Solid anti-friction devices – materials therefor – lubricant or se – Solid anti-friction device – article or material therefor
Reexamination Certificate
2000-11-08
2002-01-15
Howard, Jacqueline V. (Department: 1764)
Solid anti-friction devices, materials therefor, lubricant or se
Solid anti-friction device, article or material therefor
C508S320000, C508S441000
Reexamination Certificate
active
06339049
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART STATEMENT
This invention relates to grease compositions which are used to give lubricity to bearings, sliding parts, and junctions of a variety of equipment, and particularly to grease compositions of low dust emission which are suitable for use in bearings such as linear guides, ball screws, and rolling bearings of semiconductor manufacturing equipment, liquid crystal manufacturing equipment, electronic computers, and the like that require a clean working environment and also to bearings sealed with said grease compositions.
In regard to those grease compositions which are used in bearings such as linear guides, ball bearings, and rolling bearings to be incorporated into equipment that require a clean working environment such as semiconductor manufacturing equipment, liquid crystal manufacturing equipment, and electronic computers, the property of low dust emission is extremely important because the product value would be lost or erroneous functioning would be caused if grease becomes airborne and adheres to wafers and recording media.
Up to the present, fluorine greases of extremely low volatility composed of perfluoropolyether (PFPE) as base oil and polytetrafluoroethylene (PTFE) as thickener have generally been used in the aforementioned bearings that require a clean working environment. However, the fluorine greases have high rolling resistance because of the use of high-viscosity PFPE as base oil and become the cause of heat generation and overloaded motor. Moreover, there is a problem of the fluorine greases being inferior to greases based on mineral oil or synthetic oil in wear resistance.
To solve the aforementioned problems, several grease compositions of low dust emission have been proposed.
For example, a variety of grease compositions have been formulated from mineral oil, synthetic hydrocarbon oil, ether oil, or ester oil of a specified kinematic viscosity as base oil and metallic soap such as lithium salt of C
12
-C
24
higher fatty acids and higher hydroxyfatty acids or a urea compound such as diurea as thickener at a specified ratio, thereby increasing the hardness of grease to have the property of low dust emission and at the same time reducing the rolling resistance as described in the specifications of Japan Kokai Tokkyo Koho Hei 5-9489 (1993), Japan Kokai Tokkyo Koho Hei 8-143884 (1996), Japan Kokai Tokkyo Koho Hei 8-270747 (1996), and Japan Kokai Tokkyo Koho Hei 9-217745 (1997).
In addition to the property of low dust emission, however, grease compositions for use in bearings such as linear guides, ball screws, and rolling bearings intended for semiconductor manufacturing equipment, liquid crystal manufacturing equipment, and electronic computers are required to have not merely low rolling resistance but also a narrow range of variation in rolling resistance because the precision positioning unit of a bearing is required to function with uniform velocity during low-speed operation and with rapid follow-up during microstep feeding (both low rolling resistance and small variation in rolling resistance are sometimes referred to as “low rolling characteristics” hereinafter). Furthermore, the grease compositions are required to perform well in fretting wear resistance as microvibration generated by microfeeding at the time of use causes fretting wear.
Hardening grease in order to reduce dust emission decreases the fretting resistance and increases the resistance of grease, thus adversely affecting the low rolling characteristics. Conversely, softening grease in order to improve the fretting resistance and low rolling characteristics makes it impossible to attain the desired level of low dust emission.
No grease compositions with a good balance of low dust emission, low rolling characteristics and fretting resistance have been known thus far and what is practiced at the present time is to apply grease compositions at the sacrifice of some of the properties depending upon the end use.
OBJECT AND SUMMARY OF THE INVENTION
The present inventors have conducted intensive studies to develop grease compositions with a balanced performance with respect to low dust emission, low rolling characteristics, and fretting resistance, found that the incorporation of using lithium stearate as thickener, controlling the hardness of grease in terms of worked penetration (JIS K2220 5.3) in the relatively soft range 270-350 and using one kind or two kinds or more of compounds selected from a group of oxidized paraffins and diphenyl hydrogenphosphite in specified amounts can help to display a well-balanced performance of low dust emission, low rolling characteristics, and fretting resistance, thereby extending the lubricating period and reducing the maintenance cost, and completed this invention.
Accordingly, an object of this invention is to provide grease compositions of low dust emission which have a balanced performance in respect to low dust emission, low rolling characteristics, and fretting resistance, can extend the lubricating period, reduce the maintenance cost, and are suitable for use in a clean environment.
Another object of this invention is to provide bearings of low dust emission which are sealed with said grease compositions of low dust emission and are suitable for use in a clean environment such as semiconductor manufacturing equipment, liquid crystal manufacturing equipment, and electronic computers.
Thus, this invention relates to grease compositions which contain 10-35 mass % of (A) lithium stearate as thickener and 0.5-15.0 mass % of (B) one kind or two kinds or more of compounds selected from a group of oxidized paraffins and diphenyl hydrogenphosphite and display a worked penetration in the range 270-350.
This invention also relates to bearings which are sealed with said grease compositions.
This invention will be described in detail below.
Lubricating base oils to be used in this invention include mineral oils and/or synthetic oils. The kinematic viscosity of these base oils at 100° C. is normally 2 mm
2
/sec., preferably 3 mm
2
/sec., at its lower limit and it is normally 40 mm
2
/sec., preferably 30 mm
2
/sec., at its upper limit. The viscosity index of the base oils is normally 90 or more, preferably 95 or more.
Mineral oil is obtained by a procedure generally practiced in the process for producing lubricating oils in petroleum refining. A lubricating oil fraction obtained by atmospheric distillation or vacuum distillation of crude oil is refined by subjecting it to one or more treatments such as solvent deasphalting, solvent extraction, hydrocracking, solvent dewaxing, catalytic dewaxing, hydrorefining, sulfuric acid washing, and clay treatment.
Synthetic oils include polybutenes, poly-&agr;-olefins such as 1-octene oligomers, and 1-decene oligomers and their hydrogenated products, diesters such as ditridecyl glutarate, di(2-ethylhexyl) adipate, diisodecyl adipate, ditridecyl adipate, and di(3-ethylhexyl) sebacate, polyol esters such as trimethylolpropane caprylate, trimethylolpropane pelargonate, pentaerythritol 2-ethylhexanoate, and pentaerythritol pelargonate, alkylnaphthalenes, alkylbenzenes, polyoxyalkylene glycols, polyphenyl ethers, dialkyl diphenyl ethers, silicone oils, and mixtures of these compounds.
As a lubricating base oil of this invention, one kind of the aforementioned mineral oils and/or synthetic oils can be used singly or a mixture of two or more kinds can also be used.
Lithium stearate to be used as component A in this invention means either lithium stearate or lithium salts of a mixture of fatty acids consisting mainly of stearic acid. These lithium salts can be obtained generally by the reaction of stearic acid or a mixture of fatty acids consisting mainly of stearic acid with lithium hydroxide. Also, a mixture of fatty acids consisting mainly of stearic acid can be obtained by hydrolyzing natural oils and fats and, for example, directly separated fatty acids and hardened fatty acids obtained from beef tallow are desirable raw materials for lithium stearate.
The aforementioned directly separa
Funahashi Hiroshi
Honjo Yoshiyuki
Kaneko Tomonori
Kinoshita Hirotugu
Sakamoto Kiyomi
Arent Fox Kintner Plotkin & Kahn
Howard Jacqueline V.
THK Co. Ltd.
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