All hydraulic systems contain some contamination, and large particles are possibly present at start-up or will be introduced during its lifetime. The particles can be machining chips, Teflon® tape, weld flash, O-ring chips, wear particles, and debris from failing components. These particles enter the system through various means, including:
Often, large particle contamination is the result of something as innocuous as a mechanic snagging a piece of paper towel inside a pipe while cleaning the system.
Filters come in a wide variety of types, sizes, and capacities, and they address a wide ranging size of particle contamination. Typically, filters collect particles less than 100 µ in size. Most often, a single filter is used, but some systems have several located throughout the circuit. In most cases, the fluid contaminants being filtered have circulated through the various hydraulic components.
Unfortunately, in aircraft, machine tools, and other large systems, sensitive components such as check valves, restrictors, and relief valves are often located far from the system filter. Thus, they have little protection from large contaminants. Further, the components are often in hard to reach locations, and failure results in extensive and costly downtime. Large particles can cause component failure by blocking a flow restrictor or holding open a check valve. Safety screens, either integral to the individual component or located immediately upstream, provide an added margin of safety.
Teflon® is a registered trade name of E.I. DuPont de Nemours, Inc.