Turbomachinery bearing vibration and turbine-oil varnishing: a technical article

1. Core answer

The main vibration families seen at turbomachinery bearings are radial/lateral vibration, axial/thrust vibration, torsional vibration, casing/structural vibration, and frequency-domain classes such as synchronous 1× vibration, harmonic vibration, subsynchronous vibration, oil whirl, oil whip, rub-related vibration, broadband/random vibration, and, in machines with rolling-element bearings, bearing defect-frequency vibration.

For varnishing, the most important conclusion is this:

Varnish is not directly “caused by vibration.” Varnish is mainly caused by turbine-oil degradation: oxidation, thermal degradation, contamination, electrostatic discharge, aeration, microdieseling, additive depletion, and low solvency for polar degradation products. However, the bearing vibration type most likely to accelerate varnish formation is subsynchronous oil-film instability, especially oil whip, because it is generated inside the lubricating oil film itself and tends to produce high oil shear, high pressure fluctuation, reduced minimum film thickness, aeration, local heating, and sometimes rub or bearing wipe. If actual rotor rub or bearing wipe is included as a vibration fault, it can be even more thermally severe than oil whip, but it is better classified as a contact/failure condition rather than a normal bearing vibration mode.

There is no single industry standard that ranks “varnish production by vibration mode.” The ranking below is an engineering inference from established rotor-dynamic and lubricant-degradation mechanisms.

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2. Bearing vibration categories in turbomachinery

Critical turbomachinery is normally monitored using shaft-relative vibration probes, casing vibration sensors, axial position probes, speed/phase references, bearing metal temperature, and other protection measurements. API Std 670, for example, covers machinery protection systems measuring radial shaft vibration, casing vibration, shaft axial position, speed, phase reference, overspeed, surge detection, and critical machinery temperatures. (Accuris Standards Store)

2.1 Radial or lateral shaft vibration

This is the dominant vibration class for journal-bearing turbines, compressors, expanders, and generators. It is normally measured by orthogonal X–Y proximity probes at each bearing and displayed as:

Diagnostic view	What it shows
Overall displacement	Total shaft motion, usually in µm pk-pk or mils pk-pk
Spectrum/FFT	Frequency components such as 1×, 2×, subsynchronous bands
Orbit	Shaft center motion inside bearing clearance
Bode plot	Amplitude and phase versus speed
Cascade/waterfall	Frequency evolution during startup or coastdown
Shaft centerline plot	Static journal position and bearing load changes

Typical radial vibration sources include unbalance, shaft bow, misalignment, rub, looseness, resonance, oil whirl, oil whip, seal instability, and bearing clearance/preload problems.

2.2 Axial or thrust vibration

Axial vibration is measured along the shaft centerline. It is strongly associated with thrust bearing load variation, coupling problems, compressor surge, pressure pulsation, steam-force imbalance, thermal growth, and thrust-collar or thrust-pad distress.

In turbine thrust bearings, high axial dynamic load can produce elevated pad temperature and local oil distress. It can contribute to varnish deposition on thrust pads or oil-feed paths, but it is usually less directly connected to oil-film instability than journal-bearing oil whirl/whip.

2.3 Torsional vibration

Torsional vibration is angular oscillation of the shaft train. It may be caused by generator electrical disturbances, grid events, gear mesh excitation, reciprocating compressor torque pulsation, coupling issues, or blade-pass/drive-train interaction.

Torsional vibration is important for shaft fatigue and coupling integrity, but it is not usually a primary varnish driver unless it creates secondary bearing heating, gear distress, or oil temperature rise.

2.4 Casing or structural vibration

Casing vibration is normally measured by velocity probes or accelerometers. It may reflect rotor vibration transmitted through the bearings, pipe strain, foundation resonance, looseness, blade-pass excitation, flow-induced pulsation, or structural modes.

Casing vibration is useful for machine protection and troubleshooting, but it is less directly tied to turbine-oil varnish than shaft-relative bearing vibration and bearing temperature.

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3. Frequency-domain vibration types

3.1 Synchronous 1× vibration

1× vibration occurs at running speed. It is the classic signature of rotor unbalance, eccentricity, shaft bow, or thermal bow. In a spectrum, it appears at exactly one times shaft rotational speed.

From a varnish perspective, 1× vibration is usually low to moderate risk. It can increase bearing dynamic load and film shear, but if bearing temperatures and oil condition remain normal, 1× vibration alone does not strongly create varnish. It becomes varnish-relevant when it causes high bearing temperature, repeated minimum-film-thickness collapse, or rub.

3.2 2× and harmonic vibration

Harmonics such as 2×, 3×, and higher multiples can indicate angular misalignment, coupling distress, looseness, cracked components, distorted bearing geometry, rub, or nonlinear bearing response.

Severe 2× vibration can be more harmful than ordinary 1× unbalance because misalignment often increases bearing edge loading, thrust loading, and local thermal stress. For varnish, the risk is moderate to high only if the harmonic vibration is accompanied by elevated bearing metal temperature, oil drain temperature, or evidence of rubbing/wiping.

3.3 Subsynchronous vibration

Subsynchronous vibration occurs below running speed. In turbomachinery journal bearings, this is especially important because many serious fluid-film instabilities occur below 1×.

Subsynchronous vibration can come from oil whirl, oil whip, rub, looseness, seal instability, aerodynamic cross-coupling, or surge-related forces. Of these, oil whirl and oil whip are the most directly connected to the turbine-oil film.

3.4 Oil whirl

Oil whirl is a self-excited, forward-precessing, subsynchronous vibration caused by the hydrodynamic oil film in a journal bearing or seal. It commonly appears at about 0.39× to 0.48× running speed. Turbomachinery International describes oil whirl as a fluid-induced instability where the subsynchronous frequency tracks a constant percentage of rotative speed, typically 0.39× to 0.48×. (Turbomachinery Magazine)

Mechanically, the oil film is not only supporting the rotor; it is also applying cross-coupled dynamic forces. When these cross-coupled forces exceed the stabilizing damping, the rotor precesses around the bearing clearance.

A simplified linearized bearing-force model is:

The direct stiffness and damping terms tend to stabilize the rotor. The cross-coupled terms can inject tangential energy into the rotor orbit. When the destabilizing oil-film energy exceeds damping, oil whirl appears.

3.5 Oil whip

Oil whip is the more severe development of oil whirl. It occurs when the oil-whirl frequency locks onto a rotor natural frequency, commonly the first critical speed. Once locked, the whip frequency may remain near the critical speed even as running speed increases. NASA’s technical summary describes oil whirl/whip as lateral, forward-precessional, self-excited, subsynchronous vibration; oil whirl occurs somewhat below half rotative speed, then transitions to whip when the whirl frequency approaches the first balance resonance. (NASA Technical Reports Server)

Oil whip is usually more destructive than oil whirl because it excites a rotor bending mode and can create large deflection amplitudes. Turbomachinery International also notes that oil whip is more serious because the oil-film instability re-excites the first bending mode. (Turbomachinery Magazine)

3.6 Rub-related vibration

A rub occurs when the rotor contacts a stationary component, seal, bearing, or other close-clearance part. Rubs can generate 1×, harmonics, subsynchronous components, reverse precession, and broadband energy. They also create local frictional heating.

For varnish and oil degradation, rub is dangerous because it can produce localized hot surfaces, metallic wear debris, and oil thermal cracking. If a bearing wipe or rub is present, it can create severe local deposits. However, rub is not best described as “a type of turbine-oil vibration”; it is a mechanical contact fault that produces vibration and accelerates oil degradation.

3.7 Broadband/random vibration

Broadband vibration can indicate turbulence, cavitation, looseness, impacts, rubbing, poor oil supply, aeration, or flow disturbance. It is important for varnish because air entrainment and microdieseling can degrade turbine oil.

Microdieseling occurs when entrained air bubbles experience rapid compression, creating very high local temperatures that can carbonize oil and form sludge, tar, varnish, and oxidation products. (Lubrication Expert)

3.8 Rolling-element bearing defect frequencies

Some turbomachinery auxiliaries, small turbines, gearboxes, pumps, and motor-driven equipment use rolling-element bearings. Their vibration signatures include:

Defect frequency	Typical origin
FTF	Cage defect
BPFO	Outer-race defect
BPFI	Inner-race defect
BSF	Rolling-element defect
GMF	Gear mesh, if gearbox present

These are high-frequency mechanical fault signatures. They can produce heat and oil degradation locally, but in large turbine journal-bearing systems they are usually less relevant than oil-film instability.

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4. What varnish is in turbine-oil systems

Varnish is a thin, insoluble, often polar organic deposit that forms on oil-wetted surfaces such as servo valves, bearings, heat exchangers, filters, piping, and reservoirs. Machinery Lubrication describes turbine-oil varnish as insoluble film deposits composed of oil additives and high-molecular-weight thermo-oxidative breakdown compounds with limited solubility in the base fluid. (machinerylubrication.com)

The main varnish formation mechanisms are:

Mechanism	Technical effect
Oxidation	Produces acids, peroxides, polar oxidation products, sludge, varnish precursors
Thermal degradation	Cracks oil molecules at hot surfaces or hot microzones
Electrostatic discharge	Creates localized oil cracking and carbonaceous degradation products
Microdieseling	Compresses air bubbles, creating high-temperature micro-hotspots
Contamination	Water, particles, incompatible fluids, and wear metals accelerate oil degradation
Additive depletion	Reduces antioxidant and deposit-control reserve
Low solvency	Allows polar degradation products to precipitate from modern turbine oils

ExxonMobil identifies three major varnish mechanisms as thermal degradation, oxidation, and contamination. It also notes that poor air release can allow entrained air to be compressed in bearings or high-pressure hydraulics, causing adiabatic compression or microdieseling, which can create localized high oil temperatures that facilitate varnish formation. (Mobil)

Electrostatic discharge is another important turbine-oil degradation mechanism. Pall notes that highly refined, lower-conductivity, finely filtered fluids in high-speed turbine lubrication and hydraulic systems are increasingly associated with electrostatic discharge and varnish formation. (pall.co.uk)

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5. Why oil whirl/oil whip is the bearing vibration most associated with varnish acceleration

Oil whirl and oil whip are not merely “vibrations near the bearing.” They are oil-film-driven rotor instabilities. The lubricant is part of the forcing mechanism. That makes them more varnish-relevant than ordinary 1× unbalance.

5.1 High shear rate in the oil film

In a journal bearing, approximate shear rate is:

As whirl or whip amplitude increases, the journal orbit approaches the bearing clearance limit. Minimum film thickness decreases. Since heat generation scales approximately with (1/h^2), small reductions in film thickness can significantly increase local viscous heating.

5.2 Pressure cycling and bubble compression

Oil whirl/whip causes cyclic pressure variation in the bearing wedge. If air is entrained in the turbine oil, those bubbles can move rapidly between lower-pressure and higher-pressure regions. The simplified adiabatic compression relation is:

This helps explain why aerated oil under rapid pressure cycling can experience micro-hotspots. Microdieseling is strongly associated with oil carbonization, sludge, soot, tar, varnish, and oxidation products. (Lubrication Expert)

5.3 Loss of damping and self-excitation

A stable hydrodynamic bearing damps rotor motion. In oil whirl/whip, the oil film becomes a destabilizing energy source. This creates a feedback loop:

This is why varnish and vibration can reinforce each other. Varnish can restrict oil flow, insulate bearing surfaces, alter clearances, increase bearing temperature, and reduce effective damping; the resulting rotor instability then further stresses the oil.

5.4 Oil whip is worse than oil whirl

Oil whirl is already harmful, but oil whip is generally worse because it locks into a rotor natural frequency and can maintain large vibration amplitudes. Oil whip can excite a bending mode, increasing dynamic load at bearings and seals. Turbomachinery International explicitly describes oil whip as more destructive due to larger rotor deflection amplitudes. (Turbomachinery Magazine)

Therefore, among bearing-related vibration modes:

for varnish-acceleration risk, assuming similar oil chemistry and operating temperature.

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6. Practical ranking of vibration types by varnish risk

Rank	Vibration/fault type	Varnish risk	Reason
1	Bearing wipe, rotor rub, severe contact	Very high	Direct frictional heating, metal debris, local oil cracking, possible coking
2	Oil whip	Very high	Self-excited oil-film instability locked to rotor natural frequency; high orbit, high shear, high temperature
3	Developed oil whirl	High	Oil-film-driven subsynchronous orbit; high shear, pressure cycling, aeration sensitivity
4	Broadband vibration from aeration/cavitation/oil starvation	High	Promotes microdieseling, oxidation, unstable film, poor cooling
5	Severe axial/thrust vibration with high pad temperature	Medium to high	Dynamic thrust loading, local pad heating, deposit risk on thrust surfaces
6	Severe 2× / harmonic vibration from misalignment	Medium	Edge loading, elevated bearing temperature, possible rub
7	Synchronous 1× unbalance	Low to medium	Mainly mechanical forcing; varnish risk rises only if temperature or rub develops
8	Casing structural vibration without bearing heating	Low	Indirect relationship to oil degradation
9	Torsional vibration without bearing/gear heating	Low	Mainly shaft-train fatigue issue, not primary varnish mechanism

The most technically correct answer is:

The bearing vibration type most likely to cause or accelerate varnishing from turbine oil is subsynchronous oil-film instability, especially oil whip. If actual rub or bearing wipe is present, that contact condition can generate even more local thermal degradation, but it is a fault state rather than a normal vibration mode.

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7. Varnish may cause vibration before vibration causes varnish

In many real turbine systems, varnish is not the result of vibration. The sequence is often reversed:

oil oxidation/thermal degradation→varnish precursors→deposit formation→restricted oil flow or sticky control valves→bearing temperature rise or unstable control→abnormal vibration

This is especially important in gas turbines with common bearing and hydraulic reservoirs. ExxonMobil notes that turbines with common hydraulic and bearing reservoirs are more susceptible to varnish-related trips or no-starts than turbines with segregated reservoirs, and that mild varnish can accumulate on journal and thrust bearings with little or no immediate effect on bearing temperatures or shaft rotation. (Mobil)

So, a turbine may show servo-valve sticking, filter plugging, poor hydraulic response, or start failure before it shows a major bearing vibration alarm.

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8. Diagnostic approach

A proper diagnosis should combine vibration data, bearing temperature, operating data, and oil analysis.

8.1 Vibration indicators

For oil whirl:

where (N) is running speed. It usually tracks speed during startup and appears as a forward-precessing, nearly circular orbit.

For oil whip:

The frequency tends to lock near the first critical speed rather than continuing to scale with running speed.

Key indicators to trend:

Parameter	Why it matters
1× amplitude and phase	Unbalance, bow, thermal growth
2× and harmonics	Misalignment, rub, looseness
0.35×–0.50× band	Oil whirl suspicion
Fixed subsynchronous frequency	Oil whip or structural resonance
Orbit shape	Circular whirl, elliptical unbalance, rub distortion
Shaft centerline	Bearing load, clearance, thermal growth
Bearing metal temperature	Thermal stress and oil degradation
Drain oil temperature	Heat removed from bearing
Supply pressure/flow	Oil starvation, restriction, varnish blockage

8.2 Oil-analysis indicators

ASTM D7843 measures lubricant-generated insoluble color bodies in in-service turbine oils using membrane patch colorimetry and reports results as a ΔE value. ASTM states the test can guide users on insoluble deposit formation and is intended as a condition-monitoring trending tool. (ASTM International | ASTM)

Recommended varnish-related tests include:

Test	Purpose
MPC, ASTM D7843	Varnish potential / insoluble color bodies
RULER	Remaining antioxidant concentration
RPVOT	Remaining oxidation stability
FTIR oxidation	Oxidation products
TAN	Acid formation trend
Viscosity	Oxidation, contamination, wrong oil
Water	Hydrolysis, rust, poor demulsibility
Particle count	Contamination and wear debris
PQ / ferrous density	Large ferrous wear particles
Ultracentrifuge	Insoluble/deposit tendency
Air release / foam tests	Aeration and microdieseling risk

Mobil’s turbine oil condition-monitoring guidance also links oxidation with varnish tendency and identifies RULER, RPVOT, ultracentrifuge rating, viscosity, water, and other tests as useful condition indicators. (Mobil)

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9. Engineering controls

9.1 For oil whirl and oil whip

Correcting oil whirl/whip usually requires changing the rotor-bearing stability margin, not simply balancing the rotor.

Common actions include:

Corrective action	Technical effect
Verify correct oil viscosity	Controls film stiffness, damping, and temperature
Correct oil supply temperature	Prevents excessive viscosity or low film thickness
Restore design bearing clearance	Reduces instability and excessive orbit
Increase bearing preload or load	Improves journal stability
Use tilting-pad bearings	Reduces cross-coupled stiffness
Correct bearing geometry	Elliptical, lemon-bore, pressure-dam, or tilting-pad designs improve stability
Reduce seal cross-coupling	Seals can also drive subsynchronous instability
Improve oil cleanliness	Reduces abrasive wear and deposit nucleation
Remove varnish	Restores oil flow, heat transfer, and valve/bearing response
Eliminate aeration	Improves damping and reduces microdieseling

9.2 For varnish control

Varnish control should focus on oil chemistry, temperature, contamination, and deposit removal:

Control	Purpose
Maintain OEM oil temperature band	Avoid excessive oxidation and cold deposition zones
Use turbine oil with high oxidation/thermal stability	Slows precursor formation
Control air entrainment and foam	Reduces microdieseling
Manage electrostatic discharge	Prevents localized oil cracking
Remove water	Prevents hydrolysis and additive depletion
Control wear metals and particles	Reduces catalytic oxidation and abrasive damage
Use varnish-removal technology	Removes soluble/insoluble precursors
Trend MPC and antioxidants	Detects varnish potential early
Inspect last-chance filters and servo valves	Finds deposits before trip/no-start events

Machinery Lubrication notes that varnish problems in combustion turbines have increased due to higher operating temperatures, smaller reservoirs, cyclic service, highly refined base stocks with lower solvency, and fine filtration that can increase electrostatic charging. (machinerylubrication.com)

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10. Final technical conclusion

In turbomachinery bearings, the major vibration types are radial/lateral, axial/thrust, torsional, casing/structural, synchronous, harmonic, subsynchronous, oil whirl, oil whip, rub-related, broadband/random, and rolling-element defect-frequency vibration.

The vibration type most associated with increased varnishing from turbine oils is subsynchronous oil-film instability, particularly oil whip. It is the most varnish-relevant bearing vibration mode because the turbine oil film is itself the excitation source, and the resulting large orbit increases shear heating, pressure cycling, aeration sensitivity, minimum-film-thickness reduction, and the chance of rub or wipe.

However, the strictest technical statement is:

Vibration does not chemically create varnish by itself. Varnish forms when turbine oil degrades and its polar oxidation/thermal-degradation products exceed the oil’s solvency, then deposit on metal surfaces. Oil whip and severe oil whirl accelerate the conditions that create and deposit varnish; rub or bearing wipe can be even more severe locally; but oxidation, thermal stress, aeration/microdieseling, electrostatic discharge, contamination, and additive depletion are the root causes.