A busy lumber yard reported that a Hyster S80FT 4-ton forklift felt "tippy" and unstable when cornering, and the steering response was erratic. The operator stated that the rear end of the forklift felt like it was "skating" on wet pavement, even on dry concrete. This is a critical safety hazard, as forklifts rely on a specific center of gravity.
The technician first inspected the steering components. The tie-rod ends and steering cylinder were in good condition with no play. He then checked the rear axle assembly. The Hyster S80FT uses a "pivoting rear axle" design. The axle beam is connected to the chassis via a large central pivot pin. This allows the rear wheels to oscillate and maintain contact with the ground on uneven surfaces, while the chassis remains stable.
The technician attempted to manually pivot the rear axle by pushing down on one rear tire. The axle was completely seized solid. He removed the pivot pin cover and found that the grease zerk (fitting) was broken off, and the pivot bushing had rusted to the pin. Because the axle could not oscillate, when the forklift hit a bump or turned a corner, the entire machine would tilt instead of just the axle. This dramatically reduced the tipping stability margin, giving the operator the sensation that the truck was about to roll over.
Additionally, the technician checked the drive tire diameters. The left drive tire was a brand new solid rubber tire (diameter 860mm), while the right drive tire was worn down to 820mm. In a differential-drive forklift, a difference in tire diameter causes the machine to "crab walk" or pull to one side constantly. The smaller tire rotates faster, pushing the truck to the left. This constant "fighting" against the steering, combined with the seized axle, created the "skating" sensation.
The repair involved pressing out the seized pivot pin, installing a new greasable bushing kit, and replacing the pivot pin. The drive tires were also matched-a new pair of tires was installed to ensure equal diameter. After the repairs, the forklift passed a "tilt test" (measuring the angle at which the wheels lift off the ground), confirming that the stability margin was restored to factory safety standards.
