粗糙峰所構成之微楔形在混合潤滑之效應

Abstract

混合潤滑在一般實際應用場合上是普遍存在的一種潤滑方式，因同時牽涉到液膜潤滑與粗糙峰接觸的問題，但截至目前為止尚未有一個較為完整的潤滑模型能夠應用在混合潤滑方面所遭遇到的問題。尤其是有關於平行滑動面在某特定條件下產生液動效果的原因不明。因此本研究從微觀的觀點出發，探討粗糙峰在混合潤滑中的潤滑機制及其所造成的影響，最後並模型化其潤滑機制，建立起混合潤滑模型。藉由此混合潤滑模型，我們可以用來預測其液膜支撐力及摩擦係數，並藉由摩擦係數的預測來幫助我們準確的判斷其潤滑狀態。由本研究的結果可以歸納出以下幾點結論 : (1)平行滑動面之所以能夠產生液動支撐力的原因在兩平行面間之表面粗糙峰，在較小的表面粗度及配合適當的紋路方向時，能夠產生有效的微楔形，它能夠產生顯著的液動壓來支撐負載，並降低摩擦係數。 (2)液動支撐力的關鍵除了滑動速度、黏度、正向負載、表面粗度，以及表面紋路方向之外，另一個最重要的影響因數便是表面間距。當表面間距較大時，所產生的液動支撐力較小，當表面間距較小時，所產生的液動支撐力較大，此一間距是由兩滑動接觸表面間最高粗糙峰所決定，此一間距直接決定微楔形的最小間距並進而影響液動效應。 (3)滑動面為橫向紋路時，當某特定條件下可以產生液動支撐力，但液動支撐力有其極限，當正向壓力較大時，橫向紋路所產生的液動力便無法有效承擔正向負載。 (4)本研究利用斜襯墊軸承理論應用於計算粗糙峰所產生之液動支撐力，微楔型理論模型的特點在於從粗糙峰所產生的液動力為主，避免了粗糙峰接觸時複雜的假設及計算，有別於一般直接計算粗糙峰接觸應力為出發點的混合潤滑模型。

In engineering practice, mixed lubrication is easily found in many operation cases. Because of the coexistence of hydrodynamic film and asperity interaction, up to now, it seems to be lacking a realistic model applied to solve problems in mixed lubrication, especially the reason for generating hydrodynamic pressure between two parallel surfaces was still not clear, and the mechanism of lubrication often fails to grasp . From the macro-micro approach, the purpose of this study is to investigate the mechanism of lubrication formed by asperity, and the effects caused by the asperity in the mixed lubrication. Finally, this thesis is going to establish a mixed lubrication model to predict the hydrodynamic load carrying capacity and friction coefficient. From this model, we can determine the status of lubrication more accurately. The following conclusions are obtained in this study: (1) The main factor that hydrodynamic pressures generated between two parallel surfaces under parallel sliding in the mixed lubrication regime is the surface roughness. When the surfaces that are smooth with suitable lay directions can form effective micro-wedges, which definitely contribute significant hydrodynamic load carrying capacity to the sliding pair and lower the friction coefficient. (2) Besides the sliding speed、viscosity of lubricant、nominal pressure 、surface roughness and lay direction, the key point of the hydrodynamic load carrying capacity is the gap between the two contact surfaces. The minimum gaps or the minimum fluid film thicknesses in micro-wedges not only depend upon the heights of interacting asperities, but also upon the heights of the highest asperities on the two contact surfaces. When the gap is wide, the hydrodynamic effect is small, so as the load carrying capacity. On the contrary, when the gap is narrow, they can provide a significant hydrodynamic effect and significant load carrying capacity. (3) Although surfaces roughness is in transverse lay direction, it can produce hydrodynamic effect in some particular lubrication conditions when the nominal pressure is sufficiently high, the hydrodynamic pressure may not be sufficiently high to support the normal load. (4) The study employed the simple theory of tilting-pad bearing to confirm that the micro-wedges do really create substantial hydrodynamic pressure. The micro-wedge model is a new approach and different from the most mixed lubrication model, especially the new model avoided the complicated assumptions about the asperity contact stress problems.