# Patent

## Title
Pivot assembly bearing

## Abstract

A pivot assembly bearing used in an information storage device maintains its capabilities for a longer period of time under frequent quick starting and stopping by containing a grease composition made up of a base oil, a thickener and an extreme-pressure agent.

## Background
RELATED APPLICATIONS
This application claims all rights of priority to Japanese Patent application No. 2002-035919, filed on Feb. 13, 2002.
FIELD OF THE INVENTION
This invention relates to a pivot assembly bearing for an information storage device of information equipment. In such information equipment, information is stored in or read out from a disk 5 by means of a magnetic head 4 supported by a head actuator 3 as shown in FIG. 1. A pivot assembly bearing 1 supported by a supporting part pivots the actuator head 3 so as to move the magnetic head 4 to a predetermined position. The pivot assembly bearing supports a shaft 6 by means of an upper bearing 1-1 and a lower bearing 1-2 as shown in FIG. 2.
BACKGROUND OF THE INVENTION
There has been conventionally used a grease composition for a pivot assembly bearing of information equipment which comprises a base oil a of mineral oil, ester, carbonic acid ester type, etc., a thickener and an additive not containing an extreme-pressure agent. As a conventional hard disk is large in size and small in information density, a pivot assembly bearing has worked gently without considering actuation thereof under a severe condition.
In the case of a recent small-sized, high speed and high density hard disk, however, it is required for a pivot assembly bearing to move quickly within a short distance in a short time, so that the bearing is heavily loaded due to repetition of quick starting and stopping.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a pivot assembly bearing of long term durability against repeated quick starting and stopping.
The invention relates to a pivot assembly bearing sealed with a grease composition comprising a base oil, a thickener and an extreme-pressure agent. In particular, it has been found that an effective pivot assembly bearing of long term durability against repeated quick starting and stopping can be obtained by sealing therein a grease composition which comprises a mineral oil and a synthetic hydrocarbon oil containing a base oil having a dynamic viscosity of 40 to 80 mm2/s, preferably 50 to 70 mm2/s at 40°, a ureic thickener and an extreme-pressure agent.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of a pivot assembly of an information storage device; and
FIG. 2 is a cross section of a pivot assembly bearing.



PREFERRED EMBODIMENTS OF THE INVENTION
A mineral oil and synthetic hydrocarbon oil are preferably used for a base oil in the invention.
A mineral oil used in the invention includes one or a mixture of not less than two of a naphthenic mineral oil, paraffin mineral oil, hydrogenated mineral oil, destructively hydrogenated mineral oil, solvent-purified mineral oil and highly purified mineral oil.
It is not preferable that the dynamic viscosity at 40° of a synthetic hydrocarbon oil used in the invention is 50 mm2/s or less, or more than 70 mm2/s, because a grease composition thereof splashes over and leaks from the bearing to reduce its life, or the rotational torque thereof increases to cause an energy loss.
A synthetic hydrocarbon oil used in the invention may be prepared by mixing various commercially available poly-α-olefins (PAO) so as to meet the above mentioned requirements.
Such α-olefins include an oligomer such as a dimer, trimer, tetramer, etc. of 1-octene, 1-nonene, 1-decene, 1-dodecene, 1-tridecene, 1-tetradecene, 1-pentadecene, 1-hexadecene, 1-heptadecene, 1-octadecene, 1-nonadecene, 1-eicocene, 1-dococene and the like. Desirable in particular is an oligomer such as a dimer, trimer, tetramer, etc. of 1-decene, above all.
A mass ratio of mineral oil to synthetic hydrocarbon oil in the base oil used in the pivot assembly bearing of the invention is 1:0.5 to 2, and preferably 1:0.7 to 1.5.
A thickener used in the grease composition of the invention is a ureic compound and preferably a diurea compound having two urea bonds in a molecule. In particular, a diurea compound having an aliphatic or alicyclic group as an end group is preferable.
Such a thickener is added to adjust the consistency of a grease composition to be used, although its content is generally about 8 to 18% by mass.
An extreme-pressure agent used in the invention includes an organic molybdenum compound, organic fatty acid compound, and organic phosphorus compound, etc.
A molybdenum compound is added to the grease composition for the purpose of improving the bearing properties, i.e., the thus added compound is absorbed on the surface of bearing metal to form a film, which increases the anti-seizing properties, load bearing properties and resistance to wear even under a high-load and low-speed condition.
As a typical organic molybdenum compound used in the invention, there is preferably used a molybdenum thiophosphate represented by the following general formula:




wherein R1, R2, R3 and R4 are an alkyl group having 1 to 24 carbon atoms or an arlyl group having 6 to 30 carbon atoms, respectively, and/or molybdenum dithiocarbamate represented by the following general formula:





wherein R5, R6, R7 and R8 are an alkyl group having 1 to 24 carbon atoms, respectively, and X is O or S.

Further, there may be used dihydrocarbyl dithiophosphate metal compounds, dihydrocarbyl dithiocabamate metal compounds, naphtenic acid salts, etc., the metal being preferably zinc, tin, cadmium, antimony and the like. These metals may also be used together, if necessary. The above mentioned dihydrocarbyl dithiophosphate metal compounds include, for example, zinc dimethyl dithiophosphate, zinc butylisooctyl dithiophosphate, zinc di(4-methyl-2-pentyl) dithiophosphate, zinc di(tetrapropenyl-phenyl) dithiophosphate, zinc (2-ethyl-1-hexyl) dithiophosphate, zinc (isooctyl) ditho-phospate, zinc (ethylphenyl) dithiophosphate, zinc (amyl) dithiophosphate, zinc di-(hexyl) dithiophosphate, etc.
An organic aliphatic acid compound used in the invention includes oleic acid, naphthenic acid, abietic acid, lanolin aliphatic acid, succinic acid, amino acid derivatives, etc. Alkenyl succinic acid or its anhydride is preferable as a succinic acid compound, which conveniently absorbs on a rolling and sliding surface of the bearing material to form a film. Further, succinic acid derivatives also behave in a similar manner as described above.
Such succinic acid derivatives include succinic acid, alkyl succinic acid, succinic acid alkyl half ester, alkenyl succinic acid, succinic acid alkenyl half ester, succinimide, etc. These succinic acid derivatives may be used alone or in combination.
Phosphorous ester is preferable as an organic phosphide compound and includes those esters having a hydrocarbon group, such as trioctyl phosphite, triphenyl phosphite, tricresyl phosphite, bis-2-ethylhexyl phosphite, tridecyl phosphite, dibutyl hydrogen phosphite, tris(nonylphenyl) phosphite, dilauryl hydrogen phosphite, diphenyl monodecyl phosphite, trilauryl trithiophosphite and diphenyl hydrogen phosphite.
Further, orthophosphoric acid esters may also be used in the invention, which include tripheny phosphate, triethyl phosphate, tributyl phosphate, tris(2-ethylhexyl) phosphate, tris(2-ethylhexylyl) phosphate, tridecyl phosphate, diphenyl mono(2-ethylhexylyl) phosphate, tricresyl phosphate, trioctyl phosphate, tristearyl phosphate, etc.
An acidic phosphoric ester may also be used in the invention. Preferable acidic phosphoric esters include, for example, methyl acid phosphate, isopropyl acid phosphate, butyl acid phosphate, 2-ethylhexyl acid phosphate, isodecyl acid phosphate, dodecyl acid phosphate, lauryl acid phosphate, etc.
An amount of the extreme-pressure agent comprising the above mentioned organic molybdenum compound, organic fatty acid compound and organic phosphorus compound is 0.2 to 5% by mass and desirably in particular 1 to 2% by mass of the total grease composition.
The object of the invention is hardly achieved in an amount less than 0.2% by mass, while a large quantity of corrosive gas is inconveniently generated over an amount of 5% by mass.
The grease composition used in the invention may further comprise a conventional antioxidant, rust preventing agent, corrosion inhibitor, oiliness improving agent, etc.
The antioxidant includes phenolic or aromatic amine compounds, e.g., 2,6-di-tert-butyl-4-methylphenol as a phenolic antioxidant and p,p′-dioctyldiphenylamine as an amine antioxidant.
These antioxidants may either be used alone or as a mixture of two or more of them and are suitably added in an amount of 0.5 to 5% by mass of the grease composition.
The grease composition used in the invention may be prepared in a conventionally well-known manner.
Embodiments of the invention will be summarized as in the following.
1. A pivot assembly bearing sealed with a grease composition comprising a base oil, thickener and extreme-pressure agent.
2. The pivot assembly bearing described in the above item 1 in which the base oil containing a mineral oil and a synthetic hydrocarbon oil and having a dynamic viscosity of 40 to 80 mm2/s at 40° is used.
3. The pivot assembly bearing described in the above item 1 in which a mass ratio of mineral oil:synthetic hydrocarbon oil is 1:0.5 to 2.
4. The pivot assembly bearing described in any one of the above items 1 to 3 in which the synthetic hydrocarbon oil is a poly-α-olefin (PAO).
5. The pivot assembly bearing described in any one of the above items 1 to 4 in which the poly-α-olefin (PAO) is one or not less than two oligomers of 1-decene selected from a group consisting of a dimer, trimer, tetramer, etc.
6. The pivot assembly bearing described in any one of the above items 1 to 5 in which the thickener is a urea compound.
7. The pivot assembly bearing described in any one of the above items 1 to 6 in which the extreme-pressure agent is one or not less than two compounds selected from a group consisting of a molybdenum compound, organic fatty acid compound and organic phosphorus compound.
8. The pivot assembly bearing described in any one of the above items 1 to 6 in which the molybdenum compound is molybdenum dithiophosphate and/or molybdenum dithiocarbamate.
9. The pivot assembly bearing described in any one of the above items 1 to 6 in which the organic phosphorus compound is tricresyl phosphate.
This invention will be further described in detail in the following examples.
EXAMPLES
A base oil having dynamic viscosity of 52 to 53 mm2/s at 40° was prepared by mixing a naphthenic mineral oil and a 1-decene oligomer as a synthetic hydrocarbon oil in a mass ratio of 1:1.
Further, a base oil having a dynamic viscosity of 55 to 56 mm2/s at 40° was prepared by mixing a paraffin mineral oil and a 1-decene oligomer as a synthetic hydrocarbon oil in a mass ratio of 1:1.
An alicyclic-aliphatic mixed diurea compound as a thickener was prepared by reacting 5 moles of diphenylmethane 4,4′-diisocyanate, 3 moles of cyclohexylamine and 7 moles of stearylamine and used in an amount of 12 to 14% by mass of total grease composition.
Further, lithium stearate as a thickener was used in an amount of 12 to 14% by mass of total grease composition.
As the extreme-pressure agent, there were used molybdenum compounds of molybdenum dithiophosphate (R1, R2, R3 and R4 of the general formula as shown previously represent an alkyl group having 1 to 10 carbon atoms or aryl group having 6 carbon atoms, respectively) and molybdenum dithiocarbamate (R5, R6, R7 and R8 of the general formula as shown previously represent an alkyl group having 1 to 8 carbon atoms, respectively and X is O).
Tricresyl phosphite preferable as an organic phosphorus compound, was also used as the extreme-pressure agent.
These extreme-pressure agents were used in an amount of 1 to 2% by mass of total grease composition.
An antioxidant of the amine type and a rust preventing agent of the sulfonic acid and sulfonate type were used as a stabilizer in an amount of 2 to 5% by mass of total grease composition to yield grease products.
Compositions of these grease products are shown in Table 1.
COMPARATIVE EXAMPLES
Comparative grease compositions were prepared in a similar manner as described above without adding any extreme-pressure agent.










TABLE 1









Grease Composition
















Extreme-







pressure

4-Ball


Example
Base Oil
Thickener
Agent
Stabilizer
Test

















Ex.1
naphthen
PAO
1:1
diurea
MDC 1%
A-S-E
⊚


Ex.2
paraffin
PAO
1:1
diurea
MDC 2%
A-S-E
⊚


Ex.3
naphthen
PAO
1:1
diurea
phoshite
A-S-E
⊚


Ex.4
paraffin
PAO
1:0.8 
diurea
MDP 2%
A-S-E
⊚


Ex.5
naphthen
PAO
1:1.5 
diurea
MDC 2%
A-S-E
⊚












Ex.6
carbonic acid
St-Li
MDC 2%
A-S-E
⊚



ester


Ex.7
polyol ester
St-Li
MDP 2%
A-S-E
⊚














Com.Ex.1
naphthen
PAO
1:1
diurea
none
A-S-E
Δ












Com.Ex.2
carbonic acid
St-Li
none
A-S-E
◯



ester


Com.Ex.3
polyol ester
St-Li
none
A-S-E
X





Diurea: alicyclic-aliphatic mixed diurea compound


St-Li: lithium stearate


Phosphite: tricresyl phosphite


MDC: molybdenum dithiocarbamate


MDP: molybdenum dithiophosphate


A-S-E: antioxidant of amine type and rust preventing agent of sulfonic acid and sulfonate type


4-ball Test: diameter of wear scar caused by foul-ball test at 4.0 kg · f load and 1,200 rpm for one hour


⊚ less than 0.6μ;


◯ less than 0.7μ;


Δ less than 0.8μ;


X_0.8μ or more






Properties Test of Grease Compositions

Four-Ball Test
Four-ball test was carried out at 4.0 kg·f load and 1,200 rpm for one hour and the diameter of the wear scar was measured.
Pivoting Test of Pivot Assembly Bearing
Pivot assembly bearings were sealed with grease compositions and subjected to a pivoting test under the following condition:
test machine: 3.5 inch-HDD actuator
seek angle: full
number of seeking times: 108 seeks
frequency: 12 Hz
temperature: ordinary temperature
evaluation items: torque (before starting the test; after 107 seeks; after 5×107 seeks and after 108 seeks), and maximum torque (2 rpm for 30 seconds)
visual evaluation of lubricity after 108 seeks: excellent/good/moderate/poor
State of disassembled bearings: excellent/good/moderate/poor
Bearing properties of pivot assembly bearings of the invention are shown in Table 2.










TABLE 2









Maximum Torque (g · cm) and Appearance
Lubricity after
















Example
no seek

107 seeks

5 × 107 seeks

108 seeks

108 seeks





Ex.1
0.545
⊚
0.400
⊚
0.600
⊚
0.752
⊚
⊚


Ex.2
0.540
⊚
0.403
⊚
0.611
⊚
0.784
⊚
⊚


Ex.3
0.531
⊚
0.411
⊚
0.602
⊚
0.772
⊚
⊚


Ex.4
0.551
⊚
0.402
⊚
0.599
⊚
0.713
⊚
⊚


Ex.5
0.503
⊚
0.444
⊚
0.609
⊚
0.744
⊚
⊚


Ex.6
0.540
⊚
0.451
⊚
0.663
⊚
0.801
◯
◯


Ex.7
0.650
⊚
0.403
⊚
0.602
⊚
0.779
◯
◯


Com.Ex.1
0.536
⊚
0.424
⊚
0.864
◯
0.925
Δ
Δ


Com.Ex.2
0.543
⊚
0.477
◯
0.751
Δ
0.886
X
X


Com.Ex.3
0.650
⊚
0.441
◯
0.754
Δ
0.911
X
X





Appearance: evaluation by disassembly:


⊚ excellent;


◯ good;


Δ moderate;


X poor


Lubricity after 108 seeks:


⊚ excellent;


◯ good;


Δ moderate;


X poor






It is clear from these Tables 1 and 2 that pivot assembly bearings sealed with a grease composition comprising a base oil, thickener and extreme-pressure agent exhibit excellent results in the long-term pivoting durability test. Particularly, it is demonstrated that a pivot assembly bearing sealed with a grease composition comprising a base oil containing a mineral oil and synthetic hydrocarbon oil and having a dynamic viscosity of 40 to 80 mm2/s, and preferably 50 to 70 mm2/s at 40°; a ureic thickener; and an extreme-pressure agent can withstand long-term service even when quick starting and stopping are repeated.
It has been found that the pivot assembly bearing of the invention exhibited a stable torque over a long period of time as shown in Tables 1 and 2, which makes it possible to extend its life and withstand its tong-term service against repeated quick starting and stopping. It has also been found that the present pivot assembly bearing does not cause a fretting phenomenon.

## Claims

1. In an information storage device for storing and/or reading out information on a disk by means of a magnetic head supported by a head actuator and pivoted by a pivot assembly bearing, the improvement comprising said pivot assembly bearing being sealed with a grease composition comprising a base oil having a dynamic viscosity of 40 to 80 mm2/sec at 40° C. and containing a mineral oil and a synthetic hydrocarbon oil in a mass ratio of mineral oil:synthetic hydrocarbon oil of 1:0.5 to 1:2, a thickener and an extreme pressure agent.

2. The information storage device of claim 1, wherein the synthetic hydrocarbon oil is a poly-α-olefin.

3. The information storage device of claim 2, wherein the poly-α-olefin is at least one mixed oligomer selected from the group consisting of a dimer, trimer and tetramer.

4. The information storage device of claim 1, wherein the thickener is a urea compound.

5. The information storage device of claim 1, wherein the extreme pressure agent is at least one compound selected from the group consisting of a molybdenum compound, an organic fatty acid compound and an organic phosphorus compound.

6. The information storage device of claim 5, wherein the molybdenum compound is at least one of molybdenum dithiophosphate and molybdenum dithiocarbamate.

7. The information storage device of claim 5, wherein the organic phosphorus compound is tricresyl phosphate.