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Hydraulic fluids perform four basic functions. Their primary function is to create force and motion as flow is converted to pressure near the point of use. Second, by occupying the space between metal surfaces, the fluid forms a seal that provides a pressure barrier and helps exclude contaminants. The third function is the lubrication of metal surfaces. Finally, hydraulic fluid cools system components.

If any one of these functions is impaired, the hydraulic system will not perform as designed. Worse yet, sudden and catastrophic failure is possible. The resulting downtime can easily cost a large manufacturing plant thousands of dollars an hour. Hydraulic fluid maintenance helps prevent or reduce unplanned downtime. It is accomplished through a continuous program to minimize and remove contaminants.

Contaminants and their sources

The most common source of system impairment is fluid contamination. Contamination can exist as solid particles, water, air, or reactive chemicals. All impair fluid functions in one way or another. Contaminants enter a hydraulic system in a variety of ways. They may be built in during manufacturing and assembly processes, internally generated during normal operation, or ingested from outside the system during normal operation.

If not properly flushed out, contaminants from manufacturing and assembly will be left in the system. These contaminants include dust, welding slag, rubber particles from hoses and seals, sand from castings, and metal debris from machined components. Also, when fluid is initially added to the system, a certain amount of contamination probably comes with it. Typically, this contamination includes various kinds of dust particles and water.

During system operation, dust also enters through breather caps, imperfect seals, and any other openings. System operation generates internal contamination as well. This occurs as component wear debris and chemical byproducts from fluid and additive break down due to heat or chemical reactions. Such materials then react with component surfaces to create even more contaminants.

Contaminant interference

In broad terms, contaminant interference manifests itself as either mechanical or chemical interaction with components, fluid, or fluid additives. Mechanical interactions include blocking of passageways by hard or soft solid particles and wear between hard particles and component surfaces.

Chemical reactions include formation of rust or other oxidation, conversion of the fluid into unwanted compounds, depletion of additives (sometimes forming harmful byproducts), and production of biochemicals by microbes in the fluid.

Any of these interactions will be harmful. Without preventive measures and fluid conditioning, their negative effects can escalate to the point of component failure. One of the most common failure modes is excessive wear due to loss of lubrication.

Lubrication and wear

The pressures required in modern hydraulic systems demand tough, precisely machined components. Precision machining leaves very small clearances between moving parts. For example, it is not uncommon for control valves to have spools and bores matched and fitted within a mechanical tolerance of ±0.0002 in. (5 µm). With some of today’s electrohydraulic valves, tolerances may be even tighter, with clearances of 1 µm or less. The surface finishes on high-pressure bearings and gears can result in rolling clearances as small as 0.1 µin.

The hydraulic fluid is expected to create a lubricating film to keep these precision parts separated. Ideally, the film is thick enough to completely fill the clearance between moving parts. This condition is known as hydrodynamic or full-film lubrication, and it results in low wear rates. When the wear rate is kept low enough, a component is likely to reach its intended service life expectancy, which may be millions of pressurization cycles.

The actual thickness of a lubricating film depends on fluid viscosity, applied load, and the relative speed of the two dynamic surfaces. In many applications, mechanical loads are so high that they squeeze the lubricant into a very thin film, less than 1 µin. thick. This is known as thin-film lubrication. If loads become high enough, the asperities of the two moving parts will puncture the film. The result is boundary lubrication, which allows metal-to-metal contact and the surface wear resulting from it.

Component and system designers try to avoid boundary lubrication by making sure that fluid is of the proper viscosity. However, viscosity can change as the fluid temperature changes. Also, loads and speed may vary widely during normal operating cycles. Therefore, most hydraulic components operate at least part of the time with only boundary lubrication. When that happens, parts of moving surfaces contact each other and are torn away from the parent material. The resulting particles then enter the fluid stream and travel throughout the system. If not removed by filtration, they react with other metal parts to create even more wear.

Lubricant manufacturers continually strive to reduce potential lubrication issues by improving fluids with additives. Viscosity-index (VI) improvers are added to help keep viscosity stable as temperature changes. Antiwear additives increase film strength. For hydraulic fluids, defoamers, demulsifiers, detergents, or dispersants may be added. Rust and oxidation (R&O) inhibitors are used in most hydraulic fluids because air and water are always present to some extent.

Particle-generated wear

Symptoms of component wear are sluggish or erratic system operation, poor efficiency, and short component life. In pumps, wear first may be detected as reduced flow rate. This is because abrasive wear has increased internal clearance dimensions. Sometimes called increased slippage, this condition means that the pump is less efficient than it was when new.

When pump flow rate decreases, the fluid system may become sluggish, as evidenced by hydraulic actuators moving slower. Pressure at some locations in the system also may decrease. Eventually, there can be a sudden catastrophic failure of the pump. In extreme cases, this can occur within a few minutes after initial startup of the system.

In valves, wear increases internal leakage. The effect this leakage has on the system depends on the type of valve. For example, in flow-control valves, increased leakage usually means increased flow. In valves designed to control pressure, increased leakage may reduce the circuit pressure set by the valve. Silting interference causes valves and variable-flow pump parts to become sticky and operate erratically. Erratic operation shows up as flow and pressure surges, causing jerky motion in actuators.

Assessing contamination

Contamination control involves preventing contaminants from entering a hydraulic system. For example, filters may be placed in strategic locations throughout the system to trap any contaminants that do find their way into the fluid.

But for critical equipment, a successful contamination control program also must include regular assessment of the hydraulic fluid’s cleanliness. Often, fluid must be checked every two to six months or after every 500 or 1,000 hours of operation, depending on the equipment’s duty cycle, operating environment, and how critical it is to overall operation.

Experts also recommend that fluid be tested immediately after any maintenance event that exposes the hydraulic system to the external environment, such as when a hose or other component is replaced or fluid is added to the reservoir. Fluid replenishment can be particularly troublesome because new fluid is notorious for being dirty, often from improper storage and handling practices.

Test labs or do it yourself?

Before the advent of portable contamination detection instruments, fluid testing was conducted for only the most critical equipment and sent to a laboratory for analysis. This is still the most practical route for companies that do not require fluid testing often enough to justify purchasing their own diagnostic equipment and training their personnel. Even if a company has equipment, test labs still prove valuable for running multiple tests, interpreting results, troubleshooting, and recommending appropriate action.

Test labs also should be consulted for periodic chemical analysis of hydraulic fluid. Even if contamination has been brought to within acceptable limits, certain contaminants can alter a fluid’s chemical composition (primarily the additives) and render it ineffective. For example, additive depletion, over time, can reduce a fluid’s lubricity, oxidation resistance, or anti-foaming characteristics.

Excessive water and overheating can upset a fluid’s chemical balance relatively quickly. If excessive water is found in a fluid or a system overheats, experts recommend not only finding and correcting the source of the problem, but conducting chemical analysis of the fluid as well. Even if these problems do not occur, hydraulic fluid suppliers generally recommend having fluid analyzed chemically at regular intervals, such as annually, to identify potential problems and prevent them from occurring.

Portable particle counters and other diagnostic equipment have made it easy and convenient to monitor the fluid cleanliness of even non-critical equipment. In fact, many companies that have invested in their own particle counters and other instruments monitor the cleanliness of more equipment more often.

The higher reliability that results from this more intense preventive maintenance adds to the return on their investment. Furthermore, advanced techniques are being developed to make fluid-monitoring instruments even more sophisticated. Equipment currently is under development to continuously monitor the condition of hydraulic fluid while equipment is running.

Assessment starts with a sample

Regardless of the details of any contamination-monitoring program, its usefulness will depend on fluid sampling. Fluid samples must accurately represent the condition of the fluid within the hydraulic system. This means that the sampling technique and devices, as well as the container, must not contaminate the fluid sample.

The point from which samples are extracted should be determined by the information desired from the sample. For example, sampling fluid from a system’s pump discharge line will likely produce different results from a sample taken from a return line. Fluid from the pump discharge line is more likely to contain pump wear debris than fluid from a return line because filters would have captured pump wear debris before it would reach a return line.

Experts advise, however, that samples taken from the reservoir usually are the most unreliable. First, because a reservoir acts as a storage device, its contents have accumulated over a relatively long interval, whereas fluid from a hydraulic line is more representative of conditions at the time the sample is taken. Second, most reservoirs are designed to minimize turbulent flow so contaminants can settle to the bottom and air can rise to the top. This makes it difficult to obtain a sample with a representative concentration of water and other contaminants.

Fluid analysis techniques

Once fluid samples have been obtained, any of several methods can be used to analyze the size, concentration, and nature of the contaminants. The most common analysis techniques for hydraulic systems are particle distribution, gravimetric, ferrographic wear debris, proton induced X-ray, and water content.

Each of these tests produces different results according to the type of information desired. Therefore, they should not be viewed as competing technologies. Rather, the more tests that are conducted on a sample, the more knowledge that can be gained. But no matter which technique is employed, obtaining a pure and representative sample is essential to achieving accurate results.

Particle distribution summarizes the number of contaminant particles classified by size for a sample. Automatic particle counters have gained wide acceptance for this previously time-consuming, laborious task that produced inconsistent results. The widespread use of particle counters is a testament to their ease of use and consistent reliability. Technicians often use them at manufacturing and maintenance facilities.

Gravimetric analysis summarizes the total mass of solid particles above a given size for a specific volume of fluid. Results are reported as mass density, usually mg/l. Unlike particle counting, gravimetric analysis quantifies only solid particles, not water.

But gravimetric analysis does not indicate size distribution, so a sample may contain 25 mg/l of solid particles greater than, say, 5 µm. It does not indicate what percentage of particles is greater than 10 µm and how many are greater than 15 or even 25 µm. As with particle counters, technicians often use gravimetric analysis instruments to monitor contaminants in hydraulic systems.

Ferrographic wear debris analysis quantifies wear debris (primarily metals) in a fluid sample. Because the most highly stressed wearing parts of machine components are made of steel, wear debris usually are influenced by magnetic fields. Ferrographic analysis can be used to evaluate a system’s wear mechanisms, assess the severity of wear, and identify the predominant materials being worn away.

Proton-induced X-ray emission (PIXE) summarizes the elemental composition of solid contaminants and wear debris in a fluid. After the fluid sample is exposed to a proton beam, a computer interprets the results of the test by producing data on the entire spectrum of elements in the target, not just a single element. Neither particle counting nor gravimetric techniques can differentiate between foreign contaminants and wear debris, which makes PIXE useful for gaining insight into the nature of particles found in a fluid.

Water-content analysis determines how much water is present in a base fluid. Next to particulate matter, water, by far, is the most damaging contaminant in a hydraulic system—or any oil-lubricated system, for that matter. Higher concentrations of water in hydraulic oil accelerate wear, fluid degradation, corrosion, and reduction in service life. Therefore, once the amount of water present in a hydraulic fluid has been found, the challenge becomes determining how much can be tolerated.

The test itself uses a solution that conducts electrical current based, in part, on the amount of water contained in a sample. Measuring the current and its duration indicates the water content in the base fluid. The test can be accurate to within 10 ppm, but additive packages common to hydraulic fluid tend to produce less detailed results.

New technology enhances assessment

Whether your car has an “idiot light” or a real temperature gauge, its designers deemed the engine’s water temperature important enough to monitor it continuously. After all, it wouldn’t do much good to have your engine’s water temperature checked only when you stop for gas. It’s much more likely that a problem will occur while you are on the road rather than when you are at the gas station.

Monitoring contamination of hydraulic fluid would at first seem to be much less critical than engine water temperature. After all, fluid usually becomes contaminated gradually, so monitoring its condition frequently enough can identify problems before they cause any real harm. Engine temperature, however, can increase quickly once a problem occurs. If a hose ruptures, the water pump gives out, or the radiator leaks, the engine can quickly overheat.

Contamination, under certain conditions, also can act quickly to cause catastrophic failure in a hydraulic system. For example, if a pump ingests enough air to cause serious cavitation, it can become inoperative within days. Or if a large quantity of water flows through a system, hydraulic fluid can lose its lubricity, which will result in rapid wear of components. If either of these events occurred a few weeks before a scheduled fluid analysis, the machine could undergo costly downtime.

Granted, these types of problems happen rarely. But if the equipment costs millions of dollars or works in an operation where downtime is measured in thousands of dollars per hour, it becomes practical to continuously monitor fluid cleanliness. Companies, then, are developing systems to monitor the cleanliness of hydraulic fluid continuously while a system is running.

One such prototype system routes pressurized fluid from the pump into a tube through which light is transmitted. When the fluid is clean and relatively free of air and water, a receptor detects the amount and pattern of light transmitted through the fluid.

As the fluid becomes more contaminated, the amount and diffraction of light transmitted through the tube changes. If undissolved water or air is present, the transmitted light becomes more scattered. Calibrating the receptor to these different conditions provides an instantaneous indication of the fluid’s condition. Therefore, appropriate and immediate action can prevent a potentially catastrophic failure.

Another emerging technology enables users to go beyond particle counting and actually analyze wear debris. The system consists of hardware to generate digital photomicrographs and software to aid in analyzing the digital images. Once imported to a PC, images can be compared to those in an atlas of known wear debris using wear debris analysis software. The software also aids in characterizing descriptions, managing data, and generating reports. The analysis can be incorporated into maintenance and statistical process control software used for plant operation and quality assessment.