Grease composition, rolling bearing and lubrication method

Abstract

Provided are: a grease composition having both low noise and high-temperature and high-speed durability; and a rolling bearing in which the grease composition is sealed. The grease composition (7) contains a base oil, a thickener and an additive, the base oil is a mixed oil of a synthetic hydrocarbon oil and an ester oil, the thickener is an aliphatic diurea compound, the initial acid value of the grease composition is 2.0 mgKOH / g or more, and the additive contains sulfur (S), phosphorus (P) and zinc (Zn) elements. The total amount of these elements contained in the grease composition (7) is 0.50 mass% or more with respect to the total amount of the grease composition.

Description

Technical Field

[0001] The present invention relates to a grease composition, a rolling bearing encapsulated with the grease composition, and a method for lubricating the rolling bearing, and more particularly to a grease composition and a rolling bearing used in rolling bearings that are used at high temperatures and high speeds and require low noise and long life, such as industrial machinery engines (motors) for home appliances or industrial equipment, and drive engines for electric vehicles (EVs) or hybrid electric vehicles (HEVs). Background Art

[0002] The bearings used in the rotating support parts of motor machinery represented by electric motors have grease enclosed inside. In such bearings, the grease is usually sealed using a sealing plate (sealing component) such as a rubber seal that is a rubber material vulcanized and bonded to an iron plate or a mandrel.

[0003] In recent years, these bearings have been operating at higher temperatures and higher speeds, and require better high-temperature and high-speed durability (extending the life of the grease) while maintaining quietness. In addition, from the perspective of quietness, fretting resistance is also required.

[0004] Conventionally, grease compositions used for these purposes mainly adjust the components of base oil, thickener, antioxidant, etc. For example, Patent Document 1 proposes a grease composition using a specific ester oil in a specific diurea compound as a grease composition for a driving engine bearing. In addition, Patent Document 2 proposes a rolling bearing having improved fretting resistance by using a grease composition using a base oil having a specific dynamic viscosity as a lubricant. Prior art literature

[0005] Patent Literature Patent Document 1: International Publication No. 2013 / 015413 Patent Document 2: Japanese Patent Application Publication No. 2004-352858 Summary of the invention Problems to be solved by the invention

[0006] However, conventional knowledge has shown that grease compositions are insufficient for bearings for engines in recent years, which require further high-temperature and high-speed durability while maintaining low noise performance.

[0007] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a grease composition that achieves both low noise performance and high-temperature and high-speed durability, a rolling bearing enclosed with the grease composition, and a method for lubricating the rolling bearing. Means of solving problems

[0008] The grease composition of the present invention is a grease composition containing a base oil, a thickener and an additive, characterized in that the base oil is a mixed oil of a synthetic hydrocarbon oil and an ester oil, the thickener is an aliphatic diurea compound, and the initial acid value of the grease composition is 2.0 mgKOH / g or more. In the present invention, the initial acid value of the grease composition refers to the total acid value (TAN) before the prepared grease composition is used (for example, the grease composition or the rolling bearing enclosed with the grease composition at the time of shipment, sale, etc.).

[0009] The additive contains sulfur (S), phosphorus (P) and zinc (Zn) elements, and the total amount of these elements contained in the grease composition is 0.50 mass % or more based on the total amount of the grease composition.

[0010] The additive contains sulfur (S), phosphorus (P), zinc (Zn), barium (Ba) and calcium (Ca) elements, and the total amount of these elements contained in the grease composition is 0.50 mass% or more relative to the total amount of the grease composition.

[0011] The total amount of the elements contained in the grease composition is 1.5 mass % or less based on the total amount of the grease composition.

[0012] When the grease composition is exposed to a high temperature environment, the acid value of the grease composition changes with time, showing a behavior of decreasing from the initial acid value and then increasing.

[0013] The aliphatic diurea compound is characterized in that it is obtained by reacting a diisocyanate component with two different aliphatic monoamines.

[0014] The dynamic viscosity of the above base oil at 40°C is 40 mm 2 / s~70mm 2 / s.

[0015] The additive contains sulfur (S), phosphorus (P), zinc (Zn), barium (Ba) and calcium (Ca) elements, and the total amount of these elements contained in the grease composition is 0.50 mass% or more and 1.5 mass% or less relative to the total amount of the grease composition. The aliphatic diurea compound is an aliphatic diurea compound obtained by reacting a diisocyanate component with two different aliphatic monoamines. The base oil has a dynamic viscosity of 40 mm at 40°C. 2 / s~70mm 2 / s.

[0016] The rolling bearing of the present invention is a rolling bearing having an inner ring, an outer ring, a plurality of rolling elements interposed between the inner ring and the outer ring, and a grease composition sealed around the rolling elements, and is characterized in that the grease composition is the grease composition of the present invention.

[0017] The rolling bearing is characterized in that it is a bearing for an engine.

[0018] The lubricating method of a rolling bearing of the present invention is a lubricating method for lubricating a rolling bearing by a grease composition, wherein the grease composition contains: a base oil composed of a mixed oil of a synthetic hydrocarbon oil and an ester oil, and a thickener composed of an aliphatic diurea compound, The grease composition is characterized in that the acid value is decreased from an initial acid value and then increased.

[0019] The rolling bearing lubrication method is characterized in that the rolling bearing is lubricated in a high temperature environment of 80°C to 200°C.

[0020] The above-mentioned method for lubricating a rolling bearing is characterized in that a grease composition having an initial acid value of 2.0 mgKOH / g or more is used as the above-mentioned grease composition. Effects of the Invention

[0021] In the grease composition of the present invention, the base oil is a mixed oil of a synthetic hydrocarbon oil and an ester oil, the thickener is an aliphatic diurea compound obtained by reacting a diisocyanate component and an aliphatic monoamine component, and the initial acid value of the grease composition is 2.0 mgKOH / g or more, so as shown in the examples described below, low noise performance and high-temperature and high-speed durability (extending the life of the grease) can be achieved at the same time. In the above grease composition, the aliphatic diurea compound has excellent dispersibility in the above base oil, which contributes to low noise performance, but on the other hand, it is easy to soften (deteriorate) and leak at high speeds, making it difficult to achieve a long life. However, by setting the initial acid value of the grease composition to 2.0 mgKOH / g or more, it is possible to suppress the progress of oxidative degradation while maintaining low noise performance, thereby achieving a long life.

[0022] The additive contains sulfur (S), phosphorus (P) and zinc (Zn) elements (preferably sulfur (S), phosphorus (P), zinc (Zn), barium (Ba) and calcium (Ca) elements), and the total amount of these elements contained in the grease composition is 0.50 mass% or more relative to the total amount of the grease composition. Therefore, it is easy to make the initial acid value 2.0 mgKOH / g or more, and to form a film by adsorption or chemical reaction on the steel surface, so that oxidative degradation in a high temperature environment and the like can be easily prevented.

[0023] Since the total amount of the above elements contained in the grease composition is 1.5 mass % or less based on the total amount of the grease composition, it is easier to extend the life of the grease.

[0024] The time-dependent change in the acid value of the grease composition when the grease composition is exposed to a high temperature environment shows a behavior of gradually increasing after decreasing the initial acid value, so that the time until the acid value finally reaches the value determined to be deteriorated can be extended over a long period of time, and as a result, the progress of oxidative degradation can be suppressed. It is believed that this behavior is due to the fact that the strong acid component from the additive containing the above-mentioned element catalyzes the decomposition of hydrogen peroxide (ROOH) which is a deterioration product of the base oil, thereby temporarily reducing the acid value.

[0025] The rolling bearing of the present invention has the grease composition of the present invention sealed around the rolling elements, and therefore is suitable for use in bearings of electric machines such as motors, and can contribute to higher temperatures and higher speeds and improved reliability of the electric machines.

[0026] The lubrication method for rolling bearings of the present invention is a lubrication method for lubricating rolling bearings using a grease composition containing a base oil composed of a mixed oil of synthetic hydrocarbon oil and ester oil and a thickener composed of an aliphatic diurea compound, wherein the acid value in the grease composition decreases from an initial acid value and then increases, thereby maintaining low noise while suppressing the progress of oxidative degradation and achieving a long life. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 It is a cross-sectional view of a deep groove ball bearing as an example of the rolling bearing of the present invention. Figure 2 This is a cross-sectional view of an engine using the rolling bearing of the present invention. Figure 3 This is a graph showing the change in acid value of the grease compositions of Example 1 and Comparative Example 3 over time. DETAILED DESCRIPTION

[0028] Generally, the acid value is used to indicate the deterioration of a grease composition. Generally, the acid value of a grease composition gradually increases from the initial acid value with use, and when it reaches a certain level or more, it is judged that the oxidative degradation of the grease composition is progressing. Therefore, the grease composition is usually designed in such a way as to reduce the initial acid value. The present inventors have conducted in-depth research on grease compositions in order to achieve both low noise performance and high-temperature and high-speed durability, and found that if the initial acid value of the grease composition is set high, the progress of oxidative degradation in a high-temperature environment can be suppressed. The present invention is based on such insights.

[0029] The grease composition of the present invention contains a predetermined base oil, a predetermined thickener and additives, and has an initial acid value of 2.0 mgKOH / g or more.

[0030] The base oil used in the grease composition of the present invention is a mixed oil of a synthetic hydrocarbon oil and an ester oil. The ratio is not particularly limited, for example, in terms of mass ratio, synthetic hydrocarbon oil: ester oil = (90:10) to (10:90), preferably (80:20) to (20:80), and may be (70:30) to (30:70). For example, from the viewpoint of further high temperature stability, the ester oil may be mixed with a mass ratio of more than the synthetic hydrocarbon oil.

[0031] As the synthetic hydrocarbon oil, poly-α-olefin oil (PAO oil) is more preferred. PAO oil is a mixture of oligomers or polymers of α-olefins or isomerized α-olefins. Specific examples of α-olefins include: 1-octene, 1-nonene, 1-decene, 1-dodecene, 1-tridecene, 1-tetradecene, 1-pentadecene, 1-hexadecene, 1-heptadecene, 1-octadecene, 1-nonadecene, 1-eicosene, 1-docosene, 1-tetracosene, etc., and mixtures thereof are usually used.

[0032] Ester oil is a compound having an ester group in the molecule and being liquid at room temperature, and examples thereof include polyol ester oil, phosphate ester oil, polymer ester oil, aromatic ester oil, carbonate oil, diester oil, etc. Among them, aromatic ester oil, polyol ester oil, and polymer ester oil are preferred.

[0033] The aromatic ester oil is preferably a compound obtained by reacting an aromatic polybasic acid or a derivative thereof with a higher alcohol. Examples of the aromatic polybasic acid include: aromatic tricarboxylic acids such as trimellitic acid, biphenyltricarboxylic acid, and naphthalenetricarboxylic acid; aromatic tetracarboxylic acids such as pyromellitic acid, biphenyltetracarboxylic acid, benzophenonetetracarboxylic acid, and naphthalenetetracarboxylic acid; or derivatives thereof such as anhydrides. Examples of the higher alcohol include: aliphatic monohydric alcohols having a carbon number of 4 or more such as octanol and decanol. Examples of the aromatic ester oil include: trioctyl trimellitate, tridecyl trimellitate, tetraoctyl pyromellitate, and the like.

[0034] Polyol ester oil is preferably a compound having multiple ester groups in the molecule obtained by reacting a polyol with a monobasic acid. The monobasic acid reacted with the polyol can be used alone or as a mixture. It should be noted that in the case of oligoesters, dibasic acids can be used. Examples of polyols include trimethylolpropane, pentaerythritol, dipentaerythritol, neopentyl glycol, 2-methyl-2-propyl-1,3-propanediol, etc. Examples of monobasic acids include monobasic fatty acids with 4 to 18 carbon atoms. For example, valeric acid, caproic acid, caprylic acid, heptanoic acid, nonanoic acid, capric acid, undecanoic acid, lauric acid, myristic acid, palmitic acid, tallow fatty acid, stearic acid, decenoic acid, undecenoic acid, lindric acid, tsuduic acid, spermaceti acid, myristoleic acid, palmitoleic acid, petroselinic acid, oleic acid, elaidic acid, ascorbic acid, vaccenic acid, sorbic acid, linoleic acid, linolenic acid, sabinic acid, ricinoleic acid, etc. Examples of polyol ester oils include trimethylolpropane caprylate, trimethylolpropane nonanoate, pentaerythritol 2-ethylhexanoate, pentaerythritol nonanoate, etc.

[0035] The dynamic viscosity of the base oil at 40°C (dynamic viscosity of the mixed oil) is, for example, 30 mm 2 / s~90mm 2 / s, preferably 30mm 2 / s~70mm 2 / s, preferably 40 mm 2 / s~70mm 2 / s.

[0036] The thickener used in the grease composition of the present invention is an aliphatic diurea compound obtained by reacting a diisocyanate component with an aliphatic monoamine component. Examples of the diisocyanate component include phenyl diisocyanate, toluene diisocyanate, diphenyl diisocyanate, diphenylmethane diisocyanate, octadecane diisocyanate, decane diisocyanate, and hexane diisocyanate.

[0037] As the aliphatic monoamine component, for example, a linear amine having 6 to 24 carbon atoms (preferably 8 to 22 carbon atoms) can be used, and specific examples thereof include hexylamine, octylamine, dodecylamine, hexadecylamine, stearylamine, oleylamine, and behenylamine.

[0038] From the viewpoint of easy thickening, the aliphatic monoamine component preferably uses two different aliphatic monoamines. For example, a linear amine having 8 to 12 carbon atoms and a linear amine having 18 to 22 carbon atoms can be used.

[0039] A base grease is obtained by mixing a diurea compound as a thickener in a base oil. The base grease using a diurea compound as a thickener is produced by reacting a diisocyanate component with a monoamine component in a base oil. The amount of the thickener relative to the total amount of the base oil and the thickener (100% by mass) is, for example, 5% to 30% by mass, preferably 10% to 20% by mass, more preferably 10% to 15% by mass, and even more preferably 11% to 13% by mass.

[0040] The additive used in the grease composition of the present invention preferably contains sulfur (S), phosphorus (P) and zinc (Zn) elements. The additive more preferably further contains barium (Ba) or calcium (Ca) elements, and further preferably further contains barium (Ba) and calcium (Ca) elements. Various additives can be used as the additive used in the grease composition of the present invention.

[0041] The total amount of the above elements contained in the grease composition (for example, the total amount of the five elements when five elements are contained) is preferably 0.50 mass% or more, more preferably 0.60 mass% or more, relative to the total amount of the grease composition. The upper limit of the total amount of the above elements is not particularly limited, and is, for example, 2.0 mass%, 1.5 mass% or less, or 1.0 mass% or less.

[0042] The content of each element based on the total amount of the grease composition is specifically described. The sulfur content is, for example, 0.20 mass% to 1.0 mass%, preferably 0.40 mass% to 0.80 mass%. The phosphorus content is, for example, 0.10 mass% to 0.30 mass%, preferably 0.10 mass% to 0.20 mass%. The zinc content is, for example, 0.10 mass% to 0.30 mass%, preferably 0.10 mass% to 0.20 mass%. The barium content is, for example, 0.01 mass% to 0.20 mass%, preferably 0.01 mass% to 0.10 mass%. The calcium content is, for example, 0.01 mass% to 0.20 mass%, preferably 0.01 mass% to 0.10 mass%.

[0043] The relationship between the contents of the elements contained in the grease composition is preferably (sulfur content)>(phosphorus content or zinc content)>(barium content or calcium content).

[0044] The above elements are mainly derived from additives. Examples of additives include: phosphates such as tricresyl phosphate and triphenyl phosphate, phosphites such as tricresyl phosphite and triphenyl phosphite, extreme pressure agents such as thiophosphates, thiophosphites, zinc alkyl dithiophosphates (ZnDTP), zinc dithiocarbamates (ZnDTC), and molybdenum alkyl dithiophosphates (MoDTP); sulfonate-based rust inhibitors such as calcium sulfonates and barium sulfonates, carboxylate-based rust inhibitors such as fatty acids, cyclohexane acids, and alkyl salicylic acids as Ca salts or Ba salts; antioxidants such as sulfur compounds, etc. These additives are appropriately added according to the content of the above elements.

[0045] In the grease composition of the present invention, the initial acid value is increased due to the above-mentioned additives (e.g., metal additives), and the initial acid value is 2.0 mgKOH / g or more. Therefore, as shown in the Examples, the grease composition has a superior grease life, especially in a high temperature environment, compared with a grease composition having an initial acid value lower than that of the present invention. The initial acid value of the grease composition is preferably 2.5 mgKOH / g or more, more preferably 3.2 mgKOH / g or more, and may be 4.0 mgKOH / g or more.

[0046] The initial acid value is measured according to JIS K 2501. The initial acid value is defined as the number of milligrams (mg) of potassium hydroxide required to neutralize all acidic components contained in 1 g of a sample. The initial acid value is measured by potentiometric titration.

[0047] As described later Figure 3 As shown in FIG. 1 , when the grease composition is sealed in a rolling bearing and used in a high temperature environment, the acid value of the grease composition preferably shows a behavior of gradually decreasing from the initial acid value and then gradually increasing over time. In contrast, a conventional grease composition shows a behavior of gradually increasing the acid value from the initial acid value as it is used. Figure 3 As shown, the acid value shows a behavior of temporarily decreasing from the initial acid value and then increasing, thereby extending the time until the acid value reaches the value finally determined as deterioration. As a result, the oxidative degradation of the grease composition can be suppressed.

[0048] When the grease composition of the present invention exhibits the above-mentioned behavior, the period during which the acid value decreases depends on the environment to which the grease composition is exposed, and is, for example, a period of at least 1 hour from the start of use, preferably a period of 24 hours or more. In the change in the acid value over time, the acid value decreases, for example, by 1.0 mgKOH / g or more (preferably 2.0 mgKOH / g or more) from the initial acid value to a value lower than 1.0 mgKOH / g.

[0049] The grease composition of the present invention may further contain additives that do not contain the above-mentioned elements within a range not impairing the purpose of the present invention.

[0050] The working consistency (JIS K 2220) of the grease composition of the present invention is, for example, in the range of 220 to 300, and preferably in the range of 240 to 280.

[0051] based on Figure 1 A rolling bearing in which the grease composition of the present invention is enclosed will be described. Figure 1 It is a cross-sectional view of a deep groove ball bearing. In a rolling bearing 1, an inner ring 2 having an inner ring raceway surface 2a on the outer circumference and an outer ring 3 having an outer ring raceway surface 3a on the inner circumference are arranged concentrically. A plurality of rolling elements 4 are arranged between the inner ring raceway surface 2a and the outer ring raceway surface 3a. The rolling element 4 is retained by a retainer 5. In addition, the axial end openings 8a and 8b of the inner and outer rings are sealed by a sealing member 6, and the above-mentioned grease composition 7 is sealed at least around the rolling element 4. The inner ring 2, the outer ring 3 and the rolling element 4 are mainly composed of iron-based metal materials, and the grease composition 7 is interposed on the raceway surface of the rolling element 4 for lubrication.

[0052] In the rolling bearing 1, the iron-based metal material constituting the bearing components of the inner ring 2, the outer ring 3, and the rolling element 4 is any material generally used as a bearing material, for example: high carbon chromium bearing steel (SUJ1, SUJ2, SUJ3, SUJ4, SUJ5, etc.; JIS G 4805), carburizing steel (SCr420, SCM420, etc.; JIS G 4053), stainless steel (SUS440C, etc.; JIS G 4303), high-speed steel (M50, etc.), cold-rolled steel, etc.

[0053] The retainer 5 is formed of an iron material or a resin material. As the iron material, any material commonly used as a retainer can be used, for example, cold-rolled steel plate for retainer (SPCC; JIS G 3141), stainless steel (SUS 440C, etc.; JIS G 4303), carbon steel for retainer (JIS G 4051), high-strength brass casting for retainer (JIS H 5102), etc. In addition, as the resin material, for example, phenolic resin, polyetheretherketone (PEEK) resin, polyphenylene sulfide (PPS) resin, thermoplastic polyimide resin, polyamide-imide resin, a resin material using polyamide resin such as polyamide 66 resin and polyamide 46 resin as a resin matrix and reinforcing fibers such as carbon fiber and glass fiber and other additives can be used.

[0054] exist Figure 1In the figure, a ball bearing is exemplified as a bearing, but the rolling bearing of the present invention can also be used as a cylindrical roller bearing, a tapered roller bearing, a self-aligning roller bearing, a needle roller bearing, a thrust cylindrical roller bearing, a thrust tapered roller bearing, a thrust needle roller bearing, a thrust self-aligning roller bearing, etc.

[0055] The rolling bearing of the present invention can be used as a bearing for supporting a rotor of an engine for driving industrial machinery, electrical equipment, or electric vehicles, for example. Figure 2 An example in which the rolling bearing of the present invention is applied to an engine (motor), for example, will be described. Figure 2 : is a cross-sectional view of the structure of the engine. The engine has: a stator 12 (which is composed of an engine magnet arranged on the inner peripheral wall of the sleeve 11); a rotor 15 (which is wound on a winding 14 fixed to a rotating shaft 13); a commutator 16 (which is fixed to the rotating shaft 13); a brush holder 17 (which is arranged on an end frame 19 supported by the sleeve 11); and a brush 18 (which is accommodated in the brush holder 17). The rotating shaft 13 is rotatably supported by the sleeve 11 through a deep groove ball bearing 1 and a support structure for the bearing 1. The bearing 1 is a rolling bearing of the present invention.

[0056] As engines (motors) to which the rolling bearing of the present invention can be applied, there can be cited: engines for industrial machinery such as ventilator engines, fuel cell blower engines, cleaner engines, fan engines, servo engines, stepping engines, etc., automobile starting engines, electric power steering engines, tilt engines for steering adjustment, blower engines, wiper engines, electric window engines, etc., engines for electrical equipment, and driving engines for electric vehicles or hybrid vehicles, etc. The bearings are particularly suitable for bearings that require quietness and are used at high temperatures and high speeds, but are not limited to engine bearings.

[0057] As described above, the rolling bearing of the present invention is suitable for use under high temperature and high speed conditions. As a high temperature condition, specifically, the outer diameter temperature of the bearing outer ring is 80°C or higher, preferably 100°C or higher. The upper limit of the outer diameter temperature of the bearing outer ring is not particularly limited, for example, 200°C, preferably 180°C. As a high speed condition, specifically, the dm·n value is 30×10 4 Above, preferably 70×10 4 The upper limit of the dm·n value is not particularly limited, but is, for example, 200×10 4 , preferably 180×10 4 For example, the grease lubricated bearings of the driving engine of EV or HEV are subjected to high temperature conditions of 100℃ to 150℃ and the dm·n value is 70×10 4 ~180×10 4The rolling bearing of the present invention has excellent quietness and grease life under such a use environment.

[0058] The rolling bearing lubrication method of the present invention is a lubrication method for lubricating the rolling bearing using a grease composition, the grease composition containing: a base oil composed of a mixed oil of a synthetic hydrocarbon oil and an ester oil, and a thickener composed of an aliphatic diurea compound, and the lubrication method is characterized in that the acid value of the grease composition is lowered from the initial acid value and then increased. When lubricating the bearing, as a method for lowering the acid value of the grease composition from the initial acid value and then increasing it, for example, a method of lubricating with a single grease composition can be adopted. In addition, other lubricating compositions (other grease compositions or lubricating oil compositions) or additives can be added to the grease composition sealed in advance during the lubrication. In the above lubrication method, it is preferred to lubricate the rolling bearing under a high temperature environment of 80°C to 200°C. In addition, in the above lubrication method, as the grease composition, it is preferred to use a grease composition with an initial acid value of 2.0 mgKOH / g or more, and it is more preferred to use the above-mentioned grease composition of the present invention.

[0059] In addition, according to the findings of the present invention, the high-temperature and high-speed durability of a grease composition can be evaluated. Specifically, the evaluation method is characterized in that the initial acid value of the grease composition is measured, and the high-temperature and high-speed durability of the grease composition is evaluated based on the initial acid value. Here, evaluating the high-temperature and high-speed durability means judging the quality of the grease life when the grease composition is subjected to high-temperature and high-speed conditions. As shown in the examples described below, there is a correlation between the results of the initial acid value and the grease life in the high-temperature and high-speed durability test. Therefore, for example, it can be judged that the high-temperature and high-speed durability of a grease composition with a high initial acid value (for example, an initial acid value of 2.0 mgKOH / g or more) is better than that of a grease composition with a low initial acid value (for example, an initial acid value of less than 2.0 mgKOH / g). It should be noted that in this evaluation, it is more preferable to compare between grease compositions having the same composition of the base grease. Example

[0060] Grease compositions of the compositions shown in Tables 1 and 2 were prepared respectively. In Tables 1 and 2, the contents of base oil and thickener represent the contents (mass %) relative to the base grease (base oil + thickener). The contents of each element (S, P, Zn, Ba, Ca) represent the contents (mass %) relative to the total amount of the grease composition. It should be noted that the contents were measured by fluorescent X-ray analysis using calibration curves prepared in advance for each element. In Tables 1 and 2, PAO oil was used as the synthetic hydrocarbon oil. It should be noted that 1) to 4) below Table 1 also have the same meaning in Table 2.

[0061] <Determination of Initial Acid Value> The initial acid value of each grease composition was measured according to JIS K 2501. Specifically, it was measured by potentiometric titration. The end point was pH 11.65.

[0062] <Fretting wear test> 1.0 g of each grease composition was applied to the inside of the one-way thrust ball bearing 51204, and it was vibrated at a vibration frequency of 30 Hz, a swing angle of 12°, and an axial load of 2450 N at room temperature. The bearing wear (weight loss) was measured 2 hours after the start of the test. Regarding the evaluation of the wear amount, less than 2 mg was evaluated as "A", 2 mg or more and less than 3 mg was evaluated as "B", and 3 mg or more was evaluated as "C", which are recorded in Tables 1 and 2.

[0063] <Sound Test> A ball bearing 608 (inner diameter 8 mm, outer diameter 22 mm, width 7 mm) filled with each grease composition was prepared, and an axial load of 7.8 N was applied to the bearing for 1800 min. -1 The vibration value G (RMS value) was measured after running for 30 seconds. The vibration value G was evaluated as "A" when it was less than 25 mG, "B" when it was 25 mG or more and less than 30 mG, and "C" when it was 30 mG or more. The results are listed in Tables 1 and 2.

[0064] <High temperature and high speed durability test> 1.8 g (equivalent to 38% of the total space) of each grease composition was sealed in a ball bearing 6204 (inner diameter 20 mm, outer diameter 47 mm, width 14 mm) to produce a test bearing. The test bearing was subjected to 10,000 min at an outer diameter temperature of 150°C, an axial load of 67 N, and a radial load of 67 N. -1 The motor rotates at a speed of 10000 rpm and measures the time it takes for the motor current value to rise to +2A. The dm·n value under this condition is approximately 34×10 4 Regarding the evaluation of grease life time, more than 5000 hours is evaluated as "A", more than 3000 hours and less than 5000 hours is evaluated as "B", more than 2000 hours and less than 3000 hours is evaluated as "C", and less than 2000 hours is evaluated as "D", which are recorded in Tables 1 and 2.

[0065] [Table 1] 1) 47mm 2 / s@40℃(manufactured by イネオス Co., Ltd., デュラシン168) 2) 30mm 2 / s@40℃(manufactured by イネオス Co., Ltd., デュラシン146) 3) 112mm 2 / s@40℃(Italmatch Chemicals Co., Ltd., ケッチェェンルーブ115) 4) 70mm 2 / s@40℃(Lanxess, Hatcor H2362)

[0066] [Table 2]

[0067] As shown in Table 1, the grease compositions of Examples 1 to 4, which comprised a base oil composed of a mixed oil of a synthetic hydrocarbon oil and an ester oil, a thickener composed of an aliphatic diurea compound, and additives and had an initial acid value of 2.0 mgKOH / g or more, showed good results in all tests. In particular, Examples 3 to 4 having a higher initial acid value showed a better result in grease life than Examples 1 to 2.

[0068] As shown in Table 1, even if the composition of the base grease is the same, when the initial acid value is less than 2.0 mgKOH / g, the grease life is significantly reduced (Comparative Examples 1 to 3). In addition, as shown in Table 2, even if the initial acid value is 2.0 mgKOH / g or more, it is difficult for the grease composition that does not meet the requirements of the base grease to exhibit various properties in a good balance (Comparative Examples 4, 7, 8, and 9).

[0069] Next, in order to confirm the degradation state of the grease composition with the operation time, for Example 1 and Comparative Example 3, in the above-mentioned high-temperature and high-speed durability test, the test was stopped at a certain time interval, the grease composition in the bearing was collected, and the acid value of the operation time was measured. The results are shown in Table 3, and the graph thereof is shown in Figure 3 shown.

[0070] [Table 3]

[0071] like Figure 3 As shown, in Comparative Example 3, the acid value gradually increased over time from the initial acid value (0.3 mgKOH / g) and eventually exceeded 10 mgKOH / g. In contrast, in Example 1, the acid value decreased from the initial acid value (3.4 mgKOH / g) until the operating time was 100 hours, and then gradually increased, reaching the initial acid value after about 500 hours of operating time. Thereafter, the acid value further increased. After 2000 hours of operating time, it was still less than 10 mgKOH / g. This behavior is considered to particularly affect the life of the grease. It is to be noted that, for example, even when exposed to a high temperature environment of a thermostatic bath adjusted to 150°C for a long time, the time-dependent change in the acid value of the grease composition of Example 1 is shown as follows. Figure 3 The behavior shown.

[0072] In summary, the grease composition of the present invention achieves both low noise performance and long life under high-speed and high-temperature rotation by combining a predetermined base oil, a predetermined thickener and an initial acid value. Industrial Applicability

[0073] The grease composition of the present invention can achieve both quietness and high-temperature and high-speed durability, and therefore can be suitably used in, for example, rolling bearings for rotary electric machines, bearings for driving engines of EVs and HEVs, and the like.

[0074] Description of Reference Numerals 1 Rolling bearings 2 Inner ring 3 Outer ring 4 Rolling elements 5 Retainer 6 Sealing parts 7 Grease 8a, 8b opening 11. Sheath 12 Stator 13 Rotation axis 14 Winding 15 Rotor 16 Commutator 17 Brush Holder 18 Brush 19 End frame

Claims

1. A grease composition comprising a base oil, a thickener and an additive, characterized in that: The base oil is a mixed oil of synthetic hydrocarbon oil and ester oil, the thickener is an aliphatic diurea compound, and the initial acid value of the grease composition is 2.0 mgKOH / g or more.

2. The grease composition according to claim 1, characterized in that The additive contains sulfur (S), phosphorus (P) and zinc (Zn) elements, and the total amount of these elements contained in the grease composition is 0.50 mass % or more relative to the total amount of the grease composition.

3. The grease composition according to claim 1, characterized in that The additive contains sulfur (S), phosphorus (P), zinc (Zn), barium (Ba) and calcium (Ca) elements, and the total amount of these elements contained in the grease composition is 0.50 mass% or more relative to the total amount of the grease composition.

4. The grease composition according to claim 3, characterized in that The total amount of the elements contained in the grease composition is 1.5 mass % or less based on the total amount of the grease composition.

5. The grease composition according to claim 1, characterized in that When the grease composition is exposed to a high temperature environment, the change in the acid value of the grease composition over time shows a behavior of decreasing from the initial acid value and then increasing.

6. The grease composition according to claim 1, characterized in that The aliphatic diurea compound is an aliphatic diurea compound obtained by reacting a diisocyanate component with two different aliphatic monoamines.

7. The grease composition according to claim 1, characterized in that The additive contains sulfur (S), phosphorus (P), zinc (Zn), barium (Ba) and calcium (Ca) elements, and the total amount of these elements contained in the grease composition is 0.50 mass% or more and 1.5 mass% or less relative to the total amount of the grease composition, The aliphatic diurea compound is an aliphatic diurea compound obtained by reacting a diisocyanate component with two different aliphatic monoamines. The dynamic viscosity of the base oil at 40°C is 40 mm 2 / s~70mm 2 / s.

8. A rolling bearing comprising an inner ring, an outer ring, a plurality of rolling elements interposed between the inner ring and the outer ring, and a grease composition sealed around the rolling elements, It is characterized in that The grease composition is the grease composition according to claim 1.

9. The rolling bearing according to claim 8, characterized in that: It is a bearing for engine.

10. A method for lubricating a rolling bearing, which is a method for lubricating a rolling bearing by a grease composition, wherein: The grease composition comprises: a base oil composed of a mixed oil of a synthetic hydrocarbon oil and an ester oil, and a thickener composed of an aliphatic diurea compound. The lubrication method is characterized in that, in the grease composition, the acid value is decreased from an initial acid value and then increased.

11. The rolling bearing lubrication method according to claim 10, characterized in that: The rolling bearing is lubricated in a high temperature environment of 80°C to 200°C.

12. The rolling bearing lubrication method according to claim 10, characterized in that: As the grease composition, a grease composition having an initial acid value of 2.0 mgKOH / g or more is used.

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