A land-clearing contractor reported that a Caterpillar D6N XL crawler dozer's right track developed an abnormal "slap-and-pitch" noise during travel and slope work. The operator noticed the track was visibly sagging along the roller frame-substantially more than the 50-millimeter sag measured on the left track. The dozer also experienced frequent "track shedding" (the track derailing from the idler) when turning sharply on uneven ground, requiring manual re-racking.
The technician first recognized the symptoms of a failing track adjuster. The D6N's track is tensioned by a "Hydraulic Track Adjuster"-a spring-loaded hydraulic cylinder mounted at the front idler that extends the idler forward to maintain proper track tension. The cylinder is filled with grease under pressure, and the internal spring provides the compliance that allows the track to absorb impact from rocks and debris.
He measured the track sag at the mid-point between the front idler and the front carrier roller. The right track sagged 100 millimeters-double the specification. He located the track adjuster's grease fitting and attempted to pump grease into the adjuster to re-tension the track. The grease gun pressure would not build, and grease continuously escaped from the adjuster's rod-side weep hole, indicating an internal seal failure.
He removed the track adjuster assembly from the idler support frame. Disassembling the cylinder revealed the "Rod Seal" and "Piston Seal" were hardened, cracked, and had lost their elastic memory-classic symptoms of prolonged exposure to heat, abrasive dust ingress, and grease oxidation. The grease inside the cylinder had degraded to a thin, dark liquid that could no longer maintain a sealed column. Without the pressurized grease column, the spring could not hold the idler forward, allowing the track to sag excessively. The sag in turn caused the track's pitch to whip against the carrier rollers and sprocket, producing the "slap-and-pitch" noise and the track-shedding events.
The repair involved replacing the complete track adjuster seal kit with new OEM seals, honing the cylinder bore to remove minor scoring, and reassembling the adjuster with fresh EP-2 lithium-complex grease. The adjuster was pumped up to the specified 700-bar internal grease pressure using a hydraulic hand pump with a gauge, restoring proper idler extension and track tension to the 50-millimeter specification. The technician also inspected the right-side front idler and front carrier roller for wear-both were within specification, and no undercarriage damage had occurred from the extended sag. After reassembly, the right track held firm tension, the slap-and-pitch noise disappeared, and the dozer operated through a full day of slope work without any further track-shedding events.
