Critical speed

Critical speed

In Solid mechanics, in the field of rotordynamics, the critical speed is the theoretical angular velocity which excites the natural frequency of a rotating object, such as a shaft, propeller or gear. As the speed of rotation approaches the objects's natural frequency, the object begins to resonate which dramatically increases systemic vibration. The resulting resonance occurs regardless of orientation.

When the rotational speed is equal to the numerical value of the natural vibration then that speed is called speed.

For rotor bearing systems, critical speeds can be divided into two categories by their mode shape. Rigid body modes are spring mass damper systems, where the spring is the support bearing. Since almost all rotors have multiple bearings there are more than one rigid body mode. The second category is rotor bending modes where the shaft is the excited member in the system.

Rigid body modes for two bearing systems can be described as pitch or bounce modes. A pitch mode is a mode in which the deflection at each bearing is 180 degrees out of phase. A bounce mode is a mode in which the deflection at each bearing is in phase (phase angle near zero).

Critical speeds in rotor-bearing systems are excited by the eccentric center of gravity of the shaft. This is due to the static deflection under its own weight. The excitation force from unbalance is a function of the stiffness of the shaft and the shaft speed.

When a rotor approaches its first bending critical speed, the phase angle between the unbalance force and the resultant deflection approaches 90 degrees. Above the first critical speed, the rotor deflects 180 degrees behind the unbalance force. This does not occur for rigid body modes.