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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.