Understanding Self-Excited Vibration

1. Definition: What is Self-Excited Vibration?

Self-Excited Vibration (also known as a self-induced or unstable vibration) is a particularly dangerous type of vibration where the motion of a system induces the forces that, in turn, sustain or amplify that motion. This creates a feedback loop where the vibration can grow in amplitude, sometimes to a catastrophic level, without any corresponding increase in an external forcing frequency.

This is fundamentally different from a forced vibration, like unbalance or misalignment, where the vibration is a direct response to a specific, periodic input (the forcing frequency). In a self-excited system, the vibration creates its own driving force.

2. The Feedback Loop Mechanism

The mechanism of self-excited vibration can be summarized as follows:

1. A system (e.g., a rotor in a bearing) is in motion.
2. A small, random disturbance causes a slight displacement or change in velocity.
3. This change in motion alters the forces acting on the system (e.g., the fluid pressure in a bearing or the cutting force of a tool).
4. Critically, this altered force acts in a way that *adds energy* to the system, pushing the component further in the direction it was already going.
5. This increased motion generates an even larger force, which adds more energy, and so on.

This feedback loop causes the vibration to grow until it is either limited by non-linearities in the system (like hitting a hard stop) or it leads to failure.

3. Common Examples of Self-Excited Vibration

Several well-known phenomena in machinery diagnostics are classic examples of self-excited vibration:

• Oil Whirl and Oil Whip: The most common examples in rotating machinery. In a fluid-film journal bearing, the rotating shaft creates an oil wedge. A disturbance can cause the oil wedge itself to start rotating (whirling) around the bearing. The pressure from this whirling wedge pushes the shaft, which adds more energy to the whirl. The resulting vibration is not at the running speed, but at a subsynchronous frequency (typically 0.42-0.48X running speed).
• Chatter in Machining: In metal cutting (lathing or milling), chatter occurs when the cutting tool starts to vibrate. This vibration causes the thickness of the chip being cut to vary. The varying chip thickness, in turn, causes a fluctuation in the cutting force, and this fluctuating force can pump energy back into the tool’s vibration, causing it to grow into a violent chatter.
• Aerodynamic Flutter: The vibration of an airplane wing where the bending and twisting motion of the wing changes its aerodynamic profile. This change in profile alters the air pressure in a way that adds energy to the wing’s motion, leading to catastrophic failure if not controlled.
• Rotor Rubs: A condition where a rotor makes contact with a stationary part. The friction from the rub can heat the rotor, causing it to bow. This bowing increases the rubbing force, which increases the heat and the bow, creating a feedback loop that can lead to seizure.

4. Key Characteristics and Diagnosis

Self-excited vibrations often have distinct characteristics in the FFT spectrum:

• Non-Synchronous Frequencies: The vibration is typically not an integer multiple or harmonic of the running speed. It often occurs at a sub-synchronous frequency.
• Instability: The amplitude can be highly unstable and can grow rapidly with small changes in operating conditions (speed, temperature, load).
• Sudden Onset: The vibration may not be present at all until the machine crosses a certain speed or load threshold, at which point it can appear suddenly and with high amplitude.

Diagnosing self-excited vibration involves identifying these characteristic non-synchronous peaks and understanding the physical mechanisms that could be causing such an instability in the specific machine.

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