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.
Results
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 |
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.
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
The procedures described on this page were developed and field-tested by the Vibromera team using the Balanset-1A, a portable dual-channel balancer and vibration analyzer for on-site rotor balancing.
Professional vibration analysis instruments for diagnosing journal bearing issues. Sub-synchronous tracking and orbit analysis. Used in 50+ countries.
Nikolai Shelkovenko
Nikolai Shelkovenko is a vibration analysis engineer and the founder and CEO of Vibromera. For more than 15 years he has balanced rotating equipment in the field rather than on a test bench: mulchers, industrial fans, crushers, centrifuges, shafts and spindles. That work is what the Balanset instruments grew out of — they were designed as a tool a specialist can carry to the machine and use alone, on site, not as laboratory equipment. Vibromera was founded in 2017 and has been based in Porto, Portugal, since 2023. Development, assembly and support of the Balanset line all happen here. The flagship instrument is the Balanset-1A, a portable analyser for single- and two-plane balancing and for vibration diagnostics. Nikolai is personally involved in customer support, in working through difficult balancing cases and in the development of the software. He works with customers worldwide, in any language.
