How Filters Work to Control Contamination in Oil

Jim Fitch, Noria Corporation
Tags: contamination control

Imagine the filter inside your machine is made of fibers the size of telephone poles, stacked randomly in all directions, many layers thick. Each juncture where poles touch is a drop of super glue for support. To emulate actual operating conditions, the stack of poles is placed on a large moving and vibrating table.

Now, imagine that the contaminants inside your oil are lumps of gelatin, clumps of tar, ping-pong balls, marbles, tree branches, powdery sand, beanbags, strips of sheet metal, streams of honey, wet rags and beach balls. To begin our example, suppose that you had large containers of these different contaminants beside you as you perch on top of scaffolding hovering above the stack of telephone poles.

This is where the fun begins. It’s time to start dumping our contaminants onto the telephone poles, beginning with the ping-pong balls and marbles. As you could imagine, they don’t have much trouble navigating the maze through the openings between the poles on their way to the bottom. There would be the occasional marble or ping-pong ball that might hang up in tight spaces where poles touch or run close together. That is, until the table starts to vibrate, rock and tilt. The capture efficiency of our filter is not good for contaminants the size of marbles and ping-pong balls. In contrast, if we dropped the beach balls over the poles, we would find all of them restricted from entry. They don’t pass through the pores in our telephone pole filter.

Next, consider what would happen if we changed the order of delivery. Suppose we pour a drum of honey over the filter and give it time to coat and occlude to the poles’ surfaces. Once again, drop the ping-pong balls and the marbles. What happens to the marbles and ping-pong balls now? The high density of the marbles would carry most of them to the ground. However, the ping-pong balls would adhere tightly to the sticky honey and only a few would work their way through the poles.

Now consider the capture efficiency of our filter with powdery sand. The grains of sand that don’t make it to the bottom would likely rest on the top surface of horizontal poles. The dynamics would change considerably if the honey were dropped on the filter before the sand. Even though the average opening between the poles is more than a thousand times the average diameter of sand particles, there would still be very few grains of sand that make it to the bottom because of the adhesiveness of the honey.

Let’s switch our contaminants once again. Imagine what semisolids, like lumps of gelatin, would do compared to clumps of tar. Most of the gelatin would probably bounce, wiggle and contort its way through the poles. In contrast, the tar’s natural adherent properties give it little chance to bounce off anything. Once it contacts the pole’s surface, it stays. Neither the gelatin nor the tar is influenced by the filter’s pore size (spacing between the poles), however the gelatin squirms through to the bottom undeterred, unlike the tar.

How effective is the filter at capturing beanbags and wet rags? These interesting contaminants tend to conform to the shape of solid objects they come in contact with. It is unlikely that many would travel the distance through our telephone pole filter. However, if we engaged our rocking and vibrating table again, it is likely that the beanbags would work to the bottom, unlike the rags that are more likely to stay draped over poles.

This leaves the last set of contaminants: tree branches and strips of sheet metal. Can they make it through our filter? Even though the more narrow dimensions of these contaminants (branch diameter for instance) are much smaller than the openings between the poles, they cannot make it through the tortuous path that weaves through the poles. This is still true if we rock and vibrate the table. In fact, the tree branches and sheet metal become an integral part of the structure of our makeshift filter and affects the progressive capture efficiency of the filter.

So what does all of this have to do with lubrication and oil analysis? Well, many people have misconceptions about how filters work in their native environment. The false image is that particles are thought of as being like ping-pong balls and that filters are like screen doors, and naturally, ping-pong balls don’t fit through the mesh of a screen door.

While this is true in many cases, the dynamics of the filter in relation to real-world field contaminants and the fluid are much more complex. Let’s go back to our analogy and do some quick conversions:

The Filter:

  • Telephone poles are the fibers in filter media (cellulose, glass, synthetics, etc.).

  • Super glue is the binding element (binders) used to stick the fibers together.

  • The rocking and vibrating table simulates the pulsating environment a filter experiences: flow cycles, pressure cycles, temperature changes, and mechanical shock and vibration.

The Contaminants:

  • Ping-pong balls = finely dispersed soot and carbon insolubles.

  • Marbles = submicron hard dust and wear particles pulverized in the machine’s frictional surfaces.

  • Powdery sand = polar insolubles such as oxides, compounds and additive floc.

  • Honey = either an electrostatic charge that many filters acquire over time that attracts particles of opposite charge, or the influence of water as it preferentially coats filter fibers and causes premature blockage, particularly cellulose filters.

  • Beach balls = large particles from wear and the environment that are quickly removed by size exclusion.

  • Lumps of gelatin = amorphous polymeric globules produced from the oil caught between highly loaded frictional surfaces or from thermal degradation.

  • Beanbags = common dust and lint that floats around in the air, enters the machine and remains on a filter.

  • Wet rags = foam inhibitors of methyl silicon or acrylate that coat filter fibers.

  • Clumps of tar = sludge, an adherent mixture of polar degradation products, water, glycol, soot and/or environmental contaminants.

  • Tree branches = hose fibers, seal materials, etc.

  • Strips of sheet metal = cutting wear, delamination wear, paint chips, etc.

Most filters encounter all of these contaminants plus a host of others in varying concentrations during the filter’s life. Consider the unpredictable impact these natural contaminants have on the filter’s performance, especially when surge flow, shock and vibration are added to the environmental equation.

Finally, think about the quality of filter selected for your machine. With rare exception, any contaminant trapped in the filter is much better in the filter than in the oil.


About the Author

Jim Fitch, a founder and CEO of Noria Corporation, has a wealth of experience in lubrication, oil analysis, and machinery failure investigations. He has advised hundreds of companies on developing their lubrication and oil analysis programs. Contact Jim at

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