Bearings – Rotary bearing – Fluid bearing
Reexamination Certificate
2002-02-13
2003-09-16
Hannon, Thomas R. (Department: 3682)
Bearings
Rotary bearing
Fluid bearing
C384S907100
Reexamination Certificate
active
06619847
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ceramic dynamic-pressure bearing, a motor having a bearing, a hard disk drive, and a polygon scanner.
2. Description of the Related Art
Conventionally, a ball bearing has often been used as a bearing for the shaft of a motor serving as a drive unit of electric equipment. High-speed rotation of a motor has been rapidly implemented in precision equipment, such as peripheral equipment of a computer. In this connection, in order to obtain excellent bearing performance with low rotation-speed fluctuation and reduced noise and vibration, or in order to elongate bearing service life, a dynamic-pressure bearing, which uses fluid, such as air, as a medium, has been employed. The dynamic-pressure bearing operates in the following manner: when, for example, a spindle and a bearing member disposed so as to surround the spindle undergo relative rotation about an axis, the axis of rotation is supported by the action of fluid dynamic-pressure generated in the gap formed between the outer circumferential surface of the spindle and the inner circumferential surface of the bearing member. Also, another bearing is configured such that the thrust face of a spindle or that of a bearing member is supported by action of dynamic pressure.
When a dynamic-pressure bearing is in a high-speed rotation state, in which generated dynamic-pressure is sufficiently high, two members which face each other with a dynamic-pressure gap present therebetween do not come into contact with each other. However, at the time of starting or stopping, when rotational speed is low, sufficiently high dynamic pressure is not generated; thus, the two members come into contact with each other. Component members of such a dynamic-pressure bearing have generally been formed of a metal, such as stainless steel, and in some cases have been further coated with resin or a like material. However, the two metallic members are subject to a problem of wear or seize-up caused by mutual contact thereof at the time of starting or stopping. In order to prevent this problem, coating a metallic member with a lubricating layer, such as a resin layer, at a portion facing the dynamic-pressure gap has been attempted, resulting in failure to yield a sufficient effect. In order to attain sufficient endurance against wear and seize-up, either or both of the two members, such as either or both of the spindle and the bearing member described above, which face each other with a dynamic-pressure gap present therebetween have been formed of a ceramic, such as alumina.
3. Problems to be Solved by the Invention
However, conventionally, when a dynamic-pressure component is formed of alumina ceramic, material design cannot be said to have sufficiently considered wear or a like problem associated with starting, stopping, or a like operation mode. Also, even when a component of a dynamic-pressure bearing is formed of ceramic, a problem may arise such that vibration occurs during rotation of a spindle, thereby hindering smooth rotation of the spindle. In a dynamic-pressure bearing configured so as to support a thrust face by the action of dynamic pressure, such as a dynamic-pressure bearing configured such that a thrust face of a rotation body faces a disk-like thrust plate, when the rotation body and the thrust plate come into contact with each other at the time of starting or stopping, wear or linking (a phenomenon in which two members come into close contact due to vacuum created in the clearance therebetween) may arise.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a ceramic dynamic-pressure bearing which is not prone to wear or a like problem associated with starting, stopping, or a like operation mode and which can realize smooth rotation.
The above object of the invention has been achieved by providing a ceramic dynamic-pressure bearing having a dynamic-pressure gap formed between a first member and a second member. The first and second members undergo relative rotation about a predetermined axis of rotation to generate fluid dynamic-pressure in the dynamic-pressure gap. At least a portion of one or both of the first member and the second member including a dynamic-pressure gap definition surface formed of alumina ceramic and comprising a polished surface facing the dynamic-pressure gap. Furthermore, the alumina ceramic, which forms the dynamic-pressure gap definition surface finished by polishing, has an apparent density of 3.5-3.9 g/cm
3
.
The present invention uses alumina ceramic. Alumina is relatively inexpensive and exhibits high strength and excellent chemical stability. In the present invention, the density of the alumina ceramic is adjusted to a relatively high value of 3.5-3.9 g/cm
3
, thereby improving the absolute value of strength and wear resistance of the alumina ceramic. The material is used to form the dynamic-pressure gap definition surface, thereby effectively preventing occurrence of wear and seize-up of the dynamic-pressure gap definition surface at the time of starting and stopping when the two members are prone to come into contact with each other.
An ideally densified alumina ceramic has a density of up to 4.0 g/cm
3
. However, the present invention particularly employs a slightly lower density of 3.9 g/cm
3
as the upper limit of apparent density, for the following reason. When alumina ceramic is used as material for a dynamic-pressure bearing, the surface state of the dynamic-pressure gap definition surface of the ceramic component serving as a spindle or a bearing is important. That is, in general, fine pores are present on the surface of the ceramic component that has been subjected to polishing, and the size of such pores is considered to exert considerable influence on the state of rotation of the dynamic-pressure bearing.
Studies carried out by the present inventors have revealed that an extremely smooth dynamic-pressure gap definition surface fails to generate sufficient fluid dynamic-pressure in a dynamic-pressure gap. Insufficient dynamic pressure fails to stably support the axis of rotation, resulting in difficulty in establishing a favorable state of rotation of a dynamic-pressure bearing. Accordingly, formation of surface pores of a certain dimensional range on the dynamic-pressure gap definition surface is effective for maintaining high fluid dynamic-pressure that is generated stably at a high level.
Specifically, when pores of large size are present on the dynamic-pressure gap definition surface of the ceramic component, turbulence is generated in the fluid layer present between the spindle and the bearing upon rotation of, for example, the spindle, with the result that vibration of the spindle occurs. By contrast, when pores of small size are present on the dynamic-pressure gap definition surface of the ceramic component, adhesion easily occurs between the dynamic-pressure gap definition surfaces of the spindle and the bearing, with the result that, for example, an attempt to forcibly induce rotation in a high-friction state associated with adhesion is likely to cause occurrence of wear (hereinafter referred to as “adhesion wear”) or a like problem. When one of two members between which a dynamic-pressure gap is formed is formed of metal; for example, when the spindle is formed of a metal, seize-up may occur. Also, surface pores of excessively small size hardly contribute to generation of dynamic pressure.
The above-mentioned pores are formed on the dynamic-pressure gap definition surface mainly as a result of dropping off of grains in the course of polishing. Thus, the size (diameter) or distribution of crystal grains of alumina ceramic on the dynamic-pressure gap definition surface plays a very important role in forming surface pores in a favorable state against occurrence of the above-described problems. For example, when alumina ceramic is to be sintered for complete densification, sintering must be performed at high temperature, with the result that the growth of crystal grains beco
Ishikawa Hironobu
Yogo Tetsuji
Hannon Thomas R.
NGK Spark Plug Co. Ltd.
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