Understanding Rotor Whirl and Whip Instabilities

Definition: What are Whirl and Whip?

Oil Whirl and Oil Whip are two related and highly dangerous forms of self-excited, sub-synchronous vibration that occur in high-speed rotating machinery equipped with fluid-film (journal) bearings. They are not forced vibrations caused by issues like unbalance, but are rotor instabilities where the motion of the rotor itself generates the forces that sustain and grow the vibration. Both are characterized by the rotor shaft “whirling”—precessing forward in a large orbit—within its bearing clearance.

The Mechanism: How Does it Happen?

In a fluid-film bearing, the rotating shaft is supported by a high-pressure wedge of oil. The shaft is not in the center of the bearing but rides up one side. As the oil is dragged around by the shaft, the oil itself circulates at an average speed of slightly less than half the shaft’s surface speed.

Oil Whirl occurs when this circulating oil film begins to “push” the shaft around the bearing, causing it to precess in a large, forward orbit. The frequency of this whirl is determined by the average speed of the oil film, which is typically between 42% and 48% of the shaft’s running speed (0.42x to 0.48x). This is a classic sub-synchronous vibration signature.

Oil Whirl: The Precursor

Oil whirl is often the initial stage of the instability. Its characteristics are:

• Frequency: Appears as a distinct peak in the FFT spectrum between 0.42x and 0.48x RPM.
• Behavior: The frequency of the whirl *will increase* as the machine’s speed increases, always staying in that ~45% range.
• Severity: It can cause high but sometimes stable vibration. It may appear or disappear as machine load, speed, or oil temperature changes. While undesirable, it is not always immediately destructive.

Oil Whip: The Critical Danger

Oil Whip is a far more severe and dangerous condition that develops from oil whirl. It occurs when the speed of the machine increases to a point where the oil whirl frequency (at ~45% of running speed) becomes equal to the rotor’s first natural frequency (its first critical speed).

When this happens, the oil whirl “locks onto” the rotor’s natural frequency and excites a resonance. The characteristics of oil whip are:

• Frequency: The vibration frequency becomes “locked” at the rotor’s first natural frequency and *does not increase any further* even as the machine continues to speed up.
• Amplitude: The vibration amplitude grows very large and becomes violent and unstable.
• Behavior: Oil whip is extremely destructive and will not go away by further increasing speed. It can cause catastrophic damage to bearings, seals, and the rotor itself in a very short amount of time.

The speed at which whip begins is typically just over twice the rotor’s first critical speed. A machine experiencing oil whip requires an immediate shutdown.

How to Identify Whirl and Whip

• Spectrum Analysis: Look for a strong sub-synchronous peak. During a startup, if the peak’s frequency increases with speed, it is whirl. If the peak’s frequency “flatlines” at a certain point while the 1x running speed peak continues to increase, it has transitioned to whip.
• Orbit Plot: The shaft orbit will be a large, forward-precessing circle or ellipse, often with the 1x running speed vibration superimposed, creating a “loop-the-loop” appearance.
• Waterfall Plot: A waterfall plot from a startup test provides the clearest possible picture, showing the oil whirl frequency increasing with speed until it intersects with the first natural frequency and transitions into oil whip.

Causes and Solutions

These instabilities are complex and are influenced by bearing design, rotor geometry, oil viscosity, temperature, and load. They are not caused by unbalance and cannot be fixed by balancing. Solutions are typically design-level changes, such as:

• Changing to a more stable bearing design (e.g., a tilt-pad bearing).
• Altering the oil viscosity or temperature.
• Increasing the bearing load.
• Introducing features like grooves or dams into the bearing to disrupt the circumferential flow of the oil.

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