In the field of rotordynamics, critical speed is a common term used to describe the speed which excites the natural frequency of a rotor such as shaft-fan system, shaft-propeller system, etc. As the rotational speed approaches the rotor’s natural frequency, the system starts to resonate. This results in increasing vibration level.
Critical speeds (according to their mode shapes) can be divided into two categories: rigid body and flexible modes. Rigid body modes occur when a rotor is not adequately supported or has a low spring stiffness; it can translate or rotate as a whole without deformation. A rotor without any restraints has six rigid body modes: 3 translations in x, y, and z directions and 3 rotations about x, y, and z axes.
An example of rigid body modes can be found in a horizontal washing machine. The spin basket rotates inside the tub, which is connected by a bearing. The tub is supported by spring-damper system. A local Cartesian coordinate system can be set up in the tub with the center of gravity as o coordinate, please refer to the Figure. The spin basket rotates about z axis. Since the tub is supported by springs and dampers in x and y directions, the tub is able to move as a whole in x and y directions (translational modes) and rotate about x and y axes (rotational modes). The system experiences four rigid body modes at different speeds. The sequence of the rigid modes depends on the stiffness of the springs in x and y directions.
Rigid body modes usually occur at lower frequencies than flexible modes. Flexible modes involve shaft flexures. I’ll discuss flexible modes in the next XYO blog article.