A heavy-duty cushion-tire forklift was brought in with a dangerous symptom. Whenever the operator picked up a heavy load (over 4,000 pounds) and parked the truck, the mast would slowly tilt forward over the course of two minutes. If they were sitting in the truck with the engine running, the mast held perfectly rigid. It only drifted when the engine was off.
The shop assumed the main tilt control valve was bypassing internally and replaced it. The mast still drifted under load with the engine off. We had to look at the physics of a static hydraulic system. When the engine is running, the pump continuously feeds pressure to the valve, keeping the spool seated. When the engine shuts off, the pump stops, and the only thing holding the mast upright is the mechanical seal of the valve spool and the piston seals inside the tilt cylinders.
We chained the mast to the overhead guard to take the load off the tilt cylinders, and cracked the hydraulic fitting on the "rod-end" (the tilt-back side) of the left tilt cylinder. Oil dripped out. We then had an operator slowly push the tilt joystick forward. A solid, high-pressure stream of oil shot out of the cracked fitting, even though the cylinder wasn't supposed to be moving.
The piston seal inside the left tilt cylinder had failed. Under the extreme static weight of a 4,000-pound load, the oil on the bottom of the piston was slowly bypassing the torn seal and filling the rod-end of the cylinder. Because the rod-end was full and sealed, the bypassing oil was pushing the cylinder rod outward, causing the mast to tilt forward. With the engine running, the pump's pressure masked the bypass; with the engine off, gravity won. We replaced the piston seal pack in the left tilt cylinder, and the drift vanished.
