Spool Valves and Particulate Contamination

Precision fitting components such as solenoid vales have very tight mechanical tolerances. The clearance between the solenoid spool and housing is measured in microns (µm), or thousandths of a millimeter.

Depending on the direction of loading, a solenoid spool will operate eccentrically withing the spool housing; that is, clearance on one side of a spool will be greater than the other.

One side may have a clearance of 10µm, but the other side may have 20µm. Consequently, hard particulate matter in the lubricating oil less than the smaller clearance dimension will not necessarily inhibit operation, but will cause wear damage; however, hard particulate matter between the two clearance dimensions in diameter can be sufficiently sized to enter the spool on the side of the larger clearance and cause it to seize at the side of the smaller clearance.

The video demonstrates that for the spool valve shown, application of 6.5um diamond paste is insufficiently sized to cause the spool to seize (but the grinding sound after application would suggest wear damage is being caused). However, application of 12µm is sufficiently sized to cause immediate, and likely permanent, seizure.

If lubricating oil is kept clean, wear is minimised and seizures do not take place.