U.S. patent number 3,696,412 [Application Number 05/081,819] was granted by the patent office on 1972-10-03 for method and means for indicating tension.
This patent grant is currently assigned to Dake Corporation. Invention is credited to Roger I. Swanson.
United States Patent |
3,696,412 |
Swanson |
October 3, 1972 |
METHOD AND MEANS FOR INDICATING TENSION
Abstract
In a method and means for indicating tension on bolts, screws
and the like, a plastic member of flowable material having a
plurality of relatively thin copper wires embedded along each edge
is utilized. The member has an annular cutout portion permitting it
to be inserted between any two bearing surfaces of a threaded
fastening joint with the bolt passing through the cutout portion.
When the point is tightened to pre-load below yield, the plastic
material flows or is extruded until the joint contacts the wire.
Subsequent tightening to the desired minimum proof load will shear
the wires and material to indicate that minimum proof load has been
reached. The plastic strip material also serves as a lubricant to
reduce the amount of torque required to reach minimum proof load
and a portion is extruded into the core of the joint to act as a
lock and seal. In one embodiment, the wires form a pair of leads
which may be connected in an electric circuit so that when the
joint is tightened to pre-load below the plastic range, there is
sufficient extrusion of the plastic material to establish
electrical contact between the joint and edge wires to close the
electrical circuit, activating an electrical signal to indicate
that pre-load has been reached. BACKGROUND OF THE INVENTION This
invention relates to threaded fasteners, and, more particularly, to
a fastener tension indicating system that will provide a signal at
various levels of load tensioning in a threaded joint. It is well
recognized that a key to a successful product is many times
directly related to the success in properly tightening a threaded
joint to maximum tension load. Too little or too much will in
either event result in failure and the objective is lost. In
addition, in mass production, material costs in the way of quantity
and size of bolts, nuts, tools, etc. can often represent the
difference between commercial success or failure. With the advent
of power tools, many prior and existing proposals have used torque
as the standard for measuring "bolt tension". Although it is
theoretically feasible under ideal conditions to measure the torque
applied to a fastener and thereby accurately arrive at the
desirable tension, there are so many variables in actual practice
such as the quality of threads, galling, foreign matter, etc. that
it has proven to be unreliable as a standard for bolt tension.
Where it is still used, it is common to specify bolts larger than
required for the assembly and utilize less than their maximum
tension. While this reduces the chances of over stressing the bolt,
it also results in higher costs. With the advent of high strength
bolts, and in view of the unreliability of torque as a measure of
tension, the trend has been to obtain higher tensile loading in the
fastener, and draw the bolt up to minimum proof load and beyond.
Thus, other methods have been devised to indicate bolt tension such
as strain washers, strain gauges, bolt stretch and others. However,
all have proven to be too costly or impractical. SUMMARY OF THE
INVENTION To achieve a successful assembled product, there are
several areas of major concern. To begin with, the product must be
designed to withstand the stresses and strains of the work load.
Thus, an economical fastener system must be designed that will
pre-stress the assembly above the work load without loosening or
failing. From a commercial standpoint, an economical method and
tools for assembly and inspection must be established to insure the
proper pre-stressing of the assembly. Therefore, it is a principle
object of this invention to provide a method and means for
indicating when minimum proof load or desired bolt tension is
reached in a threaded joint at a cost well below today's unreliable
methods. It is also an object of this invention to provide a method
and means for indicating when the joint has been tightened to
pre-load before minimum proof load is reached. It is a further
object of this invention to additionally provide such benefits as,
the reduction of required torque to achieve proper joint tension; a
seal; a bonding to resist rotation if the joint relaxes; and a
method and means which is not only convenient and easy to handle by
an unskilled operator, but, in fact, will reduce the amount of
handling required thereby increasing overall production. For
achievement of these and other objects, a tension indicating tab of
flowable material is provided having a center cutout portion and
means of substantially greater hardness than said material embedded
along each edge. With the tab positioned between two bearing
surfaces in the joint, tensile loading is provided. The tab is
deformable and extrudable so that when minimum proof load is
achieved, the tab is completely sheared off between the bearing
surfaces. Preferably, the means embedded along each edge is
comprised of a wire to provide a pair of leads connected to an
electrical circuit which when closed operates an electric signal.
The tab is positioned so that the wire means is partially
interposed between the two bearing surfaces and when the joint is
tightened to a pre-selected pre-load, the tab is partially extruded
in, around and out of the joint area until electrical continuity is
established between the wire means and joint thereby closing the
electrical circuit to actuate the signal. Further tightening of the
joint causes complete shear of the tab and wire means between the
bearing surfaces thereby opening the electrical circuit to indicate
minimum proof loading. The flowable characteristics of the tab acts
as a lubricant thereby reducing the amount of torque required to
establish a positive joint and in addition acts as a sealant and
bond to resist rotation should the joint relax. To increase
production and reduce handling, a plurality of tabs may be collated
into a continuous strip.
Inventors: |
Swanson; Roger I. (Butler,
PA) |
Assignee: |
Dake Corporation (Grand Haven,
MI)
|
Family
ID: |
22166601 |
Appl.
No.: |
05/081,819 |
Filed: |
October 19, 1970 |
Current U.S.
Class: |
340/668; 73/761;
200/61.08; 174/117F; 411/548 |
Current CPC
Class: |
G01L
5/243 (20130101); F16B 31/028 (20130101) |
Current International
Class: |
G01L
5/24 (20060101); F16B 31/00 (20060101); F16B
31/02 (20060101); G08b 021/00 () |
Field of
Search: |
;340/421,213R
;174/117R,117F,117AS ;200/61.08,61.19 ;73/88E,88F,94 ;85/62 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Trafton; David L.
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows.
1. A method for indicating load tension in a threaded fastener
joint comprising the steps of:
providing a tab of flowable material having means of substantially
greater hardness than said material embedded along each edge and
means defining a cutout to permit passage of a threaded member
therethrough;
positioning said tab between two bearing surfaces in said joint so
that a portion of said embedded means is between said bearing
surfaces; and
tightening the threaded fastener to apply a tensile load to the
joint of sufficient magnitude to deform and extrude the flowable
material and embedded means to completely shear the tab when the
predetermined load has been applied.
2. The method according to claim 1 wherein said means embedded
along each edge is wire-like.
3. The method according to claim 1 wherein said means embedded
along each edge is metallic.
4. The method according to claim 2 wherein said wire-like means is
metallic.
5. The method according to claim 4 wherein said metallic wire-like
means is connected to an electric circuit having an electric source
and signal so that when said joint is tightened to a predetermined
tensile load of less magnitude than that required to shear said
tab, said flowable material is partially deformed and extruded to
establish electrical continuity between said metallic means and
joint closing said electric circuit to actuate said signal.
6. The method according to claim 1 wherein said means embedded
along each edge is of sufficient hardness in relation to the
strength of said joint to permit said joint to be tightened to
minimum proof load prior to shear of the tab and sufficiently soft
with relation to the strength of said joint to permit complete
shearing of the tab prior to the joint being tightened to its
ultimate breading point.
7. The method according to claim 6 wherein a plurality of said tabs
are collated into a continuous strip to facilitate mass production
and reduce installation time and handling.
8. The method according to claim 6 wherein one of said bearing
surfaces is characterized as having an internal and external
chamfer to permit deformation and extrusion of said material and
metallic means inwardly around the core of said joint and outwardly
away from the fastener, said inwardly extruding portion forming a
bond to restrain said joint from rotation should said joint loosen,
and further forming a seal to weatherproof said joint after
tightening.
9. The method according to claim 8 wherein said one bearing surface
is formed on a nut.
10. The method according to claim 8 wherein said one bearing
surface is formed on a bolt or screw.
11. The method according to claim 8 wherein said one bearing
surface is formed on a washer.
12. The method according to claim 8 wherein said internal chamfer
is of less angular magnitude than said external chamfer, and said
flowable material provides a lubricant in said joint reducing the
amount of torque required to tighten said joint to shear at which
point the torque increases due to the increased friction in the
joint.
13. The method according to claim 8 wherein a fastening element
means is collated onto each tab in said strip to further reduce
handling and installation time.
14. The method according to claim 13 wherein said element means is
comprised of a plurality of screws or bolts.
15. The method according to claim 13 wherein said element means is
comprised of a plurality of nuts.
16. The method according to claim 13 wherein said element means is
comprised of a plurality of washers.
17. A fastener construction comprising, in combination: at least
one member with an opening and a fastener element including a shank
extending through said opening; a shoulder-like means associated
with said shank, said member and shoulder-like means providing two
opposed bearing surfaces; a tab of flowable material having means
of substantially greater hardness than said material embedded along
each edge, and means defining a cutout portion through which said
shank is extended so that when said tab is positioned between two
bearing surfaces in said fastener construction with a portion of
said embedded means aligned between said bearing surfaces, and said
fastener construction is tightened to a predetermined load, the tab
will become deformed and extruded at said predetermined load, to
completely shear the material and embedded means when said
predetermined load has been reached.
18. The combination according to claim 17 wherein said means
embedded along each edge is wire-like.
19. The combination according to claim 17 wherein said means
embedded along each edge is metallic.
20. The combination according to claim 28 wherein said wire-like
means is metallic.
21. The combination according to claim 17 wherein said means
embedded along each edge is of sufficient hardness in relation to
the strength of said fastener construction to permit said fastener
construction to be tightened to minimum proof load prior to shear
and sufficiently soft with relation to the strength of said
construction to permit complete shearing of said tab prior to said
construction being tightened to its breaking point.
22. The combination according to claim 20 wherein said metallic
wire-like means is comprised of a malleable electric conducting
wire connected to an electric circuit having an electric source and
signal so that when said fastener construction is tightened to a
predetermined tensile load less than minimum proof load, said
flowable material will partially deform and extrude to establish
electrical continuity between one of said bearing surfaces and said
wire to close said circuit and actuate said electric signal.
23. The combination according to claim 21 wherein one of said
bearing surfaces in abutment with said tab is characterized as
having an internal and external chamfer to permit extrusion of said
material and metallic means inwardly around the core of said
fastener construction and outwardly away from the fastener
construction, said inwardly extruded portion forming a bond to
restrain said fastener construction from rotation should said
construction loosen, and further forming a seal to weatherproof
said construction.
24. The combination according to claim 23 wherein said internal
chamfer is of less angular magnitude than said external chamfer and
said flowable material provides a lubricant in said fastener
construction to reduce the amount of torque required to tighten
said fastener construction.
25. The combination according to claim 17 wherein said shank
includes a head, and one of said bearing surfaces between which
said tab is positioned is formed by the head portion of said
shank.
26. The combination according to claim 17 wherein one of said
bearing surfaces between which said tab is positioned is formed by
a nut threadable on said shank.
27. The combination according to claim 17 wherein one of said
bearing surfaces between which said tab is positioned is formed by
a washer.
28. The combination according to claim 17 wherein one of said
bearing surfaces between which said tab is positioned is formed by
said one member.
29. The combination according to claim 17 wherein said fastener
element is adapted to tighten said fastener construction, and the
bearing area of said opposed bearing surfaces is substantially
equal to the cross-sectional bearing area of said fastener
element.
30. The combination according to claim 29 wherein one of said
bearing surfaces in abutment with said tab is characterized as
having an internal and external chamfer to permit extrusion of said
material and metallic means inwardly around the core of said
fastener construction and outwardly away from the fastener
construction, said inwardly extruded portion forming a bond to
restrain said fastener construction from rotation should said
construction loosen, and further forming a seal to weatherproof
said construction.
31. The combination according to claim 30 wherein said internal
chamfer is of less angular magnitude than said external chamfer and
said flowable material provides a lubricant in said fastener
construction to reduce the amount of torque required to tighten
said fastener construction.
32. A fastener construction comprising, in combination: at least
one member with an opening and a fastener element including a shank
extending through said opening; a shoulder-like means associated
with said shank, said member and shoulder-like means providing two
opposed bearing surfaces: a continuous strip of flowable material
having means of substantially greater hardness than said material
embedded along each edge, and means defining a plurality of
openings spaced along said strip, said openings permitting passage
of a threaded member therethrough so that when one of said openings
is positioned between two bearing surfaces in a fastener
construction with a portion of said embedded means aligned between
said bearing surfaces, and said fastener construction is tightened
to a predetermined tensile load, the strip will become deformed and
extruded at a predetermined load to completely shear the material
and embedded means when said predetermined tensile load has been
reached.
33. The combination according to claim 32 wherein said embedded
means is of sufficient hardness in relation to the strength of said
fastener construction to permit said fastener construction to be
tightened to minimum proof load prior to shear of the strip and
sufficiently soft with relation to the strength of said fastener
construction to permit complete shearing of the strip prior to the
fastener construction being tightened to its ultimate breaking
point.
34. The combination according to claim 33 wherein said embedded
means along each edge is comprised of a malleable metallic
wire-like means connected to an electric circuit having an electric
source and signal so that when said fastener construction is
tightened to a predetermined tensile load of less magnitude than
that required to shear said tab, said flowable material is
partially deformed and extruded to establish electrical continuity
between said metallic means and fastener construction closing said
electric circuit to actuate said signal.
35. The combination according to claim 32 wherein a plurality of
fastening element means are collated onto said strip to further
reduce handling and installation time, each of said fastening
element means being aligned with one of said openings.
36. The combination according to claim 35 wherein said element
means is comprised of a plurality of screws or bolts.
37. The combination according to claim 35 wherein said element
means is comprised of a plurality of nuts.
38. The combination according to claim 35 wherein said element
means is comprised of a plurality of washers.
39. A fastener construction having at least two bearing surfaces
and a tension indicating device comprising at least one element
including metallic means embedded in a flowable solid material,
said element being positioned between said two bearing surfaces in
said fastener construction so that when said fastener construction
is tightened to a predetermined tensile load said element including
said metallic means will become sufficiently deformed and extruded
to completely shear said element including said metallic means,
thereby indicating that said predetermined load has been
reached.
40. The construction according to claim 39 wherein said metallic
means is wire-like.
41. The construction according to claim 39 wherein said flowable
solid material is deformed away from said metallic means embedded
therein at a second predetermined tensile load less than said first
predetermined tensile load whereby at least one of said two bearing
surfaces are in physical contact with said metallic means when said
second tensile load is reached.
42. The construction according to claim 41 wherein said metallic
means is comprised of an electrical conducting material.
43. A method for indicating load tension in a threaded fastener
joint comprising the steps of:
providing a tab of flowable material having metallic means of
substantially greater hardness than said material embedded along
each edge and means defining a cutout to permit passage of a
threaded member therethrough;
positioning said tab between two bearing surfaces in said joint so
that a portion of said embedded means is between said bearing
surfaces; and
tightening the threaded fastener to apply a tensile load to the
joint of sufficient magnitude to partially deform and extrude said
flowable material to establish electrical continuity between said
metallic means and joint.
44. A method according to claim 43 wherein said metallic means is
wire-like and connected to an electric circuit having an electric
source and signal so that when said joint is tightened to a
predetermined tensile load to establish electrical continuity
between said wire-like means and joint, said electric circuit
closes to actuate said signal.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view of the tension indicating tab proposed by
this invention;
FIG. 2 is an end view of the embodiment shown in FIG. 1 taken in
cross section along line II--II in FIG. 1;
FIG. 3 is a schematic view of the tab as shown in FIG. 1 connected
to an electric circuit;
FIG. 4 is an enlarged view of a portion of the nut shown in FIG.
3;
FIG. 5 is a cross-sectional view of a fastening joint embodying the
present invention prior to stressing the joint;
FIG. 6 is a cross-sectional view similar to FIG. 5 with the joint
tightened to a predetermined pre-load;
FIG. 7 is a cross-sectional view of the joint shown in FIGS. 5 and
6 with the joint tightened to the desired tensile load;
FIG. 8 is a cross-sectional view of an alternative embodiment of
the present invention showing a washer collated to a tab similar to
the one shown in FIG. 1;
FIG. 9 is a cross-sectional view of a washer for use as an
alternative embodiment of the present invention;
FIG. 10 is a plan view of the washer and tab shown in FIG. 8
collated into a continuous strip providing a plurality of washers
and tabs;
FIG. 11 is a cross-sectional view of an alternative embodiment of
this invention showing the collation of a plurality of threaded
bolts mounted in tabs forming a continuous strip; and
FIG. 12 is a schematic current diagram of a power fastener tool
connected to an electrical circuit in communication with the
tension indicating tab.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In order to design a product which will withstand the stresses and
strains of the work load, the designer must analyze the structure,
determine static and dynamic stresses, and design with an adequate
factor of safety. The designer knows that if the joint is fastened
and held at a load greater than the fluctuating load on the bolt,
the fastener will not fail or loosen in operation. Not
infrequently, the average designer is concerned with the modulus of
elasticity (30 .times. 10.sup.6 ) for steel. Therefore, for each
thirty thousand PSI stress of a fastener, there will be 0.001 inch
elongation of the fastener per inch of grip or portion elongated.
At 90,000 PSI, elongation will be 0.003 inch/inch regardless of the
size of fastener.
To illustrate the importance of elongation or bolt stretch, if the
grip is only one-half inch, at 90,000 PSI, the stretch is only
0.0015 inch; and at one-quarter inch grip, 0.0075 inch. Thus, even
though the fastener is tightened to pre-load, the one-quarter inch
grip joint will lose all of its load if the joint relaxes 0.00075
inch. Once the joint loses its clamp load, dropping below the
fluctuating load of the assembly, the fastener will loosen further
or be stressed and eventually fail from fatigue.
The problem thus is to obtain a joint that will not relax and a
fastener that will be stretched to the maximum amount. As indicated
in the Background of the Invention, torque has been used as a
measure of "bolt tension." However, it is well-known that if torque
is used, it is most difficult to obtain 75percent of the bolt
minimum proof load. Tests have been made on a 3/8-24 SAE 8 bolt
with various lubricants that indicated variations of 20 to 74 lb.
ft. of torque to obtain 7,100 pounds tension. Other tests show the
nut to gall and weld on a 3/4-10 bolt having 1,000 lb. ft. torque
and zero tension. Needless to say, the ideal fastener joint with
maximum stretch cannot be obtained with torque.
As an alternative to torque, the Research Counsel on Riveted and
Bolted Structural Joints permitted
one-turn-of-the-nut-from-hand-tight method of pretensioning
structural bolts. This made sense as a 1 inch - 8 bolt will stretch
0.125 in one-turn-of-the-nut on a rigid joint. However, the problem
was, and still is; when is the joint snug, and how can one indicate
the start of tensioning. With today's higher tensile bolts (120,000
PSI) even one-half turn may cause the bolt to fail in tightening.
In response to the many problems encountered in the art today,
which remain unsolved, the present invention has been developed,
and will indicate and prove maximum tension at a cost below today's
unreliable methods.
Referring now to the drawings in detail, FIGS. 1 and 2 show a
tension indicating tab 10 preferably comprised of a flowable
plastic such as polyethylene and having enlarged edge portions 12
and 14. The tab includes an arcuate cutout portion 16 which permits
clearance of a threaded bolt when the tab is inserted in the joint
between two bearing surfaces. While cutout 16 is shown having a
circular shape, it will be appreciated that various other shapes
could be used, and indeed preferred in certain applications, the
essential requirement being that the minimum diameter of cutout 16
be large enough to permit insertion of the threaded bolt. Embedded
in each enlarged edge portion of the tab is a wire 18 and 20,
preferably multi-strand copper although other materials such as
aluminum, soft steel, etc. may also be used so long as the material
is substantially harder than the flowable material of the tab and
it yields under tension prior to the joint, usually comprised of
steel.
One reason that a wire such as copper is preferred is because of
its good electrical characteristics since in many instances, wires
18 and 20 are connected to an electrical circuit 80 as shown in
FIG. 3. When a predetermined load is applied to the joint, the
flowable material partially deforms and extrudes to establish
electrical continuity between wires 18, 20 and the joint. This
closes circuit 80 and the electrical source 82 activates signal 84
to indicate the predetermined load level.
FIG. 5-7 show a particular fastening joint comprised of two members
22 and 24 joined together in positive fashion by a threaded bolt 26
having a standard hex head 28, bolt 26 being threaded into a hex
nut 30 with a washer 32 interposed between the nut 30 and member
24. In this embodiment, the tension indicating tab is shown
interposed between nut 30 and washer 32. It will be appreciated,
that the indicating tab is operable between any two bearing
surfaces such as washer 32 and member 24; bolt head 28 and member
22; or in fact the joint itself. Subsequent embodiments discuss and
show some of the alternative uses of tab 10, however for purposes
of this embodiment, the tab is interposed between nut 30 and washer
32.
Turning to FIG. 4, hex nut 30 is shown having a preferred
configuration wherein an external tapered surface 34 and internal
tapered surface 36 is provided with the bearing or seating surface
38 lying therebetween. The tapered surfaces 34 and 36 are described
in terms of chamfer, there being an external chamfer angle a and an
internal chamfer angle b. The chamfer of nut 30 is provided to
facilitate the extrusion of the tab and wires as the nut is
tightened. The chamfers provide easy flow of the plastic tab as the
nut is tightened to permit flow of the plastic internally toward
the core 37 of the nut and external thereof. In part, the plastic
acts as a lubricant reducing the amount of torque required to
tighten the nut to pre-load and the desired load; and in addition,
the flow internally towards the core 37 creates a bond in the core
between the nut 30 and bolt 26 as shown in FIG. 7 so that should
the joint relax, the bonding resits rotation of the bolt.
Furthermore, the plastic at the core acts as a sealant to
weatherproof the joint.
Preferably, the internal chamfer angle b is less than the external
angle a and it has been found that a 10.degree. and 45.degree.
relationship provides good lubricating, bonding and sealing as well
as reliable tension indicating characteristics. The ideal bearing
surface 38 of nut 30 is to maintain a stress area at least equal to
the stress area of the bolt. The outside bearing diameter shown at
39 (at the start of the outer chamfer) in FIG. 4 is preferably less
than the outer diameter of the nut and the inside diameter shown at
41 (at the start of the inner chamfer) should be large enough to
gather the wires inward as the joint is tightened. The smaller
outside bearing diameter permits use of a smaller fastener and also
will reduce the torque radius and therefore reduce the torque
required to tighten the joint.
OPERATION
The operation of tab 10 will now be described in detail with
reference to the embodiment shown in FIGS. 5-7, it being understood
that the tab can be used in conjunction with an electric responsive
device or not. In either use, tab 10 is inserted between the
bearing surface 38 of nut 30 and the external bearing surface of
washer 32. The outer periphery of cutout 16 should be relatively
close to edge portions 12 and 14 so that at least a portion of
wires 18 and 20 are positioned directly between bearing surface 38
of nut 30 and washer 32. The nut may be finger tightened so that
there is physical contact between the tab, nut and washer (as shown
in FIG. 5) prior to mechanical tightening of the joint.
Preferably, wires 18 and 20 are connected in an electrical circuit
with an electrical source and signal connected in series. The
circuit will be open at the onset since there is no electrical
continuity between wires 18 and 20. When used in conjunction with
an electrical responsive device as diagramatically disclosed in
FIGS. 3 and 12, the nut or bolt is mechanically tightened to
achieve a desired pre-load, generally in the vicinity of 10 to 50
percent minimum proof load. At pre-load, there is partial
deformation or extrusion of the enlarged edge portions 12 and 14 so
that electrical continuity is established between wires 18 and 20
and the joint as shown in FIG. 6. Once electrical continuity is
established between wires 18 and 20 an electric circuit 80 or 90
such as disclosed in FIGS. 3 and 12 is closed. In the example of
FIG. 3, the electric signal 84 indicates to the operator or tool
that pre-load has been reached. In the example of FIG. 12, the
electric circuit also controls the operation of the tool.
At pre-load, the bolt tension is in general below its inelastic or
yield limit, although very close to it. Further tightening of the
bolt will bring the bolt into its plastic range, which is beyond
yield. It is in this vicinity, that the bolt reaches its proof
load, and though the bolt will retain a certain amount of permanent
stretch when loosened, this will not affect the strength of the
bolt so long as the bolt is not stretched beyond its minimum
tensile strength. Thus, it is realized that once pre-load is
reached, sensitive tightening is required.
Having achieved pre-load, the operator or tool now tightens the
bolt further to bring it up to minimum proof load. At minimum proof
load, there is a complete shear of the plastic tab and wires. Once
complete shear is obtained, the electrical circuit 80 such as the
one shown in FIG. 3 is opened and the electric signal 84 ceases to
operate. This notifies the operator or tool that minimum proof load
has been reached and further rotation is terminated. The minimum
proof load condition is shown in FIG. 7 and is characterized by the
steel-to-steel contact between the bearing surface 40 of washer 32
and bearing surface 38 of nut 30. If a power tool 92 and circuit 90
as shown in FIG. 12 is used, the circuit (to be described
hereinafter) is designed to automatically shut the tool off at both
pre-load and minimum proof load.
It will be appreciated that when it is unnecessary to determine
pre-load, an electrical circuitry is not required to determine
proof load. The operator then can visually ascertain proof load
when complete shear of the tab and wires takes place. For this
reason, the width of tab 10 should be less than the washer diameter
so that the entire strip will be covered. Also, it has been found
that when it is unnecessary to determine pre-load, there are
applications wherein the wires are in and of themselves sufficient
for indicating proof load at shear eliminating the necessity of any
flowable material in between.
Referring now in detail to FIG. 12, an electrical circuit diagram
90 is shown which automatically turns the power tool 92 off when
pre-load and minimum proof load is reached. As override is provided
in the system so that when pre-load is reached, the tool can be
further operated to tighten the joint to minimum proof load. While
there are multifarious circuits capable of design to automatically
control the tool during fastening, the circuit shown in FIG. 12
includes an electrical source 94, an electrical signal 96, a
circuit relay 98 and plurality of switches 100, 102 and relays 104,
106, 108, and 110. Leads 112 and 114 are connected to wires 18 and
20 of tab 10 and leads 116, 118 and 120 are connected to tool 92 to
provide electric energy for the operation thereof. Before loading,
wires 18 and 20 and hence leads 112 and 114 are open. With relays
104 and 108 normally open and relays 106 and 110 normally closed,
the tool is activated by closing switch 102 thus providing power to
the tool through leads 118 and 120. The joint is thus loaded until
the plastic material is partially deformed and extruded to
establish contact between wires 18 and 20 through the joint at
pre-load. At this point, circuit relay 98 is activated to open
relays 106 and 110 causing the tool to cease operation while at the
same time circuit relay 98 closes relays 104 and 108. Further
activation of the tool is achieved by closing switch 102 providing
current flow through leads 116 and 120 causing the joint to be
tightened to minimum proof load at which point wires 18 and 20 are
served from between the joint thereby deactivating circuit relay 98
causing relays 104 and 108 to return to their normally open
position thus cutting off power to the tool. Preferably, switches
100 and 102 are arranged so that when one is open, the other is
closed. Thus, when switch 100 is open and the joint is tightened to
minimum proof load, the deactivation of circuit relay 98 which
opens relay 104 and 108 also causes relay 106 and 110 to return to
their normally closed position. However, at this point, switch 102
is open so that the tool 92 is not reactivated. An electric signal
96 may be connected in series with the circuit relay to provide
visual indication of pre-load and minimum proof loading of the
joint.
In addition to providing a signal for indicating both pre-load and
minimum proof load in the joint, the plastic tab 10 provided by
this invention along with the embedded copper wires reduces the
amount of torque up to 50 percent necessary to load the joint. That
is because the plastic, in essence, operates as a lubricant in the
joint thereby greatly reducing the possibility of galling caused by
metal-to-metal contact. Tests have shown that the reduced friction
up to cutoff (i.e. pre-load) will permit 3/4 3/4inch SAE 5 bolt to
be tensioned to pre-load (approximately 30,000 PSI) with a
conventional one-half inch capacity impact wrench. This is
significantly smaller than generally used today. In addition, at
cutoff, the contact surfaces are changed from steel, copper and
plastic to steel-to-steel. At this point, the torque can double if
the proper bearing area and stress is reached thereby providing a
built-in brake to resist the energy output of the turning power
system. This means that 100 percent loading can be reached at 50
percent the normal torque output and still have conventional
calibrated power tools not exceeding minimum tensile load.
It has been found that a seven strand copper wire 18 and 20
provides a reliable and consistent proof load indicating tab. For
one thing, the reliability of the joint is increased since seven
strands of copper wire must be cut off at four points or 28
individual check points are provided. In addition, by using
multi-strand copper wire, as the joint is tightened, the wire is
flattened and drawn both inwardly and outwardly with some of the
strands remaining between the bearing surfaces. The wire outside
the fastener remains round and as the load increases, will tend to
pull outward on the wire as the tab extrudes from the fastener. The
copper is being severely cold worked at this point or yielded to
its ultimate and will fail with little additional tension or pull.
It is the use of an embedded wire which permits the tab to remain
useful up to minimum proof load (for a given grade bolt) and
thereby permit an accurate indication at minimum proof load.
By having an approximate 45.degree. external chamfer angle a on the
outside of the fastener nut 30, the plastic will extrude outward,
tending to pull the wires free at pre-load. This can be demostrated
by heating the plastic and wire to 250.degree. F. wherein it is
found that the cutoff is at a slightly higher load than when at
0.degree. F. The reason is that the plastic will not pull on the
wire as well when the plastic is hot. That portion of the plastic
and wire extruded internally in the core area 37 provided by the
10.degree. chamfer angle b forms a lock nut around the bolt shank
and joint, and many joints remain secure even after the nut is
removed as a result thereof. The plastic in the core will also act
as a seal against the weather and elements. As a result of the
features provided by the present invention, increased speed in
fastener tightening is provided with a more reliable tensioning
method and simple quality inspection assurance. All this adds up to
total lower in place cost.
MODIFICATIONS
Turning to FIG. 9, an alternative embodiment is shown to that
described with relation to FIGS. 1-7 in that washer 50 is shown
having the same physical bearing surface characteristics as that
described with regards to nut 30 shown in FIG. 4. That is, washer
50 is characterized by having an internal tapered surface 52, a
bearing surface 54 and an external tapered surface 56. Internal
surface 52 and external surface 56 characterized by chamfer angles
c and d are similar in respect to chamfer angles a and b with
regards to nut 30 shown in FIG. 4. By providing a washer having the
characteristics as shown in FIG. 9, tab 10 could be inserted in the
same position as shown in FIG. 5 with a flat hex nut 30 acting as
one bearing surface; or in the alternative, bearing surface 54 of
washer 50 could be positioned to face member 24 which would act as
the opposite bearing surface for tab 10. Furthermore, it will be
appreciated that any of the joint elements such as the bolt, screw,
nut, washer, etc. or the joint itself could include the preferred
bearing surface comfiguration.
Referring to FIG. 8, instead of providing a single tab 10 as shown
in FIG. 1, tab 10 is collated with washer 32 by an adhesive or
other conventional means thereby reducing the amount of handling
required by the operator to reduce installation costs and time. In
fact, several alternatives are envisioned such as that shown in
FIG. 10 wherein a continuous strip of tabs and washers are collated
together to further reduce handling by the operator.
Yet another alternative is shown in FIG. 11 wherein a plurality of
screws or bolts 64 are shown to be pre-mounted in a collated strip
66 so that they can be easily and rapidly screwed into each
individual joint. It should be noted that the screw heads 68 are
characterized by having an internal chamfer e; an external chamfer
f and a bearing surface 70 similar to that described previously
with regards to nut 30 and washer 50. In this embodiment, each
individual tension indicating tab in strip 66 will be compressed
between the bolt head 68 and one of the members to be joined.
Thus, it is appreciated from the foregoing, that a variety of
independent tension indicating fastener tab or collated strips may
be provided depending on the particular application envisioned. In
addition, the bolt, nut or washer can be collated with the strip to
cut down on handling time and packaging thereby providing yet more
economy in the setup and tightening of the joint.
Other alternatives envision the use of more than two wires so that
an indication can be registered of more than one tension level
prior to shear. It is also possible that the compression media be
of such a mixture of suspended conducting particles that when
subject to a given load, the conducting particles will conduct and
complete the circuit. An example of this is gallium antimonide
which has an electrical output when subject to pressure.
Although several alternative embodiments have been shown and
described, it will be obvious to those having ordinary skill in
this art that the details of construction may be modified in a
great many ways without departing from the unique concepts
presented. It is therefore intended that the invention is limited
only by the scope of the appended claims rather than by particular
details of construction shown, except as specifically stated in the
claims.
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