Note: Descriptions are shown in the official language in which they were submitted.
~0~ 36Z
The present invention relates to a coating for reducing
the wear of two opposable stainless steel surfaces which are in
a sliding relationship with one another comprising a first
metal layer; a second metal layer; an inorganic lubricant;
and, an organic lubricant.
The first metal layer is a hard metal layer, i.e., a
metal layer having an elastic modulus of about 20,000,000 p.s.i.
or higher such as nickel and the alloys thereof, whereas the
second metal layer is a soft metal layer, i.e., a metal layer
having an elastic modulus of about 17,000,000 p.s.i. or lower
such as copper and the alloys thereof. The inorganic lubri-
cant comprises molybdenum disul~ide, graphite or combinations
thereof. The organic lubricant may be any organic lubricant
known in the art such as high molecular weight aliphatic
alcohols and aliphatic acids or the glycerides of such acids;
waxes derived from petroleum or other natural sources or
blends of such waxes with polyolefins such as polyethylene,
polypropylene and the like. Automo$ive lubricants such as
motor oils and greases may also be used as well as the art
known silicone oils and silicone greases.
It has besn discovered that the use of the organic
lubricant is optional where the first metal layer is applied
to one metallic surface and the second metal layer is applied
to the second metallic surface.
According to the present invention there is provided a
coating for reducing the wear of two opposed stainless steel
metallic surfaces disposed in sliding relationship with one
another comprising a first layer having an elastic modulus in
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a range inclusive of and higher than 20,000,000 p.s.i. and
comprised of a metal selected from a member of the group
consisting of platinum, manganese, cobalt, nickel, chromium,
beryllium, rhodiuml molybdenum, tungsten, ruthenium, iridium
and alloys thereof, a second layer having an elastic modulus
inclusive of and lower than 17,000,000 p.s.i. and comprised
of a metal selected from a member of the group consisting
of palladium, copper, gold, zirconium, silver, cadmium, tin,
lead and alloys thereof, a layer comprised of an inorganic
lubricant, a layer comprised of an organic lubricant, wherein
said first layer is applied to at least one of said metallic
surfaces and is a substrate for said second layer, said
second layer is a substrate for said inorganic lubricant
layer and said organic lubricant is the final layer.
It has been found that coatings in accordance with the
invention reduce wear between metallic surfaces and have been
found to be effective to reduce wear or galling in locknut
and bolt fasteners in which the bolt is used under high bolt
tension. Without such coatings, nut and bolt fasteners made
from A-236 stainless steel or other stainless steels have a
tendency to gall after continued assemblies and reassemblies
even when these operations are performed at room temperatures.
Hence, such fasteners have a limited reuse, i.e., generally
less than 20 on-off cycles. However, with coatings of the
invention such fasteners can be
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assembled and reassembled without galling for 50 or more reuse
cycles.
The features of the invention may be further understood
by reference to the drawings in which:
Fig. 1 illustrates a lock-nut and bolt each having a
coating in accordance with the present invention;
Fig, 2 illustrates a bolt for use in combination with
an uncoated lock-nut wherein said bolt has a coating in accord-
ance with the present invention;
Fig. 3 illustrates a ~rontal elevation in section of
an off-set lock-nut having a coating thereon in accordance with
the present invention;
Fig. 3A is a plan view of the lock-nut of Fig. 3;
Fig. 4 illustrates two opposed metallic surfaces one
of which is coated in accordance with the present invention; and
Fig. 5 illustrates two opposed metallic surfaces both
of which are coated in accordance with the present invention.
~ self-locking threaded fastener, e.g./ a lock-nut,
comprises a device having means built into a threaded nut to pre-
vent the nut from unloosening during use in an environment wherea high degree of vibration is encountered such as in internal com-
bustion engines, aircraft or automotive assemblies.
There is some difficulty occasioned where the lock-nut
is used in applications where constant re-use is required since
with prior art lock-nuts and bolts employed in fastening opera-
tions and made of high strength materials like A-286 stainless
steel only about 10 to 20 reuse or off-on cycles are possible
before excessive wear and/or thread damage renders them unsuit-
able for use, particularly in application where an automatic
torque wrench is used to apply a high torque to the nut or bolt
to intentionally leave a high residual stress on the bolt and ob-
tain vptimum fastening. Examples of such applications are the
~astening of access panels for electronic components in aircra`ft
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or access panels in turbojet engine assemblies where ordinary
maintenance requires regular removal of the panels for replace-
ment or overhaul of components. As can be readily appreciated
there would be a considerable advantaye in being ahle to extend
the reuse life of lock-nut and bolt fasteners in such applications.
The means built in to the lock-nut to p~event the nut
from unlooseniny comprises a friction device that permits the nut
to be turned on a bolt with a moderate amount of torque which
provides frictional resistance to unloosening when the threaded
connection is subject to vibration, and which permits assembly
without undue tightening. The friction on the nut be~ore it is
screwed into a fastening position is generally reflected in the
torque values required to screw the nut down into such fastening
position and is sufficiently high to keep the lock-nut from un-
loosening due to vibration when in place. The friction means in
the lock-nut generally comprises a polymeric washer sealed into
the nut such as a polYamide or nylon compound or a polyimide such
as Vespel ~a trademarked compound produced and sold by E.I. DuPont
deNemours and Company, Inc.). Friction means are also provided
by crimping or offsetting one end of a nut so that when looking
down into the threaded portion of the nut at least one end is in
an oval, elliptical, oblate or prolate configuration instead of
being ordinarily round. Offsetting a nut is generally the method
emp~oyed for manufacturing lock-nuts that are to be used in high
temperature applications since polymeric materials tend to decom-
pose upon pr~longed exposure to temperatures in excess of 500F.
Either of these friction means (i.e., offsetting or the provi$ion
of a polymeric washer) is designed to supply enough friction in
the thread engagement area of the lock-nut and bolt combination
to avoid loosening under vibration.
Thus, in trying to devise a way of increasing the
number of reuse cycles of a lock-nut and bolt assembly means,
there must be provided not only means that would prevent excessive
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wear or damage to the threads of the nut and bolt fastener but
also a sufficient amount of friction must be maintained in the
fastener assembly throughout successive reuse cycles so that the
nut will not work loose in a vibration environment.
; Some prior art references disclose various coatings on
nut and bolt fasteners and metal substrates, such as, ~or instance,
pp. 122 and 133 of Fasteneering Cat:alog "L" copyright 1947 of
the Central Screw Company, 3501 Shields Avenue, Chicago, Illinois,
United States Patents No. 3,244,625, Silwones; No. 2,122,915,
Olson; No. 3,099,083, Delong; No. 3,522,177, Benz; No. 3,311,493,
Schunemann; British Patents No 341,923, Parsons; No. 811,573;
and Canadian Patent No. 617,081, Nelson.
It is therefore an object of the invention to provide
means and methods for increasing the number of reuse cycles of
a nut and bolt fastener where the bolt is placed under high stress
or tension in the fastening operation.
It is a further object of the invention to provide
means and methods for increasing the number of reuse cycles of a
lock-nut and bolt fastener where the bolt is under high stress
in a fastening operation in a manner such that desired friction
between the lock-nut and bolt is maintained.
It is also an object of the present invention to pro-
vide a coating and methods for reducing the wear of two opposable
metallic surfaces that are to operate in an abutting relationship
with one another as well as methods for making such coating.
It is a further object of the present invention to pro-
vide improved fastener assemblies having slidably engageable mem-
bers and methods of making the same~
These and other objects are achieved according to the
present invention wherein one embodiment comprises providing a
coating on the threads of a lock-nut and/or bolt for reducing the
wear and maintaining desired friction b~tween the threads of the
lock-nut and the threads of the bolt.
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~O~C)3~'~
According to the invention a coating is provid~d for
reducing the wear of two opposable metallic surfaces comprising
a first metal layer; a second metal layer; an inoxganic lubri-
cant; and, an organic lubricant.
The aforementioned coating layers are applied to the
surfaces of the metal in such a fashion so that ~he first metal
layer is a substrate for the second metal layer or this metal
layer is applied to one metallic surface and the second metal
layer to the other metallic surface. Where the first metal
layer is a substrate for the second metal layer it is intended by
this expression to indicate that the second metal layer is placed
on the first metal layer. The inorganic lubricant is placed on
the second metal layer whether this second metal layer is placed
on the first layer or applied directly to one metallic surface
and the fixst laver to the other metallic surface.
The organic lubricant is optional when the first metal
layer is applied to one metallic surface and the second metal
layer is applied to the remaining metallic surface.
As mentioned, when the first metal layer is employed
as a substrate for the second metal layer, the inorganic lubri-
cant is placed on the second metal layer. The organic lubricant
is then employed as a final layer by which it is m~ant that the
final layer is placed on either the aforementioned uncoated metal-
lic surface or coated surface or both.
The ~irst metal layer is a hard metal, i.e., a metal
having an elastic modulus of about 20,000,000 p.s.i. and higher
~i.e., a range inclusive of and greater than about 20,000,000
p.s.i.) or an elastic modulus in the range of from about
20,000,000 p.s.i. to about 75,000,000 p.s.i. The hard metal in-
cludes a member selected from the group consisting of the follow-
ing metals, the elastic modulus of each in p.s.i. unit being given
parenthetically a~ter such metals: platinum (21,000,000);
manganese (23,000,000); cobalt (30,000,000); nickel (30,000,000);
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~OSV3~
chromium (36,000,000); beryllium (37,000,000); tungsten (50,000,0003;
ruthenium (60,000,000); iridium (75,000,000) and alloys thereof.
Where metals having an elastic modulus as high as rhodium or higher
are used, better results would be obtained if a soft metal such as
copper, copper-nickel layers or a copper-nickel alloy was used
as an undercoat. Nickel and its alloys are preferred as the first
metal layer.
The second metal layer is a soft metal, i.e.~ a metal
having an elastic modulus of about 17,000,000 p.s.i. and lower
(i~e., a range inclusive of and lower than about 17,000,000 p.s.i.
or an elastic modulus in the range of from about 17,000,000 p.s.i.
to about 2.5 milLion p.s.i. The soft metal includes a member
selected from the group consisting of the following metals, the
elastic modulus of each being in p.s.i. units parenthe~ically
after such metal: palladium (17,000,000); copper (16,000,000);
gold (12,000,000); zirconium (11,000,000); silver (11,000,000);
cadmium (8,000,000); tin (6,000,00Q); lead (2,600,000) and alloys
thereo such as, for example, the bronze alloys and brass alloys,
i.e., the alloys of copper and tin and copper and zinc. Copper
and its alloys are preferred as the second metal layer.
The inorganic lubricant comprises molybdenum disulfide
or graphite, however, molybdenum disulfide is preferred although
mixtures of molybdenum disulfide and graphite may also be used.
The organic lubricant coating layer is a member selected
from the group consisting of cetyl alcohol, lauric acid, normally
solid paraffin wax, microcrystalline wax, carnauba wax, spermacetti
wax and blends thereof or blends with polyolefins such as poly-
ethylene; polypropylene and the like. The preferred organic lubri-
cants comprise lauric acids, cetyl alcohol, or lauric triglycer-
-30 ides which may be further characterized as aliphatic alcohols or
aliphatic acids having about 10 to about 20 carbon atoms and the
glycerides, especially the triglycerides of such acids. Petrolçum
based lubricants may also be used such as automotive motor oils
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~'~)S~36~
and grease. Silicone lubricants known in the art may also be used
such as silicone oils and silicone greases.
Referring to Fig. 1 a bolt 10 is illustrated havi~g
coated threads 12 which engage coated threads 1~ of a lock-nut
16 having a Vespel polymeric washer 18, which also engages coated
threads 12, and a protective outer washer 20. The nut-bolt assem-
bly provides securement ~or such as plates lOa and lOb. The ~oat-
ing on threads 12 is a first layer comprising nickel whereas the
coating on the threads 14 is a second layer comprising copper which
is also a substrate for an inorganic lubricant layer comprising
molybdenum disulfide. The coated thread 12 and/or coated thread
14 may optionally have an organic lubricant as a final coat.
It has been discovered that a lock-nut and bolt as de-
scribed in Fig. 1 can be usad in a minimum of 50 on-off re-use
cycles in a fastening operation where applied torque is sufficient
to generate the bolt tension described for the assembly in question
without so galling or wearing the thread surfaces as to render the
nut and bolt combination unsuitable for use during such reuse
cycles, i.e., exhibi~ing undue galling. The desired friction fit
between washer 18 and threads 12 is maintained throughout the 50
reuse cycles.
Fig. 2 illustrates a high strength bolt 22 having coated
threads 24 for use in combination with a mating nut, such as a lock-
nut. ~he threads 24 having a coating thereon in accord with the
present invention in which a first metal layer comprising nickel
is coated onto the bolt thread, a second metal layer comprising
copper is coated over the first metal layer, an inorganic lubri-
cant comprising molybdenum disulfide is coated over said second
metal layer and an organic lubricant comprising cetyl alcohol is
applied over the inorganic lubricant. Bolts of Fig. 2, when made
from such materials as A-286 or other stainless steels and assembled
on plain uncoated nuts of the same material, can be used through
50 reuse cycles without undue galling. Also, if the mating nut is
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:1051D3GZ
a lock-nut with a nylon of Vespel insert, it continues to give
adequate locking torque throughout the 50 reuses.
Fig. 3 illustrates a lock-nut 26 having threads 28,
said lock-nut having been offset by crimping one end thereof 30
so that said offset end 30 is elliptical having one elliptical
diameter shorter in length than the round diameter at uncrimped
nut portion 32. The lock-nut is mounted in channel 34, the side-
walls of which are slotted to receive lock-nut retaining clip 36.
Fig. 3A is a plan view of nut 26 showing crimped end
30. The crimped nut acts as a lock-nut since the shortened
elliptical diameter at the crimped end 30 enables the nut to
frictionally engage a bolt.
Threads 28 are coated with a first metal layer compris-
ing nickel, said first metal layer being coated in turn with a
second metal layer comprising copper, said second metal layer in
turn being coated with an inorganic lubric~nt comprising molybdenum
disulfide and said inorganic lubrican~ comprising molybdenum di-
sulfide in turn being coated with an organic lubriçant such as
cetyl alcohol.
The lock-nut illustrated in Fig. 3 and Fig. 3A, even if
made from materials like A-286 or other stainless steels that tend
to gall when installed in tightened relationship to uncoated bolts
of the same material, can be used through 50 re-use cycles without
undue galling or loss of friction locking torque.
Fig. 4 illustrates two opposable sliding metallic sur-
faces 38 and 40 having a first metal layer 42 comprising: nickel
coated on metallic surface 38, said first metal layer having a
second metal layer 44 coated thereon comprising copper, said
second metal layer 44 having an inorganic lubricant 46 coated
thereon comprising molybdenum disulfide, said inorganic lubricant
46 having an organic lubricant 48 coated thereon comprising cetyl
alcohol. Metallic surface 38 may be a nut thread whereas metallic
surface 40 may be a bolt thread.
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~050362
Fig. 5 illustrates a set of two opposable metallic ~ur-
faces 50 and 52 that may be placed in a sliding relationship with
one another, said metallic surface 50 having a first metal layer
54 comprising nickel coated thereon, said metallic surface 52 hav-
ing a second metal layer 56 comprising copper coated thereon, said
second metal layer 56 having an inorganic lubricant 58 comprising
molybdenum disul~ide coated thereon. Metallic surface 50 may be
a nut thread and metallic surface 52 may be a bolt thread. As
discussed above in connection with the Fig. 1 fastençr, use of an
organic lubricant on the molybdenum disulfide surface is, in this
instance, optional.
It has been discovered that in accord with the present
invention the metallic surfaces illustrated in Fig. 4 and Fig. 5,
even if the parts are made of materials that tend to gall on re-
peated assemblies, may be placed in a sliding relationship with
one another without binding or galling and that the wear thereon
will be substantially reduced, even if there is a large amount of
pressure at the interface of such coated metallic surfaces and
even if there are repeated assemblies and disassemblies of the
parts. The coatings on such metallic surfaces prevent excessive
wear and galling, and when the coating is applied on the threads
of lock-nut and bolt fastening devices made of such materials as
A-286 or 13-8 stainless steels, the parts continue to operate
satisfactorily even after 50 reuses.
The metal coating is applied preferably by electroplat-
ing although other coating methods may be used such as flame
spraying, hot melt coating, hot dip coating, cladding, fluidized
bed coating, vacuum metallizing, cathode sputtering, chemical re-
duction, surface alloying by coating followed by heating in vacuum
and all of the art known equivalents thereof.
The metals that may be coated according to the present
invention comprise stainless steels including the austentitic,
martensitic and ferritic types as defineed in Metals Handbook,
_g_
~)S~36i~
Vol. I, Properties and Selection of Metals, published by the
American Society for Metals, Copyright 1961. By way of example,
stainless steel such as 13-8 stainLess steel containing 13% chrome,
8% nickel and minor amounts of molybdenum or A-286 stainless steel
containing about 25% nickel and about 16% chromium may be used as
the substrate to which the coating is applied especially in that
aspect of the invention which comprises the application of the
coating to lock nuts and/or bolts. Other stainless steels include
inter alia 18-8 stainless and Waspaley (Tradename, Special Metals,
Inc.).
Prior to the use of the coating of the present invention
severe galling resulted from reusing lock-nut-bolt combinations
subject to high bolt stress. The galling usually occurred between
approximately the 10th and 20th reuse cycle in stainless steel nut
and bolt combinations, the galling being a deformation of the
thread surfaces such as scoring and chippin~ and in some instances
breaking and fusing of the threads. The coating of the invention
has extended the reuse of such nut and bolt assemblies up to a
minimum of 50 cycles without rendering the nut and bolt assemblies
unsuitable for use during such reuse cycles.
Thin coatings are desirably used in lock-nut and bolt
combinations since this type of coating facilitates application
to standard screw threads without having to modify the screw
~hread dimensions to prevent binding of the lock-nut on the bolt.
Generally speaking, thin coatings such as those applied
to lock-nut bolt fastening devices comprise a first metal lay~r
such as nickel preferably of a thickness from about 0.0001 to
about 0.00015 inch, the second metal layer such as copper, of a
thickness preferably from about OoOOOl to about 0.0002 inch, the
inorganic lubricant such as molybdenum disulfide of a thickness
preferably from about 0.0001 to about 0.0004 inch and the inorganic
lubricant may be applied in any thickness from about 0.0001 inch on
up since the coating is deformable under use so that an excess will
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~5~36Z
not cause any difficulty.
When the coating of this invention is applied ~o a lock-
nut and/or bolt, the lock-nut can be a lock-nut ha~ing a polymeric
washer sealed into the nut or a nut that has been offset.
The coating may be subjected to elevated temperature up
to 600F. and still effectively function as a coating that will
prevent excessive wear when applied to a lock-nut and/or bolt ex-
posed to such temperature. Also, sufficient friction is maintain-
ed between the locking element of the lock-nut and bolt so that
the nut will not loosen when exposed to conditions o~ extreme
vibration.
The coating of the invention may also be used on other
sliding applications such as metal to metal bearing surfaces,
piston rings, piston-cylinder wall interfaces, zippers and snap-
fasteners in addition to lock-nut and lock-nut and bolt combina-
tions.
The following examples are illustrative.
EXAMPLES 1, 2 and'3
In all three Examples, 1/4 - 28 bolts of stainless steel
A-286 conforming to NAS 1004-18A and offset 1/4 - 28 type lock-
nuts made of the same material were coated in accord with the
present invention and the locking (break) torque remained after
some 50 reuse cycles. Three nuts and bolts are coated and tested
for each Example.
In all Examples, the molybdenum disulfide coating was
Everlube 642 (Registered Trademark, Everlube Corporation,
6940 Farmdale Avenue, North Hollywood, California), a solid film
lubricant conforming to Mil-L-46010(A) which is a formulation of
molybdenum disulfide, metallic oxides and salts dispersed in a
resinous binder system soluble in a solvent comprising dioxane or
a mixture of 50% methyl ethyl ketone and 50% toluenes The molyb-
denum disulfide coating is applied to the metal part by dipping and
is then cured by baking at about 400 F. for one houx.
--11--
~L~S0362
Before the metal coat or coats are applied to the bolts
or nuts, the plain metal parts are blasted with 120 mesh grit for
approximately 30 minutes to produce a frosty finish. The bolts or
nuts are then nickel plated followed by rinsing in water and
neutralizing in alkali. In Example 3, nickel plated nuts are
transferred to the copper plating solution and after copper coat-
ing the parts are again rinsed and dri~d. In Examples 1 and 2,
the plain A-286 nuts are directly copper-coated after grit blasting.
Both the nickel and copper coatings are applied by
standard electrolytic methods to a coating thickness of 0.00015"
plus or minus 0.0005" in Example 1. The nickel coat is app~ied
to an A-286 bolt and the mating A-286 nut was copper-plated and
given a molybdenum disulfide coat on the copper. In Example 2,
the same combination is used but an additional cetyl alcohol coat
is applied on the molybdenum disulfide coated surface. To apply
the cetyl alcohol coat, about 0.6 to about l pound of cetyl alcohol
is first dissolved in one gallon of trichloroethylene. The parts
to be coated are then immersed in the lubricant solution after
which the solution is allowed to drain dry before handling.
In Example 3, all four coatings, the nickel coating, the
copper coating, the inorganic lubricant and the cetyl alcohol are
applied to a plain A-286 nut while the A-286 mating bolts are un-
treated.
In all three cases, laboratory tests show that 50 reuse
cycles are obtained without galling, with minimum wear at the
thread surface and with retention of adequate locking (break) torque.
The nuts employed in these Examples are 1/4 - 28 lock-
nuts which are held in an aluminum housing for the purpose of the
test. Normally 2 1/2 bolt threads extend through the top of the
nut during each reuse cycle where a cycle comprises turning the
bolt into the nut until the bolthead is flush with the surface oF
the housing and then backing the bolt off until no locking torque
is de~eloped between the nut and bolt. When the head of the bolt
-12-
~SV36;~
is screwed down into an abutting relationship with the housing sur-
face a torque wrench is employed and the torque measured right up
until the bolthead seats on the housing surface. This is defined
as the maximum on torque. Further torque then is applied to seat
the bolt at about 60 inch pounds. After this seating torque is
applied to the bolt an unseating torque is applied to the bolt to
remove the bolt from the lock-nut so that the bolthead is barely
abutting the surface of the housing. The break torque is then
measured which is the torque required to move the bolt away from
the nut and/or the housing after unseating. Again, this measure-
ment is made with a torque wrench. A power wrench is then employ-
ed to drive the bolts onto and off the nuts during subsequent
cycles. A 500 R.P.M. modified Rockwell Manufacturing Company air-
operated wrench with a tightening clutch adjustable to 60 inch
pounds is used as the power wxench. After every five off-on re
use cycles with the power wrench, the maximum on torque and ~he
minimum break is again measured and the process repeated up and
through 50 on-off cycles.
The foregoing Examples illustrate that by employing the
coating of the invention a minimum of 50 reuse cycles were ob-
tained without galling and with minimum wear at the thread inter-
face.
Although the invention has been described by reference
to some embodiments it is not intended that the invention be
limited thereby but that modifications thereof are intended to be
included within the spirit and broad scope of the foregoing dis-
closure and the following claims.
