Why hydraulic filters are so important and which ones you should not risk omitting? Print

Any hydraulic circuit can benefit from the use of filters. However, cost and other factors, such as available space, limit the number of filters that can be designed into any given system. The best systems are strategically engineered to ensure that fluid is filtered correctly at each stage of its journey through the circuit.

The ultimate filtration system cleans fluid before it enters the pump, after it leaves the pump, before it returns to the reservoir, and while it is in the reservoir. In reality, engineers must decide which filters are essential, and which ones they can afford to leave out.

So when specifying the ideal hydraulic filtration circuit just isn’t an option, determining which filters will do the greatest good for an application (and for the least amount of money) becomes the task at hand.


Breather filters are placed on hydraulic reservoirs to prevent atmospheric contamination from entering. Air enters the reservoir whenever the fluid level drops. This occurs during normal cycling of the machine, and when fluid cools down during idle periods. Breathers are designed to stop particles larger than 3 µm from entering the system. This is a sensible, affordable solution for any hydraulic system, but by all means cannot be the only filter on a hydraulic system. Breather filters probably offer a greater benefit per dollar than any other hydraulic filtration technique. If a reservoir sealed from the atmosphere is not practical, filter breathers should be specified for the overwhelming majority of applications.

Benefit: High

Suction Strainers

Suction strainers, are typically found in the hydraulic fluid reservoir. Their only real use is to keep cigarette butts, moths, nuts, bolts, and even wedding rings. These large size contaminants easily can be eliminated instead by keeping the reservoir sealed and by using a filler-breather and return-line filter.

Benefit: Medium

Suction Filters

Suction filters, normally placed between the reservoir and the pump, and are designed to remove particles sized from 10 to 250 µm. They are easier to service and less expensive than many other types of filters. However, to prevent restricting the pump’s suction line (which could cause cavitation), they tend to be substantially larger than similar return or pressure filters with the same flow rating.

Benefit: Medium

Return Filters

Return line filters usually have low-pressure housings, which are less expensive than their high-pressure counterparts. Return-line filters collect contaminants from the oil as it returns to the reservoir. They can be positioned almost anywhere within the return line circuit, either mounted inline or built into the reservoir. Return line filters are especially effective at removing contaminants that enter the hydraulic system through actuators, such as piston rods. When extended, the oily surface of the rod collects dirt, which can be carried into the hydraulic system when the rod retracts.

Benefit: High

High Pressure Filters

High-pressure filters, clean the fluid delivered by the pump. This protects expensive downstream components, such as servo valves and actuators. High-pressure filters offer protection from catastrophic pump failure and are a worthwhile investment for high-performance systems.

Benefit: High