Solid anti-friction devices – materials therefor – lubricant or se – Lubricants or separants for moving solid surfaces and... – Sulfurized compound of indeterminate structure – which is a...
Reexamination Certificate
1999-04-09
2001-02-06
McAvoy, Ellen M. (Department: 1764)
Solid anti-friction devices, materials therefor, lubricant or se
Lubricants or separants for moving solid surfaces and...
Sulfurized compound of indeterminate structure, which is a...
C508S459000, C508S537000
Reexamination Certificate
active
06184186
ABSTRACT:
The present invention relates to the use of friction modifiers to reduce micropitting of metal surfaces such as gear teeth, and to lubricant compositions comprising friction modifiers.
Micropitting is a type of surface damage which occurs predominantly in rolling-sliding contacts of hard steel surfaces. Sometimes called “frosting”, “greystaining” or “peeling” it typically occurs in rolling element bearings and most often on gear teeth, where it poses a significant practical problem. Micropitting may lead to higher noise, to significant rapid wear and to more serious surface damage, such as scuffing and even to tooth fracture in gears. Conventional lubricants are used to reduce friction when metal surfaces move in contact with each other but they do not prevent the occurrence of micropitting. Original equipment manufacturers require lubricants which can lead to a reduction in the amount of micropitting when compared with the conventional lubricants. It is an object of the present invention to meet this need.
The awareness of micropitting within the lubricant additives industry has increased significantly. A micropitting test has been established by the FZG Institute in Germany and is called the FZG micropitting test. This test is run on gears sets with the same metallurgy and surface profile/roughness as gears used in the field. The conditions of the test (high load/low speed) are the optimum conditions for micropitting to occur. Equipment manufacturers believe that the FZG micropitting test correlates well with field experience.
The FZG micropitting test is carried out using a standardized FZG test rig according to CEC L-07-A-71, with C type case hardened gears of minimum 0.4 Ra surface roughness. The test has a stepwise phase to investigate build up of micropitting and an endurance phase to investigate resistance to micropitting. The stepwise phase runs from load stage 5 to load stage 10, each stage lasting 16 hours. The profile of the gears is measured prior to testing and during the test. The variation from the original gear profile (the profile deviation) is calculated. Also evaluated are the percentage micropitting (the percentage of gear tooth which is micropitted) and the weight loss from the gears. After the stepwise phase the endurance phase is run for 80 hours at load stage 8 and then at load stage 10 until failure. Again, the deviation from the original profile (maximum 20 microns), the level of micropitting and the weight loss are measured. A result which would be particularly acceptable to the industry would be a pass at load stage 10 in the stepwise phase of the test. This corresponds to a profile deviation of less than 7.5 &mgr;m, micropitting of less than 15% (approx) and weight loss of less than 15 mg (approx) after load stage 10. Extended performance in the endurance phase is also desirable.
The present invention is based on the surprising appreciation that certain friction modifiers may be included in lubricant compositions with the result that an improvement in micropitting performance is observed when the lubricant compositions are used, i.e. there is reduced micropitting. Accordingly, the present invention concerns the use of at least one friction modifier to reduce micropitting of a metal surface, which comprises lubricating the metal surface with a lubricant composition comprising the at least one friction modifier, wherein the at least one friction modifier is selected such that micropitting is reduced when the metal surface is so lubricated.
The metal surface may be the surface of a gear tooth, in which case the at least one friction modifier may be added to a formulated gear lubricant composition.
In the present specification the term “friction modifier” is used to describe additive compounds which are conventionally used in lubricant compositions to reduce friction. The friction modifiers which are useful in practising the present invention are all known in the art.
In accordance with the present invention it has been found that only certain friction modifiers may be used to give the desired technical effect of reduced micropitting. The efficacy of any given friction modifier in reducing micropitting may be assessed by comparing the amount of micropitting observed when a metal surface is lubricated with a lubricant composition comprising the friction modifier with the amount of micropitting observed when an identical metal surface is lubricated (under the same conditions) using the corresponding lubricant composition from which the friction modifier of interest has been omitted. The FZG micropitting test may be used to assess the relative performance of lubricant compositions.
Another way of identifying suitable friction modifiers is by reference to the friction coefficient of lubricants including them. It has been found that the at least one friction modifier may be selected such that, when measured at 130° C. using a high frequency reciprocating rig (HFRR) under the conditions described in SAE Technical Paper 961142, a lubricant which comprises the friction modifier and which has a viscosity grade of ISO 220 has a coefficient of friction of 0.100 or less. The HFRR test may thus be employed as a screen for useful friction modifiers. Lubricant compositions which have a viscosity grade of ISO 220 and which are useful in screening friction modifiers may be prepared by blending a conventional sulphur- and phosphorus-containing gear additive package with a base oil having a viscosity of between 1.98×10
−4
to 2.42×10
−4
m
2
/s (198 to 242 cSt) at 40° C. Suitable additive packages include those comprising from 15-75 wt %, preferably from 45-65 wt %, of a sulfurized isobutylene, from 0-25 wt %, preferably from 3-15 wt %, of a phosphorus-containing antiwear agent, from 0-60 wt %, preferably from 5-25 wt % of a carboxylic-type or Mannich-type ashless dispersant from 0-20 wt %, preferably from 1-10 wt % of corrosion and rust inhibitors, from 0-20 wt %, preferably from 1-10 wt %, of surface active agents and diluent oil. Such additive packages are commercially available. The additive package is used at conventional treat rates. A suitable base oil to use in formulating the compositions includes a blend of 51 wt % ESSO 600SN and 49 wt % of 2500 Brightstock. Useful additive package are described in EP-A-0744456 and EP-A-0812901.
A number of different classes of friction modifiers have found to be useful in the present invention. Mention may be made of phosphonate esters, phosphite esters, aliphatic succinimides, molybdenum compounds and acid amides.
Useful phosphonate esters include O,O-di-(primary alkyl)acyclic hydrocarbyl phosphonates in which the primary alkyl groups are the same or different each independently containing 1 to 4 carbon atoms and in which the acyclic hydrocarbyl group bonded to the phosphorus atom contains 12 to 24 carbon atoms and is a linear hydrocarbyl group free of acetylenic unsaturation. These compounds thus comprise O,O-dimethyl hydrocarbyl phosphonates, O,O-diethyl hydrocarbyl phosphonates, O,O-dipropyl hydrocarbyl phosphonates, O,O-dibutyl hydrocarbyl phosphonates, O,O-diiso-butyl hydrocarbyl phosphonates, and analogous compounds in which the two alkyl groups differ, such as, for example, O-ethyl-O-methyl hydrocarbyl phosphonates, O-butyl-O-propyl hydrocarbyl phosphonates, and O-butyl-O-isobutyl hydrocarbyl phosphonates, wherein in each case the hydrocarbyl group is linear and is saturated or contains one or more olefinic double bonds, each double bond preferably being an internal double bond. Preferred are compounds in which both O,O-alkyl groups are identical to each other. Also preferred are compounds in which the hydrocarbyl group bonded to the phosphorus atom contains 16 to 20 carbon atoms. A preferred friction modifier in this class is dimethyloctadecyl phosphonate. Phosphonate esters useful in the present invention are described in U.S. Pat. No. 4,158,633.
Useful phosphite esters are described in WO88/04313. These include dihydrocarbyl hydrogen phosphites in which the hydrocarbyl groups are the same o
Barber Rodney I.
Paterson Craig R.
Ryan Helen T.
Ethyl Petroleum Additives, Ltd
Hamilton Thomas
McAvoy Ellen M.
Moore James T.
Rainear Dennis H.
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