Filtration for servo systems

The most effective way to reduce life cycle costs of an oil hydraulic system, regardless of the types of valve used, is through close attention to contamination control. A wide range of information on the subject exists, but in a nutshell, the ideal system filter arrangement for industrial servo systems should use:

  • a 15 µm (ß15 ≥ 75) high-pressure filter without bypass just before the valve or critical parts of the valve, such as the pilot,
  • a 3 µm ( ß3 ≥ 75) low-pressure filter in the return or bypass line, and
  • a filter in the tank breather that is at least as fine as the finest filter in the system.

These recommendations have their roots in the fact that most servo and proportional valves can accept the odd particle up to 25 µm, allowing the pressure filter to protect the valve from catastrophic failure. The real work is done by the low-pressure filter in reducing the small-particle contamination, which is the prime contributor to component wear and silt formation.

Assuming the filters are properly sized, installed, and maintained, the aim should be to limit oil contamination to the maximum levels shown in the table.

Note that these are maximum contamination levels, and with proper care and regular filter changes, significantly lower levels can and should be achieved. Attention also must be paid to other factors that contribute to oil condition problems, such as high temperatures, high tank humidity, and "dirty" new oil.

ISO solid sontamination code
Old ISO 4406
New ISO 4406
Maximum recommended code*
16/13
19/16/13
Recommended code for long life
15/12
18/15/12
* May vary for some applications and models, please check documentation

This information was provided by Martyn Waddington, Moog Inc., East Aurora, N.Y.