Transfusion of knowledge: Fluid is a hydraulic system's 'lifeblood'

Hydraulic systems are expected to perform a tremendous amount of work in industrial facilities. In many cases, the hydraulic system is the heart of the operation, and hydraulic fluid its lifeblood. Reasonable attention, careful selection of lubrication, and a few simple steps can maintain efficient operation and trouble-free performance of hydraulic machinery.


Key Concepts


  • Not all OEM-approved oils may perform adequately in a system.

  • Premium quality oils prolong the life of hydraulic components.

  • Regularly monitor the condition of the hydraulic fluid.

    Review recommendations
    Select quality oil
    Maintain system performance
    Monitor fluid conditions
    When in doubt, ask
    More Info:

    4 Steps to hydraulic and system maintenance

    Hydraulic systems are expected to perform a tremendous amount of work in industrial facilities. In many cases, the hydraulic system is the heart of the operation, and hydraulic fluid its lifeblood. Reasonable attention, careful selection of lubrication, and a few simple steps can maintain efficient operation and trouble-free performance of hydraulic machinery.

    Review recommendations

    Correct fluid specifications can be found in the original equipment manufacturer's (OEM) service manual. Many major OEMs also now offer websites that provide equipment information and specifications. In critical applications, certain oils and fluids evaluated and approved by the OEM are identified as having met stringent performance requirements. It is important to be aware of these specifications.

    Many hydraulic fluid manufacturers publicize their OEM performance approvals. However, lubrication performance varies significantly, even among those fluids approved to meet OEM specifications.

    There are still other hydraulic fluids in the market that have not been approved by any OEM, but which claim to "meet the requirements" of the OEM specification. Suppliers of these fluids may have conducted in-house tests similar to those conducted by the OEM. In such cases, the supplier is qualifying the fluid's performance without OEM approval. Some suppliers take this minimal approach to hydraulic oil formulation while others take the proper steps to make sure their oils are the best available.

    Customer service among lubricant suppliers varies significantly as well. Some hydraulic oil suppliers offer strong technical support and advisory support to operators for the optimal hydraulic fluid, filter size, and filter placement.

    A few suppliers even provide on-site oil reclamation services to remove particles and water from hydraulic fluids, extending their useful life. These services can be a pragmatic, convenient solution, especially when large volumes are involved.

    Select quality oil

    It is not only important to select the correct fluid based on OEM recommendations, but also to select a premium quality hydraulic fluid. Using premium quality usually offers better wear protection in adverse conditions.

    The performance of hydraulic oils depends on the quality of the base oil and the effectiveness of its additives. With prolonged pump life, increased system efficiency and reduced premature component failure, machinery downtime is reduced and plant productivity maximized.

    Hydraulic oil selection should also focus on the lubricant's key properties: viscosity, the viscosity index, and the pour point. Viscosity is the most important property of hydraulic oil, indicating its resistance to flow. Using the correct viscosity is essential to achieving the lowest possible incidences of metal-to-metal contact within the system, which leads to premature wear and eventually component failure.

    Several measurement units are applied to viscosity, including centipoise, centistokes, Saybolt Universal Seconds, ISO grades, AGMA grades, and SAE grades (Fig. 1). In each of the measurement systems, higher numbers indicate thicker or more viscous fluids. Check the OEM fluid viscosity specifications for your equipment.

    Temperature affects fluid viscosity. Two fluids can have the same viscosity at 210° F but differ entirely at 0 F. Viscosity index (VI) reflects the effect of temperature fluctuations on fluid viscosity. The higher the VI, the less change in viscosity with temperature fluctuations. Conventional hydraulic oils typically have a VI between 90° F and 110° F (Fig. 2).

    A high VI lubricant, above 110° F, is not critical when a hydraulic system operates within a narrow range of temperatures, such as machinery on a plant floor. When the range is wide, such as in outdoor applications, using fluid with an extra high VI can be very important.

    Cooler temperatures will require a hydraulic fluid with a low pour point, the lowest temperature at which it will flow. If the fluid does not flow at startup, premature pump failure can occur. Ideally, the fluid pour point should be at least 20° F below the lowest expected operating temperature. Once the correct viscosity and pour point requirements are determined, the operator must also consider the fluid's ability to protect system components and handle various system conditions.

    Maintain system performance

    Many operators assume that all hydraulic oils are alike, but this is incorrect. Apart from the variance in quality and performance between different manufacturers' fluids, there are more important considerations that hold true.

    Lower quality oils generally cost less but do not yield adequate value. Subsequent expenses incurred through limited wear protection, increased maintenance expense, and longer machine downtimes arguably outweigh the initial benefits of using lower cost or lower performing oils. Investing in a high quality lubricant can help operators realize a reduction in maintenance and repair costs, increased wear protection, and higher productivity levels.

    Hydraulic pumps are precision instruments in every sense of the word. They are expected to provide years of trouble-free operation. Their sustained efficiency depends on the absence of abnormal wear throughout their life. Abnormal wear causes increased internal slippage of the oil in the pump, resulting in power loss, reduced output, and increased oil operating temperatures.

    Contaminants in pumps and system components are extremely small and difficult to detect. Even small amounts of varnish or sludge formed by oxidation of the oil, or contaminants such as rust, dirt, or water can cause trouble. If left unattended, contaminants accumulate in the system in significant amounts over time.

    Flushing can remove small amounts of contaminants, but it is less effective in removing the solid or semi-solid deposits from heavily sludged hydraulic systems. Such systems need to be dismantled and cleaned mechanically, which adds to the total cost of operation.

    The presence of water and contaminants impedes filtration in poorly formulated oils, leading to higher filter costs and poor system cleanliness. Operators need oils that have good filterability and don't rapidly degrade under the severe conditions present in hydraulic systems. Fluid cleanliness through proper filtration and the use of high-grade hydraulic oils is imperative.

    Monitor fluid conditions

    The hydraulic system's condition also can affect the fluid selection. If a system leaks, oil must be added more frequently than normal. Operators of leaky equipment often choose lower quality, lower priced fluids in the hopes that constant replenishment will maintain the overall condition of the system. This generally proves to be false.

    Frequent oil changes and top-offs reduce the need for oxidation stability because the oil is not in service for very long periods. But other qualities, such as filterability, wear, and corrosion protection are independent of oil change intervals. Inattention to these other properties can lead to more frequent replacement of filters and components.

    Air leakage into hydraulic systems can cause the oil to oxidize quickly. Air is usually due to leakage in the oil suction line or low levels in the oil reservoir, which permit the pump suction inlet to be exposed. Oxidation of hydraulic fluids is a major concern as it shortens the life of the oil.

    The rate of oxidation is accelerated by contaminants such as cutting oils, greases, dirt, moisture, paint, joint compounds, and insoluble oxidation products. Metals, particularly copper, are catalysts for oil oxidation and the presence of water greatly enhances the catalytic effect.

    In turn, oxidation increases the viscosity of the oil and corrodes metals in the hydraulic system. This leads to the formation of deposits and degradation by-products in the fluid, which can then cause sluggish operation, wear, and plugging of lines.

    Any moisture that enters the hydraulic system, whether through contamination or condensation, may emulsify the fluid under violent churning conditions (Fig. 3). Emulsion promotes the collection of grit and dirt, affects the proper function of valves, increases wear and corrosion, promotes oil oxidation, depletes additives, and plugs ultra-fine filters. Most hydraulic equipment manufacturers prefer or specify the use of hydraulic oils that separate water rapidly in order to address this fairly common contaminant problem.

    Hundreds of additives can improve the oxidation stability of fluids. The ASTM oxidation test is a valuable tool for rooting out the poorest of these additives, but the fluid service life cannot be predicted from ASTM oxidation data alone. A fluid rated at 2,000 hours in this test will not necessarily have double the service life of a 1,000-hour oil. Only the use of reliable oil monitoring test programs can determine which fluid will last the longest in any particular hydraulic system.

    Even with machines operating at low load conditions, the use of a high quality product is recommended. Protecting components against wear under low load conditions can be as difficult as under high load conditions because many anti-wear additives are not activated under milder operating conditions.

    When in doubt, ask

    Maintaining a hydraulic system in optimal running condition involves many important decisions, most of which should be based on careful deliberation of many factors. This can be a labor-intensive project, and many operations are not adequately staffed to deal with this.

    Advice from a quality lubricant supplier is one of the best options available to an equipment operator. A supplier should provide technical support regarding the properties of fluids and recommendations regarding preventive maintenance, oil and equipment diagnostics, consulting and training and fluid reconditioning services.

    More Info:

    Questions about hydraulic oils may be directed to Dennis Woodley at 630-820-0025, visit , or call Drew Johnson at 713-546-6429. Article edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, .

    4 Steps to hydraulic and system maintenance

    Review OEM recommendations to select the correct fluid.

    Select a high quality fluid with good physical and chemical stability.

    Properly maintain fluid cleanliness for best hydraulic performance.

    Routinely monitor hydraulic system and fluid condition to detect contamination and degradation problems early.

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