Additionally, most reservoirs are designed with cleanout covers, illustrated earlier in figure 10-23, to assist in inspection and maintenance.

ACCUMULATORS. - Accumulators, being designed like cylinder actuators, are similarly repaired using the same techniques. Caution must be exercised to ensure that the pneumatic pressure has been relieved before disassembly of an air-operated accumulator.

FILTERS. - Maintenance of filters is relatively simple since it mainly involves cleaning the filter housing and replacing or cleaning the filter elements. Replace the element on filters, using the micronic (paper) element, and clean the elements on filters using the porous metal elements according to the applicable technical manuals.

Completely test the filters that have been cleaned and repaired before reinstalling them in the system. This test includes pressure setting of the relief valve, operation of the contamination indicators, leakage tests, and proof pressure test. Consult the technical manual for the equipment or the filter design for the test information.

HYDRAULIC SYSTEM CONTAMINATION

Contamination is the director indirect cause of more hydraulic system failures than any other single source; therefore, contamination prevention is a major concern for all who operate, service, and maintain hydraulic systems.

A small mistake involving injection of contaminants can result in damage to equipment that cannot have a money value placed upon it; for example, a hydraulic in a line tester that contains contaminated fluid is used to service construction equipment. This can result in damage to expensive equipment, loss of CESE costing thousands of dollars, or injury and loss of life to personnel on the jobsite.

For further reading, NAVEDTRA 12964 (latest edition) is an excellent publication on the subject of hydraulic contamination (see your ESO for this correspondence course).

Classes of Contamination

The two general contamination classes are as follows:

1. Abrasives. This includes such particles as dust, dirt, core sand weld spatter, machining chips, and rust.

2. Nonabrasives. This includes things that result from oil oxidation and soft particles worn or shredded from seals and other organic components.

The mechanics of the destructive action by abrasive contaminants are clear. When the size of the particles circulating in the hydraulic system is greater than the clearance between moving parts, the clearance openings act as filters and retain such particles. Hydraulic pressure then embeds these particles into the softer materials; the reciprocating or rotating motion of component parts develops scratches on finely finished surfaces. Such scratches result in increased tolerances and decreased efficiency.

Oil-oxidation products, usually called sludge, have no abrasive properties; nevertheless, sludge may prevent proper functioning of a hydraulic system by clogging valves, orifices, and filters. Frequent changing of hydraulic system liquid is not a satisfactory solution to the contamination problem. Abrasive particles contained in the system are not usually flushed out, and new particles are continually created as friction products; furthermore, every minute remnant of sludge acts as an effective catalyst to speed up oxidation of the fresh fluid. (A catalyst is a substance that, when added to another substance, speeds up or slows down chemical reaction, but is itself unchanged at the end of the reaction.)

Origin of Contaminants

The origin of contaminants in hydraulic systems can be traced to the following areas:

PARTICLES ORIGINALLY CONTAINED IN THE SYSTEM. These particles originate during fabrication of welded system components, especially in reservoirs and pipe assemblies. The presence is minimized by proper design; for example, seam-welded overlapping joints are preferred; arc welding of open sections is usually avoided. Hidden passages in valve bodies, inaccessible to sandblasting, are the main source of core sand entering the system. Even the most carefully designed and cleaned casting occasionally frees some sand particles under the action of hydraulic pressure. Rubber hose assemblies always contain some loose particles, most of which can be removed by flushing; others withstand cleaning and are freed later by the action of hydraulic pressure and heat.

Rust or corrosion initially present in a hydraulic system can usually be traced to improper storage of replacement materials and component parts. Particles can range in size from large flakes to abrasives of