Solid anti-friction devices – materials therefor – lubricant or se – Lubricants or separants for moving solid surfaces and... – Organic -co- compound
Reexamination Certificate
1999-12-21
2001-05-15
McAvoy, Ellen M. (Department: 1764)
Solid anti-friction devices, materials therefor, lubricant or se
Lubricants or separants for moving solid surfaces and...
Organic -co- compound
C508S465000, C508S485000
Reexamination Certificate
active
06232278
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a lubricating grease composition and, in particular, to a grease composition for ball bearings.
2. 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-ROM's, 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 the increase in the recording density in turn requires the improvement of the precision of bearings and an increase in the number of revolutions of such motors. The latter would be accompanied by an increase in the temperature of bearings during practical use.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a lubricating grease composition, which 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.
According to an aspect of the present invention, there is provided 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
14
fatty acid.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The inventors of this invention have found that the foregoing object of the present invention can effectively be accomplished by the use of a soap having a quite uniform fibrous structure, as a thickening agent. Such a soap having a quite uniform fibrous structure 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
14
fatty acid.
In this respect, hydroxystearic acid and C
6
to C
14
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. Those having low purity comprise fatty acids and unsaturated fatty acids, whose carbon atom number differs from each other. In this respect, if the content of fatty acids other than the desired ones is high in a commercially available product, the grease composition obtained using such a product has characteristic properties, which vary widely. Thus, the present invention employs, as the hydroxystearic acid (for instance, 12-hydroxystearic acid) and the C
6
to C
14
fatty acid, the purity each of which 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 14. 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 accordingly, it is necessary to heat the resulting soap to a temperature of not less than 260° C. to thus solubilize the same. However, most of the base oils used in the invention has a flash point lower than that temperature and therefore, there is a danger of catching a fire during the preparation of the grease composition. On the other hand, a lithium salt of fatty acid selected having a carbon atom number of not less than 15 is quite similar to those observed for the lithium salt of hydroxystearic acid as the principal fatty acid component and therefore, the addition thereof cannot ensure any conspicuous effect of the simultaneous use of these two components or it is not necessary to daringly incorporate it into the grease composition. 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 C
6
to C
14
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(&agr;-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.
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patent: 4582616 (1986-04-01), Kita et al.
patent: 4749502 (1988-06-01), Alexander et al.
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patent: 5714444 (1998-02-01), Yokouchi et al.
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patent: 8-209176 (1996-08-01), None
patent: 8-270747 (1996-10-01), None
Kimura Hiroshi
Shibayama Atsushi
Kyodo Yushi Co. Ltd.
McAvoy Ellen M.
Oblon, Spivak, McClelland, Maier & Neustadt. P.C.
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