Sources of Hydraulic Contamination

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There are four sources of contamination in hydraulic fluid.

1. Built-in contamination. This is contaminate that was left in the system when it was assembled. It can range from a piece of teflon tape to a piece of welding slag.
2. Contaminated new oil. Contaminate is introduced during the manufacture and subsequent handling of oil. If this is not removed, it enters the system.
3. Ingressed contamination. This contaminate can enter with air flowing into the reservoir through the breather cap, or it can ride in on a cylinder rod. No rod seal can totally prevent the entrance of particles. Also, whenever a line is disconnected or the system is opened in any way, there is the potential for contamination to ingress.
4. Internally generated contaminate. Particles removed from the interior surface of the components will circulate in the system until they are removed. Each impact of one of these particles with a surface causes more damage. This phenomenon is known as the wear regeneration cycle.

Internally generated contamination causes damage in the following ways:

Abrasive wear. The particles that break loose from the hardened surfaces of the components are very hard. These particles bridge across the clearance between two moving surfaces and abrade one or both surfaces.
Adhesive wear. As discussed in the previous section, high temperature reduces oil viscosity and thins the film between moving parts. These parts then adhere, or stick together, and damage results.
Fatigue wear. Particles that bridge a clearance can cause a stress concentration. This stress concentration eventually causes a crack to form (Fig. 8.4), and this crack spreads until part of the surface breaks away. This type of surface failure is called a spall.
Erosive wear. Particles in a stream of moving fluid erode away the surface of the metering edge (Fig. 8.5).
Cavitation wear. When the pump compresses fluid that contains air bubbles, the bubbles implode. The resulting shock wave impacts the surface and, over time, these impacts cause damage. Cavitation wear and aeration wear are sometimes discussed as separate wear phenomena, but they are quite similar.
Corrosive wear. Chemical attack of the surface causes a loss of material. A simple example is water condensation in the reservoir that causes the metal to rust. More complex reactions than the oxidation of iron occur, and all these reactions are grouped under corrosive wear.

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