Understanding Oil Whirl

Definition: What is Oil Whirl?

Oil whirl is a type of self-excited, unstable vibration that occurs in machines equipped with fluid-film (journal) bearings, such as large turbines, compressors, and pumps. It is a form of fluid-induced instability where the oil film supporting the shaft begins to push the shaft around the bearing clearance in a forward circular motion. This “whirling” motion is a sub-synchronous vibration, meaning it occurs at a frequency below the machine’s running speed (1X).

Characteristics of Oil Whirl

Oil whirl has several distinct and identifiable characteristics in vibration data:

• Frequency: The most prominent feature is a large-amplitude vibration peak at a frequency slightly less than half of the running speed, typically between 0.4X and 0.48X (i.e., 40% to 48% of the shaft’s rotational speed). For example, in a machine running at 3000 RPM (50 Hz), oil whirl would appear at approximately 1200-1440 RPM (20-24 Hz).
• Direction: The vibration is primarily radial (horizontal and vertical) and is often highly directional.
• Orbit Plot: When viewed on an orbit plot from X-Y proximity probes, oil whirl appears as a large, forward-precessing, and often distorted (non-circular) orbit with a single, well-defined internal loop.
• Behavior: Oil whirl is not tied to a specific speed. If the machine’s speed is increased, the frequency of the whirl will also increase, always maintaining its characteristic ratio of ~0.4X – 0.48X of the new running speed. This is a key differentiator from a structural resonance, which occurs at a fixed frequency regardless of shaft speed.

The Mechanism: How Does Oil Whirl Occur?

Oil whirl is caused by the dynamics of the hydrodynamic oil wedge that supports the shaft in a journal bearing. Under normal operation, the rotating shaft drags oil into a wedge-shaped gap, creating a pressure field that lifts and supports the shaft. The shaft does not sit in the center of the bearing but is offset slightly.

The oil in this wedge is also circulating around the bearing at roughly half the speed of the shaft. If the bearing is lightly loaded or has excessive clearance, the stabilizing forces can be weak. A small disturbance can cause the shaft to be “captured” by the circulating oil film, which then begins to push the shaft in a circular path around the bearing. This creates a self-sustaining vibration that can grow to very high amplitudes, often limited only by the bearing clearance itself (i.e., the shaft starts impacting the bearing).

Oil Whip: The More Severe Form

If the machine’s speed increases to the point where the oil whirl frequency (~0.4X – 0.48X) coincides with one of the rotor’s natural frequencies (a critical speed), the condition becomes much more severe. This is called oil whip.

• Locked Frequency: The vibration “locks on” to the rotor’s natural frequency and does not increase further as the machine speed increases.
• High Amplitude: The resonant condition causes the vibration amplitude to become extremely high and often destructive.
• Danger: Oil whip is a very dangerous and unstable condition that can lead to catastrophic failure of the machine.

Common Causes and Solutions

• Causes: Lightly loaded bearings, excessive bearing clearance, incorrect oil viscosity (too low), high oil pressure, or a machine design that places a critical speed at approximately twice the running speed.

– Solutions: Solutions aim to disrupt the unstable oil film. This can include increasing the bearing loading, changing to a different oil viscosity, or redesigning the bearings to have anti-whirl features (e.g., lemon-bore, pressure dam, or tilting-pad bearings).

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