JP2010195849A - Grease composition, and pivot bearing for use in hard disk drive - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a grease composition which is capable of improving the durability and reliability under a high temperature environment (of the order of 60-80°C) and is capable of suppressing and reducing the increase in a revolution torque also under a low-temperature environment (of 0°C or below). <P>SOLUTION: The grease composition contains a base oil, a thickener and a viscosity index improver, wherein the base oil includes an ester oil as the principal component, a kinetic viscosity at 40°C is set to 40-60 mm<SP>2</SP>/sec, and a kinetic viscosity at 0°C is set to 550-650 mm<SP>2</SP>/sec, and the viscosity index improver is made to be polymethyl methacrylate and a content of polymethyl methacrylate is set to 1-5 mass% relative to the whole grease composition. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a grease composition for lubrication, and a pivot bearing for a hard disk drive in which the grease composition is enclosed. Specifically, it is possible to keep the lubrication state well at both high and low temperatures. It relates to the improvement of grease lubrication.

  2. Description of the Related Art Conventionally, lubrication is generally performed in various mechanical devices including a rotation mechanism in order to prevent malfunctions of the rotation mechanism (for example, increase in rotational torque, temperature rise, seizure, etc.). For example, for bearings that support a rotating shaft, the bearing ring (rotating ring and stationary ring), rolling elements (balls and rollers) of the bearing, and the reduction of friction and wear at the part where the cage contacts each other, Lubrication is performed to prevent seizure, extend fatigue life, and reduce rotational torque. Bearing lubrication is often performed by enclosing a lubricant inside the bearing, and can be roughly classified into oil lubrication and grease lubrication depending on the type of lubricant used at that time.

In general, oil lubrication has the characteristics that the fluidity of the lubricant and the cooling effect on the equipment are high, and that it has better lubrication performance than grease lubrication. Has a drawback that it easily leaks to the outside of the bearing. On the other hand, grease lubrication can suppress leakage to the outside of the bearing, simplify the bearing and its peripheral structure, and has features (excellent cleanliness and maintainability) that are maintenance-free.
For example, air conditioners (more specifically, their small motors) and hard disk drives (similarly, pivot parts) are required to have excellent lubrication performance, while severe cleanliness and maintainability are also required for lubrication. . Therefore, grease lubrication is widely used in air conditioners and hard disk drives (hereinafter also referred to as HDDs) in consideration of their excellent cleanliness and maintainability. The improvement of reliability and reliability is aimed at.

And in order to raise the lubrication precision by such grease lubrication further, the improvement of the component composition is added about the grease composition conventionally used for lubrication.
For example, Patent Document 1 discloses an example of a grease composition that is composed of a base oil and a thickener and that has the following characteristics. That is, the base oil has a kinematic viscosity at 40 ° C. set to 80 to 300 mm ² / sec and contains at least 10% by mass (wt%) of ester oil, while the thickener is In addition, it contains 5 to 30 wt% of diurea or polyurea, which is a characteristic in composition.

  By making the grease composition (base oil and thickener) into such a component composition, for example, even when a small motor or the like is heated to a high temperature (about 60 to 80 ° C.), the oil film is sufficient This makes it possible to satisfactorily lubricate the small motor and the like. In addition, although the invention described in Patent Document 1 assumes a small motor of an air conditioner as the main lubrication object, considering the configuration of the grease composition, not only the small motor but also other high temperatures It is estimated that sufficient effects can be achieved even in applications that become.

JP 2002-338982 A

However, when a grease with the above-described component composition is used to lubricate a member with a wide operating temperature range (for example, a HDD pivot bearing), the lubrication performance depends on the usage environment (specifically, the low temperature environment). It has been proved that it is not possible to fully exhibit. For example, in an HDD device mounted in an in-vehicle navigation system or the like, the starting temperature may be −20 ° C. or lower, but the base oil of the grease composition is 1000 mm even if it has a kinematic viscosity at 0 ° C. ² / sec or more. Therefore, in a low temperature environment of 0 ° C. or lower, an oil film is not sufficiently formed on the HDD pivot bearing (specifically, the contact surface between the race, the rolling element, and the cage), and the rotational torque of the bearing does not increase. As a result, there is a tendency that the swing accuracy of the swing arm is lowered, resulting in problems such as a decrease.
That is, when the HDD pivot bearing is lubricated with a grease composition containing a base oil having a high kinematic viscosity, it is possible to improve durability in a high temperature environment (about 60 to 80 ° C.). However, on the other hand, there is a problem that the rotational torque increases in a low temperature environment (0 ° C. or less), which easily causes a failure of the HDD device.

  The present invention has been made in order to solve such problems. The object of the present invention is to improve durability and reliability in a high temperature environment (about 60 to 80 ° C.) and to improve the durability in a low temperature environment (0 ° C.). The present invention also provides a grease composition that can suppress and reduce an increase in rotational torque, and provide a hard disk drive pivot bearing that uses the grease composition for lubrication.

In order to achieve such an object, the grease composition according to the present invention includes a base oil, a thickener, and a viscosity index improver, and the base oil is mainly composed of ester oil. The kinematic viscosity at 40 ° C. is set to 40 mm ² / sec or more and 60 mm ² / sec or less, and the kinematic viscosity at 0 ° C. is set to 550 mm ² / sec or more and 650 mm ² / sec or less. Yes.
In this case, the viscosity index improver may be polymethyl methacrylate, and the content thereof may be set to 1% by mass or more and 5% by mass or less of the entire grease composition.

  In order to achieve the above object, the pivot bearing for a hard disk drive according to the present invention is a swing arm that is rotated when information is read from and written to the hard disk in the hard disk drive. In order to lubricate its constituent members (for example, races, rolling elements and cages), any one of the above grease compositions is enclosed inside.

According to the grease composition of the present invention, 40-60 mm ^{2 /} sec to kinematic viscosity of the base oil at 40 ° C., and sets the 550~650mm ^{2 /} sec at and 0 ° C., a thickener in the base oil (an example As a diurea compound) and a viscosity index improver (polymethyl methacrylate (PMMA)), both in a high temperature environment (about 60 to 80 ° C.) and a low temperature environment (0 ° C. or less) An oil film can be sufficiently formed on the object to be lubricated (for example, an HDD pivot bearing). Accordingly, it is possible to improve durability and reliability in a high temperature environment, and to suppress and reduce an increase in rotational torque (for example, HDD pivot bearing torque) in a low temperature environment. That is, by using the grease composition for lubrication, the HDD pivot bearing can always be kept in a good lubrication state, and can be rotated with high accuracy over a long period of time.

The figure which shows the component composition and lubricating performance (each result of a durability test and a torque test) of the grease composition which concerns on one Embodiment of this invention by the comparison with a known grease composition. The figure which shows the relationship of the penetration degree with respect to the addition amount of the viscosity index improver (PMMA) in the grease composition which concerns on Example 1 shown in FIG. 1, and the grease composition which concerns on the comparative example 1. FIG. The figure which shows the relationship of the base oil kinematic viscosity with respect to the temperature of the grease composition which concerns on Example 1 shown in FIG. 1, and the grease composition which concerns on the comparative example 1. FIG. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram for explaining a configuration example of an HDD pivot bearing according to an embodiment of the present invention. A top view and (c) are half sectional views showing the whole pivot bearing unit composition.

Hereinafter, the grease composition according to the present invention will be described with reference to the accompanying drawings.
Such a grease composition is enclosed in various mechanical devices having a rotation mechanism, and is a lubricant for preventing malfunctions of the rotation mechanism (for example, increase in rotational torque, temperature rise, seizure, etc.). Used as (grease). Particularly, a mechanical device having a severe difference in operating environment temperature, for example, various mechanical devices used in a high temperature environment of about 60 to 80 ° C. to a low temperature environment of 0 ° C. or less (that is, a temperature difference of about 60 to 80 ° C.). However, the mechanical device (more specifically, its constituent members) to be lubricated is not limited.

  Therefore, in the present embodiment, as one of lubrication targets by such a grease composition, in a hard disk drive (Hard Disc Drive (hereinafter also referred to as HDD)), information is read from the hard disk, and information is stored in the hard disk. A pivot bearing (hereinafter simply referred to as a bearing) for supporting a swing arm that is rotated when writing is assumed. In addition, as a use of HDD, the case where it mounts as a memory | storage device in a personal computer, various portable terminals, etc. can be assumed, but in this embodiment, it mounts as a memory | storage device in a vehicle-mounted navigation system as an example. Assuming that

  4A to 4C show an example of the configuration of the HDD. In this case, the HDD includes a magnetic disk (hard disk) 2 for recording information (data) and the magnetic disk 2. Is provided with a spindle motor 4, a swing arm 8 having a magnetic head 6 attached to the tip thereof, and a voice coil 10 which is provided at the base end of the swing arm 8 and rotationally drives it. The swing arm 8 is pivotally supported on a base Bs of the HDD via a pivot bearing unit U so as to be rotatable. When the swing arm 8 is rotationally driven by the voice coil 10, the magnetic head 6 with respect to the rotating magnetic disk 2. Is translated (traced). Thereby, in the HDD, information can be read from the magnetic disk 2 via the magnetic head 6 or information can be written (recorded) to the magnetic disk 2.

In the configuration shown in FIG. 4C, the pivot bearing unit U includes a shaft 12 erected on the base Bs of the HDD, a sleeve 18 on which the swing arm 8 is sheathed, and the shaft 12 and the sleeve 18. Pivot bearings 14 and 16 interposed between them are provided.
Further, the pivot bearings 14 and 16 are configured to freely roll between a pair of race rings (for example, inner races 14 a and 16 a and outer races 14 b and 16 b) arranged to face each other so as to be relatively rotatable. A plurality of incorporated rolling elements (balls 14c, 16c) and retainers 14d, 16d for rotatably holding the rolling elements one by one are provided. A sealing member (same as the non-contact type shields 14e and 16e) for sealing the inside of the bearing is interposed between the races, thereby preventing foreign matter (for example, dust) from entering the outside of the bearing. In addition to being prevented, leakage of the grease composition sealed inside the bearing to the outside of the bearing is prevented.

Note that the pivot bearings 14 and 16 are not particularly limited to the configuration shown in the figure, and can be configured arbitrarily according to the purpose and conditions of use of the HDD. That is, any configuration may be used as long as the swing arm 8 can be pivotally supported.
For example, a composite configuration with a plurality of rolling bearings as shown in FIG. 4C may be used, or a single configuration with a single rolling bearing may be used. Moreover, what is necessary is just to apply arbitrary types for a holder | retainer according to the kind of rolling element. For example, when the rolling element is a ball, a crown type (see FIG. 4 (c)) or a corrugated type can be applied. A type such as a cage type can be applied. Similarly, as the sealing member for sealing the inside of the bearing, in addition to the non-contact type shields 14e and 16e shown in FIG. 4C, a contact type seal, a non-contact type seal, and the like can be applied. It is. Further, the cage may be omitted, and the sealing member may be omitted.

  The pivot bearings 14 and 16 have inner rings 14 a and 16 a fitted on the shaft 12, and the outer rings 14 b and 16 b are fitted on the sleeve 18 between the shaft 12 and the sleeve 18. The swing arm 8 mounted on the sleeve 18 is pivotally supported so as to be rotatable. At that time, an annular spacer 20 is fitted to the inner peripheral portion of the sleeve 18 so as to be interposed between the pivot bearings 14 and 16. As a result, the pivot bearings 14 and 16 are positioned and fixed at a predetermined position with a predetermined preload applied, and can be stably rotated without rattling, and the swing arm 8 can be rotated smoothly with good responsiveness. Can be made. FIG. 4C shows an example of the configuration of the shaft 12 that forms a cylindrical structure (hollow structure) in which the axial center portion penetrates along the extending direction. A solid structure is also possible. It is also optional to provide a flange portion on the outer periphery of the shaft.

  The grease composition is surrounded by pivot bearings 14 and 16 (for example, inner and outer rings 14a, 16a, 14b, and 16b and sealing members 14e and 16e, and rolling elements 14c and 16c and cages 14d and 16d are incorporated therein. The inner and outer rings 14a, 16a, 14b, 16b, the rolling elements 14c, 16c, and the cages 14d, 16d are lubricated, and friction, wear, seizure, etc. are applied to the contact parts. The fatigue life of the inner and outer rings 14a, 16a, 14b, 16b and the rolling elements 14c, 16c, etc., and the rotational torque of the pivot bearings 14, 16 are reduced.

In the present embodiment, the grease composition includes a base oil, a thickener, and a viscosity index improver.
The base oil is mainly composed of ester oils, in kinematic viscosity at 40 ° C. is 40 mm ^{2 /} sec or more, and with are set to 60 mm ^{2 /} sec or less (40~60mm ^{2 /} sec), a kinematic viscosity at 0 ℃ Is set to 550 mm ² / sec or more and 650 mm ² / sec or less (550 to 650 mm ² / sec). In addition, the base oil kinematic viscosity in this case shows the numerical value after adding a viscosity index improver to the base oil which has ester oil as a main component.

Further, as the thickener, a diurea compound is applied, and ester oil is used so that the penetration degree of the grease composition of the diurea compound is set within a predetermined numerical range (for example, about 270). What is necessary is just to add to the base oil which makes a main component.
As the viscosity index improver, polymethyl methacrylate (PMMA) is applied, and the content thereof is 1 mass% (wt%) or more and 5 mass% or less (1 to 5 wt%) of the entire grease composition. ), A diurea compound is added as a thickener to a base oil mainly composed of ester oil, and PMMA is added.

  By making the grease composition into such a component composition, the inner and outer rings 14a, 16a of the pivot bearings 14, 16 can be used both in a high temperature environment (about 60 to 80 ° C) and in a low temperature environment (below 0 ° C). , 14b, 16b, rolling elements 14c, 16c and cages 14d, 16d can sufficiently form an oil film. Accordingly, it is possible to improve the durability and reliability of the pivot bearings 14 and 16 in a high temperature environment, and to suppress and reduce the increase in the rotational torque of the pivot bearings 14 and 16 in a low temperature environment. . That is, the pivot bearings 14 and 16 can always be kept in a good lubrication state, and can be rotated with high precision over a long period of time.

Here, the lubrication performance of the grease composition according to the present invention, specifically, the durability performance at high temperature and the torque performance at low temperature when the bearing is lubricated with the grease composition, are verified. A test was conducted. The test contents and test results will be described below. In the following description, the durability performance verification test at high temperature is simply referred to as “endurance test”, and the torque performance verification test at low temperature is simply referred to as “torque test”.
For the durability test and the torque test, a total of four types of grease compositions are prepared, one of which is a sample of the grease composition according to the present invention (hereinafter referred to as Example 1), and the remaining three. Was a sample of a grease composition to be compared with Example 1 (referred to as Comparative Example 1, Comparative Example 2, and Comparative Example 3).

In Example 1, an ester oil having a kinematic viscosity at 40 ° C. of 60 mm ² / sec and a kinematic viscosity at 100 ° C. of 580 mm ² / sec is used as a base oil, and a diurea compound is added as a thickener to the base oil. At the same time, polymethyl methacrylate (PMMA) was added as a viscosity index improver so that its content was 3% by mass (wt%) of the whole Example 1. At that time, the kinematic viscosity of the base oil (ester oil) is the value after the addition of the viscosity index improver (PMMA), and the thickener has a mixing consistency of 270 in Example 1 as a whole. Was added to the base oil (ester oil).

On the other hand, Comparative Example 1 uses an ester oil having a kinematic viscosity at 40 ° C. of 80 mm ² / sec and a kinematic viscosity at 100 ° C. of 1400 mm ² / sec as a base oil. It was produced by adding a diurea compound as a thickener to the base oil so that the degree was 265. Comparative Example 2 uses an ester oil having a kinematic viscosity at 40 ° C. of 30 mm ² / sec and a kinematic viscosity at 100 ° C. of 550 mm ² / sec as a base oil. It was produced by adding a diurea compound as a thickener to the base oil so as to be 280. That is, in Comparative Example 1 and Comparative Example 2, no viscosity index improver is added, and the above kinematic viscosities of the base oil (ester oil) indicate the values of the base oil itself.

On the other hand, Comparative Example 3 uses a mineral oil having a kinematic viscosity at 40 ° C. of 60 mm ² / sec and a kinematic viscosity at 100 ° C. of 600 mm ² / sec as a base oil, and a diurea compound as a thickener in the base oil. As a viscosity index improver, PMMA was added so that the content thereof was 3 wt% of the whole Comparative Example 3. At that time, the kinematic viscosity of the base oil (mineral oil) is the value after the addition of the viscosity index improver (PMMA), and the thickener has a mixing consistency of 275 as a whole in Comparative Example 3. To base oil (mineral oil).

These four types of grease composition samples (Example 1, Comparative Examples 1 to 3) were each placed in the same amount (2 mg) in the same configuration of the rolling bearing (HDD pivot bearing: SR168B manufactured by Nippon Seiko Co., Ltd.). Each of the bearings was rotated for the same time under the same conditions, and the Anderon value was measured for each endurance test and the torque value was measured for each bearing in the torque test, and these were compared and verified. In the following description, the bearing in which Example 1 is enclosed is referred to as the present bearing 1, and the bearings in which Comparative Examples 1, 2, and 3 are enclosed are referred to as comparative bearings 1, 2, and 3, respectively.
In the durability test, the ambient temperature was set to 80 ° C., an axial load of 7 N was applied, the bearing 1 and the comparative bearings 1 to 3 were rotated for 100 hours at a rotation speed of 1800 rpm, and the test of each bearing was started. Anderon values were measured before and after 100 hours.
On the other hand, in the torque test, the ambient temperature is set to 0 ° C., the axial load is 7 N, the bearing 1 and the comparative bearings 1 to 3 are rotated at a rotation speed of 1800 rpm, and the rotation is stabilized. The torque value of each bearing at the time when the fluctuation of the value converged (settled) was measured.

  FIG. 1 shows the test results of the durability test and the torque test for each bearing (this bearing 1, comparative bearings 1 to 3), that is, for each sample of the grease composition (Example 1, Comparative Examples 1 to 3). Yes. At that time, as an endurance test result, the increase in the Anderon value in Comparative Examples 1 to 3 (Comparative Bearings 1 to 3) when the increase in the Anderon value in Example 1 (the present bearing 1) is 1. The amount (relative value with respect to Example 1, that is, the endurance time ratio) is illustrated, and as a torque test result, Comparative Examples 1 to 3 in the case where the torque value in Example 1 (the bearing 1) is set to 1 are shown. The torque value (relative value with respect to Example 1, ie, torque ratio) in (Comparative bearings 1 to 3) is shown.

As is clear from FIG. 1, in Comparative Example 1, the durability time ratio in a high-temperature environment (80 ° C.) is 0.9 (90% of Example 1), which is not much inferior to Example 1. However, the torque ratio in a low temperature environment (0 ° C.) was 2.3 (2.3 times that in Example 1), and the torque increased dramatically in the low temperature environment.
In Comparative Example 2, the torque ratio in a low-temperature environment (0 ° C.) is 0.8 (80% of Example 1 (that is, 20% reduction)), which can be said to be superior to Example 1. However, the durability time ratio in a high-temperature environment (80 ° C.) is only 0.4 (40% of Example 1 (ie, a little less than half)), and the durability is significantly reduced in the high-temperature environment (vibration, etc.). Markedly increased).
Similarly, in Comparative Example 3, the torque ratio in a low-temperature environment (0 ° C.) was 1.1 (10% increase of Example 1) and was not much inferior to Example 1, but the high-temperature environment The durability time ratio at the lower temperature (80 ° C.) was only 0.6 (60% of Example 1 (ie, slightly more than half)), and the durability was greatly reduced (vibrations and the like were increased) in the high temperature environment.

  That is, compared with Example 1, the kinematic viscosity is high at both 0 ° C. and 40 ° C. In the case of Comparative Example 1, the torque rises greatly in a low temperature environment (0 ° C.), and the kinematic viscosity at any of the above temperatures. In Comparative Example 2, which is lower than Example 1, it was verified by each test that the durability was significantly lowered under a high temperature environment (80 ° C.). Moreover, it was verified by each test that Example 1 using an ester oil as a base oil has higher durability in a high temperature environment (80 ° C.) than Comparative Example 3 using a mineral oil as a base oil.

Next, in order to verify the optimum content (addition amount) of the viscosity index improver (PMMA) with respect to the grease composition (base oil and thickener), the following test (referred to as “PMMA addition test”) was performed.
In the PMMA addition test, Example 1 and Comparative Example 1 (specifically, these base oils) used in the above-described durability test and torque test were used as bases, respectively, and these contained viscosity index improvers (PMMA). Measure the penetration of the grease composition (8 types each, total of 16 types) added in 7 stages until the content reaches 15% by mass from the untreated state (0% by mass). The blend penetration was compared and verified for each sample.

  FIG. 2 shows the test results of the PMMA addition test for each sample of Example 1 and Comparative Example 1. As is clear from FIG. 2, the sample based on Comparative Example 1 whose kinematic viscosity is higher than Example 1 at 0 ° C. and 40 ° C. has a greater effect on the penetration. That is, when PMMA is added as a viscosity index improver, the decrease in the penetration of the sample (that is, the grease composition) when the addition amount is increased is more severe in Comparative Example 1 than in Example 1. Moreover, until the content of PMMA reached 5% by mass of the entire sample, the mixing penetration of any sample of Example 1 and Comparative Example 1 was stable without decreasing.

Therefore, the optimum content (addition amount) of the viscosity index improver (PMMA) with respect to the grease composition (base oil and thickener) is 5% by mass or less of the entire grease composition. It was verified by. Although not specifically shown, if the content of PMMA is less than 1% by mass of the entire sample, the effect of improving the kinematic viscosity of the sample is diminished, so that the grease composition (base oil and thickening) The content (addition amount) of the viscosity index improver (PMMA) with respect to the agent) is optimally set to 1% by mass (wt%) or more and 5% by mass or less (1 to 5% by weight). .
Similarly, a viscosity index improver (PMMA) can be obtained by considering that the effect of improving the kinematic viscosity can be sufficiently exerted without lowering the penetration of the grease composition (base oil and thickener). ) Is preferably 10,000 or more.

Furthermore, in order to verify the improvement performance of the kinematic viscosity with respect to the temperature when the viscosity index improver is added to the grease composition, the relationship between the base oil kinematic viscosity with respect to the temperature of Example 1 used in the durability test and the torque test described above is shown. A test for comparison with Comparative Example 1 (hereinafter referred to as “kinematic viscosity test”) was performed.
In the kinematic viscosity test, Example 1 (in which PMMA was added as a viscosity index improver to ester oil) and Comparative Example 1 (in which no viscosity index improver was added to ester oil) used in the durability test and torque test described above. Base oil movement of grease compositions (5 types each, total 10 types of samples) in which the temperature of the product is changed from a low temperature environment (less than 0 ° C to about -30 ° C) to a high temperature environment (about 60 to 80 ° C) The viscosity was measured, and the base oil kinematic viscosity was compared and verified for each sample.

  FIG. 3 shows the test results of the kinematic viscosity test for each sample of Example 1 and Comparative Example 1. As is clear from FIG. 3, Example 1 has a lower kinematic viscosity in a low temperature environment than Comparative Example 1, whereas the kinematic viscosity in a normal temperature environment (about 0 to 60 ° C.) and in a high temperature environment. large. That is, in Example 1, although the kinematic viscosity in the low-temperature environment is smaller than that in Comparative Example 1, the effect of the addition of the viscosity index improver (PMMA) causes Comparative Example 1 in the normal temperature environment and the high temperature environment. It was verified by a kinematic viscosity test that a kinematic viscosity equivalent to or higher than that can be maintained.

Above, the durability test and a torque test, if more considering the test results of PMMA added test and kinematic viscosity testing, 550～650Mm the kinematic viscosity of the base oil at 40 ℃ 40~60mm ² / sec, and at 0 ° C. ² and a thickener (as an example, a diurea compound) is added to the base oil, and the total content of the viscosity index improver (same as above, polymethyl methacrylate (PMMA)) becomes 1 to 5 wt%. The grease composition thus added is lubricated by enclosing the grease composition therein, so that it can be used in both a high temperature environment (about 60 to 80 ° C.) and a low temperature environment (0 ° C. or less). The pivot bearing can always be kept in a good lubrication state, and can be rotated with high accuracy over a long period of time.

2 Magnetic disk 4 Spindle motor 6 Magnetic head 8 Swing arm 10 Voice coil 12 Shafts 14 and 16 Pivot bearings 14a, 16a, 14b, 16b Race rings 14c, 16c Rolling elements 14d, 16d Cages 14e, 16e Sealing member 18 Between the sleeves 20 Za Bs HDD base

Claims (3)

A grease composition comprising a base oil, a thickener, and a viscosity index improver,
The base oil is mainly composed of ester oil, has a kinematic viscosity at 40 ° C. of 40 mm ² / sec or more and 60 mm ² / sec or less, and has a kinematic viscosity at 0 ° C. of 550 mm ² / sec or more. And a grease composition characterized by being set to 650 mm ² / sec or less.   The grease composition according to claim 1, wherein the viscosity index improver is polymethyl methacrylate, and the content thereof is 1% by mass or more and 5% by mass or less of the entire grease composition. . In a hard disk drive, a pivot bearing for pivotally supporting a swing arm that is rotated when reading information from the hard disk and writing information to the hard disk,
A hard disk drive pivot bearing characterized in that the grease composition according to claim 1 or 2 is enclosed therein.

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