Abstract

Misalignment in journal bearings is a common industrial problem that affects the general performance of the bearing. This includes a significant effect on the load carrying capacity of the bearing in addition to the change in the level and shape of the pressure distribution and film thickness. This study considers extreme cases of misalignment where an introducing of bearing axial profile modification to prevent edge contacts is investigated in detail. A general 3D model of misalignment for the case of a finite length bearing is considered in the analysis where the Reynolds equation is solved numerically based on the finite difference method. Furthermore, in this thesis static and dynamic characteristics of finite length misaligned journal bearing for laminar flow type are also investigated. Consequently, the effect of bearing modification (chamfering of the bearing) on these characteristics is studied. The numerical results of the problem have been verified with the related literatures at different stages of the solution and excellent agreement has been found. The verification of the misalignment model, for example, has shown that the maximum difference in minimum film thickness is less than 0.007%. Also, the difference in the maximum pressure and angle of cavitation is lower than 0.1% and the difference in the dynamic characteristics is less than 1% for the practical values of the eccentricity ratio.

The results reveal that, in the case of severe misalignment levels, a sharp drop in the lubricant film thickness or even a direct contact may occur between the shaft and the bearing at the edges of the bearing. The use of bearing profile modification leads to an improvement in the levels of the film thickness, a reduction in the maximum pressure and an increase in the load carrying capacity of the bearing. For example the 3D misalignment, causes an increase in 𝑃𝑚𝑎𝑥 of 67.7% and a reduction in 𝐻𝑚𝑖𝑛 of 79.3% in comparison

with the aligned case. While modifying the bearing profile reduces 𝑃𝑚𝑎𝑥 by 22.7% and increases 𝐻𝑚𝑖𝑛   by 171.7% in comparison with the misaligned case.

The results of the dynamic coefficients have shown that the 3D misalignment affects these coefficients. In general, the axial profile modification improves the values of these coefficients in comparison with the misaligned case. Particularly, when the chamfer height (relative to the clearance) does not exceed 0.5 and the chamfer length (relative to the length of bearing) is less than 0.3.