Sorting Out Filters – Particle Size and Shape Both Matter

One important step on the path to solving any problem is understanding the enemy and the risk it poses.  The enemy, in this case particles, come in a wide variety of sizes and shapes.  A particle’s effect on the ultimate use of the part it contaminates may depend not only on its size, but on it’s shape as well.  Various manufacturing processes produce particles that have differing size and shape profiles.

Knowing the manufacturing process used to manufacture a part or family of parts to be cleaned can give you a good start toward understanding filtration needs.  For example, drilling operations typically produce long spiral chips (particles) while lathe turning operations on hard materials produce “C” shaped chips.  Grinding and lapping operations typically produce more symmetrical particles which are often mixed with remnants of the abrasive.  Finally, buffing operations commonly produce “flakes” of the substrate mixed with fibers from the buffing wheels and buffing compound residues.  Although not an exhaustive list, these few examples demonstrate that particles are far from generic and may range in size from a fraction of a micron in the case of buffing or lapping residues up to several inches in one or more dimension from parts with drilled holes.  Microscopic analysis of residues collected from typical parts is a good way to start to understand the enemy.  Most filter and machine suppliers are prepared to provide help in this analysis

Once one knows the characteristics of the particles likely to be generated by the manufacturing process, the next step is to understand what effect a particle of a known size and shape may have in the environment where the cleaned part will be used.  In some applications, for example, a stray cotton fiber (as from a buffing wheel) may have little or no consequence while in the same application, a much smaller particle of carbide or diamond abrasive lodged between two closely toleranced, mating parts (like a piston) may be fatal.  Filtration that will remove the fibers is quite simple while removing the abrasive particles may be a much more difficult task.

Filter Media A and B

Filter media A and B will both allow particle 1 to pass. Both media will stop particle 2. Media A will allow particle 3a to pass if it is oriented as shown but the same particle, 3b, in a different orientation will not pass. Media B will stop particle 3 in any orientation.

When selecting an appropriate filter and media it is important to view particles as three dimensional objects, not as round balls varying in size.  Most filter media is rated on its ability to stop a particle with a particular maximum dimension but not all.  If a particle is long and skinny and happens to arrive at the filter with its long axis perpendicular to the porous filter media, it may pass through even though its longer dimension may far exceed the rating of the media.  A different media with similar rating but requiring particles to navagate a more complex path to pass may prove effective in stopping the same particle.  The proper selection of filter media may also have significant impact on filter life depending on its ability to allow particles of no consequence to pass while retaining those which are not a problem in the final application of the part being cleaned.  This and many other topics about filtration will be discussed in upcoming blogs.

An excellent discussion of the consequences of particle size and shape in the selection of filter media and their varying effect in the use environment of the part being cleaned can be found at Particle Profiles and Measurement.  Please take time to read it if you have a further interest in understanding particles and their properties.

– FJF –

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