What are Flexible Rotors?

Flexible rotors are rotating parts that change shape (deform) when rotational speeds approach or exceed their natural resonant frequencies. Unlike rigid rotors, this causes the mass distribution to change with speed. Therefore, balancing at low speeds may be insufficient to eliminate the vibrations the rotor will experience at high operating speeds. Therefore, balancing flexible rotors is generally performed at speeds close to or above their operating speeds. This process is critical for accurately simulating the dynamic behavior of the part under operational conditions.

This dynamic nature of flexible rotors makes the balancing process more complex and often requires the application of multi-plane balancing techniques. These types of rotors are widely used in fields such as the energy sector (wind turbines), aerospace, and high-speed machinery. At MBS Balance, we offer machines specifically designed for these demanding applications, with in-situ balancing and high-speed testing capabilities. On-site balancing eliminates potential errors resulting from assembly and disassembly by balancing the rotor on its own bearings and in its own operating environment, ensuring the most accurate results.

Two fundamental approaches stand out in balancing flexible rotors: the modal balancing method and the influence coefficient method. In the modal method, the rotor is balanced separately in the planes corresponding to each bending mode it excites; generally N or N+2 correction planes are used for the Nth mode. In the influence coefficient method, the effect of each plane on each measurement point is derived as a matrix using trial weights, and the solution is calculated. These procedures are defined within the scope of ISO 21940-12.

The critical point is that the rotor is brought up to operating speed by passing it through the critical speeds while the vibration level is monitored in each mode region. This may require special high-speed balancing facilities and a vacuum tunnel. MBS Balance offers on-site balancing and high-speed testing solutions for flexible parts such as wind turbine shafts, compressors, and large generator rotors, eliminating error caused by assembly and disassembly and ensuring minimum vibration under real operating conditions.