A demolition contractor reported that a Doosan DX140LCR-5 excavator's right track had developed a progressive travel fault over a two-week period. When the operator commanded both tracks forward at full travel, the right track moved at roughly 70 percent of the left track's speed, causing the machine to pivot in a tight left circle. When climbing a soil pile or traveling up a slight grade, the right track would lose power first and slow further, requiring the operator to command full travel on the left track and reduced travel on the right track to maintain a straight path. A visible oily film was present on the right final drive housing's exterior, with fresh drips accumulating on the track frame.
The technician first verified the symptom by driving the machine forward and reverse at full travel command on level ground - the right track moved visibly slower than the left. He then inspected the right final drive - the planetary gearbox and hydraulic travel motor assembly mounted at the track's drive sprocket. The exterior was coated with a dark oily film, and fresh drips were forming at the seal's outer edge. He checked the final drive's gear-oil level through its inspection plug - the level was significantly below specification, and the oil that drained out was milky and discolored, indicating hydraulic-oil contamination.
The DX140LCR-5's final drive is a compact planetary gearbox coupled to a bent-axis hydraulic travel motor. The motor's output shaft passes through a seal into the gearbox, where it drives the planetary gear set that provides the high reduction ratio required for track drive. The seal must prevent the gearbox's gear oil from leaking outward while also preventing the motor's hydraulic oil from leaking inward past the shaft. When this seal fails, the two fluids can migrate in both directions - gear oil leaks outward onto the housing, and hydraulic oil leaks inward into the gearbox, thinning the gear oil and reducing its lubricating capacity.
He removed the final drive from the track frame and disassembled the motor-gearbox interface. The "Output Shaft Seal" - a rotary shaft seal pressed into the gearbox's input housing - was visibly hardened and cracked, with a portion of the sealing lip torn away. The gearbox's planetary gear set showed minor scoring from the thinned gear oil's reduced lubricating capacity, but the gears themselves were serviceable. The hydraulic travel motor's internal components (piston block, cylinder barrel, valve plate) were inspected - no internal damage, but the motor's case drain was carrying traces of gear oil, confirming the bidirectional fluid migration.
The root cause was traced to seal aging accelerated by heat. Final drive seals are subjected to continuous rotational contact and to the heat generated by the gearbox and motor during operation. Over extended service (the machine had approximately 5,500 hours), the seal's elastomer had hardened and lost its elastic sealing capability. The eventual cracking and lip failure had allowed fluid migration in both directions - gear oil outward, hydraulic oil inward.
The repair involved replacing the output shaft seal with a new unit, thoroughly flushing the gearbox to remove the contaminated gear oil and any hydraulic-oil residue, and refilling the gearbox with fresh specified gear oil to the correct level. The planetary gear set was inspected and reassembled with fresh lubricant. The travel motor's hydraulic connections were inspected for external leaks (none found). After reassembly and bleeding, the right track's travel speed was restored to match the left, the excavator tracked straight during a full travel test, and the final drive's exterior remained dry - no further oil seepage. Final drive gear-oil level and condition checks were added to the machine's 500-hour service checklist.
