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Varnish: Does Your Hydraulic System Need a Cardiologist?
The Curious Similarity Between Arterial Plaque and Hydraulic Varnish
Article by William Gillette (LogiLube, LLC)
An admittedly tongue-in-cheek addition to my hydraulic varnish series.
After spending far too much time thinking about hydraulic oil varnish and reflecting on my own experience with cardiovascular bypass surgery 15 years ago, I recently had an uncomfortable realization:
A hydraulic system may have more in common with the human cardiovascular system than most reliability engineers would care to admit.
Think about it.
A human circulatory system has a pump, a fluid, a network of arteries and smaller passages, control mechanisms, filtration and chemical processes that keep everything functioning.
A hydraulic system has…well…pretty much the same architecture.
The heart becomes the hydraulic pump.
Blood becomes hydraulic oil.
Arteries become hoses and tubing.
Capillaries become tiny valve clearances.
Kidneys and the liver provide at least a loose analogy to filtration and contaminant removal.
And unfortunately, both systems can suffer from deposits accumulating where you really don't want them.
In humans, we worry about atherosclerotic plaque.
In hydraulic systems, we worry about varnish.
Before the cardiologists start sending me angry emails, let's acknowledge that the chemistry and biology are obviously very different. Atherosclerosis is an extraordinarily complex inflammatory disease involving lipids, immune cells, arterial walls and many other biological mechanisms. Hydraulic varnish is the product of lubricant degradation, oxidation products and insoluble contaminants.
This is an engineering analogy—not medical guidance.
But from a maintenance perspective, the resemblance is fascinating.
The problem often starts in the fluid
Healthy blood must maintain an extraordinarily controlled chemical environment.
So must hydraulic oil.
Fresh hydraulic oil contains a carefully formulated package of base oils and additives intended to provide oxidation resistance, anti-wear protection, corrosion protection, viscosity control and other properties.
But hydraulic oil doesn't live an easy life.
It is repeatedly subjected to:
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elevated temperatures,
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localized hot spots,
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high shear forces,
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entrained air,
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water contamination,
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catalytic metals,
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electrostatic discharge,
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particulate contamination,
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and thousands of hours of continuous circulation.
Over time, the lubricant begins to degrade.
Oxidation reactions generate increasingly polar degradation products. Some initially remain dissolved or suspended in the oil. Eventually, however, the lubricant's ability to keep these degradation products in solution can be exceeded.
That's when things become interesting.
And expensive.
Soft contaminants begin depositing on wetted surfaces.
Welcome to varnish.
Your servo valve may be the hydraulic equivalent of a coronary artery
The largest hydraulic hose generally isn't where varnish causes the greatest operational problem.
The real trouble occurs in tight-clearance components.
Servo valves.
Proportional valves.
Spool valves.
Pump controls.
Bearings.
Small lubrication passages.
Anywhere that tiny changes in clearances can produce significant changes in machine behavior.
This is where the cardiovascular analogy becomes particularly useful.
A relatively small restriction in the wrong place can create a disproportionately large problem.
Varnish deposits can cause valves to become sluggish or sticky. Valve hysteresis may increase. Spools may fail to return properly. Response times can change. Pumps may operate less efficiently.
Eventually the maintenance organization may receive that universally helpful failure report:
"Hydraulics acting weird."
At which point someone changes the valve.
The machine returns to service.
Everyone congratulates themselves.
Six months later, another valve begins sticking.
Why?
Because replacing the valve treated the symptom.
The contaminated lubricant environment remained unchanged.
The new valve was installed directly into the same circulating reservoir of degradation products that damaged the old one.
Imagine replacing a section of plumbing without asking what is circulating through the plumbing.
Perhaps your hydraulic oil needs a cholesterol test
Medicine has learned an important lesson about cardiovascular risk: waiting for catastrophic symptoms isn't a particularly elegant monitoring strategy.
Nobody says:
"Let's wait until the coronary artery is completely blocked and then we'll check the blood."
Yet industrial maintenance programs occasionally use essentially this strategy with lubrication systems.
A machine runs normally.
Then a valve starts sticking.
Then a pump behaves strangely.
Then filters begin plugging.
Then maintenance pulls apart a component and discovers a brown lacquer-like deposit.
Only then does someone ask:
"Could this oil be varnishing?"
Perhaps we should turn the diagnostic process around.
Instead of waiting for deposits, monitor the conditions that precede deposit formation.
That means looking beyond traditional elemental wear analysis alone.
Depending on the application, varnish-management programs may consider tools such as:
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Membrane Patch Colorimetry (MPC),
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antioxidant monitoring,
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acid number,
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viscosity,
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dielectric properties,
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oxidation indicators,
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particle contamination,
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water contamination,
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oil temperature,
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and changes in operating conditions.
The objective isn't simply determining whether the lubricant is "good" or "bad."
The objective is identifying trajectory.
Is the fluid becoming progressively less stable?
Is oxidation accelerating?
Are insoluble degradation products increasing?
Are operating temperatures creeping upward?
Is contamination increasing?
In other words:
Is this hydraulic system developing high cholesterol?
Metaphorically speaking, of course.
Hydraulic systems have lifestyle problems too
Doctors spend a lot of time discussing risk factors.
Hydraulic systems have them as well.
Excessive temperature is the industrial equivalent of a terrible lifestyle.
Every sustained increase in lubricant temperature accelerates chemical reactions and can shorten lubricant life.
Air entrainment introduces another set of problems.
Water contamination introduces another.
High shear creates another.
Poor reservoir design can make matters worse.
Electrostatic discharge can create localized oil degradation.
And contamination provides surfaces and catalysts that can accelerate undesirable chemistry.
Then there is the lubricant's antioxidant package.
These additives are essentially sacrificial chemistry. Their job is to interrupt oxidation processes and protect the base oil.
But they don't live forever.
As operating hours accumulate, antioxidant reserves become depleted.
Eventually, the lubricant becomes increasingly vulnerable to oxidation.
And once degradation products begin accumulating, the system can enter a self-reinforcing cycle.
Deposits increase.
Heat transfer can deteriorate.
Valves begin sticking.
Operating efficiency declines.
Maintenance interventions increase.
More fresh oil may be added.
But unless the root condition is addressed, the underlying problem remains.
Don't wait for the hydraulic heart attack
Perhaps the most useful lesson from the cardiovascular analogy isn't chemical at all.
It is philosophical.
Prevention is cheaper than intervention.
The best time to discover a varnish problem isn't when a servo valve sticks during production.
It is months earlier when the oil begins showing a measurable change in oxidation stability, contamination, temperature behavior or insoluble formation.
That is where modern condition monitoring becomes particularly interesting.
Traditional oil analysis provides enormously valuable laboratory information.
But periodic sampling gives us snapshots.
Increasingly, inline sensors, automated sampling and edge analytics allow us to watch the lubricant continuously—and automatically collect laboratory samples when the machine detects something unusual.
Instead of asking:
"What did the oil look like when we sampled it last month?"
We can begin asking:
"What is happening to the oil right now?"
That is a fundamentally different maintenance philosophy.
Maybe machines need preventative medicine
Nobody would seriously suggest that a hydraulic power unit needs a cardiologist.
Although after seeing the inside of some badly varnished valve bodies, I'm not completely ruling it out.
But the underlying lesson is useful.
A healthy hydraulic system depends on maintaining the health of its circulating fluid.
And just as cardiovascular problems often develop long before dramatic symptoms appear, hydraulic varnish can develop quietly inside a machine long before it causes a recognizable failure.
By the time the valve sticks, the real problem may have been developing for thousands of operating hours.
So perhaps the next time someone opens a hydraulic valve and finds that familiar amber-brown coating, don't just ask:
"Why did this valve fail?"
Ask the more important question:
"What has been happening inside this machine's circulatory system?"
Because sometimes the hydraulic system isn't suffering from a bad valve.
It's suffering from bad blood chemistry.
And unfortunately, there isn't a statin for that.
At least not yet.
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