12.10 Higher Order Vibration Modes
The previous sections assumed a simple translational motion of the machine and its supports in
the vertical direction. This is sometimes called the “bounce” mode. Higher order, rotational
modes are also possible, including pitch and roll (Figure 10). These modes are not desirable and
hopefully are of a lower magnitude than the bounce mode. Proper location of machinery on the
isolators can minimize the excitation of these modes. Ideally the machine should be located on
its base frame and isolators so that excitation forces act through the center of gravity of the
system, so as not to excite these rotational modes. Large (wide) inertia blocks can also be used
to control these modes.