Hydraulic presses in Oriented Strand Board (OSB) mills are central and indispensable to OSB production. They exert an immense, uniform force required to compress wood strands and resin into durable panels, operating under exacting temperature and pressure conditions. The performance of hydraulic presses depends critically on the quality and condition of the hydraulic oil.

The Vital Role of Hydraulic Oil

Hydraulic oil in OSB presses serves multiple roles:

• Power Transmission: Hydraulic oil transmits power from pumps to press cylinders, enabling precise compression.
• Lubrication: Reduces friction in pumps, valve spools, and cylinders.
• Heat Transfer: Acts as a coolant, absorbing and dissipating heat from critical components.
• Sealing and Contamination Control: Prevents contamination ingress, maintaining system integrity.

Hydraulic Oil Failure: Oxidation and Varnish

Despite its crucial role, hydraulic oil is susceptible to failure, especially due to oxidation and subsequent varnish formation. Oxidation, a reaction with oxygen accelerated by heat, pressure, moisture, and catalytic metals, depletes antioxidants and generates harmful byproducts.

When oxidation takes hold, varnish becomes the silent killer of hydraulic precision.

Varnish, an insoluble, sticky deposit formed from these oxidation byproducts, accumulates on critical components, especially servo and proportional valves. This buildup is analogous to cholesterol plaque in arteries, restricting fluid flow, reducing responsiveness, and increasing operational risks.

Operational Impact of Oil Degradation

When hydraulic oil fails:

• Press Performance Suffers: Reduced valve responsiveness leads to inconsistent press forces, resulting in poor board quality and defective products.
• Maintenance Costs Escalate: Varnish buildup necessitates frequent component replacements, system flushes, and increased downtime.
• Energy Efficiency Drops: Oxidized, varnish-contaminated oil increases viscosity and blocks oil flow channels, raising power demands and operating costs.
• Safety and Environmental Risks Increase: Potential leaks and compromised components present significant hazards. Oxidized oil is known to deteriorate seals, leading to more leaking and increased risk.

Targeting Varnish at Its Source for Lasting Results

To combat these challenges, Fluitec and ExxonMobil developed Mobil Solvancer, an innovative oil-soluble cleaner. Mobil Solvancer dissolves varnish deposits effectively, analogous to a solvent clearing blocked pipes, immediately restoring system responsiveness. It also provides long-term protection, minimizing varnish recurrence, improving servo valve response times, and extending equipment life.

Inside the GP Clarendon Hydraulic Recovery Journey

GP Clarendon OSB Mill experienced significant varnish buildup in hydraulic systems after a prolonged shutdown, despite using Mobil DTE 25 and DTE 25 Ultra oils. Frequent servo valve failures were costing around $40,000 per quarter.

Identifying the Root Cause of Hydraulic Varnish

Analysis revealed high varnish levels indicated by elevated Membrane Patch Colorimetry (MPC) values (66dE). The mill implemented a 5% treatment rate of Mobil Solvancer (approximately 40 drums) combined with enhanced kidney-loop filtration.

Results Achieved

Within 2.5 months, remarkable improvements were observed:

• MPC values dropped from 66dE to 26dE.
• Ultra Centrifuge (UC) ratings improved from 4 to 1.
• Servo valve failures decreased from six per quarter to zero, showcasing substantial reliability improvements.

Ensuring Long-Term Reliability and Oil Health

With proven success, GP Clarendon scheduled a complete system oil change and plans to install permanent high-efficiency filtration in March 2025, ensuring long-term system integrity and performance.

Key Findings and Operational Takeaways

Effective management of hydraulic oil condition is crucial for maintaining optimal productivity and reliability in OSB mills. Mobil Solvancer demonstrates exceptional performance, significantly reducing varnish deposits, enhancing system efficiency, reducing maintenance costs, and ensuring consistent product quality.

As demonstrated by GP Clarendon’s experience, proactive maintenance coupled with Mobil’s industry-leading hydraulic oils can transform operational reliability in industrial hydraulic systems.

Hydraulic systems are used to transmit force from one point to another via a fluid. This fluid is usually hydraulic oil, and the concept is based on Pascal’s law. Hydraulics are present in nearly all industries and play a critical role in enhancing operational efficiency. In this article, we will take a deep dive into hydraulic oils and explore them in more detail. 

What is Hydraulic Oil?

One of the primary functions of hydraulic oil is to transmit power or energy. However, this is not its only function. Some of its primary functions include:

• Transferring pressure and motion energy
• Transferring forces and moments when used as a lubricant
• Minimization of wear to sliding surfaces under boundary friction conditions
• Minimization of friction
• Protection of components against corrosion (ferrous and non-ferrous metals)
• Dissipation of heat
• Suitability for a wide range of temperatures, good viscosity-temperature behavior
• Prolonging the life of machinery

However, hydraulic oils also have secondary and tertiary functions.

Some of the secondary characteristics of hydraulic oils include high aging stability, good thermal stability, inertness to materials, compatibility with metals and elastomers, good air separation, low foaming, good filterability, good water release, good shear stability (in the case of non-Newtonian fluids), and more.

On the other hand, some of the tertiary characteristics include low evaporation due to low vapor pressure, toxicologically harmless, ecologically safe, and low flammability (fire resistance), all of which depend on how the fluid is formulated. 

History of Hydraulic Oil

The principle of hydraulics has been around for a very long time. In fact, according to Hard Chrome Specialists, it may even date back to around 6000 BC, when ancient Mesopotamians and Egyptians used water for irrigation. Fast forward to the modern day, where hydraulics have been heavily influenced by Blaise Pascal, Joseph Bramah, Daniel Bernoulli, and William George Armstrong. A snapshot is shown in Figure 1 below.

Figure 1. History of Hydraulics as per Hard Chrome Specialists

Importance of Hydraulic Oil

Hydraulics forms part of the field of fluid technology, which can be further subdivided into hydrostatics and hydrodynamics.

For hydrostatic systems, the transfer of energy requires static pressure; hence, the pressure is high, but the flow rate is low. Fluids designed for these applications are known as hydraulic oils.

For hydrodynamic systems, the kinetic energy of the flowing fluid is utilized, resulting in low pressure but high flow rates. Fluids designed for these applications are known as power transmission oils.

As explained earlier, Pascal’s law forms the basis of hydraulics. Using the principle of the hydrostatic displacement machine, Pascal’s Law states that, “Pressure applied anywhere to a body of fluid causes a force to be transmitted equally in all directions. The force acts at right angles to any surface within or in contact with the fluid”.

Hydraulic systems utilize hydraulic oils to transmit power or energy to other applications, making them the second most crucial group of oils, after engine oils, due to their widespread use. As a result of their primary application, they can help save energy, reduce maintenance intervals and wear, increase machine life, and overall provide users with significant savings.

Types of Hydraulic Oil

There are many types of hydraulic oils due to the number of applications in which hydraulics are used. Generally, they can be classified as shown in Figure 2 below.

As can be seen above, there isn’t just one type of hydraulic oil. Depending on the application, different standards are associated with each type. The two main groups are hydrostatic applications and hydrodynamic/hydrokinetic applications.

The hydrodynamic applications mainly include ATFs (Automatic Transmission Fluids).

On the other hand, for the hydrostatic applications, the mobile systems refer to UTTO (Universal Tractor Transmission Oil) and STUO (Super Tractor Universal Oil).

These hydrostatic applications can be further categorized into mineral oil-based hydraulic fluids, fire-resistant hydraulic fluids, environmentally acceptable hydraulic fluids, and food-grade lubricants. Each of these has its associated standards and regulations.

For Mineral oil-based hydraulic fluids, there are different classifications, as shown in Figure 3 below.

For fire-resistant oils, their classifications include those shown in Figure 4 below.

For Environmentally Acceptable lubricants (particularly water-free, rapidly biodegradable hydraulic fluids), their classifications are shown in Figure 5 below.

For food-grade lubricants, their classifications include:

• NSF H1 – These are colorless, odorless, tasteless, non-toxic, and are certified for incidental contact with food
• NSF H2 – These are lubricants that can be used in food processing, but only where there is no contact with food.

(Other categories of NSF grades exist, but these do not apply directly to hydraulic oils.)

Chemical Composition of Hydraulic Oil

Due to the unique nature of hydraulic oils, they are formulated differently from other oils. Typically, it follows the regular oil formulation of Base oil + additive to give the finished product. However, many various combinations occur depending on the application for which it is being formulated.

Base Oils Used in Hydraulic Oil

Similar to other oils, any base oil can be used to create hydraulic oils. However, depending on the application in which it is being used, the type of base oil will differ. The following table gives a summary of the types of base oils used for various hydraulic oils.

Additives in Hydraulic Oil

Generally, hydraulic oils are composed of 95-98% base oil and roughly 2-5% additives. One major distinction of hydraulic oils is that they can be either zinc-containing or zinc-free (also known as ashless). The ZnDTP molecule is responsible for antiwear properties, but this does not mean that zinc-free oils do not contain some form of antiwear additive.

Zinc-free oils are formulated with no zinc and minimal concentrations of phosphorus and sulphur. These zinc-free oils are formulated for special applications where the presence of zinc could react negatively with the environment, such as equipment containing mixed metals or silver.

There are other additives used for hydraulic oils, which are classified as either surface-active additives or base-oil-active additives.  

Surface-active additives include steel/iron corrosion inhibitors, rust inhibitors, metal deactivators, wear inhibitors, friction modifiers, and detergents or dispersants.

Base active additives include: antioxidants, defoamers, VI Improvers, and pour point improvers.

Common Contaminants in Hydraulic Oil

Hydraulic systems are known for very tight clearances. As such, any form of physical contaminant can easily clog the valves or lines, leading to system failure. Keeping hydraulic oils clean is of paramount importance. Figure 6 illustrates typical clearances for hydraulic components, as well as the film thickness for various components.

The contaminants that exist in hydraulic systems can be either internally generated or externally consumed. Typically, dirt from external sources or metal wear (internally generated) form the major contaminants for hydraulic oils.

However, they are also susceptible to gaseous or liquid contaminants that can enter the system through system processes or external factors during oil handling before it enters the system. Figure 7 shows some cleanliness categories for various components.

Testing and Analyzing Hydraulic Oil Composition

There are several basic tests that should be used to determine the condition and health of hydraulic oils. These include:

• Viscosity (ASTM D445) – Generally, if this value falls below or above10-15%, there is cause for concern. Any increase in viscosity (outside of the system limits) can lead to the system experiencing higher pressures. Conversely, any decrease in viscosity outside of the limits will not allow for the full transfer of power through the fluid.
• Water content (ASTM D6304) – Too much water in a system is always a bad thing (except in a swimming pool). In particular, if the water content starts trending upwards of 500 ppm, the source of water ingression should be found and eliminated at once. This can hinder the transmission of power in the system, making it less efficient.
• Presence of wear metals (ASTM D5185-05) – These values will differ depending on the system in which the hydraulic oil is being operated. It would be a good idea to contact the OEM about the limits for the wear metals for your system to ensure that no irregular wear is occurring. The presence of these wear metals may also act as catalysts for other reactions, potentially leading to the degradation of the oil.
• Particle Count (ISO 4406) – This also depends on your system, as varying levels of cleanliness are typically aligned with different systems. However, with hydraulic systems, there is usually some guidance on the tolerance levels. The presence of these particles can hamper the transmission of power in the system or block clearances.

Specialty tests for hydraulic oils also exist. These include tests for monitoring antiwear and extreme pressure, foaming, or oxidation stability characteristics of the oil. For determining the antiwear or extreme pressure properties, tests such as the Vickers Vane pump test, 4 Ball test, and FZG rating test can be used.

Properties and Characteristics of Hydraulic Oil
Hydraulic oils must be able to withstand particular conditions and still perform their primary function of transferring power from one point to another.  As such, they have characteristics that make them unique from regular oils.

Viscosity of Hydraulic Oil

The viscosity of an oil is one of the most essential characteristics, especially for hydraulic oils, as they must transfer power. As such, the viscosity-temperature characteristic of hydraulic oils is also critical. As the temperature of the oil increases, its viscosity decreases (or becomes thinner). Similarly, if the temperature of the oil decreases, its viscosity will increase (or become thicker).

For hydraulic oils, some manufacturers plot their viscosity against temperature to help customers determine the ideal viscosity for their system based on the system’s operational temperatures. Figure 8 shows a chart for Shell Tellus S2MX, illustrating the varying viscosities as the temperature changes.

Choosing the wrong viscosity can cripple power transmission before the system even starts.

For instance, if the system is running at 60°C and the oil needs to have a viscosity of 30cSt, then an ISO VG 68 would be the most ideal oil. However, if the system is running at 40°C, and the viscosity needs to be 30cSt, then an ISO 46 oil would be more appropriate.

The relationship between viscosity and temperature is known as the viscosity index. For hydraulic oils, the higher the viscosity index, the less susceptible the viscosity is to changes in temperature. As a reference, mineral base oils have a natural viscosity index (VI) of 95-100, while synthetic ester-based base oils have a VI of 140-180, and polyglycols have a natural VI of 180-200.

Oxidation and Thermal Stability

The TOST (Turbine Oil Oxidation Stability Test) is usually used to determine the oxidation stability of an oil. Although the test name mentions “turbines”, it can also be applied to hydraulic oils.

Another test that can be used to evaluate whether oxidation has taken place or not would be the RULER® test, which quantifies the remaining antioxidants in the oil. Overall, determining the oxidation and thermal stability of the oil provides the user with an average estimate of the oil’s life expectancy when subjected to environmental extremes.

Foam and Air Release Properties

Due to the operating environment of hydraulic oils, air tends to become trapped in them. This can become a problem as it can easily lead to cavitation inside pumps (the most prevalent form of wear for these systems). Therefore, hydraulic oils must have good air release and anti-foaming properties.

Good air release allows the dissolved or trapped air to coagulate and rise to the surface, where it can then be dissipated. This is where the issue of foam “arises” as it further impedes the oil’s ability to form a full wedge between the two surfaces in any system.

Demulsibility and Water Content

Demulsibility refers to the ability of oil to repel water. Typically, hydraulic oils are designed to operate in environments with some water or high humidity, where water can easily enter the oil.

For hydraulic oils containing detergents or dispersants (DD), fluids (such as water) or other fine contaminants are usually held in suspension. Therefore, the demulsibility test, in which water and oil are mixed and then allowed to separate, will not be effective in determining the water separation characteristic of these hydraulic oils. Filtration should be used for these DD oils, which become contaminated with water.

Water in hydraulic oil isn’t harmless — it’s a silent trigger for failure.

On the other hand, for those oils that do not contain detergents or dispersants, the demulsibility test (ASTM D1401) can be performed. For this test, equal parts of oil and water are mixed at a specific temperature to create an emulsion and then allowed to separate. The amount of oil, water, and emulsion is recorded at 5-minute intervals.

If the viscosity of the oil is less than 90 cSt and there is 3 mL of emulsion or less after 30 minutes, the oil is acceptable. If the oil has a viscosity greater than 90cSt, the result is taken at the end of 60 minutes (if the value of the emulsion is less than 3 mL, it is acceptable). The results are usually recorded in the format: mL oil / mL water / mL emulsion (time recorded in minutes).

Corrosion Protection

Many hydraulic systems contain copper metals, brass, or bronze, especially in cooling systems, pumps, bearing elements, or guides. Therefore, hydraulic oils must be resistant to copper corrosion, as this could compromise the entire system. One such test, which can be used to identify the corrosivity of these oils, is the Copper Strip Corrosion test.

In this test, the copper strip is placed in hydraulic oil for a typical duration of 3 hours at 100°C. The results can be quantified based on the level of discoloration, which correlates with the degree of corrosion.

A similar type of test can also be done with steel / ferrous corrosion. In this case, the oil is mixed with distilled water or artificial seawater and stirred constantly for 24 hours at 60°C while the steel rod is submerged in the mixture.  Afterwards, the steel rod is examined for corrosion and allocated ratings accordingly.

Maintenance and Testing of Hydraulic Oil

Keeping hydraulic oils clean is critical to their operation, as any contaminant can interfere with the amount of power that they can transmit. These oils are also subjected to harsh conditions, so monitoring their quality will help to ensure that they provide the maximum efficiency for the system in which they are working.

Importance of Regular Maintenance

Hydraulic oil systems are notoriously known for leaks. Sometimes this revolves around failed seals or the improper use of material for the actual system, which cannot tolerate the existing conditions. Despite the root cause of the leak, it is essential to perform regular inspections on hydraulic equipment, as a leak can lead to a loss of power, potentially delaying or causing work in progress to come to an unexpected halt.

Hydraulic leaks are also detrimental to the environment, particularly if they seep into the ground or waterways. Hence, it is crucial to perform regular checks and maintenance on hydraulic systems to prevent harm to the environment.

A single unnoticed hydraulic leak can halt production and harm the environment.

Some other factors to consider regarding the maintenance of hydraulic fluids include maintaining the temperature and oil levels at the expected system values, as well as keeping the hydraulic oil clean to avoid contaminants.

Users should also be performing routine oil analysis (to catch any changes to the oil, which may lead to detrimental effects). Additionally, routine inspections can include checking noise levels, shock loads, filtration, vibration, leakage, fluid odor, color, and the presence of foaming. These additional methods can prove beneficial for intercepting early failures.

Common Methods for Monitoring Hydraulic Oil Condition

When monitoring hydraulic oils, several key characteristics to pay attention to include viscosity, AN (Acid Number), Water content, the presence of wear metals, and contaminants.

Any change in viscosity can affect the transfer of power, while an increase in Acid Number (AN) can indicate the degradation of the oil. On the other hand, the presence of any contaminant can also impact the performance of the oil, possibly leading to its degradation while acting as a catalyst.

Alternatively, the presence of wear metals can also indicate that wear is occurring on the inside of the hydraulic equipment. It may initiate a physical maintenance check to determine the extent of the wear.

Steps for Changing Hydraulic Oil

When changing hydraulic oil, it is important to note the previous condition of the oil. If there is a high concentration of contaminants, the system should ideally be flushed before introducing a new batch of oil. This prevents the new oil from also becoming contaminated and degrading at an accelerated rate.

Additionally, some physical contaminants may have also become lodged in the tighter clearances. Hence, it is always a good idea to perform a flush on the system, ensuring that it is clean before a new batch of oil is used.

Proper Storage and Handling

Hydraulic oils must be clean, and depending on the system, they have very specific cleanliness requirements. As such, when storing hydraulic oils, special care should be taken to ensure that the rooms are clean, the decanting equipment is clean (free from dirt or other contaminants), and a filter cart is used when decanting new oil into a system. Hydraulic oils do not mix well with other oils, and dedicated systems or equipment are required for decanting these oils.

Troubleshooting Common Issues

One of the main issues with hydraulic oils is their susceptibility to contamination. Contaminants can be in the form of physical particles, liquids, or gases.

In the case of gases, this usually leads to cavitation (one very common challenge with hydraulic oils) or an increase in the presence of foam. Hydraulic oils can also become contaminated with water, which affects their ability to transfer the required amount of power.

If your pump sounds like marbles, air or cavitation is already stealing performance.

Typically, technicians have reported hearing the “sound of marbles” within pumps that are experiencing cavitation. In these cases, there is usually an air leak or air entering the system where it is not intended to. This can be an issue with the intake or suction part of the pump, where the oil levels are low enough to allow for air to enter the system and then become trapped. In these cases, a baffle plate can be placed inside the reservoir at a 60-degree angle to trap some of the air bubbles.

On the other hand, when water is present in the hydraulic oils, depending on the concentration, a vacuum dehydrator or regular filtration system can be used to help remove the water.  Subsequently, for hydraulic oils that contain a high level of physical contaminants, a filtration system can also be used to help remove them from the system.

Selecting the Right Hydraulic Oil

With numerous hydraulic oils available on the market, selecting the one that best suits your system can be a challenging task. Several factors should be considered when choosing the most suitable hydraulic oil.

Factors to Consider When Choosing Hydraulic Oil

As with many oils, the first set of factors to consider when selecting an oil are the operational limits of the system and the oil. Typically, OEMs can guide users accordingly in this area, as they pay attention to the operating temperatures, pressures of the system, as well as the required viscosity of the oil.

Next, they need to consider the environment in which they are working. Are there any possibilities of the oil entering waterways or the soil? In such cases, environmentally friendly lubricants should be used.  They also need to determine if these oils will be exposed to harsher environmental conditions than regular ones. If this is the case, then they may consider utilizing synthetic oils instead of mineral oils.

Compatibility with Equipment and Seals

Most mineral hydraulic oils are compatible with seals and some metals. However, certain hydraulic oils are not. HFC fluids (Water polymer fire-resistant hydraulic fluids with water content >35%) react aggressively with tin and cadmium. As such, silicone rubber and Teflon are the materials utilized when these fluids are in use.

On the other hand, HFD fluids (water-free, synthetic fire-resistant hydraulic fluids) attack aluminum and aluminum alloys in the presence of friction stresses. Therefore, the only materials used with HFD oils are Viton and Teflon.

Due to the polar nature of environmentally friendly ester fluids, this causes significant swelling of conventional standard elastomers. On the other hand, Epikote and DD paints are resistant to HEPG (water-free, rapidly biodegradable polyalkylene glycols that are soluble in water), HFC, and HFD fluids to a certain extent. Therefore, the inside of tanks or other exposed surfaces should not be coated with these materials to avoid corrosion.

Environmental Regulations for Hydraulic Oil

Environmental lubricants are typically classified into three main categories:

Biodegradability – measure of the breakdown of a chemical (or chemical mixture) by microorganisms. This can be Primary (where there is a loss of one or more active groups that renders the compound inactive regarding that function) or Ultimate biodegradation (also known as mineralization, where the chemical compound is converted to carbon dioxide, water, and mineral salts).

Two additional operational properties that define biodegradability are inherently biodegradable (showing evidence of biodegradability in any test for biodegradability) and readily biodegradable (where a fraction of the compound is ultimately biodegradable within a specific time frame, as specified by a test method).  Figure 9 shows a table of internationally standardized test methods for measuring biodegradability.

Aquatic Toxicity – This refers to the effects of a chemical on organisms that live in water and is determined using organisms representing three trophic levels: algae or plants (primary producers), Invertebrates (primary consumers or secondary producers), and vertebrates (secondary consumers).

Acute toxicity is determined by exposing fish to a series of concentrations of a chemical over a short period. The concentration that is lethal to 50% of the test fish is calculated and expressed as the LC50 value.

On the other hand, Chronic Toxicity covers a longer exposure time. It examines the effects on hatching, growth, and survival to determine the NOEC (No Observed Effect Concentration) values, LOEC (Lowest Observed Effect Concentration), or ECx values, where x is a percentage and the concentration of a chemical at which that percentage of the population shows some effect. 

As seen in Figure 10, this is the list of OECD (Organization for Economic Co-operation and Development) Aquatic Toxicity Tests.

Bioaccumulation – this refers to the accumulation of chemicals within the tissues of an organism over time. Depending on the degradation rate of the chemical, this can lead to a buildup in the organism over time, ultimately resulting in adverse biological effects. The bioaccumulation potential of a compound is directly related to its water solubility. Figure 11 presents a summary of the bioaccumulation potential by base oil type.

 Overall, when investigating the environmentally acceptable properties of lubricants, we can compare their behavior based on their base oil type, as shown in Figure 12 below.

Additionally, a comparison of their features by type is illustrated in Figure 13.

Several labelling programs provide guidance for EALS. These are summarized below in Figure 14.

Tips for Storing and Disposing of Hydraulic Oil

Hydraulic oils are easily contaminated, particularly during storage and handling. Due to the tight clearances in hydraulic systems and their function of transmitting power, any contaminant can cause an issue with the system’s efficiency. As such, it is crucial to store and dispose of these oils properly.

Every ounce of contamination prevented during storage saves hours of troubleshooting later.

Proper storage of hydraulic oils includes keeping the containers closed and protected from the elements, as this prevents dirt particles from entering easily. These oils should not be stored in an area that is not covered or exposed to the elements, as this increases the risk of contamination. For any oil that is decanted into smaller containers, filters should be used during decanting (into a clean container) and upon decanting into the equipment to minimize the transfer of particles from the outside.

When hydraulic oil reaches the end of its life, it must be disposed of properly. In different countries, various rules and regulations govern this disposal. In many countries, a certificate of custody and chain of transfer is required when moving used hydraulic oils from the equipment site to the site where they are disposed of. Some waste oil removal companies ask that the hydraulic oils be separated from other oils, especially in cases where these will be re-refined to create new oils.

Global Market Size and Growth Projections

In 2021, the global size of the hydraulic fluid market was $7.6 billion, with a projected compound annual growth rate (CAGR) of 5.2% from 2022 to 2030. On the other hand, in 2021, the US hydraulic fluids market was valued at $654.5 million, as shown in Figure 15 below. They have an estimated compound annual growth rate (CAGR) of 4.8% from 2022 to 2030.

Key Drivers and Challenges in the Market

An increase in construction due to greater global demand for infrastructure is noted as one of the key factors driving the demand for hydraulic fluids. Previously, during the pandemic in 2020, there were supply chain issues that affected the delivery of base oils to many markets. Additionally, the Russia-Ukraine war has also led to supply shortages, disrupting the regular market demand.

Regional and Industry-specific Trends in Hydraulic Oil Consumption

While the global hydraulic fluid market size was $7.6B in 2021, the most prevalent sector was construction (25.3%) as seen in Figure 16 below. As initially noted, the construction sector has seen a significant increase, but this should not distract from the prevalence of hydraulics in many other industries, including metal and mining, oil and gas, Agriculture, Automotive, aerospace, and defense, to name a few. 

Emerging Technologies and Their Impact on the Market

As regulations regarding emissions and their impact on the environment become more stringent, we are seeing some movement towards EALs (Environmentally Acceptable Lubricants) and even synthetic hydraulic lubricants. Users are more aware of the environmental implications of hydraulic oils entering waterways and the overall CO2e associated with mineral oils compared to synthetic lubricants.

References

1. Mang, T., & Dresel, W. (2007). Lubricants and Lubrication 2nd Edition. Weinheim: WILEY-VCH Verlag GmbH & Co. KGaA.
2. Hard Chrome Specialists. (2025, March 01). The History of Hydraulics. Retrieved from Hard Chrome Specialists: https://hcsplating.com/resources/hydraulic-systems-guide/history-of-hydraulics/
3. National Lubricating Grease Institute. (2025, March 01). Grease Glossary H1/H2/H3/3H/HX1. Retrieved from National Lubricating Grease Institute: https://www.nlgi.org/grease-glossary/h1-h2-h3-3h-hx1/
4. Totten, G. E. (2006). Handbook of Lubrication and Tribology Volume I Application and Maintenance Second Edition. Boca Raton: CRC Press, Taylor & Francis Group.
5. United States Environmental Protection Agency, Office of Wastewater Management, Washington, DC 20460. (2011, November). Environmentally Acceptable Lubricants, EPA 800‐R‐11‐002. Retrieved from U.S. Environmental Protection Agency: https://www3.epa.gov/npdes/pubs/vgp_environmentally_acceptable_lubricants.pdf
6. European Commssion. (2025, April 05). Aquatic toxicity. Retrieved from The Joint Research Centre: EU Science Hub: https://joint-research-centre.ec.europa.eu/projects-and-activities/reference-and-measurement/european-union-reference-laboratories/eu-reference-laboratory-alternatives-animal-testing-eurl-ecvam/alternative-methods-toxicity-testing/validated-test-methods-h
7. Houston, M. (2015). New EPA Regulations for Environmentally Acceptable Lubricants and their effect on the dredging industry. Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies’ “Dredging Summit and Expo 2015”, (pp. 384-391).
8. Grand View Research. (2022). Hydraulic Fluids Market Size, Share & Trends Analysis Report By Base Oil (Mineral Oil, Synthetic Oil, Bio-based Oil), By End-use (Construction, Oil & Gas, Agriculture, Metal & Mining), By Region, And Segment Forecasts, 2022 – 2030. San Francisco: Grand View Research.

Industries that rely on hydraulic systems are often some of the heaviest consumers of lubricants, primarily due to their tendency to leak. While best practice would suggest that they identify and fix the leaks, many companies are either unable or unwilling to address the root cause, electing instead to collect, reclaim, and re-use the fluid.

Introduction to Hydraulic Fluid Reclamation

Reclamation has received renewed focus recently due to supply chain disruptions in the lubricants market, in conjunction with an increased awareness of the environment and the carbon footprint of capital-intensive industries.

However, the subject of oil reclamation is not new. Reclaiming lubricating oils was addressed as far back as 1922 in a technical article published by the US Bureau of Standards (now Nation Institute of Standards & Technology – NIST)¹.

Of course, today’s lubricants and hydraulic fluids are very different from those used in the 1920s, but with appropriate care and attention, it is possible to reclaim lubricants successfully.

Before attempting to reclaim any lubricant, we need to start with a fundamental truth: lubricating oils do not last forever!

Both base oils and additives degrade through various processes, including oxidation, thermal stress, hydrolysis, shearing, neutralization (acid-base reaction), and absorption (Figure 1).

Many of these processes result in a permanent physical or chemical change to the lubricant that cannot easily be reversed, rendering the lubricant unfit for further use.

The Process and Importance of Oil Analysis

Reclamation involves much more than filtering the oil through a filter cart or filter skid and measuring particle count and moisture levels. While ensuring reclaimed fluids are clean and dry is essential, we must also determine if the oil’s fundamental properties are unchanged.

This requires that any reclaimed oil be subjected to a range of oil analysis tests beyond the standard tests run on in-service oil samples. Table 1 shows an ideal test slate for evaluating the health and cleanliness of reclaimed hydraulic fluid, along with the rationale for each test.

When collecting fluid to be reclaimed, the fluid must be isolated from other contaminants such as different grades or types of oils, process fluids, water, cleaning agents, etc. The captured oil should be collected and stored in a dedicated holding tank, ideally isolated from the ambient plant environment.

Because of the cost of performing some of the more advanced tests listed in Table 1, the fluid should be collected and batch-processed in sufficient quantity to justify the reclamation and testing expense.

Challenges and Considerations in Fluid Reclamation

Reclamation usually involves physical processes to remove unwanted contaminants, mostly particles and water. This can be done through a combination of mechanical filtration and vacuum dehydration to remove water down to low levels.

Centrifuges have also been used in the past with good effect but, in general, have been found to be less effective with newer base oil formulations. Suppose small amounts of sludge or other degradation by-products are present.

In that case, these can be removed using cellulose depth media filters or ion-exchange resins, provided the fluid does not show an overall high varnish potential and still has appropriate amounts of antioxidant additives present as measured by Linear Sweep Voltammetry (LSV).

Once cleaned and tested, the reclaimed fluid should be quarantined in the same or a separate holding tank before use. The quality assurance (QA) testing results should be retained as a baseline against which future in-service oil samples can be evaluated.

One of the most common questions when discussing reclaiming degraded fluid is the potential for re-additization of the oil. This is a logical question since additives typically degrade before base oil degradation. Adding third-party, aftermarket additives should not be taken lightly and is usually not recommended.

Additive manufacturers and oil formulators spend considerable time, money, and resources ensuring the functionality of their lubricants, carefully balancing the chemistries between different types of additives, which, unless carefully controlled, can produce severe, deleterious results.

If supported by the original oil manufacturer, including their technical and formulation teams, re-additizing an oil is possible. Still, the risk versus the reward should be carefully evaluated before proceeding.

While the list of tests outlined in Table 1 may seem daunting, proceeding without adequate testing can be a recipe for disaster.

Without the appropriate (healthy) additives, base oils can rapidly degrade when put back into service, creating sludge, varnish, and other deposits, which can become very difficult to displace in pumps, valves, hoses, lines, and hydraulic actuators.

Likewise, failure to correctly identify chemical contamination with other fluids or lubricants can result in aeration, foaming, and loss of demulsibility, all of which can destroy hydraulic pumps through cavitation.

The loss of base oil integrity can also change the fluid’s bulk modulus, which will change its compressibility.

This can cause changes in overall system control and impact the volumetric efficiency of hydraulic pumps, which in turn can impact cycle times. Since most hydraulic pumps require antiwear (AW) additives, QA testing should ensure that the fluid has sufficient AW additives left to meet the requirements of the OEM pump manufacturer.

While reclaiming hydraulic fluid may seem like a sound, financial, and environmental decision, extreme caution should be employed, including ensuring that the fluid meets or exceeds the oil suppliers’ and OEM’s minimum performance standards. When in doubt, you may be better served to recycle – or better yet, fix the leaks!

References

1. W. H. Herschel and A. H. Anderson Technologic Papers of the Bureau of Standards Volume 17, 1922-1924 p93-108

For professionals who work with machinery, it is vital to grasp the potential risks of fluid injection injuries. Although these incidents are rare, they are a significant concern, especially for personnel who are engaged in performing lubrication tasks. Let’s simplify the facts to understand why these injuries matter and how we can prioritize safety.

A few years ago, the International Fluid Power Society held a webinar on preventing and managing fluid injection injuries. The webinar highlighted some concerning facts from a study conducted by emergency department doctors at New York Methodist Hospital, revealing some serious statistics.

In North America, approximately 600 fluid injection incidents occur every year. This number may seem low, but it raises some concerns about maintenance staff working in the industrial sector.

Many doctors may underestimate these injuries because they’re rare, creating a problem since time is crucial in addressing them. On average, people wait about 8 to 9 hours before seeking medical help, assuming the injury isn’t severe. Unfortunately, waiting too long can worsen the situation and may even lead to amputation of limbs.

How Do Fluid-Injection Injuries Happen?

A question arises: Where do these injuries come from?

Most of these injuries occur from high-pressure grease guns and associated systems, making up 57% of cases.

Paint, hydraulic oil, and similar fluids follow at 18%, with diesel fuel injectors contributing 14%. Professionals who perform lubrication activities must understand the reasons behind these incidents to ensure their safety while working with machines.

Have a look at some of the worrying facts:

• The likelihood of amputation (losing a body part) after a fluid injection injury is, on average, 48%.
• If the pressure exceeds 7000 PSI, this risk approaches 100%. This highlights the need to ensure the utmost care, especially when dealing with high-pressure greasing systems.
• Another highly concerning factor is the duration it takes to seek medical assistance. If more than 10 hours pass, the likelihood of amputation can increase up to 100%. It is, therefore, crucial to act promptly to minimize the damage.

Injuries caused by fluid injections are often severe and usually need surgical intervention to fix the wound and avoid further damage caused by the fluid. Professionals working on high-pressure hydraulic machines and grease systems should spread this vital information and ensure everyone within their team knows how to stay safe.

How to Prevent High-Pressure Fluid Injection Injuries

Now, let us discuss how we can prevent these workplace-related injuries. First, be aware of the working hazards, be careful, take all the necessary precautions, let others know about the risks, and follow all the safety rules.

Understanding the dangers and taking simple precautions can help create a safe environment for lubrication tasks. Safety should always be the priority at the workplace.

Some basic steps must be implemented to ensure our workplaces are safe.

Always wear the appropriate PPEs, including hand gloves and protective eyewear, while performing the lubrication tasks.

While working with hydraulic systems, it’s important to be cautious and never underestimate the potential risks. If any signs of a fluid injection injury are noticed, like swelling or redness, seek medical consultation immediately without any delay. Waiting can worsen the injury, which should be avoided through immediate action.

Professionals who use high-pressure grease guns for the greasing task must be aware of the maximum pressure that grease guns can generate in case of overpressure.

Extra care must be taken when dealing with high-pressure grease guns and systems. It is also essential to be aware of the potential risks, including the chemical properties of the fluids being handled, and take the proper steps to avoid getting hurt.

It is important to let others know about safety regarding fluid handling. Consider having toolbox talks and safety awareness campaigns about fluid handling.

If everyone within the company knows and cares about safety, the chances of getting hurt by fluid injections are significantly reduced.

In conclusion, safety is everyone’s responsibility, especially when working with hydraulic machines. Understanding the associated risks, taking simple preventive measures, and spreading awareness can create a safer work environment for everyone within the organization. Let’s make safety a habit and ensure we go home safe and sound at the end of the day.

Despite their complexity, hydraulic systems are forgiving in nature. In most cases, they can perform well for a long time before any significant or catastrophic failure occurs.

Unfortunately, this “creep to failure” quality can promote apathy in production and maintenance attitudes toward failure prevention, efficiency optimization, and service-life management of these workhorse systems.

The most critical component in any hydraulic system is, unequivocally, its hydraulic-fluid medium. Hydraulic fluid is an engineered component designed to perform three major tasks simultaneously:

1. Act as a solid to transfer and amplify power.
2. Lubricate a system’s moving parts.
3. Absorb and dissipate frictional heat buildup within the system.

If the hydraulic system is to perform efficiently, the hydraulic fluid must be kept scrupulously clean and contamination-free. Knowing and acting on this simple requirement provides the maintenance planner with an easy, laser-focused strategy to manage and prevent most hydraulic system failures for literally pennies a day.

Hydraulic-fluid contamination can be present in three forms, i.e., as a solid particle, water, or air, which can seriously affect the fluid’s effectiveness and damage the components it serves.

Maintaining hydraulic fluid in an optimum condition requires a preventive strategy focused on measuring, controlling, and preventing the introduction of all three forms of contamination.

Solids Contamination

Hydraulic systems are not “dirt” tolerant. Cylinders and spool-valve assemblies are designed with precise operating tolerances that can be closer than .0002″, or 5 microns in size (the size of a human red blood cell).

For comparison, a human hair is approximately .0035″ in diameter or 80 microns. Most solids manifest as grit or dirt of 100 microns or more in size that, if allowed to enter a hydraulic system, will damage machined surfaces and hydraulic seals.

In a close-tolerance environment, solid particles set up in a three-body abrasion state that will easily score mated machined surfaces, creating rapid bearing and component surface wear. This leads to undesirable fluid bypass, causing reduced hydraulic operating efficiency.

Solid contaminants can induce a variety of problems. They include valve stiction, in which valves can become “sticky” and inefficient due to static friction; increased fluid viscosity that results in sluggish operation (due to excess fluid thickness); and unwanted fluid leakage, where “nicked” and scored cylinder seals and wipe surfaces allow the contaminant to be pushed, or “weep,” past the seal.

If the equipment is new or freshly rebuilt, all hoses, lines, and fittings must be cleaned internally. This is best accomplished using a compressed-air-projectile cleaning tool/system that shoots cleaning-wad material through the hoses and fittings to remove all dirt and swarf before connecting all lines and performing an initial system flush.

Once cleaning and flushing are complete, the system filters can be changed, and the reservoir and system filled with correct-viscosity hydraulic oil are ready for service.

Solid contamination can also find its way into new hydraulic fluid before delivery by the supplier or manufacturer, especially if bulk transferred with dirty transfer hoses and equipment. When receiving new oil—especially bulk oil—always perform an oil-analysis test to detect solids and water contamination.

New oil for use in high-pressure hydraulic systems should test, at least, at an ISO cleanliness level of 16/14/11 or better. To prevent receipt of unacceptable hydraulic fluid, set up a cleanliness contract with your oil supplier based on a contracted minimum cleanliness level. The supplier would then be bound to provide proof of cleanliness upon delivery.

Other preventable primary sources of maintenance-introduced solids contamination include:

• improperly stored oil (loose or open lids, incorrect container orientation, etc.)
• non-dedicated or dirty transfer equipment is used to transfer the oil into the equipment reservoir
• reuse of dirty “leaked” oil
• “open to air” reservoir due to missing fill-port or missing reservoir breather
• lack of filter maintenance, causing dirty oil to bypass into the system
• incorrect filter rating (a 100-micron filter rating provides zero system protection)
• poor housekeeping practices (cleanliness is godliness when handling lubricants and hydraulic fluids).

Water Contamination

Hydraulic fluid is naturally hygroscopic (water-retentive) and can entrain moisture until its saturation point. Most hydraulic fluids’ saturation point is reached around 300 ppm, or at a 0.04% concentration level.

Water is a universal contaminant that can deplete vital oil additives and react with them to create corrosive acids that can attack system components.

In addition, water can reduce lubricant film strength and its ability to release air, thus increasing the chances of wear, corrosion, and cavitation (see “Air Contamination” section below). In high-heat applications, water can also “boil off “and create significant inefficiency in the hydraulic-power transfer motion.

Typical preventable water contamination sources include:

• incorrect outdoor lubricant storage hot-cold cycle condensation
• “open to air” reservoir allowing ingression of wash down and process water.

Water can be present (and visually detected) in a free state (separated from the oil in an unstable form) and an emulsified state (in a stable form, the fluid appears cloudy).

It must be dealt with by using a polymeric-style filter media designed to absorb the water as it passes through the filter, through vacuum distillation to “boil off “the oil, and/or through dehumidification of the air in the reservoir headspace with the use of desiccant breathers.

Air Contamination

Air contamination causes problems when entrained in the fluid or oil. In this form, air bubbles less than one millimeter in diameter are dispersed throughout the fluid, reducing fluid viscosity and resulting in a lack of full film strength, causing premature component wear.

Entrained air can also reduce oil’s bulk modulus, causing a lack of efficiency and control due to the sponginess of the oil condition and increasing the heat load, resulting in fluid deterioration and system erosion or cavitation wear.

As for air bubbles, those over one millimeter can create foam, quickly depleting any hydraulic fluid antifoam additive and causing the fluid to oxidize.

Typical preventable-air-contamination causes include:

• over/under-filled lubricant reservoir(s)
• clogged inlet/suction filters
• clogged reservoir breather
• restricted inlet line
• loosely clamped inlet lines
• pump-shaft seal failure.

Fortunately, contamination problems are easily preventable and inexpensive to implement.

The following recommendations can be used as a checklist or starting point for an improved PM approach to hydraulic system preventive maintenance.

• Implement a fluid cleanliness standard.
• Store all lubricants in a cool, dry place and practice FIFO (First In-First Out) stock rotation.
• Practice good housekeeping and ensure reservoirs are kept clean of all dirt and debris.
• Cap all hydraulic hoses and manifolds during handling and maintenance.
• Wad-clean all lines before initial system startup.
• Flush all lube systems before startup and change oil and filter(s).
• Use a dedicated filter cart (one for each hydraulic fluid type) with quick-connect fittings to transfer and clean hydraulic fluid before it enters a reservoir.
• Install external sight gauges marked with high- and low-fluid markers on reservoirs to check for water contamination and correct fluid levels (leakage detection).
• Have the machine operator regularly check for system reservoir levels and leaks.
• Repair all system leaks as soon as they are discovered.
• Use polymeric oil filters and desiccant reservoir-style air breathers.
• Specify cylinder-rod wipers and replace all worn actuator seals.
• Regularly perform oil-analysis testing in hydraulic fluids for cleanliness and additive efficacy.

Bottom Line

Remember the hardworking hydraulic systems in your plant. They are the lifeblood of many manufacturing operations. Their effectiveness is not just a financial concern; it’s also a safety concern. With basic care, common sense, and little expense, you can ensure these systems are efficient, reliable, and long-lived.

Previously published in The RAM Review.

Many questions surround hydraulic oil selection, from viscosity grades to base oil types to additives. This article answers pressing questions like: How do temperatures influence oil choice? Should you use zinc-based or non-zinc fluids? Should engine oils be used in hydraulics? And why does oil cleanliness matter so much?

How Do Weather Conditions Affect the Choice of Hydraulic Oils?

It might affect your choice in two ways: viscosity and viscosity index. Hydraulic oil viscosity tends to vary between ISO 32-100, and part of the decision process may include the ambient temperature, which can affect the bulk oil operating temperature.

As the viscosity (thickness) of the oil is highly temperature dependent, colder climates may require thinner oils, while hotter parts of the world may require thicker oils.

The viscosity index is also a fundamental consideration. This index measures the oil’s change in viscosity with temperature variation. You can classify hydraulic oils into two primary categories based on their viscosity index:

• Standard Viscosity Index: Typically ranges from 80 to 110.
• High Viscosity Index: Ranges from 120 up.

Oils with a high viscosity index are beneficial when stable viscosity is required at elevated temperatures. This stability is especially crucial when the ambient temperature is high or the operating machinery, like plastic injection molding machines, imposes high thermal stress on the oil.

Mobile equipment with high power density is another example where a high viscosity index hydraulic fluid would be advantageous.

When Should I Choose Zinc or Non-Zinc Hydraulic Oil?

The zinc in hydraulic oils is usually related to the antiwear package and essential oil components primarily responsible for the pump’s wear protection. There are primarily two categories:

• Zinc-Based: Contains zinc or ZDDP, a widely-used antiwear additive found in many engine and hydraulic oils.
• Zinc-Free: Oils without zinc. In some cases, oils may be advertised as “zinc-free” when elemental analysis shows traces of zinc – this is likely due to zinc-containing detergents rather than the antiwear additive.

It’s essential to understand that while zinc is a popular additive, it comes with challenges. Zinc toxicity can pose environmental concerns, especially if machinery, such as cranes, operates over water. In such cases, a zinc-free formulation might be preferable.

Additionally, zinc tends to break down at very high temperatures, leading to sludge formation. Thus, many of the newer “ashless” antiwear additives produce less sludge, making them a better choice in high-temperature situations.

What Kind of Hydraulic Fluids Are on the Market, and What Is the Best?

This question doesn’t have a straightforward answer. The “best” choice often hinges on specific applications and scenarios. So, let’s discuss the various hydraulic fluids available in the market.

Hydraulic fluids can be primarily classified based on their base oil into the following categories:

1. Mineral-Based Hydraulic Oils: These are the most prevalent type, chosen in approximately 90 to 95% of applications. The inherent properties of mineral oil can be enhanced using additives. For instance, if a high viscosity index is desirable, formulators usually add viscosity index improvers to the mineral oil to achieve the desired performance.
2. PAO (Polyalphaolefin) Synthetics: These are synthetic hydraulic oils. The primary advantage of transitioning from a high VI mineral to a PAO synthetic is primarily evident in frigid temperatures, where they carry a pour point advantage. For example, the pour point performance of neat PAO is in the order of -60°C at the lower viscosity grades. Thus, a fully synthetic hydraulic oil might be the ideal choice when machinery operates in exceptionally frigid environments, like wind turbines in northern Canada or mine sites in Russia.
3. Synthetic & Natural Esters: These are frequently found in eco-friendly lubricants. If there is a requirement for a biodegradable hydraulic fluid, it will likely be an ester-based fluid, whether derived from vegetable sources or synthesized. Natural esters usually have reduced oxidation stability relative to mineral oils, while synthetic esters offer excellent temperature performance and low varnish formation.
4. Phosphate Esters: Lubricants often consist of polyol esters or phosphate esters in high-temperature scenarios demanding fire-resistant properties.

On top of the base oil type, different oils will contain varying additive combinations (see zinc versus zinc-free above), which will cause the performance to vary.

So deciding which is “best” is not so straightforward – but rather, the conditions and application should be matched to the product.

Why Do Some Hydraulic Fluids Have an SAE Grade While Some Have an ISO Grade?

Navigating through the world of hydraulic oils, you’ll encounter various classifications that can be somewhat perplexing. In the industrial sector, the typical viscosity grades for hydraulic oils are identified as ISO grades like 32, 46, 68, or 100. However, if your domain is mobile equipment, you might stumble upon hydraulic oils labeled with SAE grades such as 10W or 30, 40.

The distinction between these two types is rooted in their formulation traditions. In the realm of mobile equipment, there was a history of creating “cheap and dirty” formulations. This process involves taking an engine oil and reducing the concentration of its additive package.

Despite this reduction, the oil retains many characteristics of an engine oil, including antiwear properties and the presence of detergents, which are uncommon in industrial hydraulic oils. This type of oil is often called a “cutback engine oil,” and it’s why you’ll find mobile equipment hydraulic oils using the SAE (Society of Automotive Engineers) viscosity grading system.

Nowadays, having an SAE viscosity grade does not necessarily mean that the hydraulic fluid is a cutback engine oil, but rather can signify it is designed for mobile equipment applications.

One of the significant differences between hydraulic oils tailored for mobile equipment versus those for industrial applications lies in their detergency level. Detergents in hydraulic oil play a pivotal role in how the oil interacts with water.

Oils with detergency will typically hold onto water—a characteristic that might benefit systems with small sumps where the maintenance team is likely to remove contaminants by performing an oil change. Conversely, in industrial systems where the reservoirs are significantly larger, the desired property is for the oil to allow water to separate, forming a distinct layer that can be easily drained off.

Can I Use Engine Oil as Hydraulic Oil?

While technically, this is possible, it’s generally not advisable. To understand why, let’s examine engine oils’ distinct functions and formulations compared to hydraulic oils.

Engine oils are crafted to withstand the harsh conditions of combustion engines. They need to handle contaminants like soot, water, and acids, which are byproducts of combustion. This is why engine oils typically have a high Total Base Number (TBN) and include dispersants to cope with these elements—features not commonly found in hydraulic oils.

In contrast, hydraulic oils are designed with different priorities in mind. Where an engine oil is over-engineered to retain contaminants until the next oil change, hydraulic systems often require the opposite.

For instance, in many industrial applications, you’d want water to separate from the oil easily so it can be removed, ensuring the smooth functioning of the hydraulic system.

Employing engine oil in a hydraulic system could lead to suboptimal performance. The engine oil’s propensity to hold onto water and other contaminants might disrupt the hydraulic system’s operation, which could be designed to purge these contaminants efficiently.

Is Oil Cleanliness Important to Hydraulic Oils?

In the world of hydraulics, the cleanliness of oil is not just a preference—it’s a mandate for the longevity and efficient operation of the system. An excellent demonstration of the effects of cleanliness on the life of hydraulic assets can be found in publicly accessible life extension tables, demonstrating a direct correlation between oil cleanliness and component lifetime.

Consider this: by improving the ISO cleanliness code of hydraulic oil from, let’s say, 24/22/19 to 19/17/14, the theoretical life extension is 4x – a remarkable return on investment considering the typical costs for oil filtration versus the capital cost of a new hydraulic system.

This significant life extension is primarily due to reducing contaminants that can cause wear and tear on the system’s components.

Contaminants are a primary contributor to wear in hydraulic systems, but their impact doesn’t stop there. Hydraulic controls, particularly servo valves, operate under tight tolerances, making them highly sensitive to contamination.

The presence of contaminants can lead to ‘silting’ or ‘servo sticking,’ where the valves do not actuate smoothly. This issue can cause a delay or ‘lag’ in the system’s response, where the actual movement does not align with the commanded action, leading to operational errors.

It is common to hear about deposits or varnish in turbine oils, but can other oils also suffer from varnish? The short answer is yes. Once an oil (regardless of its application or type) has been exposed to thermally stressful conditions, contaminants, and severe operating environments, it will degrade, possibly producing deposits.

Hydraulic systems are susceptible to deposit formation due to the prevalent use of close-tolerance components and requirements for ultra-clean operating fluids.

Today, like many other industrial lube systems, hydraulic fluids are subjected to greater amounts of stress than in the past. This is the case across many industries, from injection molding and steel mills to off-road and construction equipment. To reduce cost, weight, and space, reservoirs are shrinking.

A recent survey of plastic injection molding machines revealed that a major manufacturer had reduced the oil reservoir volume in the same machine by 40% over the last ten years.

To improve performance, valves have tighter clearances while system pressures are increasing. Engineering hydraulic systems with increased power outputs and productivity also require higher speeds and shorter cycle times.

OEMs are also designing systems with reduced noise levels and lower carbon footprints, which require hydraulic oils to have great efficiency. These evolutionary changes in hydraulic system design place further demands on hydraulic oils.

End-users also share some responsibility for the increased stress on hydraulic oils. Their attention to leak reduction inadvertently results in fewer additions of new oil top-offs. Also, many hydraulic machines are operated at a higher rate than originally designed.

The confluence of system design changes and more severe operating conditions is more oxidative stress and less recovery time for the oil. Oxidation is now the most common form of fluid failure.

Lubricant manufacturers have responded to these stressful conditions by using more highly refined base oils with lower levels of wax, sulfur, and aromatics. Antiwear additives are also being upgraded.

The dominant antiwear additive technology in hydraulic oils remains zinc dialkyldithiophosphate (ZDDP). New formulations, however, are evolving to higher performing, more environmentally friendly, ashless antiwear technologies.

Since hydraulic fluids can directly impact the energy efficiency of the overall hydraulic system, formulations are being designed to provide improved energy efficiency. This is accomplished through improved viscometrics.

Shear-stable, high Viscosity Index (VI) hydraulic oils have shown energy efficiency improvements across multiple applications. Studies have shown that these advanced formulations may reduce energy by up to 10% in some applications. Shear-stable VI improvers are typically polymethacrylate polymers and can make up to 15% of the overall hydraulic oil formulation.

Finally, more antioxidants are used in hydraulic oils, producing a more oxidatively robust product that is less prone to deposits. Even with these changes, however, we still see hydraulic oil varnish occurring more frequently than in the past.

What Is Hydraulic Oil Varnish?

Varnish is a thin deposit in a lubrication system that is difficult to wipe away and comprises mainly organic residue. Its chemical composition is widely varied and classified by chemistry and degradation mechanisms. The terms varnish and sludge are often used interchangeably, but they mean different things.

Sludge can be considered easy to wipe, gooey in nature, and usually contains moisture. Conversely, varnish has a more cured, shiny appearance and is not easy to wipe. These definitions help users identify whether they have sludge or varnish in their system.

Varnish is primarily caused by the continual process of oxidation, which is accelerated by temperature, various metallic components, and gaseous catalysts.

Oxidation was originally defined as a reaction involving the combination with oxygen. However, its definition has now been expanded to include any reaction in which electrons are transferred from a molecule.

Oxidation is the predominant reaction a lubricant undergoes in service, accounting for significant lubricant performance problems.

Contaminants will also increase the rate of reaction. In hydraulic oils, there are three different types of contamination: dirt contamination (silt, wear metals), water, and air.

Oxidation is the major source of viscosity increase, varnish formation, sludge and sediment formation, additive depletion, base oil breakdown, filter plugging, loss of foam resistance, loss of demulsibility, acid number increase, rust, and corrosion. Deposits, however, are most often the link between oil degradation and machine performance.

Varnish can cause a wide range of operational challenges in lube systems. These can include; increased wear rates experienced due to the “sandpaper” effect. Heat exchanger efficiency declines as varnish creates an insulating layer on the tubes.

Oil flow is subsequently impaired, potentially causing starvation issues. Filters may blind off quickly as they become slimed. As such, oil life is shortened due to the reactive nature of these deposits.

Servo valves are the most sensitive component impacted by varnish. Often the oil flow to these components is intermittent, increasing the likelihood of deposits forming in these components. A leading cause of valve hysteresis and chatter due to the increased friction required to move varnished components.

Some of the operational impacts of a hydraulic system are:

• Valve lock resulting in high levels of static friction
• Loss of clamp speed or cycle time impacting production
• Reduced repeatability and precision impacting scrap rate
• Increased set-up and interchangeability times due to additional maintenance and variance adjustments impacting production.

An increased amount of energy required to move these valves results in major energy losses for the overall system. Additionally, due to the loss of hydraulic control of the system, they are often replaced prematurely, costing more maintenance dollars. Since the issue causing the varnish was not initially solved, then the newly replaced valves will also become coated with varnish and can fail more rapidly.

The Critical Role of Condition Monitoring

Failure Modes and Effects Analysis (FMEA) has been a dominant part of reliability programs since its conceptualization in the 1940s. FMEA provides a systematic method of evaluating failure modes, their causes, and their effects. This should be the basis of any oil analysis program.

Why perform an oil analysis test if it does not detect a lubricant’s failure mode? Conversely, if one knows the primary failure modes, shouldn’t your condition monitoring program be set up to detect this failure? Unfortunately, many hydraulic oil analysis programs are not aligned with the fluid’s primary failure mode, resulting in unexpected problems.

Case Study: “But my oil analysis reports look good!”

A plastic injection molding facility started having machine performance problems that increased their scrap rate and reduced productivity. The maintenance manager increased their oil sample test slate and frequency to ensure it wasn’t an oil-related issue. Most of the machines came back in acceptable condition. Here’s an example report from Machine 15, which uses AW46 hydraulic oil in their presses:

This “acceptable” oil analysis report gave the maintenance manager a false sense of security. During the next maintenance activity, the maintenance manager noticed varnish deposits on his valve components and in the hydraulic oil reservoir. This prompted further analysis.

A voltammetry (RULER) test was used to measure the antioxidant health in the fluid, as seen in Figure 1.

Next, the maintenance manager performed a Membrane Patch Colorimetry (ASTM D7843) test on the in-service hydraulic oil. The result was a DE of 61, determined to be in the critical range.

The additional analysis revealed the source of the plastic injection molding machine performance problems – oxidation and varnish development.

What can we learn from this case study?

The maintenance manager thought that he was testing for oxidation and varnish tendencies. He mistakenly thought that elemental spectroscopy would indicate the health of his ZDDP additive package and that acid number and viscosity would identify oxidation. Here’s why those tests were ineffective at identifying oxidation and deposit formation:

Elemental spectroscopy is a powerful oil analysis tool and is effective for identifying metallic additive elements. For example, it can identify the presence of zinc to determine if the formulation is ashless. However, this test does not measure additive health. The ZDDP molecule may no longer be intact and effective. It may have degraded to form sulfates and phosphates. However, the zinc and phosphorus elements will remain in solution until they eventually settle out as sludge.

Acid Number has been integral to hydraulic oil analysis for decades. The final component in an oxidation reaction is acid, so this test has shown value in measuring the results of oxidation. However, because Group II and III base oils have lower solubility, the degradation products may not remain in the oil long enough to be converted to an acidic molecule. Also, acids will only start to develop in the oil once all the antioxidants have been depleted, which is often too late.

Viscosity increases may be due to oxidation; however, by the time a noticeable change has occurred, many other problems have already started.

Appropriate tests for detecting oxidation and its effects on hydraulic oils

To adequately predict hydraulic oil failure due to oxidation, the following tests are recommended:

• Voltammetry, RULER (ASTM D6971) – This test identifies the antioxidant health of both primary and secondary antioxidants used in hydraulic oils.
• Molecular Spectroscopy using Fourier Transform Infrared (FTIR) – Several ASTM methods can be of value in monitoring hydraulic oils.
  • ASTM D7414 measures oxidation
  • ASTM D 7412 measures antiwear additives
  • An ASTM draft method is currently drafted to measure acid number
  • FTIR is also effective at measuring phenolic antioxidants
• Membrane Patch Colorimetry (ASTM D7843) – This test method effectively determines a hydraulic oil’s propensity to form varnish. Its results are reported on the CIE LAB ΔE scale, as can be seen below in Fig. 3

• Colorimetric Analysis – A revised version of the MPC test, colorimetric analysis is based on research by Dr. Sasaki and Prof. Honda. In addition to varnish potential, it provides further insight into the mode and rate of degradation.
• Ultracentrifuge – Extracting and qualitatively measuring the amount of degradation products through high centrifugal forces has successfully predicted the onset of varnish.
• Air release – measures the fluid’s ability to dissipate air bubbles which can be of value, especially in the areas where hydraulic oil is required to do the work of moving a valve or lifting a bearing shaft. The deterioration of air release values is linked to both hydraulic oil degradation and contamination.

Solving the Deposit Challenge for Hydraulic Oils

One of the easiest and most cost-effective methods of eliminating deposits from hydraulic oils is using a solubility enhancer. Solubility enhancers should be considered under the following conditions:

1. The solution must be added when the machine is in operation.
2. It must be compatible with the in-service oil and other system materials.
3. It cannot affect the oil’s interaction with contaminants, inhibit corrosion, or antiwear performance. Therefore, surface active chemistries such as dispersants or detergents should never be used.
4. It must also work under intense conditions without breaking down.
5. Does not give in to oxidative stress.
6. Using the solubility enhancer eliminates the need to install any other mitigation system.
7. Works under extreme oxidative conditions and does not produce deposits when burnt.

The Fluitec team developed Solvancer® technology (patent pending) after the exhaustive categorization of the various deposit chemistries in hydraulic systems. Three essential properties were studied during this categorization; polarity, hydrogen bonding, and dispersive forces.

Solvancer has been optimized to consider these three characteristics to ensure that lube deposits are quickly dissolved into the lubricant. It is typically added at a rate of 3-5% to an in-service lubricant and has no adverse effects on the performance of the fluid. It goes to work immediately to dissolve deposits and prevent further varnish from forming.

The case study below shows the immediate and long-term impact of adding Solvancer to an in-service oil. The graph also shows the results from a 6-week accelerated oxidation test (120°C) without and with Solvancer.

Hydraulic oils are subjected to increased thermal and mechanical stress in modern hydraulic systems causing system deposits and fluid failure. Hydraulic oil formulations are being tweaked to accommodate this more stressful environment; however, problems persist.

The leading reason hydraulic oil users experience performance problems with their fluid is inadequate oil analysis testing. Most condition monitoring programs do not measure oil degradation and deposit formation. By upgrading your oil analysis package to include monitoring your antioxidant system’s health and deposits’ formation, many hydraulic oil degradation problems can be avoided.

A deposit-free hydraulic oil can exist with a solubility enhancer such as Solvancer®, which will not affect the hydraulic oil’s performance properties. The RULER and MPC tests can effectively monitor the health of the lubricant to amply warn operators of the presence of deposits in hydraulic systems. These can be coupled with FTIR, Air release, and elemental spectroscopy to provide a full suite of tests.

Adapted from the paper, “Managing hydraulic oil deposits by using novel solubility enhancing technology” written by; Jo Ameye, Greg Livingstone & Cristian Soto of Fluitec, presented at OilDoc 2023.

The old battle cry in the hydraulics world is “flow makes it go.” However, pressure determines the ability of actuators to move heavy loads. Therefore, flow and pressure together define the total power of the hydraulic system.

Hydraulic Oil Viscosity

Hydraulic oil must contain the correct viscosity, or the system cannot develop that power or operate properly. Viscosity is the most crucial property of lubrication, yet it is shrouded in mystery and confusion.

Viscosity is a measurement of a fluid’s internal resistance to flow. This definition is based on a specific liquid’s molecules having more or less internal friction.

The typical viscosities of hydraulic fluids are ISO VG 32, 46, and even 68, but they are available in an even wider range of ISO VG 22 to 100, which are also used in hydraulics. Selecting the correct viscosity is a balancing act that requires juggling two conflicting requirements.

Effects of Operating Temperature

This is because oil viscosity reduces rapidly with the increasing temperature of the oil and increases with decreasing temperature of the oil. This means that operating temperature is king. Controlling operating temperature reigns as the predominant viscosity-affecting variable.

Too hot oil will easily bypass through pumps, valves, and cylinders. This bypassing causes additional heat and a slowdown of the system. Cold, thick oil will cause the system to operate sluggishly or erratically. The main issue with cold oil is pump cavitation.

When the viscous oil restricts the fluid’s flow enough at the pump’s inlet, the pressure drops low enough (vacuum increases) that the liquid can’t exist as oil but vaporizes or boils into a gas. The little gas pockets released not only do not lubricate well but when exposed to high pressure at the pump outlet, the cavity walls of the bubbles collapse or implode.

This reaction generates tons of pounds of force per square inch and can produce destructive micro jets of oil that collide against the internal pump surfaces at extremely high velocities. Catastrophic pump failure and distribution of pump fragments throughout the system will occur.

Hydraulic oil must be light enough at very low starting temperatures to be easily sucked into the pump, preventing cavitation. But it must also be heavy enough at high operating temperatures to avoid wear and internal leakage inside the pump, resulting in inefficient operation. Be careful not to get too heavy or too light of oil, or it could result in excessive heat.

Address this dilemma by using high-viscosity index fluids. With high VI hydraulic fluids, the optimum operating range of acceptable viscosities spreads over a wider temperature range, and the temperature range for the highest overall efficiency is also wider. So you save energy. To make a high VI fluid (with a VI considerably more than 100), the oil supplier can use a high VI base oil, viscosity modifiers, or both.

Individual pump manufacturers specify their own viscosity requirements and operating temperature ranges. As a good rule of thumb, I recommend these temperature mile markers.

• Minimum start-up temperature – 40°F
• Ideal operating temperature – 120°F
• Max temperature for 100% life – 140°F
• System noise may increase due to thinner oil.
• Oil life is cut in half at 140°F – 155°F.

Operating Temperature as a System Health Indicator

Hydraulic oil starts breaking down at around 140°F. For every 15° above 140°, the life of the oil is cut in half. Hydraulic oil life is halved at 155°F and halved again at 170°F due to the temperature dependence of oil and its reaction rate, as per the Arrhenius equation.

So remember that the site glass thermometer/ temperature gauge on your reservoir is more than a shiny piece of costume jewelry on the side of your tank. It is a critical tool for maintaining the viscosity of your oil and the reliability of your system.

Record and rigorously track the system’s operating temperature, which may vary with season and reservoir location. Regard it as a vital system health indicator and troubleshooting tool.

Reliability Solutions offers hydraulic training with the 3H approach to learning. The Hybrid, Heuristic, Hands-on program provides a balanced blend of computer-based pre-work, live online training of fundamentals, applied assignments to be completed on your equipment, and a live, hands-on, onsite session with our hydraulic demonstrators. For more information, contact Katie Dickson at katiedickson@reliabilitysolutions.net.