Hydraulic fluid is one of the most important components of a hydraulic system. It performs multiple functions, such as power transmission, lubrication, heat transfer, and conveyance of sludge, wear debris, and contamination. With the important roles played by fluids, proper fluid selection is critical in maximizing performance and life of hydraulic pumps, motors, and other components. To make the right choice, a variety of fluid properties must be considered, along with other factors such as operating parameters, system requirements, environmental conditions, and regulations.
Hydraulic components and the hydraulic fluid work together to run the hydraulic system. Because the fluid is the medium by which power is transmitted to perform any usable work, hydraulic systems simply cannot operate without sufficient fluid.
With the critical role of hydraulic fluid, appropriate fluid type, viscosity, and quality are essential requirements for fluid selection. A hydraulic system with a poorly matched fluid may operate, but deliver substandard performance, and ultimately could lead to catastrophic failures. Improper fluid selection can cause various undesirable results, such as decreased system efficiency, lack of lubrication, reduced fluid and component life, corrosion, erosion, sludge and varnish formation, and excessive heat generation.
In addition to fluid properties and quality, contamination also affects system performance significantly. Contamination can generally originate from four sources: contaminated oil, built-in contamination, ingressed contamination, and internally-generated contamination. It can lead to decreased efficiency, component wear, and other adverse impacts. Studies indicate that more than 70 percent of hydraulic system failures are due to contamination and can reduce hydraulic efficiency as much as 20 percent before a system malfunction is recognized [1].
Various fluid properties affect the fluid’s ability to perform different functions. Viscosity, which describes a fluid’s resistance to flow, is the most important. It accounts for hydrodynamic/boundary lubrication, volumetric efficiency, mechanical efficiency, cavitation, quantity of lubricants\ reaching lubricated parts, heat generation, and many other properties like air release, heat dissipation, and filterability.
Low viscosity fluids provide thin film thickness, leading to boundary lubrication conditions, which can result in metal-to-metal contact and damage system components. For example, when two moving metal surfaces contact each other with inadequate lubrication, excessive wear can occur due to cold welding, as shown in Figure 1, and damage components. Low viscosity also reduces volumetric efficiency of pumps and motors through increased internal leakages.
The rate of air release varies based on different viscosities and temperatures. At a given temperature, air is released faster with lower viscosity fluids, as shown in Figure 3. As the temperature increases, air is also released faster for each fluid.
Viscosity itself is affected by temperature, with contributing factors of environment temperature, operating temperature, and system design. Suitable viscosity grade fluid needs to be selected for each application based on the operating temperatures. The fluid viscosity at operating temperature must meet the viscosity recommendations of the system components, primarily the pump. Minimum, normal, and maximum operating s need to be considered for selecting the fluid viscosity grade.
Overall efficiencies of hydraulic components are related to mechanical efficiency and volumetric efficiency. Mechanical efficiency is related to frictional losses and drag due to fluid viscosity, and volumetric efficiency relates to internal leakages. Both volumetric and mechanical efficiencies depend on the viscosity of the fluid. As shown in Figure 4, volumetric efficiency increases with increased viscosity and mechanical efficiency increases with decreased viscosity. The particular range of viscosity at which overall efficiency is maximum is typically selected as the optimum range for the specific components. Viscosity recommendations should be considered for all system components, but viscosity recommendations for pumps and motors should be given prime importance.
The Eaton-Vickers 35VQ25 pump test was developed to demonstrate a fluid’s ability to protect components from wear, thereby confirming long-term use in various operating conditions, as shown in Figure 7. The test was adopted by the American Society for Testing and Materials (ASTM) with the designation of ASTM D6973 (Standard Test Method for Indicating Wear Characteristics of Petroleum Hydraulic Fluids in a High Pressure Constant Volume Vane Pump).
The 35VQ pump test can be used to evaluate anti-wear properties of hydraulic fluids. Higher performance fluids can dramatically reduce wear and extend the life of components, as represented in Figure 8.
A properly selected fluid meets the requirements of the various properties mentioned above in a balanced manner. Because identifying and interpreting all fluid requirements for a common user is difficult, Eaton has developed a full-fledged specification that cover requirements of hydraulic fluids and stipulate base stock requirements, physical properties, and performance requirements for both conventional and zinc-free hydraulic fluids, along with material compatibility with rubber materials. Fluids meeting this specification have been considered good quality hydraulic fluids[2]. Most oil/ additive manufacturers follow Eaton’s specification and print the same on their catalogs and on oil containers, so that the common users can identify quality lubricants easily.
Eaton has developed fluid recommendations with for Eaton hydraulic products, such as “Hydraulic Fluid Recommendations,” which provides basic guidelines for selecting hydraulic fluid. The document provides viscosity recommendations and cleanliness requirements for Eaton hydraulic products[3].
Numerous lubricants other than conventional hydraulic fluids are used in hydraulic systems, such as motor oil, automatic transmission fluids, universal tractor transmission oil (UTTO), and super tractor oil universal (STOU). Some of these are formulated with mineral/ petroleum-base oils, while others are synthetic-base such as those using a polyalphaolefin (PAO) base. Most Eaton components are rated with these fluids.
While hydraulic pumps are generally designed to operate with mineral-base fluids, alternative fluids are sometimes used for applications where certain special properties are essential, perhaps even more important than hydraulic system performance.
For specific applications such as environmentally friendly and fireresistant applications, certain alternate fluids such as phosphate esters, polyol esters, polyether polyols, polyalkylene glycols, vegetable oil base fluids, water glycol, and invert emulsions, are sometimes used. For example, if fire resistance is critical, fireresistant properties may be the most important factor. Similarly, in environmentally sensitive applications, the focus may be on biodegradability and toxicity. Other fluid types, such as food-grade fluids and military specification fluids, are sometimes used in hydraulic applications.
Each alternative fluid may have certain advantages for the specified application, but their fundamental properties, such as lubrication, coefficient of friction, pressure viscosity coefficient, vapor pressure, specific gravity, and low temperature properties, may not be equivalent to those of mineral-base lubricants. Therefore, hydraulic components, especially pumps and motors may need to be de-rated to account for these adverse factors.
What happens if an improperly matched fluid is used in a pump? The answer can vary, depending on the degree of mismatch. As noted earlier, fully formulated fluids should have balanced properties. Excessive variation in any given parameter can cause direct or indirect impacts. For example, a fluid incompatible with certain rubber materials may cause failure of gaskets and other components, as shown in Figure 9.
A single issue can cause catastrophic failure. For example, poorly maintained systems may fail catastrophically due to one or more reasons. Contamination, along with incompatible fluids, can lead to component failures, as shown in Figure 10. Any undesirable matter in the fluid is a contaminant and could be particulate matter, water, air, or other lubricants. The Eaton document “The Systemic Approach to Contamination Control” provides more information on contamination and its control[1].
With all the properties to consider, fluid selection may seem like a daunting process. For assistance, contact your Eaton representative or Eaton’s lubricant specialist for pump/motor ratings with different alternate fluids. As an additional resource, the Eaton Lubricant Help Desk can offer assistance in defining fluid requirements, identifying available documentation, and other technical support.