Fineran, Charles A. (East Orange, NJ)
Silwones, Steven S. (New Providence, NJ)

05/435889

10/21/1975

01/23/1974

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Amerace Corporation (New York, NY)

508/103

428/677, 411/929, 411/930, 411/914, 428/623, 411/548, 411/272, 428/685, 411/947, 428/679, 428/624

F16B33/06; F16B37/04; F16B39/284; F16B39/34; F16B33/00; F16B39/00; C10M5/00; C10M7/00

252/12,12.2,12.4,12.6 29/195C,195Y 85/1C 151/7,14R,14.5

3380843	Bearing	April 1968	Davis
3634129	METHOD OF LUBRICATING METAL FASTENERS WITH AN AQUEOUS LUBRICANT COMPOSITION	January 1972	Benz
3705450	BEARING MATERIAL	December 1972	Morisaki
3729292	ANTI-FRICTION BEARING PAD	April 1973	Heck
3791970		February 1974	Tubb
3793195	COATED BEARING SURFACES	February 1974	Betts

Gantz, Delbert E.

Vaughn I.

Craig, Richard Bender Michael A. S.

What is claimed is

1. A coating for reducing the wear of two opposed stainless steel metallic surfaces disposed in sliding relationship with one another comprising:

2. The coating of claim 1 wherein:

3. A coating for reducing the wear of two opposed stainless steel metallic surfaces disposed in sliding relationship with one another comprising:

4. The coating of claim 3 where said first layer metal comprises nickel and said second layer metal comprises copper,

5. The coating of claim 4 where said organic lubricant is selected from at least one member of the group consisting of aliphatic alcohols and aliphatic acids having from about 10 to about 20 carbon atoms and the glycerides of such acids.

6. A fastener assembly comprising first and second slidably threadably engageable stainless steel members and lubricant structure therefor including a first layer comprised of a metal having a modulus of elasticity inclusive of and greater than about 20,000,000 p.s.i. disposed on said first member, a second layer comprised of a metal having a modulus of elasticity inclusive of and lower than about 17,000,000 p.s.i. disposed on said second member and a third layer comprised of an inorganic lubricant disposed on said second layer.

7. The fastener assembly claimed in claim 6 wherein said first layer metal is selected from a member of the group consisting of manganese, cobalt, platinum, nickel, chromium, beryllium, rhodium, molybdenum, tungsten, ruthenium, iridium, and alloys thereof and wherein said second layer metal is selected from a member of the group consisting of palladium, copper, gold, zirconium, silver, cadmium, tin, lead and alloys thereof.

8. The fastener assembly claimed in claim 7 wherein said inorganic lubricant is selected from at least one member of the group consisting of molybdenum disulfide and graphite.

9. The fastener assembly claimed in claim 8 wherein said inorganic lubricant comprises molybdenum disulfide.

10. The fastener assembly claimed in claim 8 wherein said first layer metal comprises nickel.

11. The fastener assembly claimed in claim 9 wherein said second layer metal comprises copper.

12. The fastener assembly claimed in claim 9 wherein said first layer metal comprises nickel and wherein said second layer metal comprises copper.

13. A fastener assembly comprising first and second stainless steel slidably threadably engageable members and lubricant structure therefor disposed between said first and second members and including a first layer comprised of a metal having a modulus of elasticity inclusive of and greater than about 20,000,000 p.s.i., a second layer comprised of a metal having a modulus of elasticity inclusive of and lower than about 17,000,000 p.s.i. disposed on said first layer, a third layer comprised of an inorganic lubricant disposed on said second layer and a final layer comprised of an organic lubricant.

14. The fastener assembly claimed in claim 13 wherein said first layer metal is selected from a member of the group consisting of manganese, cobalt, platinum, nickel, chromium, beryllium, rhodium, molybdenum, tungsten, ruthenium, iridium, and alloys thereof and wherein said second layer metal is selected from a member of the group consisting of palladium, copper, gold, zirconium, silver, cadmium, tin, lead and alloys thereof.

15. The fastener assembly claimed in claim 14 wherein said final layer is disposed on said third layer.

16. The fastener assembly claimed in claim 14 wherein said inorganic lubricant is selected from at least one member of the group consisting of molybdenum disulfide and graphite.

17. The fastener assembly claimed in claim 16 wherein said inorganic lubricant comprises molybdenum disulfide.

18. The fastener assembly claimed in claim 14 wherein said organic lubricant is selected from at least one member of the group consisting of aliphatic alcohols and aliphatic acids having from about 10 to about 20 carbon atoms and the glycerides of such acids.

19. The fastener assembly claimed in claim 17 wherein said first layer metal comprises nickel.

20. The fastener assembly claimed in claim 17 wherein said second layer metal comprises copper.

21. The fastener assembly claimed in claim 17 wherein said first layer metal comprises nickel and wherein said second layer metal comprises copper.

22. The fastener assembly claimed in claim 21 wherein said organic lubricant is selected from at least one member of the group consisting of aliphatic alcohols and aliphatic acids having from about 10 to about 20 carbon atoms and the glycerides of such acids.

23. The fastener assembly claimed in claim 21 wherein said organic lubricant comprises cetyl alcohol.

BRIEF SUMMARY OF THE INVENTION

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 lubricant comprises molybdenum disulfide, 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. Automotive lubricants such as motor oils and greases may also be used as well as the art known silicone oils and silicone greases.

It has been 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.

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-286 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 assembled and reassembled without galling for 50 or more reuse cycles.

BRIEF DESCRIPTION OF THE DRAWINGS

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 accordance with the present invention;

FIG. 3 illustrates a frontal 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.

DETAILED DESCRIPTION OF THE INVENTION

A self-locking threaded fastener, e.g., a lock-nut, comprises a device having means built into a threaded nut to prevent the nut from unloosening during use in an environment where a high degree of vibration is encountered such as in internal combustion 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 operations 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 unsuitable 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 obtain optimum fastening. Examples of such applications are the fastening of access panels for electronic components in aircraft or access panels in turbojet engine assemblies where ordinary maintenance requires regular removal of the panels for replacement or overhaul of components. As can be readily appreciated there would be a considerable advantage in being able to extend the reuse life of lock-nut and bolt fasteners in such applications.

The means built in to the lock-nut to prevent the nut from unloosening 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 before 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 unloosening 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 employed for manufacturing lock-nuts that are to be used in high temperature applications since polymeric materials tend to decompose upon prolonged exposure to temperatures in excess of 500°F. Either of these friction means (i.e., offsetting or the provision 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 re-use cycles of a lock-nut and bolt assembly means, there must be provided not only means that would prevent excessive 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 re-use 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, for instance, pp. 122 and 133 of Fasteneering Catalog L copyright 1947 of the Central Screw Company, 3501 Shields Avenue, Chicago, Ill., U.S. Pat. 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 Pats. No. 341,923, Parsons; No. 811,573; and Canadian Pat. 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 provide 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 provide improved fastener assemblies having slidably engageable members 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 between the threads of the lock-nut and the threads of the bolt.

According to the invention a coating is provided for reducing the wear of two opposable metallic surfaces comprising a first metal layer; a second metal layer; an inorganic lubricant; and, an organic lubricant.

The aforementioned coating layers are applied to the surfaces of the metal in such a fashion so that the 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 first layer 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 lubricant is placed on the second metal layer. The organic lubricant is then employed as a final layer by which it is meant that the final layer is placed on either the aforementioned uncoated metallic surface or coated surface or both.

The first 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 includes a member selected from the group consisting of the following metals, the elastic modulus of each in p.s.i. unit being given parenthetically after such metals; platinum (21,000,000); manganese (23,000,000); cobalt (30,000,000); nickel (30,000,000); chromium (36,000,000); beryllium (37,000,000); tungsten (50,000,000); 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 parenthetically 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 (6,000,000); lead (2,600,000) and alloys thereof 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 polyethylene, polypropylene and the like. The preferred organic lubricants comprise lauric acids, cetyl alcohol, or lauric triglycerides 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. Petroleum based lubricants may also be used such as automotive motor oils 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 having coated threads 12 which engage coated threads 14 of a locknut 16 having a Vespel polymeric washer 18, which also engages coated threads 12, and a protective outer washer 20. The nutbolt assembly provides securement for such as plates 10a and 10b. The coating 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 described in FIG. 1 can be used in a minimum of 50 on-off reuse cycles in a fastening operation where applied torque is sufficient to generate the bolt tension desired 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., exhibiting 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. The 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 lubricant 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 assemblied on plain uncoated nuts of the same material, can be used through 50 reuse cycles without undue galling. Also, if the mating nut is 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 sidewalls 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 comprising 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 lubricant comprising molybdenum disulfide and said inorganic lubricant comprising molybdenum disulfide in turn being coated with an organic lubricant 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 reuse cycles without undue galling or loss of friction locking torque.

FIG. 4 illustrates two opposable sliding metallic surfaces 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.

FIG. 5 illustrates a set of two opposable metallic surfaces 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 having a second metal layer 56 comprising copper coated thereon, said second metal layer 56 having an inorganic lubricant 58 comprising molybdenum disulfide 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 fastener, 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 repeated 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 electroplating 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 reduction, 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 Handbook, austentitic, martensitic and ferritic types as defined in Metals Handbood, 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 Waspaloy (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 chipping 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 thread 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 layer 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 0.0001 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 not cause any difficulty.

When the coating of this invention is applied to a lock-nut and/or bolt, the lock-nut can be a lock-nut having a polymeric washer sealed into the nut or a nut that has been offset.

The coating may be subjected to elevated temperature up to 600°F and still effectively function as a coating that will prevent excessive wear when applied to a lock-nut and/or bolt exposed to such temperature. Also, sufficient friction is maintained between the locking element of the lock-nut and bolt so that the nut will not loosen when exposed to conditions of 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 combinations.

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, Calif.), 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% toluene. The molybdenum disulfide coating is applied to the metal part by dipping and is then cured by baking at about 400°F for one hour.

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 coating the parts are again rinsed and dried. 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 applied 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 1 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 untreated.

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 21/2 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 developed between the nut and bolt. When the head of the bolt is screwed down into an abutting relationship with the housing surface 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 measurement is made with a torque wrench. A power wrench is then employed 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 wrench. After every five off-on reuse cycles with the power wrench, the maximum on torque and the 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 obtained without galling and with minimum wear at the thread interface.

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 disclosure and the following claims.