Earlier this month, I explained the meaning behind the ISO 4406 contamination code, so now I’m explaining why it matters, and how you can apply it to your application. The code describes the quantity of 4-, 6- and 14 -μm particles in a milliliter of hydraulic fluid—because these particle sizes are considered the most damaging to pumps, valves and motors in a hydraulic system.

But why not 1-, 20- and 40- μm particle sizes, or even some other arbitrary particle sizes? Previous standards measured 2-, 5- and 15- μm particles, while even previous to that, just 5- and 15-μm particles were measured. There are two major reasons for the 4-, 6- and 14- μm particles sizes, although it’s taken decades to decide upon these numbers.

The first reason is simply that 4-, 6-, and 14- μm particles are considered the most damaging because they are large enough to cause wear and small enough to fit into tight clearances. A 2-μm particle, for example, is only 0.00008 in. wide, and it simply too small to cause damage. It harmlessly passes through any and all clearances with sufficient space on either side. Particles larger than 14 μm are still small enough to fit through tight clearances, because a 20-μm particle, for example, is still only 0.0008 in. wide.

Although a 20-μm particle can cause damage, for sure, this brings us to the second reason the ISO 4406 code is what it is. The larger particles in a hydraulic system are extremely easy to remove; pretty much anything this side of cheese cloth can remove 20-μm particles with a fair level of efficiency (Disclaimer: extreme exaggeration, do not use cheese cloth to filter your hydraulic fluid). Because most hydraulic systems are operating with 3- to 10-μm (absolute) filters, particles larger than 14 μm should be of little worry, as fewer of them will exist within the fluid.

Higher pressure hydraulic systems are susceptible to increased wear from particle contamination because the particles are forced through small clearances. Elevated pressure pushes harder on the particles. At 5000 psi, a 6-μm particle could actually be pushed into a 5-μm clearance, where at 2000 psi, this simply may not be possible. For this reason, the ISO code must be reduced for high pressure systems, and this is achieved with finer filtration.

The target ISO code of your system must reflect the most contamination sensitive component. If, for some reason, you’re using a gear pump to provide flow to a servovalve that is controlling a gerotor motor, it is the servo valve that dictates the quality of filtration. Gear pumps and gerotor motors are fairly insensitive to contamination, but servovalves are one of the most extreme examples requiring as little particle contamination possible.

To summarize, the ISO 4406 contamination code uses 4-, 6- and 14- μm particle sizes because they are the most damaging sizes within hydraulic fluid that exist with any significant volume. Smaller particles simply do not cause enough damage, and larger particles are so easily filtered out. Next, I’ll wrap up this mini-series on filtration basics with a discussion of filter beta ratios.