Higher Order Vibration Modes

          In previous topics we assumed a simple translational motion of the machine and its supports in vertical direction. This is some times is called the "bounce" mode. Higher order, rotational modes are also possible, including pitch and roller (Refer Fig.) These modes are not desirable and hopefully are of a lower magnitude than the bounce mode. Proper location of machinery on the isolators can minimise the excitation of these modes. Ideally the machine should be located on its base frame and isolators so that excitation forces act through the centre 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.