Maintaining clean fluid in any hydraulic system requires capturing and removing contaminants, this is typically done by filtering the fluid. But to be done successfully, filtration must not disrupt fluid flow or substantially increase the pressure drop within the system. It’s a delicate balance between system design and efficiency.

Research and development efforts for hydraulic filtration of the future focuses on three major areas:

  • developing the technology to produce finer fibers in filter media,
  • engineering the geometries 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.

Filtration basics

The simplest form of filter is a sieve, sometimes called a screen or strainer. The sieve works by allowing fluid and small particles to pass through, but trapping particles larger than the holes in the screen. Depth media, the popular choice for hydraulic filters, consist of multiple strands of fibers laid down randomly. The layers effectively reduce the size of passages through which fluid flows, thereby trapping particles smaller than a single layer of screen. Depth filters then have the potential for higher efficiency and to hold more dirt than simple strainers.

Two basic principles are commonly applied to improve efficiency of depth filters. One can either add more layers to the filter in an attempt to catch any particles that make their way through the surface layers, or make the pore spaces finer by compacting the media during the manufacturing process. Modern filter media design often combines layered combinations of coarse media with finer grades. The intent is to capture the larger dirt particles on the courser surface layer, and trap the finer particles within the finer media.

Improving efficiency by increasing the number of layers or making pores finer, however, can have some undesirable consequences. Both methods can lead to an increased differential pressure across the filter as it fills up with dirt. When this occurs, hydraulic filters generally have a bypass circuit. This causes oil to be routed around the filter, rather than through it, once the pressure across the filter builds to a predetermined level. As pressure builds across the filter over time, caused by contamination, the filter element must be periodically changed to prevent dirty oil from bypassing the filter.

Finer fibers hold more dirt

To improve element life and reduce pressure drop across the filter element, filter manufacturers are continually looking for materials that have finer fibers. The goal is to provide more pore spaces to capture the dirt without decreasing the fluid flow area, Figure 1. Traditionally, most depth filtration media were made using cellulose (paper) fibers. Today, many hydraulic filters are made with man-made fibers of smaller diameter, Figure 2.

Future technology will most likely continue to produce even finer fibers. For example, cellulose fibers typically range in diameter as small as 15 μm. Synthetic fibers, however, routinely are manufactured with diameters below 1μm. 

But research and development efforts for hydraulic filters are not limited to new media materials. Manufacturers also are experimenting with new geometries and orientations of media to increase filter capacity and rigidity of construction. For example, Donaldson Co. is developing fluted construction, Figure 3,which provides alternating flow paths and allows more filter media per unit volume. In the future, this type of construction could replace the pleated designs that are used so extensively in hydraulic filters.