A landscaping contractor reported that a Bobcat E35 mini excavator's left track had developed visible excessive sag - measuring more than 40 millimeters of droop from the top of the track pads to the tensioner assembly's sag point (spec maximum is roughly 15 millimeters). When the machine traveled over rough ground or side slopes, the track would repeatedly derail from the idler and carrier rollers, and the operator had to stop frequently to manually re-seat the track onto the undercarriage. A rhythmic "banging" noise was also audible from the undercarriage's front section when the machine traveled over uneven ground.
The technician first inspected the track tensioner system. Mini excavators achieve track tension mechanically via a hydraulic-grease cylinder mechanism that uses a manual grease gun to pump grease through a one-way valve, pushing the idler forward and tensioning the track. The further the idler advances, the tighter the track. He attempted to grease the left track tensioner's fitting with a grease gun - the grease would not enter, and pressure would not build. This indicated the "Track Tensioner Grease Cylinder" had failed: either the internal one-way valve was blocked or the cylinder seals had failed, allowing grease to pass straight through the cylinder without moving the piston.
He removed the left track to access the tensioner assembly. With the track removed, he disassembled the grease cylinder and confirmed the cylinder seal failure - grease flowed freely through the fitting and out the rod side, indicating the main piston seal had failed. The cylinder's grease fitting was also packed with hardened debris, preventing new grease entry. He further inspected the front idler - the "Front Idler" assembly rotates on a yoke supported by bushings. The idler's yoke bushings showed significant wear, with approximately 2.5 millimeters of radial play (spec maximum is 1 millimeter), and the idler wobbled noticeably under load.
The root cause was traced to seal degradation and maintenance neglect. Track tensioner grease cylinder seals are made of synthetic rubber that degrades over time from exposure to grease moisture, continuous heat from the machine's undercarriage, and abrasive soil particles migrating onto the cylinder rod. The machine's service history showed no track tensioner cylinder seal replacement, and no regular track tension inspection. As the cylinder seals degraded, the track lost tension; the sagging track then imposed additional stress on the idler yoke bushings, which wore rapidly from the continuous banging of a loose track.
The repair involved replacing the track tensioner grease cylinder with a new unit, replacing the worn front idler yoke bushings with new units, and inspecting the rear sprocket and carrier rollers (which were in good condition). Hardened grease and debris lodged in the fitting were recovered from inside the undercarriage during the replacement. Track tension on both sides of the machine was set to Bobcat's specified sag - the left track had deviated the most because the cylinder had failed completely, but the right track was also approaching spec limit and required re-adjustment. The new tensioner cylinder pushed the idler into position when properly greased, establishing correct tension. After the repair, the track remained on the undercarriage with correct sag, no track derailment occurred even on side slopes, and the rhythmic banging noise from the front undercarriage was eliminated. Track tension inspection was added to the machine's 250-hour checklist.
