threading operation or bushing 7 (as shown in Figure 7).
Further, assembling bolt 10 with housing 20 as shown in Figure 14 is
significantly quicker than that of the current designs. Housing 20 does not
necessarily
need to be locked into position with shims S or locking nut 6. Rather, housing
20 is
merely suitably rotated into position. Thus, checking the zero or changing
load
indicator 30 from off to on or vice versa can be much quicker and can be
performed
by an operator in the field. Finally, bolt 10 does not necessarily need to be
during
assembly in order to work-harden or "set" head 11 as end yielding will not
adversely
effect the performance of load indicator 30.
Another improvement is that the housing 20 is symmetrical and can be
comprised of hardened materials such as low carbon 4140 steel, 4340 steel, B-7
steel
and the like. Such materials result in improved shock resistance superior to
that of
current designs.
For elevated temperature applications, it is a simple matter to find stock
materials to which the coefficient of thermal expansion is suitably similar to
the
material of bolt 10. In many cases, it is possible to simply use the same
material that
is used for bolt 10 to make housing 20.
Conical washer 33 may suitably establish a consistent locking pressure in the
"on" position and remove the variables and inaccuracies associated with the
operator
when determining the appropriate locking tightness. Conical washer 33, as
shown in
Figures 11, 13, and 14, additionally suitably supplies a constant force to
take out
"back lash" caused by internal and external threads 14, 22. Washer 33 suitably
pulls
housing 20 snugly against the upper thread faces of thread 14, 22, but does
not lock
them into position as retaining shims 5 or lock nut 6 do. Conical washer 33
also
suitably acts as a shock absorber between the housing 20 and bolt 10.
Additionally,
the precision of the fit between plug 2 and gauge pin 34 can be increased,
thus
reducing inaccuracies that arise due to the "looseness" of current designs.
While the principles of the invention have been described in illustrative
embodiments, many modifications of structure, arrangement, proportions, the
elements, materials and components, used in the practice of the invention and
not
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specifically described may be varied and particularly adapted for a specific
environment and operating requirement without departing from those principles.
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