Specifications

26 Catalog WSC/WDC-4
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Bearings
Since the impeller shaft must be sized to support the static, rotational and torsional loads applied
by the impeller, as impellers become larger, shafts must also become proportionally larger. These
factors also come into play in the design or selection of a bearing. The primary criteria used in
bearing design are:
1. The load-per-unit of bearing area.
2. The relative velocity of the two bearing surfaces.
3. The bearing dimensions.
4. The viscosity of the lubricating oil.
Notice that item 2 returns to the phenomenon of tip speed. Surface velocity is simply the tip
speed of the inner bearing surface or shaft with respect to the outer bearing surface as illustrated
in the following diagram.
Figure 10, Bearing Loading
A hydrodynamic bearing is basically two infinite surfaces passing over one another with a velocity
equal to the surface velocity.