10/27/2026
If the controls on a machine have started to feel sticky or jerky, especially first thing in the morning, the valve might be telling you something about the oil.
A directional valve moves a ground spool inside a bore, with very small clearances. Two things commonly gum that up.
1. Silt. Particles too fine to see settle into the gap around the spool, more so when it sits in one position for a while. The spool sticks, then lets go with a jerk.
2. Varnish. Oil that has run hot for long enough breaks down and leaves a sticky film on metal. It builds up in exactly the tight spaces where the spool needs to move, and it tends to come out of the oil as it cools. That's why the problem is often worst at start-up.
Freeing up or replacing the valve fixes the symptom. If the oil is still dirty or still running hot, it'll be back. An oil analysis with a particle count and a varnish test will usually show which one you're dealing with.
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