A utility contractor reported that a Cat 308 CR mini excavator had developed a progressive right-track travel fault over a two-month period. When the operator commanded full forward travel with the machine loaded and the boom lowered, the right track would intermittently slip - losing traction briefly before re-engaging - and the machine would veer leftward, requiring constant corrective steering input from the left travel lever. A rhythmic "clunking" was audible from the right final drive housing during loaded travel cycles, and the machine's right-track travel speed had degraded noticeably compared with the left side.
The technician first verified the machine's track tension was correct on both sides, and that both tracks showed even wear patterns - ruling out the simple causes that mimic a failed final drive. He then removed the right final drive's cover and inspected the internal planetary assembly.
He understood the 308 CR's travel architecture. Each track is driven by a dedicated "Travel Final Drive" - a compact planetary gearbox mounted inside the track frame, with a hydraulic travel motor coupled to the gearbox's input shaft through a spline connection. The gearbox reduces the motor's rotational speed while multiplying torque, delivering the slow, high-torque output required for the sprocket to drive the track chain. The planetary assembly consists of a sun gear, several planet gears mounted on a "Planet Pin" (a small shaft the planet gears rotate around), and a ring gear. When the planet pin wears, the planet gears develop radial play, producing the vibration and "clunking" the operator described.
The right final drive's "Planet Pin" showed visible wear - the pin's surface had worn into an oval profile at the point where the planet gear's bushing rotated around it, with the oval's long axis roughly 2.4 millimeters wider than the pin's nominal diameter (spec maximum is 1 millimeter of clearance). The planet gears' bushings showed corresponding wear, and the sun gear showed minor tooth-profile wear consistent with the planetary gears' uneven loading. He further inspected the "Drive Sprocket's Mating Hub Spline" - the spline connection between the final drive's output shaft and the drive sprocket. The spline showed visible wear on several teeth, with a dark, metallic-sludge residue at the spline's base indicating abrasive wear particles had accumulated within the connection. The worn spline was allowing the sprocket to intermittently rotate relative to the output shaft under high torque - producing the "slip-and-reengage" behavior the operator described.
The root cause was traced to planet pin wear compounded by output spline degradation. The planet pin had worn naturally over extended service (the machine had approximately 4,200 hours), and the resulting planetary gear play had accelerated the load cycling on the output spline, which had then begun to rock under high-torque conditions and wear the spline teeth.
The repair involved replacing the right final drive's planet pin, planet gears, and bushings with new components, replacing the sun gear (its tooth profile showed wear), replacing the drive sprocket's mating hub with a new hub (the worn spline was not field-repairable), and thoroughly flushing the final drive's internal cavity to remove all metallic sludge and debris. The final drive was refilled with fresh specified gear oil to the correct level. The left final drive was inspected for comparison (planet pin and bushings within service limits). After reassembly, a loaded travel test showed the machine tracking straight with no drift, the rhythmic clunking eliminated, and right-track travel speed restored to match the left side across a full travel test on level ground. Final drive gear-oil level and condition checks were added to the machine's 500-hour preventive maintenance checklist.
