A contractor reported that the left track on a Komatsu PC200-8M0 excavator would frequently become loose and "de-track" (jump off the sprocket) during sharp turns, especially when working on a slope. The operator tried to grease the track adjuster (tensioner) daily, but the track would loosen again within an hour of operation.
The technician jacked up the left track frame and removed the cover for the track adjuster assembly. The Komatsu PC200-8M0 uses a grease-type tensioner. The adjuster consists of a hydraulic cylinder that pushes against a large mechanical recoil spring. The technician noticed that the lock nut on the adjuster yoke was backed off, and there was a trail of dried grease leaking from the rear of the cylinder rod.
He disassembled the adjuster. The U-cup seal inside the cylinder was cracked and hardened. This allowed the grease to bypass the piston and leak out through the rod seal. Because the grease leaked out, the spring pressure was no longer amplified by the hydraulic cylinder, causing the track to slacken under load.
Furthermore, the technician found that the "Anti-Retreat Check Valve" in the grease fitting was stuck open. This valve is designed to hold the grease pressure, preventing the track from loosening even if the seal weeps slightly. A piece of dirt had jammed the check ball, preventing it from seating.
The repair involved rebuilding the adjuster cylinder with a new polyurethane seal kit. The check valve was cleaned and verified to hold pressure. The adjuster was re-installed, and the track was re-tensioned. The "Sag Measurement" (distance from the bottom of the track roller to the track links) was set to 320mm as per the service manual. The machine operated for a week without any further track loosening or de-tracking incidents.
