How Varnish on Bearing Surfaces Affects the Coefficient of Friction

A practical tribology article for turbomachinery, journal bearings, thrust bearings, and lubricated sliding contacts

In turbomachinery lubrication, varnish is often discussed as a cleanliness, oil degradation, or servo valve reliabilityproblem. But in rotating equipment, varnish is also a friction problem.

When varnish deposits form on bearing surfaces, thrust pads, journal bearing babbitt, tilting-pad components, bearing housings, seals, and other oil-wetted metallic surfaces, they can change the local coefficient of friction dramatically. This does not always happen in a simple way. Sometimes varnish may initially behave like a thin organic film that slightly smooths the surface. But as it becomes thicker, harder, more polar, thermally aged, oxidized, carbonized, or contaminated with fine particles, it can shift the contact from a controlled hydrodynamic regime toward mixed lubrication, boundary lubrication, higher shear stress, stick-slip behavior, and abnormal heat generation.

In simple words:

Varnish converts a well-lubricated precision bearing surface into a chemically active, sticky, uneven, thermally unstable surface.

That is why the coefficient of friction can increase, temperature can rise, oil film thickness can reduce, and a bearing that was designed to run in full-fluid-film lubrication may begin to show symptoms similar to overload, poor oil viscosity, misalignment, or insufficient oil flow.

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1. First: what is coefficient of friction?

The coefficient of friction, usually shown as µ, is the ratio between the friction force and the normal load.

µ = Friction Force / Normal Load

In lubricated bearings, this value is not fixed. It depends on:

• Lubrication regime
• Oil viscosity
• Surface roughness
• Sliding speed
• Bearing load
• Temperature
• Oil chemistry
• Surface chemistry
• Contamination
• Film thickness
• Bearing material
• Additive condition
• Deposit formation

For example, in a properly operating journal bearing or thrust bearing under hydrodynamic lubrication, the shaft or collar is separated from the bearing surface by a pressurized oil film. In this condition, friction is mostly due to viscous shear of the oil film, not metal-to-metal rubbing.

But when varnish forms, the interface changes.

The friction is no longer only controlled by the oil viscosity and hydrodynamic film. It is now affected by a third body: the varnish layer.

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2. Varnish is not just “dirt”

Varnish is not the same as normal hard dirt, sand, rust, or metallic debris. In turbine oils and compressor oils, varnish is usually formed from oxidation and degradation by-products of the lubricant.

These degradation products can be:

• Polar organic molecules
• Oxidized hydrocarbons
• Resin-like material
• Sludge precursors
• Soft sticky films
• Lacquer-like deposits
• Thermally aged carbonaceous material
• Oxidized additive residues
• Fine agglomerates of insoluble degradation products
• Organic deposits mixed with inorganic contamination

The important point is this:

Varnish has chemistry.

It is usually polar, adhesive, surface-active, and strongly attracted to metal surfaces. Bearing surfaces, especially those under high temperature, high pressure, low flow, or electrostatic/polar conditions, can become preferred deposition zones.

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3. Why bearing surfaces are sensitive to varnish

Bearings are designed with very small clearances and very controlled surface geometry. Even a very thin deposit can disturb the intended oil film behavior.

In journal bearings and thrust bearings, the bearing surface is not only a support surface. It is part of a fluid-dynamic machine. Its geometry creates pressure in the oil film.

For example:

• A journal bearing needs the correct clearance and eccentricity to generate a hydrodynamic wedge.
• A thrust bearing pad needs the correct tilt to generate a pressure wedge.
• A tilting-pad bearing needs free pad movement and correct pivot response.
• Oil grooves and feed holes must distribute oil properly.
• Surface finish must support stable oil film formation.

When varnish deposits form on these areas, they can change:

• Local clearance
• Surface roughness
• Oil film thickness
• Pad tilt behavior
• Oil flow distribution
• Heat transfer
• Wettability of the surface
• Boundary film behavior
• Local load carrying area
• Shear resistance at the interface

Therefore, varnish can affect friction both directly and indirectly.

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4. How varnish increases coefficient of friction

4.1 Varnish increases surface roughness

A clean bearing surface has a controlled finish. It may look smooth to the eye, but microscopically it has peaks and valleys. The oil film is designed to separate the moving surfaces so that these asperities do not contact each other.

When varnish deposits on the bearing, it rarely forms a perfectly uniform layer. It can appear as:

• Streaks
• Patches
• Islands
• Brown/yellow films
• Dark localized deposits
• Glazed areas
• Sticky regions
• Raised deposits
• Hardened spots
• Edge deposits
• Deposits near oil grooves or low-flow areas

This uneven layer increases the effective surface roughness.

When surface roughness increases, the ratio of oil film thickness to surface roughness decreases. This ratio is often discussed using the lambda ratio:

Lambda ratio = Oil Film Thickness / Composite Surface Roughness

When lambda is high, lubrication is more hydrodynamic.
When lambda drops, the bearing enters mixed lubrication.
When lambda becomes very low, boundary lubrication and asperity interaction dominate.

Varnish can reduce lambda by increasing roughness and reducing film thickness at the same time.

The result is:

More asperity interaction → more friction → more heat → more varnish formation.

This becomes a self-accelerating loop.

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4.2 Varnish reduces effective bearing clearance

In hydrodynamic bearings, clearance is not a random number. It is essential for oil film formation, oil flow, cooling, and stability.

A varnish layer may be only a few microns thick, but that is enough to matter in precision bearings.

If varnish deposits on the loaded zone of a journal bearing or on thrust bearing pads, it can reduce the local oil film thickness. Reduced film thickness means higher shear rate and higher probability of mixed lubrication.

A simplified view:

Clean surface → designed clearance → stable oil film → low friction

Varnished surface → reduced local clearance → thinner oil film → higher friction

In severe cases, varnish can create high spots. These high spots carry more load than intended. Local pressure rises, oil film collapses locally, and the coefficient of friction increases sharply.

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4.3 Varnish changes surface energy and oil wettability

Bearing surfaces are not only mechanical surfaces; they are also chemical surfaces.

Clean metal, babbitt, steel, bronze, and coated bearing materials interact differently with turbine oil and additives. When varnish covers these surfaces, the oil no longer interacts with the original bearing material. It interacts with the varnish layer.

This can change:

• Oil wetting behavior
• Additive adsorption
• Boundary film formation
• Shear behavior near the surface
• Local oil residence time
• Heat transfer behavior

A varnished surface may become more adhesive and less predictable. Instead of the oil forming a stable lubricating interface with the engineered bearing material, the oil is now shearing against an oxidized organic film.

The varnish layer may have a higher shear strength than the intended boundary film. Higher shear strength means more resistance to sliding.

More resistance to sliding means a higher coefficient of friction.

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4.4 Varnish can disturb additive performance

Turbine oils are usually not heavily additized like gear oils, but they still depend on carefully balanced additives:

• Antioxidants
• Rust inhibitors
• Demulsifiers
• Antifoam agents
• Metal passivators
• Sometimes antiwear chemistry, depending on application

The bearing surface may rely on the correct interaction between oil chemistry and surface chemistry. Varnish can interfere with that interaction.

It can:

• Cover the original metallic surface
• Adsorb polar additives
• Trap degraded additives
• Create chemically active sites
• Reduce the effectiveness of protective films
• Hold contaminants at the surface

This is very important in boundary or mixed lubrication moments, such as:

• Startup
• Shutdown
• Turning gear operation
• Low-speed operation
• High load transients
• Temporary oil starvation
• Rapid load changes
• High-temperature excursions

During these conditions, the equipment may need surface-active chemistry to protect the bearing. If varnish has already contaminated the surface, friction can rise faster and protection can become less reliable.

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4.5 Varnish can act as a sticky third body

In tribology, a third body is material trapped between two contacting or near-contacting surfaces. It can be beneficial or harmful depending on its properties.

Varnish can become a harmful third body because it may be:

• Sticky
• Soft at higher temperature
• Hard after aging
• Chemically polar
• Adhesive to metal
• Non-uniform in thickness
• Mixed with fine particles
• Thermally unstable
• Difficult to shear consistently

A clean oil film shears predictably. A varnish-contaminated interface does not.

This can cause:

• Higher breakaway friction
• Stick-slip motion
• Unstable pad movement
• Local temperature spikes
• Frictional chatter
• Increased vibration
• Abnormal orbit behavior
• Fluctuating bearing metal temperatures

In tilting-pad bearings, this is especially important. The pad must tilt freely and respond dynamically to load and speed. If varnish deposits around pivots, pad backs, contact areas, or oil feed regions, the bearing may lose some of its designed freedom of movement.

A small increase in friction at the pad support can become a large operational issue.

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5. Varnish and the Stribeck curve

The Stribeck curve explains the relationship between coefficient of friction and lubrication regime.

It usually shows three main regimes:

1. Boundary lubrication
2. Mixed lubrication
3. Hydrodynamic lubrication

In hydrodynamic lubrication, friction is mainly controlled by viscous shear of the oil film. The surfaces are separated.

In mixed lubrication, part of the load is carried by the oil film and part by asperity contact.

In boundary lubrication, the surfaces are mostly interacting through very thin boundary films.

Varnish can shift the bearing operating point on the Stribeck curve.

A bearing that should operate safely in the hydrodynamic zone can be pushed toward mixed lubrication because varnish:

• Increases roughness
• Reduces film thickness
• Creates local high spots
• Changes surface chemistry
• Restricts oil flow
• Increases temperature
• Reduces oil viscosity locally
• Disturbs pad movement
• Increases shear resistance

This means the coefficient of friction may rise not because the oil viscosity was wrong, but because the bearing surface condition has changed.

This is a very important diagnostic point.

Sometimes the oil analysis report may show “acceptable viscosity,” but the bearing still runs hot because the surface is no longer clean and hydraulically correct.

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6. Friction increase creates heat — heat creates more varnish

This is one of the most dangerous feedback loops in turbine oil systems.

The sequence can be:

1. Oil oxidation creates soluble varnish precursors.
2. These polar degradation products deposit on bearing surfaces.
3. The deposit increases friction.
4. Friction increases local temperature.
5. Higher temperature accelerates oil oxidation.
6. More oxidation creates more varnish precursors.
7. More varnish deposits on hot surfaces.
8. The bearing becomes hotter and less stable.

This can become a repeating cycle:

Varnish → friction → heat → oxidation → more varnish → more friction

This is why a bearing with varnish symptoms may not recover simply by changing oil or improving filtration for particles only. The oil chemistry and the surface deposits both need to be addressed.

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7. Why varnish may cause bearing temperature instability

A varnished bearing does not always show one simple temperature increase. Sometimes it shows unstable temperature behavior.

The bearing temperature may:

• Slowly trend upward
• Show saw-tooth temperature patterns
• Increase after load changes
• Increase after shutdown/startup cycles
• Improve temporarily and then return
• Differ between pads in the same bearing
• Show one thrust pad hotter than others
• Show sudden step changes
• Show sensitivity to oil supply temperature

This happens because varnish is not uniformly distributed. It may deposit more in specific areas:

• Loaded zone
• Low-flow zones
• Hot zones
• Dead zones
• Oil groove edges
• Thrust pad leading or trailing regions
• Pivot contact areas
• Drain areas
• Areas with repeated thermal stress

If varnish causes one area to carry more load or experience more shear, the local coefficient of friction rises there first. That local hot spot can then become the preferred area for more deposit formation.

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8. Effect on journal bearings

In journal bearings, varnish can affect friction in several ways.

8.1 Loaded zone deposits

The loaded zone is where oil film pressure is highest. If varnish forms there, it can disturb the oil wedge and increase local shear.

This can cause:

• Higher bearing metal temperature
• Reduced minimum oil film thickness
• Increased eccentricity
• Increased frictional power loss
• Local wiping risk
• Higher sensitivity to load changes

8.2 Oil groove and feed hole deposits

If varnish forms around oil grooves or feed holes, oil distribution may become less effective.

Reduced oil supply can increase friction because:

• Cooling is reduced
• Oil film becomes thinner
• Local temperature rises
• Oil viscosity drops locally
• More mixed lubrication occurs

8.3 Surface roughness and wiping tendency

Once varnish increases roughness or traps fine particles, the surface can become more aggressive. The bearing may move from smooth hydrodynamic sliding into mixed lubrication.

In severe cases, this can contribute to wiping, smearing, or distress of the babbitt surface.

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9. Effect on thrust bearings

Thrust bearings are extremely sensitive to surface condition because the pads depend on correct oil wedge formation.

Varnish can affect thrust bearings by:

• Disturbing pad tilt
• Changing the surface profile
• Increasing shear stress
• Reducing cooling
• Creating local hot spots
• Causing uneven load sharing between pads
• Increasing friction at pivot contacts
• Restricting oil flow through grooves or spray nozzles

A thrust pad with varnish may not generate the same oil film pressure as a clean pad. If one pad runs hotter, its oil viscosity drops locally, which further reduces film thickness. That pad can then become even hotter.

This is why varnish in thrust bearings can become a serious reliability concern.

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10. Effect on tilting-pad bearing movement

Tilting-pad bearings depend on freedom of movement. The pads must tilt correctly to establish the oil wedge.

Varnish can deposit on:

• Pad backs
• Pivot contact areas
• Retaining hardware
• Levelling plates
• Pad support surfaces
• Oil supply areas
• Drain areas

If the pad does not move freely, the bearing may lose its designed self-adjusting behavior.

This may cause:

• Uneven pad loading
• Higher friction
• Higher pad temperatures
• Reduced dynamic stability
• Changes in vibration response
• Increased risk during startup/shutdown

The coefficient of friction in this case increases not only at the shaft-to-pad oil film interface, but also at mechanical support interfaces where small movements are required.

This is a critical point:

Varnish can increase friction in places where engineers do not normally think about friction.

Not only on the main bearing surface, but also behind the pad and around the support geometry.

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11. Boundary friction during startup and shutdown

Most turbomachinery bearings are designed to operate under hydrodynamic lubrication at normal speed. But during startup and shutdown, speed is low and oil film generation is limited.

At low speed, the bearing may temporarily operate in boundary or mixed lubrication.

If the surface is clean, smooth, and chemically protected, this short period is usually acceptable.

If the surface is varnished, the situation changes.

Varnish can increase:

• Breakaway friction
• Startup torque
• Local rubbing
• Adhesive shear
• Stick-slip
• Thermal spikes

Repeated starts and stops can therefore accelerate bearing distress when varnish is already present.

This is one reason why peaking units, cyclic service turbines, standby machines, and machines with frequent start-stop operation can be vulnerable to varnish-related friction problems.

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12. Varnish hardness matters

Not all varnish behaves the same.

Fresh varnish may be soft, sticky, and resin-like.
Aged varnish may become harder and more lacquer-like.
High-temperature deposits may become darker, harder, and more carbonaceous.
Varnish mixed with particles may become abrasive.

Therefore, the effect on coefficient of friction depends on the varnish condition.

Soft varnish

Soft varnish may increase adhesive friction. It behaves like a sticky layer that resists sliding.

Possible symptoms:

• Stick-slip
• Temperature fluctuation
• Slow pad response
• Increased breakaway friction
• Deposit smearing

Hard varnish

Hard varnish may increase roughness and create high spots.

Possible symptoms:

• Higher local contact pressure
• Mixed lubrication
• surface distress
• abnormal wear
• pad temperature rise

Carbonized varnish

Carbonized or thermally stressed deposits may behave more like hard contamination.

Possible symptoms:

• Dark deposits
• polishing or scratching
• local overheating
• increased particle generation
• severe surface distress

This is why visual inspection of deposits matters. A light amber film and a dark carbonized deposit do not represent the same tribological risk.

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13. Varnish mixed with particles: the worst case

Varnish alone is already a problem. But varnish mixed with particles can be worse.

Sticky varnish can trap:

• Oxidation insolubles
• Wear debris
• Rust particles
• Seal material
• Carbonaceous particles
• Inorganic contamination
• Fine dust
• Additive residues

This creates a deposit with both adhesive and abrasive behavior.

Then the surface friction can increase through two mechanisms:

1. Adhesive friction from sticky organic varnish
2. Abrasive or ploughing friction from trapped particles

This can strongly increase friction and wear risk.

In this condition, particle count alone may not explain the problem because some harmful particles are immobilized on surfaces as part of the deposit layer.

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14. Thermal insulation effect of varnish

Bearing surfaces must transfer heat to the oil and surrounding metal structure. Varnish is generally a poor thermal conductor compared with metal.

A varnish layer can act as a thermal barrier.

This means:

• Heat generated at the interface is removed less efficiently
• Local surface temperature increases
• Oil near the surface becomes hotter
• Local viscosity decreases
• Oil film thickness reduces
• Friction increases further

This effect is very important because the bearing metal temperature sensor may not always capture the exact highest local surface temperature. The actual micro-contact or near-surface hot spot can be hotter than the measured bulk bearing temperature.

So varnish can create a hidden thermal problem before the temperature alarm clearly shows it.

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15. Varnish and oil viscosity: a local problem, not only bulk oil viscosity

Engineers often check viscosity and say, “The oil viscosity is normal.”

But the bearing does not operate based only on the laboratory viscosity at 40°C or 100°C. It operates based on the local viscosity inside the bearing oil film, at the actual local temperature and shear condition.

If varnish increases friction, local temperature rises. When local temperature rises, oil viscosity decreases. When viscosity decreases, oil film thickness decreases. When film thickness decreases, friction may increase further.

Therefore, even with normal laboratory viscosity, the bearing may experience poor local film conditions.

This is why varnish can create a friction problem even when standard oil properties still look acceptable.

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16. Why coefficient of friction can increase suddenly

In some cases, varnish problems appear gradual. In other cases, symptoms seem sudden.

This can happen because varnish has solubility behavior.

Turbine oil degradation products may remain dissolved when oil is hot. But when oil cools, or when the oil becomes saturated, these polar compounds can come out of solution and deposit.

Possible triggers include:

• Oil temperature reduction
• Shutdown and cooling
• Cold start
• Oil change or top-up compatibility effects
• Antioxidant depletion
• Change in oil formulation
• Water contamination
• Electrostatic discharge
• Fine particle contamination
• Local hot spots
• Filter changes
• Reservoir circulation changes
• Long residence time in cooler zones

A machine may therefore operate for a long time with soluble varnish potential in the oil. Then, after a trigger event, deposits form on sensitive surfaces and friction increases.

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17. Why varnish-related friction is often misdiagnosed

Varnish-related friction problems can look like many other problems.

They may be misdiagnosed as:

• Misalignment
• Overload
• Low oil flow
• Wrong oil viscosity
• Cooler problem
• Bearing design issue
• High ambient temperature
• Shaft instability
• Poor balancing
• Instrumentation error
• Oil contamination only
• Normal aging

The reality may be that varnish is reducing oil film reliability and increasing friction at the bearing surface.

This is why oil analysis, bearing temperature trends, vibration trends, inspection photos, and deposit analysis should be interpreted together.

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18. How to detect varnish contribution to friction increase

No single test gives the full answer. A practical investigation should combine oil analysis, machine symptoms, and physical inspection.

18.1 Oil analysis indicators

Useful tests include:

• MPC / membrane patch colorimetry
• RULER / LSV antioxidant measurement
• RPVOT
• TAN by ASTM D664
• FTIR oxidation/nitration trends
• Particle count
• Water content
• Demulsibility
• Foam and air release
• Elemental analysis
• Ferrous density, where applicable
• Patch microscopy
• SEM/EDS for deposit or patch analysis, where needed

MPC is especially useful because it indicates varnish potential. However, MPC should be interpreted carefully. It is not just a number. The patch appearance, Delta L, Delta a, Delta b, and visual character of the deposit can give important clues.

18.2 Machine condition indicators

Look for:

• Increasing bearing temperature
• Temperature instability
• Pad-to-pad temperature differences
• Higher oil drain temperature
• Startup temperature spikes
• Abnormal orbit changes
• Increased subsynchronous vibration
• Changed phase behavior
• Increased frictional power loss
• Reduced coast-down time
• Higher turning gear load
• Repeated servo or control valve issues in the same oil system

18.3 Inspection indicators

During shutdown or inspection, look for varnish on:

• Journal bearing loaded zone
• Thrust pads
• Pad leading/trailing edges
• Oil grooves
• Feed holes
• Drain areas
• Pad backs
• Pivot areas
• Levelling plates
• Bearing housing
• Servo valves
• Reservoir walls
• Cooler tubes
• Filter housings

Take photos under consistent lighting. Varnish pattern is very important.

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19. Practical friction mechanisms caused by varnish

The coefficient of friction can increase through several mechanisms at the same time:

Mechanical mechanisms

• Increased roughness
• Reduced clearance
• Local high spots
• Mixed lubrication
• Boundary contact
• Restricted pad movement
• Restricted oil flow
• Load concentration
• Particle entrapment

Chemical mechanisms

• Polar deposit attraction
• Additive adsorption
• Surface chemistry alteration
• Oxidized organic film formation
• Acidic degradation products
• Poor boundary film stability

Thermal mechanisms

• Increased shear heating
• Reduced heat transfer
• Lower local viscosity
• Thermal softening of deposits
• Accelerated oxidation
• More deposit formation

Dynamic mechanisms

• Stick-slip
• Pad motion restriction
• Uneven load sharing
• Vibration changes
• Orbit instability
• Saw-tooth temperature behavior

This is why varnish is not only an oil cleanliness issue. It is a system reliability issue.

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20. Can varnish ever reduce friction?

In a very limited and temporary situation, a very thin organic film may appear to smooth a surface or reduce direct metallic interaction. But this should not be considered beneficial in turbomachinery bearings.

Why?

Because varnish is not a controlled engineered coating. It is uncontrolled degradation material.

It has:

• Unknown thickness
• Unknown hardness
• Unknown shear strength
• Unknown chemistry
• Unknown thermal stability
• Unknown adhesion strength
• Unknown contamination content
• Unknown effect on oil flow
• Unknown effect on bearing geometry

A bearing manufacturer designs the surface finish, clearance, geometry, and material. Varnish randomly modifies all of them.

So even if an early thin film does not immediately increase friction, it is still a warning sign. As varnish matures, oxidizes, thickens, and traps particles, it usually becomes harmful.

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21. Why varnish is dangerous in high-speed turbomachinery bearings

High-speed turbomachinery bearings depend on stable oil films. The margin between safe hydrodynamic operation and distress can become small under high load, high temperature, or transient operation.

Varnish is dangerous because it attacks several safety margins at once:

• Reduces film thickness margin
• Increases friction margin
• Reduces cooling margin
• Reduces surface cleanliness margin
• Reduces pad mobility margin
• Reduces oil chemistry margin
• Reduces thermal stability margin

A small varnish film may not look dramatic visually, but in a bearing operating with micron-level oil films, it can be significant.

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22. Field example: why a bearing runs hotter after varnish formation

Imagine a thrust bearing pad in a steam turbine.

Originally:

• Oil supply is clean.
• Pad tilts correctly.
• Oil wedge forms properly.
• Surface is smooth.
• Heat is removed by oil flow.
• Coefficient of friction remains stable.

After varnish formation:

• Deposit forms near the hot loaded zone.
• Surface roughness increases.
• Pad tilt becomes less ideal.
• Oil film becomes thinner.
• Local shear stress increases.
• Local temperature rises.
• Oil viscosity decreases locally.
• More mixed lubrication occurs.
• Friction increases further.
• More varnish deposits in the hot area.

The operator may see:

• One pad temperature higher than others
• Slowly increasing thrust bearing temperature
• Alarm during load increase
• Temperature drop after shutdown, then repeat after restart
• Dark amber/brown deposits during inspection

The root cause is not simply “dirty oil.” It is a combined oil chemistry, surface deposit, and tribological friction problem.

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23. The relationship between varnish, friction, and wear

Friction and wear are related, but they are not the same.

A bearing can have increased friction before severe wear appears. This is why temperature can be an early warning.

The progression may be:

1. Varnish potential increases in oil.
2. Thin deposits form on bearing surfaces.
3. Coefficient of friction increases locally.
4. Bearing temperature rises.
5. Oil film becomes less stable.
6. Mixed lubrication increases.
7. Wear begins.
8. Debris increases.
9. Deposits trap debris.
10. Surface distress accelerates.

By the time wear debris becomes obvious, the friction problem may already be advanced.

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24. Why particle filtration alone may not solve varnish-related friction

Traditional mechanical filtration removes particles. It is necessary, but it may not be sufficient for varnish.

Varnish precursors can be soluble or submicron. They may not be captured effectively by normal particle filters while they are dissolved in hot oil.

If the oil still contains soluble varnish precursors, clean bearing surfaces can become re-contaminated.

This is why varnish management should consider:

• Soluble varnish precursors
• Insoluble varnish deposits
• Oil oxidation stability
• Antioxidant depletion
• Acidic degradation products
• Thermal stress
• Water contamination
• Surface deposit removal
• Long-term oil chemistry control

The objective is not only to clean particles. The objective is to restore oil chemistry and prevent re-deposition on critical surfaces.

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25. How to reduce varnish-related friction risk

25.1 Control varnish potential before deposits form

The best strategy is prevention.

Monitor:

• MPC trend
• Antioxidant depletion
• TAN trend
• RPVOT trend
• FTIR oxidation
• Water content
• Particle count
• Temperature history
• Operating mode changes

Do not wait until bearing temperatures increase.

25.2 Keep oil cool, dry, and chemically stable

Oxidation accelerates with temperature. Water and contamination can worsen degradation. Air entrainment, microdieseling, electrostatic discharge, and poor reservoir design can also contribute to oil stress.

Good practices include:

• Proper oil temperature control
• Dry oil
• Good breathers or nitrogen blanketing where appropriate
• Minimizing air entrainment
• Avoiding electrostatic discharge
• Correct filter selection
• Adequate reservoir residence time
• Proper oil sampling
• Avoiding incompatible top-ups

25.3 Use varnish removal technology that addresses soluble and insoluble material

For turbine oil varnish control, the technology should not only capture visible sludge. It should reduce varnish potential in the oil and help dissolve and remove deposits over time.

For high-value turbomachinery, long-term online lubricant chemistry management is usually more reliable than occasional emergency cleanup.

25.4 Inspect bearing surfaces during outages

During major inspection or turnaround, the bearing surface can tell the truth.

Look for:

• Amber/brown films
• Dark deposits
• Sticky areas
• Local high spots
• Patchy deposits
• Deposits near oil grooves
• Pad discoloration
• Uneven pad temperature history
• Signs of wiping or polishing
• Deposit thickness differences between pads

Do not clean the bearing before documenting the deposit pattern. The pattern helps diagnose the mechanism.

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26. Key diagnostic question

When a bearing runs hot, many people ask:

“Is the oil viscosity correct?”

That is a good question, but not enough.

A better question is:

“Is the bearing still operating with the surface condition, clearance, chemistry, and oil film behavior it was designed for?”

Varnish can make the answer no.

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27. Practical summary

Varnish on bearing surfaces can increase coefficient of friction by:

• Increasing surface roughness
• Reducing effective oil film thickness
• Reducing local clearance
• Creating high spots
• Increasing adhesive shear
• Disturbing oil wettability
• Interfering with additive function
• Restricting pad movement
• Reducing oil flow and cooling
• Acting as a thermal barrier
• Trapping abrasive particles
• Shifting operation from hydrodynamic to mixed or boundary lubrication
• Creating local hot spots
• Accelerating oxidation and further varnish formation

The most important point is this:

Varnish does not only sit on the bearing surface. It changes the tribological behavior of the bearing.

It changes friction, heat generation, oil film stability, surface chemistry, and dynamic response.

For turbomachinery, this means varnish should not be treated as a cosmetic deposit or just an oil analysis number. It should be treated as a direct threat to bearing reliability, because once the coefficient of friction starts increasing, the machine may enter a dangerous cycle of:

higher friction → higher temperature → lower oil viscosity → thinner oil film → more varnish → more friction.

That is why turbine oil varnish control is not only about clean oil.

It is about protecting the bearing’s designed friction regime.