GREASE COMPOSITION FOR ROLLING BEARINGS, AND ROLLING BEARING

Description

TECHNICAL FIELD

The present invention relates to a grease composition for rolling bearings and a rolling bearing filled with the grease composition.

BACKGROUND ART

The main shaft of a machine tool is preferably rotated at high speed to increase machining efficiency, and various lubrication technologies are applied to its bearings. As lubrication methods suitable for high-speed rotating main shafts, methods such as oil mist lubrication, air-oil lubrication, and jet lubrication are known. However, such lubrication methods require ancillary equipment such as compressed air and oil supply devices; this is one of the causes of increasing the initial cost and running cost of machine tools. In contrast, grease lubrication can be said to be a preferable lubrication method with little need for maintenance.

Rolling bearings used in machine tools are often used in high-speed rotation ranges exceeding dmn 800,000 (dmn: the product of the ball pitch circle diameter (mm) and the rotational speed (min⁻¹)), and angular ball bearings and cylindrical roller bearings are used.

Also, in machine tools, high preload is sometimes applied to increase the rigidity of the main shaft; the PV value of the rolling contact part of the inner ring or outer ring often becomes 100 MPa·m/s or more, and load-carrying capacity under high-speed rotation conditions is also required for bearings for machine tools. Furthermore, as the rotation speed increases, the rolling surface of the bearing becomes hot; with unreliable grease, various disturbances can trigger a chain reaction of increased heat generation in the bearing and a decrease in the lubricating oil film, resulting in seizure.

To address such problems, heat-resistant thickeners and base oils are used, or antioxidants are added. For example, in consideration of heat resistance, Patent Literature 1 discloses a grease composition having a worked penetration of 220 to 320, in which a urea compound is blended as a thickener in a base oil having a kinematic viscosity at 40° C. of 15 to 40 mm²/s so that the urea compound accounts for 9 to 14% by mass of the total grease composition. Patent Literature 2 discloses a grease for high-speed bearings, which is obtained by blending a non-urea grease using a complex amido-lithium soap having an amide bond in the molecule as a thickener into a urea grease using a urea-based compound as a thickener.

Also, in machine tools, water-soluble coolant used during machining may enter the bearing. Mixing water particles into the oil film of the lubricated part makes it easier for metal contact to occur, and flaking and seizure occur, leading to the end of the bearing's life. It also causes rust, resulting in abnormal noise and premature bearing damage, necessitating bearing replacement in a short time.

As methods for suppressing such specific flaking and seizure that occur early, for example, Patent Literature 3 proposes a method of adding a passivation agent to a grease composition; Patent Literature 4 proposes a method of adding bismuth dithiocarbamate.

CITATION LIST

Patent Literatures

• • Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2006-29473
  • Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2009-275176
  • Patent Literature 3: Japanese Unexamined Patent Application Publication No. Hei 03-210394
  • Patent Literature 4; Japanese Unexamined Patent Application Publication No. 2005-42102

SUMMARY OF INVENTION

Problems to be Solved by the Invention

However, in recent years, with the downsizing and speed increase, operating conditions and usage conditions of rolling bearings have become even more severe, and the number of rolling bearings for machine tool main shafts used at high-speed rotation with a dmn value of 1.7 million or more is increasing. Along with this, the increase in heat generation on the rolling surface promotes grease thermal degradation, significantly shortening the grease life; since the number of times the rolling elements pass through the rolling surface increases, early flaking and seizure occur when coolant is mixed in, and there is a high possibility of reaching the end of life. With such an increase in rotational speed, machine tool rolling bearings are desired that can sufficiently cope with high-speed rotation, such as having excellent lubricity and high-temperature durability, and can prevent flaking and seizure on the rolling surface due to metal contact when coolant is mixed in, and prevent bearing damage due to rust generation.

Under such circumstances, an object of the present invention is to provide a grease composition that addresses the same problems as rolling bearings for machine tools; it aims to provide a grease composition that is excellent in seizure resistance under high-speed conditions, and also excellent in flaking resistance and seizure resistance when containing water. The present invention also aims to provide a rolling bearing filled with the grease composition.

Means for Solution of the Problems

The present invention provides the following grease composition and the rolling bearing filled with the grease composition.

• • 1. A grease composition for rolling bearings, comprising
    • a base oil;
    • as a thickener, a complex amido-lithium soap;
    • as antioxidants, 0.5 to 5% by mass of an amine-based antioxidant and 0.5 to 5% by mass of a phenol-based antioxidant, based on the total mass of the composition; and
    • as rust inhibitors, 0.1 to 5% by mass of a carboxylic acid-based rust inhibitor, 0.1 to 5% by mass of a carboxylate-based rust inhibitor, and 0.1 to 3% by mass of an amine salt of a fatty acid, based on the total mass of the composition, wherein
    • a 60-stroke worked penetration is 260 to 320 as measured according to JIS K 2220 7.
  • 2. The grease composition according to 1, wherein the thickener is a mixture of a lithium salt of an N-alkyl-substituted monoamide acid, a lithium salt of a dibasic acid, and an N-alkyl-substituted diamide.
  • 3. The grease composition according to 1, wherein the amine-based antioxidant is p,p′-dioctyldiphenylamine, and the phenol-based antioxidant is benzenepropanoic acid, 3,5-bis(1,1-dimethylethyl)-4-hydroxy-, C7-9 side-chain alkyl esters.
  • 4. The grease composition according to 1, wherein the carboxylic acid-based rust inhibitor is a combination of 2-tetrapropenyl succinic acid half ester and tetrapropenyl butanedioic acid, the carboxylate-based rust inhibitor is zinc naphthenate, and the amine-based rust inhibitor is a fatty acid amine salt.
  • 5. A bearing for a machine tool, comprising the grease composition according to any one of 1 to 4.

Advantageous Effects of Invention

The present invention can provide a grease composition and a bearing for machine tools that are excellent in seizure resistance under high-speed conditions, and in flaking resistance and seizure resistance when containing water. The grease composition of the present invention is also excellent in seizure resistance at high temperatures.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of the adhesion evaluation test.

FIG. 2 shows the test container used in the centrifugal oil separation test.

FIG. 3 shows the testing machine used in the centrifugal oil separation test.

DESCRIPTION OF EMBODIMENTS

<Base Oil>

The type of base oil used in the present invention is not particularly limited. Examples include mineral oils typified by naphthenic and paraffinic oils, synthetic hydrocarbon oils typified by polyalphaolefin (PAO) and polybutene, ether-based synthetic oils typified by alkyl diphenyl ether, ester oils, silicone oils, and various synthetic oils such as fluorinated oils. The synthetic oil may be a so-called biomass oil produced from biological resources derived from animals and plants. For example, biomass ester oil synthesized from various fatty acids and alcohols derived from vegetable oil, and biomass hydrocarbon oil using vegetable oils such as palm oil, corn oil, and soybean oil can also be used. The base oil may be used alone, or two or more types may be mixed. Of these, mineral oils and synthetic hydrocarbon oils are preferred, mixed oils of mineral oil and synthetic hydrocarbon oil are more preferred, and mixed oils of mineral oil and PAO are even more preferred.

The kinematic viscosity at 40° C. of the base oil of the present invention is preferably 10 mm²/s or more, and more preferably 20 mm²/s or more, from the viewpoint of securing the oil film thickness of the base oil in the lubrication part and preventing metal contact to improve durability. From the viewpoint of suppressing heat generation due to stirring, it is preferably 80 mm²/s or less, and more preferably 40 mm²/s or less.

When the base oil of the present invention is a mixed oil of mineral oil and PAO, the mass ratio of mineral oil to PAO is preferably mineral oil:PAO=30:70 to 50:50, and more preferably 35:65 to 45:55, from the viewpoints of oil supply performance and oxidative degradation of the base oil.

The content of the base oil in the composition of the present invention is preferably 49 to 77% by mass, more preferably 56 to 74% by mass, and even more preferably 67 to 74% by mass, based on the total mass of the composition, in consideration of adhesion and use in a high-speed environment. By including the base oil within such a range, the grease remains in the lubrication part, and leakage during high-speed operation can be suppressed, so that improvement in seizure life can be expected.

<Thickener>

The complex amido-lithium soap, which is the thickener of the present invention, is synthesized from an aliphatic dicarboxylic acid, an aliphatic monoamine, lithium hydroxide, and the like. A thickener containing a complex amido-lithium soap obtained by reacting lithium hydroxide with a mixture of N-octadecylsebacamic acid, which is a reaction product of an aliphatic dicarboxylic acid and an aliphatic monoamine, sebacic acid, and N,N′-dioctadecyl decanediamide (that is, a mixture of lithium N-octadecylsebacamate, lithium sebacate, and N,N′-dioctadecyl decanediamide) is particularly preferred.

The composition of the present invention preferably has a penetration of 260 to 300 so as to be suitable for use in a high-speed environment such as machine tool bearings. This range is also suitable for use at high temperatures. 260 to 290 is more preferable. The content of the thickener in the composition of the present invention may be any amount suitable for adjusting the penetration to the above range, and is, for example, 21.5 to 27% by mass, preferably 23 to 26% by mass, based on the total mass of the composition. In this specification, the term “penetration” refers to the 60-stroke worked penetration measured according to JIS K 2220 7.

From the viewpoint of oil supply performance, the grease composition of the present invention preferably does not contain a urea-based thickener.

<Antioxidant>

The grease composition of the present invention contains 0.5 to 5% by mass of an amine-based antioxidant and 0.5 to 5% by mass of a phenol-based antioxidant, based on the total mass of the composition.

The content of the amine-based antioxidant in the composition of the present invention is 0.5 to 5.0% by mass. A sufficient oxidative degradation preventing effect is exhibited at 0.5% by mass or more; on the other hand, even if it exceeds 5.0% by mass, the effect reaches a plateau. 0.5 to 3% by mass is preferred, and 0.8 to 2% by mass is more preferred.

The content of the phenol-based antioxidant in the composition of the present invention is 0.5 to 5.0% by mass. A sufficient oxidative degradation preventing effect is exhibited at 0.5% by mass or more; on the other hand, even if it exceeds 5.0% by mass, the effect reaches a plateau. 1 to 3% by mass is preferred, and 1.5 to 2.5% by mass is more preferred.

The amine-based antioxidant that can be used in the present invention is preferably selected from the group consisting of p,p′-dioctyldiphenylamine, N-phenyl-α-naphthylamine, alkylated phenyl-α-naphthylamine, and mixtures thereof. Of these, p,p′-dioctyldiphenylamine is particularly preferred.

The phenol-based antioxidant that can be used in the present invention is preferably selected from the group consisting of 2,6-di-tert-butyl-4-methylphenol, pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate), benzenepropanoic acid, 3,5-bis(1,1-dimethylethyl)-4-hydroxy-, C7-9 side-chain alkyl esters, octadecyl 3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate, and mixtures thereof. Of these, benzenepropanoic acid, 3,5-bis(1,1-dimethylethyl)-4-hydroxy-, C7-9 side-chain alkyl esters is particularly preferred.

In particular, combination of p,p′-dioctyldiphenylamine and benzenepropanoic acid, 3,5-bis(1,1-dimethylethyl)-4-hydroxy-, C7-9 side-chain alkyl esters is preferred from the viewpoint of suppressing oxidative degradation of grease. Particularly, a combination of 0.8 to 2% by mass of p,p′-dioctyldiphenylamine and 1.5 to 2.5% by mass of benzenepropanoic acid, 3,5-bis(1,1-dimethylethyl)-4-hydroxy-, C7-9 side-chain alkyl esters is preferred.

<Rust Inhibitor>

The grease composition of the present invention contains 0.1 to 5% by mass of a carboxylic acid-based rust inhibitor, 0.1 to 5% by mass of a carboxylate-based rust inhibitor, and 0.1 to 3% by mass of an amine salt of a fatty acid, based on the total mass of the composition.

The content of the carboxylic acid-based rust inhibitor in the composition of the present invention is 0.1 to 5% by mass. Sufficient water resistance and rust prevention are exhibited at 0.1% by mass or more; on the other hand, even if it exceeds 5% by mass, the effect reaches a plateau. 0.2 to 3% by mass is preferred, and 0.2 to 1% by mass is more preferred.

The content of the carboxylate-based rust inhibitor in the composition of the present invention is 0.1 to 5% by mass. Sufficient water resistance and rust prevention are exhibited at 0.1% by mass or more; on the other hand, even if it exceeds 5% by mass, the effect reaches a plateau. 0.2 to 3% by mass is preferred, and 0.2 to 1% by mass is more preferred.

The content of the amine salt of a fatty acid in the composition of the present invention is 0.1 to 3% by mass. Water resistance and rust prevention are exhibited at 0.1% by mass or more; on the other hand, even if it exceeds 3% by mass, the effect reaches a plateau. 0.3 to 2.5% by mass is preferred, and 0.5 to 2% by mass is more preferred.

Examples of the carboxylic acid-based rust inhibitor that can be used in the present invention include, as monocarboxylic acids, for example, straight-chain fatty acids such as lauric acid and stearic acid, and saturated carboxylic acids having a naphthene nucleus; as dicarboxylic acids, for example, succinic acid derivatives such as succinic acid, alkylsuccinic acids, alkylsuccinic acid half esters, alkenylsuccinic acids, alkenylsuccinic acid half esters, and succinimides, tetrapropenyl butanedioic acid, hydroxy fatty acids, mercapto fatty acids, and sarcosine derivatives. Among them, 2-tetrapropenyl succinic acid half ester and tetrapropenyl butanedioic acid are preferred.

Examples of the carboxylate-based rust inhibitor that can be used in the present invention include metal salts of fatty acids, naphthenic acids, abietic acid, lanolin fatty acid, alkenylsuccinic acid, and amino acid derivatives. Examples of the metal element include cobalt, manganese, zinc, aluminum, calcium, barium, lithium, magnesium, and copper; among them, zinc naphthenate is particularly preferred.

Examples of the amine salt of a fatty acid that can be used in the present invention include alkoxyphenylamine, amine salts of fatty acids, and partial amides of dibasic carboxylic acids, and amine salts of fatty acids are preferred.

In particular, from the viewpoint of suppressing water mixing into the oil film, a combination of a carboxylic acid-based rust inhibitor containing an alkenylsuccinic acid half ester, zinc naphthenate, and an amine salt of a fatty acid is preferred. More particularly, a combination of 2-tetrapropenyl succinic acid half ester, tetrapropenyl butanedioic acid, zinc naphthenate, and an amine salt of a fatty acid is preferred.

Particularly, a combination of 0.2 to 0.8% by mass of a carboxylic acid-based rust inhibitor containing an alkenylsuccinic acid half ester, 0.2 to 0.8% by mass of zinc naphthenate, and 0.8 to 1.2% by mass of an amine salt of a fatty acid is preferred. More particularly, a combination of 0.1 to 0.4% by mass of 2-tetrapropenyl succinic acid half ester, 0.1 to 0.4% by mass of tetrapropenyl butanedioic acid, 0.2 to 0.8% by mass of zinc naphthenate, and 0.8 to 1.2% by mass of an amine salt of a fatty acid is preferred.

As the composition of the present invention, a grease composition for machine tool bearings is particularly preferred, comprising

• • a base oil that is a mixed oil having a kinematic viscosity at 40° C. of 20 to 40 mm²/s, with the mass ratio of mineral oil to PAO being 35:65 to 45:55,
  • a thickener that is a complex amido-lithium soap that is a reaction product of a mixture of N-octadecylsebacamic acid, sebacic acid, and N,N′-dioctadecyl decanediamide with lithium hydroxide,
  • antioxidants that include 0.8 to 2% by mass of p,p′-dioctyldiphenylamine and 1.5 to 2.5% by mass of benzenepropanoic acid, 3,5-bis(1,1-dimethylethyl)-4-hydroxy-, C7-9 side-chain alkyl esters, based on the total mass of the composition, and
  • rust inhibitors that include 0.1 to 0.4% by mass of 2-tetrapropenyl succinic acid half ester, 0.1 to 0.4% by mass of tetrapropenyl butanedioic acid, 0.2 to 0.8% by mass of zinc naphthenate, and 0.8 to 1.2% by mass of an amine salt of a fatty acid, based on the total mass of the composition, wherein
  • a 60-stroke worked penetration is 260 to 290 as measured according to JIS K 2220 7.

In addition to the above components, the grease composition of the present invention can further contain additives commonly used in grease compositions. Examples of such optional additives include extreme pressure agents and waxes.

Examples of the extreme pressure agent include phosphorous acid esters such as triphenyl phosphorothioate, triphenyl phosphite, and triethyl phosphite, and glycerin fatty acid esters such as glycerol monocaprate and glycerol monostearate.

Examples of the wax include monoamide wax, ester wax, ketone wax, and polyethylene oxide wax.

The content of such optional additives is, for example, 0.1 to 5% by mass, preferably 0.3 to 3% by mass, based on the total mass of the composition.

The grease composition of the present invention is used for rolling bearings. In particular, it is preferably used for rolling bearings of machine tool bearings, and more particularly for main shaft support portions of rolling bearings of machine tools. In addition to machine tool bearings, it can also be applied to bearings that slide and rotate at high speed, similar to machine tool bearings. It can also be applied to bearings that slide and rotate at high speed and high temperature.

EXAMPLES

Examples 1 to 3 and Comparative Examples 2 to 5

A mixture of N-octadecylsebacamic acid, sebacic acid, and N,N′-dioctadecyl decanediamide was reacted with lithium hydroxide in a base oil, heated and cooled to obtain a base grease. Additional base oil was added so that the 60-stroke worked penetration shown in Table 1 was achieved, each additive was added at the ratio shown in Table 1, and dispersed with a three-roll mill to obtain the grease compositions of Examples 1 to 3 and Comparative Examples 2 to 5. The respective blending ratios and types are as shown in Table 1. The numerical values for each component in the table represent mass % based on the total mass of the composition, except that the numerical values for mineral oil and PAO represent mass ratios, and the unit of kinematic viscosity is mm²/s.

Comparative Example 1

A base grease containing lithium 12-hydroxystearate in a base oil was used, and the same procedure as above was performed to obtain the grease composition of Comparative Example 1.

The obtained grease compositions of Examples and Comparative Examples were evaluated by the following test methods and test conditions.

1. Thermal Hardening Test (Evaluation of Seizure Resistance at High Temperature)

The test grease is uniformly applied to a 60×80×1 mm SPCC-SD steel plate at a thickness of 2 mm, left to stand in an air circulating constant temperature bath at 160° C. for 300 hours, and the penetration after the test is measured.

<Evaluation>

• • ∘: Penetration of 150 or more
  • x: Penetration of less than 150
    2. Adhesion Evaluation Test (Evaluation of Seizure Resistance under High-Speed Conditions)

The test grease 4 in tablet form is placed on an SPCC-SB steel plate shown in FIG. 1, the steel plate is rotated under the following conditions, and the remaining amount of grease (g) after the test is measured.

• • Rotational speed: 1000 rpm
  • Test time: 3 min
  • Temperature: 25° C.

<Evaluation>

• • ∘: Remaining amount of grease of 0.045 g or more
  • x: Remaining amount of grease of less than 0.045 g

3. Oil Supply Performance Test (Evaluation of Seizure Resistance)

The grease in tablet form is placed on a round 5C filter paper, left to stand in an air circulating constant temperature bath at 40° C. for 168 hours, and the oil supply rate is calculated by the following formula from the weight change of the filter paper before and after the test.

Oil supply amount (%)=(Amount of oil bleeding into filter paper after test (g)/Amount of grease before test (g))×100

<Evaluation>

• • ∘: Oil supply rate of 30% or more and less than 50%
  • x: Oil supply rate of less than 30% or 50% or more

4. Centrifugal Oil Separation Test (Evaluation of Seizure Resistance Under High-Speed Conditions)

The grease 4 to be tested is accommodated in the main body 3 of the test container 2 whose inside is partitioned by a mesh 1 (FIG. 2), and this test container 2 is connected to the tip of the arm 5 of the centrifugal oil separation tester 6 (FIG. 3) equipped with the arm 5 rotating around the rotation axis S, and the arm 5 is rotated under the following conditions, and a centrifugal force is applied to the grease in the test container 2 as indicated by the dotted line in FIG. 3. The degree of oil separation is calculated from the weight of the oil separated after the test is completed.

• • Test time: 168 hours
  • Rotational speed: 2000 rpm
  • Temperature: 40° C.
  • Distance L between mesh 1 and center of rotation axis S: 46.5 mm

Degree of oil separation (%)=(Amount of separated oil (g)/Amount of grease before test (g))×100

<Evaluation>

• • ∘: Degree of oil separation of 15% or more and less than 30%
  • x: Degree of oil separation of less than 15% or 30% or more
    5. Grease Water Absorption Test (Evaluation of Flaking Resistance and Seizure Resistance when Containing Water)

The water absorption (%) is calculated as measured according to the Japanese National Defense Standards NDS K 2756B.

<Evaluation>

• • ∘: Water absorption of less than 25%
  • x: Water absorption of 25% or more

	TABLE 1
				Compar-	Compar-	Compar-	Compar-	Compar-
				ative	ative	ative	ative	ative
	Exam-	Exam-	Exam-	Exam-	Exam-	Exam-	Exam-	Exam-
	ple 1	ple 2	ple 3	ple 1	ple 2	ple 3	ple 4	ple 5

Thickener	Type	Complex	Complex	Complex	Lithium	Complex	Complex	Complex	Complex
		Amido-	Amido-	Amido-	Soap	Amido-	Amido-	Amido-	Amido-
		lithium	lithium	lithium		lithium	lithium	lithium	lithium
		Soap	Soap	Soap		Soap	Soap	Soap	Soap
	Amount of Thickener	25	26	23	12	25	25	25	28

Base Oil	Type	Mineral Oil	40	40	40	40	40	40	40	40
		PAO	60	60	60	60	60	60	60	60

Kinematic Viscosity (40° C.), mm2/s

34

34

34

34

34

34

34

34

Anti-	Amine-based	p,p′-Dioctyl-	1	1	1	1	—	1	1	1
oxidant	Antioxidant	diphenylamine
	Phenol-based	C7-9 Branched alkyl	2	2	2	2	2	—	2	2
	Antioxidant	ester of 3-(3,5-di-
		tert-butyl-4-hydroxy-
		phenyl)propanoate
Rust	Carboxylic	2-Tetrapropenyl succinic	0.14	0.14	0.14	0.14	0.14	0.14	—	0.14
Inhibitor	Acid	acid half ester
		Tetrapropenyl	0.24	0.24	0.24	0.24	0.24	0.24	—	0.24
		Butanedioic Acid
	Carboxylate	Zinc Naphthenate	0.38	0.38	0.38	0.38	0.38	0.38	—	0.38

Fatty Acid Amine Salt

1.01

1.01

1.01

1.01

1.01

1.01

—

1.01

Worked Penetration

275

260

300

275

275

275

275

240

Performance	Thermal Hardening Test	◯	◯	◯	◯	X	X	◯	◯
Evaluation	Adhesion Evaluation Test	◯	◯	◯	X	◯	◯	◯	X

	Oil Supply	◯	◯	◯	X	◯	◯	◯	◯
	Performance Test
	Centrifugal Oil	◯	◯	◯	X	◯	◯	◯	◯
	Separation Test
	Grease Water	◯	◯	◯	◯	◯	◯	X	◯
	Absorption Test