Lubricating grease composition for bearings of electronic devices

Abstract: A lubricating grease composition comprises a base oil and a thickening agent, wherein the thickening agent comprises (a) a lithium salt of hydroxystearic acid and (b) a lithium salt of a C 6 to C 14 fatty acid. The grease composition is useful for the improvement of the acoustic characteristics of bearings, in particular, the reduction of noise and the elongation of the service life of the bearings. Accordingly, the grease composition can be used for lubricating bearings, in particular, those used in spindle motors for the operation of recording devices such as those for computer hard disks and CD-ROM's.

Description: Field of the Invention The present invention relates to a lubricating grease composition and, in particular, to a grease composition for ball bearings. Description of the Prior Art As grease compositions used for bearings, in particular those used in spindle motors for operating recording devices such as those for computer hard disks and CD-ROMs, there have conventionally been used, for instance, grease compositions which comprise an ester oil such as dioctyl sebacate or a pentaerythritol ester as a base oil and a thickening agent such as lithium stearate or lithium hydroxystearate. However, the acoustic life of these bearings has generally been insufficient. This is because the recording density of these devices has recently been increased and this in turn requires improvement of the precision of bearings and an increase in the rate of revolution of such motors. The latter would be accompanied by an increase in the temperature of bearings during practical use. US-A-5714444 and GB-A-2278612 disclose greases for bearings of electronic apparatus, the greases exhibiting reduced splashing and reduced noise and comprising base oil and a thickener which comprises Li soap of a C 12 - C 24 fatty acid having no hydroxyl group and Li soap of C 12 - C 24 fatty acid having a hydroxyl group; they have practical examples only of the use of Li stearate as the non-hydroxy soap component of the thickener. Summary of the Invention. It is an object of the present invention to provide a lubricating grease composition which is useful for improvement of the acoustic characteristics of bearings, in particular for reduction of noise, and for elongation of the service life of the bearings. The inventors have found that this object can effectively be accomplished by the use as a thickening agent of a soap having a quite uniform fibrous structure, and that such a soap can be formed by the simultaneous use of (a) a lithium salt of hydroxystearic acid (such as 12-hydroxystearic acid) and (b) a lithium salt of a C 6 to C 10 fatty acid. Accordingly the present invention provides a lubricating grease composition which comprises a base oil and a thickening agent, wherein the thickening agent comprises (a) a lithium salt of hydroxystearic acid and (b) a lithium salt of a C 6 to C 10 fatty acid. Description of the Preferred Embodiments Hydroxystearic acid and C 6 to C 10 fatty acids may, in general, be commercially available and they are put on the market in the form of products having various degrees of purity. A low purity product has fatty and unsaturated fatty acids of differing carbon atom numbers; if the content of fatty acids other than the desired ones is high in commercially available products, the grease compositions obtained using such products can have characteristic properties which vary widely. The present invention preferably employs hydroxystearic (for instance, 12-hydroxystearic) and C 6 to C 10 fatty acids whose purity is not less than 80% (by weight) to eliminate the foregoing drawbacks. The lithium salt of fatty acid used in the invention as the foregoing component (b) has a carbon atom number ranging from 6 to 10. This is because a lithium salt of fatty acid used herein having a carbon atom number of not more than 5 has a low solubility in the base oil and it would be necessary to heat the resulting soap to a temperature of not less than 260°C to solubilize the same; since most of the base oils used in the invention have flash points lower than this, there would then be a danger of a fire during preparation of the grease composition. On the other hand, a lithium salt of fatty acid of high carbon atom number is quite similar to the lithium salt of hydroxystearic acid as the principal fatty acid component and therefore, the addition thereof cannot ensure any conspicuous effect for the simultaneous use of these two components. In particular, if the carbon atom number of the fatty acid exceeds 20, the solubility thereof in the base oil is extremely high, it does not ensure a desired thickening effect at all, and the amount of the soap required for achieving a desired penetration is increased to a considerably high level as compared with that required when the lithium salt of hydroxystearic acid is used alone (i.e., 100%). This in turn leads to an increase in the production cost. In the lubrication grease composition of the invention, the molar ratio of the lithium hydroxystearic acid (a) to the lithium salt of fatty acid (b) preferably ranges from 5:95 to 95:5 and more preferably 10:90 to 95:5. The base oil used in the lubricating grease composition preferably has a kinematic viscosity, as determined at 40°C, ranging from 5 to 200 mm 2 /s and more preferably 10 to 100 mm 2 /s. Examples of the base oils usable in the lubricating grease composition of the invention include ester type synthetic oils such as carboxylic acid ester compounds and polyol ester oils; ether type synthetic oils such as alkyl diphenyl ethers; synthetic hydrocarbon oils such as poly(α-olefin) type oils; and mineral oils such as paraffinic mineral oils. Among these base oils, particularly preferred are those comprising ester oils in an amount of not less than 10% by mass and more preferably not less than 50% by mass based on the total mass of the base oil. The content of the thickening agent in the lubricating grease composition preferably ranges from 5 to 40% by mass and more preferably 7 to 20% by mass on the basis of the total amount of the grease composition. As has been described above in detail, the grease composition of the present invention is useful for the improvement of the acoustic characteristics of bearings, in particular the reduction of noise and the elongation of the service life of the bearings. Accordingly, the grease composition can be used for lubricating bearings, in particular those used in spindle motors for operating recording devices such as those for computer hard disks and CD-ROM s. The present invention will be described in more detail with reference to the following Examples, but the present invention is not restricted to these specific Examples at all. Example 1 In this Example, a pentaerythritol ester oil (kinematic viscosity as determined at 40°C: 33 mm 2 /s) was used as a base oil and a fatty acid was reacted with lithium hydroxide in the base oil as will be detailed below to give a grease composition (200 g each). The amount of the fatty acid was set at 10% by mass and that of lithium hydroxide was adjusted in such a manner that the amount thereof was 5% higher than the saponification value practically determined. The fatty acid used herein had a purity of not less than 80%. The process used for preparing the grease composition was as follows: a) A base oil was mixed with a fatty acid in a beaker, followed by heating the mixture to about 80°C to thus solubilize the latter in the base oil and to form a solution A; b) Lithium hydroxide was mixed with 5 times of distilled water in a beaker and then the beaker was heated to about 80°C to give a solution B; c) The solution B was introduced into the solution A and they were reacted with one another for about 5 minutes; and d) After the completion of the reaction, the reaction system was heated to dissolve the resulting soap in the base oil (maximum temperature: 235°C), followed by allowing the system to stand to cool the reaction system to room temperature and once passing it through a three-roll mill (clearance: 10 to 20 µm) to knead the same and to thus give a desired grease composition. Example 2 A pentaerythritol ester oil (kinematic viscosity as determined at 40°C: 33 mm 2 /s) was used as a base oil and grease compositions (200 g each) were prepared by the method detailed below. The content of the soap in the grease composition was adjusted to 10% by mass. a) A base oil and a lithium soap were dispensed into a single beaker and the mixture was heated to 235°C; and b) After the dissolution of the soap in the base oil, the resulting solution was allowed to stand to cool the same down to room temperature and then once passed through a three-roll mill (clearance: 10 to 20 µm) to give each desired grease composition. The lubricating grease compositions prepared in the foregoing Examples 1 and 2 were inspected for characteristic properties by the following methods: Acoustic Life Testing Method The following bearing was operated for a predetermined period of time under conditions specified below, using each grease composition, then the level of Anderon (quantity of sound) and the magnitude of noise were determined using an Anderon tester (AD-SN-4 available from Sugawara Kenkyusho Co., Ltd.). The results were evaluated by points based on the magnitude of noise and Anderon levels thus determined, and compared with those observed for a standard product (Sample 10 of Comparative Example 1, Sample 11 of Comparative Example 2 or Sample 16 of Reference Example). (1) Test Conditions : Bearing used: 608VV; test temperature: 100°C; rate of revolution: 1800 rpm; test time: 500 hours; and amount of filled grease: 0.35 ml (corresponding to about 40% of the dead volume of the bearing). (2) Anderon Test (determination of the level of Anderon and the magnitude of noise): These quantities were determined at 1800 rpm for a test time of 120 sec. Evaluation : The grease compositions were evaluated according to the following criteria, with respect to the standard product: A: A grease composition, which has a point greater than 1.1 times that observed for the standard product; B: A grease composition, which has a point ranging from 0.9 to 1.1 times that observed for the standard product; and C: A grease composition, which has a point of less than 0.9 time that observed for the standard product. Method for Confirming the Length and Width of Soap Fibers The length and width of soap fibers were determined by observation under an electron microscope (LEM-2000 available from Akashi Seisakusho Co., Ltd.). The resulting grease sample was diluted 5 to 20 times with vaseline and the diluted sample was applied onto a metal mesh for electron microscopic observation. Then the metal mesh was put in a glass dish filled with a solvent (such as n-hexane) to thus remove the oil components from the grease sample. The metal mesh was withdrawn from the glass dish, dried and fitted to the electron microscope for the observation of soap fibers. The magnification of the microscope was adjusted to x6000, followed by selection of 5 fibers among those having a typical size and mainly constituting the soap present in one visual field to determine the lengths and widths thereof and determination of the average of these measured values. The results thus obtained are summarized in the following Tables. Example 1 Comparative Example 1 Samples 1 2 3 4 5 6 7 8 9 Principal Fatty Acid Hydroxystearic acid 70 70 50 95 70 30 70 80 70 Auxiliary Fatty Acid Caproic acid (C6) 30 -- -- -- -- -- -- -- -- Caprylic acid (C8) -- 30 50 5 -- 70 -- -- -- Capric acid (C10) -- -- -- -- 30 -- -- -- -- Lauric acid (C12) -- -- -- -- -- -- 30 20 -- Myristic acid (C14) -- -- -- -- -- -- -- -- 30 Worked Penetration 197 193 196 200 192 232 198 206 200 Dropping Point (°C) 217 209 212 205 207 216 203 -- 200 Acoustic Life A A A A A A A A A Length of Fiber (µm) 0.45 0.68 0.55 0.95 0.88 0.59 0.94 0.92 1.05 Width of Fiber (µm) 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 Comparative Example 1 Samples 10 11 12 Principal Fatty Acid Hydroxystearic acid 100 70 70 Auxiliary Fatty Acid Caprylic acid (C8) -- -- -- Palmitic acid (C16) -- -- 30 Acetic acid (C2) -- 30 -- Worked Penetration 219 -- 205 Dropping Point (°C) 196 -- 198 Acoustic Life B -- B Length of Fiber (µm) 1.84 0.25 1.75 Width of Fiber (µm) 0.05 0.01 0.05 The symbol "--" appearing in the columns entitled "Worked Penetration", "Dropping Point" and "Acoustic Life" means that the corresponding sample was not inspected for these properties. The hydroxystearic acid used in Example 1 and Comparative Example 1 is 12-hydroxystearic acid (purity: 85%). In addition, other fatty acids used have a purity of not less than 90%. Regarding the size of the soap fiber, the samples 1 to 9 and 11 were thin and short as compared with the fibers of the sample 10, while the sample 12 had a size similar to that of the sample 10. Example 2 Samples 1 2 3 4 5 Principal Soap Li salt of hydroxystearic acid 70 70 70 50 30 Auxiliary Soap Li salt of caproic acid (C6) 30 -- -- -- -- Li salt of caprylic acid (C8) -- 30 -- -- -- Li salt of capric acid (C10) -- -- 30 50 70 Li salt of lauric acid (C12) -- -- -- -- -- Li salt of myristic acid (C14) -- -- -- -- -- Worked Penetration 221 213 209 202 203 Acoustic Life A A A A A Length of Fiber (µm) 0.65 0.84 0.86 0.68 0.85 Width of Fiber (µm) 0.02 0.03 0.03 0.02 0.02 Example 2 Comparative Example 2 Example 2 Samples 6 7 8 9 10 Principal Soap Li salt of hydroxystearic acid 10 95 70 70 70 Auxiliary Soap Li salt of caproic acid (C6) -- -- -- -- -- Li salt of caprylic acid (C8) -- -- -- -- 15 Li salt of capric acid (C10) 90 5 -- -- 15 Li salt of lauric acid (C12) -- -- 30 -- -- Li salt of myristic acid (C14) -- -- -- 30 -- Worked Penetration 265 206 212 204 204 Acoustic Life A A A A A Length of Fiber (µm) 0.90 0.75 0.90 1.08 0.94 Width of Fiber (µm) 0.02 0.05 0.03 0.03 0.03 Comparative Example 2 Samples 11 12 13 14 15 16 Principal Soap Li salt of hydroxystearic acid 100 70 70 70 0 100 Auxiliary Soap Li salt of capric acid (C10) -- -- -- -- 100 -- Li salt of stearic acid (C18) -- -- 30 -- -- -- Li salt of behenic acid (C22) -- -- -- 30 -- -- Li salt of acetic acid (C2) -- 30 -- -- -- -- Worked Penetration 221 -- 221 273 437 250 Acoustic Life B -- B C -- B Length of Fiber (µm) 1.70 -- 1.55 1.86 2.63 1.60 Width of Fiber (µm) 0.05 -- 0.05 0.05 0.16 0.05 The symbol "--" appearing in the columns entitled "Worked Penetration" and "Acoustic Life" means that the corresponding sample was not inspected for these properties. The lithium hydroxystearate used in Example 2 and Comparative Example 2 is lithium 12-hydroxystearate (purity: 85%). In addition, other fatty acids used have a purity of not less than 90%. The sample of Reference Example 16 was prepared using lithium 12-hydroxystearate having a purity of about 70% and a 80:20 (weight ratio) mixture of pentaerythritol ester oil and a diester oil as the base oil. The kinematic viscosity of the base oil was 26.0 mm 2 /s at 40°C and the dropping point of the grease composition was found to be 190°C. Regarding the size of the soap fiber, the samples 1 to 10 were thin and short as compared with the fibers of the sample 11, while the samples 13 to 16 had a size similar to or greater than that of the sample 11.

Claims: A lubricating grease composition which comprises a base oil and a thickening agent, wherein the thickening agent comprises (a) a lithium salt of hydroxystearic acid and (b) a lithium salt of a C 6 to C 10 fatty acid. A composition according to claim 1 wherein the molar ratio of the lithium salt of hydroxystearic acid (a) to the lithium salt of fatty acid (b) ranges from 5:95 to 95:5. A composition according to claim 1 or 2 wherein the kinematic viscosity of the base oil as determined at 40°C ranges from 5 to 200 mm 2 /s. A composition according to claim 3 wherein the kinematic viscosity of the base oil as determined at 40°C ranges from 10 to 100 mm 2 /s. A composition according to any preceding claim wherein the base oil comprises not less than 10% by weight of an ester oil. A composition according to claim 5 wherein the base oil comprises not less than 50% by weight of an ester oil. A composition according to claim 1 wherein the molar ratio of the lithium salt of hydroxystearic acid (a) to the lithium salt of fatty acid (b) ranges from 5:95 to 95:5, the kinematic viscosity of the base oil as determined at 40°C ranges from 5 to 200 mm 2 /s and the base oil comprises not less than 10% by weight of an ester oil. A composition according to any preceding claim wherein the content of the thickening agent ranges from 5 to 40% by weight on the basis of the total weight of the composition. A composition according to claim 8 wherein the content of the thickening agent ranges from 7 to 20% by weight on the basis of the total weight of the composition.

Inventors: KIMURA HIROSHI, SHIBAYAMA ATSUSHI, KIMURA, HIROSHI, SHIBAYAMA, ATSUSHI
Applicants: KYODO YUSHI, KYODO YUSHI CO., LTD.
Application year: 1999
Main classification: C10M123
IPC codes: C10M123, C10M169, C10M117