The right chemistry minimizes operating problems and saves money.

David Oesterle
Product Manager,
Hydraulic and Industrial Gear Additives
The Lubrizol Corp.
Wickliffe, Ohio

Using the wrong hydraulic fluid can have severe and costly consequences, such as catastrophic pump failure.

Using the wrong hydraulic fluid can have severe and costly consequences, such as catastrophic pump failure.


Hydraulic fluids are the workhorses of hydraulic systems. They transmit power, lubricate and help cool equipment, and prevent rust and corrosion. But hydraulic fluids are not one-size-fits-all commodities. Fluid manufacturers choose from an array of base oils and additives to tailor characteristics to specific applications.

For instance, some fluids resist oxidation and thermal degradation in high-temperature equipment, such as plastic injection-molding machines. Others need to resist fire, biodegrade, or operate in wet conditions, requiring different formulations.

Not all additive packages or combinations provide equal performance, so it is important to understand how additives work in specific base oils. Reputable suppliers conduct laboratory bench testing and hydraulic-pump testing before introducing fluids into the marketplace.

And make no mistake, there are financial consequences to using the wrong fluid. In addition to the cost of prematurely replacing the fluid, there are expenses associated with the labor to drain old fluid and add new, lost productivity while equipment is down, fluid disposal costs, and the negative effects of wear and corrosion on equipment.

Here are three key parameters to know and discuss with fluid suppliers.

1. Viscosity. This is the most important property to consider when selecting a fluid. Viscosity is the strength of the fluid's cohesive force. It determines fluid friction and the drag moving parts exert, which pulls fluid between metal surfaces. Viscosity at the equipment's operating temperature determines the rate at which fluid flows through a bearing — and with it bearing friction and load-carrying capacity.

Fluid with the proper viscosity quickly distributes to moving surfaces — a key to long pump life. Fluid with lower-than-recommended viscosity can cause internal pump leakage and raise operating temperatures. Conversely, fluid with viscosity that is too high will be sluggish and not spread well throughout the hydraulic system. Of course, be sure to follow the equipment manufacturer's fluid guidelines.

Mineral-based hydraulic oils tend to become less viscous at higher temperatures and thicken at lower temperatures. Such viscosity changes are common in outdoor mobile equipment such as excavators, cherry pickers, and backhoes. Viscosity modifiers help alleviate the effects of temperature fluctuations. These are typically long-chain polymers that minimize viscosity changes despite temperature swings. Viscosity modifiers lessen the need for individual winter and summer-grade oils.

2. Base oil. Traditionally, hydraulic fluids have been produced from solvent-neutral mineral oils (called Group I base oils). But the trend is to move toward hydro-processed base oils (Group II, III, and higher). As group number increases, so does cost. But higher-group-number base oils can extend equipment life due to lower volatility, better oxidation resistance, and less sensitivity to water in the system.

In addition, higher-group-number base oils contain less sulfur and have more saturated hydrocarbon molecules, both of which are better for equipment. Saturate level refers to the number of double bonds — or unsaturated molecules — present in an oil. Unsaturated molecules in Group I base oils serve as reaction points for oxidation, which can ultimately shorten hydraulic-oil life. Higher group-number base oils, in contrast, have fewer unsaturated molecules. Combined with efficient antioxidants, the result is better oxidative stability.

3. Additive package. Hydraulic fluids are made up of about 99% base oil and 1% or less additives. But that one percent is critical. Factors that influence additive selection include performance, compatibility, color, odor, and economics.

As mentioned previously, additives provide a number of properties, including antiwear and corrosion protection. They can also contain a demulsifier to separate water from the oil, so water can be drained from the system. This helps extend fluid life. And additives provide good thermal and oxidation stability. Properly formulated hydraulic oils provide all these properties under a wide variety of conditions.

Several industry trends are affecting the additives in hydraulic fluids. One is a move toward smaller hydraulic systems, where the fluid spends less time in the system's reservoir. That means less time to release air and control foam. Air contamination affects precision in hydraulic actuators, and dispersed air bubbles in the fluid — aeration — can result in severe pump erosion. Compact hydraulic systems also have less time to cool down, so equipment tends to run hotter.

As a result, additives in smaller systems must work harder because they have less time to interact with contaminants such as dirt and metal particles. The additives also have less time to demulsify water that inevitably gets into hydraulic fluid through condensation or leakage. Unchecked, water in the fluid plugs filters and causes corrosion and pump wear.

Smaller hydraulic systems simply use less oil, so additives must have reserve performance. Additive suppliers continue to develop additives for the harsh environment and pressures of compact hydraulic systems. Examples include more thermally stable antiwear chemistry and better antioxidants to improve performance. These additives, in combination with higher quality base oils, have resulted in hydraulic fluids better suited to small systems.

Another cost-saving trend is extending the drain intervals — longer fluid life. In such cases, the additive package must have enough power in reserve — what is termed chemistry — to provide performance and durability despite severe applications and long overdue maintenance.

Long-life hydraulic oils must also meet stringent OEM filtration requirements that include the ability to filter effectively when contaminated by water and sludge generated by oxidation and thermal degradation. Many hydraulic formulations commonly use detergents to keep systems clean.

Other hydraulic equipment trends that impact fluids and additives include:

  • Higher pressures and power density that drive up temperature, increasing the chance of varnish deposits and sludge.
  • Environmental sensitivities such as biodegradability. Because additives are used at low percentages, the base oil plays a large role. Many biodegradable hydraulic oils are derived from vegetable oils or esters.
  • Reconditioning fluids. There are many different ways users can reclaim, recycle, and recondition hydraulic fluids. But they must understand the impact of reconditioning fluids. New additives mixed in the oil must be compatible with existing additives — which is not always the case. And topping off fluids with new additives may invalidate OEM warranties.

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