Tagged: used oil analysis

Determining the Root Causes of Oxidation in Lubricants

Finally, we’ve arrived at the point where we can effectively determine the root cause. It is critical that the analyst understands oxidation and has knowledge of the evidence needed before embarking on the root cause journey. As noted in the first part of this article, the question we should ask is, “How could?”.

We hypothesize that oxidation is occurring. In a complete root cause analysis, we should hypothesize the occurrence of all the degradation mechanisms and eliminate them with evidence-based data.

There are two main ways in which oxidation can occur either through the presence of oxygen and temperature over the normal operating temperature of the system or if there is a less-than-adequate presence of antioxidants.

If we follow our line of questioning with the presence of oxygen and temperature and again ask, “How could?” we can get two primary responses. Either there was an air leak in the system, or the system was being pushed beyond its operating limits.

If we further investigate the air leak into the system, we ask, “How could it?” again. There are two main ways: either there are damaged components, or a less-than-adequate system design allows air to enter the system.

If we follow the pathway of investigating “how could” the system be pushed beyond its operating limits, then we can come up with two hypotheses. Either an increase in production was required, or there was a malfunction of the components, which caused strain on the other components.

Both of these hypotheses are physical and can be investigated further, but we will focus on the lubricant aspect of this article. Hence, we will follow the questioning surrounding the less-than-adequate presence of antioxidants.

We begin with the question, “How could we have a less-than-adequate presence of antioxidants?”. From the information gathered in this article, we know this can result from free radicals or less than adequate lubricant specifications.

We will investigate the “Presence of free radicals” first.

“How could we have the presence of free radicals?” Free radicals can emerge as a result of chemical reactions.

“How could these chemical reactions produce free radicals?” There are two main ways in which this can occur. Either the lubricant got contaminated, which introduced catalysts for these chemical reactions, or adverse operating conditions gave rise to these chemical reactions.

Then, we must ask again, “How could we have contamination?” Contamination can occur if leaks are getting into a closed lubrication system or if there is ingress of foreign material from the environment.

Our line of questioning continues when we ask, “How could we have leaks in a closed lubrication system?”. These can result from damaged components or seals allowing leaks into the system or if the system is less than adequately designed.

These are physical attributes of the system, so we will go back to investigating the lubricant aspect.

This is where we get to ask our famous question, “How could we have ingress of foreign material from the environment?”. Ideally, this can be classified in three ways;

There are openings which are allowing materials to enter the system or
Wrong lubricant was placed in the system or
Contaminated lubricant was placed in the system

Let’s investigate all three aspects, starting with the openings allowing foreign material to enter the system. There are two main ways in which this can occur. Either the openings were not closed after use, or the safety latches malfunctioned.

Suppose the openings were not closed after use. In that case, there is a possibility that there were less than adequate inspections to verify that these were closed after use or a less than adequate procedure for the task being completed which required the opening of the hatch.

On the other hand, if the safety latches malfunctioned, this could result from less than adequate checks to verify the functioning of the safety latches.

In these cases, the root causes are not the physical elements but rather the systemic reasons for these procedures not being adequately performed.

Now we investigate the second central hypothesis, “How could the wrong lubricant be placed in the system?” While there are many ways in which this can occur, we have narrowed it down to two main areas.

Either there were less than adequate checks to verify that the technician received the correct lubricant, or there were less than adequate procedures to dispatch the correct lubricant from the warehouse. We will not go further into these two as they are now systemic causes that must be addressed.

Onto the third hypothesis of “How could a contaminated lubricant be placed in the system?”. There are two main avenues for this to occur. Either there were improper storage and handling procedures, or there needed to be more adequate procedures to verify the cleanliness of the lubricant before entering the system.

The other hypothesis stemming from the “less than the adequate presence of antioxidants” is having “less than adequate lubricant specifications.” Let’s investigate this one a bit further.

“How could we have a less than adequate lubricant specification?” Typically, this can result from the lubricant not being blended properly or less than adequate antioxidant levels, which were inappropriate to protect the lubricant.

Now, the line of questioning changes to “Why?” as we have gone past the physical element and some decision-making was involved in this hypothesis. We must ask, “Why wasn’t the lubricant blended properly?”

This can result from less than adequate procedures to ensure the quality of the lubricant by the supplier or less than sufficient checks for the proper blending mix being processed.

These are factors one should consider when receiving any lubricant from their supplier.

On the other hand, if we follow the line of questioning of “How could there be a less than adequate antioxidant level to protect the lubricant?” we can come up with the following.

Either the operating environment caused the antioxidants to be depleted at a higher rate. This would be as a result of a harsh but normal operating environment. In this case, we may be unable to make those environmental changes (without the OEM’s consent).

Or the antioxidants used were not suited to the operating conditions. This is where the line of questioning again shifts to “Why were they not suited?”. This could result from inadequate information in choosing the right lubricant suited for the system.

What Is the Real Root Cause of Oxidation?

From the logic tree that we have created, we can see that there is no sole root cause for oxidation. It can stem from various causes, including physical, human, and even systemic roots. The main takeaway from this exercise is to acknowledge that root causes are not limited to physical causes, such as leaks in the system.

Instead, the actual root causes can be linked to systemic areas of concern where there may not have been enough information to guide the analyst in choosing the most ideally suited lubricant for the application. There are also root causes related to the lubricant not being appropriately blended.

It is critical to thoroughly investigate the real root causes when the lubricant becomes degraded to avoid being stuck in the loop of constantly experiencing degradation.

For more info on other methods, check out the book Bob Latino, and I authored called “Lubrication Degradation – Getting Into the Root Causes,” published by CRC Press.

References:

Ameye, Jo, Dave Wooton, and Greg Livingstone. 2015. Antioxidant Monitoring as Part of a Lubricant Diagnostics – A Luxury or Necessity. Rosenheim, Germany. February 2015.

Latino, Bob, Sanya Mathura. 2021. Lubrication Degradation – Getting into the Root Causes. CRC Press, Taylor & Francis.

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Lubricant Degradation

How do I know if Oxidation is occurring?

What Evidence is needed to prove that oxidation has occurred / is occurring?

However, understanding the oxidation process is just one part of the puzzle. When performing an investigation, we also need to know what factors or characteristics should be present. Additionally, we need to prove that their presence confirms our hypothesis of whether or not oxidation is occurring. This is where the line of evidence-based questioning plays a significant role.

When oxidation occurs, it is usual to see the presence of aldehydes, ketones, hydroperoxides, and even carboxylic acids. These can be confirmed using the FTIR (Fourier Transform Infrared test).

Typically, one will also find some deposits in the system. These deposits can be further characterized and tested to determine their nature using FTIR. Their presence, however, may be confirmed using the MPC (Membrane Path Colorimetry, ASTMD7843) test.

Identifying the presence of the deposits and/or the compounds listed above can lead to the conclusion that oxidation has occurred.

Another critical characteristic of oxidation is the depletion of antioxidants. This can be easily identified by utilizing the RULER® (Remaining Useful Life Evaluation Routine) test. This test quantifies the remaining antioxidants in the oil and gives the value for the amines and phenols (which is very important, especially in synergistic mixtures).

As such, one can detect the trend in the depletion of antioxidants and implement measures to prevent this before they become depleted.

The main tests to assist in determining the presence of Oxidation include:

RULER (Remaining Useful Life Evaluation Routine) levels less than 25% compared to new oil. This value represents the level of antioxidants in the oil. Hence, low levels indicate that the antioxidants are decreasing, possibly due to oxidation. This test can accurately give information on whether oxidation is currently occurring in the oil before deposits are formed.

An increase in acid number indicates the presence of acids resulting from oxidation. However, it must be noted that this change in acid number only occurs after oxidation has taken place. Hence this test is not a good indicator to determine if oxidation is occurring; instead, it is more definitive in letting us know that oxidation has already occurred.

Rapid color changes – darkening of the oil due to the deposits being present. While color is not the best indicator, in some instances, the darkening of the oil can provide a bystander to ask whether something is occurring in the oil. It is not a definitive test for the presence of oxidation.

FTIR test (Fourier Transform Infrared) for the presence of insolubles formed during the oxidation reaction. This can accurately determine the presence of any compound to assist us in determining whether oxidation is occurring.

MPC (Membrane Patch Colorimetry) levels outside the normal range (above 20). This lets us know that insoluble deposits are present in the oil. One must note that there may be instances where the deposits might not appear in the MPC test. As such, this should not be a standalone test to determine the presence of deposits.

RPVOT (Rotating Pressure Vessel Oxidation Test) levels are less than 25% compared to new oil (this is the warning limit). This is the industry standard, but this test does not have a high repeatability value in that if the same test were performed on identical samples, the values would be different. Additionally, the value (reported in minutes) is not easily translated into the environment of the components.

These tests provide us with the evidence we need to determine the presence of oxidation when performing the root cause analysis on the component’s failure.

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Lubricant Degradation

How Can Oxidation Occur in Lubricants?

Typically, when an oil undergoes degradation, the first culprit to be blamed is oxidation. We often hear that the oil has oxidized, producing varnish, leading to its degradation. While this simple statement may seem plausible, it is not the only way oil can degrade.

If an oil has undergone oxidation, the real question we should be asking is not how much varnish has been produced but what caused the oxidation in the first place?

In this article, we will explore the various ways in which an oil can degrade via oxidation. However, as you know from previous articles, other degradation methods exist.

How Can Oxidation Occur?

Before diving further into the root cause of oxidation, one must first fully understand how oxidation occurs. When truly investigating a root cause for a failure, we should start with the question “How could?” rather than “Why?”.

This line of questioning heavily influences the answers. The “How could?” responses stem from a more evidence-based approach.

On the contrary, if we question “Why?” this is more opinionated and can mislead the investigation towards a biased opinion rather than the facts.

This leads us to the main question, “How can oxidation occur?”.

According to Ameye, Wooton, and Livingstone, 2015, oxidation occurs when there is any reaction in which electrons are transferred from one molecule. Ideally, in oxidation, during the initiation phase, free radicals are formed, which in turn produce more free radicals.

A free radical is a molecular fragment with one or more unpaired electrons which are accessible and can easily react with other hydrocarbons, as explained by Ameye, Wooton, and Livingstone, 2015.

After the initiation phase, which has the free radicals, the propagation phase follows, in which the antioxidants react with these free radicals to make them more stable. This is part of the reaction in which there is usually a drastic depletion of antioxidants or where the oil becomes sacrificial.

The antioxidants act as a barrier to protect the base oil from oxidizing. However, they can no longer protect the base oil once they become depleted. This leads to the termination phase, where the remaining free radicals attack the base oil.

As a result, this gives rise to the condensation phase, where we begin to physically notice the changes in the oil’s viscosity and the presence of insoluble by-products. These are the deposits that are known are lube oil varnish to some but can further be defined by their chemical composition.

Understanding how oxidation occurs can assist us in determining the root cause when an oil degrades. It allows us to identify the different stages to further help us determine if it is indeed oxidation that is occurring or not.

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Lubricant Degradation

Which Degradation Mechanism Is Affected?

My previous article published in Precision Lubrication covered six degradation mechanisms: oxidation, thermal degradation, microdieseling, electrostatic spark discharge, additive depletion, and contamination.

Upon further investigation, there are only three mechanisms where selecting the correct lubricant will impact the degradation mode. These are; oxidation, microdieseling, and electrostatic spark discharge. The properties of the lubricant can easily influence each of these degradation mechanisms.

When selecting a lubricant, especially for rotating equipment, one of the critical areas of importance is the performance of the antioxidants. When formulated, oils must be balanced to protect the components in various aspects.

Thus, some oils that boast a high level of antioxidants may suffer from low levels of antiwear, or these increased levels can react with other components to reduce the performance of the oil. During oxidation, antioxidants are depleted at an accelerated rate which can lead to lube oil varnish. Hence, the choice of lubricant can influence this degradation mechanism.

A good trending test, in this case, would be the RULER test to accurately quantify and trend the remaining useful antioxidants for the oil. This test can easily distinguish and quantify the type of antioxidant rather than providing an estimate of the oxidation, as with the RPVOT test.

It has been noted that oils with an RPVOT of more than 1000 mins have a low reproducibility value which can mislead users during trending of lubricant degradation. Corrosion inhibitors, not just antioxidants, have also influenced the RPVOT values. Thus, there are better tests for monitoring the presence of antioxidants and helping operators to detect the onset of possible lube oil varnish.

On the other hand, during microdieseling, entrained air can lead to pitting the equipment’s internals and eventually the production of sludge or tars depending on whether the entrained air experiences a high or low implosion pressure.

If bubbles become entrained in the lubricant and do not rise to the surface, this can directly result from the lubricant’s antifoaming property. The antifoaming property is essential when selecting an oil, especially for gearboxes. Typically, OEMs will have recommendations for their components that should be followed.

Another degradation mechanism that can be influenced by lubricant selection is electrostatic spark discharge. This mechanism occurs when the lubricant accumulates static electricity after passing through tight clearances. These then discharge at the filters or other components inside the equipment, providing sharp points or ideal areas to allow static discharge.

This is frequently seen in hydraulic oils due to the very tight clearances within the equipment. If fluid conductivity is above 100 pS/m, the risk of static being produced is reduced. Some OEMs also provide particular values the lubricant should meet for this property.

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Oil Properties

How Do Lubricant Additives Work?

Each additive works differently to produce its function on the base oil and the overall finished lubricant. This section will explore how each of the lubricant additives works and some of the challenges they may experience.

Pour Point Depressants

As noted above, the pour point depressants help control the flow of the lubricant. This is achieved by modifying the wax crystals present in the lubricant’s base oil. At lower temperatures, the liquid usually has trouble being poured due to the presence of wax molecules in the base oil¹.

There are two main types of pour point depressants, namely;

Alkylaromatic polymers adsorb on the wax crystals as they form, thus preventing them from growing and adhering to each other. This effectively controls the crystallization process and ensures the lubricant can be poured.
Polymethacrylates co-crystallize with wax to prevent crystal growth.

While these additives do not entirely prevent wax crystal growth, they lower the temperature at which these rigid structures are formed. These additives can achieve a pour point depression of up to 28°C (50°F); however, the common range is typically between 11-17°C (20-30°F).

Solubility thresholds may limit the use of this type of additive to achieve the desired effect on the base oil.

VI Improvers

These additives are typically long-chain, high-molecular-weight polymers that change their configuration in the lubricant based on temperature⁴. When the lubricant is in a cold environment, these polymers adopt a coiled form to minimize the effect on viscosity. On the other hand, in a hot environment, they will straighten out, allowing the oil to produce a thickening effect.

While it is more desirable to use high molecular weight polymers (since they provide a better thickening effect), these long-chain molecules are also subject to degradation due to mechanical shearing. Therefore, a balance must be reached between the molecular weight and shear stable service condition.

Another challenge for formulators is to balance the polymer’s tendency to shear with the expected viscosity thickening due to oxidative processes and the viscosity thinning due to the dilution of fuel¹.

Friction Modifiers

These usually compete with the antiwear and extreme pressure additives (and other polar compounds) for surface room. However, they become activated at temperatures when the AW and EP additives are not yet active. Thus, they form thin mono-molecular layers of physically adsorbed polar soluble products or tribochemical friction-reducing carbon layers, which exhibit a lower friction behavior than AW and EP additives².

There are different groups of friction modifiers based on their function. Some are mechanically working FMs (solid lubricating compounds, e.g., Molybdenum disulfide, graphite, PTFE, etc.), adsorption layers forming FMs (e.g., fatty acid ester, etc.), tribochemical reaction layers forming FMs, friction polymer forming FMs and organometallic compounds.

Defoamants (Antifoam)

When foam forms in the lubricant, tiny air bubbles become trapped either at the surface or on the inside (called inner foam). Defoamants work by adsorbing on the foam bubble and affecting the bubble surface tension. This causes coalescence and breaks the bubble on the lubricant’s surface¹.

For the foam that forms at the surface, called surface foam, defoamants with a lower surface tension are used. They are usually not soluble in base oil and must be finely dispersed to be sufficiently stable even after long-term storage or use.

On the other hand, inner foam, which is finely dispersed air bubbles in the lubricant, can form stable dispersions. Common defoamants are designed to control surface foam but stabilize inner foam².

Oxidation Inhibitors

As noted above, antioxidants are usually deployed during the propagation phase to neutralize the scavenging radicals or decompose the hydroperoxides³. There are two main forms of antioxidants: primary and secondary antioxidants.

Primary antioxidants, also known as radical scavengers, remove radicals from oil. The most common types are amines and phenols.

Secondary antioxidants are designed to eliminate peroxides and form non-reactive products in the lubricant. Some examples include zinc dithiophosphate (ZDDP) and sulphurized phenols.

Mixed antioxidant systems also exist where two antioxidants have a synergistic relationship. One example is the relationship between phenols and amines, where phenols deplete early during oxidation while amines deplete later. Another example is using primary and secondary antioxidants to remove radicals and hydroperoxides.

Rust and Corrosion Inhibitors

Rust and Corrosion inhibitors are usually long alkyl chains and polar groups that can be adsorbed on the metal surface in a densely packed formation of hydrophobic layers.

However, this is a surface-active additive, and as such, it competes with other surface-active additives (such as antiwear or extreme pressure additives) for the metal surface. There are two main groups for corrosion additives: antirust additives (to protect ferrous metals) and metal passivators (for non-ferrous metals²).

Rus inhibitors have a high polar attraction to metal surfaces. They form a tenacious, continuous film that prevents water from reaching the metal surface. It must also be noted that contaminants can introduce corrosion into an oil, just as organic acids are produced.

Detergents and Dispersants

Detergents are polar molecules that remove substances from the metal surface, similar to a cleaning action. However, some detergents also provide antioxidant properties. The nature of a detergent is particularly important as metal-containing detergents produce ash (typically calcium, lithium, potassium, and sodium)¹.

On the other hand, dispersants are also polar, and they keep contaminants and insoluble oil components suspended in the lubricant. They minimize particle agglomeration, which in turn maintains the oil’s viscosity (compared to particle coalescing, which leads to thickening). Unlike detergents, dispersants are considered ashless. They typically work at low operating temperatures.

Antiwear Additives

These are typically polar with long chain molecules that adsorb onto the metal surfaces to form a protective layer. This can reduce friction and wear under mild sliding conditions. Usually, these additives are formed from esters, fatty oils, or acids, which can only work at low or moderate levels of stress within the system.

The most common form of antiwear is ZDDP, which is used in engine or hydraulic oils. On the other hand, an ashless phosphorus type of antiwear also exists for systems that require that characteristic, and tricreysl phosphate is the usual choice.

Extreme Pressure Additives

Since extreme pressure additives only become active when higher temperatures or heavier loads are on a system, they have earned the name “Anti-scuffing additives.”

Unlike antiwear additives, extreme pressure additives react chemically with the sliding metal surfaces to form relatively insoluble surface films. This reaction only occurs at higher temperatures, sometimes between 180-1000°C, depending on the type of EP additive used¹.

It must be noted that even with the presence of EP additives in a lubricant, there will still be some wear during the break-in period as the additives have yet to form their protective layers on the surfaces.

EP additives must also be designed for the system they protect as different metals have varying reactivity (EP additives designed for steel-on-steel systems may not be appropriate for bronze systems as they are not as reactive with bronze).

EP additives also contribute to polishing the sliding surfaces as they experience the most significant chemical reaction when the asperities are in contact and the localized temperatures are at their highest. They tend to be created from compounds containing sulphur, phosphorus, borate, chlorine, or other metals⁴.

Do Lubricant Additives Degrade Over Time?

As noted earlier, most additives can deplete over time as they get used up in their various functions. Antiwear and rust protection additives continuously coat the surfaces of the interfacing metals.

This can cause their initial concentrations to decrease over time until it reaches a point where the concentration of the additive is too low to offer any protection. In this case, it has not degraded but depleted.

In earlier years, there used to be prevalent issues with the separation of additives from the finished lubricant due to filtration. However, with the evolution of technology and better practices, this is no longer a common problem operators face.

In the past, operators would notice frequent clogging of their filters and subsequent reduction of additive concentrations, rendering the oil unprotected. It was common to notice additives settling to the bottom of a drum of oil after standing still for some time.

In essence, lubricant additives do not really degrade over time; rather, their concentrations get depleted, which assists in the lubricant degrading faster than a finished lubricant with higher additive concentrations.

Innovation and Future Trends for Additives

What does the future look like for additives within our industry? Will they go away completely?

From my estimations, we’re a long way from that happening. The lubricant industry has evolved over the years, with many advances from the chemical side, which has developed better-suited additives, and the OEM side, which has pushed the chemists to develop lubricant additives that can adapt to equipment changes.

OEMs are creating more components that can withstand higher temperatures, increased pressures, and more demanding environments. Lubricants must also be developed for this specific use, and additive technology will continue to evolve as these boundaries are pushed.

We are also being driven towards more environmentally friendly products, and additives are also on that list. Most of the metals used in the production of additives (such as EP or AW additives) are toxic to the environment, and alternatives are being discovered.

In the field of tribology, there has also been continued research into ways of reducing friction and wear. This is coupled with research into the interaction of varying surfaces and ways lubricants can effectively reduce the coefficient of friction, leading to increased energy efficiency and fuel efficiency in some cases.

Lubricant additives will be around for some time as everything that moves needs to be lubricated, and base oils do not have all the required properties to handle varying temperatures and other conditions that the machine encounters.

While their structure will change to adapt to provide a more environmentally friendly impact, their functions will also evolve based on their future requirements.

References

1 Bruce, R. W. (2012). Handbook of Lubrication and Tribology, Volume II Theory and Design, Second Edition. Boca Raton: CRC Press.

2 Mang, T., & Dresel, W. (2007). Lubricants and Lubrication – Second Completely Revised and Extended Edition. Weinheim: WILEY-VCH GmbH.

3 Livingstone, G., Wooton, D., & Ameye, J. (2015). Antioxidant Monitoring as Part of Lubricant Diagnostics – A Luxury or a Necessity?

4 Pirro, D. M., Webster, M., & Daschner, E. (2016). Lubrication Fundamentals – Third Edition Revised and Explained. Boca Raton: CRC Press.

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Oil Properties

What are the types of Lubricant Additives?

There are many types of lubricant additives, and various formulations exist from different suppliers. In this section, we will cover the most common additives found in finished lubricants.

Pour Point Depressants

All liquids have a particular temperature at which they can effectively flow. The liquid’s viscosity and current temperature determine how quickly it moves. As the name implies, this type of additive can assist in lowering the temperature at which the lubricant flows¹.

VI Improvers

This should not be confused with Pour Point Depressants. Viscosity Index Improvers are also known as Viscosity Modifiers². They assist the lubricant in increasing its viscosity at higher temperatures, allowing lubricants to operate in wider temperature ranges.

Friction Modifiers

When two surfaces rub against each other, friction is formed. Depending on the type and extent of friction, some surfaces can experience welding and even adhesive wear. This is where friction modifiers can help by reducing frictional forces associated with stick-slip oscillations and noises.

Defoamants (Antifoam)

Some lubricants succumb to foam being created in their systems. When foam is made, it significantly impacts the functions of the lubricant and can lead to excessive wear due to lack of lubrication (they disrupt the surface of the lubricant), cavitation (due to the presence of air bubbles), and even increased oxidation (due to presence of air trapped in the system). Foam can also affect the ability of a liquid to transfer heat or cool. Defoamants or antifoam additives reduce the amount of foam being produced.

Oxidation Inhibitors (Antioxidants)

Oxidation occurs in most lubricants. During the oxidation process, free radicals emerge, propagating to form alkyl or peroxy-radicals and hydroperoxides, which eventually react with others to form oxidation by-products. During the propagation phase, antioxidants are usually deployed to neutralize the free radicals or decompose the hydroperoxides³. As such, these additives are sacrificial in nature, as they protect the base oil from oxidation by being depleted.

There are many types of antioxidants, including phenolics and aromatic nitrogen compounds, hindered phenols, aromatic amines, zinc dithiophosphates, and a couple of others.

Rust and Corrosion Inhibitors

If oxygen and water are present at a location containing iron, then rust can be formed. Corrosion affects the non-ferrous metals in the presence of acids in the lubricant¹. Most pieces of equipment succumb to rust and corrosion quite easily, so these inhibitors were developed to mitigate these effects by forming protective layers on the surfaces of the equipment.

Detergents and Dispersants

These two often get confused as they usually work together to prevent deposits from accumulating in the oils. Detergents neutralize deposit precursors (especially in engine oils), while dispersants suspend the potential sludge or varnish-forming materials⁴.

Antiwear Additives

Antiwear additives reduce friction and wear, especially during boundary lubrication conditions. They are designed to reduce wear when the system is exposed to moderate stress².

Extreme Pressure Additives

Extreme Pressure additives are usually confused with antiwear additives, or the names are used interchangeably. However, extreme pressure additives begin to work when the system experiences high stress and try to prevent the welding of moving parts, unlike antiwear additives, which work when the system experiences moderate stress.

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Oil Properties

Why Do We Need Lubricant Additives?

Lubricants keep the world turning. Once something moves, a lubricant should be present to reduce friction or wear between the surfaces. But what makes lubricants so unique in our industry? Is it just the base oil?

No, this is where the power of lubricant additives truly shines, an area many overlook.

Why Do We Need Lubricant Additives?

Before getting into the world of additives, let’s step back to the basics: why are they needed? A lubricant is composed of base oil and additives. Depending on the type of oil, different ratios of additives will be used for the various applications. Additionally, each Lubricant OEM will have its unique formula for its lubricant.

To simplify this, we can think of making a cup of tea. The first thing we need is some hot water in a cup. This can be our base oil. It can be used on its own (some people drink hot water or use it for other purposes), but if we want to make a cup of tea, we must add stuff.

Depending on the purpose for which you’re drinking the tea, you may choose a particular flavor. Perhaps peppermint for improved digestion or to help improve your concentration or chamomile to keep you calm.

These flavors can represent the various types of oils: gear oils, turbine oils, or motor oils. Different blends are suited for different applications.

Now, while we’ve added the tea bag to the hot water (and some people can drink tea like this), others need to add sweetener or milk. These are the additives to the base oil (hot water).

Depending on the preference of the person drinking the tea, there will be varying amounts of sweetener (honey, stevia, or sugar) and varying amounts of milk (regular, low-fat, oat, dairy-free). The combinations are endless!

The same can be said of additives in finished lubricants. Depending on the type of oil (tea flavor, think gear or turbine oil) and its application (the person drinking the tea, with dietary preferences of being dairy-free or sugar-free), the combination of lubricant additives and their ratios will differ. The percentage of additives can vary from 0.001 to 30% based on the type of oil.

Additives have three main functions in a finished lubricant. They can;

Enhance – improve some of the properties of the base oil
Suppress – reduce some of the characteristics of the base oil
Add new properties – introduce new features to the base oil

The finished lubricant will have properties from the base oil and additives combined.

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Oil Properties

Factors That Affect Oil Viscosity

Similar to the molasses and water examples above, different factors can affect the viscosity of a liquid. For instance, water can assume other states depending on the temperature.

If water is at its freezing point (0°C), it can turn to ice but remains liquid at room temperature (around 20-30°C). Then, at 100°C, it can turn into a vapor. Its viscosity can change depending on the influencing factors.

Four factors affect oil viscosity:

Temperature
Pressure
Shear rate
Oil type, composition, and additives

Temperature

As seen with the example of the water above, when the temperature decreases, the water can turn to ice. Similarly, for lubricants, as the temperature drops, the viscosity increases. This means the oil will get thicker or more resistant to flow at lower temperatures. Likewise, as the oil heats up, it can become thinner.

This is similar to a block of ice melting as temperatures increase. Its viscosity will decrease, and the ice will turn to water. In this case, the internal molecules gain more energy with the increase in temperature, lowering the internal friction within the fluid. As such, the viscosity also decreases.

Since the oil’s viscosity will change with temperature, most OEMs will supply a temperature–viscosity chart for their equipment to help ensure the correct viscosity is used depending on the operating temperature.

In the figure below, gear oils of varying viscosities are plotted against the temperature for a particular piece of equipment. OEMs will typically specify the optimum operational viscosity range for their equipment.

It is then up to the lubrication engineers to determine the ideal viscosity based on the conditions of their equipment (this can vary depending on the application).

Figure 1: Temperature – Viscosity chart for Shell Omala S2 G (Source: Shell Lubricants TDS)

From the figure above, one can see that at 40°C, most of the gear oil grades correspond with their viscosities (ISO 68 corresponds with a 68 viscosity). However, at 0°C, an ISO 68 gear oil can become 1000 cSt, while at 90°C, this same grade of oil is around 11cSt.

Interestingly, all of the oils listed here can achieve a viscosity of 100cSt but at different temperatures, as shown below:

32°C – ISO 68
40°C – ISO 100
47°C – ISO 150
55°C – ISO 220
63°C – ISO 320
68°C – ISO 460
75°C – ISO 680

Temperature is a significant influencing factor of viscosity, but it is not the only factor.

Pressure

The effects of pressure on a lubricant’s viscosity are often overlooked. However, the viscosity-pressure behavior has become part of the calculation for elastohydrodynamic films. In these cases, oil viscosity can rapidly increase with pressure.

One such instance occurs with metal-forming lubricants, which are subjected to high pressures such that the oil’s viscosity can increase tenfold (Mang & Dresel, 2007). As the pressure increases, viscosity also increases, protecting the surface in these lubricant films.

The very definition of viscosity alludes to pressure’s impact on Newtonian and non-Newtonian fluids. For example, with Newtonian fluids (regular lubricating oils), the shear rate is proportional to the applied shear stress (pressure) at any given temperature.

As seen above, the viscosity can be determined once the temperature remains the same. However, Non-Newtonian fluids, such as greases, only flow once a shear stress exceeding the yield point is applied (Pirro, Webster, & Daschner, 2016).

Hence, this is why the observed viscosity of grease is called its apparent viscosity and should always be reported at a specific temperature and flow rate.

Shear Rate

For Newtonian fluids, viscosity does not vary with shear rate (Pirro, Webster, & Daschner, 2016). In fact, per the definition of viscosity for Newtonian fluids (regular lubricating oils), viscosity is a constant proportionality factor between the shear force and shear rate. Thus, even when subjected to greater shear forces, the viscosity will not change for Newtonian fluids.

On the other hand, for non-Newtonian fluids, the viscosity is influenced by the shear rate. Some non-Newtonian fluids can include; pseudoplastic fluids, dilatant fluids, and a Bingham solid, the effects of shear rate on these fluids are shown in the figure below.

A Bingham solid is a plastic solid such as grease that only flows above a particular yield stress. It can be seen that pseudoplastic fluids decrease viscosity with an increasing shear rate, while dilatant fluids show an increase in viscosity with an increasing shear rate. (Hamrock, Schmid, & Jacobson, 2004)

Figure 2: Characteristics of different fluids as a function of shear rate vs. viscosity (a) and shear rate vs. shear stress (b). Source: Fundamentals of Fluid Film Lubrication by Hamrock, Schmid & Jacobson, page 102.

The shear of a lubricant can influence its shear rate. Typically, longer-chain polymer viscosity index improvers can shear over time. When this happens, it can result in a decrease in oil viscosity. Similarly, non-Newtonian fluids, such as grease, experience a decrease in viscosity as a function of shear rate (Totten, 2006).

Another essential characteristic to note is whether a material is thixotropic or rheopectic. For a thixotropic material, if it is placed under a continuous mechanical load over a period of time, the viscosity will appear to decrease over this time.

However, the original viscosity is restored after a specific rest period, as shown in the figure below. On the other hand, for rheopectic materials, continuous shearing causes the viscosity to increase. (Mang & Dresel, 2007).

Figure 3: Flow characteristics of a thixotropic lubricant (Source: Lubricants and Lubrication edited by Theo Mang and Wilfried Dresel, page 30)

Oil Type, Composition, and Additives

Various oil types, compositions, and additives can influence a lubricant’s viscosity. For instance, the five groups of base oils all have varying characteristics, as shown in the figure below. One can note the differing viscosities for the various groups.

Figure 4: Base Stock property comparison (Source: Lubricants and Lubrication edited by Theo Mang and Wilfried Dresel, page 13)

When a finished lubricant is made, it usually consists of a base oil and additives. Hence, the base oil will have a significant role in determining the final viscosity of the oil. However, with the advent of Viscosity Index Improvers, desired viscosities can be engineered regardless of the base oil type being used.

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Lubricant Degradation

Can Lube Oil Varnish be Eliminated?

Varnish can be likened to cholesterol in the human body. It can build up in our arteries and eventually clog those, causing restrictions in blood flow to our heart which may lead to a heart attack.

Humans cannot simply change their blood to remove the cholesterol build-up. However, cholesterol is controlled through proper diet, exercise, and with some condition monitoring in the form of blood tests to help gauge the presence of it in the bloodstream. Similarly, a couple of approaches can be used to reduce the varnish build-up or eliminate it.

As per Livingstone et al. (2011), the lifecycle of varnish is critical. Particular attention should be paid to the double arrows between the stages of Solubility to Varnish formation in the figure below.

This means that even after varnish has been deposited, it can be solubilized back into the oil. This can only occur if conditions are met per Hansen’s Solubility principles where the solvent and degradation products meet using the three parameters of Polarity, Hydrogen Bonding, and Dispersive Forces as discussed in “The Hansen Solubility Principles and Its Relation to Varnish” (2022).

The Varnish Lifecycle as per Livingstone et al. (2011)

Varnish exists in various forms and can consist of differing compositions. Hence, it is essential to understand the characteristics of the varnish being formed in a system before attempting to eliminate it.

There are certain technologies, such as solubility enhancers or specifically engineered filtration media, which can be effective at removing lube oil varnish. However, this technology is heavily reliant on the type of varnish being formed and can be customized as per the system accordingly.

Solubility enhancers can solubilize the varnish back into the oil solution. When these deposits are reintroduced into the oil, they can be removed using resin-based filtration. In this method, the media is specifically designed to allow for the adsorption and removal of the varnish which presently exists in the oil.

When these methods are used together, they can prove quite effective and prevent manufacturing plants from experiencing unwanted downtime.

To summarize, it is of utmost importance to first understand the characteristics of the varnish being produced in your equipment before attempting to remove it from your system.

There is no cookie-cutter method to eliminate varnish from a system as it is a complex deposit. Similar to practices we observe with our bodies in the instances of cholesterol build-up, we can employ methods of dissolving the varnish and removing it while monitoring for possible recurrences in the future.

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References:

Livingstone, Ameye, & Wooton. (2015.). Antioxidant Monitoring as Part of Lubricant Diagnostics – A Luxury or a Necessity? OilDoc, Rosenheim, Germany.

Livingstone, Overgaag, & Ameye. (2011). Advanced removal Techniques for Turbine oil Degradation Products. Powergen Milan.

Mathura, S. (2020). Lubrication Degradation Mechanisms (CRC Press Focus Shortform Book Program) (1st ed.). CRC Press.

The Hansen Solubility Principles and its Relation to Varnish. (2022, July 31). Fluitec International. https://www.fluitec.com/the-hansen-solubility-principles-and-its-relation-to-varnish/

Lubricant Degradation

Is Oil Analysis the Only Method of Varnish Detection?

Varnish will deposit in layers and adhere to the metal surfaces inside the equipment. As it continues to deposit, the layers will eventually accumulate until it reaches a point whereby it can cause significant changes to the clearances of the components.

There have been instances where shafts in rotating pieces of equipment have been moved due to the build-up varnish. This is where vibration analysis can be instrumental.

When the vibration analysis method is used, it can detect any small changes in the alignment of the shaft in rotating equipment. As varnish continues to build on the inside of the component, vibration analysts can detect if the shaft observes some misalignment over a period.

This may be easy to miss as sometimes the varnish which has built up can be wiped away, causing the shaft to resume its proper alignment. Thus, these technologies should be used in tandem before conclusions are made about the presence of varnish.

Another detection method that can be employed is the monitoring of temperature fluctuations. As stated earlier, varnish can form an insulating layer trapping heat. There have been case studies that demonstrate that bearings experiencing varnish tend to display temperature increases.

Typically, these temperature patterns assume a saw-tooth pattern where temperatures rise continuously as the varnish builds up. The varnish becomes wiped away, and the temperature is reduced drastically.

This saw tooth pattern of temperature variation is characteristic of varnish formation. In some cases, the formation of localized deposits on bearing surfaces may cause temperature escalations without a corresponding MPC increase. In this case, the bulk oil may not show any degradation, yet temperature excursions may be experienced at the bearing surface.

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