Learning Objective: Identifiy the types of hydraulic contamination found in naval aircraft.

There are many different forms of contamination, including liquids, gasses, and solid matter of various composition, size, and shape. Normally, contami-nation in an operating hydraulic system originates at several different sources. The rate of its introduction depends upon many factors directly related to wear and chemical reaction. Contamination removal can reverse this trend. Production of contaminants in the hydraulic system increases with the number of system components. The rate of contamination from external sources is not readily predictable. A hydraulic system can be seriously contaminated by poor maintenance practices that lead to introducing large amounts of external contaminants. Poorly maintained SE is another source of contamination. 

Contaminants in hydraulic fluids are classified as particulate and fluid contamination. They may be further classified according to their type, such as organic, metallic solids, nonmetallic solids, foreign fluids, air, and water.

The type of contamination most often found in aircraft hydraulic systems consists of solid matter. This type of contamination is known as particulate contamination.

The size of particulate matter in hydraulic fluid is measured in microns (millionths of a meter). The largest dimensions (points on the outside of the particle) of the particle are measured when determining its size. The relative size of particles, measured in microns, is shown in figure 4-2. Table 4-1 shows the various classes of particulate contamination levels.

Contamination of hydraulic fluid with particulate matter is a principal cause of wear in hydraulic pumps, actuators, valves, and servo valves. Spool-type electrohydraulic valves have been used in particle contamination experiments. The valves are easy to control and respond rapidly to repositioning. In these experiments, the valves were operated with both ultraclean and contaminated hydraulic fluids. The experiments proved that wear is accelerated by even small amounts of contamination. Contamination increases the rate of erosion of the sharp spool edges and general deterioration of the spool surfaces. Because of the extremely close fit of spools in servo valve housings, the valves are particularly susceptible to damage or erratic operation when operated with contaminated hydraulic fluid.

Figure 4-2. Graphic comparison of particle sizes.