A reach truck was acting erratically. It would lift a load, hold it for five seconds, and then slowly drop an inch. The lift cylinders were rebuilt, and the seals were perfect. The issue was traced to the directional control valve, but not in the way one might expect.
We removed the spool from the valve body and looked at the static O-rings that seal the valve bank. Instead of being round and smooth, the O-rings had "nibbles" or small chunks taken out of their surface, looking like a beaver had chewed on them.
This is "O-Ring Nibbling" caused by high-frequency pressure ripple. The hydraulic pump creates a pulse with every piston stroke. In certain pressure ranges, the O-ring vibrates against the sharp edge of the machined groove in the valve body. This vibration chews away tiny pieces of rubber. These "nibs" create microscopic paths for oil to bypass the spool, causing the drift. The fix wasn't just a new O-ring, but installing a PTFE (Teflon) backup ring that resists this nibbling effect, preventing the rubber from vibrating against the sharp metal edge.
