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Based on the operating condition and mechanical properties of the rotor, and from a balance perspective, rotors are commonly divided into two categories: rigid rotors and flexible rotors. 　　Generally speaking, any rotor whose operating speed is much lower than the first-order bending critical speed of the rotor is considered a rigid rotor ； A rotor whose operating speed is close to or exceeds its first-order bending critical speed is considered a flexible rotor. 　　In the ISO 1925-1981 standard for \"balancing vocabulary\" established by the International Organization for Standardization, as well as in China’s ZBY033-82 standard for \"terms and expressions related to testing machines,\" a rigid rotor is more precisely defined as a rotor that can be balanced at any speed lower than its operating speed, on one or any two selected reference planes; and after balancing, its imbalance does not exceed the specified balance requirements significantly at its maximum operating speed, at any speed lower than the operating speed, or under conditions close to actual operating conditions. All rotors other than these are classified as flexible rotors. 　　What we usually refer to as a rigid rotor is one with a high degree of stiffness; the dynamic deflection generated by the unbalanced centrifugal inertial forces is so small that it can be ignored during the operation and balancing of the rotor. Flexible rotors are much more complex because they undergo flexural deformation during operation and balancing.

To put it simply, the rotational speed of a rigid rotor is below the first-order critical speed, while that of a flexible rotor is above the first-order critical speed