When the rotational speed is equal to the numerical value of the natural vibration then that speed is called the natural vibration.
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 residual unbalance. Residual unbalance is the remaining unbalance after a rotor has been balanced. The excitation force from unbalance is a function of unbalance and 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 occurr for rigid body modes.