What is oil whirl in journal bearings?

Oil whirl is a self-excited vibration in fluid-film (journal) bearings caused by the oil wedge pushing the shaft in a forward precessional orbit. It occurs at approximately 0.42–0.48× shaft speed (often cited as ~0.43×). Oil whirl amplitude increases with speed and can become unstable if uncorrected.

What is oil whip and how does it differ from oil whirl?

Oil whip occurs when oil whirl frequency coincides with the rotor's first natural frequency (first critical speed). At this point, the whirl 'locks on' to the natural frequency and does not increase further with speed. Oil whip is a severe instability condition with potentially destructive amplitude levels.

How do I distinguish oil whirl from other sub-synchronous vibrations?

Oil whirl is identified by: (1) frequency at ~0.42–0.48× shaft speed, (2) forward precession orbit, (3) frequency tracks shaft speed proportionally, (4) amplitude typically increases with speed. Other sub-synchronous sources like rub or loose components typically occur at exact sub-harmonics (1/2×, 1/3×) and may show reverse precession.

What causes oil whirl onset?

Oil whirl onset depends on: (1) shaft speed exceeding the stability threshold, (2) light bearing load (lightly loaded bearings are more prone), (3) low oil viscosity (from high temperature or wrong oil grade), (4) excessive bearing clearance, and (5) misalignment or unbalance affecting the bearing load distribution.

How to prevent or fix oil whirl and oil whip?

Solutions include: (1) increase bearing load or reduce clearance, (2) use anti-whirl bearing designs (tilting pad, pressure dam, lemon bore), (3) change oil viscosity (grade or temperature control), (4) reduce operating speed below instability threshold, (5) improve alignment and balance to stabilize the bearing load pattern.

Free Engineering Tool

Journal Bearing Defect Frequencies

Calculate oil whirl frequency range, oil whip indicators, shaft frequency, and sub-synchronous vibration characteristics for plain/sleeve journal bearings.

Oil Whirl Oil Whip Sub-synchronous

Results

Oil Whirl Frequency Range

—

Typical Oil Whirl (at selected ratio)

—

Shaft Frequency (1×)

—

2× Shaft (misalignment check)

—

Key Frequency Summary

Frequency	Hz	CPM	Indicator

⚠️ Oil Whip: If operating speed exceeds approximately 2× the first critical speed, oil whirl may lock onto the rotor natural frequency, producing oil whip — a severe instability condition. Monitor sub-synchronous components closely.

Oil Whirl

Oil whirl is a self-excited vibration in fluid-film journal bearings. The oil film wedge generates a force that drives the shaft into a forward precessional orbit at a frequency below shaft speed:

The typical whirl ratio is ~0.43, but ranges from 0.42 to 0.48 depending on bearing geometry, load, oil viscosity, and clearance.

Oil Whip

Oil whip occurs when the oil whirl frequency coincides with the rotor’s first bending critical speed. At this point, the sub-synchronous vibration “locks on” to the critical speed and no longer tracks shaft speed:

Oil whip is a potentially destructive condition requiring immediate attention.

Shaft Frequency

Key Vibration Indicators

Frequency	Pattern	Indication
0.42–0.48× shaft	Sub-synchronous, forward precession	Oil whirl — bearing instability onset
Locked sub-sync	Fixed frequency regardless of speed	Oil whip — locked to 1st critical
1× shaft	Synchronous	Normal unbalance response
2× shaft	Second harmonic	Misalignment, looseness, or non-linear response

Example — Steam Turbine at 3000 RPM

Given: Shaft speed = 3000 RPM, Whirl ratio = 0.43

Shaft freq = 3000 / 60 = 50.0 Hz

Oil whirl range = 0.42×50 to 0.48×50 = 21.0 – 24.0 Hz

Typical whirl = 0.43 × 50 = 21.5 Hz (1290 CPM)

2× shaft = 100.0 Hz

Diagnostic tip: Oil whirl tracks shaft speed (frequency changes proportionally). Oil whip does NOT track speed — it remains locked to the critical speed. This is the primary way to distinguish between the two conditions during run-up or coast-down.