The Future of Hydraulic Oil Filtration

Philip Johnson, Donaldson Company, Inc.

Achieving proper cleanliness levels in today’s hydraulic systems requires the successful capture and removal of contaminants. Filtration must be done in a way that doesn’t disrupt the flow of oil, or unduly increase the pressure drop within the system. It’s a delicate balance between system design and efficiency.

The future of hydraulic filtration focuses on three major areas:

  • Developing finer fiber technology to be used in filter media,
  • Engineering the various layers/arrangements of filter media for maximum effect, and
  • Improving the overall construction and design of the filter to maximize the useful filtration area for optimal performance.
Cellulose Fibers
Cellulose Fibers
Man-made Fine Fibers
Man-made Fine Fibers
Figure 1. Finer media
fibers increase dirt-holding
capacity while improving
flow conditions.

Manufacturing Efficiency Into Filters

One of the simplest forms of filter media is a sieve or strainer. Depth media, the popular choice for hydraulic filters, consists of random layers of fiber strands to create a multi-layered sieve. Depth filtration technology is designed to have higher efficiencies and hold more dirt than strainers.

There are two basic principles that are commonly applied to improve efficiency of depth filters: (1) add more layers to the filter in an attempt to catch any particles that pass through the surface layers, and (2) make the pore spaces finer by compacting the media during the manufacturing process.

Modern filter media design often interlaces layered combinations of coarse media with finer grades - with the idea that the larger dirt particles are captured on the surface, while the finer particles are trapped deeper within the media.

Improving efficiency through increased layers or finer pore sizes can have undesirable consequences. Both methods can lead to an increased differential pressure across the filter, which may shorten filter life and lead to an early filter change.

Finer Fiber Technology

To improve element life and reduce pressure drop across the filter element, filter manufacturers are continually looking for materials with finer fibers to provide more pore spaces in which to capture the dirt, while increasing the fluid flow area (Figure 1).

Traditionally, most depth filtration was made with cellulose fibers (paper media). Today, many hydraulic filters are made using man-made fibers with smaller diameter strands. Future filter media technology will most likely continue to develop even finer fibers (Figure 2 and Figure 3).

Evolving Media Surfaces

Hydraulic filter manufacturers strive to create leading-edge filter media. Recently, a fluted media was developed which provides alternating flow paths and allow more filter media per unit volume. In the future, this type of media could replace the pleat technology found in conventional filters (Figure 4).

Comparison of Steady Flow and Pulsating Flow Conditions
Figure 5. Comparison of Steady Flow and Pulsating Flow Conditions

Filter Efficiency

Filtration efficiency is normally expressed as the ratio of dirt entering a filter compared to the dirt exiting the filter, of a specified micron (µm) size. Filter testing is completed in a laboratory so the results can be compared and statistically verified.

Filtration tests attempt to accurately control the quality of the test dust, the flow rates, temperatures, measuring equipment and many other variables to ensure the repeatability of the test. The test, called a multi-pass filter test, produces a Beta (ß) rating for the filter at a given micron size. For example, a ß10=75 filter removes 74 of 75 of the particles greater than 10 microns entering the filter.

It’s understandable, but nevertheless disappointing, to find that filters do not always perform in the field exactly as the laboratory tests would suggest. Issues contributing to poor filter performance can include: pulsating flows, flow rate, start-up contamination and overall design flaws within the hydraulic system.

In the laboratory, filters are tested under steady-state flow conditions. The pulsating flow of real-life applications can significantly reduce performance. For this reason, it’s advisable to avoid conditions of pulsating flow when installing a filter into a circuit design (Figure 5).

Efficiency vs. Differential Pressure
Figure 6. Efficiency vs. Differential Pressure

Not only can efficiency vary with pulsating flow, but different configurations of media can also cause different efficiencies at increasing differential pressures. For example, Figure 6 depicts two elements that have the same micron rating (average efficiency) but differing maximum and minimum efficiencies, and consequently, varying dirt-holding capacity. (The final point on the graph depicts element collapse.) Effective filtration is as much a function of good system design as the quality of the filter used.

Controlling Contamination Levels

Contamination levels in a system are a function of many factors, such as the amount of contaminant in the oil at start-up, the rate of contaminant ingression, etc. For most hydraulic systems, the prevention of contamination is more cost-effective than the cure. To control contamination, troubleshooting the system design will yield dramatic results. Here are a few troubleshooting tips:


It’s unfortunate that many hydraulic system breathers consist of an open cover or tube, or at best a filler cap without a proper filter element inside. No unfiltered air should be allowed to enter a hydraulic system.

Wherever possible, oil reservoirs should have adequate breather filters installed; ambient air carries considerable quantities of contaminants.

A quality breather filter with an absolute efficiency of Beta 10µm(c) = 75 or better will suffice for most circumstances.

A quality 10µm liquid filter will typically be more efficient at capturing fine dirt particles when applied in air filtration applications. In humid conditions, breathers should remove water (desiccant).

Reservoirs and Drum Storage

  • Good reservoir design will ensure that any water or heavy dirt settles to a small area or standpipe at the base of the reservoir, which can be drained periodically. Water left in the oil leads to bacteria growth and chemical degradation.

  • Oil drums are best stored on their sides so that the bungholes are submerged. This prevents standing water or humidity drawn into the drum through breathing caused by temperature changes.

  • Good reservoir design will provide return line diffusers, adequate baffles and sufficient volume to settle out heavy dirt particles, water and any entrained air.

  • Oil should be regularly recycled or used to prevent long-term degradation.

  • Ideally, oil should be filtered going into and out of the reservoir.

Of course, the perfect filter and the perfect filter media will do wonders for the hydraulic system. But when it comes to achieving optimal cleanliness levels, one cannot rely on filter efficiency ratings alone.

As discussed in this article, several other factors can influence a system’s integrity. Proper handling and a solid system design go a long way toward solving dirty oil problems. Each situation is different.

Climates, environments and cleanliness requirements vary and should be taken into consideration. However, a good fluid power distributor will be able to accurately diagnose systems and recommend which media choices, filter types and filter positioning are right for every application.

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