隨著超精密產業之加工精度要求越來越高，氣靜壓軸承已逐漸取代滾珠軸承以及油壓軸承，成為超精密工具機之元件核心之ㄧ。雖然早期有研究探討氣靜壓軸承，但其節流器多採用孔口節流而非多孔質節流，但多孔質節流型式可使得氣靜壓軸承之承載能力與剛性提升，因此本研究將探討工具機模組內之多孔質氣靜壓止推軸承與軸頸軸承；採用計算流體力學軟體Fluent 進行模擬分析，藉流場可視化觀察軸承氣膜內之壓力分佈，並變更多項設計參數以探討其對軸承承載能力、剛性、表面平均壓力之關係。
　　在分析氣靜壓軸承前，應確認需使用可壓縮流或不可壓縮流體設定較為恰當，故首先分析氣體壓縮性對氣靜壓止推軸承之影響，由模擬結果與物理現象分析判斷，氣靜壓軸承需使用可壓縮流計算模擬較為恰當。接著比較多孔質材料之流阻對氣靜壓止推軸承之影響，本研究所採用材料為多孔質石墨和多孔質氧化鋁，結果顯示因多孔質氧化鋁之流阻值較小，故若多孔質材料採用多孔質氧化鋁分析止推軸承時其承載能力較大，隨著轉速不同，兩者相差約10 至13 倍左右；最後分析多孔質柱塞厚度對氣靜壓止推軸承之影響，由結果顯示當多孔質柱塞厚度越薄其止推軸承之承載能力越大，數據顯示當多孔質柱塞厚度由10 mm 變更至4 mm 時，止推軸承承載能力可提升約6%。
　　進一步將氣靜壓止推軸承之模擬方法延伸至氣靜壓軸頸軸承，首先為了要探討剛性與氣膜間隙厚度之關係，故分析主軸偏心率對氣靜壓軸頸軸承之影響；結果顯示當氣膜厚度越薄時其剛性提升約19 ~　39 %，且當主軸轉速越快時，因動壓效應之因素故其剛性越明顯的提升。考慮模擬的真實性，本研究將機殼內表面與主軸表面之間的氣膜間隙稱之為機殼氣隙，而多孔質柱塞與主軸表面之間的氣膜間隙稱之為軸承氣隙，比較機殼氣隙與軸承氣隙對氣靜壓軸頸軸承之影響；由結果顯示當機殼氣隙越薄其主軸表面平均壓力越高，而軸承氣隙越薄則主軸表面平均壓力越低，例如當軸承氣隙由10 μm變薄至4 μm時，其主軸表面平均壓力下降了55 %，故機殼氣隙不能太厚且軸承氣隙不能太薄，將可提升主軸表面之平均壓力。本研究變更一系列設計參數，進而提升氣靜壓止推軸承以及軸頸軸承之平均壓力與剛性，並且觀察高轉速下動壓效應之物理現象，可供未來研發工具機或其他相關領域者之參考所用。

　　This study investigates the performances of the porous-media aerostatic thrust and journal bearings via numerical simulation. Based on the finite-volume method and the pressure-velocity coupling SIMPLE scheme, this work utilizes the CFD software Fluent to solve the three dimensional, compressible Navier-Stokes equations for calculating the pressure and velocity fields inside the bearings. The effects of the air compressibility, the flow resistance of porous medium, the thickness of porous medium insert, the bearing gap, the housing gap, the eccentric ratio, and the rotational speed of the spindle on the characteristics of bearing such as the pressure distribution, the load carrying capacity, the surface average pressure and the stiffness are evaluated.
　　At first, the appropriateness of compressible or incompressible for the　analysis on aerostatic bearing is examined carefully. The computed results　of thrust bearing reveal that, the compressible approach should be adopted　for analyzing the aerostatic bearings. Later, porous graphite and porous　alumina are selected for comparing the pressure distribution of thrust　bearing. Because the porous graphite’s fluid resistance is larger, so the　load　capacity of porous-alumina bearing is considerably bigger comparing that　of the porous-graphite bearing by 10 to 13 times for various spindle speeds. Additionally, the thickness of porous-medium insert is considered in　analyzing its effect on aerostatic bearings. The numerical results illustrate that the load capacity increases about 6% when the thickness of porous medium varies form 10 mm to 4 mm.
　　Subsequently, the simulation approach of the thrust bearing extends to
the journal bearings. At first, the analysis on the eccentric ratio of spindle for journal bearing shows that, the stiffness enlarges about 19~39% for a thinner gap. To enhance the practical validity of the simulation, the study also takes into account the ratio of the housing gap to bearing gap. The housing gap is the distance between the inner case surface and spindle surface, and the bearing gap is defined between the bottom surface of the
porous-medium insert and spindle surface. The calculations of journal bearing reveal that the average pressure on spindle surface becomes larger
when the housing gap is thinner. Also, the average pressure on spindle surface reduces 55% when bearing gap decreases form 10 to 4 μm. These results show the average pressure on spindle surface may reach the peak value when the housing gap is not too thick and the bearing gap is not too thin. In summary, the CFD study considers many parameters for enhancing the pressure and the stiffness of thrust bearing and journal bearing. Also, the physical phenomenon of dynamic-pressure effect inside the porous bearing is analyzed thoroughly for the high-speed spindle case, which can serve as an important design reference for engineering applications.

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