A heavy-duty cushion tire forklift was dropping loads dangerously. It would hold a 5,000-pound pallet at eye level perfectly fine on flat concrete. But the moment the operator drove over a rough expansion joint or a dock plate, the mast would violently drop two inches, jolting the load, before locking back up. The shop rebuilt the lift cylinder, thinking the piston seal was bypassing on shock. It didn't fix the drop.
We looked at the hydraulic schematic for the lower circuit. To prevent a mast from free-falling if a hose bursts, there is a "lowering control valve" (often called a counter-balance or lowering valve) mounted directly on or near the lift cylinder. To lower the mast, the operator pushes the joystick, which sends pilot pressure to open this valve, allowing oil to leave the cylinder.
We removed the lowering valve and found the hardened steel poppet seat had a microscopic crack across it. When the mast was static, the spring held the poppet tight against the seat, and it held the load. But when the forklift hit a dock plate, the massive upward G-force shock caused the lift cylinder to momentarily spike in pressure. This shock wave unseated the cracked poppet for a fraction of a second. The pressurized oil instantly escaped past the seat, dropping the mast two inches until the spring snapped the poppet shut again. Replacing the $150 lowering valve eliminated the shock drop entirely.
