microscopic dimensions (remember the discussion earlier on the size of a single micron). Proper preservation of stored parts is helpful in eliminating corrosion.

PARTICLES OF LINT FROM CLEANING MATERIAL. These can cause abrasive damage in hydraulic systems, especially to closely fitted moving parts. In addition, lint in a hydraulic system packs easily into clearances between packings and contacting surfaces, leading to component leakage and decreased efficiency. Also, lint helps clog falters prematurely.

PARTICLES INTRODUCED FROM OUTSIDE FORCES. Particles can be introduced into hydraulic systems at points where either the liquid or certain working parts of the system (e.g., piston rods) are at least in temporary contact with the atmosphere. The most common danger areas are at the refill and breather openings and at cylinder rod packings. Contamination arising from carelessness during servicing operations is minimized by the use of an approved dispensing cart using proper filters and filler strainers in the filling adapters of hydraulic reservoirs. Hydraulic cylinder piston rods incorporate wiper rings and dust seals to prevent the dust that settles on the piston rod during its outward stroke from being drawn into the system when the piston rod retracts. Similarly, single-acting actuating cylinders incorporate an air filter in the vent to prevent ingestion of airborne contamination during the return stroke (refer back to view A of figure 10-13).

PARTICLES CREATED WITHIN THE SYSTEM DURING OPERATION. Contaminants created during system operation are of two general types: mechanical and chemical. Particles of a mechanical nature are formed by wearing of parts in frictional contact, such as pumps, cylinders, and packing gland components. Additionally, overaged hydraulic hose assemblies tend to breakdown inside and contaminate the system. These particles can vary from large chunks of packings and hose material down to steel shavings of microscopic dimensions that are beyond the retention potential of system filters.

The chief source of chemical contaminants in hydraulic liquid is oxidation. These contaminants are formed under high pressure and temperatures and are promoted by the catalytic action of water and air and of metals, like copper or iron oxides. Oil-oxidation products appear initially as organic acids, sludge, gums, and varnishes-sometimes combined with dust particles as sludge. Liquid soluble oxidation products tend to increase liquid viscosity, while insoluble types form sediments and precipitates, especially on colder elements, such as heat exchanger coils.

Liquids containing antioxidant have little tendency to form gums under normal operating conditions; however, as the temperature increases, resistance to oxidation diminishes. Hydraulic liquids that have been subjected to excessively high temperatures (above 250°F) break down in substance, leaving minute particles of asphalting suspended in the liquids. The liquid changes to brown in color and is referred to a decomposed liquid. This explains the importance of keeping the hydraulic liquid temperature below specified levels.

The second contaminant producing chemical action in hydraulic liquids is one that permits these liquids to establish a tendency to react with certain types of rubber. This causes structural changes in the rubber, turning it brittle, and finally causing its complete disintegration. For this reason, the compatibility of system liquid with seals and hose material is an important factor.

PARTICLES INTRODUCED BY FOREIGN LIQUIDS. One of the most common foreign-fluid contaminants is water, especially in hydraulic systems that require petroleum base oils. Water, which enters even the most carefully designed systems by condensation of atmospheric moisture, normally settles to the reservoir bottom. Oil movement in the reservoir disperses the water into fine droplets; agitation of the liquid in the pump and in high-speed passages forms an oil-water-air emulsion. Such an emulsion normally separates out during the rest period in the system reservoir; but when fine dust and corrosion particles are present, the emulsion is catalyzed by high pressures into sludge. The damaging action of sludge explains the need for water-separating qualities in hydraulic liquids.

Control of Contamination

Filters (discussed earlier) provide adequate control of the contamination problem during all normal hydraulic system operations. Control of the size and amount of contamination entering the system from any other source must be the responsibility of the personnel who service and maintain the equipment; therefore, precaution must be taken to ensure that contamination is held to a minimum during service and maintenance. Should the system become excessively contaminated, the filter element should be removed and cleaned or replaced.