ANTI-FRETTING ADDITIVES FOR NON-LUBRICATED CONTACT SURFACES

Abstract

An anti-fretting rust preventative solution includes a rust preventative fluid and an anti-fretting additive dissolved in the rust preventative fluid. The anti-fretting additive includes at least one compound that is surface-active with steel to produce a low-shear velocity accommodation layer in a metal-to-metal interference fit. The anti-fretting rust preventative solution can be provided in a bearing assembly with metal-to-metal interference fits, as well as other applications where fretting wear may otherwise occur.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application No. 61/614,364, filed Mar. 22, 2012, the entire contents of which are hereby incorporated by reference.

BACKGROUND

The present invention relates to treatments for non-lubricated contact surfaces such as “fixed” metal-to-metal joints in bearing assemblies, for example a shaft fitted into the bore of an inner ring, or the outer surface of an outer ring fitted into a housing. These “fixed” joints are typically press-fitted or shrink-fitted together and generally intended to eliminate movement therebetween. With small-bore size bearings, it is usually not a challenge to achieve adequate interference fits (sometimes referred to as tight fits) to prevent or inhibit fretting wear. However, achieving sufficient interference fits with large bore bearings can be significantly more difficult. Repetitive oscillatory movements of small amplitude can occur between the surfaces of the interference fit due to certain torque and/or vibration occurrences during operation of the bearing assembly. In these circumstances, fretting wear is initiated by adhesion and rupture of contacting asperities. Fretting wear can also be amplified by oxidation or corrosion and then manifest as abrasive wear. Thus, fretting wear is a common concern for interference fits for inner and outer rings in large bore bearings. Fretting wear can further result in seizure of components by corrosion and abrasion damage and/or by adhesion and welding between the parts.

Because the contact surfaces are not fitted with a running clearance and generally must not rotate relative to one another, commercially available greases or lubricants cannot be used to prevent the fretting wear. As such, these contact surfaces represent a “dry” friction situation. Typically, the only substance present in the metal-to-metal joint is a thin coating of rust preventative oil applied after manufacturing. However, the rust preventative oil does not have properties that significantly protect the surfaces from fretting wear damage. A standard practice used by some bearing manufacturers to reduce the risk of fretting wear of large bore bearings is to apply a coating of a low friction material, covering the metal bearing ring surface that is susceptible to the fretting wear. For example, a Teflon®-based coating or a chromium-based coating may be provided such that true metal-on-metal contact is avoided. Although these coatings are believed to be effective at reducing the risk of fretting wear, adding a coating to the bearing ring(s) is costly and adds complexity to the manufacturing process.

SUMMARY

In one aspect, the invention provides an anti-fretting rust preventative solution that includes a rust preventative fluid and an anti-fretting additive dissolved in the rust preventative fluid. The anti-fretting additive includes at least one compound that is surface-active with steel to produce a low-shear velocity accommodation layer in a metal-to-metal interference fit.

In another aspect, the invention provides a bearing assembly with anti-fretting properties. The bearing assembly includes a shaft, an inner ring secured to the shaft with a first interference fit, a housing including a bore, an outer ring secured in the bore of the housing with a second interference fit, and an anti-fretting rust preventative solution provided in at least the first and second interference fits. The anti-fretting rust preventative solution includes a rust preventative fluid and an anti-fretting additive dissolved in the rust preventative fluid. The anti-fretting additive including at least one compound that is surface-active with steel to produce a low-shear velocity accommodation layer.

Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates conformal fretting wear test friction results of 5 percent tri-isopropyl borate in a rust preventative oil.

FIG. 2 illustrates conformal fretting wear test temperature results of 5 percent tri-isopropyl borate in a rust preventative oil.

FIG. 3 is an illustrative summary of the conformal fretting testing of 5 percent tri-isopropyl borate in a rust preventative oil.

FIG. 4 is a perspective view of a conventional bearing assembly.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways.

FIG. 4 illustrates a typical bearing assembly 10 including an inner ring 12, a shaft 14 secured within a bore 12A of the inner ring 12, an outer ring 16, and a housing 18 having a bore 18A in which the outer ring 16 is secured. The inner ring 12 and the outer ring 16 have a low friction coupling therebetween, such as rolling elements (not shown). The bore 12A of the inner ring 12 has an interference fit with the shaft 14 to inhibit relative rotation between the inner ring 12 and the shaft 14. Likewise, the outer ring 16 has an interference fit with the bore 18A of the housing 18 to inhibit relative rotation between the outer ring 16 and the housing 18. The interference fits can be achieved by press-fitting or shrink-fitting. In either case, the interference fit between the inner ring 12 and the shaft 14 and the interference fit between the outer ring 16 and the housing 18 both result in metal-to-metal surface contact. In some constructions, the bearing assembly 10 can be a large size bearing assembly in which the bore 12A of the inner ring 12 is greater than about 200 mm. It should be understood that certain applications may require only one bearing ring, and thus a single metal-to-metal interference fit.

To prevent or inhibit corrosion on at least the inner and outer rings 12, 16, a rust preventative (RP) fluid is provided on the surfaces of the inner and outer rings 12, 16. The RP fluid may also be provided on one or both of the shaft 14 and the housing 18. The RP fluid can be a hydrocarbon-based fluid (or water-based rust preventative fluid) that wets the surfaces of the inner and outer rings 12, 16. Hydrocarbon-based fluids can include petroleum-based fluids (e.g., a thin oil with kerosene-like consistency) or non-petroleum-based fluids (e.g., vegetable or plant oils, synthetics, etc.) or water soluble oil or water-based synthetic rust preventative solution. Examples of suitable RP's include but are not limited to Quaker FERROCOTE® 5856 BF and Quaker FERROCOTE® 5856 BF T1. To provide an anti-fretting property to the metal-to-metal surface contacts, at least one additive is added to the RP to form a solution. Such additives can include at least one of a boron compound (e.g., boric acid, salts of boric acid such as sodium borate, a borate ester, a boronic ester, or a bonnie ester) and an extreme pressure (EP) additive (e.g., molybdenum dithiophosphate “MoDTP”). For example, the additive can include at least one tri-alkyl borate where the alkyl group is any straight or branched alkyl group similar to but not limited to ethyl, n-butyl, iso-butyl, tert-butyl, n-pentyl, iso-pentyl, etc. (e.g., tri-alkyl borates like tri-isopropyl borate or tri-propyl borate). Regardless of the exact type, the anti-fretting additive stays in solution in the RP fluid so that fretting wear protection is provided when at least one of the inner ring 12 and the shaft 14, which are assembled into the first interference fit, and at least one of the outer ring 16 and the housing 18, which are assembled into the second interference fit, are wetted with the RP. In some constructions, one or both of the inner ring 12 and the shaft 14 and one or both of the outer ring 16 and the housing 18 are steel, and the anti-fretting additive can be surface-active with steel (i.e., subject to a chemical reaction in a steel-on-steel concentrated contact) to produce a shear deformation layer, or “low-shear velocity accommodation layer”, such that when two surfaces move relative to each other, the sliding interface resides in the low-shear velocity accommodation layer. Low-shear velocity accommodation layers are discussed in detail in Y. Berthier, M. Godet & M. Brendle (1989): Velocity Accommodation in Friction, Tribology Transactions, 32:4, 490-496, which is incorporated by reference herein. For example, this reference describes in detail the differences between simple thick film lubricants, which operate only through fluid shear according to predictable fluid dynamic principles, and “dry” friction, which operates through numerous, more complex mechanisms to accommodate movement between two bodies or surfaces. In dry friction, an oil film may be present, but the mechanisms of velocity accommodation do not adhere to the simple fluid principle of shear alone. As used herein, “velocity accommodation layer” refers to a layer between two bodies or surfaces that enhances the ability for movement (i.e., non-zero velocity) therebetween. In some constructions, boron compounds dissolved in the RP fluid can be surface-active with steel to produce a velocity accommodation layer of boric acid. Also, MoDTP as an additive is surface-active with steel to produce a velocity accommodation layer of molybdenum disulfide MoS₂. Boron compounds such as tri-isopropyl borate and tri-propyl borate and some EP additives like MoDTP have been observed in the inventors' labs to function well as velocity accommodation layers in the kind of reciprocating contact that causes fretting. Experimental testing has shown this methodology to be extremely effective at inhibiting fretting wear without sacrificing the inherent corrosion protection of the RP.

Two methods of testing were used to determine the efficacy of the additives and also demonstrate the use. The first was the Fafnir Fretting Oxidation test (ASTM D-4170-97), which is a standard test for fretting wear. The second was an oscillating conformal fretting test that emulates a ring on shaft application. Although not limiting, the testing disclosed herein was conducted on four different solutions. The four solutions were created by combining (e.g., dissolving) one of two additives (tri-isopropyl borate and 2-ethylhexyl molybdenum dithiophosphate) as a solute at 5 percent by total volume with one of two RP fluids (FERROCOTE® 5856 BF T1 and FERROCOTE® 5856 BF) as a solvent. The FERROCOTE® 5856 BF T1 RP fluid is the same as or similar to the FERROCOTE® 5856 BF, further diluted with a solvent. The tests were replicated at least twice. The Fafnir test was run according to the ASTM procedure and the conformal fretting wear test was run according to the following parameters:

• • Clearance: ˜0.254 mm (0.010 inches)
  • Inner Ring Dia.: ˜49.23 mm (1.938 inches)
  • Outer Ring Bore: ˜49.48 mm (1.948 inches)
  • Contact Width: ˜13.07 mm (0.515 inches)
  • Load: ˜4480N (1000 lbs)
  • Contact Stress: ˜53 MPa (7.7 ksi)
  • Inner Ring Surface Finish: ˜μm (pin)
  • Outer Ring Surface Finish: ˜0.813 μm (32 pin)
  • Oil: 0.1 mL of FERROCOTE® 5856BF in the contact
  • Oscillation: 5 degrees at 13.3 Hz
  • Duration: 22 hours or when test is suspended by temperature (100 deg. C.) or friction (110 lbs).

The Fafnir test is most commonly used as a grease test so the preparation was modified to give testing conditions closer to that of the actual rust preventative conditions in the field. Samples were ultrasonically cleaned in Hexanes and Isopropanol. A thin layer of FERROCOTE® 5856 BF or FERROCOTE® 5856 BF plus 5 percent tri-isopropyl borate was applied to the surfaces of the inner and outer ring. All excess was wiped off or allowed to drip off (e.g., for 5 minutes). Once the inner ring was mounted on the Falex spindle, a 0.1 mL drop of the test solution was placed on the top of the test ring prior to mounting the outer ring. Multiple tests were conducted on each solution.

The Fafnir test measures weight loss. The larger the weight loss, the larger the fretting wear. The results are tabulated here.

TABLE 1
Average weight loss:	Baseline 5856BF T1	3.475 mg
	5% TIPB/5856BF T1	0.875 mg
	5% MoDTP/5856BF T1	−0.075 mg

	TABLE 2
	Average weight loss:	Baseline 5856BF	1.075 mg
		5% TIPB/5856BF	0.250 mg
		5% MoDTP/5856BF	0.450 mg

ASTM D-4170 is designed to test greases and the statistical evaluation in the documentation is correct for grease. This testing was performed on oil rust preventatives rather than grease, so we rely on statistical data drawn from the measurements. The range of values found for the studies with FERROCOTE® 5856BF T1 for the baseline sample was 1.8 mg. The range for the baseline sample in the FERROCOTE® 5856BF tests was 0.38 mg. The trend is similar in both tests that the additives did reduce fretting wear.

The result of the conformal fretting wear test is a time to the suspension of the test due to friction or temperature. When the test conditions reach the cutoff for friction or temperature the film protecting the surfaces from fretting has failed. The longer the film lasts, the better the protection. The results are summarized in Table 3 below.

TABLE 3
Falex Test Results - Time to 65 lbs Friction Cutoff
		Time (hrs)	Time (hrs)
	Solution	Test 1	Test 2
	RP Baseline	2.1	2.0
	RP w/5% TIPB	10.5	8.9
	RP w/5% MoDTP	11.4	10.5

Since the additized RP solution is intended to be first and foremost a corrosion inhibitor, the rust preventative properties of the additized RP solution were tested. A long-term humidity cabinet test was also conducted according to ASTM 1748. Samples coated with a solution of 5 percent by volume tri-isopropyl borate in Quaker FERROCOTE® 5856 T1 did not display corrosion after four months exposure to 120 degrees Fahrenheit at 90 percent relative humidity. Thus, it was determined that the anti-fretting additive had no detrimental effect on the corrosion prevention ability of the RP fluid itself Other additives, such as the MoDTP, are lubricant additives and because of their use in bearings are understood to have no ill effect on the corrosion protection of an RP fluid.

RP fluids additized as described herein can be direct replacements for RP fluids already in use. The amount of the additives in an RP solution can be about, or at least about, 0.1, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 percent by total volume (v/v), and about or less than about, 90, 80, 70, 60, 50, 40, 30, 20, or 10 percent by total volume (v/v), but other amounts can also be effective. In some constructions, the amount can be between about 1 percent and about 10 percent by total volume (v/v), and more particularly, can be about 5 percent by total volume (v/v) in some constructions. Although there is not necessarily an upper limit, at very high concentrations of additive, the rust preventing properties of the base RP fluid may begin to be sacrificed. In other words, the additives will not cause corrosion, but there should be a minimum amount (e.g., about, or at least about, 10, 20, 30, 40, 50, 60, 70, 80 or 90 percent) of the base RP fluid necessary to perform the basic function of inhibiting corrosion. The additized RP solution can be applied in any manner as RP fluids are typically applied. In some constructions, the parts are dipped and allowed to drain. Although FERROCOTE® 5856BF and FERROCOTE® 5856BF T1 are given as examples herein, there is not necessarily a preference between the two, and those of ordinary skill in the art will realize that other known RP fluids can also be modified into solutions with anti-fretting additives as described herein.

Another aspect of equal importance is that false brinelling, or fretting can occur between the rolling elements and the raceways of a bearing prior to installation in the application. An anti-fretting RP solution as described herein can also reduce the risk of this type of wear occurring during shipping and storage, for example. The fretting wear prevention due to the additives in RP solutions as described herein may also be used to mitigate fretting wear in other situations besides interference fits of bearing rings, and in metal components besides bearings altogether. Shipping of metallic parts often requires some form of fretting wear protection—either a lubricant such as grease or a physical barrier like plastic or cardboard. The fretting wear protection from an anti-fretting RP solution as described herein can be great enough to be used alone in place of these other forms of fretting wear protection at a lower cost.

Claims

1. An anti-fretting rust preventative solution comprising:

a non-lubricating rust preventative fluid; and

an anti-fretting additive dissolved in the rust preventative fluid, the anti-fretting additive including at least one compound that is surface-active with steel to produce a low-shear velocity accommodation layer which operates under dry friction principles in a metal-to-metal interference fit.

2. The anti-fretting rust preventative solution of claim 1, wherein the rust preventative fluid is a hydrocarbon-based fluid.

3. The anti-fretting rust preventative solution of claim 1, wherein the at least one compound is surface-active with steel to produce a low-shear velocity accommodation layer of boric acid.

4. The anti-fretting rust preventative solution of claim 3, wherein the compound includes at least one of a borate ester, a boronic ester, and a borinic ester.

5. The anti-fretting rust preventative solution of claim 3, wherein the at least one compound includes tri-alkyl borate where the alkyl group is one of an isopropyl group and an n-propyl group.

6. The anti-fretting rust preventative solution of claim 1, wherein the at least one compound is surface-active with steel to produce a low-shear velocity accommodation layer of molybdenum disulfide.

7. The anti-fretting rust preventative solution of claim 6, wherein the at least one compound includes molybdenum dithiophosphate.

8. The anti-fretting rust preventative solution of claim 1, wherein the amount of the anti-fretting additive is at least 0.5 percent and less than 50 percent by volume.

9. The anti-fretting rust preventative solution of claim 1, wherein the amount of the anti-fretting additive is between about 1 percent and about 10 percent by volume.

10. The anti-fretting rust preventative solution of claim 1, wherein the amount of the anti-fretting additive is about 5 percent by volume.

11. A bearing assembly with anti-fretting properties, the bearing assembly comprising:

a ring secured to a mating component with a non-lubricated metal-to-metal interference fit;

an anti-fretting rust preventative solution provided in at least the interference fit, the anti-fretting rust preventative solution including a non-lubricating rust preventative fluid and an anti-fretting additive dissolved in the rust preventative fluid, the anti-fretting additive including at least one compound that is surface-active with steel to produce a low-shear velocity accommodation layer which operates under dry friction principles.

12. The bearing assembly of claim 11, wherein the rust preventative fluid is a hydrocarbon-based fluid.

13. The bearing assembly of claim 11, wherein the at least one compound is surface-active with steel to produce a low-shear velocity accommodation layer of boric acid.

14. The bearing assembly of claim 13, wherein the at least one compound includes at least one of a borate ester, a boronic ester, and a borinic ester.

15. The bearing assembly of claim 13, wherein the at least one compound includes tri-alkyl borate where the alkyl group is one of an isopropyl group and an n-propyl group.

16. The bearing assembly of claim 11, wherein the at least one compound is surface-active with steel to produce a low-shear velocity accommodation layer of molybdenum disulfide.

17. The bearing assembly of claim 16, wherein the at least one compound includes molybdenum dithiophosphate.

18. The bearing assembly of claim 11, wherein the amount of the anti-fretting additive is at least 0.5 percent and less than 50 percent by volume.

19. The bearing assembly of claim 11, wherein the amount of the anti-fretting additive is between about 1 percent and about 10 percent by volume.

20. The bearing assembly of claim 11, wherein the amount of the anti-fretting additive is about 5 percent by volume.

21. The bearing assembly of claim 11, wherein the ring is an inner ring secured with the mating component, a shaft, with the interference fit, which is a first interference fit, and wherein the bearing assembly further comprises an outer ring secured in a bore of a housing with a second interference fit; and the anti-fretting rust preventative solution is further provided in the second interference fit.