A manufacturing plant reported that a Hyster S120FT LPG forklift was making a loud "clacking" noise during the final 1 meter of the lift. The mast would shudder, and the operator felt a distinct "bump" through the controls. The noise stopped as soon as the lift stopped.
The technician inspected the mast assembly. The lift chains are anchored to the top of the inner mast channel and run over a sheave wheel (roller) at the top of the mast. Upon inspection, the sheave wheel had deep grooves worn into the metal, and the lift chains showed signs of "jumping" – skipping teeth on the sheave.
The root cause was determined to be uneven chain elongation. Chains stretch over time, but if one side stretches more than the other, the tension becomes unbalanced. The anchor pins that connect the chains to the mast cylinders were also found to be worn on one side, allowing the chain to sit at an angle. When the mast reached full extension, the slack in the stretched chain combined with the misaligned anchor pin caused the chain to ride up over the sheave ridge rather than seating in the groove.
This is a dangerous condition that can lead to chain failure and load drop. The repair involved replacing both lift chains as a set to ensure equal tension, replacing the sheave wheel bearings, and installing new hardened steel anchor pins. The mast channels were lubricated, and the chain tension was adjusted to the manufacturer's specification. The "clacking" noise was eliminated, and the mast extended smoothly.
