In normal operation of a solenoid valve, the O-rings in the valve create a smooth transition while the plunger or spool is in motion. Because the O-rings are in direct contact with the valve chamber walls, in order for the plunger/spool to begin to move, it must first overcome the stiction between the O-rings and the chamber walls. Once in motion, the plunger/spool must overcome the sliding or dynamic friction between the O-rings and walls, however since the dynamic friction is usually significantly less than the nominal level of stiction, it is generally not of concern. By design the magnetic and spring forces in the solenoid valve are sufficient to overcome the nominal level of static friction and, depending on the design specifics of the solenoid valve, generally have excess force sufficient to overcome about 2 to 2.5 times nominal stiction. Once the stiction has been overcome, the plunger/spool will continue to move.
However, a common and troublesome problem can occur when the valve is at rest, i.e., stationary, for a length of time because the stiction between the O-ring and the valve chamber walls increases over time from its nominal level until it reaches some maximum value. It is possible that the increases in stiction may reach a level where the forces generated by the solenoid coil and/or spring (in the case of a spool valve) are no longer sufficient to overcome the increased stiction. Consequently, when the valve is called upon to close, it is unable to do so. However, if the excessive stiction is overcome, stiction reverts to its original nominal level, though it does begin to increase again once the valve stops moving.
Stay tuned for more entries on stiction. I will also be presenting a white paper at GCPS/AIChE conference on 5/1/13 in San Antonio, TX.