Misalignment
Misalignment in bearings can occur due to improper mounting. Misalignment often results in metal-to-metal contact in the bearing components with high contact forces. This gives rise to noise and high temperatures in the bearing. The misalignment is broadly classified in the following categories:
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Parallel misalignment: The axes of the bearings are parallel to each other but the bearing centers are offset in the lateral direction (see Figure 7-5).
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Angular misalignment: The axes of the bearings are not perfectly parallel. Rather, they are slightly inclined relative to each other (see Figure 7-6). Bearing centers are still aligned in the direction of the rotor axis.
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A combination of the above two.
Figure 7-5: Parallel misalignment.
Figure 7-6: Angular misalignment.
The film thickness in a hydrodynamic bearing changes as an effect of its misalignment with the rotor axis. As a result, the pressure distribution in the bearing also gets affected. Let u0 and θ0 be the parallel and angular misalignments, respectively. Then the net change in the film thickness due to misalignment can be calculated by
Here, X is the coordinate on the journal/collar surface, Xc is the center of the bearing, and nref is the material normal of the journal/collar surface. Therefore, the height of the bearing from the reference surface will be given by
where h0 is the film thickness without the misalignment in the bearing.