A large grocery distribution center reported that a Crown PE4500 electric pallet jack developed a loud, low-frequency "growl" from the drive wheel when loaded, and the truck consistently pulled to the left when traveling on flat concrete. Operators also noted that with a 2-ton pallet load, the drive wheel occasionally "chattered" and the truck momentarily lost drive torque, requiring the operator to momentarily reverse to regain traction.
The technician first checked the drive wheel's tire tread and surface condition-the polyurethane tread was evenly worn, with no flat spots. He elevated the truck's front end and attempted to rock the drive wheel side-to-side. The wheel exhibited 4 millimeters of lateral play (spec is under 0.5 millimeters), indicating severe bearing wear. He then spun the drive wheel by hand; it produced a rough, gritty resistance with audible clicking, confirming bearing failure.
He disassembled the drive wheel assembly. The "Outboard Drive-Wheel Bearing" (a heavy-duty tapered roller bearing) had completely lost its grease, and the race surfaces were brinelled and pitted. The bearing's cage had partially disintegrated, with loose roller elements migrating inside the bearing housing. The bearing failure had allowed the drive wheel to wobble slightly under load, changing the tire's contact patch and causing the truck to pull toward the side of greater friction-the left side in this case. The growling noise was the direct result of the damaged bearing rollers grinding against the pitted race surfaces.
The technician also inspected the "Traction Motor Drive Coupling"-a rubber-bushed flexible coupling between the traction motor's output shaft and the drive wheel's hub. The rubber bushings were cracked and partially disintegrated, allowing intermittent slippage between the motor shaft and the wheel hub. Under heavy load, the coupling would momentarily slip (causing the chatter and the torque loss the operator experienced) before re-engaging. The coupling failure was a secondary result of the bearing failure: the wobbling drive wheel had subjected the coupling to cyclic shock loads that rapidly degraded the rubber bushings.
The repair involved replacing the failed drive-wheel outboard bearing (along with its matching inboard bearing, since both had the same service life), replacing the complete traction-motor drive coupling with a new unit, and inspecting the traction motor's output shaft splines (clean, no wear). The drive wheel's polyurethane tire was undamaged and reused. The motor's gearbox oil was inspected and found to be clean at the correct level. After reassembly, the drive wheel spun freely with zero lateral play, the truck tracked straight with no pulling, and the drive wheel operated silently under a full 2-ton load with no chatter or torque loss.
