everything that rotates
                        needs to be balanced


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Vibration Information for rotating machinery applications.


Flexible Modes

Monday, July 25, 2011

Mode shapes are divided into two categories: rigid body and flexible modes. Rigid body modes have been covered in the previous article. Rigid body modes do not involve any deformation of the rotor, only translation and rotation. On the other hand, flexible modes involve deformation of the rotor.

Let’s consider a disk on a shaft, suspended by two bearings, please refer to the Figure. When the rotor (disk and shaft) spins up, the rotational speed approaches the first natural frequency of the system. Very large vibration amplitude can be observed. Vibration amplitude is controlled by the magnitude of damping, which is related to ability to dissipate vibrational energy. If there is no damping involved, the rotor will fail instantly. When the speed of the rotor is at the same speed as the natural frequency of the system, The system is said to be “in resonance”. The speed is called critical speed. The first mode shape looks like a bent shaft. Rotors are designed to be running below or above critical speed.  

If we keep increasing the speed of the rotor, the rotor passes through the first critical speed. Vibration amplitude continues to decrease. When speed approaches the second natural frequency, the vibration amplitude is higher. The second mode shape is different from the first mode shape. If we increase the speed further, the rotor will approach the third critical speed and so on. Notice that the lowest vibration level is at the bearings since the shaft cannot move freely at these supports.

A rotor could experience many modes, depending on the operating speed. Mode shapes are influenced by the bearing/support arrangements, disk arrangements, gyroscopic effects, etc. For example: An overhung system has different mode shapes, as compared to a disk suspended by two bearings. The mode shapes of overhung systems are harder to predict as gyroscopic effects act to change the rotational natural frequency.  

Static balancing are more difficult to perform in flexible rotors due to deformation of the rotor (mode shapes). Multi-plane balancing (more than 2-plane balancing) may be required to balance the system.



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