Bearings – Rotary bearing – Fluid bearing
Reexamination Certificate
2002-03-06
2003-12-09
Footland, Lenard A. (Department: 3682)
Bearings
Rotary bearing
Fluid bearing
C384S902000, C384S913000
Reexamination Certificate
active
06659647
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ceramic dynamic pressure bearing, a motor with the bearing, a hard disk device, and a polygon scanner.
2. Description of the Related Art
Conventionally, a bearing for a motor which is a drive source of an electric appliance is in a form of a ball (or a “ball bearing”).
Recently, a precision instrument such as peripheral device of computer has a motor that is rapidly increased in speed. For obtaining an excellent bearing capability (with reduction in: non-uniformity at low speed, abnormal noise, and vibration) and for keeping longevity, a dynamic pressure bearing is used. The dynamic pressure bearing is the one that uses fluid (such as air) as medium.
For example, the following dynamic pressure bearing is provided:
A main shaft and a bearing section (surrounding the main shaft) rotate around an axis. A fluid dynamic pressure is caused to a gap between an outer periphery of the main shaft and an inner periphery of the bearing section. The thus caused fluid dynamic pressure supports a rotation shaft.
Moreover, there is provided another bearing having a thrust surface (of the main shaft or the bearing section) that is supported with dynamic pressure.
At high speed with sufficient dynamic pressure, the dynamic pressure bearing is unlikely to cause contact between members facing each other across the dynamic pressure gap. Contrary to this, at low speed (such as starting and shutdown of rotation), the dynamic pressure bearing is likely to cause the contact between the members, due to insufficient dynamic pressure.
For component part of the above dynamic pressure bearing, a metal such as stainless metal, and a metal coated with resin and the like were generally used as material. The above metallic material are, however, likely to cause failures such as wear and seizure, attributable to the contact between the members at staring or shutdown. For preventing the wear and the seizure, a lubricant layer such as resin was applied to a section (of the member) facing the dynamic pressure gap, leaving insufficient effect.
For preventing the wear and the seizure securely, the members (the main shaft and the bearing) facing each other across the dynamic pressure gap are made of ceramic such as alumina.
The conventional dynamic pressure bearing with the dynamic pressure part made of the alumina ceramic is, however, not paid attention to in terms of material design, in other words, in respect of machining finish (accuracy/precision). Especially, the gap (between the outer periphery of the main shaft and the inner periphery of the bearing) causing a radial dynamic pressure is likely to cause a local wear attributable to low machining accuracy/precision of the outer periphery (of the main shaft) and the inner periphery (of the bearing section). Moreover, the low machining accuracy/precision of the outer periphery (of the main shaft) and the inner periphery (of the bearing section) may cause harmful effect on the dynamic pressure, losing uniformity and stability of rotation.
BRIEF SUMMARY OF THE INVENTION
It is an object of the present invention to provide a ceramic dynamic pressure bearing that is unlikely to cause wear, seizure and the like at low speed at starting, shutdown and the like of rotation.
It is another object of the present invention to provide the ceramic dynamic pressure bearing that achieves a preferable rotation.
According to a first aspect of the present invention, there is provided a ceramic dynamic pressure bearing, comprising: a first member having a substantially cylindrical outer periphery which is formed with a gap forming surface for causing a radial dynamic pressure, and a second member having an inner periphery defining a substantially cylindrical through hole which is formed with a gap forming surface for causing the radial dynamic pressure. The first member is inserted into the through hole of the second member in such a manner as to form a radial gap between the gap forming surface of the first member and the gap forming surface of the second member. The first member and the second member make a rotation relative to each other so as to cause a fluid dynamic pressure at the radial gap. The first member is composed of an alumina ceramic comprising: an aluminum in a range from 90% to 99.5% by weight, where the figures in % are an Al
2
O
3
conversion, and an oxide sintering assistant in a range from 0.5% to 10% by weight, where the figures in % are an oxide conversion. The outer periphery of the first member has a cylindricity not larger than 1.0 &mgr;m, and a roundness not larger than 0.5 &mgr;m which is measured in an arbitrary cross section perpendicular to an axis of the first member. The second member is composed of the alumina ceramic comprising: the aluminum in the range from 90% to 99.5% by weight, where the figures in % are the Al
2
O
3
conversion, and the oxide sintering assistant in the range from 0.5% to 10% by weight, where the figures in % are the oxide conversion. The inner periphery defining the through hole of the second member has a cylindricity not larger than 1.5 &mgr;m, and a roundness not larger than 1.0 &mgr;m which is measured in an arbitrary cross section perpendicular to an axis of the second member.
According to a second aspect of the present invention, there is provided a motor comprising the ceramic dynamic pressure bearing as described above. The ceramic dynamic pressure bearing is used for bearing an output section of the motor.
According to a third aspect of the present invention, there is provided a hard disk device, comprising: a motor and a hard disk rotatably driven by the motor. The motor comprises the ceramic dynamic pressure bearing as described above.
According to a fourth aspect of the present invention, there is provided a polygon scanner, comprising: a motor and a polygon mirror rotatably driven by the motor. The motor comprises the ceramic dynamic pressure bearing as described above.
The roundness and the cylindricity are those specified, respectively, in item
3
and item
4
in “Attached Table” in JIS B 0021 (1984), where JIS stands for Japanese Industrial Standard.
For securing accurate/precise measurement of the roundness and the cylindricity, the following measures may be taken:
The roundness and the cylindricity of the inner surface of the through hole of a second member are measured with a conventional profile measurement device for measuring profile of the inner surface. Hereinabove, the cylindricity is measured in cross sections (in required and sufficient number for securing accuracy/precision) perpendicular to the axis of the through hole of the second member.
Likewise, the roundness and the cylindricity of the outer surface of a first member may be measured with the conventional profile measurement device for measuring profile of the outer surface. Hereinabove, the cylindricity is measured in cross sections (in required and sufficient number for securing accuracy/precision) perpendicular to the axis of the first member.
If a hereinafter described dynamic pressure recess (groove) is to be formed, the roundness and the cylindricity are evaluated by excluding the area covering the dynamic pressure recess (groove).
According to the inventor of the present invention, the following points may be important in order to prevent the local wear from a radial dynamic pressure gap forming surface, and to secure uniform and stable dynamic pressure as well as rotation:
To keep the machining accuracy/precision of the radial dynamic pressure gap forming surface not lower than a predetermined level. More specifically, the cylindricity of the inner surface of the through hole of the second member are kept not larger than 1.5 &mgr;m, while the roundness of the inner surface of the through hole of the second member in the arbitrary cross section perpendicular to the axis are kept not larger than 1.0 &mgr;m. On the other hand, the cylindricity of the outer surface of the first member are kept not larger than 1.0 &mgr;m, while the roundness of the fir
Ishikawa Hironobu
Sugiyama Atsutoshi
Yogo Tetsuji
Footland Lenard A.
NGK Spark Plug Co. Ltd.
Sughrue & Mion, PLLC
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