The rotating rotor of the PD-35, like other rotor systems, has its own forms and frequencies of oscillation. When the excitation frequency coincides with one of the natural frequencies, vibration amplification is possible.
For an engine of this class, this is one of the important tasks during design. Designers calculate critical modes in advance and provide for safe passage through such zones.
"Pervy Tekhnichesky" explains what solutions Perm designers have incorporated into the promising power plant to avoid such unpleasant effects.
Why a metal shaft starts to "sound"
On a stationary engine, the rotor appears rigid. But during rotation, it experiences loads and can undergo small elastic displacements.
Any rotor system has its own forms and frequencies of oscillation. As it approaches the resonant frequency, the amplitude of vibration can increase. Therefore, it is impossible to completely eliminate oscillations — it is necessary to control their level.
Here, the rotor can be compared to a guitar string. The string does not have to remain motionless: it is important that its oscillations do not get out of control.
Dangerous speeds are passed in seconds
The rotor does not immediately reach its operating speed. During startup, it sequentially passes through various rotation modes, and along with them — critical frequencies.
Therefore, the calculation is performed for the entire operating range. Engineers determine the natural frequencies of the rotor system, evaluate its stiffness, and take into account the characteristics of the supports and bearings.
A separate task is damping. It allows dissipating oscillation energy and limiting their amplitude.
A critical mode itself does not mean engine destruction. Turbomachine rotors are designed to pass through such zones. The task is to prevent unacceptable vibration amplification during acceleration and deceleration.
The task of the rotor system is to control vibration in all modes
The more complex and powerful the turbomachine, the more important its rotor dynamics become. Changing the operating mode affects the nature of the rotor system's oscillations, so different combinations of rotational frequency, stiffness, and support characteristics are taken into account during design.
Therefore, designers consider the rotor, supports, and bearings as a single dynamic system. A change in the stiffness of one element can affect the dynamic characteristics of the entire system.
As a result, the engine is not designed on the principle that "the shaft should not vibrate at all": it is impossible to completely eliminate oscillations in a real rotor system. The requirement is different — vibration must remain controlled in all modes, including passing through critical frequencies.