The Perils of Ignoring Low or Fluctuating Oil Levels

Jim Fitch, Noria Corporation

I’ve seen people shrug off the importance of fluctuating oil levels when they inspect sight glasses.  After giving a lot of thought to this phenomenon plus plenty of research, I’ve concluded that it’s a very bad practice for many reasons.  Those reasons are the subject of my article. My hope is that I can open your eyes to the need to investigate what’s behind oil level changes and of course take corrective action when warranted (most of the time). 

A sight glass is an information portal with a purpose. Through it, the oil and the machine can telegraph critical messages related to changing operating conditions. These messages, when seen and understood, offer forewarnings of adverse future events including catastrophic machine failure.

To you, the inspector or operator, these messages present an opportunity to impede the progress of failure. Too often people instead opt to postpone or dismiss the need for action including troubleshooting and fixing the offending condition. True, you may not be facing a sudden death concern but how do you really know? There are consequences to deferred maintenance.

Why allow ignorance and inaction rob your oil and machine components of thousands of hours of service life? Proactive maintenance means fixing the roof when the sun is shining. Not rushing around looking for drip buckets to put under leaks during the deluge of a rainstorm.

A quick glance at this sight glass might be interpreted as nothing of concern. If the aeration and foam was allowed to dissipate and release out of the oil, the actual oil level would be well below the crosshairs of the gauge. Air and foam can significantly distort reality to produce an apparent oil level that is well above the actual level. Many machines that are lubricated by slingers, flingers, oil rings, collars, oil dip, and similar devices can be significantly affected by minor changes in oil level. An astute inspector will understand this and top up the sump before moderate to severe starvation occurs.

Where Is the Risk the Highest?

Many accelerated life tests (ALT) have been conducted by researchers on the effects impaired lubrication conditions have on machine service life, especially bearings and gears. Ultrasound, bearing metal temperature and wear debris analysis have been used to track and gauge the onset of partial or full starvation conditions. Your ears alone can pick up that distinct metallic sound from mechanical friction that is transmitted when oil levels go down.

The image below illustrates how impaired oil film thickness from partial oil starvation causes mechanical two-body abrasion and adhesion. The same thing occurs due to misalignment, low oil viscosity, starts and stops. When surfaces rub and asperities collide, material is excavated, and particles are released to the oil (wear debris). A bearing for instance, will only allow so much wear debris to be released before a changeout is required. The same is true for gears, hydraulic pumps, pistons, and cams. The more friction, the more wear the shorter the bearing life — similar to the tread on your tires.

Harm Done to Gearing

Splash lubricated gearing requires deeper gear tooth immersion depths for higher speeds and loads. This translates to greater friction and wear penalty when oil levels drift down due to misting and leakage. This will show up as radial-direction scuffing/scoring marks on the addendum and dedendum surfaces of the gear teeth. 

Many of the shaft bearings on these gearboxes are fed oil by splash and/or slinger action. Some use paddles gears. Oil travels down grooves or troughs to the bearing cavities. The oil flow rate depends on the splash depth of the gears and slingers. Meaning, when the oil level is low so is the splash feed rate. Both the frictional surfaces of the gears and the bearings can be sharply affected.

Harm Done to Bearings

In rolling element bearings, partial starvation can reduce oil film thickness by 50% or more. This means the oil feed rate to the load zone is slowed or restricted. The bearings run hotter due to resulting friction and lack of heat transfer to the oil. The mechanical rubbing and friction pushes antiwear and anti-scuff additives into action, assuming they are in the lubricant’s formulation. These additives are sacrificial, meaning they die as they try to suppress abrasive and adhesive wear. The more pronounced the starvation condition the faster they are consumed.

It's all about film thickness. If there is clearance between surfaces in relative motion, anti-scuff additives aren’t consumed by the process. Heat generation is diminished and the heat that does occur is washed out by the healthy supply of oil flowing through the bearing. We all know that heat can rapidly reduce the life of our oil’s additives and basestock. The impact that film thickness has on bearing wear and service life is shown in the data plot of the figure below.

Churning of Reservoirs and Sumps

High flow rate hydraulic and circulating oil systems require large reservoirs to dampen and diffuse the oil movement from tank returns. This prevents excessive aeration caused by vortex oil movement and lapping. Lapping refers to air being tucked into the body of the oil due to turbid oil surface conditions. 

Low tank oil levels increase the churning and lapping that leads to entrained air. Correct oil levels reduce air entrainment and enables buoyant air bubbles to rise to the surface and dissipate from the oil. Conversely, air that remains entrained due to churning low oil levels can sharply reduce the oxidative life of the oil and its additives.

This aeration can compound starvation conditions further (see the diagram below). Further, aerated oil can induce cavitation in pumps and bearings as well as varnish from microdieseling, etc.

Similar issues occur in smaller wet-sump bearings and gears. Low oil levels not only churn air into the oil but also concentrate the heat in the remaining smaller volume. The air acts like an insulator impairing the dissipation of heat outward to the walls of the machine, oil reservoir or cooler. The amount of heat transfer is a function of the flow rate. Starvation slows flow and heat transfer. This increases thermal stress on the oil and the machine.

The condition goes viral as the elevated heat spurs aeration and foam of the oil. See the chart in the figure below that shows the relationship between rising temperature (induced by low oil level) and foam tendency.

Oil Level and Energy Consumption

Anything that increases oil and bearing temperature will, by default, increase energy consumption.  It is that simple. This includes too much oil as well as too much viscosity, both lead to churning energy losses.

As previously mentioned, too little oil level can reduce film thickness causing mechanical friction, heat and energy consumption. Getting the target oil level right and maintaining good oil level control is a smart strategy to reduce energy consumption. The benefit is instant.

Low Oil Level Increases Oil Consumption

You may wonder why oil level has anything to do will oil consumption. It’s a good question. There are several reasons. 

  • Low oil level causes more turbulence and more oil mist. Mist in gear boxes and bearings is a major cause of unnecessary oil consumption. Oil is lost to the ambient air.
  • Low oil level concentrates heat, catalytic metal particles and other contaminants. These conditions put stress on critical oil additives like oxidation inhibitors, leading to premature base oil oxidation. These same conditions (particles, heat, oxidized oil) will distress and shorten the life of seals and packings, leading to oil leakage.
  • Low oil level means low additive reserve needed to suppress oil degradation. This leads to shorter oil life. Conversely, when oil tops ups are routinely performed, additives become refreshed and contaminants become diluted. It’s like a booster shot. 

Don’t make light of all the other costs associated with frequent oil changes. Some studies have stated that the real cost of an oil change can exceed 40 times of the cost of the oil itself. Compounding this are the disturbance risk from oil changes including the fishbowl effect.

Distortions of True Oil Level

Bullseye sight glasses can play tricks on you. Pay close attention to ensure that you are viewing the true oil level. It is not uncommon for sight glasses to become stained or fouled, thus giving an illusion of oil at the correct level.

This is especially true if you are too far from the sight glass with poor lighting. Such a false reading can be catastrophic.

In the case of a columnar level gauge, remove the clear tube and clean it using an appropriate cleaner. If the stain is persistent, replace it. Plugged vents in columnar sight glasses can also lead to faulty oil level readings.

What do Changes in Oil Level Mean?

A sudden change in oil level, either up or down, is a telegraphed alert that something is wrong. And, this “something” could potentially be serious. The frequent examination of quality oil level sight glasses by trained inspectors is a sound condition monitoring practice. Perhaps the most important of all lubrication-related inspections.

Oil Too High:

When oil levels rise above the acceptable range usually something new has been added.  But there are other options too:

  • Too much makeup fluid. Adding makeup fluid without carefully watching sight glasses can cause over-lubrication.
  • Oil drain back. If fluid is topped off while the machine is running, the oil level can climb when it stops and oil drains back from gears, bearings, oil galleries and distant zones or oil ways.
  • Aeration and foam. Such conditions can double or triple the apparent oil level.
  • Internal leakage. Various sources of internal leakage can cause other nearby fluids to invade the sump. These include coolant, washdown fluids, fuel, heat transfer fluid, hydraulic fluid, grease and process fluids. Oil analysis can identify these invading fluids.

Oil Too Low:

This is usually caused by leakage, but there are other reasons.  Here’s a short list.

  • Out-leakage. This is an alert to examine the machine for any visual sign of oil leakage to external surfaces (oil exiting the machine).
  • Internal leakage. If no out-leakage is observed, are there other internal pathways and compartments where the oil might have gone? Look for rising fluid levels in these zones and compartments.
  • Gear climbing. Oil lifters, including paddle gears, slingers/flingers and the rotation of moving parts (gearing in particular) all draw oil out of sumps and reservoirs and lower the apparent oil level during machine operation.
  • Oil pump out. After startup, oil reservoirs can go down as the pump fills system lines and cavities such as gear cases, bearings, oil galleries and distant oil ways.
  • Bleed purge. Hydraulic and circulating systems often have bleed valves that when opened allow trapped pockets of air to purge and become replaced by oil. This draws the oil level down the reservoir.
  • Aeration and foam. Foam, in particular, lowers the liquid portion of level gauges, sometimes substantially. When the foam collapses (e.g., when the machine is at rest) the correct level should return unless foam was pushed out of vents and other headspace openings.
  • Excessive misting and volatilization. This is a form of out-leakage from various causes, such as the wrong oil (e.g., wrong viscosity), high temperature, excessive agitation, headspace vacuum, atomization/sprays or aeration.

Oil Too High or Too Low:

The following conditions can cause the oil level to appear to be too high or too low.

  • Air currents. Strong air currents moving across vents and headspace openings can alter the oil level in the gauges. When the air currents stop the oil level returns to normal.
  • Mechanical agitation. High speed mechanical agitation, including rotation, can push oil towards or away from the level gauge ports.
  • Temperature. Hot oil and machine temperatures cause thermal expansion of lines, reservoirs and galleries. Extreme cold conditions do just the opposite.
  • Cylinder actuation. Both single-acting and double-acting hydraulic cylinders change the oil level in the tank up or down depending the direction of actuation.
  • Reservoir pressure. Some reservoirs have positive headspace pressure (nitrogen or instrument air for instance) and others have negative pressure (extraction fans for instance). This can move the oil level slightly.
  • Sump or machine level. If the sump or the machine is off level, perhaps on a slope, this will affect the apparent oil level of the sight glass. This is common with mobile equipment.

Can this level gauge see foam? Emulsified water? Oxidized oil? Correct oil level? Any concerns about the installation hardware? Ventilation? Sediment build-up between the elbow and the sump? Correct oil level?

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About the Author

Jim Fitch, a founder and CEO of Noria Corporation, has a wealth of experience in lubrication, oil analysis, and machinery failure investigations. He has advise...