GB2278612A

GB2278612A - Grease composition for bearings of electronic computers

Info

Publication number: GB2278612A
Application number: GB9410283A
Authority: GB
Inventors: Michiharu Naka; Jun Kuraishi; Kenichi Iso; Tetsuo Tsuchiya; Takahiro Ozaki
Original assignee: NSK Ltd; Showa Shell Sekiyu KK
Current assignee: NSK Ltd; Showa Shell Sekiyu KK
Priority date: 1993-05-20
Filing date: 1994-05-20
Publication date: 1994-12-07
Anticipated expiration: 2014-05-20
Also published as: JPH06330070A; JP3290752B2; GB2278612B; GB9410283D0

Abstract

The grease composition comprises at least one a base oil selected from a mineral oil and a synthetic hydrocarbon oil, and a thickener prepared by mixing a lithium soap containing no hydroxyl group in its chemical structural formula and a lithium soap containing a hydroxyl group in its chemical structural formula. The grease composition of the present invention shows a decrease on the amount of its scattering at an ambient temperature as well as a high temperature, and improves the sound quality and friction property of bearings of electronic computers. Mixtures of Li stearate and 12-hydroxy-stearate are disclosed.

Description

GREASE COMPOSITION FOR BEARINGS OF ELECTRONIC COMPUTERS The present invention relates to a grease composition.

Specifically, the present invention relates to a grease composition suitable, for example, for bearings of electronic computers, and more particularly to the improvements of a grease composition on its scattering, friction torque, and sound quality.

Low scattering, small friction torque, and better sound or acoustic quality have been appreciated as requisite properties for superior grease compositions for bearings of electronic computers. Among the requisite properties, the most important one is that the scattering of a grease composition from bearings is small as much as possible, since scattered grease will soil recording media, causing the occurrence of errors.

A sodium soap type grease (sodium complex soap-mineral oil type grease using a mineral oil as base oil and using a sodium complex soap as thickener) which is well known as ANDOK CX (trade name) has almost exclusively been used for the past more than 20 years due to its good reputation for scattering property as a conventional grease composition for bearings of electronic computers.

Also, a typical latest prior art grease composition is disclosed in Unexamined Japanese Patent Publication No. 59489.

However, the sodium complex soap-mineral oil type grease is not good in the sound and vibration qualities at the initial rotation of bearings since the dispersion of the soap in greases is inferior and heterogenous. Further, there was a problem that bearing cages become easy to sound since the sodium complex soap has a high water absorbing property and thus the grease hardens with the lapse of time to worsen its fluidity in bearings and produce inferior lubrication.

Accordingly, when importance is attached particularly to low ambient noise and small vibration, low ambient noise and small torque type greases represented by lithium soapester types have been used. However, there was a defect that the lithium soap-ester type greases are easy to scatter so that when the greases were used as they were, recording media are most likely to be soiled. While magnetic fluid seals can be used in combination to prevent the scattering, they are expensive, bring about an increase of the cost of products, and make the miniaturization of devices difficult due to required spaces for fitting the magnetic fluid seals.

Further, a grease composition for bearings of electronic computers is disclosed in Unexamined Japanese Patent Publication No. 5-9489 mentioned above which comprises 70 to 80 % by weight of at least one base oil having a kinematic viscosity of 8 to 180 mm2/s at 40"C and selected from the group consisting of a mineral oil, synthetic hydrocarbon oil, and polyphenyl ether oil and 20 to 30 % by weight of lithium salt of a higher fatty acid or f to C24 containing no hydroxyl group in its chemical structural formula, added with a specific rust preventive agent. However, the grease composition had a defect that the amount of scattering of the grease composition increases at a high temperature of about 70"C which is a highest working temperature for the bearings of electronic computers while the grease composition favorably exhibits a small scattering at an ambient temperature.

Accordingly, an object of the present invention is ts provide a grease composition for bearings of electronic computers which maintains the small scattering property of the conventional grease composition at an ambient temperature, has a small scattering even at a high temperature of about 70"C, and is excellent both in sound quality and friction torque property.

Fig. 1 shows a schematic illustration of an apparatus used for determining the amount of scattering of a grease composition.

Fig. 2 shows a schematic illustration of an apparatus for determining the friction torque of a bearing.

For the purpose of the present invention, the term excellent scattering is intended to mean that the amount of a grease composition which is scattered is small.

The grease composition of the present invention is characterised by comprising 70 to 80k by weight of at least one base oil having a kinematic viscosity of 8 to 180 mm2/s at 400C and selected from the group consisting of a mineral oil and synthetic hydrocarbon oil, and 20 to 80t by weight of a thickener composed of lithium salt of a higher fatty acid of C2 to C24 and lithium salt of a higher hydroxyfatty acid of C12 to 024.

Further, the grease composition of the present invention preferably contains 10 to 40k by weight of lithium salt of a higher hydroxyfatty acid in the thickener and has an unworked penetration of 190 to 250.

The grease composition of the present invention contains a lithium soap as thickener. The lithium soap has a relatively high lubricating ability among thickeners and can uniformly and thoroughly be dispersed in greases to provide excellent sound quality and friction torque property. The thickener in the present inventIon is used in form or a mixture of a lithium soap containing no hydroxyl group in its chemical structural formula (hereinafter referred to as "lithium soap lA)") with a lithium soap containing hydroxyl group in its chemical structural formula 'hereinafter referred to as "lithium soap (B)"). Accordingly, the oil separation of the grease composition can be decreased since a micelle structure formed by the lithium soap and base oil is more stabilized to produce a compact structure. As a result, the amount of scattering of the grease is decreased.

However, it has been found that even when the oil separation was small, the amount of scattering of the grease does not considerably decrease at a temperature condition of 700C unless the mixing ratio of the lithium soap A) with the lithium soap fB) is in a prescribed range.

When the mixing ratio of the lithium soap (3) is creased in the thickener in particular, the yield penetration will aiso increase to form a hard grease composition, and problems will occur in its sound quality and friction torque property.

Tt had been found by the present inventors to be efficient to use the thickener for the lithium soap grease in a larger amount than usual and use the thickener in a structure of soap short fibre for roduclng a superior grease composition. ks a result of further study it has now been found that a grease composition having an excellent scattering property at a high temperature can be prepared by using the lithium soap (A) and lithium soap (B) as thickener, and using a mineral oil, synthetic hydrocarbon oil, or mixture thereof as base oil, leading to the present invention The base oil which can be used in the present invention includes a mineral oil and synthetic hydrocarbon oil, and they can be used alone or in combination. When both of them are used in combination, the grease scattering at a high temperature is improved in particular. The mixing ratio of the mineral oil in the base oil is preferably 30 to 70% by weight, and most desirably about 50% by weight. As the mineral oil, a highly refined oil from a so called petroleum is preferable, but its viscosity index is not critical. The synthetic hydrocarbon oil which can be in the present invention includes polymerized olefin oils such as a poly-a-olefin and liquid polybutene, and aromatic alkyl oils. Among the oils, a poly--olefin, for example, an oligomer of 1-Decene (CH,(CH2)7CH=CH2) and a copolymer of methylene with an ot-olefin are preferable.

In the present invention, a base oil having a kinematic viscosity of 8 to 180 mm2/s at 400C can be used for preparing the grease composition. When the kinematic viscosity is lower than 8 mm2/s, production of a grease is difficult since the base oil tends to evaporate. On the other hand, when the kinematic viscosity exceeds 180 mm2/s, not only the friction torque is increased but also te amount of scattering of the grease is unfavourably increased. As a result of the research by the present inventors, it has been confirmed that a lower base oil viscosity is rather advantageous from the viewpoint of the scattering. Considering tis fact, a kinematic viscosity of 16 to 100 mm/s at 40 C is preferred.

The thickener which can be used in the grease composition of the present invention is lithium salt of a higher fatty acid of C12 2 to C24 which has a good dispersibility in oils and from which a grease composition having an excellent sound quality can readily be produced. The salt of a higher fatty acid is required to be a mixture, for example, of a lithium soap (A) containing no hydroxyl group in its structural formula such as lithium stearate and lithium salt of a beef tallow fatty acid with a lithium soap (B) containing a hydroxyl group in its structural formula such as lithium 12-hydroxy stearate In a prescribed mixing ratio. The reason for this is that the amount of scattering of the grease composition at a high temperature can considerably be decreased by using, for example, the lithium stearate and lithium 12-hydroxy stearate In a suitable combination in a prescribed range of mixing ratio. When the mixing racio of a lithium soap (B) containing hydroxyl group is too high, the amount of the scattering particularly at a high temperature remarkably increases.

The amount 0r the thickener comprising lithium salt of a higher fatty acid of C12 to C24 to be blended with a base oil used for the preparation of the grease composition of the present invention affects the scattering property of the grease composition. In conventional lithium soap t,te greases, the amount of a nickener to be blended is generally 8 to 18% by weight.

- contrast, the amount of the thickener to be blended in ne present invention Is adjusted In a range of 20 to 30% by weight, and preferably in a range of 23 to 27% by weight based on repeated experimental results. When the amount of the thickener to be blended is more than 30% by weight or less than 20% by weight, the scattering property at a high temperature becomes inferior.

Further, the amount of the lithium soap type thickener to be blended is an important factor not only for the scattering property cf a grease composition but also for improvements of sound quality and friction torque property.

Since the scattering of grease means that fine particles produced by the rotation of bearings are scattered, a suitable value of hardness is required of a grease composition to improve the scattering property.

On the other hand, when the grease composition is excessively hard, its fluidity in bearings will decrease and cause adverse influences such as the generation of noise at bearing cages. In other words, the hardness of the grease composition is also a factor which considerably influences the scattering property and sound quality. As a result of the research by the present inventors on the yield penetration of grease compositions, it has been concluded that it is desirable to control the unworked penetration of the grease composition to be in the range of 190 to 250. When the unworked penetration is lower than 190, the grease composition is excessively hard and remarkable deterioration of the sound quality and torque property occurs. On the other hand, when the unworked penetration exceeds 250, the amount of scattering of the grease composition unfavourably increases and recording media in electronic computers will be soiled.

According to the present invention, a grease composition for bearings of electronic computers is provided which exhibits an excellent scattering property not only at an ambient temperature but also at a high temperature of 700C and is usef or improving the sound quality and friction torque of bearings.

Examples The present invention will be described in further detail with reference to Examples and Comparative Examples. However, it shouid be understood that the invention is not limited to those Examples.

Examples 1 throw 18 and Comparative Examples 1 through 15 Experiments were conducted to determine the yield penetration (unworked penetration and 60 strokes worked penetration), amount of scattering of the grease composition, bearing sound, and friction torque by using grease compositions of Examples 1 through 18 shown in Tables 1 and 2, and grease compositions of Comparative Examples 1 through 15 shown in Tables 3 and 4, respectively.

Table 1

Example Composition and Property 1 2 3 4 5 6 7 8 9 10 11 Thickener (g) Lithium stearate 225 175 150 225 175 150 175 175 175 175 175 12-hydroxy lithium stearate 25 75 100 25 75 100 75 75 75 75 75 Base oil (g) Mineral oil - - - 750 750 750 750 - - - 750 Poly-α-olefin oil 750 750 750 - - - - 750 750 750 Base oil kinematic viscosity 30 30 30 30 30 30 8 16 90 180 180 (mm/s &commat; 40 C) Unworked penetration 219 216 203 218 228 205 225 229 218 229 220 60 strokes Worked penetration 221 220 207 222 232 208 227 232 224 232 224 Prop- Scattering particles 120 100 145 130 140 150 110 120 110 160 200 25 C larger than 0.3 m erty 2500 1050 1400 2450 1500 1650 1150 1100 1080 1700 1950 (number/0.01 ft ) 70 C Sound Kinematic friction 12.7 13.7 14.7 13.7 13.7 15.7 10.8 11.8 14.7 16.7 16.7 torgue (N.m) (x10-5) 2.3 0.8 0.6 2.4 1.0 0.8 2.0 1.5 0.7 0.5 0.4 Oil separation 100 C, 24 hr, wt % Table 2

Example Composition and Property 12 13 14 15 16 17 18 Thickener (g) Lithium stearate 210 140 225 175 175 175 150 12-hydroxy lithium stearate 90 60 25 75 75 75 100 Base oil (g) Mineral oil - - 225 375 525 225 375 Poly-α-olefin oil 700 800 525 375 225 525 375 Base oil kinematic viscosity 30 30 30 30 30 30 30 (mm/s &commat; 40 C) Unworked penetration 224 225 229 225 222 200 231 60 strokes Worked penetration 229 230 231 227 231 207 237 Prop- Scattering particles 150 200 90 40 65 85 70 25 C larger than 0.3 m erty 1700 2300 850 500 700 750 800 (number/0.01 ft ) 70 C Sound Kinematic friction 17.6 12.7 13.7 13.7 14.7 14.7 14.7 torgue (N.m) (x10-5) Oil separation 100 C, 1.1 2.3 0.1 0.1 0.1 0.5 0.1 24 hr, wt % Table 3

Comparative Example Composition and Property 1 2 3 4 5 6 7 8 Thickener (g) Lithium stearate 250 75 75 - 175 245 150 350 12-hydroxy lithium stearate - 175 175 250 75 105 - Mineral oil - - 750 - 750 - - Poly-α-olefin 750 750 - 750 - 650 850 650 Base oil (g) oil Diester oil Polyol ester oil Base oil kinematic viscosity 30 30 30 30 200 30 8 16 (mm/s &commat; 40 C) Unworked penetration 218 209 205 211 220 209 250 240 60 strokes Worked penetration 226 212 208 215 223 213 255 246 120 110 130 7000 220 130 800 350 Prop- Scattering particles 25 C larger than 0.3 m erty 7000 4500 5200 12500 4000 1550 8800 7700 (number/0.01 ft ) 70 C Sound x x x Kinematic friction 13.7 15.7 16.7 13.7 25.5 26.5 11.8 25.5 torgue (N.m) (x10-5) 2.3 0.6 0.7 2.3 1.0 1.2 2.8 1.4 Oil separation 100 C, 24 hr, wt % Table 4

Comparative Example Composition and Property 9 10 11 12 13 14 15 Thickener (g) Lithium stearate 70 250 250 175 175 175 150 12-hydroxy lithium stearate 30 - - 75 75 75 75 Mineral oil - 525 750 375 375 - Poly-α-olefin Base oil (g) oil 900 225 - 375 375 Diester oil 750 Polyol ester oil 750 Base oil kinematic viscosity 90 180 180 180 180 11 30 (mm/s &commat; 40 C) Unworked penetration 255 219 225 275 175 221 218 60 strokes Worked penetration 260 222 229 279 179 228 226 Prop- Scattering particles 300 220 250 230 120 7200 7000 25 C larger than 0.3 m erty 8300 4800 7000 4500 1200 9700 9550 (number/0.01 ft ) 70 C Sound x # Kinematic friction 12.7 13.7 14.7 14.7 19.6 15.7 15.7 torgue (N.m) (x10-5) 2.0 2.2 2.5 0.1 0.1 2.3 2.2 Oil separation 100 C, 24 hr, wt % In the Tables, the unworked penetration and 60 strokes worked penetration were determined with respect to the yield penetration according to the methods described in JIS K2220 (5.3) (ASTM D 217) and the oil separation was determined according to the method described in JIS K2220 (5.7) (FS 321 (Federal Test Method Standards No. 791 Method No. 321)).

Determination of values of each physical properties of the amount of scattering of the grease composition, bearing sound, and friction torque were carried out as follows.

A grease composition as test specimen in an amount of 19 mg was enclosed in a ball bearing having a single row of balls located in a deep groove, provided with a non contact rubber seal which was completely degreased with an organic solvent in advance and had an inside diameter of 5 mm and outside diameter of 14 mm, and tests were performed under the conditions of pre-load of 14.7 N at an ambient temperature (about 250C) and 700C (only for the amount of scattering).

An apparatus for determining the amount of scattering of the grease composition is illustrated in Fig. 1. The apparatus was placed under the condition of an ambient temperature of 700C, the bearing A which enclosed a grease composition was rotated at 3600 rpm for 20 min, fresh air was flowed through the vessel B, in the direction indicated by the arrows, and the number of scattered particles of the grease composition having a size larger than 0.3 jim contained in the air was determined by using a particle counter taking a measurement for every 0.01 cubic feet (ft3) volume of the air passing. In Tables 1, 2, 3, and 4, the amounts of grease scattered from the grease composition 20 min after the start of bearing rotation are shown.

The sound of the bearings was determined by judging whether sound was emitted from a bearing cage by using an Anderon meter when a bearing as test specimen was rotated at 1800 rpm. Results of the judgements are shown in Tables 1 through 4 in which symbol 0 indicates that the sound was not emitted, A indicates scarcely emitted, and x indicates occasionally or continually emitted from the bearing cage.

An apparatus for determining friction torque is illustrated in Fig. 2. Bearing A as test specimen was rotated at 3600 rpm, the friction torque values determined by using a strain gauge C were recorded on a recorder, and the torque values when they were almost stabilized 10 min after the start of rotation are shown in Tables 1, 2, 3, and 4. In Fig. 2, symbol D indicates an air spindle, E an arbor, F an aluminium cap, and G indicates an air bearing.

As will be found from the results shown in the Tables, the grease compositions of Examples 1 through 18 were all excellent both in the scattering property and sound quality, while the grease compositions of Comparative Examples 1 through 15 had a defect at least either in the scattering property or sound quality.

While comparing the data shown in the Tables with each other, the relationships of the thickener, kinematic viscosity of base oil, class of base oil, blending ratio of base oil and thickener, or yield penetration with properties of grease compositions are examined as follows.

With respect to the thickener, when Comparative Examples 1, 4, 7, 8, 10, and 11 in which lithium stearate or lithium 12-hydroxy stearate was used singly were compared with Examples 1 through 18 in which both of them were used in admixture, the amount of scattering of the grease composition at 700C in the Examples are considerably smaller than those in the Comparative Examples. In Comparative Example 4 in particular in which lithium 12-hydroxy stearate was used on its own, the amount of scattering of the grease composition even at 250C is much larger than in any of the Examples even at 700C, vindicating that sole use of the thickener is inappropriate. Further, when the ratio of lithium 12hydroxy stearate in the thickener increased, the scattering amount also increased. In Comparative Examples 2 and 3 in which the mixing ratio of lithium 12hydroxy stearate in the thickener was 70% by weight, the scattering amount at 700C considerably increased while the oil separation was low as compared with Examples 1, 4, 7, 8, and 13. Still further, when lithium stearate and lithium 12-hydroxy stearate were used as a mixture in a certain range cf mixing ratio, the scattering amount at 700C was able to decrease. This is considered to be based on the fact that a compact and stabilised grease structure was formed by mixing a lithium soap (A) with a lithium soap (B) in an appropriate mixing ratio.

With respect to the kinematic viscosity of the base oil, when Examples 1 through 18 in which the kinematic viscosity at 400C is 8 to 180 mm2/s were compared with Comparative Example 5 in which the kinematic viscosity at 00 was increased up to 200 mm/s, the scattering amount at 70 C increased, and sound qua Icy and torque property in the Comparative Example were inferior to those in the Examples, while the grease compositions in the Comparative Example showed a low oil separation.

With respect to the class of the base oil, the grease compositions in Examples 1 through 18 In which a mineral oil, poly-α-olefin oii, or their mixture was used exhibited excellent scattering amount of grease compositions at an ambient temperature and at 700C. When compared with Comparative Examples 14 and 15 in which a diester oil or polyol ester oil was used, it can be found that the grease compositions In Examples 1 through 18 exhibited remarkably better scattering results than the Comparative Examples. From these results, it has been found, with respect to the base cil like the thickener, that the use of a mineral oil, poly-a-olefin, or their mixture is effective for decreasing the amount of scattering of the grease composition.

With respect to the blending ratio of the base oil with the thickener, when Examples 12 and 13 were compared with Comparative Examples 6 through 9, the grease compositions in Comtarative Example 6 in which the amount of the thickener was 35% by weight in the grease composition had problems in the sound quality and torque property, while Comparative Example - in which the amount of the thIckener was 10% by weight had a problem in the scattering property, even if lithium stearate and lithium 2-hydroxy stearate were sea cv mixture From this result, it can be understood that the amount of the thickener in the grease composition is Important to mprove the properties of scattering, sound, and torque.

Further, in Examples 14 through 18 in which a mineral oil and poLy-a-olefin oil were used as base oil, and lithium stearate and lithium 12-hydroxy stearate were used as a thickener, by mixtures respectively, the amount of scattering at 700C particularly decreased.

With respect to the yield penetration (unworked penetration and 60 strokes worked penetration), when Example 15 in which the yield penetration of the grease composition were in a range of 190 to 250 was compared with both Comparative Example 12 in which the yield penetration was higher than 250 and Comparative Example 13 in which the yield penetration was lower than 180, with the chemical composition of the grease composition being all the same, It can be found that the scattering amount of the grease composition at 700C increased in Comparative Example 12, and the sound quality and friction torque property were worse in Comparative Example 13.

Claims

CLAIMS 1. A grease composition for bearings of electronic computers comprising 70 to 80 % by weight cf at least one base oil having a kinematic viscosity of 3 to 180 mm/s at 40 C and selected from the group consisting of a mineral oil and a synthetic hydrocarbon oil, and 20 to 30 % by weight of a thickener composed of lithium salt of a higher fatty acid of C12 to C? and lithium salt of a higher hydroxyfatty acid of C12 to C24.
2. The grease composition for bearings of electronic computers according to claim 1 wherein the grease composition contains 20 to 30 k by weight of a thickener comprising 10 to 40 by weight of lithium salt of a higher fatty acid of C. 2 to C24 containing a hydroxyl group in its chemical structural formula and 60 to 90 k by weight of lithium salt of a higher fatty acid of C12 to C24 containing no hydroxyl group in its chemical structural formula.
3. The grease composition for bearings of electronic computers according to claim i or claim 2 wherein the grease composition has an unworked penetration of 190 to 250.

The crease composition for bearings of electronic computers according to claim 3, 2 sr 3 wherein the basic oil contains both of the mineral oil and synthetic hydrocarbon oil.