Page 73 - Steam Turbines Design, Applications, and Rerating

54 Chapter Three

Figure 3.2 High-speed, lightly
loaded, unstable bearing. (RMT,
Wellsville, N.Y.)

namic forces multiplied by the area must be equal and opposite to the
external load W. Likewise, the sum of all horizontal forces must be zero,
which can occur only for attitude angles that approach 90°.
Since a downward load is supported by a horizontal displacement,
any downward force perturbation will result in a horizontal displace-
ment, which will result in a horizontal force, which in turn produces a
vertical displacement, and so on. Thus, the bearing generates unstable
cross-coupling forces that actually drive the rotor and cause it to
vibrate at a frequency that is normally in the range of 40 to 50 percent
of running speed.
Conversely, a relatively low speed, heavily loaded (i.e., low Sommer-
feld number), stable journal bearing is illustrated in Fig. 3.3. Note that

Figure 3.3 Low-speed, heavily
loaded, stable bearing. (RMT,
Wellsville, N.Y.)