A warehouse had a terrifying near-miss with a heavy-capacity, cushion-tire forklift. The operator was unloading a trailer. He raised the forks, tilted the mast back to carry the load, and suddenly, the entire mast assembly slammed violently forward, dropping the load and pinning the operator's legs against the steering column. The only reason he wasn't crushed was because the load hit the trailer bed before the mast flattened him.
When the mast slammed forward, the immediate assumption is that the tilt cylinders failed-specifically, that the piston rods snapped or the mounts ripped off the carriage. But when we got the forklift into the shop and looked at the tilt cylinders, they were perfectly intact. The hydraulic lines weren't leaking. The mast had slammed forward because the mechanical mast lock had been bypassed.
Every sit-down counterbalance forklift equipped with a tall, triplex mast has a heavy-duty mechanical latch-usually a massive steel hook or a heavy pin. When the mast is tilted all the way back to its maximum rearward angle, this latch is supposed to automatically catch on a striker plate, physically locking the mast to the frame. This is a critical safety feature. If you are driving with a heavy load and hit a pothole, the momentum of the load can generate thousands of pounds of forward force. The tilt cylinders alone are not strong enough to hold that load back during a sudden impact; the mechanical lock takes the stress.
On this forklift, we found that the spring on the mast latch had broken months ago. The latch was stuck in the "open" position, completely useless. Operators, in a rush to make their pallet counts, had been driving the truck for weeks without the mast mechanically locked to the frame. When the operator hit a severe dock plate lip while carrying a heavy load, the hydraulic fluid compresses slightly under the shock. The mast tilted forward just an inch, enough for the load's momentum to take over, and the entire mast slammed down. We replaced the latch mechanism and the spring, but the real fix was firing the maintenance supervisor who allowed a truck with a visibly broken safety latch to remain in service. On tall-mast forklifts, you must physically verify that latch clicks into the striker plate before you move an inch.
