Clean hydraulic fluid is essential for proper operation and acceptable component life in all hydraulic systems. Although every effort must be made to prevent contaminants from entering the system, contaminants that do find their way in must be removed. Filtration devices are installed at key points in fluid power systems to remove the con­ taminants that enter the system along with those that are generated during normal operations of the system.

The filtering devices used in hydraulic systems are commonly referred to as strainers and filters. Since they share a common function, the terms are often used interchange­ ably. As a general rule, devices used to remove large particles of foreign matter from hydraulic systems are referred to as strainers, while those used to remove the smallest particles are called filters.


Strainers are used primarily to catch only very large particles and will be found in applications where this type of protection is required. Most hydraulic systems have a strainer in the reservoir at the inlet to the suction line of the pump. A strainer is used in lieu of a filter to reduce its chance of being clogged and starving the pump. How­ ever, since this strainer is located in the reservoir, its maintenance is frequently neglected. When heavy dirt and sludge accumulate on the suction strainer, the pump soon begins to cavitate. Pump failure follows quickly.


The most common device installed in hydraulic systems to prevent foreign particles and contamination from remaining in the system are referred to as filters. They may be located in the reservoir, in the return line, in the pressure line, or in any other location in the system where the designer of the system decides they are needed to safe­ guard the system against impurities.

Filters are classified as full-flow or proportional flow. In full-flow types of filters, all of the fluid that enters the filter passes through the filtering element, whereas in pro­ portional types only a portion of the fluid passes through the element.

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