This invention relates to a lubricating oil for use in rolling contact or rolling and sliding contact systems such as roller bearings and gears, and in particular it relates to a lubricating oil for use in rolling contact or rolling and sliding contact systems where a load (weight) is applied.

Japanese Laid-open Patent 2008-179669 proposes, as a lubricating oil that can be used for example in the bearings of high-speed main spindles having ceramic ball roller-bearings run in harsh environments of high speeds and large loads, a lubricating oil composition for ceramic lubrication wherein the base oil, being at least one kind of oil selected from mineral oils and/or synthetic oils, contains at least one kind of additive selected from the group consisting of acid amides obtained by reacting amines with saturated monocarboxylic acids of 12 to 30 carbons or unsaturated monocarboxylic acids of 18 to 24 carbon, sarcosinic acids, or aspartic acid derivatives. This lubricating oil composition is an excellent lubricating oil composition exhibiting satisfactory cooling characteristics and having high rust prevention properties, a high level of thermal oxidation stability and high extreme pressure properties, even when used for high-speed main spindles with ceramic ball roller- bearings in machine tools and the like, which are operated in a harsh environment of high speeds and large loads, but the emphasis has been placed on the combination of additives. A difficulty has been that, in order to obtain superior lubrication performance as appropriate to changes in the conditions of use, it is necessary to vary the combination of additives and to respond to the changing conditions of use through specific combinations of additives. The use of a base oil with superior lubrication performance through the base oil itself has therefore been investigated.

In Tribologist, Vol. 53, No. 10, page 653 it has been shown that a lubricating oil which forms an EHL (Elasto-Hydrodynamic Lubrication) oil film and so prevents interference between protuberances on sliding surfaces can be used as a lubricating oil for use in rolling contact or rolling-sliding contact systems such as roller bearings or gears, and especially as a lubricating oil for use in rolling contact or rolling- sliding contact systems under a load (weight) . According to this article, the important elements in a lubricating oil which forms an EHL oil film are the minimum oil film thickness in line contact and the pressure-viscosity coefficient. The minimum oil film thickness is the minimum oil film thickness of the line contact gap, and so is the minimum thickness of the film of oil that is present in the line contact gap. It signifies the minimum condition for maintaining lubrication. The pressure- viscosity coefficient is a coefficient showing the relationship between the pressure applied in the contact system and the viscosity of the lubricating oil. It is the numerical value expressed by α in the Hamrock-Dowson formula, and the larger the value the more the viscosity increases as the pressure increases. It shows a trend whereby the oil film thickness is maintained according to the elasticity. In the above article it is assumed that, in a lubricating oil forming such an EHL oil film, a base oil having a chemical structure in which the viscosity- increase under high pressures is large (and so having a large pressure-viscosity coefficient) is advantageous for fatigue life, and there have been reports that naphthene- based mineral oils which have a large high-pressure viscosity and traction coefficient and a low viscosity index have a better fatigue life than paraffin-based mineral oils. However, there have been reports that high viscosity index mineral oils also have a better fatigue life than paraffin-based mineral oils, and so the evaluations of the various base oils have diverged.

In Journal of Lubrication Technology, Transactions of ASME, 99 (Apr.), 264 (1977) it is disclosed that a lubricating oil which forms an EHL (elasto-hydrodynarnic lubrication) oil film plays a role in preventing interference between protuberances on sliding surfaces in roller bearings, and the Hamrock-Dowson formula relating to point contact minimum oil thickness (Hmin: dimensionless minimum oil film thickness) and central oil film thickness (Hc: dimensionless central oil film thickness) is shown.

The theme of this invention is to resolve the aforementioned problems of the prior art by offering, as a lubricating oil for use in rolling contact or rolling and sliding contact systems such as roller bearings and gears, and in particular a lubricating oil for use in rolling contact or rolling and sliding contact systems where a load (weight) is applied, a lubricating oil which has a large minimum oil film thickness, a high pressure- viscosity coefficient and a large pressure-velocity product (PV value) . This invention relates to the following lubricating oils .

(1} Lubricating oil .containing a base oil in which the naphthene component (%C _N ) is from 30 to 60, the aromatic component (%C _A ) is less than 10, and the remaining portion is a paraffin component (%C _P ) .

(2) Lubricating oil containing a base oil in which the naphthene component (%C _N ) is from 38 to 54, the aromatic component (%C _a ) is less than 10, and the remaining portion is a paraffin component (%Cp) .

(3) Lubricating oil in accordance with the aforementioned (1) or (2) wherein the base oil has a central oil film thickness at 80 ⁰ C, measured by means of an optical-type EHL oil film thickness measuring device, of not less than 130 nm.

(4) Lubricating oil in accordance with any of the aforementioned (1) to (3) wherein the base oil has a pressure-viscosity coefficient (average) at 80 ⁰ C, calculated from the central oil film thickness measured by means of an optical type EHL oil film thickness measuring device, of not less than 9.0 GPa ^"1 .

(5) Lubricating oil in accordance with any of the aforementioned (1) to (4) wherein the base oil is a hydrorefined naphthene-based base oil. (6) Lubricating oil in accordance with any of the aforementioned (1} to (5) wherein the lubricating oil is a lubricating oil used in rolling contact or rolling and sliding contact systems.

The lubricating oil forming the subject of this invention is a lubricating oil for use in rolling contact or rolling and sliding contact systems such as roller bearings and gears, and in particular a lubricating oil for use in rolling contact or rolling and sliding contact ""* Z) *"*

systems where a load (weight) is applied. The elements subject to lubrication in the spindles, bearing materials and bearing parts which constitute the rolling contact or rolling-sliding contact systems are lubricated elements comprised of materials such as the steels and ceramics generally used in rolling contact or rolling-sliding contact systems such as roller bearings and gears.

The base oil used in this invention is a base oil in which the naphthene component (%C _M ) is from 30 to 60, the aromatic component (%C _A ) is less than 10, and the remaining portion is a paraffin component (%Cp), but preferably is a base oil in which the naphthene component (%C _S ) is from 38 to 54, the aromatic component (%C _A ) is less than 10, and the remaining portion is a paraffin component (%C _P ) . "%C _H " is the proportion of naphthene- based constituent carbon in accordance with the n-d™M of ASTM D-3238 (ring analysis) . "%C _A " is similarly the proportion of aromatics-based constituent carbon. "%C _P " is similarly the proportion of paraffin-based constituent carbon and is calculated from the following formula (I) .

[%C _P ] = 100 - ([%C _A ] + [%C _H ]) ... (I)

The hydrocarbon composition of the usual naphthene- based base oils is a naphthene component (%C _H ) of from 30 to 50, an aromatic component (%C _R ) of from 10 to 20 and a paraffin component (%C _P ) of from 35 to 50. Paraffin-based base oils have a naphthene component (%C _N ) of from 20 to 35, an aromatics component (%C _A ) of from 0 to 10 and a paraffin component (%C _P ) of from 60 to 70. The base oil composition of this invention is a base oil having a hydrocarbon composition midway between these. Because the base oil of this invention increases the oil-film forming properties and the pressure-viscosity coefficient, it is therefore exhibiting the fact that it is something different from the general theory of the prior art which claims that aromatics-based and naphthene-based oils are better.

It is possible to use for the base oil of this invention those of the aforementioned composition from base oils used as the base oils of lubricating oils. There is no restriction as to origin, refining method or the like. The base oils that can be used are the mineral oils known as highly refined base oils and synthetic oils. Since the base oils that belong to API (American Petroleum Institute) base oil categories of Group 1, Group 2, Group 3, Group 4 and Group 5 may or may not fall within the aforementioned ranges of composition, it is possible to select one kind alone from the base oils belonging thereto or a mixture of several kinds for use as the base oil of this invention.

Good base oils for use in this invention are those with a density of from 0.80 to 0.95 g/cm ³ , but preferably from 0.85 to 0.93 g/cm ³ . Those with a kinematic viscosity (40 ⁰ C) of from 22 to 100 mm ² /s, but preferably from 22 to 68 mm ² /s, a number average molecular weight of from 300 to 550 but preferably from 320 to 480, and a kinematic viscosity (100 ⁰ C) of from 4 to 200 mmVs but preferably from 5 to 8 mm ² /s are suitable. The viscosity index may be selected freely according to the objective, but will generally be from 40 to 160 and preferably from 80 to 130. Particularly suitable as base oils for use in this invention are those in which the central oil film thickness at 80 ⁰ C, measured by means of an optical type EHL oil film thickness measuring device, is not less than 130 nm, and preferably not less than 150 nm. The method of measuring the central oil film thickness is the method described later.

Suitable bases oil for use in this invention are those as used in lubricating oils for high-speed main spindles which have a pressure-viscosity coefficient (average) at 80 ⁰ C, calculated from the central oil film thickness measured by means of an optical type EHL oil film thickness measuring device, of not less than 9.0 GPa ^'1 , and preferably not less than 9.5 GPa ^"1 , and so can increase the pressure-viscosity coefficient and increase the pressure-velocity product (PV value) . The method of calculating the pressure-viscosity coefficient is the method described later. The important factor which influences lubrication properties is the "minimum oil film thickness (Hmin)" formed on the lubrication surface. There are several methods for measuring the oil film thickness, and the measured values which can be measured are the "minimum oil film thickness {Hmin)", the "central oil film thickness (Hc)" and so on. Of these, the "minimum oil film thickness (Hmin)" is the oil film thickness of the area where the oil film formed on the lubrication area is the minimum thickness, and a procedure is necessary to find the area of minimum thickness from data obtained by means of measurements. In contrast, the "central oil film thickness (Hc)" is the oil film thickness obtained as is from data for the central area of ball contact. The procedure is simpler and measurements can be taken in a shorter time. As described in Journal of Lubrication

Technology, Transactions of ASME, 99 (Apr.), 264 (1977) (page 274), Hmin and Hc are expressed by approximation formulas and have almost a proportional relationship, so that there is basically no difference whether properties are determined by either Hmin or Hc. For this reason, in this invention the readily measurable "central oil film thickness (Hc)" is measured as an indicator for the "minimum oil film thickness (Hmin)", and the characteristics of the base oils and lubricating oils are expressed by means of the "central oil film thickness (Hc)".

The method of measuring the oil film thickness adopted in this invention is the method of computing the EHL oil film thickness by means of optical interferometry. The basic principles of the measurements are as follows.

Part of the white light which is radiated onto the leading edge (centre) of a contact steel ball from above a glass disk in point contact with the steel ball is reflected back by a chromium film which is coated on the disk, and the rest of the light travels through a silica layer and the oil film, and returns by reflecting on the steel ball. The interference stripes thereby produced are taken to a computer via a spectrometer and a high- resolution CCD camera, and the oil film thickness is thus computed.

The film thickness obtained in this method of measurement is the thickness of the centre of the contact area (central oil film thickness) , and consequently the "pressure-viscosity coefficient" is calculated from Formula (IV) and Formula (V) described below.

Suitable base oils for use in this invention as a base oil for use in lubricating oils for high-speed main spindles are those in which the PV value calculated from the maximum load (P) and the maximum number of rotations (V) in the undermentioned Formula (II) as obtained in Shell 4-ball extreme pressure tests using ceramic balls is not less than 40 x 10 ⁴ and preferably not less than 50 x 10 ⁴ . The method of calculating the PV value is described below.

PV value - (P) x (V) ... (II)

As preferred instances for the base oil used in this invention, mention may be made of highly refined naphthene-based base oils. In general, instances with a naphthene component (%C _N ) of from 30 to 50 are called naphthene-based base oils, but for the highly refined naphthene-based base oils used in this invention it is possible to use those which are naphthene-based base oils which are further refined and so have the naphthene component (%C _N ) and the aromatics component (%C _A ) adjusted to within the previously mentioned ranges. The method of refining is one which has as its objective not only removal of the sulphur component and other impurities but also the cracking and removal of the aromatics component. There are situations where solvent refining and so on will do, but hydrorefining is preferred. It is preferable if the hydrorefining goes through stages of hydrocracking, vacuum distillation, solvent dewaxing and hydrofinishing.

Hydrorefined naphthene-based base oils are those with a lowered %C _A , by virtue of the hydrorefining. As the %C _H , %C _Λ and %C _P of such hydrorefined naphthene-based base oils fall within the aforementioned ranges, it is preferable to use base oils of such composition as the base oils of this invention. Base oils where the %C _N , %C _A and %C _P fall within the aforementioned ranges as in the aforementioned hydrorefined naphthene-based base oils are used in an amount such that they form the main constituent as material for the lubricating oil of this invention. The blend proportion of the aforementioned base oils in the lubricating oil of this invention is not specially limited. They are used in the proportion of being the rest after incorporating the amounts of the various additive components described below, but it is desirable if the blend proportion on the basis of the total amount of the lubricating oil is from 70 to 90% by weight and preferably from 75 to 85% by weight.

In the aforementioned base oils of this invention, the minimum oil film thickness is large, the pressure- viscosity coefficient is high and the pressure-velocity product is large, so that it is possible to make up a lubricating oil with the aforementioned base oils alone, but in order to improve its performance and characteristics as a lubricating oil, or with objectives such as extending its life, it is possible to blend lubricating oil additives with the aforementioned base oils and so use them as a lubricating oil composition. In this case, it is preferable if the blending is done so that the minimum oil film thickness, pressure-viscosity coefficient and pressure-velocity product (PV value) of the lubricating oil composition blended with the additives will fall within the aforementioned ranges. For the aforementioned lubricating oil additives capable of being blended in lubricating oils compositions of this invention it is possible to use the lubricating oil additives generally used as additives for use in lubricating oils. For example mention may be made of ordinary extreme pressure agents, anti-oxidants, metal deactivators, oiliness improvers, defoamers, pour point depressants, viscosity index improvers, rust inhibitors, demulsifiers and other known lubricating oil additives. Phosphorus compounds may be added as the extreme pressure agents to be added to the lubricating oil of this invention. These can impart further wear resistance and extreme pressure properties. As examples of phosphorus compounds suitable for this invention mention may be made of phosphate esters, acidic phosphate esters, amine salts of acidic phosphate esters, phosphite esters, phosphorothionates, zinc dithiophosphates, phosphorus- containing carboxylic acids and phosphorus-containing σarboxylate esters, but in particular phosphorylated carboxylic acids or phosphorylated carboxylate esters are preferred. As an example of phosphorylated carboxylic acids mention may be made of β-dithiophosphorylated propionic acids.

As examples of the anti-oxidants that may be used in this invention mention may made of amine-based antioxidants, phenol-based anti-oxidants, sulphur-based antioxidants and phosphorus-based anti-oxidants. These antioxidants may be used as they are in the forms used in practice in normal lubricating oils. These anti-oxidants may be used alone or in plural combinations in the range 0.01 to 5% by weight in terms of the total amount of the lubricating oil.

As examples of the metal deactivators that may be used in this invention mention may made of benzotriazole derivatives, benzoimidazole derivatives, benzothiazole derivatives, benzooxazole derivatives, thiadiazole derivatives and triazole derivatives. These metal deactivators may be used alone or in plural combinations in the range 0.01 to 0.5% by weight in terms of the total amount of the lubricating oil.

As examples of oiliness improvers that may be used in this invention, it is possible for example to blend in fatty acid esters of polyhydric alcohols. For example, it is possible to use partial or complete 1 to 24-carbon saturated or unsaturated fatty acid esters of polyhydric alcohols such as glycerols, sorbitols, alkylene glycols, neopentyl glycols, trimethylolpropanes, pentaerythritols and xylitols. These oiliness improvers may be used alone or in plural combinations in the range 0.01 to 5% by weight in terms of the total amount of the lubricating oil.

As examples of defoaming agents that may be used to impart defoaming characteristics in this invention, mention may be made of organosilicates such as dimethylpolysiloxanes, diethyl silicates and fluorosilicones and non-silicone-based defoaming agents such as polyalkylacrylates. These defoaming agents may be used alone or in plural combinations in the range 0.0001 to 0.1% by weight in terms of the total amount of the lubricating oil.

In this invention, it is possible to add pour point depressants and viscosity index improvers in order to improve the low-temperature flow characteristics or the viscosity-temperature characteristics. As examples of the pour point depressants that can be used mention may be made of polymethacrylate-based polymers. As to the amount added, they may be used alone or in plural combinations in the range 0.01 to 5% by weight in terms of the total amount of the lubricating oil. In polymethacrylates used as pour point depressants the average molecular weight is normally of the order of 100,000 and the molecular weight distribution is small. The effect in improving the pour point varies according to the length of the side-chain alkyl groups, so that in a base oil with a high pour point, long side chains will be effective and in base oils with a low pour point short side chains will be effective.

As examples of viscosity index improvers, mention may be made of non-dispersant viscosity index improvers as exemplified by polymethacrylates and olefin polymers such as ethylene-propylene copolymers, styrene-diene copolymers, polyisobutylene and polystyrene, and dispersant-type viscosity index improvers in which nitrogen-containing monomers are copolymerised with these. As to the amount added, they may be used in the range 0.05 to 20% by weight in terms of the total amount of the lubricating oil. The polymethacrylates used for viscosity index improvers have an extremely wide range of average molecular weights from 10,000 to 1,500,000, and as regards the molecular structure there are two types: the non-dispersant and the dispersant types. The dispersant type includes those which have polar groups at the terminals and thus impart oil-film forming characteristics and detergent-dispersant characteristics. Poly (meth) acrylates containing hydroxyl groups are preferred for the viscosity index improvers used in this invention. These poly (meth) acrylates containing hydroxyl groups are copolymers, and are copolymers in which the constituent monomers are alkyl (meth) acrylates having alkyl groups of 1 to 20 carbons and vinyl monomers containing hydroxyl groups.

As specific examples of the aforementioned alkyl (meth) acrylates (a) having alkyl groups with 1 to 20 carbons, mention may be made of {al) alkyl (meth) acrylates having alkyl groups with 1 to 4 carbons: for example, methyl (meth} acrylate, ethyl (meth) acrylate, n- or iso-propyl (meth) acrylate, n-, iso- or sec-butyl (meth) acrylate;

(a2) alkyl (meth) acrylates having alkyl groups with 8 to 20 carbons: for example, n-octyl (meth) acrylate, 2- ethylhexyl (meth) acrylate, n-decyl (meth) acrylate, n- isodecyl (meth) acrylate, n-undecyl (meth) acrylate, n~ dodecyl (meth} acrylate, 2-methylundecyl (meth) acrylate, n- tridecyl (meth) acrylate, 2-methyldodecyl (meth) acrylate, n- tetradecyl (meth) acrylate, 2- methyltridecyl (meth) acrylate, n-pentadecyl (meth) acrylate, 2-methyltetradecyl (meth) acrylate, n- hexadecyl (rneth) acrylate, and n-oσtadecyl (meth) acrylate, n-eicosyl (meth) acrylate, n-docosyl (meth) acrylate, methacrylate of Dobanol 23 [mixture of C-12/C-13 oxoalcohols made by Mitsubishi Chemical (Ltd.)] and methacrylate of Dobanol 25 [mixture of C-13/C-14 oxoalcohols made by Mitsubishi Chemical Company Ltd.];

(a3) alkyl (meth) acrylates having alkyl groups with 5 to 7 carbons: for example, n-pentyl (meth) acrylate and n- hexyl (meth} acrylate.

Of the aforementioned (al) ~ (a3) , the preferred substances are those belonging to (al) and (a2), and (a2) is further preferred. Also, the preferred substances of the aforementioned {al), from the standpoint of the viscosity index, are those with 1 to 2 carbons in the alkyl groups. The preferred substances of the aforementioned (a2), from the standpoint of solubility in the base oil and low-temperature characteristics, are those with 10 to 20 carbons in the alkyl groups, and further preferred are those with 12 to 14 carbons.

The aforementioned vinyl monomers (b) containing hydroxyl groups which constitute the copolymers with the alkyl (meth) acrylates having alkyl groups of 1 to 20 carbons are vinyl monomers containing one or more than one hydroxyl group (preferably one or two) in their molecules. As specific examples mention may be made of

(bl) hydroxyalkyl (2 to 6 carbons) (meth) acrylates : for example, 2-hydroxyethyl {meth) acrylate, 2 or 3- hydroxypropyl (meth) acrylate, 2-hydroxybutyl (meth) acrylate, l-methyl-2-hydroxyethyl (meth) acrylate;

(b2) mono or di-hydroxyalkyl (1 to 4 carbons) substituted (meth) acrylamides: for example, N,N-dihydroxymethyl (meth) acrylamide,

N,N-dihydroxypropyl (meth} acrylamide, N-N-di-2- hydroxybutyl (meth) acrylamide;

(b3) vinyl alcohols {formed by hydrolysis of vinyl acetate units) ; (b4) alkenols of 3 to 12 carbons: for example, (meth)allyl alcohol, crotyl alcohol, isocrotyl alcohol, 1-octenol, 1-undecenol; {b5) alkenediols of 4 to 12 carbons: For example, l-buten-3-ol, 2-buten-l-ol, 2-butene- 1,4-diol;

(b6) hydroxyalkyl (1 to 6 carbons) alkenyl (3 to 10 carbons) ethers: 2-hydroxyethylpropenyl ether;

{b7) aromatic monomers containing hydroxyl groups: o-, m- or p-hydroxystyrene; (b8) polyhydric (from trihydric to octahydric) alcohols, for example: alkane polyols, intramolecular or intermolecular dehydrates thereof, alkenyl (3 - 10 carbons) ethers of sugars (e.g. glycerine, _L O

pentaerythritol, sorbitol, sorbitan, diglycerine, sucrose} or (meth) acrylates of sugars (e.g. sucrose (meth)allyl ether);

{b9) vinyl monomers containing hydroxyl groups and polyoxyalkylene chains, for example: mono {meth) acrylates or mono (meth) allyl ethers of polyoxyalkylene glycols (alkylene group of from 2 to 4 carbons, degree of polymerisation from 2 to 50} or polyoxyalkylene polyols {polyoxyalkylene ethers (alkyl groups of from 2 to 4 carbons, degree of polymerisation from 2 to 100) of the aforementioned trihydric to octahydric alcohols} {e.g. polyethylene glycol (degree of polymerisation from 2 to 9) mono (meth) acrylates, polypropylene glycol (degree of polymerisation from 2 to 12) mono (meth) acrylates, polypropylene glycol (degree of polymerisation from 2 to 30) mono (meth) allyl ethers}.

Of the above mentioned (bl) to (b9) , from the standpoint of effect of improving the viscosity index the preferred type is (bl} , and 2-hydroxy-ethyl methacrylate in particular.

The respective proportions in isomers constituting the copolymers of poly (meth} acrylates containing the aforementioned hydroxyl groups are preferably, from the standpoint of the viscosity index, as follows. The lower limit of the aforementioned constituent

(a) is preferably 50% by weight but more preferably 75% by weight. The upper limit is preferably 95% by weight but more preferably 85% by weight.

The lower limit of the aforementioned (al) is preferably 0% by weight and more preferably 1% by weight. The upper limit is preferably 20% by weight and more preferably 10% by weight. The lower limit of the aforementioned (a2) is preferably 50% by weight and more preferably 70% by weight. The upper limit is preferably 95% by weight and more preferably 90% by weight. The lower limit of the aforementioned (b) is preferably 5% by weight and more preferably 7% by weight, but especially preferable is 11% by weight. The upper limit is preferably 50% by weight and more preferably 30% by weight, but especially preferable is 15% by weight. The lower limit of the total of the aforementioned

(a) + (b) is preferably 55% by weight and more preferably 82% by weight. The upper limit is preferably 100% by weight .

The hydroxyl value of the poly (rneth) acrylates containing hydroxyl groups and used in this invention as an additive is 10 to 100, but preferably 20 to 50 and more preferably 25 to 35. Measurement of the hydroxyl value denotes the number obtained by measuring in accordance with JIS K3342 (1961) , and it shows the amount of hydroxyl groups in an additive.

For the rust inhibitors used in this invention it is possible to use, for example, at least one kind of additive selected from acid amides, sarcosinic acids or aspartic acid derivatives having mainly a rust inhibiting effect. These rust inhibitors may be used alone or in plural combinations within the range 0.01 to 0.1% by weight in terms of the total amount of the lubricating oil .

Suitable examples of the aforementioned acid amides are acid amide compounds in which saturated monocarboxylic acids of 12 to 30 carbons or unsaturated monocarboxylic acids of 18 to 24 carbons have been reacted with amines, and mention may be made of such as lauric acid amide, myr±stic acid amide, palmitic acid amide, stearic acid amide, isostearic acid amide and oleic acid amide. PolyalJcylpolyamides obtained by reaction with polyalkylamines, for example carboxylic acid amides such as isostearic acid triethylene tetramide, isostearic acid tetraethylene pentamide, isostearic acid pentaethylene hexamide, oleic acid diethylene triamide and oleic acid diethanolamide may also be used. The aforementioned sarcosinic acids are derivatives of glycine as shown in the undermentioned General Formula (1} .

O

CH3

In Formula 1 above, R denotes a straight-chain or branched alkyl group or alkenyl group of from 1 to 30 carbons.

As a specific example of the aforementioned sarcosinic acids, mention may be made of (Z) -N-methyl-N~ (l-oxo-9-octadecenyl} glycine as in the undermentioned Formula (2) .

O

CH3-(CH2)8 =(CH2)8 -C -H -GH2—COOH ⁽²⁾

[ GH3 The aforementioned aspartic acid derivatives are those shown by the undermentioned General Formula (3) .

In the aforementioned General Formula (3) , Xi and X2 are each hydrogen or alkyl groups of 3 to 6 carbons which may be the same or different, or hydroxyalkyl groups. More preferable is if they are respectively a 2- methylpropyl group or a tertiary-butyl group.

X ₃ is an alkyl group comprised of 1 to 30 carbon atoms, or an alkyl group having ether bonds, or a hydroxyalkyl group. Good examples are where it is an octadecyl group, an alkoxypropyl group, or a 3- hydrocarbon oxyalkyl group in which the number of carbons of the hydrocarbon is 6 to 18 and the number of carbons of the alkyl group is 3 to 6, but more preferably it is a cyclohexyloxypropyl group, a 3-octyloxypropyl group, a 3- isooctyloxypropyl group, a 3-decyloxypropyl group, a 3- isodecyloxypropyl group, a 3-dodecyloxypropyl group, a 3- tetradecyloxypropyl group or a 3-hexadecyloxypropyl group. X ₄ is a saturated or unsaturated carboxylic acid group comprising 1 to 30 carbon atoms, or an alkyl group comprising 1 to 30 carbon atoms, or an alkenyl group, or a hydroxyalkyl group. For example, a propionic acid group or a propionylic acid group is good.

The aforementioned aspartic acid derivative should have an acid value as determined by JIS K2501 of 10 to 200 mgKOH/g, but more preferably 50 to 150 mgKOH/g. The aspartic acid derivative is used in the amount of approximately 0.01 to 5% by weight, but preferably approximately 0.05 to 2% by weight, in terms of the total amount of the lubricating oil. The amount of the aforementioned acid amides, sarcosinic acids and aspartic acid derivatives is not specially limited, but, in terms of the total amount of the lubricating oil, is 0.01 to 5% by weight, preferably 0.05 to 4.5% by weight, more preferably 0.05 to 4% by weight, even more preferably 0.05 to 3.5% by weight, and yet more preferably 0.05 to 3% by weight. If the amount thereof is less than 0.01% by weight, there is a risk that the prevention of corrosion will be inadequate, whilst if it exceeds 5% by weight, there is a risk that the demulsification and foaming properties will be reduced.

The demulsifiers that can be used in this invention may be those of the prior art used as normal lubricating oil additives, for example polyoxyethylene- polyoxypropylene condensates, reverse forms of polyoxyethylene-polyoxypropylene block polymers, and ethylenediamine polyoxyethylene-polyoxypropylene block polymers. As to the amount thereof added, they may be used in the range, in terms of the total amount of the lubricating oil, of 0.0005 to 0.5% by weight.

The lubricating oil of this invention includes the aforementioned base oil of the invention, but as regards the properties of the base oil itself, since it has the characteristic that the minimum oil film thickness is large, the pressure-viscosity coefficient is high and the pressure-velocity product (PV value) is high, a lubricating oil which contains such a base oil will have the characteristic that the minimum oil film thickness is large, the pressure-viscosity coefficient is high and the pressure-velocity product (PV value) is high.

What is meant here by saying that the minimum oil film thickness is large is that the minimum oil film thickness in a system of rolling contact or rolling- sliding contact where a load (weight) is applied is large. Also, what is meant by saying that the pressure- viscosity coefficient is high is that in a system where a load (weight) is applied, the viscosity coefficient increases when the pressure in the form of the load (weight) increases, and by virtue of this the aforementioned minimum oil film thickness can be maintained in a large state.

Also, the pressure-velocity product is the product of the pressure in the form of the load (weight) and the velocity corresponding to the rolling or rolling-sliding, and is expressed as the PV value already mentioned. What is then meant by saying that the pressure-velocity product is high is that, in a rolling contact or rolling- sliding contact system where the pressures and/or velocities are large, the aforementioned minimum oil film thickness is maintained in a large state.

For this reason, if the lubricating oil of this invention is used as a lubricating oil for use in rolling contact or rolling-sliding contact systems such as roller bearings or gears, an EHL (elastohydrodynamic lubrication) oil film will be formed and interference between protuberances on sliding surfaces can be prevented. In particular, if it is used as a lubricating oil for use in rolling contact or rolling-sliding contact systems where a load (weight) is applied, the EHL oil film will be formed even when the load (weight) is applied, and interference between protuberances on sliding surfaces can be prevented.

The lubricating oil of this invention is contains a base oil in which the naphthene component (%C _N ) is from 30 to 60, the aromatic component (%C _a ) is less than 10, and the remaining portion is a paraffin component (%C _P ) , and thereby it is possible to obtain, as a lubricating oil for use in rolling contact or rolling and sliding contact systems such as roller bearings and gears, and in particular a lubricating oil for use in rolling contact or rolling and sliding contact systems where a load

(weight) is applied, a lubricating oil which has a large minimum oil film thickness, a high pressure-viscosity coefficient and a large pressure-velocity product (PV value) . The invention is explained in specific detail below by means of Examples and Comparative Examples, but the invention is not limited to only these Examples.

The base oils used in the Examples and Comparative Examples were as follows. Base Oil A Hydrorefined naphthene-based base oil

Base Oil B Highly refined naphthene-based base oil Base Oil C Kaphthene-based base oil Base Oil D Group I base oil Base Oil E Group II base oil Base Oil F Group II base oil

Base Oil G Alkylnaphthalene base oil Base Oil H Group III base oil Base Oil I GTL (XHVI) base oil Base Oil J : PAO-6 base oil

The categories of measurement and the methods of measurement of the properties in the Examples and Comparative Examples were as follows. (1) %C _H : Naphthene-based constituent carbon ratio

(%) in accordance with ASTM D3238

(2) %C _A : Aromatics-based constituent carbon ratio {%) in accordance with ASTM D3238

(3) %C _P : Paraffin-based constituent carbon ratio (%) in accordance with ASTM D3238

(4) Acid number : Acid number (mgKOH/g) in accordance with JIS K2501

The categories of measurement and the methods of measurement of the properties in the Examples and Comparative Examples were as follows.

(1) Density : Density at 15 ⁰ C (g/cm ³ } in accordance with JIS K2249

(2) Kinematic viscosity at 40 ⁰ C (Vk40) : Kinematic viscosity at 40 ⁰ C (ramVs) in accordance with JIS K2283 (3) Kinematic viscosity (VkIOO) : Kinematic viscosity at 100 ⁰ C {mmVs} in accordance with JIS K2283

(4) Viscosity index : Viscosity index in accordance with JIS K2283

(5) Number average molecular weight : Number average molecular weight in accordance with ASTM D3238

As to the characteristics of the lubricating oils in the Examples and the Comparative Examples, a Shell 4-ball wear test was carried out in accordance with the test method standardised in ASTM D4172, and the lubrication properties of each lubricating oil composition were evaluated. Previous Shell 4-ball wear tests have been carried out with test conditions of a comparatively low number of revolutions (sliding velocity) of 1200 min ^"1 to 1800 mirf ¹ , but in consideration of actual conditions of use the following more rigorous test conditions were applied. The rate of increase of the measured oil temperature, the maximum torque, the friction coefficient and the fixed ball wear mark diameter were used as indicators to evaluate the lubrication performance.

By way of evaluation of the lubrication properties of the ceramic and steel balls, a Shell 4-ball wear test was carried out as follows to evaluate performance of the lubricating oils and a Shell 4™ball extreme-pressure test was also carried out as follows on lubricating oils that had superior lubrication properties, being blended with additives.

Shell 4-Ball Wear Test Test balls : The rotating ball was made of a ceramic

(8! ₃ N ₄ ) and the fixed balls were made of bearing steel (SUJ-2) .

Load (P) : 40 kgf (= 392 N) Number of rotations (V) : 10,000 min ^"1 Duration of test : 30 seconds

Temperature : Room temperature (at start of test) Measurement : In the period from the start of the test to the end, the torque maximum value (kgf "cm} , the torque fluctuation value (kgf ^* cm) and the wear mark diameter (mm) in the SUJ-2 after completion of the test were measured. Shell 4-Ball Extreme Pressure Test

Test balls : The rotating ball was made of a ceramic (Si ₃ N ₄ ) and the fixed balls were made of bearing steel (SUJ-2) .

Load (P) : 40 to 60 kgf (392 to 588 N)

Number of rotations (V) : 6,000 to 12,000 min ^"1

Duration of test : 30 seconds Temperature ; Room temperature

Measurement : The number of rotations and the test load were varied and the maximum load (P) and maximum speed (V) at which seizing did not occur were obtained. The PV value was calculated from these values by means of the following Formula (III) . An assessment can be made that the higher its PV value, the better are its extreme pressure-resisting properties.

PV value = (P) x (V) ... (Ill)

Measurement of Oil Film Thickness: The oil film thickness of the sample oils was measured under the following conditions by using an optical type EHL oil film thickness measuring apparatus made by PCS Instruments Ltd.

The oil film thickness of the lubricating oil is measured by means of a steel ball and the contact behaviour on a glass plate which rotates. Part of the light which is radiated from above the rotating glass disk onto the area in contact with the steel ball is reflected back by a chromium film which is coated on the surface of the glass, and the rest of the light travels through a silica layer and the oil film, and returns by reflecting on the steel ball. The interference stripes thereby produced are taken to a computer via a spectrometer and a high-resolution CCD camera, and the oil film thickness is thus measured. Measurement Conditions

Velocity : 0 4.4 m/s

Load : 20 N

Oil temperature : 80 ⁰ C The aforementioned method of measurement of the Examples follows the previously mentioned ASTM method of measurement, but the measurement is so done that, in conformity with the application (operating conditions) of the lubricating oil used, the test conditions are varied so as to increase the relationship with actual machines as far as practicable. Comparison with the previously mentioned ASTM method of measurement is as shown in Table 1 below.

Table 1

Notes to Table 1:

ISL : Initial Seizure Load WL : Welding Load LWI : Load Wear Index

With all these indicative values, the higher they are the better the extreme pressure (EP) properties.

In Table 1, the "load" goes up in steps and in the tests to obtain the seizure limit loads, the seizure load varies considerably according to the lubricating oil, and so has been designated as "any". Calculation of Pressure-Viscosity Coefficient at 80°C

The pressure-viscosity coefficient at 80 ⁰ C is calculated using the following formula from the central oil film thickness measured by means of the aforementioned optical type EHL oil film thickness measuring device.

The pressure-viscosity coefficient is obtained by calculation from the measured values of the central oil film thickness as shown in Hamrock, B-J, Dowson, D.: "Isothermal Elastohydrodynamic Lubrication of Point Contacts, part III", Journal of Lubrication Technology, Transactions of ASME, 99 (Apr.), 264 (1977).

The lubricating oil forms an EHL (elastohydrodynamic lubrication) oil film in the bearing and performs a role in preventing interference between protuberances of the sliding surfaces. The point-contact central oil film thickness (Hc: dimensionless central oil film thickness) according to the Hamrock-Dowson formula is shown by formula (IV) .

Hc = 2 . 69U 0.67Q0.53tøp0 - 067 (1-0.61e-0.731c :iv)

k = a/b Ellipticity parameter

(In the case of a true circle, k = 1) U = u ηo /E' R) Velocity parameter W = w/(E'R ² ) Weight parameter G = otE ^r Material parameter

E' Elastic modulus of test balls R Radius of test balls (m) ηo Viscosity of lubricating oil at atmospheric pressure (mPa)

U Sliding velocity (m/s) W Load (N) α Pressure-viscosity coefficient

The pressure-viscosity coefficient is shown by Formula (V) from the definition formula of the material parameter of the above mentioned Formula (IV) .

α = G/E' ... (V)

Using Formula (IV) , the material parameter "G" is calculated from the measured oil film thickness (Hc) .

Next, the pressure-viscosity coefficient α is obtained by calculation from Formula (V) .

In Formula (V) , focusing on the property values of the lubricating oil shows that the viscosity η ₀ in the velocity parameter U and the pressure-viscosity coefficient α in the material parameter G are the factors which influence the central oil film thickness.

Given that the viscosity ηo is included in the velocity parameter, the central oil thickness varies in proportion to the power of 0.67 of the viscosity, so that the greater is the atmospheric pressure viscosity at the lubricating oil temperature at the inlet of the rolling contact element, the more the oil film thickness increases, and the more the bearing life increases. In other words, it is preferable to have a small variation in viscosity in relation to temperature (high viscosity index) . In the case of the pressure-viscosity coefficient α included in the material parameter the oil film thickness varies in proportion to the power of 0.53. In general, according to the Braus formula Tribologist, Vol. 53, No. 10, page 653 which shows the relationship between viscosity and pressure, the viscosity under high pressure becomes higher the higher the pressure-viscosity coefficient α is, so that the bearing fatigue life improves the more the lubricating oil has a large α.

η _P = η ₀ exp(αP) ... (VI)

P : Pressure on lubrication surface (load) η _P : Lubricating oil viscosity under high pressure

Examples Examples 1 to 6, Comparative Examples 1 to 4 :

Examples 1 to 6 and Comparative Examples 1 to 4 used the previously described base oil compositions A to J. The properties and the measured values of the lubricating oil characteristics are shown in Tables 2 and 3.

Table 2

Table 3

In Tables 2 and 3, if it is assumed that a pass point is a torque maximum value of not more than 2.0 kgf ^• cm but preferably not more than 1.9 kgf -cm, a torque fluctuation of not more than 0.2 kgf *cm but preferably not more than 0.15 kgf -cm, a wear mark diameter of not more than 0.65 mm but preferably not more than 0.5 mm, a central oil film thickness (80°C) of not less than 130 nm but preferably not less than 150 nm, and a pressure- viscosity coefficient (average) at 80 ⁰ C calculated from the central oil film thickness of not less than 9.0 GPa ^'1 but preferably not less than 9.5 GPa ^"1 , the lubricating oils of Examples 1 to 6 reach the pass line and the lubricating oil of Example 1 in particular has achieved desirable results, and it is shown that even the base oil on its own may be used as a lubricating oil. Example 7 :

Example 7 used the Base Oil A of the previously mentioned Example 1, blended with which were the additives shown in Table 3, thus giving Example 7. The composition and properties of this lubricating oil and the measured values of the lubricating oil characteristics are shown in Table 4. Comparative Example 5 was a commercial lubricating oil. Its properties and the measured values of the lubricating oil characteristics are shown in Table 4. Table 4

As can be seen from Table 4, the lubricating oil of Example 7 has achieved excellent lubricating oil properties in that the central oil film thickness is large, the pressure-viscosity coefficient is high and the pressure-velocity product (PV value) is large. This invention can be used as a lubricating oil for use in rolling contact or rolling and sliding contact systems such as roller bearings and gears, and in particular as a lubricating oil for use in rolling contact or rolling and sliding contact systems where a load {weight) is applied.