Hydraulic Strainers and Filters

To keep hydraulic components performing correctly, the hydraulic liquid must be kept as clean as possible. Foreign matter and tiny metal particles from normal wear of valves, pumps, and other components are going to enter a system. Strainers, filters, and magnetic plugs are used to remove foreign particles from a hydraulic liquid and are effective as safeguards against contamination. Magnetic plugs, located in a reservoir, are used to remove the iron or steel particles from a liquid.

Strainers. A strainer is the primary filtering system that removes large particles of foreign matter from a hydraulic liquid. Even though its screening action is not as good as a filter’s, a strainer offer less resistance to flow. A strainer usually consists of a metal frame wrapped with a fine-mesh wire screen or a screening element made up of varying thickness of specially processed wire. Strainers are used to pump inlet lines where pressure drop must be kept to a minimum.

Figure 2-12 shows a strainer in three possible arrangements for use in a pump inlet line. If one strainer causes excessive flow friction to a pump, two or more can be used in parallel. Strainers and pipe fittings must always be below the liquid level in the tank.

Filters. A filter removes small foreign particles from a hydraulic fluid and is most effective as a safeguard against contaminates. Filters are located in a reservoir, a pressure line, a return line, or in any other location where necessary. They are classified as full flow or proportional flow.

Full-Flow Filter (Figure 2-13). In a full-flow filter, all the fluid entering a unit passes through a filtering element. Although a full-flow type provides a more positive filtering action, it offers greater resistance to flow, particularly when it becomes dirty. A hydraulic liquid enters a full-flow filter through an inlet port in the body and flows around an element inside a bowl. Filtering occurs as a liquid passes through the element and into a hollow core, leaving the dirt and impurities on the outside of the element. A filtered liquid then flows from a hollow core to an outlet port and into the system.

A bypass relief valve in a body allows a liquid to bypass the element and pass directly through an outlet port when the element becomes clogged. Filters that do not have a bypass relief valve have a contamination indicator. This indicator works on the principle of the difference in pressure of a fluid as it enters a filter and after it leaves an element. When contaminating particles collect on the element, the differential pressure across it increases. When a pressure increase reaches a specific value, an indicator pops out, signifying that the element must be cleaned or replaced.

Proportional-Flow Filters
(Figure 2-14). This filter operates on the venturi principle in which a tube has a narrowing throat (venturi) to increase the velocity of fluid flowing through it. Flow through a venturi throat causes a pressure drop at the narrowest point. This pressure decrease causes a sucking action that draws a portion of a liquid down around a cartridge through a filter element and up into a venturi throat. Filtering occurs for either flow direction. Although only a portion of a liquid is filtered during each cycle, constant recirculation through a system eventually causes all of a liquid to pass through the element. Replace the element according to applicable regulations and by doing the following:

• Relieve the pressure.
• Remove the bowl from the filter’s body.
• Remove the filter element from the body, using a slight rocking motion.
• Clean or replace the element, depending on its type.
• Replace all old O-ring packings and backup washers.
• Reinstall the bowl on the body assembly. Do not tighten the bowl check the appropriate regulations for specifications, as some filter a specific torque.
• Pressurize the system and check the filter assembly for leaks.

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