A heavy-duty forklift was brought in with a violent symptom. The operator could tilt the mast back perfectly fine. But when they tried to tilt the mast forward with a load, the truck violently shuddered, the hydraulic pump screamed, and a hydraulic line blew off its fitting. The shop replaced the tilt control valve spool, assuming it was sticking. The problem persisted exactly as before.
We looked at the schematic for the tilt circuit. To prevent a mast from slamming forward uncontrollably when the operator tilts forward under a heavy load, the rod-end of the tilt cylinder has a "cross-port relief valve." If the pressure gets too high, this valve pops and dumps oil back to the tank. We pulled the cross-port relief valve from the tilt cylinder manifold. It was physically jammed open with a tiny, translucent flake.
The forklift had recently had a tilt cylinder rebuilt. The rebuilder had used Teflon backup rings on the piston. During installation, a tiny burr on the cylinder bore had shaved off a microscopic flake of the Teflon ring-what hydraulics mechanics call a "Teflon seed." This seed traveled through the hose and lodged perfectly in the poppet of the cross-port relief valve, holding it open.
When the operator tilted back, the oil flowed into the bore-end of the cylinder, and the open relief valve just allowed the oil on the rod-end to escape back to the tank easily. But when tilting forward, the oil was being pumped *into* the rod-end. Because the relief valve was jammed open, the pump was trying to fill the cylinder while the oil simultaneously dumped straight back to the tank through the stuck valve. The pump was deadheaded against its own bypass, creating a massive pressure spike that blew the hose. We fished the Teflon seed out of the relief valve with a pick, and the tilt operated smoothly. When a relief valve fails instantly after a cylinder rebuild, always hunt for the microscopic seal debris.
