A pharmaceutical distribution center reported that a Crown FC 5252-50 5,000-pound-capacity electric counterbalance forklift had developed a loud grinding noise from the front (drive) end of the truck. The operator also reported the forklift pulling to the right during travel, and the left drive wheel occasionally feeling as if it "lost grip" during turns. The left drive tire showed accelerated wear on its inner shoulder.
The technician first elevated the front of the truck and manually rotated each drive wheel. The right wheel turned smoothly with minimal resistance. The left wheel produced a gritty, grinding resistance with audible clicking - confirming internal damage in the left drive wheel assembly. He inspected the left drive tire and noted flat-spot wear consistent with the wheel locking intermittently during travel.
He removed the left drive wheel assembly from the axle housing. On the FC 5252, each drive wheel is driven by a "Drive Motor" - a permanent-magnet DC motor whose output shaft drives the wheel through a pinion-and-gear reduction inside the axle housing. Disassembling the housing revealed the "Drive Wheel Bearing" (a sealed double-row ball bearing supporting the wheel hub on the axle housing) had completely seized. The bearing's internal rollers had flat-spotted and the race surfaces were severely pitted. Loose bearing debris had migrated into the axle housing, contaminating the gear oil.
He further inspected the drive motor's pinion gear - the small gear on the motor's output shaft that engages the larger bull gear inside the axle housing to provide the reduction ratio. The pinion showed abnormal wear on its teeth, with several teeth showing pronounced scoring and metal removal. The scoring was consistent with the pinion operating under misaligned load (caused by the seized bearing allowing the motor shaft to shift slightly under torque load).
The root cause was traced to water ingress into the bearing cavity. The drive wheel bearing's seal had been compromised - likely from a prior forklift impact with a dock plate that had partially dislodged the bearing seal - allowing water and cleaning-fluid residue from the pharmaceutical facility's wash-down procedures to enter the bearing cavity. Over time, the water had washed out the bearing grease and initiated corrosion, leading to progressive bearing failure. The debris from the failed bearing had then contaminated the axle housing's gear oil and accelerated the pinion gear's wear.
The repair involved replacing the left drive wheel bearing, replacing the drive motor's pinion gear and its mating bull gear, thoroughly flushing the axle housing to remove all bearing debris and contaminated gear oil, and refilling the housing with fresh specified gear oil. The right-side drive assembly was inspected proactively - its bearing and pinion showed minor wear but were serviceable. Both drive tires were inspected; the right tire was in good condition, while the left tire's flat-spot wear was within acceptable limits. After reassembly, both drive wheels rotated smoothly with equal resistance, the grinding noise disappeared, and the forklift tracked straight during a loaded test cycle. The drive wheel bearing seal condition was added to the truck's 250-hour inspection checklist given the facility's wash-down environment.
