A 5-ton capacity forklift was experiencing severe juddering when lifting near its maximum rated load. The lift cylinder was holding, but the mast was physically binding and groaning. The mechanic suspected internal cylinder bypass.
We checked the lift chains. Forklift chains are ISO 80 grade (tensile strength 800 MPa). The chains consist of inner and outer links with a specific pitch (e.g., 3/4 inch or 19.05 mm). As chains endure shock loads, the pins wear against the bushings, causing the chain to stretch.
We applied 500 lbs of tension to the chain and measured across 12 links (pitch x 12 = 228.6 mm). The measurement was 233.2 mm. This is an elongation of 2.01%. In heavy-duty chain engineering, 2% elongation is the absolute discard limit. Beyond 2%, the chain has undergone plastic deformation-the steel has passed its yield point and will not stretch elastically; it will snap.
More importantly, a 2% elongation means the chain pitch no longer matches the pitch of the mast roller sprockets. The chain was trying to climb out of the sprocket grooves under a 10,000 lb load, causing the violent judder. Continuing to operate would have sheared the sprocket teeth or snapped the chain, dropping the load. We replaced both chain sets (costing $800) and re-shimmed the mast, saving the$5,000 mast rails and roller bearings from certain destruction.
