Directed Lubrication in Double Thrust Bearings — and What Varnish Does to These Critical Areas
The image shows directed lubrication arrangements for double thrust bearings, designed specifically to prevent bulk oil from contacting the thrust collar. This is an important detail in turbomachinery because the thrust collar is a high-speed rotating surface. If excessive oil is allowed to contact it, the result is not “better lubrication”; it can become oil churning, aeration, heat generation, foaming, oxidation acceleration, and eventually varnish formation.
The drawing is very valuable because it teaches one major lesson:
In high-speed thrust bearing systems, oil must be delivered exactly where it is needed and removed immediately after doing its job.
When varnish starts forming, the areas shown in this drawing become some of the first places where small deposits can create large reliability problems.
1. What the image is showing
The drawing shows a double thrust bearing arrangement with a rotating thrust collar between two sets of thrust pads or thrust bearing elements.
The oil is supplied from the top and directed into the bearing zones. After passing through the loaded and unloaded thrust faces, the oil must drain quickly through dedicated drain holes or drain ports.
The purpose of this design is to avoid flooding the collar with bulk oil.
The key notes in the drawing are extremely important:
“Total drain area should provide for a flow velocity below 0.6 m/s.”
And in the second arrangement:
“Drain ports equally spaced around collar periphery. Total area should provide a flow velocity less than 0.15 m/s.”
This means the design is not only about supplying oil. It is equally about removing oil at a controlled velocity.
In thrust bearings, poor drainage is often more dangerous than insufficient supply because trapped oil can become a source of heat, air entrainment, oxidation, and deposit formation.
2. Why directed lubrication is used in thrust bearings
In high-speed turbomachinery, thrust bearings are not simply “oil-filled cavities.” The oil has to perform several functions at the same time:
It must create an oil film between the thrust pad and collar.
It must remove heat generated by shearing.
It must carry away wear particles and oxidation by-products.
It must avoid excessive residence time in hot zones.
It must leave the bearing housing without causing turbulence, foaming, or recirculation.
Directed lubrication helps because the oil is introduced close to the required lubrication zones rather than flooding the entire cavity. This reduces unnecessary oil contact with the rotating collar.
The thrust collar can behave almost like a disk pump. If bulk oil contacts the collar, the rotating surface can drag the oil, throw it outward, whip it, aerate it, and generate heat. That extra heat accelerates oxidation. Oxidation accelerates varnish. Varnish then worsens drainage and heat transfer. The result can become a self-reinforcing failure loop.
3. What happens when varnishing starts in these areas?
Varnish is not just a “dirty oil” issue. In thrust bearing areas, varnish becomes a clearance, heat transfer, drainage, and control issue.
The most affected locations in the image are:
- Thrust collar surfaces
- Thrust pad faces
- Oil supply passages
- Drain holes and drain ports
- Bearing housing pockets
- Lower stagnant zones
- Pad supports, pivots, and leveling mechanisms
- Oil outlet regions and return paths
Each area reacts differently when varnish forms.
4. Varnish on the thrust collar
The thrust collar is one of the most sensitive surfaces in the arrangement.
The collar is rotating at shaft speed. It is exposed to high shear, high surface speed, and elevated oil temperature. When oil degradation products become polar and insoluble, they tend to deposit on metallic surfaces, especially where temperature, residence time, and surface polarity support deposition.
When varnish forms on the collar, several problems can develop.
First, the collar surface becomes less clean and less thermally efficient. Varnish is a poor heat conductor compared with metal. Even a very thin deposit can act like an insulating layer. This reduces heat transfer from the collar to the oil.
Second, the collar surface roughness and surface energy change. The thrust pad depends on stable hydrodynamic oil-film formation. If the collar surface becomes coated with sticky oxidation products, the oil film may become less stable during start-up, shutdown, low-speed turning, or transient load changes.
Third, varnish can create localized hot spots. These hot spots accelerate oil oxidation even more. Once this starts, the collar area can become both a victim and a generator of varnish.
In severe cases, the thrust bearing temperature may show a saw-tooth pattern: temperature rises due to restricted oil flow, deposit drag, or unstable film; then drops after load or flow changes; then rises again as varnish redeposits or softens and moves.
5. Varnish on thrust pad faces
The thrust pad face is where hydrodynamic film formation is critical.
In a properly operating thrust bearing, the pad tilts slightly and forms a wedge-shaped oil film. This wedge carries the axial load without metal-to-metal contact.
When varnish forms on pad faces, the following can happen:
The pad surface loses its designed smoothness.
The oil wedge becomes less predictable.
The minimum oil film thickness can reduce.
The bearing may run hotter.
The pad may show wiping, polishing, discoloration, or localized distress.
Varnish does not need to be thick to cause trouble. A thin, sticky, brown or amber film can be enough to interfere with the thermal and hydrodynamic behavior of the pad.
A major practical issue is that varnish may not deposit evenly. It often deposits in patterns related to oil flow, pad temperature, load zone, and residence time. Therefore, one pad may look much worse than another.
In double thrust arrangements, this can create confusion because the active thrust side and inactive thrust side may show different deposit patterns depending on machine operating conditions, thrust direction, load changes, and axial movement.
6. Varnish in oil supply passages
The drawing shows oil supply entering from above and being directed to the bearing areas.
These supply passages are critical because directed lubrication depends on accurate oil delivery. If varnish deposits inside these passages, the problem is not only contamination; it becomes a hydraulic restriction.
Possible effects include:
Reduced oil flow to one side of the bearing.
Uneven flow distribution between pads.
Delayed oil supply during start-up.
Increased oil temperature at the pad outlet.
Localized overheating of one thrust face.
False confidence from normal header pressure.
This last point is important.
The main oil header pressure may look acceptable, but the actual oil distribution inside the thrust bearing may be poor. Small restrictions at nozzles, orifices, grooves, or feed holes can create serious local starvation while the main system instruments still look normal.
This is why varnish in directed lubrication systems is dangerous. Directed lubrication is efficient, but it is also more sensitive to small restrictions than a simple flooded system.
7. Varnish in drain holes
The left side of the drawing mentions drain holes at the bottom and says the total drain area must keep flow velocity below 0.6 m/s.
This is one of the most important parts of the image.
The drainage system is designed to let oil escape without backing up into the collar area. If varnish forms around drain holes, the effective drain area reduces.
When the drain area reduces, the oil level inside the bearing cavity may rise. Then bulk oil may contact the rotating collar, which is exactly what the design was trying to prevent.
Once the collar starts contacting bulk oil, the following can occur:
Oil churning increases.
Bearing temperature increases.
Air entrainment increases.
Foam tendency increases.
Oxidation accelerates.
The oil spends more time in hot zones.
Varnish generation accelerates.
Drain restriction is one of the most underestimated varnish-related problems in thrust bearing housings.
A small amount of deposit around drain holes can change the oil behavior dramatically because drainage depends strongly on open area, geometry, and flow path cleanliness.
8. Varnish in peripheral drain ports
The right-side arrangement shows drain ports equally spaced around the collar periphery, with total area designed for velocity less than 0.15 m/s.
This lower velocity requirement indicates that the design is trying to create very gentle oil removal around the collar, avoiding turbulence and recirculation.
When varnish forms in these peripheral drain ports, several problems may appear:
The flow becomes uneven around the collar.
Some areas drain well, while others retain oil.
Oil can recirculate near the collar.
Local hot oil pockets develop.
Air bubbles may remain trapped longer.
Oxidation products deposit faster in stagnant regions.
This can create an asymmetric thermal condition around the thrust bearing. One thermocouple may show higher temperature than another, or temperature may fluctuate with load and speed.
In practice, these peripheral ports can become partially coated internally. From outside, the bearing may look normal, but inside the drainage geometry may no longer match the original design intent.
9. Varnish in lower stagnant zones
In the left figure, the lower half of the housing is especially important because it includes drain holes and return paths.
Varnish likes areas with:
Low flow velocity
Long oil residence time
Cooler surfaces where insoluble material can precipitate
Hot/cold cycling
Dead legs and pockets
Poor flushing action
The lower housing areas can become varnish traps. Deposits may collect, soften when the oil gets hot, then move downstream. This can cause intermittent contamination events.
This is why some machines show unstable MPC or patch results. The oil sample may look better or worse depending on whether deposits are currently dissolved, suspended, or sitting quietly in the housing.
For turbine oils, this is one reason why hot, representative sampling is so important. A cold sample may miss soluble varnish precursors or misrepresent the real deposit risk inside the bearing housing.
10. Varnish around pad pivots and leveling mechanisms
Many thrust bearings use tilting pads. The pad must tilt freely to create the correct hydrodynamic wedge.
If varnish deposits around pivots, supports, leveling plates, or equalizing links, the pad may not move freely.
This is a serious mechanical reliability issue.
A pad that cannot tilt correctly may develop:
Uneven load sharing
Higher metal temperature
Poor oil wedge formation
Edge loading
Localized wiping
Increased vibration or axial instability
Abnormal thrust bearing temperature response
This is where varnish becomes more than an oil cleanliness problem. It becomes a mechanical freedom-of-movement problem.
A thrust bearing can have enough oil flow and acceptable oil pressure, but still operate poorly if the tilting mechanism is sticky because of varnish.
11. Varnish and heat transfer
Every area in the drawing is connected to heat removal.
The oil enters, absorbs heat, and must leave. Varnish interferes with this in three ways:
First, it reduces heat transfer from metal surfaces to oil.
Second, it restricts oil flow and drainage.
Third, it increases oil churning and residence time.
The result is higher local oil temperature.
Higher oil temperature accelerates oxidation. Oxidation creates more polar degradation products. These products form more varnish. More varnish creates more heat.
This is the classic varnish loop:
Varnish → restricted flow/drainage → higher temperature → faster oxidation → more varnish
In thrust bearings, this loop can become very aggressive because the collar and pads operate in a high-shear environment.
12. Why drain velocity matters
The drawing gives specific drain velocity guidance. This is not a random design note.
High drain velocity can cause turbulence, splashing, air entrainment, and poor separation. Low, controlled velocity helps oil leave the bearing area smoothly.
When varnish reduces the drain area, the same oil flow must pass through a smaller opening.
That means velocity increases.
So even if the pump flow has not changed, varnish can cause the drain velocity to exceed the design intent.
For example:
Original drain area is clean.
Oil drains smoothly.
Varnish reduces effective drain area by 30–50%.
Velocity through the remaining open area increases.
Oil backs up or becomes turbulent.
The collar starts contacting more bulk oil.
Temperature rises.
Varnish accelerates.
This is why drain holes and ports should not be treated as simple “holes.” They are functional parts of the bearing lubrication design.
13. What symptoms may appear in operation?
When varnish affects the areas shown in the drawing, the plant may observe several symptoms.
Typical symptoms include:
Increasing thrust bearing metal temperature
Higher oil outlet temperature
Temperature instability or saw-tooth behavior
Higher bearing drain temperature
Foaming or air entrainment in the reservoir
Dark deposits in bearing drains or inspection covers
Sticky brown deposits on pads or collar
Slow temperature recovery after load changes
Different temperatures between active and inactive thrust sides
Abnormal axial position behavior
More frequent filter plugging
MPC increasing even when particle count looks acceptable
RULER antioxidant depletion accelerating
TAN slowly increasing or becoming unstable
The important point is that varnish symptoms may appear mechanical before they appear chemical.
The machine may first complain through temperature, axial behavior, or bearing distress before the oil analysis looks catastrophic.
14. What oil analysis can reveal this problem?
For this type of bearing arrangement, particle count alone is not enough.
Particle count measures hard and soft particles above certain sizes, but varnish risk is often related to soluble and submicron oxidation products.
A strong turbine oil varnish investigation should include:
MPC by ASTM D7843
This indicates varnish potential and should include patch color, not only the number.
RULER by ASTM D6971
This shows antioxidant depletion, especially phenolic and aminic antioxidant health.
RPVOT by ASTM D2272
This gives oxidation stability reserve but should not be used alone.
TAN by ASTM D664
This tracks acidic degradation products.
Water content by Karl Fischer
Water accelerates additive depletion, corrosion risk, and oil degradation.
FTIR
Useful for oxidation, additive trends, and contamination patterns.
Patch membrane inspection
Very useful when combined with microscopy, SEM/EDS if inorganic contamination is suspected.
Oil color and visual inspection
Not a varnish test by itself, but useful when trended with other data.
For the bearing in this image, I would never rely only on ISO particle count and TAN. The real risk may be hidden in MPC, RULER, and deposit inspection.
15. What should be inspected during shutdown?
During inspection of this type of double thrust bearing, the following areas should be checked carefully:
Thrust collar surface condition
Pad face cleanliness and wiping marks
Pad edge deposits
Pad pivot freedom
Equalizing mechanism movement
Oil feed holes and nozzles
Oil grooves
Drain holes
Peripheral drain ports
Lower housing pockets
Return oil path
Evidence of oil pooling around the collar
Dark or sticky deposits in low-flow areas
Any burnt, lacquer-like, amber, brown, or black deposits should be treated seriously.
A key mistake is cleaning the bearing and closing the machine without asking:
Why did deposits form here?
Cleaning removes the evidence, but it does not remove the root cause.
16. Why varnish is especially dangerous in directed lubrication systems
Directed lubrication systems are efficient because they deliver oil only where needed. But this also means they rely heavily on small passages, correct geometry, and clean flow paths.
In a flooded system, small deposit restrictions may be less obvious at first.
In a directed lubrication system, a small restriction can cause immediate maldistribution.
That is why varnish in directed lubrication systems can create:
High sensitivity to deposit thickness
Uneven oil delivery
Local hot spots
Restricted drainage
Thermal instability
False normal readings from main oil pressure
Directed lubrication gives better efficiency, but it demands better oil chemistry management.
17. Practical root causes of varnish in these areas
Varnish formation around thrust bearings is usually not caused by one single factor. It is normally a combination of oil stress, operating condition, and system design.
Common root causes include:
High oil temperature
Long oil residence time
Hot spots in bearing areas
Poor reservoir turnover
Air entrainment
Foaming
Water contamination
Antioxidant depletion
Wrong oil top-up
Mixing incompatible oils
Electrostatic discharge in filters
Microdieseling in hydraulic control circuits
Poor varnish removal strategy
Long operation with degraded oil
Insufficient oil analysis frequency
Poor flushing after previous degradation events
In thrust bearings, drainage and churning must be added to this list. Poor drainage can create the thermal stress that accelerates varnish formation.
18. What happens if varnish is ignored?
If varnish continues to grow in the areas shown in the image, the consequences can become severe.
The first stage is usually higher operating temperature.
The second stage is unstable temperature or increasing alarm frequency.
The third stage is reduced bearing safety margin.
The fourth stage may be thrust pad distress, wiping, or collar damage.
The final stage can be forced outage, thrust bearing failure, axial position trip, or major turbomachinery damage.
In critical machines, thrust bearing distress is not a small lubrication issue. It can threaten the whole rotor train.
19. Corrective actions
The response should be both mechanical and chemical.
Mechanical actions
Inspect and clean all drain holes and drain ports.
Confirm that the total drain area is restored.
Check whether oil is pooling around the collar.
Inspect pad movement and pivot freedom.
Verify oil feed nozzles and passages.
Check bearing metal temperatures before and after cleaning.
Confirm oil flow distribution.
Inspect reservoir return flow and deaeration behavior.
Check for foaming and air release problems.
Oil analysis actions
Run MPC, RULER, RPVOT, TAN, Karl Fischer water, FTIR, particle count, and membrane patch inspection.
Trend results instead of judging one sample.
Compare active and historical data.
Sample hot and from a representative live zone.
Avoid relying on reservoir bottom samples only.
Lubricant chemistry actions
Remove soluble and insoluble varnish precursors.
Do not only remove particles.
Do not use chemical additives that only dissolve deposits back into the oil without removing degradation products.
Control water and air ingress.
Restore antioxidant stability if the oil is still recoverable.
Replace oil only when oil chemistry and deposit condition justify it.
20. Why varnish removal must target soluble varnish too
One of the biggest mistakes in varnish control is thinking that varnish is only a solid contaminant.
In turbine oils, many varnish precursors are soluble when the oil is hot. They may not appear as visible particles. When the oil cools or when the saturation limit is exceeded, they can precipitate and deposit on metal surfaces.
That means a normal particle count does not prove the oil is safe from varnish.
For the bearing arrangement shown in the image, soluble varnish precursors are dangerous because they can travel through the oil system and deposit exactly in hot, low-flow, high-shear, or polar surface areas.
Therefore, the treatment method must address the oil chemistry, not just the particle count.
21. Khash practical interpretation of the image
This drawing is not only a bearing design figure. It is a warning.
The designer is saying:
Do not flood the thrust collar.
Do not allow oil to remain trapped.
Do not allow high drain velocity.
Do not let oil churn around the collar.
Remove the oil after it has done its job.
When varnish appears, it attacks exactly these design intentions.
It restricts the passages.
It reduces the drain area.
It makes pads sticky.
It insulates hot surfaces.
It increases oil residence time.
It changes the thermal behavior of the thrust bearing.
So, when we see varnish in a thrust bearing housing, we should not only say, “The oil is dirty.”
We should say:
The lubrication geometry is being changed by deposits.
That is the real danger.
22. Final message
In double thrust bearing arrangements with directed lubrication, varnish is dangerous because it does not need to cause full blockage to create failure risk.
A thin layer of varnish can reduce heat transfer.
A small deposit can disturb oil distribution.
A partially restricted drain hole can increase oil level around the collar.
A sticky pivot can prevent correct pad tilting.
A varnished drain path can increase oil residence time and oxidation.
This is why varnish in turbomachinery thrust bearings must be treated as a system reliability problem, not only an oil cleanliness problem.
The image teaches a powerful reliability principle:
Oil supply is only half of lubrication. Controlled oil removal is the other half.
And when varnish blocks, coats, or changes these areas, the bearing may still receive oil — but it may no longer receive lubrication.
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