 |
Description  |
|
|
BACKGROUND OF THE INVENTION
This invention relates generally to the field of nut and bolt locks and
more particularly to a self-locking fastener with a pressure-rolled
resilient, deformable plastic patch bonded to the fastener thread surface.
Self-locking threaded fasteners of the type having a locking patch of
thermoplastic resin bonded to the thread surfaces have found wide
commercial application. This broad demand requires a manufacturing process
for high volume production in a rapid and economical manner while
maintaining high quality standards. Some important requirements for
self-locking fasteners of the type described include sufficient adhesion
between the plastic patch and the fastener thread surface, consistent
patch material distribution, and controlled patch height. These qualities
produce more consistent torque properties than fasteners in which these
parameters are less accurately controlled. Inherent in the above
requirements is the need to control the dimensional characteristics of the
plastic patch in order to produce a uniform patch size and shape for
repeatable fastener torque performance. Previous attempts at forming a
free-form liquid pool of melted thermoplastic resin on the thread surface
of a fastener, such as disclosed in U.S. Pat. No. 3,294,139 to Preziosi
and 3,498,352 to Duffy, have certain inherent disadvantages. Included
among these disadvantages are difficulty in confining the plastic patch to
a definite and predictable configuration, localized buildup of excess
patch material which can be easily sheared off, and difficulty in
producing a sufficient and predictable amount of adhesion between the
plastic patch and the thread surface. Shearing of the material occurs when
the fastener containing the plastic patch is engaged in a mating thread
causing loosely bonded particles to separate from the threaded surface and
finally settle in the workpiece. Examination of fasteners which have been
produced by the processes described in the above-noted patents under
actual commercial conditions reveal wide variations in the size and shape
of the plastic patches, many having an uneven, lumpy surface, or even a
"spattered" appearance, with portions of the resin being in the form of
small droplets separate from the main plastic body. Any such disparities
in physical properties result in corresponding performance disparities,
such as unpredictable locking torque values, decreased reusability and
possible contamination of the environment in which the locking fastener is
used due to the shearing off of excess patch material. Attempts have been
made to confine the size and shape of the plastic locking patch, such as
disclosed in U.S. Pat. Nos. 3,634,577 to Kull and 3,787,222 to Duffy, but
the methods disclosed therein and the resulting locking fasteners produced
thereby are not completely satisfactory. Either the process is relatively
expensive, such as in the case of the Kull process which requires the use
of matching molds or dies brought into contact with the threaded fastener
to constrain the free flow of fused plastic material, or the resulting
locking fastener does not produce a repeatable, predictable and sufficient
amount of locking torque for all applications. For example, Kull's plastic
patch material is confined to a height below the thread crests and thus
forms an interrupted patch, which is effectively a series of individual
patches between thread flanks. Duffy's patch material, on the other hand,
extends an uncontrolled distance above the thread crests at the central
line of the patch.
SUMMARY OF THE INVENTION
Accordingly, it is a general purpose and object of the present invention to
provide a novel apparatus and method for producing a threaded fastener
having a locking patch of thermoplastic resin bonded to the thread surface
in a rapid, economical, simple and reliable manner. It is a further object
to provide a threaded fastener having a locking patch formed thereon of
variable yet confined proportions for providing more predictable and
repeatable locking torques, greater reusability of the locking fastener
and for reducing contamination of the surrounding environment due to
shearing off of excess material from the locking patch. It is yet another
object to confine the maximum radial height of the plastic patch a
controlled distance above the thread crests to the basic major diameter of
the fastener thread by the application of rolling pressure thereto. It is
still a further object of the present invention to provide more consistent
patch material distribution by the application of rolling pressure to the
plastic patch. It is yet another object of the present invention to
provide increased adhesion of the thermoplastic resin to the fastener
thread surface by the application of rolling pressure to the plastic
patch.
These and other objects are accomplished according to the present invention
by a self-locking fastener of the type including external threads, each
having a root, a crest and a flank therebetween, and having a resilient,
deformable patch of thermoplastic material applied in powder form and
fusion bonded to the thread surface over a preselected circumferential
extent less than 180.degree.. The thermoplastic material is subjected to
rolling pressure forcing the material to a controlled radial height above
the thread crests.
A method for making such fasteners includes the steps of heating the
external thread surface to a temperature at least equal to the melting
temperature of the thermoplastic material; depositing the material in
powder form on the heated external thread surface over a preselected
circumferential extent less than 180.degree. thereby fusion bonding the
material to the external thread surface; and applying rolling pressure to
the material to form the patch, whereby the material is forced to the
controlled radial height above the thread crests.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of apparatus for producing a patch type
locking fastener in accordance with the present invention;
FIG. 2 is an elevation view of a magnetic fixture shown in FIG. 1;
FIG. 3 is an enlarged perspective view of the heating and powder dispensing
apparatus shown in FIG. 1;
FIG. 4 is a plan view of the powder dispensing apparatus shown in three
positions, A, B and C;
FIG. 5 is an enlarged perspective view of the water spray and pressure
rolling apparatus shown in FIG. 1;
FIG. 6 is an elevation view of the roller adjusting apparatus;
FIG. 7 is an enlarged front elevation view of the pressure rolling
apparatus of FIGS. 1 and 5 including the composite resilient material,
showing a typical fastener with plastic material on its surface before,
during and after rolling;
FIG. 8 is a perspective view of a fastener with the formed plastic patch
after pressure rolling; and
FIG. 9 is an enlarged fragmentary sectional view taken on the line 9--9 of
FIG. 7 showing the distribution of the plastic material on the fastener
threads at approximately the center of the patch.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIGS. 1-7, a preferred embodiment of the apparatus for
practicing the present invention is shown. FIGS. 8 and 9 illustrate the
resulting locking fastener produced. A fastener 10 having a threaded shank
12 is shown carried on a conveyor 14. The process will be described with
reference to a single fastener, but it should be understood that the
apparatus will accommodate, successively, a large number of fasteners in
an assembly line manner. The fastener is prepared for application of a
thermoplastic resin in powder form by first being degreased to remove any
oil and grease in the threads and body of the fastener and is then
mechanically cleaned, such as by wheelabrating, to provide a substantially
clean surface free of foreign material. Wheelabrating is a well known
mechanical agitation process generally used throughout the fastener
industry for removing extraneous material from fasteners. Fastener 10 is
then loaded onto conveyor 14 at one end in some suitable manner, either
manually or automatically by means not shown, by placing the fastener on
one row of magnetic fixtures 16 transverse to the longitudinal axis of
conveyor 14. As shown in FIG. 2, each fixture 16 includes a pair of
members 18 and 20 fixed to a base 22. The members and base are all made of
a magnetizable material such as sheet steel. A permanent magnet 24 is
secured between members 18 and 20 in some suitable manner, such as by bolt
26 and nut 28. The top portion of member 20 is configured to accept
fastener 10 thereon in such a manner that it bears securely against
vertical member 18 for transportation through the various stations of the
apparatus. Member 20 is preferably horizontally disposed on the top
surface in order to allow fastener 10 to be mechanically knocked off the
fixture at the end of the process, as will be more fully described
hereinafter. FIG. 3 shows the tip end of threaded shank 12 extending
beyond the outermost one of a row of fixtures 16 so that fastener 10 is in
proper position to receive the plastic powder on its surface. Conveyor 14
is driven at a variable range of speeds from approximately 5 to 24 feet
per minute by suitable conventional driving means (not shown).
Fastener 10 is first moved to heating apparatus 30 where threaded shank 12,
which is to receive the thermoplastic material, is heated by passing it
between a top 32 and a bottom 34 inductive heating coil, raising the
surface temperature to between approximately 475.degree. F. to 525.degree.
F., which temperatures are above the melting temperature of the
thermoplastic material. It should be understood that the temperature of
the interior material of the bolt will vary according to the bolt
diameter, but conduction of heat from the surface to the interior will
cause the material adjacent the surface to be at approximately the same
temperature as the surface. In the present apparatus, coils 32 and 34 are
approximately 18" long and are powered by a 15 kw high frequency Lapel
induction heater 36. The speed of conveyor 14 and energy imparted to coils
32 and 34 are selected to cause the fastener to become heated to its core,
so that the internally heated metal acts as a heat reservoir to supply
heat to the fastener surface after it leaves the coils. This continuing
source of heat has been found to aid the melting and subsequent fusion
bonding of the thermoplastic material to the fastener threaded surface.
After being heated, fastener 10 is advanced to powder dispensing apparatus
38. Referring to FIGS. 3 and 4, apparatus 38 includes a linear vibratory
feeder 40 having an adjustable trough 42 mounted on feeder 40, a powder
storage hopper 44 above trough 42 and a return chute (not shown) below
conveyor 14 for catching excess powder. Plates 46 and 48 are mounted on
either side of trough 42 and are capable of being moved in both directions
along the axis of conveyor 14 to increase or decrease the width of trough
42, as can be seen in FIGS. 4A, 4B and 4C. Apparatus 38 is mounted for
rotation in a horizontal plane so that the angle between the front edge of
trough 42 and the longitudinal axis of conveyor 14 is adjustable. This
feature, along with the width adjustability of trough 42, allows careful
control of the dimensions and shape of the plastic patch which is formed
on fastener 10. The frequency of vibratory feeder 40 is adjustable in
order to control the amount of powder deposited on fastener 10. The
thermoplastic material which is deposited on threaded shank 12 is stored
in fine powder form in hopper 44. The thermoplastic material may be one of
a number of materials possessing among its general properties, high
tensile strength and elongation when in a solidified state. In the present
embodiment Dumont Chemical Co. DURBEL-MA-12 nylon powder is considered
suitable for use. The powder falls through opening 50 at the bottom of
hopper 44 onto trough 42, which dispenses a controlled amount of powder
onto bolt shank 12. At least one full thread at the tip end of fastener 10
should not contain any thermoplastic material, facilitating entry of the
fastener into a mating threaded workpiece. Trough 42 is inclined slightly
upwardly from back to front toward bolt shank 12 and thus the vibratory
action of a high frequency oscillating motor (not shown) causes the powder
to advance up the inclined trough and fall due to gravity onto threaded
shank 12 of fastener 10 as it passes under trough 42. A shroud or cover 52
is placed over conveyor 14 adjacent apparatus 38 to prevent external air
currents from affecting the flow of powder onto bolt shank 12. The powder
melts upon contacting threaded shank 12 and thereupon starts to solidify
from the surface inwardly toward the root of the thread upon exposure to
ambient air as the fastener advances on conveyor 14. Due to the nature of
gravity feeding the powder onto threaded shank 12, the thermoplastic
material is generally uniformly tapered from a minimum thickness at the
edges to a maximum thickness at the center, as can be seen in FIG. 7 (A).
In this free form state, the thermoplastic material generally extends an
uncontrolled distance above the crests on threaded shank 12. After the
powder is deposited, fastener 10 is transported a predetermined distance
by conveyor 14 in order to allow the thermoplastic material to solidify at
and adjacent the surface. In order to speed the solidification process, a
spray pipe 54, which dispenses a water-mist spray, may be placed
downstream from apparatus 38, such as by slidingly mounting it on a beam
55, as shown in FIG. 5. It is emphasized that spray pipe 54 is basically
an optional feature for the purpose of shortening conveyor 14.
Referring now to FIGS. 5, 6 and 7, there is shown pressure roll patch
forming apparatus 56. FIG. 7 shows a progressive advancement of fastener
10 through a pair of rollers where the thermoplastic material is formed
into a shaped patch 58. In the first position (A), fastener 10 is shown
with the partially solidified plastic patch on threaded shank 12 prior to
entering apparatus 56. It should be noted that prior to rolling, the patch
material is generally of uneven shape extending an uncontrolled distance
above the threads of the fastener, with the thickness of material being
greatest proximate the center. Apparatus 56 includes top roller 60 and
bottom roller 62. Each roller is a circular disc generally made from metal
or a rigid material and is approximately 67/8" in diameter in the present
embodiment. It should be understood that rollers 60 and 62 could be any
suitable size for producing sufficient force to properly form patch 58,
and may vary with screw size. An annular resilient member 64 is mounted
around the circumference of top roller 60 and a similar annular resilient
member 66 is mounted around the circumference of bottom roller 62.
Resilient members 64 and 66 are each fabricated by laminating a 3/32"
thick strip of Cohrlastic 400 silicone sheet rubber on top of a 1/2"
thick strip of Cohrlastic R-10470, medium silicone sponge rubber. The
sponge rubber provides resiliency to follow the thread form and the sheet
rubber provides a smooth, closed pore surface for more uniform patch
appearance. Silicone rubber is preferred for its general heat resistant
properties. It should be understood that other suitable, generally
resilient materials could be used for members 64 and 66 with equally
successful results. Rollers 60 and 62 are initially separated by a
predetermined fixed distance in order to allow fastener 10 to pass through
the rollers without being dislodged from fixtures 16. For example, a
typical initial separation of surfaces 68 and 70 of respective members 64
and 66 for a nominal 3/8" I.D. fastener is on the order of 1/32". Rollers
60 and 62 are driven at a rotational speed such that the tangential
velocity of surfaces 68 and 70 is approximately equal to the linear speed
of conveyor 14. The rotational speed and initial spacing of the rollers
can be adjusted by apparatus which will be described hereinafter. As shown
in position (B) in FIG. 7, fastener 10 passes between rollers 60 and 62,
and surface 68 of top resilient member 64 contacts the upper half of
threaded shank 12 producing a generally vertical downward force on the
partially solidified thermoplastic material. Surface 70 of bottom
resilient member 66 similarly contacts the lower half of threaded shank 12
providing an equal reactive force. Surface 68 first contacts threaded
shank 12 proximate one tapered edge of the thermoplastic material and
thereupon rolls across the surface exerting radially inward pressure on
the material to compact it and force it into the root of the threads and
against the surface of shank 12, forming patch 58. The rollers displace
excess thermoplastic material above the crests of the threads and compress
it into the thread roots and circumferentially along the thread flanks,
thus controlling and limiting the height of formed patch 58, as shown in
position (C) in FIG. 7. This forming operation imparts a generally
rectangular shape to patch 58. It has been found that the radially
outermost surface of patch 58 so formed by rollers 60 and 62 does not
exceed a value equal to the basic major diameter of the fastener thread.
Referring to FIG. 9, the actual diameter of crests 61 of the fastener
thread is less than the basic major diameter of the fastener thread. For
example, a nominal 3/8-16" bolt has a basic major diameter of 0.375 inches
while actual major diameters typically range from approximately 0.366 to
0.368 inches. Pressure rolling of the thermoplastic material reduces the
maximum radial extent of surfaces 63 on patch 58 to an envelope which does
not exceed the basic major diameter of the fastener thread equal to 0.375
inches. Typical maximum diameters taken at surfaces 63 on a number of
nominal 3/8-16" bolts ranged from approximately 0.367 to 0.371 inches. It
should be noted that the radial distance from the longitudinal axis of the
fastener to surfaces 63 is greater than the corresponding radial distance
from that axis to the thread crests at a point diametrically opposite
surfaces 63. This point is noted to indicate that the peripheral
configuration of the fastener at patch 58 is not completely circular, and
therefore the maximum diameters taken at surfaces 63 indicated above are
effective diameters for comparison with the basic major diameter. The
uniformity of patch dimensions including restricting the patch to a
controlled height substantially equal to the envelope of the basic major
thread diameter, has been found to produce greater uniformity in
prevailing torque test results as well as acceptable prevailing torque
values, both upon initial tightening and after the first and fifth
removals. This method of testing is an industry-wide standard for
measuring the effectiveness of prevailing torque fasteners. The pressure
produced by the forming operation creates a more closely packed patch and
also forces the molten material into more intimate contact with the heated
fastener surface improving patch adhesion. It should be noted that no
primer material is applied to the fastener surface, as is disclosed, for
example, in the aforementioned patents to Duffy.
Referring to FIG. 6, the adjustment and driving mechanism for rollers 60
and 62 is shown. Bottom roller 62 is shown mounted to a gear 72 by a shaft
74 journaled for rotation in apparatus 56. Top roller 60 is connected to a
gear 76 by a shaft 78 journaled for rotation in a plate 80. Plate 80 is
movable in a vertical direction with respect to apparatus 56 by means of
elongated slots 82, 84 and 86, and adjusting bolts 88, 90 and 92,
respectively. By adjusting the vertical position of plate 80, the desired
amount of initial separation between surfaces 68 and 70 of resilient
members 64 and 66 can be obtained. It should be understood that bottom
roller 62 could be similarly vertically adjustable for finer control and
adjustment of the initial separation. The initial separation of members 64
and 66 for a 3/8-16" bolt, for example, should ideally be adjusted so that
when the bolt passes between them, members 64 and 66 are each compressed
to a thickness of approximately 1/16" to 3/32". The amount of initial
separation of members 64 and 66 will vary with bolt size, but ultimate
compression of the members to the above-noted range of thicknesses will
produce acceptable results for all bolt sizes. A driving gear 94 is
mounted to a variable speed motor (not shown) by a shaft 96 journaled for
rotation in a plate 98 having slots 100, 102, 104 and 106 formed therein.
The speed of the motor is adjusted to drive rollers 60 and 62 at the
proper tangential speed to match the linear velocity of conveyor 14.
Adjusting bolts 108, 110, 112 and 114 cooperate with the slots to allow
linear movement of plate 98. Gear 94 is formed to mesh with gears 72 and
76, respectively, to drive rollers 60 and 62. When plate 80 is moved
vertically, changing the vertical separation of surfaces 68 and 70, plate
98 containing driving gear 94 must be moved into engagement with gears 72
and 76.
Referring again to FIG. 1, after fastener 10 passes through rollers 60 and
62 it is transported by conveyor 14 to the underside where it is forced
off of magnetic fixtures 16 by an appropriate apparatus, such as a
stationary wiper (not shown), and falls into a quench tank 116 where it is
cooled.
Having thus described the present invention, some of the many advantages
should now be readily apparent. The apparatus and method disclosed affords
a relatively simple and economical means for producing self-locking
fasteners of the type having a thermoplastic resin patch formed on the
thread surface. Pressure rolling produces controlled uniform patch
dimensions from fastener to fastener including patch height, thus insuring
uniform performance characteristics. Greater predictability and uniformity
of prevailing torque test results, in accordance with customary industry
standards, are obtained with fasteners made in the disclosed manner. In
addition, the pressure rolling step insures greater adhesion of the
thermoplastic patch material to the threaded shank portion of the
fastener, thus insuring fewer break-away type of patch failures.
Contamination of the surrounding environment is also minimized because
there is no loose or excess material on the formed patch which can be
sheared off by the mating thread.
Obviously, many modifications and variations of the present invention are
possible in light of the above teachings. It is therefore to be understood
that within the scope of the appended claims the invention may be
practiced otherwise than as specifically described.
* * * * *
|
|
 |
|
 |
Description |
|
