Effects of Micro-Wedges Formed by Asperities on Mixed Lubrication
Date Issued
2005
Date
2005
Author(s)
Chen, Chang-Hung
DOI
zh-TW
Abstract
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.
Subjects
粗糙峰
微楔形
表面間距
摩擦力
混合潤滑
Asperity
Micro-Wegde
Mixed Lubrication
Gap
Friction
Type
thesis
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