Fig. 1(a). Three-body mechanical interactions can result in interference. (b) Two-body wear is common in hydraulic components. (c) Hard particles can create three-body wear to generate more particles. (d) Particle effects can begin surface wear.

Fig. 2(a). In highly magnified representation, ideal lubrication forms full film between two surfaces. (b) During boundary lubrication, metal asperities of surfaces make physical contact and are torn off.

Fig. 3. Even accumulations of soft particles can cause silting interference if they are the same size as the clearance dimension.

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, which provides a pressure barrier and helps exclude contaminants. A third function - often misunderstood - is lubrication of metal surfaces. The fourth and final function provided by hydraulic fluid is cooling of 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.

Aside from human interference, 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:

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 also generates internal contamination. This occurs as component wear debris and chemical byproducts from fluid and additive breakdown 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, Figure 1, include blockage of passages by hard or soft solid particles, and wear between particles and component surfaces.

Chemical reactions include: formation of rust or other oxidation, conversion of the fluid into unwanted compounds, depletion of additives - sometimes involving 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 sturdy, precisely matched components. And precision machining leaves very small clearances between moving parts. For example, it is not uncommon for control valves to have pistons and bores matched and fitted within a mechanical tolerance of ±0.0002 in. (two ten-thousandths of an inch). In metric units, this is about 5 µm (five millionths of a meter). In modern electrohydraulic devices, tolerances may be even tighter and clearances can be less than 1 mm. 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, Figure 2. This condition is known as hydrodynamic or full-film lubrication and 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 elastohydrodynamic (EHD) or thin-film lubrication. If loads become high enough, the film will be punctured by the asperities of the two moving parts. The result is boundary lubrication.

Component and system designers try to avoid boundary lubrication by making sure that fluid has the proper viscosity. However, viscosity changes 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.

Fluid chemists continually try to minimize potential lubrication problems by improving fluids with additives. Viscosity-index (VI) improvers are added to help keep viscosity stable as temperature changes. Antiwear additives increase film strength. If very heavy loads will be applied, the fluid should contain an extreme pressure (EP) additive that reacts with metal surfaces to form a hard protective film. For fluids in circulating systems, defoamant, demulsifier, detergent, or dispersant may be added. Rust and oxidation (RO) inhibitors are used in most hydraulic fluids because air and water are always present to some extent.

Particle-generated wear

The symptoms of wear are diminished system performance and shorter component life. In pumps, wear may first 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 start-up 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 valves used to control flow, 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, Figure 3, 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.

Water contamination of liquids

Water in oil-base fluids can be just as destructive as particle contaminants. And water exclusion is very difficult to accomplish. Because of its affinity for other liquids, water is present in some concentration in most hydraulic systems. The hygroscopic nature of liquids causes them to pick up a certain amount of water simply from contact with humid air. When condensation occurs in a reservoir, with subsequent mixing into the base liquid, more water can be added to the system. Water can even enter with new oil. An oil barrel stored outdoors in a vertical position is likely to have rainwater collect around its bung. With changes in ambient temperature, some of this moisture can be sucked into the barrel. Eventually, this water enters the system when the reservoir is filled from that barrel.

Besides these natural phenomena, there are several system- and maintenance-related sources of water. In machine tool applications, a good deal of water-base coolant can enter hydraulic systems through breather caps and imperfect seals. Worn and damaged heat exchangers can allow cooling water to leak through seals and ruptured lines into the oil system - and vice versa.

Each fluid has its own saturation level for water. Below the saturation level, water will be completely dissolved in the other fluid. For oil-base hydraulic fluids, the saturation level is likely to be in the range of 100 to 1,000 parts per million (0.01% to 0.1%) at room temperature. At higher temperatures, the saturation level is greater.

Above the saturation level, water becomes entrained, meaning that it takes the form of relatively large droplets. This also is called free water. Sometimes these droplets combine and precipitate to the bottom of the reservoir. At other times, due to churning or other mixing action, undissolved water is emulsified so that it exists as very fine droplets suspended in the oil.

Water's mechanical effects

When water concentration in hydraulic oil reaches 1% or 2%, the response of a hydraulic system may be affected. If water alters the hydraulic fluid's viscosity, the operating characteristics of the hydraulic system change. When the rate of water influx is swift, poor system response could be the first indication that water is present.

Cavitation is another symptom of water in the fluid. Because the vapor pressure of water is higher than hydrocarbon liquids, even small amounts of water in solution can cause cavitation in pumps and other components. This occurs when water vaporizes in the low pressure areas of components, such as the suction side of a pump. Vaporization is followed by the subsequent violent collapse of vapor bubbles against metal surfaces in these areas. The loud characteristic sounds of cavitation may be noticeable when this happens. The result is cavitation damage on the interior surfaces of hydraulic components because the metal has fatigued.

Emulsified water in oil

Tiny water droplets may be emulsified or suspended in oil-base fluids. Evidence of this is when the fluid appears cloudy or milky. Sometimes an oil/water emulsion is so tight that it is very difficult to separate the two fluids - even with the addition of a coalescing agent formulated to do this. While this is desirable in emulsion-type hydraulic fluids, it is highly undesirable for ordinary oil-base fluids. Some fluid additives encourage emulsification, while others (demulsifiers) discourage it. The viscosity of a water emulsion can be much different from its original base liquid. As noted earlier, lower viscosity reduces the thickness of lubricating films, leading to increased wear of surfaces in dynamic contact.

If free water is present in hydraulic fluid and the system operates at temperatures below 32° F, ice crystals may form. These crystals can plug component orifices and clearance spaces. In hydraulic systems, this will cause slow or erratic response.

Without fluid analysis to warn of water's presence and appropriate control measures, water content probably will increase to the point where these and other symptoms appear. Other symptoms include evidence of chemical-reaction products and eventually, component failures.

Chemical reactions due to water

Water reacts with almost everything in a hydraulic system. Water promotes corrosion through galvanic action by acting as an electrolyte to conduct electricity between dissimilar materials. The most obvious sign is rust and other oxidation that appears on metal surfaces. The inside top surface of the reservoir often is the first place rust becomes visible, but such rust still may go undetected unless the reservoir is drained and opened for servicing. Also, the time it takes for rust to form depends partly on the original surface treatment used to protect the metal used to build the reservoir.

Unfortunately, before rust is noticed in the reservoir, water probably has damaged other system components. An inspection of failed bearings and other components may point to corrosion damage. Corroded aluminum and zinc alloys could have a whitish oxide film. Steel bearing and gear surfaces may show signs of rust and pitting.

Water's reaction with oxidation inhibitors produces acids and precipitates. These water-reaction products also increase wear and interference. At high operating temperature (above 140° F), water reacts with and actually can destroy zinc-type antiwear additives. For example, zinc dithiophosphate (ZDTP) is a popular boundary lubricant added to hydraulic fluid to reduce wear in high-pressure pumps, gears, and bearings. When this type of additive is depleted by its reaction with water, abrasive wear will accelerate rapidly. The depletion shows up as premature component failure, resulting from metal fatigue and other wear mechanisms.

Water frequently can act as an adhesive that causes smaller contaminant particles to clump together in a larger mass. These gluey masses may slow down a valve spool, or cause it to stick in one position. Or these clumps could plug component orifices. In any case, the result is erratic operation or a complete system failure.