US20170130157A1

US20170130157A1 - Grease composition, rolling bearing, and motor

Info

Publication number: US20170130157A1
Application number: US15/347,490
Authority: US
Inventors: Yusuke Asai; Yusuke Enomoto; Shintaro TAKATA
Original assignee: Minebea Co Ltd
Current assignee: MinebeaMitsumi Inc
Priority date: 2015-11-10
Filing date: 2016-11-09
Publication date: 2017-05-11

Abstract

A grease composition according to an embodiment includes a base oil, a thickener and 0.5% by weight or more of a dispersing agent, the dispersing agent including at least one of a carboxylic acid metal salt, a metal salt of a polycarboxylic acid and a copolymer of a carboxylic acid metal salt and a hydrocarbon compound.

Description

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2015-220400 filed in Japan on Nov. 10, 2015 and Japanese Patent Application No. 2016-209200 filed in Japan on Oct. 26, 2016.

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a grease composition, a rolling bearing, and a motor.
2. Description of the Related Art
A grease composition is used for lubrication in a rolling bearing applied to a fan motor and other motors of a consumer electronics product and a communication device. The grease composition contains a base oil, a thickener, and additives. Examples of the grease composition include a urea grease containing a urea compound as the thickener. The urea grease has excellent heat resistance and thus a rolling bearing using the urea grease has excellent acoustic properties even when the rolling bearing is used at high temperature.
Among grease compositions containing a base oil, a thickener, and additives, for example, Japanese Laid-open Patent Publication No. 2004-224823 discloses a urea grease containing a urea compound as the thickener and at least one of carboxylic acids, carboxylic acid salts, and esters as the additive (a rust inhibitor).
Among the grease compositions containing a base oil, a thickener, and an additive, Japanese Laid-open Patent Publication No. 2004-339448 discloses a urea grease containing the base oil containing 30% by mass (% by weight) or more of an aromatic ester oil and containing 5% by mass to 35% (% by weight) by mass of a diurea compound.
The temperature of the environment in which fan motors are used is becoming higher due to increase of the amount of heat generated by consumer electronics products and communication devices. Therefore, the environmental temperature in which the rolling bearings are used is becoming higher. Consequently, it is required for urea greases to be free of thermal deterioration even when used under an environmental temperature higher than conventional. However, conventional urea greases as disclosed in Japanese Patent Application Laid-open No. 2004-224823 and Japanese Patent Application Laid-open No. 2004-339448 have insufficient heat resistance and thus the urea grease agglomerates under high temperature environments. Consequently, acoustic properties become deteriorated when the grease is used in a rolling bearing. Therefore, further improvement is required.
The present invention was made in view of the above problem and an object of the present invention is to provide a grease composition that shows less thermal deterioration even when the grease composition is used under high temperature environment and a rolling bearing and a motor that have excellent acoustic properties even when used under high temperature environment.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least partially solve the problems in the conventional technology.
A grease composition according to an embodiment includes a base oil, a thickener and 0.5% by weight or more of a dispersing agent, the dispersing agent including at least one of a carboxylic acid metal salt, a metal salt of a polycarboxylic acid and a copolymer of a carboxylic acid metal salt and a hydrocarbon compound.
The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view illustrating a fan motor including a rolling bearing according to an embodiment of the present invention;

FIG. 2 is a sectional view illustrating the rolling bearing according to an embodiment of the present invention;

FIG. 3 is a graph illustrating the experimental results of embodiments of the present invention and comparative examples; and

FIG. 4 is a graph illustrating the experimental results of embodiments of the present invention and comparative examples.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A grease composition, a rolling bearing, and a motor according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings. The following embodiment, however, does not limit the present invention. In each drawing, the same reference number will be assigned to the same or corresponding element and a repeated description will be omitted.
FIG. 1 is a sectional view illustrating a fan motor 10 including rolling bearings 20 according to an embodiment of the present invention. FIG. 2 is a sectional view illustrating the rolling bearing 20 according to an embodiment of the present invention.
The fan motor 10 according to an embodiment of the present invention includes a rotor shaft 11, rolling bearings 20, a stator 12, an impeller 13, and a casing 16. The rotor shaft 11 is held so as to be rotatable by the rolling bearing 20. The impeller 13 has a rotor housing 14 and blades 15 provided on the outer periphery of the rotor housing 14.
The rolling bearing 20 includes an inner ring 21, an outer ring 22, spherical rolling elements 23, a retainer 24, and sealing members 25. The internal space of the rolling bearing 20 sealed by the sealing members 25 is filled with a grease composition G. The rolling bearing 20 according to the embodiment of the present invention may be applied not only to the fan motor but also to other types of motors.
Subsequently, the grease composition G according to the embodiment of the present invention will be described. The grease composition G according to the embodiment contains a base oil and a thickener, and further contains at least any one of a carboxylic acid metal salt, a metal salt of a polycarboxylic acid, and a copolymer of a carboxylic acid metal salt and a hydrocarbon compound as a dispersing agent. As a result of the intensive studies about the grease composition, the inventors of the present invention verified that, when the rolling bearing with a conventional grease composition is used under high temperature environment (for example, 140° C. or higher) over a long period of time, the acoustic properties of the rolling bearing deteriorates due to coarsening of the agglomerates in the grease composition. The inventors of the present invention found out that, by adding at least one of a carboxylic acid metal salt, a metal salt of a polycarboxylic acid, and a copolymer of a carboxylic acid metal salt and a hydrocarbon compound as the dispersing agent in the grease composition, the coarsening of the agglomerates could be reduced even when the rolling bearing is used under a high temperature environment over a long period of time and thus the deterioration in the acoustic properties could be reduced. The grease composition G according to the embodiment of the present invention was accomplished based on this finding.
There is no limitation as to the type of the base oil. Synthetic hydrocarbon oils, alkyl ether oils, alkyl diphenyl ether oils, ester oils, mineral oils, fluorine oils, silicone oils, and other oils, which are commonly used as the grease base oils, can be used singly or in combination. The content of the base oil may be, for example, 70% by weight or more and 90% by weight or less.
Examples of the synthetic hydrocarbon oils may include normal paraffins, isoparaffins, and poly-α-olefins such as polybutenes, polyisobutylenes, 1-decene oligomers, and oligomer of 1-decene and ethylene. Examples of the ester oils may include diester oils such as dibutyl sebacate, di-2-ethylhexyl sebacate, dioctyl sebacate, dioctyl adipate, diisodecyl adipate, ditridecyl adipate, ditridecyl tallate, and methyl acetyl ricinoleate; aromatic ester oils such as trioctyl trimellitate, tri-2-ethylhexyl trimellitate, tridecyl trimellitate, tetraoctyl pyromellitate, and tetra-2-ethylhexyl pyromellitate; polyol ester oils such as trimethylolpropane caprylate, trimethylolpropane pelargonate, pentaerythritol-2-ethyl hexanoate, and pentaerythritol pelargonate; and carbonic acid ester oils. Examples of the alkyl diphenyl ether oils may include mono alkyl diphenyl ethers, dialkyl diphenyl ethers, and poly alkyl diphenyl ethers. Among the above base oils, the aromatic ester oils are preferable and can be used singly or in combination. In particular, the base oil prepared by mixing tri-2-ethylhexyl trimellitate and tetra-2-ethylhexyl pyromellitate is preferably used.
As the thickener, a non-urea compound or a urea compound can be used. However, from the viewpoint of the heat resistance and the acoustic properties, the urea compound is preferably used. The content amount of the thickener may be, for example, 10% by weight or more and 30% by weight or less.
Examples of the non-urea compound may include metal soaps and polytetrafluoroethylene resins and the like. The metal soap is synthesized from an aliphatic monocarboxylic acid such as stearic acid or an aliphatic monocarboxylic acid containing at least one hydroxyl group such as 12-hydroxystearic acid and an alkaline earth metal hydroxide. A complex metal soap synthesized from the aliphatic monocarboxylic acid and a dibasic acid such as an aliphatic dicarboxylic acid can also be used.
As the urea compound, urea compounds such as diurea compounds, triurea compounds, and polyurea compounds can be used. In particular, the diurea compound is preferably used from the viewpoint of the heat resistance and the acoustic properties (quietness). The diurea compound can be represented by following Formula (1).
R₁—NHCONH—R₂—NHCONH—R₃ (1)
Here, R₁, R₂and R₃are hydrocarbon groups selected from aliphatic hydrocarbon groups, alicyclic hydrocarbon groups and aromatic hydrocarbon groups. Specifically, R₁, R₂and R₃may be aliphatic hydrocarbon groups, alicyclic hydrocarbon groups, or aromatic hydrocarbon groups and there is no limitation as to the carbon numbers of these groups. R₂is preferably an aromatic hydrocarbon group and more preferably an aromatic hydrocarbon group having one or two phenyl groups as substituents. As raw materials used at the time of the synthesis of these urea compounds, an amine compound and an isocyanate compound are used. As the amine compound, aliphatic amines represented by hexylamine, octylamine, dodecylamine, hexadecylamine, octadecylamine, stearylamine, and oleylamine, cycloaliphatic amines represented by cyclohexylamine, and, in addition, aromatic amines represented by aniline, p-toluidine, and ethoxyphenylamine are used. As the isocyanate compound, phenylene diisocyanate, tolylene diisocyanate, diphenyl diisocyanate, diphenylmethane diisocyanate, octadecane diisocyanate, decane diisocyanate, and hexamethylene diisocyanate are used. An aliphatic-aromatic diurea compound synthesized from the aliphatic amine and the aromatic amine used as the amine starting material and the aromatic isocyanate is preferably used.
Although there is no limitation as to the carboxylic acid used in the metal salt of a carboxylic acid contained as the dispersing agent, the carboxylic acid is preferably an aliphatic carboxylic acid. The aliphatic carboxylic acid may be a saturated aliphatic carboxylic acid or an unsaturated aliphatic carboxylic acid. Further, there is no limitation as to the polycarboxylic acid used in the metal salt of a polycarboxylic acid as well. As a carboxylic acid constituting the polycarboxylic acid, an aliphatic carboxylic acid is preferable as well. The aliphatic carboxylic acid may be a saturated aliphatic carboxylic acid or an unsaturated aliphatic carboxylic acid. As the carboxylic acid constituting the polycarboxylic acid, an unsaturated aliphatic carboxylic acid is more preferably used. Examples of the unsaturated carboxylic acids include acrylic acid, crotonic acid, isocrotonic acid, 3-butenoic acid, methacrylic acid, angelic acid, tiglic acid, 4-pentenoic acid, 2-ethyl-2-butenoic acid, 10-undecenoic acid, and oleic acid. Examples of the saturated dicarboxylic acids include oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, and sebacic acid. Examples of the saturated carboxylic acids include formic acid, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, enanthic acid, caprylic acid, pelargonic acid, capric acid, lauric acid, myristic acid, palmitic acid, margaric acid, and stearic acid. Examples of the unsaturated dicarboxylic acids include fumaric acid, maleic acid, and itaconic acid. The polycarboxylic acid constituted by these carboxylic acids may be polymers of the monocarboxylic acids and polymers of the dicarboxylic acids. In particular, the polymer of the unsaturated carboxylic acid having one or two carboxy groups is preferable. The metal salts of these carboxylic acids may form copolymers with hydrocarbon compounds. In other words, the grease composition may contain the copolymer of a carboxylic acid metal salt and a hydrocarbon compound as the dispersing agent. The grease composition may also be constituted by containing at least any one of a carboxylic acid metal salt, a metal salt of a polycarboxylic acid, and the copolymer of a carboxylic acid metal salt and the hydrocarbon compound. Specific examples of the carboxylic acid metal salt, the metal salt of the polycarboxylic acid, or the carboxylic acid metal salt forming the copolymer of the carboxylic acid metal salt and the hydrocarbon compound include at least one salt selected from alkali metal salts and alkaline earth metal salts. As the alkali metal salts, sodium salts, lithium salts, and potassium salts and the like are preferable, and as the alkaline earth metal salts, magnesium salts and calcium salts are preferable. Examples of the hydrocarbon compound polymerized (reacted as polymerization) with the carboxylic acid metal salt in the copolymer of the carboxylic acid metal salt and the hydrocarbon compound include isobutylene, propylene, isoprene, and butadiene.
As the weight average molecular weight of the polycarboxylic acid, the weight average molecular weight (Mw) in terms of polyethylene glycol measured by gel permeation chromatography (GPC) is preferably 5,000 or more and 200,000 or less, more preferably 7,000 or more and 80,000 or less, and further preferably 9,000 or more and 16,000 or less.
In the grease composition G according to the embodiment of the present invention, the carboxylic acid metal salt or the metal salt of the polycarboxylic acid that is the dispersing agent is preferably contained in an amount of 0.5% by weight or more. In the case in which the content of the carboxylic acid metal salt or the metal salt of the polycarboxylic acid is 0.5% by weight or more, it becomes possible to improve heat resistance, reduce thermal deterioration, and provide excellent acoustic properties. It is noted that when the dispersing agent is a copolymer of the carboxylic acid metal salt and the hydrocarbon compound, the copolymer is preferably contained in an amount of 0.5% by weight or more. In the grease composition G according to the embodiment of the present invention, the content of the dispersing agent is preferably 10% by weight or less.
The grease composition G can contain antioxidants, anti-friction agents, metal deactivators, rust inhibitors, oiliness agents, viscosity index improvers, and the like as other additives, if necessary.
In the grease composition, the rolling bearing, and the motor according to the present embodiment constituted as described above, the grease composition contains the dispersing agent described above and thus the grease composition has high heat resistance and exhibits less thermal deterioration even used under high temperature environment. Therefore, the rolling bearing exhibits excellent acoustic properties. Here, the grease composition can reduce coarsening of the agglomerates when used under high temperature environment over a long period of time because the grease composition contains the dispersing agent and thus the rolling bearing with such grease composition exhibits excellent acoustic properties even under high temperature use.
Any embodiment or improvement constituted by appropriately combining the components described above is also included in the scope of the present invention. Further modified examples can be readily derived by those skilled in the art. A broader aspect of the present invention is not limited to the above embodiment and various modifications are possible.
For example, in the embodiment described above, the grease composition may contain the sodium salt of the carboxylic acid or the polycarboxylic acid. Further, The grease composition may also contain a metal salt other than a sodium salt of a carboxylic acid or a polycarboxylic acid as the dispersing agent. Aside from the sodium salt of the carboxylic acid or the polycarboxylic acid, examples of the metal salt of the carboxylic acid or the polycarboxylic acid include lithium salts, potassium salts, magnesium salts, and calcium salts. In this case, the grease composition may contain at least one kind of a metal salt of a carboxylic acid or a polycarboxylic acid. When the grease composition contains the metal salt of the carboxylic acid or the polycarboxylic acid, the same effect as the effect of the grease composition G described above is obtained. The constitution of the copolymer of the carboxylic acid metal salt and the hydrocarbon compound may be a sodium salt or a metal salt other than a sodium salt. When the grease composition contains two or more kinds of dispersing agents selected from the group of a carboxylic acid metal salt, the metal salt of the polycarboxylic acid, and the copolymer of the carboxylic acid metal salt and the hydrocarbon compound, the dispersing agents are preferably contained in an amount of 0.5% by weight or more in total.

Embodiments and Comparative Examples

Subsequently, Embodiments according to the present invention and Comparative Examples will be described. In Embodiments and Comparative Examples, various grease compositions were prepared and properties of the grease compositions were studied. In Table 1 and Table 2, the grease compositions of Embodiments and Comparative Examples are shown, where ticks (checkmarks) indicate that ingredients of the corresponding rows are contained in the grease compositions of the corresponding Embodiments and Comparative Examples.
The grease compositions of Embodiments 1 to 20 and Comparative Examples 1 to 9 were prepared by mixing the base oil, the thickener, and the dispersing agent according to the components shown in Tables 1 and 2. As the base oil, the ester oil (TOTM (trioctyl trimellitate)+TOPM (tetraoctyl pyromellitate)), PAO (poly-α-olefin) and a mineral oil were used. As the thickener, the mixture of an aromatic urea compound and an aliphatic urea compound were used as the diurea compound A (Diurea A in Tables 1 and 2) and a mixture of an aliphatic urea compound and an alicyclic urea compound were used as the diurea compound B (Diurea B in Tables 1 and 2). As the dispersing agent, a sodium salt of polyacrylic acid is used as the metal salt of the polycarboxylic acid (A in Tables 1 and 2), a copolymer of sodium maleate and isobutylene is used as the copolymer of the carboxylic acid metal salt and the hydrocarbon compound (B in Tables 1 and 2), and sebacic acid sodium salt is used as the carboxylic acid metal salt (C in Tables 1 and 2). In Tables 1 and 2, figures listed in percentage represent the contents of the dispersing agents in % by mass. More specifically, the grease compositions of Embodiments 1 to 20 contained Dispersing agent A, B, or C in a range of 0.5% by weight to 10.0% by weight. The grease compositions of Comparative Examples 1 to 3 contained no dispersing agent. The grease compositions of Comparative Examples 4, 5 and 8 contained Dispersing agent A, B or C in an amount of 0.1% by weight, respectively. Furthermore, the grease compositions of Comparative Examples 6 and 7 contained polyacrylic acid ammonium salt (D in Table 2) and polyacrylic acid amine salt (E in Table 2) as polycarboxylic acid salts, which are other than the metal salts, in an amount of 2.0% by weight, respectively. Furthermore, the grease composition of Comparative Example 9 contained sebacic acid ammonium salt (F in Table 2), which is a carboxylic acid salt but not a metal salt, in an amount of 2.0% by weight. Each prepared grease composition was subjected to a stationary heating test described below, and thereafter the size of agglomerates was evaluated.

TABLE 1

(Embodiments)

			Embodi-	Embodi-	Embodi-	Embodi-	Embodi-	Embodi-	Embodi-
			ment 1	ment 2	ment 3	ment 4	ment 5	ment 6	ment 7

Base	Ester	TOTM +	✓	✓	✓	✓	✓	✓	✓
oil	oil	TOPM
	PAO	50 cSt
	Mineral	—
	oil
Thickener	Diurea	Aromatic +	✓	✓	✓	✓	✓	✓	✓
	A	aliphatic urea
	Diurea	Aliphatic +
	B	alicyclic urea
Dispersing	A	Polyacrylic acid	0.5%	1.0%	2.0%	5.0%	9.0%	10.0%
agent		sodium salt
	B	Copolymer of							0.5%
		maleic acid
		sodium salt and
		isobutylene
	C	Sebacic acid
		sodium salt

Maximum diameter of agglomerates	80	20	20	20	20	20	80
after stationary heating test (μm)

			Embodi-	Embodi-	Embodi-	Embodi-	Embodi-	Embodi-	Embodi-
			ment 8	ment 9	ment 10	ment 11	ment 12	ment 13	ment 14

Base	Ester	TOTM +	✓	✓	✓	✓	✓
oil	oil	TOPM
	PAO	50 cSt						✓	✓
	Mineral	—							✓
	oil
Thickener	Diurea	Aromatic +	✓	✓	✓	✓	✓
	A	aliphatic urea
	Diurea	Aliphatic +						✓	✓
	B	alicyclic urea
Dispersing	A	Polyacrylic acid
agent		sodium salt
	B	Copolymer of	1.0%	2.0%	5.0%	9.0%	10.0%	2.0%	2.0%
		maleic acid
		sodium salt and
		isobutylene
	C	Sebacic acid
		sodium salt

Maximum diameter of agglomerates	20	20	20	20	20	40	50
after stationary heating test (μm)

			Embodi-	Embodi-	Embodi-	Embodi-	Embodi-	Embodi-
			ment 15	ment 16	ment 17	ment 18	ment 19	ment 20

Base	Ester	TOTM +	✓	✓	✓	✓	✓	✓
oil	oil	TOPM
	PAO	50 cSt
	Mineral	—
	oil
Thickener	Diurea	Aromatic +	✓	✓	✓	✓	✓	✓
	A	aliphatic urea
	Diurea	Aliphatic +
	B	alicyclic urea
Dispersing	A	Polyacrylic acid
agent		sodium salt
	B	Copolymer of
		maleic acid
		sodium salt and
		isobutylene
	C	Sebacic acid	0.5%	1.0%	2.0%	5.0%	9.0%	10.0%
		sodium salt

Maximum diameter of agglomerates	60	20	20	20	20	20
after stationary heating test (μm)

TABLE 2

(Comparative Example)

Compar-	Compar-	Compar-	Compar-	Compar-	Compar-	Compar-	Compar-	Compar-
ative	ative	ative	ative	ative	ative	ative	ative	ative
Example 1	Example 2	Example 3	Example 4	Example 5	Example 6	Example 7	Example 8	Example 9

Base	Ester	TOTM +	✓			✓	✓	✓	✓	✓	✓
oil	oil	TOPM
	PAO	50 cSt		✓	✓
	Mineral	—			✓
	oil
Thickener	Diurea	Aromatic +	✓			✓	✓	✓	✓	✓	✓
	A	aliphatic urea
	Diurea	Aliphatic +		✓	✓
	B	alicyclic urea
Dispersing	A	Polyacrylic acid				0.10%
agent		sodium salt
	B	Copolymer of					0.10%
		maleic acid
		sodium salt and
		isobutylene
	C	Sebacic acid								0.10%
		sodium salt
	D	Polyacrylic acid						2.0%
		ammonium salt
	E	Polyacrylic acid							2.0%
		amine salt
	F	Carboxylic acid									2.0%
		ammonium salt

Maximum diameter of agglomerates	250	500	500	130	130	200	190	150	230
after stationary heating test (μm)

The size of the agglomerates was evaluated by carrying out the stationary heating test in which each grease composition was left stationary in an environment of 160° C. for 500 hours, and thereafter observing the size of the agglomerates with an optical microscope. At this time, the maximum diameter of the agglomerates was measured by visual observation covering an area of 300 μm square with a magnification of 200 times. The results of the experiment are listed in Table 1 and 2.
As listed in the heat resistance evaluation results in Tables 1 and 2, in Embodiments 1 to 20, the maximum diameters of the agglomerates were less than 100 μm even when the grease compositions were left in a high temperature environment of 160° C. over a long period of time. In contrast, in Comparative Examples 1 to 9, the agglomerates in the grease compositions were coarsened to a maximum diameter of 100 μm or larger.
In addition, for the grease compositions of Embodiments 3, 9 and 16 and Comparative Examples 1 and 4, viscosity was measured as evaluation of heat resistance. Specifically, the prepared grease compositions of Embodiments 3, 9 and 16 and Comparative Examples 1 and 4 were subjected to the stationary heating test under each temperature condition of 120° C., 140° C., and 160° C. for 500 hours and thereafter viscosity of each grease composition were measured. The viscosity was measured using a rheometer under a condition of temperature of 25° C., shear rate of 1/s, and gap of 0.2 mm. The experimental result is illustrated in FIG. 3. In FIG. 3, the horizontal axis represents the stationary test temperature and the vertical axis represents the viscosity. Seen from the results illustrated in FIG. 3, although the difference of the viscosities between Embodiments and Comparative Examples in the results of the stationary heating test at 120° C. was not large, the grease compositions of Embodiments 3, 9 and 16 exhibited significantly lower viscosities than the grease compositions of Comparative Examples 1 and 4 after the stationary heating test under high temperature environments of 140° C. and the 160° C. Namely, it was ascertained that the grease compositions of Embodiments 3, 9 and 16 exhibited less thermal deterioration even under high temperature environments exceeding 120° C. and thus the properties of the grease compositions were maintained.
For the grease compositions of Embodiments 3, 9 and 16 and Comparative Examples 1 and 4, a rotational heating test was carried out as a heat resistance test. In the rotational heating test, rolling bearings were filled with each of grease composition and rotated at temperatures of 120° C., 140° C. and 160° C., at a rotation speed of 3000 rpm for 500 hours and thereafter the acoustic property of each of the rolling bearings was verified.
Specifically, the test grease corresponding to an amount of 25% to 35% of the bearing internal volume was provided in a steel shielded ball bearing (inner diameter 8 mm, outer diameter 22 mm, and width 7 mm). The ball bearing was mounted in a housing and a shaft was inserted into the bearing bore, followed by coupling the shaft to the rotating shaft of a test motor. The rotational heating test was carried out in a manner that each of ball bearings was rotated by the inner ring under the respective test temperature condition.
After the rotational heating test, the evaluation of the acoustic property was performed by measuring the Anderon values of the ball bearings for M band (300 Hz to 1,800 Hz) using an Anderon meter (manufactured by Sugawara Laboratories Inc.). The frequency of M band (300 Hz to 1,800 Hz) is considered to be unpleasant sound for humans. The experimental results are illustrated in FIG. 4. In FIG. 4, the horizontal axis represents the test temperature and the vertical axis represents the Anderon value. As illustrated in FIG. 4, the grease composition of Embodiments 3, 9 and 16 exhibited significantly lower Anderon values after the rotational heating test than those of the grease compositions of Comparative Examples 1 and 4 even under high temperature environments of 140° C. and 160° C. and their excellent acoustic properties (quietness) were confirmed. For example, in the case of rotational heating test at 160° C., the Anderon values after the test were about 5 for both Embodiments 3, 9, and 16. This value is less than ⅓ of the Anderon value of Comparative Example 1 (about 17) and less than ½ compared with the Anderon value of Comparative Example 4 (about 13).
As listed in Table 2, after the stationary heating test is carried out, the grease compositions of Comparative Examples 1 and 4 have maximum diameters of the agglomerates of 250 μm and 130 μm, respectively, which exceed the size of 100 In addition, the grease compositions of Comparative Examples 1 and 4 showed Anderon values of more than 10 after the rotation under heating test at 160° C., and thus unpleasant sound was heard. Consequently, as the grease composition, a grease composition containing the dispersing agent so that the maximum diameter of the agglomerates generated after the stationary heating test in which the grease composition is left in an environment of 160° C. for 500 hours is 100 μm or less is preferable.
As can be seen from Embodiments and Comparative Examples described above, the dispersing agent in the present invention is contained in the grease composition to minimize coarsening of the agglomerates caused by thermal deterioration of the thickener such as a urea compound.
According to the present invention, a grease composition that minimizes thermal deterioration when the grease composition is used in high temperature environment as well as a rolling bearing and a motor that have excellent acoustic property when used in high temperature environment can be provided.
Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited to them but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.

Claims

What is claimed is:

1. A grease composition including:

a base oil;

a thickener; and

0.5% by weight or more of a dispersing agent,

the dispersing agent including at least one of a carboxylic acid metal salt, a metal salt of a polycarboxylic acid and a copolymer of a carboxylic acid metal salt and a hydrocarbon compound.

2. The grease composition according to claim 1, wherein at least one of the carboxylic acid metal salt and the metal salt of the polycarboxylic acid is an alkali metal salt or an alkaline earth metal salt, the alkali metal salt including at least one of a sodium salt, a lithium salt, and a potassium salt, the alkaline earth metal salt including at least one of a magnesium salt and a calcium salt.

3. The grease composition according to claim 1, wherein the carboxylic acid metal salt forming the copolymer of the carboxylic acid metal salt and the hydrocarbon compound is an alkali metal salt or an alkali earth metal salt, the alkali metal salt including at least one of a sodium salt, a lithium salt, and a potassium salt, the alkaline earth metal salt including at least one of a magnesium salt and a calcium salt.

4. The grease composition according to claim 1, wherein a content of the dispersing agent is 10% by weight or less.

5. The grease composition according to claim 1, wherein the thickener comprises a diurea compound represented by Formula (1):

R₁—NHCONH—R₂—NHCONH—R₃ (1)

where R₁, R₂, and R₃are hydrocarbon groups selected from aliphatic hydrocarbon groups, alicyclic hydrocarbon groups, and aromatic hydrocarbon groups.

6. A rolling bearing comprising the grease composition according to claim 1.

7. A motor comprising the rolling bearing according to claim 6.

8. A grease composition comprising:

a base oil;

a thickener comprising a diurea compound represented by Formula (1):

R₁—NHCONH—R₂—NHCONH—R₃ (1)

where R₁, R₂, and R₃are hydrocarbon groups selected from aliphatic hydrocarbon groups, alicyclic hydrocarbon groups, and aromatic hydrocarbon groups; and

0.5% by weight or more and 10% by weight or less of a dispersing agent,

the dispersing agent including at least one of a sodium salt of sebacic acid, a sodium salt of polyacrylic acid and a copolymer of maleic acid sodium salt and isobutylene.

9. A rolling bearing comprising the grease composition according to claim 8.

10. A motor comprising the rolling bearing according to claim 9.