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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, eventually transitioning to oil whip if uncorrected.

Oil whip occurs when oil whirl frequency coincides with the rotor's first natural frequency (first bending critical speed). At this point, the whirl frequency "locks on" to the natural frequency and does not increase further with speed.

Key difference: oil whirl tracks shaft speed (proportional), oil whip is fixed at the critical speed regardless of operating speed. Oil whip is a severe instability condition with potentially destructive vibration levels.

Oil whirl is identified by:

• Frequency at ~0.42–0.48× shaft speed
• Forward precession orbit (shaft whirls in same direction as rotation)
• Frequency tracks shaft speed proportionally — this is key
• Amplitude typically increases with speed

Other sub-synchronous sources like rub or loose components typically occur at exact sub-harmonics (½×, ⅓×) and may show reverse precession.

Oil whirl onset depends on several factors:

• Shaft speed exceeding the stability threshold
• Light bearing load (lightly loaded bearings are more prone)
• Low oil viscosity (high temperature or wrong oil grade)
• Excessive bearing clearance (worn bearings)
• Misalignment or unbalance affecting load distribution

Solutions include:

• Increase bearing load or reduce clearance
• Use anti-whirl bearing designs (tilting pad, pressure dam, lemon bore)
• Change oil viscosity (grade or temperature control)
• Reduce operating speed below instability threshold
• Improve shaft alignment and balance to stabilize bearing load
• Add external damping or squeeze-film dampers