Bearing grease composition

Abstract: A bearing grease composition used under clean atmosphere, as in a bearing of hard disc drive, which has small dust generation (scatter), a long life, excellent acoustic performance and low torque. It has been found that especially the bearing grease composition comprising a carbonate having a branched alkyl group of 13 to 15 carbon atoms and a metallic soap shows remarkable characteristics.

Description: FIELD OF THE INVENTION The present invention relates to a bearing grease composition for hard disc drive or the like. More particularly, the invention relates to a bearing grease composition for spindle motors which are used under clean environment, as in memories such as hard disc drive (HDD) or floppy disc drive (FDD) in computers. BACKGROUND OF THE INVENTION In general, performances required for a bearing grease composition used in memories, for examples, HDD or FDD in computers are that dust generation (scatter) is low, torque is small, acoustic performance is excellent and life is long. In particular, in HDD used under a clean atmosphere, fine particles of gaseous oil or grease scattered from the inside of a bearing during revolution contaminates the surface of a disc, resulting in cause of wrong operation. Therefore, it is considered to be most important to suppress the amount of scatter. A sodium complex soap-based grease comprising a mineral oil as a base oil or a lithium soap-based grease comprising as a base oil, a synthetic ester (diester oil or polyol ester oil) which is a reaction product of an organic acid and an alcohol has conventionally been used as HDD bearing grease. The sodium complex soap-based grease comprising a mineral oil as a base oil has been used for a long time with appreciation that the amount of scatter is small. However, the grease had the problems that dispersion of a thickening agent in the grease is poor, and it is difficult to form a homogeneous mixture, so that acoustic and vibrating performances during evolution of the bearing is not good; moisture absorption is high, and the grease cures with the passage of time, so that flowability of the grease in the bearing becomes poor, causing defective lubrication. Further, the lithium soap-based grease comprising as a base oil, a synthetic ester (diester or polyol ester oil) which is a reaction product of an organic acid and an alcohol had no problem due to that dispersibility of the lithium soap is good, and low torque property was good. However, the lithium soap-based grease tends to scatter,and if the grease is used as it is, there is the great possibility to damage a hard disc. Therefore, in order to prevent the damage, the grease is used in combination with an expensive magnetic fluid seal which is used in a motor. This rather results in increase of cost of a motor and prevents the motor from being miniaturized. Further, for example, due to the demand of high speed revolution and high precision of the motor, this grease does not sufficiently withstand acoustic performance and torque, disorder has sometimes occurred in acoustic performance or torque. SUMMARY OF THE INVENTION As a result of extensive investigations to overcome the above-described problems in the prior art, it has been found that properties required for a bearing grease composition which is used under a clean environment as in, for example, memories such as HDD or FDD are satisfied by using a metallic soap-based grease comprising as a base oil a carbonate containing an organic carbonate represented by the formula (I) described hereinafter. The present invention has been completed based on this finding. Accordingly, an object of the present invention is to provide a grease composition which can maintain stable low torque property, low noise property and low scatter property even at high speed revolution. According to the present invention, there is provided a bearing grease composition comprising components (a) and (b) where (a) is a carbonate compound represented by the following formula (I) wherein R and R', which may be the same or different, each represent a branched alkyl group having 13 to 15 carbon atoms; and (b) consists of an alkali metal salt and/or an alkaline earth metal salt, which are synthesized from a hydroxide of an alkali metal or an alkaline earth metal and a higher hydroxyfatty acid having 10 or more carbon atoms or a higher hydroxyfatty acid having at least one hydroxyl group and having 10 or more carbon atoms. The branched alkyl group R and R' in the carbonate compound represented by the formula (I) is represented by the following formula (II): wherein n is a number of 13 to 15 and m is a number of 0 to 6. The representative examples of the branched alkyl group are groups represented by the following formulae (III), (IV) and (V): The bearing grease composition of the present invention preferably comprises 70 to 95 parts by weight of component (a) of the carbonate compound, and 5 to 30 parts by weight of component (b) of the alkali metal salt and/or alkaline earth metal salt. If the amount of component (b) of the alkali metal salt and/or alkaline earth metal salt is less than 5 parts by weight, a worked penetration becomes soft, and as a result, the grease composition leaks or scatters during revolution of a bearing, resulting in possibility of contaminating HDD, FDD and the like. On the other hand, if the above amount of component (b) exceeds 30 parts by weight, the grease composition becomes too hard, and flowability of grease in the bearing becomes poor. As a result, there is the possibility of causing defective lubrication. The bearing grease composition according to the present invention can contain, in addition to the components (a) and (b) described above, lubricants other than the component (a), and various additives, as a third component. Examples of the lubricants used in the present invention include mineral oils, synthesized hydrocarbon oils, ether oils and ester oils, which are generally used as a base oil of grease compositions. The additives used in the present invention are additives generally used in grease composition, and the examples thereof include antioxidants and rust inhibitors. The grease composition of the present invention may further contain one or more of thickening agents. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention described in detail below. (1) According to the preferred embodiments of the present invention, a bearing grease composition preferably comprises components (a) and (b) where (a) is a carbonate compound represented by the following formula (I) wherein R and R' which may be the same or different each represent a branched alkyl group having 13 to 15 carbon atoms; and at least one of R and R' is wherein n is an integer of 9, 10 or 11; and (b) consists of an alkali metal salt and/or an alkaline earth metal salt, synthesized from a hydroxide of an alkali metal or an alkaline earth metal and a higher fatty acid having 10 or more carbon atoms or a higher fatty acid having at least one hydroxyl group and having 10 or more carbon atoms wherein said bearing grease composition comprises 70 to 95 parts by weight of component (a) and 5 to 30 parts by weight of component (b). (3) The bearing grease composition comprises 70 to 95 parts by weight of the carbonate compound represented by formulas (I) and (II) and 5 to 30 parts by weight of the member selected from the group consisting of an alkali metal salt and/or an alkaline earth metal salt, synthesized from a hydroxide of an alkali metal or an alkaline earth metal and a higher hydroxyfatty acid having 10 or more carbon atoms or a higher hydroxyfatty acid having at least one hydroxyl group and having 10 or more carbon atoms. (4) The bearing grease composition for hard disc drive includes preferably a plurality of carbonate compounds. (5) The bearing grease composition for hard disc drive in which the carbonate compound preferably has a dynamic viscosity of 10 to 50 mm 2 /sec at 40 °C (6) Preferably, the bearing grease composition further comprises one or more thickening agents. (7) The bearing grease composition as in any of the above-described aspects (1) to (6) comprising further lubricants and/or additives. The carbonate compound can be used alone or as mixtures thereof. The present invention will now be described in more detail below by referring to the following examples, but the invention should not be limited thereto. EXAMPLES 1 TO 6 Those examples are the preparation example of the typical bearing grease composition consisting of component (a) and component (b). A carbonate represented by the following formula (VI): that is, in the formula (I), R is the branched alkyl group represented by the following formula (IV): and R' is the branched alkyl group represented by the following formula (V): (this carbonate is hereinafter referred to as "carbonate oil A") alone, or a mixed oil of a carbonate containing commercially available organic carbonate having different structure of an alkyl group (CYCRANT T-394, trade name, a product of Mitsui Petrochemical Industries, Ltd.) (this ester is hereinafter referred to as "carbonate ester oil B") and the carbonate ester oil A was mixed with lithium stearate and/or lithium 12 hydroxystearate in the proportions as shown in Table 1 such that the sum of those was 100 mass%. The resulting mixture was heated to 220-230 °C while stirring until the whole of mixture became liquid. The liquid thus obtained was poured into a stainless steel vessel at a depth of 3-5 mm. The liquid was cooled at 50 °C or less and then homogenized with three rollers to obtain a grease composition. EXAMPLES 7 AND 8 Grease compositions were obtained in the same manner as in Examples 1 to 6 above except that an alkyl diphenyl ether or a polyol ester was further used as a third component other than components (a) and (b) in the proportion as shown in Table 1. COMPARATIVE EXAMPLES 1 AND 2 Two kinds of commercially available greases in which a base oil and a thickening agent are known were used for the sake of comparison. In Table 2, reference sign + means that such a compound is contained. COMPARATIVE EXAMPLE 3 AND 4 Grease compositions were obtained in the same manner as in Examples 1-6 except that a base oil and a lithium soap were used in the proportions as shown in Table 2. The grease compositions obtained in the above Examples and Comparative Examples were measured for a worked penetration and a dropping point, and were subjected to motor characteristic test, under the same conditions. The results obtained are shown in Tables 1 and 2. The worked penetration was measured according to JIS K2220, 5.3, and the dropping point was measured according to JIS K2220, 5.4. The motor characteristic test was conducted such that a bearing having included therein a grease composition to be tested was incorporated in a spindle motor, the motor was rotated at 10,000 rpm at a normal temperature, and noise generated, amount of evaporation (scatter) and rotating torque were measured. Noise was measured with a microphone at a place which is 30 cm apart from a hub edge of the motor during rotation of motor. The amount of evaporation (scatter) was determined by the difference between weight of motor before rotation and weight of motor after rotation. The torque was determined by measuring electric current value at the rotation of motor with an ammeter, and the torque stability was determined by the difference between the maximum electric current value and the minimum electric current value. The judgement results according to performances required for a bearing grease composition for HDD or the like on each evaluation item are shown in Table 1 (Examples) and Table 2 (Comparative Examples). The smaller the value, the better the low noise property; the smaller the amount of evaporation (scatter), the better the low scatter property; the lower the value, the better the low torque property; and the smaller the fluctuation, the better the torque stability. The tests were evaluated with the following four grades. A: Particularly excellent B: Excellent C: Moderate D: Poor Example 1 2 3 4 5 6 7 8 Thickening agent StLi 25 25 10 15 20 20 12OH St-Li 15 10 5 Base oil Carbonate A 75 85 38 90 90 80 70 70 Carbonate B 37 ADE 10 POE 10 Viscosity of base oil (40°C ) 18 18 50 18 18 18 25 21 Worked Penetration (25 °C ) 198 203 196 305 247 181 210 205 Dropping point (°C ) Motor characteristic 197 194 198 193 193 195 197 195 Test Low noise property A A B B B A B B Low scatter property A B A B B B A A Low torque property A A B A A A B B Torque stability A A B B B A A A Total evaluation A A B B B A B B Notes: StLi : Lithium stearate 12OH St-Li : Lithium 12 hydroxystearate ADE : Alkyl diphenyl ether POE : Polyol ester oil COMPARATIVE EXAMPLE 1 2 3 4 Thickening agent StLi + 25 25 12OH St-Li + Na-Complex + Base oil Carbonate ester A Carbonate ester B 75 Diester oil + 38 POE + 37 Mineral oil + Viscosity of base oil (40 °C ) 26 145 18 130 Worked penetration 250 205 197 181 Dropping point (°C ) 194 >260 197 199 Motor characteristic test Low noise property C D D B Low scatter property B A D A Torque C B B D Torque stability D C D C Total evaluation C C D C Notes: Na-Complex : Sodium complex soap 12OH St-Li : Lithium 12 hydroxystearate POE : Polyol ester oil Unit of base oil viscosity shown in Tables 1 and 2 is mm 2 /S. As shown in Table 1 above, the bearing grease composition of the present invention shows low evaporation (scatter), low noise, low torque property and stable torque.

Claims: A bearing grease composition comprising components (a) and (b) where (a) is a carbonate compound represented by the following formula (I) wherein R and R', which may be the same or different, each represent a branched alkyl group having 13 to 15 carbon atoms; and at least one of R and R' is wherein n is an integer of 9, 10 or 11; and (b) consists of an alkali metal salt and/or an alkaline earth metal salt, synthesized from a hydroxide of an alkali metal or an alkaline earth metal and a higher fatty acid having 10 or more carbon atoms or a higher fatty acid having at least one hydroxyl group and having 10 or more carbon atoms wherein said bearing grease composition comprises 70 to 95 parts by weight of component (a) and 5 to 30 parts by weight of component (b). The bearing grease composition of claim 1 wherein said component (a) includes a plurality of carbonate compounds each represented by said formula (I), wherein said plurality of carbonate compounds may be the same compound or a mixture of compounds. The bearing grease composition of claim 1 further comprising one or more thickening agents. The bearing grease composition of claim 1 further comprising lubricants and/or additives. The bearing grease composition of claim 1, further comprising oil based lubricants. The bearing grease composition of claim 1, further comprising additives. The bearing grease composition of claim 1, wherein component (a) has a dynamic viscosity of 10 to 50 mm 2 /sec at 40°C. Use of a bearing grease composition for memories comprising components (a) and (b) where (a) is a carbonate compound represented by the following formula (I) wherein R and R', which may be the same or different, each represent a branched alkyl group having 13 to 15 carbon atoms; and (b) consists of an alkali metal salt and/or an alkaline earth metal salt, synthesized from a hydroxide of an alkali metal or an alkaline earth metal and a higher fatty acid having 10 or more carbon atoms or a higher fatty acid having at least one hydroxyl group and having 10 or more carbon atoms wherein said bearing grease composition comprises 70 to 95 parts by weight of component (a) and 5 to 30 parts by weight of component (b).

Inventors: AKIYAMA MOTOHARU, MIYAMOTO YASUHIRO, OKAMURA SEIJI, AKIYAMA, MOTOHARU, MIYAMOTO, YASUHIRO, OKAMURA, SEIJI, Akiyama, Motoharu, c/o Minebea Co., Ltd., Miyamoto, Yasuhiro, c/o Nippon Grease Co., Ltd., Okamura, Seiji, c/o Nippon Grease Co., Ltd.
Applicants: MINEBEA KK, MINEBEA CO., LTD.
Application year: 1999
Main classification: C10M169
IPC codes: C10M117, C10N30, C10N40, C10M105, C10N50, C10N10, C10M169