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Description  |
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STATE OF THE ART
The use of a yieldable friction material, such as polymerized resin, acting
between opposed thread surfaces of mated threaded fasteners has long been
known and at present has developed into the basis for very substantial
commercial activity.
From the outset, while many materials have been tested or used, the
material gaining the most widespread acceptance is a nylon polymer,
typically nylon 6 or 11.
Early patents disclosing this concept were Boots U.S. Pat. No. 2,462,603
and Brutus U.S. Pat. No. 2,520,127, both filed in the 1940's, in which
nylon plugs or pellets were seated in holes or recesses formed in the
thread surface of one fastener and extending into interfering relation
with threads of a mating fastener.
In 1950 Villo U.S. Pat. No. 3,093,177 disclosed a nylon member applied to a
local area of an unmodified thread form by heat and pressure, thus
eliminating the necessity for drilling or otherwise forming a hole or
recess for receiving the nylon pellet.
A significant development in this art is illustrated by James U.S. Pat. No.
2,928,446, filed in the mid 1950's. This patent discloses a resin deposit
preferably a vinyl tripolymer, applied as a liquid solution and dried. The
liquid is applied while the threaded fastener (a stud) is rotated so that
the deposit extends around 360.degree., and is spaced from the end of the
stud to facilitate initial engagement.
In the 1960's, patent applications were filed resulting in U.S. Pat. Nos.
3,294,139 (Preziosi), 3,416,492 (Greenleaf) and Burke, et al., 3,452,714,
all of which taught the formation of a friction deposit by melting or
fusing a thermoplastic resin (nylon) powder onto the threaded portion of a
fastener. Preziosi deposited the powder on a fastener and subsequently
heated the assembly to melt the resin. Greenleaf and Burke formed the
deposit by directing the thermoplastic resin powder onto a fastener
previously or concurrently heated to resin fusion temperature. Both
suggested forming an arcuate deposit extending completely around the
fastener.
BRIEF SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention the previously known
method is modified primarily by positively rotating the fasteners at a
substantial speed as they traverse the particle deposit zone.
The particles may be applied from one side of the line of advancing
fasteners, or the streams of particles may be directed from opposite
sides.
The stream of thermoplastic particles is conveniently formed by an air flow
through a tubular supply conduit, one end of which may be flattened to
form an elongated discharge nozzle at the end of the tube. This nozzle
projects the stream of air borne particles in a thin fan-shaped form.
Where the elongation of the slot is parallel to the direction of advance
of the fasteners it defines an elongated zone of deposition which is of
limited vertical dimension. The nozzle may be adjusted about the
horizontal axis of the tube, and control the horizontal dimension of the
zone in the direction of advance of the fasteners, as well as its vertical
dimension, which controls the dimension of the deposit of particles
axially of the fastener, or the width of the ring of friction material.
It will be appreciated that the deposit of particles may also be controlled
by varying the speed of spin or rotation of the fasteners, by varying the
rate of advance of the fasteners through the application zones, and by
controlling the quantity shape and dimension of the spray which the nozzle
emits.
One of the difficulties which has been troublesome in the method as
practiced in the past is the relatively high temperature necessary to fuse
the particles at the outer surface of the deposit, since the heat for this
purpose is required to traverse the thickness of the deposit. During
application of the particles, the source of the heat necessary to fuse the
particles is of course limited to the previously heated thread surfaces.
As a result of this it was necessary to heat the fastening elements, or
the threaded portions thereof, to a temperature substantially above the
fusion temperature of the thermoplastic material. Where the thermoplastic
material is nylon having a fusion temperature of approximately 400.degree.
F., it was often necessary to heat the fastener to a temperature of as
high as 650.degree. F. to ensure that the particles at the surface of the
deposit are properly fused together. This temperature besides being
wasteful of energy damages certain metallic coatings commonly provided on
threaded members, such for example as zinc coatings, as well as the resin.
In the present invention the deposit of thermoplastic particles is built up
in sequentially applied applications. Accordingly each application of
particles is fused or at least substantially fused prior to the succeeding
application. While the variables above referred to may be controlled to
produce a multiplicity of sequentially applied deposits, the advantages of
the present invention are realized when the length of the application
zone, the rate of advance, and the rate of spin are correlated such as to
produce a deposit extending completely around the fastener. It is
contemplated that as many as ten applications may be provided.
Since the coating of particles deposited at each passage is very thin, the
particles are more completely fused before the next application, and a
completely fused mass of nylon is obtained with a much lower temperature
of the fastener.
A feature of the present invention is the preliminary heating of the
fasteners as for example in economic gas-fired furnaces. The fasteners may
be brought to a temperature of 200.degree.-300.degree. F. which reduces
the energy required to raise the threaded surfaces to the required fusion
temperature. Final heating may conveniently be accomplished by flameless
heating, such for example as by induction heating.
While the use of gas-fired heaters alone is not to be excluded, it is
preferable to avoid flame producing heaters immediately adjacent the
particle application zone, to avoid the possibility of igniting the powder
spray.
Significant advantages of forming the deposits of friction material in a
continuous ring or band extending completely around the fastener are
realized. In the first place, the deposit need have only approximately a
small fraction (5-15%) of the radial dimension or depth of a localized
patch at one side of the fastener. This is in part because the localized
patch must take up the sum of the radial clearance existing at both sides
of the fastener, with respect to a mating threaded member. The ring patch,
acting at all diametrically related zones, is required to take up only the
radial clearance existing at one side between concentric mating threaded
fasteners. Secondly, the ring patch is most efficient to provide a seal
between the mated threaded fasteners. Finally, the frictional resistance
to turning is effective throughout the complete 360.degree. zone.
The apparatus for carrying out the method as described in the foregoing
comprises essentially the provision of a pair of relatively larger
diameter wheels having soft compressible tires or rims engageable with the
heads of the fasteners as they advance into the particle application zone.
These wheels are driven so that the surfaces engaging opposite sides of
the bolt head move in opposite directions so as to produce relatively
rapid rotation or spinning of the fasteners on their own axes as they
traverse the application zone. The rate of advance of the individual
fasteners through the zone is controlled by selecting differential rates
of rotation of the wheels.
It is noted that the fasteners may normally be advanced by the flexible
belts with hexagonal heads of the fasteners in abutment. The differential
rates of rotation of the wheels is selected such that the fasteners
advance through the application zone at a rate somewhat greater than the
rate at which they are advanced by the belts. This separates the heads of
the bolts for non-interfering rotation as they traverse the particle
application zone.
The wheels are positioned above the belt in position to engage the sides of
the heads of the fasteners, which remain supported vertically by the
flexible belts.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an elevational view of a fastener produced by the present method.
FIG. 1A is an enlarged detail of a single thread groove.
FIG. 2 is a diagrammatic elevational view of the apparatus.
FIG. 3 is a fragmentary enlarged sectional view on the line 3--3, FIG. 2.
FIG. 4 is an enlarged plan view of the apparatus at the application shown.
FIG. 5 is an enlarged sectional view on the line 5--5, FIG. 4.
FIG. 6 is a diagrammatic illustration of generally axially separated bands
of friction material.
FIG. 7 is a fragmentary enlarged view of a deposit only on the non-load
bearing thread surface.
FIG. 8 is a diagrammatic view of application of friction material to only
the non-load bearing thread surfaces.
FIG. 9 shows a modified spray nozzle.
DETAILED DESCRIPTION
The present invention is concerned with the production of externally
threaded fasteners provided with friction material effective to retain the
threaded fasteners in engagement with mating fastening parts. As
illustrated in FIG. 1 an example of such a fastener is a bolt 10 having a
hexagonal head 12 and a threaded shank 14. At an annular zone spaced both
from the head 12 and the end 16 of the bolt there is a deposit 18 of
friction material which as seen in FIG. 1 may fill the bottoms of the
thread spaces of the threaded shank and this material as will subsequently
be described, is applied by directing a stream of particles of
thermoplastic friction material such as nylon against the heated threaded
surfaces of the fasteners while the fastener is rotated to cause the
deposit of particles to be built up in a successive series of
applications. While the friction material is applied in the form of a
powder or relatively fine particles, the temperature of the thread
surfaces onto which the thermoplastic material is directed is sufficient
to produce fusion of the particles. This not only provides a fused bond
between the deposit and the thread surfaces ut it also causes the deposit
to become in effect a solid continuous body in the thread groove. In
practice the final deposit presents an outwardly concave surface in a
direction traverse to the thread groove, and the depth of the deposit is
such that its radial dimension measured from the bottom of the thread
groove is only a small fraction of the thread height as measured from root
to the crest of the thread.
In fact, in accordance with the present invention, and as best seen in FIG.
1A, the radial dimension of the mid-portion of the resin deposit 18 as
measured from the bottom of the thread groove is indicated at d and is
5-15% of the full depth of the thread groove. Since this deposit is built
up of a plurality of sequential applications, it is considered more
accurate to describe the deposit as of approximately uniform thickness on
the thread flanks.
A significant difference in the operation of an annular deposit of
360.degree. to form a ring or band as compared to a deposit located at one
side of the fastener will now be considered.
Where the deposit is localized at one side of the fastener, it will be
apparent that the initial engagement between the fasteners at the zone
occupied by the local patch or deposit results in a relative lateral
displacement, bringing the threads into metal-to-metal contact at the zone
180.degree. from the center of the deposit. At this time, in the absence
of axial loading, the thread portions in the zone of the deposit are
generally centered between adjacent threads on which the resin deposit
appears. At the opposite side, the metal-to-metal thread contact occurs at
both sides of the threads.
The deposited resin as disclosed herein is yieldable, but it is essentially
incompressible and accordingly is displaced as required by the relative
massive forces developed by subjecting the fasteners to the axial load for
which they are designed. It will be understood also that there may be
widely differing clearance between mating threads, and this clearance does
not occupy the same locations with different thread types. For example,
root clearance may vary widely.
When the fasteners having the localized resin deposits at one side of the
fastener come under load, the resin is initially substantially evenly
divided between both sides of each thread in the zone of the deposit as
determined by the full metal-to-metal contact between threads at the zone
opposite the deposit. Under load however, axial displacement of the
flowable resin is required. This displacement results in flow which is
partly circumferential, distributing the resin laterally beyond the edges
of the local deposit. This circumferential displacement is not symmetrical
since the relative rotation between the fasteners carries more of the
material in the direction of rotation of the untreated fastener. There is
however some reverse flow, depending on the amount of resin deposit and
the geometry of the threads.
In addition there is a flow of resin axially, across the crests of the
threads, from the decreasing thread space between the thread flanks which
sustain the loading into the increasing thread space at the other sides of
the threads.
As the loading displaces resin from between load bearing thread surfaces,
the fasteners tend to resume a concentric relationship, with substantially
solid metal-to-metal contact, or the equivalent thereof, around the full
360.degree..
Under these conditions a substantial excess of resin in the deposit is
readily accepted, since the material is displaced ahead of the female
fastener, displaced circumferentially beyond the original side edges of
the deposit, and, under load, is displaced axially across the thread
crests into the thread spaces between unloaded thread surfaces.
The situation existing when the resin deposit extends completely and
uniformly around the treated fastener is quite different. In this case,
initial engagement between the threads in the deposit zone leaves the
fasteners in concentric relation, with substantially equal thickness of
resin deposit at both sides of the threads, prior to axial loading.
Under axial loading, there is no unoccupied clearance space for
circumferential displacement of resin, and after the spaces between thread
crests and adjacent thread roots are filled, the only available space to
accommodate material displaced from the spaces between loaded thread
surfaces is across the thread crests into the enlarging spaces between the
non-load bearing thread surfaces. Under the axial loading normally
anticipated, the friction material is nearly completely displaced to
provide essentially solid metal to metal abutment between the loaded
thread flanks around 360.degree. of the fasteners.
At the same time, frictional opposition to loosening of the fasteners is
uniform throughout the entire 360.degree. and the concentric fastener
relationship is maintained.
A high production method of producing threaded friction fasteners 10 is
illustrated in FIGS. 2-5.
Referring to these figures, the threaded fasteners are advanced with the
threaded shanks 14 extending vertically downwardly and the heads 12
supported on flexible belts 20 and 22. Belts 20 and 22 may conveniently be
formed of metal and are advanced by rollers 24 and 26. Intermediate
lengths of the belts 20 and 22 may be supported by suitable means not
shown so that the series of bolts 10 are advanced horizontally as for
example to the right as seen in FIG. 2.
As previously noted, the fasteners 10 may advantageously be heated to
elevated temperatures somewhat less than required to produce fusion of the
subsequently applied particles of thermoplastic material before final
heating. This may be accomplished by passing the fasteners through a
gas-fired preliminary heater 27. As the fasteners 10 are further advanced
by the belts 20 and 22 they traverse a final heating station indicated
diagrammatically at 28 which may conveniently be induction heating
apparatus. The heaters are capable of accurate control so as to raise the
temperature of the threaded shanks of the fasteners to the minimum
temperature required to produce fusion throughout of the deposit of
particles of thermoplastic material. The preliminary heating of the
fasteners in economic gas-fired furnaces substantially reduces the load of
the induction heater.
It will be understood that the induction heaters may be located at one or
both sides of the line of advancing fasteners as required.
Immediately after the fasteners are brought to the required minimum
temperature for producing uniform fusion of the thermoplastic particles
into a continuous condition, the belts advance the heated fasteners to the
particle application zone indicated generally at 30 in FIG. 2. In this
zone, as more clearly seen in FIG. 4, there are provided two rotating
wheels 32 and 34 positioned by rotating spindles 36 extending vertically
and supporting the wheels for rotation in a horizontal plane and
engageable at their peripheries with the heads 12 of the fasteners 10.
The wheels 32 and 34, as best illustrated in FIG. 5 are provided with soft
compressible rims or tires 37 which are engageable with the hexagonal
heads of a plurality of fasteners for a substantial distance along the
line of advance. Since the portions of the wheels engaging the bolts are
moving in opposite directions as illustrated by the arrows they positively
rotate or spin the fasteners about the axes by rates determined by the
speed of rotation and diameter of the wheels.
In addition the speed of rotation of the wheel 34, which is in the
direction of advance of the fasteners to the right in FIG. 4 exceeds the
speed of rotation of the wheel 32 so that while engaged between the
opposing wheels 32, 34, the fasteners are advanced through the particle
application zone 30 at a speed slightly in excess of the speed of advance
of the belts 20, 22. Accordingly, while the hexagonal heads of the bolts
may be in abutment as they enter the application zone, they are separated
during the traverse of the zone sufficient to permit independent rotation
of the hexagonal heads without interference.
At the application zone 30 nozzles 38 are provided at one or both sides of
the line of advance of the fasteners. Conveniently the nozzles 38 may
comprise tubular portions 40 having flattened ends 42 which provide
elongated ports through which the thermoplastic particles are directed in
a generally diverging fan-shaped spray against the sides of the fasteners.
As previously stated, the longitudinal dimension of the application zone as
determined by the configuration and angular position of the nozzles 38,
and the speeds of rotation of the wheels 32 and 34 are selected such that
the deposit 16 of fluid material is built up in successive applications of
powdered material so that successive applications of powdered material are
onto previously fused portions of the deposit.
With this arrangement it is found that satisfactory fused deposits may be
produced on threaded fasteners whose initial temperature at the beginning
of application of the particles is substantially below what was previously
found to be required where the deposits were essentially the result of a
single application of particles of thermoplastic material.
As described above it is found that the benefits of the improved method of
deposition in accordance with the present invention require that the
entire depositions extend completely around the threaded shank, and
preferable comprise at least two successive applications of the particles
of thermoplastic material.
In a specific example of the present invention, the deposition was made
intermediate the ends of the threaded portion of bolts having a thread
diameter of 0.250 inches. The particles of thermoplastic material were
supplied by opposed nozzles at opposite sides of the line of advance of
the fasteners having a dimension longitudinal of the line of advance of
approximately one-half inch. The rate of advance of the fasteners through
the application zone was ten feet per minute, which causes the individual
fasteners to traverse the application zone in 0.50 seconds. The rate of
rotation of spin of the individual fasteners as they traverse the
application zone was 90 rpm. This results in 1.5 revolutions of the
fasteners as they traverse the application zone, and with two opposed
nozzles, produces a total deposition built up by approximately three
successive applications of the thermoplastic particles.
Since substantially only axial flow of the displaced friction material is
possible, the construction diagrammatically illustrated in FIG. 6 is
useful. Here, the annular 360.degree. deposits of resin are applied in a
plurality of axially separated bands. The threaded shank 50 is seen to
have three annular bands indicated at 52, 54 and 56. The spaces 58 between
adjacent bands provide spaces into which friction material may be
displaced axially with a minimum distance of displacement. This is
important because minimum displacement provides for elastic return flow,
which provides the ability of the fastener to be removed and replaced
several times while retaining the frictional opposition to flow.
It will be understood of course that the showing of FIG. 6 is only
diagrammatic. In practice the bands 52, 54 and 56 may be spaced apart by
the bands 58 in which the deposit is substantially thinner than in the
bands 52, 54 and 58. This result can be achieved by applying the powder
through a slot which is elongated axially of the fastener, and providing
thin wires or the like across the slot to at least concentrate the flow to
the desired zones.
Referring now to FIG. 7 there is illustrated a further embodiment of the
invention. Here the fastener shank 60 has a 360.degree. band or ring of
friction material applied primarily to the sides of the thread convolution
in the band at the non-load bearing side thereof. In the figure, the resin
is seen at 62 applied to the non-load bearing sides of threads, the load
bearing sides 66 being shown as substantially devoid of friction material.
In practice of course, where the deposits are built up by powder
deposition, the load bearing surfaces 66 may receive a limited amount of
material, insignificant as compared to the deposit on the non-load bearing
surfaces on which the deposits 62 are applied.
In FIG. 8 there is illustrated apparatus for producing the fasteners of
FIG. 7. Here the fastener shank 60, extending down from the head 68, has
threads 64 having surfaces 66 which are under axial loading when the
fasteners is engaged with a mating female fastener. The fasteners are
heated as seen in FIG. 2, and the resin powder is directed upwardly by
passages 70 provided in a nozzle head 72. Passages 70 may be nearly
parallel to load bearing surfaces 66, or even at a greater angle so that
little or no powder is applied directly to the bottom of the thread
grooves. By controlling orifice size, rate of flow, fastener advance and
rotation, and fastener temperature, the degree to which deposition may be
limited to the non-load bearing thread surfaces may be determined.
With this arrangement, the fasteners engage initially with the load bearing
thread surfaces in metal-to-metal contact, and flow or displacement of
friction material is minimized, while still providing adequate resistance
to loosening.
While the friction material is applied as separate particles of
thermoplastic resin, the temperature of the threaded article on which the
particles are deposited is sufficient to cause the particles to fuse into
a solid continuous mass in each thread groove, and to become fuse bonded
to the adjacent thread surface. It is of course appreciated that
continuity of the deposit applies only to the material in each thread
groove convolution.
It is desired to emphasize that where reference is made to a substantially
uniform thickness of the deposit of friction material on the thread
surface as measured perpendicular thereto over the major portion of the
thread surface from adjacent the root to adjacent the crest of the thread,
this intended to differentiate sharply from the shape of deposit which has
become familiar in the formation of localized patches of limited
circumferential extent, in which the deposit fills a substantial part of
the thread groove and has a great variation in thickness as measured as
described. It will also be apparent that in the embodiment of the
invention illustrated diagrammatically in FIG. 6, the thickness of the
deposit on the thread surfaces varies axially from bands 52, 54 and 57,
where it is of maximum thickness, to the intervening annular zones 58,
where it is of minimum or even negligible thickness.
FIG. 9 suggests a modification of nozzle which will produce this type of
deposit. Nozzle 80 has a vertically elongated outlet port 82 traversed by
one or more wires 84 which serve to separate the particles into vertically
separated streams. As the fastener is positively rotated as above
described, the particles form the axially separated rings or bands 52, 54
and 56. While there may be a small amount of material deposited in the
zone or zones 58, it is negligible.
The reduction in temperature of the fastener required in the annular
deposit extending around the fastener is a very important feature of the
present invention. Unfortunately there is no practical way of measuring
this temperature, besides which it will vary with the dimensions of the
fastener. Accordingly, the minimum temperature for any particular
operation must be arrived at empirically. Once determined, however, the
conditions which produce the most efficient results will be a matter of
record and can readily be repeated.
It is emphasized that this procedure not only avoids the use of
temperatures sufficiently high to injure fasteners, such for example as
specially treated fasteners, but also represents a substantial saving in
the energy required to heat the fasteners.
It will be noted that certain aspects of the method disclosed herein are
useful in the application of friction material which does not require
fusion.
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