Case study
Oil Whirl Speed Sweep Trip Decision Case Study
Mechanical engineering case study on diagnosing oil whirl and possible oil whip during a speed sweep using subsynchronous order, orbit direction, bearing temperature, trip limits and validation evidence.
This case study follows a journal-bearing machine that develops a growing subsynchronous vibration during a commissioning speed sweep. The important decision is not whether the plot contains a sub-1x peak. The important decision is whether the sweep should continue, hold, trip or release the machine for operation.
The scenario is generic but realistic: a high-speed process fan with fluid-film bearings is being tested after bearing inspection and oil-system maintenance. The rotor has acceptable 1x vibration, but a subsynchronous component appears as speed increases.
Case Context
| Item | Field value |
|---|---|
| Bearing type | fluid-film journal bearing |
| Speed sweep range | 3000\ \text{rpm} to 4800\ \text{rpm} |
| First bending natural frequency | 32\ \text{Hz} |
| Shaft-relative trip level | 60\ \mu\text{m}_{pp} subsynchronous |
| Hold criterion | growth above 40\ \mu\text{m}_{pp} or forward-precessing orbit |
| Speed reference | once-per-revolution reference |
| Primary evidence | waterfall spectrum, orbit plot, shaft centerline and bearing temperature |
| Initial oil inlet temperature | 58^{\circ}\text{C} |
The team already checked the proximity-probe gap and slow-roll runout. The subsynchronous component is not explained by electrical runout, aliasing or a missing tachometer reference.
The diagnostic chain uses three independent checks:
- frequency evidence from the waterfall spectrum and order tracking;
- shaft-relative motion evidence from filtered and unfiltered orbit plots;
- operating evidence from bearing metal temperature, shaft centerline position and oil-system state.
The decision is conservative because oil whirl is a stability problem. A machine can remain below hard clearance contact while still moving toward an unsafe operating state.
Speed Sweep Evidence
The speed sweep produces the following measurements:
| Speed | Running frequency | Subsynchronous peak | Peak order | Subsynchronous amplitude | Bearing metal temperature |
|---|---|---|---|---|---|
| 3000\ \text{rpm} | 50\ \text{Hz} | 22.5\ \text{Hz} | 0.45x | 12\ \mu\text{m}_{pp} | 63^{\circ}\text{C} |
| 3600\ \text{rpm} | 60\ \text{Hz} | 27.0\ \text{Hz} | 0.45x | 22\ \mu\text{m}_{pp} | 67^{\circ}\text{C} |
| 4200\ \text{rpm} | 70\ \text{Hz} | 31.5\ \text{Hz} | 0.45x | 41\ \mu\text{m}_{pp} | 72^{\circ}\text{C} |
| 4800\ \text{rpm} | 80\ \text{Hz} | 32.0\ \text{Hz} | 0.40x | 68\ \mu\text{m}_{pp} | 78^{\circ}\text{C} |
The first three points are consistent with whirl tracking a fractional order of running speed. The last point is different: frequency stops increasing with shaft speed and remains near the known rotor natural frequency.
The running frequency is:
At 4200\ \text{rpm}:
The subsynchronous order is:
That is consistent with oil whirl while the peak tracks a fractional order of speed.
Oil Whip Lock-In Check
At 4800\ \text{rpm}, a pure 0.45x whirl component would be expected near:
Instead, the measured peak remains near:
This is close to the first bending natural frequency:
A simple lock-in margin is:
Engineering comment: the evidence has shifted from ordinary oil whirl toward possible oil whip. The peak no longer follows the same fractional order; it has locked near the rotor mode while amplitude and temperature continue to rise.
The apparent order shift is:
That change is not a harmless plotting artifact if the tachometer reference is stable and the waterfall bins are fine enough. It means the analyst should stop treating the component as a simple order line and should check whether the subsynchronous peak has locked to a structural or rotor mode.
Trip and Hold Decision
The amplitude growth ratio from 3600\ \text{rpm} to 4200\ \text{rpm} is:
At 4200\ \text{rpm}, the hold criterion is already reached:
At 4800\ \text{rpm}, the trip level is exceeded:
The bearing clearance is:
The peak orbit radius at the trip point is approximately half the peak-to-peak motion:
The clearance fraction is:
The orbit is not using all available clearance, but the instability is growing and has crossed the protection criterion. Clearance fraction alone is not enough to continue the sweep.
The bearing metal temperature increase over the sweep is:
The temperature rise does not prove oil whirl by itself, but it supports the instability diagnosis because it occurs with growing subsynchronous motion and a shift toward lock-in. A defensible decision table is:
| Evidence | Interpretation | Action |
|---|---|---|
| 0.45x component below 40\ \mu\text{m}_{pp} | possible oil whirl, still below hold criterion | continue only with live monitoring |
| 41\ \mu\text{m}_{pp} at 4200\ \text{rpm} | hold criterion reached | stop acceleration and stabilize |
| 32\ \text{Hz} lock-in near f_n | possible oil whip | do not sweep through casually |
| 68\ \mu\text{m}_{pp} at 4800\ \text{rpm} | trip level exceeded | trip or abort test |
| rising temperature and forward orbit | instability evidence reinforced | inspect bearing and oil system |
Corrective Action and Validation
The team stops the sweep, holds the machine below 4200\ \text{rpm} and reviews the bearing and oil system. The first corrective actions are:
- restore oil inlet temperature to the approved range;
- confirm bearing load and preload;
- inspect clearance and bearing surface condition;
- review shaft centerline movement for abnormal lift;
- repeat the sweep with the same probes, reference mark, filtering and trip logic.
After oil temperature correction and bearing-preload adjustment, the repeat sweep shows a 0.44x to 0.46x component below 24\ \mu\text{m}_{pp} up to 4500\ \text{rpm}, no lock-in at 32\ \text{Hz} and stable bearing temperature. The machine is released only to 4500\ \text{rpm} pending a longer endurance run.
The release evidence should include:
- raw tachometer or once-per-revolution reference quality;
- waterfall spectra before and after correction with identical scaling;
- filtered orbits at the subsynchronous component and unfiltered orbits for rub or impact review;
- slow-roll runout compensation record;
- shaft centerline trend during the sweep;
- bearing temperature and oil inlet temperature trend;
- hold, trip and alarm setpoints used during the test.
The corrected sweep is not a permanent acceptance proof. It supports a controlled release to a reduced operating ceiling while the team collects endurance evidence. A final release would require stable vibration, stable temperature, repeatable centerline position and no reappearance of lock-in over the full required operating envelope.
Assumptions and Limits
This example assumes the speed reference is reliable, the proximity probes are inside their calibrated linear range and the bearing is a fluid-film journal bearing. The same logic should not be copied directly to rolling-element bearings, magnetic bearings or machines where a subsynchronous component is generated by a known non-rotor source.
The simplified lock-in margin is a screening check, not a full rotor-dynamic stability model. A full investigation may require bearing coefficients, oil viscosity at operating temperature, rotor model validation, seal-force review, foundation flexibility checks and OEM limits. The field decision, however, still needs a clear rule: when subsynchronous amplitude grows, locks to a mode and crosses hold or trip criteria, continuing the sweep is not justified by clearance fraction alone.
Common Mistakes
Do not balance a growing subsynchronous instability as if it were simple 1x unbalance. Do not call every sub-1x peak oil whirl without checking bearing type, order trend, orbit direction, oil temperature and repeatability.
Another mistake is continuing the sweep because the orbit radius is still below bearing clearance. Oil whirl and oil whip are stability problems. The decision should use amplitude growth, lock-in behavior, bearing temperature, shaft centerline position and predefined hold or trip criteria.