JP4213714B2 - Sintered oil-impregnated bearing - Google Patents

Description

  The present invention relates to a sintered oil-impregnated bearing in which a friction coefficient of a sliding surface supporting a shaft that slides at high speed is maintained low for a long period of time, and particularly suitable as a bearing for a spindle motor used in an optical disc apparatus or the like. Related to a sintered oil impregnated bearing.

A bearing for a spindle motor used in a disk rotating device such as a CD-ROM has a sliding bearing structure in which two bearing surfaces are arranged at an interval in the axial direction. The two bearing surfaces are either one in which two sintered oil-impregnated bearings are press-fitted into the housing, or bearing surfaces are provided at both ends, and an intermediate relief portion having a larger diameter than the shaft is provided in the middle portion. It is configured by press-fitting a sintered oil-impregnated bearing into the housing. The spindle motor rotates at a high speed of several thousand revolutions per minute, and when such a bearing is used, an unbalanced load due to the mounting of the disk is applied, causing a whirling around the shaft surface in the circumferential direction. In a bearing, it becomes difficult to form an oil film on the bearing surface, and the friction coefficient gradually increases, so that problems such as rapid progress of abrasion tend to occur.
Conventionally, in order to deal with such problems, a lubricating oil which is formed by forming a porous sintered bearing with a bronze alloy or a copper iron alloy having excellent compatibility with the shaft and wear resistance, and impregnating the bearing. Are improved variously as shown in Patent Documents 1 to 4 below.
In Patent Document 1, the base oil of the bearing oil contains a polyolefin and a polyol ester.
The bearing oil of Patent Document 2 has a poly-α-olefin or a hydride thereof as a base oil, and is a two-quantity condensed fatty acid having an —OH group or a fatty acid having an —OH group and a fatty acid not having an —OH group. It is the structure which mix | blended ester of estolide (estolide) which is an oxy fatty acid oligomer more than a body, and alcohol.
Patent Document 3 discloses a lubricating oil having a structure in which a poly-α-olefin or a hydride thereof is used as a base oil, and a phosphate ester is blended with the base oil.
Patent Document 4 is a prior application by the applicant of the present application, and describes a lubricating oil having a structure in which a poly-α-olefin is used as a base oil and an aliphatic monohydric alcohol having 14 to 20 carbon atoms (for example, oleyl alcohol) is blended. .
According to the documents 1 to 4, the sintered oil-impregnated bearing impregnated with the above oil has a low friction coefficient on the bearing surface, enables operation at a low current, and can maintain the state for a long time.
Patent Document 1: Japanese Patent Laid-Open No. 9-125086 (second page, etc.)
Patent Document 2: Japanese Patent Laid-Open No. 10-36870 (second page, etc.)
Patent Document 3: Japanese Patent Laid-Open No. 11-269475 (second page, etc.)
Patent Document 4: Japanese Patent Laid-Open No. 2002-310156 (second page, etc.)
However, with an increase in the number of revolutions of the spindle motor and a demand for power saving, there is a further demand for improvement in sliding characteristics such as low friction on the bearing element.

From such a background, the present inventors have repeatedly studied in order to obtain more preferable findings than the configuration of the prior invention (Patent Document 4). The present invention provides a sintered oil-impregnated bearing that is capable of further reducing the friction coefficient of the bearing in response to the above-described demand and that is excellent in durability that can sustain a low friction state for a long period of time. With the goal.
In order to achieve the above object, the sintered oil-impregnated bearing of the present invention is a sintered oil-impregnated bearing in which pores of a porous sintered bearing are impregnated with a lubricant, and the lubricant is a base oil composed of poly-α-olefin. And 2-ethylhexyl hydroxystearate. With this configuration, low friction on the sliding surface of the bearing can be maintained for a long time, and power consumption of a power source such as a spindle motor is reduced.
In the sintered oil-impregnated bearing, the lubricating oil contains polyisobutylene in the range of 0.1 to 3% by mass, polybutene in the range of 0.1 to 3% by mass, and poly in the range of 0.5 to 3% by mass. It is preferable to contain at least one of alkyl methacrylates. The lubricating oil may contain a corrosion inhibitor (metal deactivator), an antioxidant and the like, as in the conventional case.

  FIG. 1 is a schematic cross-sectional view showing the structure of a spindle motor used in a bearing performance evaluation test.

Hereinafter, the structure of the sintered oil-impregnated bearing of the present invention will be described in detail.
The sintered oil-impregnated bearing has a sintered bearing formed of a sintered porous metal and a lubricating oil impregnated in the pores of the sintered bearing, and the lubricating oil is necessary for the base oil that is a main component thereof. Depending on the type, various additives are blended. Common additives include oiliness agents, lubricity improvement (viscosity index improvement) agents, corrosion inhibitors, antioxidants, extreme pressure agents, detergent dispersants, pour point depressants, etc., but will be described below. Additives are particularly effective in the present invention. Each component of the sintered bearing and the lubricating oil will be described below.
(1) Sintered bearing In this invention, the bearing material which comprises a sintered bearing is a sintered porous metal material, As the composition, it is bronze type, iron type, iron-copper type, bronze-iron, brass -Iron-based materials are applicable. The spindle motor bearing is particularly preferably a bronze-iron composite material or a brass-iron composite material, which is a composite material in which a copper alloy is dispersed in an iron-based structure. The effective porosity of the bearing material may be determined and changed as appropriate according to the structure of the bearing, and is usually set to about 5 to 25% (volume percentage). Sintered oil-impregnated bearings have a shape that forms one smooth sliding surface that is continuous over the entire inner diameter, or a shaft that has sliding surfaces at both ends in the axial direction of the inner diameter, and the inner diameter supports between them. Includes those with a large intermediate relief shape intermediate part, those with oil grooves and dynamic pressure grooves formed on the sliding surface, those with sliding surfaces on both the inner diameter part and both end faces in the axial direction, etc. . Such a sintered bearing can be manufactured by a general powder metallurgy manufacturing method including each step of compacting raw metal powder, sintering in a non-oxidizing atmosphere, sizing, and oil impregnation. A bearing in which the amount of pores opened in the sliding surface is reduced by sizing is preferable. However, it is necessary to consider sizing performed by secondary processing or the like so that the pores are not completely crushed. When the dynamic pressure groove is provided, its shape can be appropriately selected from various shapes such as those parallel to the axial direction and herringbone pattern, but it is desirable that the groove surface has few pores.
(2) Lubricating oil a. Base oil In the present invention, poly-α-olefin is used as the base oil. Poly-α-olefin has the lubrication characteristics, heat resistance, low temperature characteristics, and thermal stability necessary for lubricating oil for sliding bearings, and is superior to various mineral oils. The composition and properties of the poly-α-olefin to be used are not particularly limited, but it is very effective to use kinematic viscosity as an index for selection. If the kinematic viscosity of the poly-α-olefin used is too low, the load bearing capacity and volatility resistance of the oil-impregnated bearing may be inferior. If the kinematic viscosity is too high, the fluidity of the lubricating oil decreases and the oil-impregnated bearing There is a risk that the friction coefficient in the initial stage of operation increases. From this, the kinematic viscosity range of the poly-α-olefin suitable for use can be 5 to 100 mm ² / sec at 40 ° C. Alternatively, the viscosity index can be used as another index. A material having a high viscosity index is preferable because it retains lubrication characteristics and high-temperature stability, and can be selected based on a viscosity index of 110 or more. In this connection, considering the viscosity characteristics in the low temperature range, it is desirable to select and use a pour point of −30 ° C. or lower.
b. Oiliness Agent The lubricating oil for bearings of the present invention contains 2-ethylhexyl hydroxystearate as an oiliness agent.
2-ethylhexyl hydroxystearate is a condensation reaction product of hydroxystearic acid and 2-ethylhexyl alcohol, and belongs to octyl hydroxystearate known as an oil agent added to cosmetics in the industry. Octyl hydroxystearate is liquid or semi-pasty at room temperature depending on the structural isomerism, but in the present invention, 2-ethylhexyl hydroxystearate that is liquid at room temperature is preferably used. Examples include 2-ethylhexyl 12-hydroxystearate.
When 2-ethylhexyl hydroxystearate is blended with the base oil poly-α-olefin, the slip characteristics are improved by the adsorption of the lubricating oil to the metal surface of the bearing. In addition, when oil-impregnated bearings are used as bearing elements to support the motor shaft, the change in the friction coefficient during operation is less than that of conventional lubricants, and the initial friction coefficient can be maintained for a long period of time. It is done. This effect increases as the content increases, and becomes extremely remarkable when the content ratio is about 5% by mass or more with respect to the entire lubricating oil, exceeding the lubricating characteristics of the conventional lubricating oil. However, when the content ratio exceeds 20% by mass, the increase in the content is not effective. Therefore, considering the economical aspect, the content ratio with respect to the total amount of the lubricating oil is preferably 5 to 20% by mass.
c. Lubricant (viscosity index) improver The bearing lubricating oil of the present invention preferably contains at least one of polyisobutylene, polybutene, and polyalkylmethacrylate as an additive for improving lubricity.
Polyisobutylene and polybutene: (—CH ₂ C (CH ₃ ) ₂ —) _n , (—CH ₂ CH (C ₂ H ₅ ) —) _n , (—CH (CH ₃ ) CH (CH ₃ ) —) _n is Although it does not contribute to lowering the friction coefficient, it has the effect of forming a thick lubricating film on the sliding surface, and suppresses metal contact between the bearing surface and the shaft, thereby improving the wear resistance and durability of the bearing. Contribute. These may be from viscous liquids to rubber-like solids depending on the molecular weight. In the present invention, those that dissolve in the base oil are used, and those having a molecular weight of about 800 to 20,000 are preferred. A mixture of different molecular weights may be used. For example, a mixture of two types of polyisobutylene having a molecular weight of up to about 5,000 and about 5,000 or more can be suitably used. The ratio with respect to the total amount of the lubricating oil is preferably in the range of 0.1 to 3 mass%. When both polyisobutylene and polybutene are added, or when different molecular weights are mixed and used, it is preferable to adjust the total amount to be within the above range. If the content ratio is less than 0.1% by mass, the effect is not sufficiently exerted, and if it exceeds 3% by mass, the increase in the content is not effective.
Polyalkylmethacrylate reduces the metal contact of the sliding part by adsorption to the metal surface. Furthermore, it acts as a friction polymer due to molecular changes accompanying temperature rise due to sliding, and has the effect of reducing friction under heavy load conditions. The alkyl group of the polyalkyl methacrylate may be a linear saturated hydrocarbon or a branched chain, and preferably has an alkyl group having 1 to 18 carbon atoms and a molecular weight such as polymethyl methacrylate and polyethyl methacrylate. Of 10 ^{4 to} 1.5 × 10 ⁶ is used. It is preferable to blend in a range of 0.5 to 3% by mass with respect to the total amount of the lubricating oil, and if the content is less than 0.5% by mass, the effect is not sufficiently exhibited, and if it exceeds 3% by mass, Since it becomes easy to form sludge by the action | operation of a shaft | shaft, there exists a possibility of reducing durability.
d. Other additives contained in the lubricating oil The lubricating oil for bearings of the present invention does not need to contain anything other than the aforementioned additives, but depending on the material of the sintered bearing, the usage environment of the sintered oil-impregnated bearing, etc. Additives such as anticorrosion (metal deactivation) agents and antioxidants can be blended.
Examples of the corrosion inhibitor include benzotriazole and derivatives thereof, and these have an action of forming an inactive film on the metal surface and an antioxidant action.
Examples of the antioxidant include phenol compounds such as 4,4′-methylene-bis (2,6-di-tert-butylphenol) and 2,6-di-tert-butyl-4-methylphenol; And amine compounds such as octylphenyl) amine and phenyl-α-naphthylamine.
The addition amount of the corrosion inhibitor and the antioxidant may be set so that the ratio to the total amount of the lubricating oil is about 0.05 to 2% by mass.

（油含浸）
前述で製造した焼結軸受に、実施例１〜９及び比較例１〜２の各潤滑油を真空含浸し、実施例１〜９及び比較例１〜２の各焼結含油軸受を得た。
（性能試験）
前述で得た各焼結含油軸受を用いて、図１に示すようなスピンドルモーターの構造を有する試験装置を構成した。
詳細には、上部が開口したカップ状ハウジング２の円筒形状の内径に、焼結含油軸受１を圧入した。この試験装置のハウジング２の内底部は、シャフト４末端の球面をスラスト受けするためのスラスト受け部３を構成している。ハウジング２の外周には、コイル７を巻回したコアが備えられている。シャフト４の直径は３ｍｍであり、シャフト４の上部には、マグネット５を内周に有したロータヨーク６及びロータ８が固定されている。従って、この試験装置は、コイル７に通電することによってロータヨーク６がシャフト４を軸として回転する。
尚、上記試験装置において、試験条件として回転アンバランスが１ｇ・ｃｍとなるように、ロータ８に錘を接着樹脂で固着した。
上記試験装置の回転数が１００００ｒｐｍになるようにコイル７に印加する電圧を調節してロータ８を回転し、回転初期にコイル７に流れる初期電流値（単位：Ａ）を測定した。回転数が変化しないように電圧を調節しながら運転を続けると、装置の運転を初めてから１時間以内に回転は安定化し、電流値は初期電流値より多少低い値で一定になるので、この時の運転条件を保持して運転を継続した。この後、ロータ８の回転数が５００ｒｐｍ以上低下する（９５００ｒｐｍ未満になる）時までの運転時間（単位：ｈ）を測定した。得られた初期電流値及び運転時間を表２に示す。
初期電流値は、焼結含油軸受の摺動面の摺動摩擦が増加するに従って大きくなるので、摺動特性の評価目安となる。又、上記運転時間は、軸受の初期の摺動特性を維持可能な時間に相当し、耐久性の評価目安となる。

（評価）
上記試験装置の運転条件は、焼結含油軸受の実際の使用状況より過酷であり、実用を勘案した評価に十分な条件と言える。
実施例１〜３の焼結含油軸受は、いずれも、初期電流値が比較例１，２のものに比べて低く、摺動摩擦が少ないことを示している。加えて、回転数が低下するまでの運転時間がかなり長く、摺動特性が維持される寿命が長いことが理解される。一方、実施例８においては、初期電流値は実施例１〜３と同様に低いが、運転時間が相対的に短い。これらのことから、潤滑油にヒドロキシステアリン酸２−エチルヘキシルを添加することによって摺動摩擦を低減することができ、その配合割合を５質量％程度以上にすることによって、摺動摩擦が低い初期の摺動特性を維持可能な運転時間を従来の潤滑油より長くすることが可能であることが解る。
又、実施例４〜７の結果から、更に潤滑性向上剤としてポリアルキルメタクリレートやポリイソブチレンを潤滑油に添加することによって、運転時間つまり寿命が更に長くなることが解る。但し、実施例６，７の結果を比較すると、ポリアルキルメタクリレートの添加量が過剰であると、回転数が低下するまでの運転時間がかえって短くなり、潤滑特性の劣化が速くなる。Hereinafter, the sintered oil-impregnated bearing of the present invention will be described in more detail with reference to examples.
(Sintered bearing)
Sintered bearings were manufactured using iron-copper sintered alloys as bearing materials. Specifically, each simple metal powder of iron, copper, zinc and tin is mixed, and the composition is Cu: 55% by mass ratio, Zn: 6%, Sn: 2%, Fe and other metal impurities: the balance A metal mixed powder was prepared. This metal mixed powder was compacted to obtain a compact having a bearing shape and a density of 6.6 Mg / m ³ . This molded body was sintered at a temperature of 780 ° C. in a mixed gas atmosphere of hydrogen and nitrogen to obtain a sintered body made of a porous sintered alloy. This sintered body had a metal structure in which a copper alloy phase was dispersed in an iron (ferrite) matrix and had an effective porosity of 20%. Sizing the obtained sintered body, as shown in FIG. 1, there are bearing portions at both ends in the axial direction, and an intermediate portion having an inner clearance shape larger in inner diameter than the bearing portion is formed between them. A bearing 1 was obtained.
(Lubricant)
Each component was mixed according to the compounding ratio of Table 1, and the lubricating oil of Examples 1-9 and Comparative Examples 1-2 was prepared. In Table 1, PAO is a poly-α-olefin (with a viscosity grade equivalent to ISO VG18), HSEH is 2-ethylhexyl 12-hydroxystearate, PAMA is a non-dispersed polyalkyl methacrylate (alkyl carbon number: average 12, molecular weight) : About 70,000), PIB is polyisobutylene (molecular weight: about 10,000), OA is oleyl alcohol, and the blending ratio of each component is expressed by mass%. Moreover, the component mix | blended as "others" is 0.1 mass% of 2, 6- di-tert- butyl-4-methylphenol as an antioxidant, and 1.1 mass% of benzotriazole as an anticorrosion agent.
Accordingly, the lubricating oils of Examples 1 to 3 are based on poly-α-olefin, 0.1% by weight of phenolic antioxidant, 1.1% by weight of benzotriazole anticorrosive and 5 to 20% by weight. % In the range of 2-ethylhexyl hydroxystearate. Further, the lubricating oils of Examples 4 and 5 further contain polyisobutylene as a lubricity improver, and the lubricating oil of Example 6 contains polyisobutylene and polyalkyl methacrylate as lubricity improvers. The lubricating oil of Example 7 contains more polyalkyl methacrylate than Example 6.
The lubricating oil of Example 8 has a lower content of 2-ethylhexyl hydroxystearate than Example 1. The lubricating oil of Example 9 has a higher content of 2-ethylhexyl hydroxystearate than Example 3. The lubricating oil of Comparative Example 1 does not contain 2-ethylhexyl hydroxystearate. The lubricating oil of Comparative Example 2 corresponds to the lubricating oil described in JP-A No. 2002-310156, does not contain 2-ethylhexyl hydroxystearate, and contains 10% by mass of oleyl alcohol instead.

(Oil impregnation)
The sintered bearings produced above were vacuum impregnated with the lubricating oils of Examples 1 to 9 and Comparative Examples 1 to 2, and the sintered oil impregnated bearings of Examples 1 to 9 and Comparative Examples 1 to 2 were obtained.
(performance test)
Using each sintered oil-impregnated bearing obtained above, a test apparatus having a spindle motor structure as shown in FIG. 1 was constructed.
Specifically, the sintered oil-impregnated bearing 1 was press-fitted into the cylindrical inner diameter of the cup-shaped housing 2 opened at the top. The inner bottom portion of the housing 2 of this test apparatus constitutes a thrust receiving portion 3 for thrust receiving the spherical surface at the end of the shaft 4. A core around which the coil 7 is wound is provided on the outer periphery of the housing 2. The diameter of the shaft 4 is 3 mm, and a rotor yoke 6 and a rotor 8 having a magnet 5 on the inner periphery are fixed to the upper portion of the shaft 4. Therefore, in this test apparatus, when the coil 7 is energized, the rotor yoke 6 rotates about the shaft 4.
In the above test apparatus, a weight was fixed to the rotor 8 with an adhesive resin so that the rotational unbalance was 1 g · cm as a test condition.
The rotor 8 was rotated by adjusting the voltage applied to the coil 7 so that the rotation speed of the test apparatus was 10,000 rpm, and the initial current value (unit: A) flowing through the coil 7 was measured at the initial stage of rotation. If the operation is continued while adjusting the voltage so that the rotation speed does not change, the rotation stabilizes within one hour from the beginning of the operation of the device, and the current value becomes constant at a value slightly lower than the initial current value. The operation conditions were maintained and the operation was continued. Thereafter, the operation time (unit: h) until the rotation speed of the rotor 8 decreased by 500 rpm or more (below 9500 rpm) was measured. Table 2 shows the obtained initial current value and operation time.
Since the initial current value increases as the sliding friction of the sliding surface of the sintered oil-impregnated bearing increases, it becomes an evaluation standard for the sliding characteristics. The operation time corresponds to a time during which the initial sliding characteristics of the bearing can be maintained, and serves as an evaluation standard for durability.

(Evaluation)
The operating conditions of the test apparatus are severer than the actual usage of the sintered oil-impregnated bearing, and can be said to be sufficient conditions for evaluation in consideration of practical use.
Each of the sintered oil-impregnated bearings of Examples 1 to 3 has an initial current value lower than those of Comparative Examples 1 and 2, indicating that there is little sliding friction. In addition, it is understood that the operation time until the rotational speed is lowered is considerably long, and the life in which the sliding characteristics are maintained is long. On the other hand, in Example 8, the initial current value is low as in Examples 1 to 3, but the operation time is relatively short. From these facts, sliding friction can be reduced by adding 2-ethylhexyl hydroxystearate to the lubricating oil, and the initial sliding with low sliding friction can be achieved by setting the blending ratio to about 5% by mass or more. It can be seen that the operation time during which the characteristics can be maintained can be made longer than that of the conventional lubricating oil.
Further, from the results of Examples 4 to 7, it is understood that the operation time, that is, the service life is further increased by further adding polyalkyl methacrylate or polyisobutylene to the lubricating oil as a lubricity improver. However, when the results of Examples 6 and 7 are compared, if the amount of polyalkyl methacrylate added is excessive, the operation time until the rotational speed decreases is shortened, and the deterioration of the lubricating characteristics is accelerated.

  According to the present invention, there is provided a sintered oil-impregnated bearing having a sliding characteristic in which a sliding friction is small and a low friction state can be maintained for a long period of time by devising a composition of a lubricating oil impregnated in pores of a porous bearing. As a result, a sintered oil-impregnated bearing suitable for a spindle motor mounted on a disk rotating device such as a CD-ROM or DVD-ROM can be provided, which contributes to power saving and performance improvement of the disk rotating device. be able to. In addition, the applications of sintered oil-impregnated bearings are expanded.

Claims (10)

  A sintered oil-impregnated bearing in which pores of a porous sintered bearing are impregnated with a lubricating oil, wherein the lubricating oil contains a base oil composed of poly-α-olefin and 2-ethylhexyl hydroxystearate. Sintered oil-impregnated bearing.   2. The sintered oil-impregnated bearing according to claim 1, wherein a content ratio of the 2-ethylhexyl hydroxystearate is 5% by mass or more based on the whole lubricating oil.   The sintered oil-impregnated bearing according to claim 1 or 2, wherein a content ratio of the 2-ethylhexyl hydroxystearate is 20% by mass or less with respect to the entire lubricating oil.   The sintered oil-impregnated bearing according to any one of claims 1 to 3, wherein the lubricating oil further contains at least one lubricity improver selected from the group consisting of polyisobutylene, polybutene, and polyalkylmethacrylate. The sintered oil-impregnated bearing according to claim 4, wherein a content ratio of the polyisobutylene and / or polybutene is in a range of 0.1 to 3% by mass with respect to the entire lubricating oil. The sintered oil-impregnated bearing according to claim 4 or 5, wherein a content ratio of the polyalkyl methacrylate is in a range of 0.5 to 3 mass% with respect to the entire lubricating oil.   The porous sintered bearing is composed of a sintered porous metal material having a composition selected from the group consisting of bronze, iron, iron-copper, bronze-iron, and brass-iron, and has an effective porosity of 5 to 25%. The sintered oil-impregnated bearing according to any one of claims 1 to 6.   The sintered oil-impregnated bearing according to any one of claims 1 to 6, wherein the porous sintered bearing is made of a bronze-iron composite material or a brass-iron composite material. The poly-α-olefin has a kinematic viscosity of 5 to 100 mm ² / sec at 40 ° C, a viscosity index of 110 or more, and a pour point of -30 ° C or less. The sintered oil-impregnated bearing described.   The sintered oil-impregnated bearing according to any one of claims 1 to 9, which is used as a spindle motor bearing.

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