How Silt Lock Can Destroy Hydraulic Valves
When hard or soft particles invade the fine clearance between a hydraulic valve spool and its bore, the force required to move the spool increases. In the worst case scenario, the spool can become stuck. This phenomenon is known as silt lock.
Furthermore, if the spool valve's actuator is an AC solenoid, often the first indication the valve is silt-locked is when the solenoid burns out. To understand why, it's necessary to learn some electrical theory.
In DC solenoids, the electrical current is constant for a given voltage. The wire diameter and number of turns in the coil are the only variables that affect resistance (all other things equal).
When voltage is applied to a DC solenoid, the current draw rises from zero to the maximum value that can pass through the coil, regardless of the position of the solenoid plunger in relation to the coil.
What Is Silt Lock?
Silt lock is when micron-sized particles (silt) become lodged between the hydraulic valve spool and the bore. Silt particles migrate into the clearances between the spool and bore, increasing friction when the valve is actuated. When more and more of these silt particles become lodged in the clearances, it eventually results in silt lock. Silt lock stops production, increases valve maintenance costs and slows production due to sluggish response. Setting up a lubrication program to control contamination will prevent this from happening. The presence of varnish on valve spools and bores tightens the interference fit (annular clearance), reducing the particle size affecting contaminant lock. The varnish also has adherent properties that stick the particles to the silt lands. The longer a valve holds pressure without actuation, the longer the available time for the valve to silt up (and sludge up). Most stiction-related valve failures occur just after a long dwell time. Large amounts of silt-sized particles in the 2- to 6-micron range have a tendency to grow dramatically in population as oils age. These clearance-sized particles increase the propensity of contaminant lock. Water has a tendency to preferentially coat particles. Two such particles in contact will cling together (like wet sand), aggravating the silt-lock risk considerably.
However, AC coils behave very differently than DC coils. The resistance or impedance of an AC coil is lowest when the solenoid is open, i.e., when the plunger is out. Impedance increases as the plunger is pulled into the closed position. As a result, the current draw of an AC solenoid is highest when the solenoid is open (plunger out) and lowest when the solenoid is closed (plunger in).
The high current draw of an open AC solenoid is known as inrush current, while the current draw when the solenoid is closed is called holding current. AC solenoids can only dissipate the heat generated by their holding current. This means it's very important for the plunger to close completely when an AC solenoid is energized.
In other words, the high, inrush current generates more heat than can be continuously dissipated by the solenoid. Therefore, if the plunger is not able to be completely pulled into its coil, due to a mechanical problem with the valve, for example, then the insulation around the coil windings will burn, and the coil will short out.
Armed with this knowledge, if you were to observe an AC valve solenoid drawing 2.5 amps, compared with 0.65 amps for identical solenoids on other parts of the machine, it would indicate either a supply voltage problem or a mechanical problem such as silt lock is preventing the plunger from completely closing. As a result, you would see inrush current, not holding current, which means solenoid coil burnout is imminent.
If silt lock is the root cause, then replacing a burned out solenoid is a waste of time. Replacing the entire valve will buy some time until it too becomes silt-locked. The solution, of course, is to get the contamination problem under control.
The nice thing about DC solenoids is they don't burn out if the plunger doesn't completely close due to silt lock or any other reason. However, they will burn out if the wrong voltage is applied. For example, if you connect a 12-volt DC solenoid to a 24-volt supply, the current draw of the coil doubles due to its fixed resistance, so it will burn out for sure.
Similarly, over-voltage of an AC solenoid tends to drive too much current through the coil, causing it to overheat, while under-voltage can reduce the power of the solenoid to the point where it can no longer close. As you now know, if an AC solenoid doesn't close, the coil is subjected to the damaging overheating effects of continuous inrush current.
Given that DC solenoids are inherently more reliable than their AC cousins, why use AC solenoids at all? In an industrial application, specifying AC solenoids eliminates the need for a step-down transformer/rectifier, which is necessary if DC solenoids are to be used. However, this can be false economy in the long run. In some industrial applications, the faster response of AC solenoids may be desirable. The typical response time of a wet-pin AC solenoid is 20 to 30 milliseconds, compared with 45 to 70 milliseconds for a wet-pin DC solenoid.
A wet-pin solenoid allows oil from the tank gallery of the valve to flood the solenoid housing, whereas the plunger's push pin in an air-gap or dry-pin solenoid is dynamically sealed from the tank gallery.
Rapid Cycling Concerns
Another problem presented by the inrush current characteristics of AC solenoids is the possibility of overheating due to rapid cycling. Each time the solenoid is closed, it is subject to the heating effect of the high, inrush current. If the solenoid is switched on and off too rapidly, the successive inrush currents can generate more heat than can be dissipated, leading to failure of the coil. Still, an AC solenoid can be cycled quite rapidly. For instance, a class H solenoid, which has insulation rated to 180 degrees C, can be safely switched twice per second. However, a DC solenoid with class F insulation rated to 150 degrees C can be cycled four times per second without any risk of overheating.
A wet-pin solenoid has a number of advantages, but the main one is better heat dissipation. Oil from the tank gallery acts as a heat sink for the solenoid. The faster the solenoid cycles, the better the oil circulation around the solenoid and the greater the cooling effect.
The oil-flooded solenoid housing also eliminates problems associated with moisture ingression, and because there is no dynamic seal around the plunger's push pin, there's one less seal that can fail. Wet-pin solenoids are also quieter, due to the damping effect of the oil around the plunger. For these reasons, wet-pin solenoids are used almost exclusively on hydraulic spool valves these days.
In conclusion, there are many compelling reasons to set and maintain a high level of fluid cleanliness in hydraulic systems. If the valves on your hydraulic machines are equipped with AC solenoids, I've just given you another one.
