A quarry contractor reported that a Hyundai HL955A wheel loader had developed a progressive front-axle fault over a two-month period. The operator first noticed dark gear-oil residue streaked across the outside of the left front wheel and tire - a condition that had not been present when the machine was running normally. As the fault progressed, the left front hub housing would climb noticeably above the right-side hub's temperature during extended travel - hot enough that the operator could not rest a hand on the housing for more than a few seconds. The loader also produced a faint grinding sound from the left front hub during travel, and the front axle's oil level in the differential housing would periodically require topping up.
The technician first understood the HL955A's front-axle architecture. The loader uses a "Planetary-Final-Drive Front Axle" - a rigid axle housing with differential and two wheel ends, each wheel end containing a "Planetary Hub Reduction" - a compact planetary gear set mounted inside the hub housing that provides the final speed reduction and torque multiplication between the axle's half shaft and the wheel. The hub housing is lubricated by gear oil contained within the hub cavity, and the oil is retained by a "Wheel Hub Seal" - a rotary lip seal mounted at the hub's inboard edge that prevents gear oil from leaking outward along the axle's rotating surface. When the hub seal's lip wears, hardens, or is damaged by abrasive contamination, gear oil escapes outward and lubricant is lost from the hub cavity - causing the planetary gears and hub bearing to run with inadequate lubrication, which produces heat, wear, and the grinding noise the operator described.
He verified the symptom by inspecting the left front hub's exterior. Fresh gear-oil residue was visible streaked across the wheel's inner face, and the hub housing's exterior was measurably warmer than the right-side hub after a 15-minute loaded travel test. He listened to the hub with a mechanic's stethoscope during travel and confirmed a faint grinding noise from the hub's bearing area. He then removed the hub's inspection plug and checked the gear-oil level - the hub's oil level was significantly below specification, with the remaining oil showing a dark, burnt appearance and containing fine metallic particles.
He removed the wheel and hub assembly for inspection. The "Wheel Hub Seal" showed visible failure - the seal's rubber lip had hardened, cracked, and partially separated from the seal's metal casing, allowing gear oil to escape continuously along the axle's rotating surface. The seal's spring had also failed, allowing the lip's contact pressure against the axle surface to drop below the specified value. He further inspected the "Hub Bearing" and the planetary gear set. The hub's tapered-roller bearing showed visible distress - the bearing's rollers showed pitting and flat-spotting, and the bearing's raceways showed scoring consistent with inadequate lubrication. The planetary gear set showed corresponding wear on the gear teeth and on the planet carrier's bushings.
The root cause was traced to wheel hub seal failure compounded by hub bearing degradation. The hub seal's lip had hardened and cracked over extended service in the quarry's abrasive dust environment (the machine had approximately 5,800 hours), allowing gear oil to escape along the axle's rotating surface and out of the hub cavity. The resulting lubricant loss had caused the hub bearing and planetary gears to run with inadequate lubrication - producing the heat, the grinding noise, and the progressive bearing and gear wear the technician observed.
The repair involved replacing the wheel hub seal with a new unit, replacing the hub's tapered-roller bearings with new bearings and adjusting the bearing preload to specification, and replacing the planetary gear set's worn components (planet gears, planet pin, and bushings) with new parts. The hub cavity was thoroughly flushed with solvent to remove all metallic debris and degraded gear oil, and the hub was refilled with fresh specified gear oil to the correct level. The axle's differential housing was inspected and refilled with fresh gear oil (the differential's internal components showed no significant wear). After reassembly, a 30-minute loaded travel test showed the left front hub's temperature matching the right-side hub within normal variation, no gear-oil leakage at the hub seal, and the grinding noise eliminated. Wheel hub seal inspection and hub oil level checks were added to the loader's 250-hour preventive maintenance checklist, and annual hub seal replacement was scheduled for machines operating in abrasive dust environments.
