A civil-infrastructure contractor reported that a JCB 220X crawler excavator had developed a progressive undercarriage fault over a two-week period. The machine's right-side track had lost its correct tension gradually across several shifts - the track visibly sagging below the machine's bottom rollers during operation, and producing a rhythmic slapping sound as the loose track contacted the track frame. The operator reported that even after manually pumping additional grease into the track adjuster's grease fitting to re-tension the track, the track would lose tension again within one or two shifts. Fine grease residue was also visible around the track adjuster cylinder's external housing, indicating a grease leak was occurring somewhere within the adjuster assembly.
The technician first understood the 220X's track-adjuster architecture. Each track is tensioned by a "Track Adjuster Cylinder" - a single-acting hydraulic cylinder mounted at the front of the track frame, with its rod end connected to the track's front idler. The cylinder's cap-end chamber is filled with high-pressure grease, which pushes the piston outward, extending the rod and moving the idler forward to tension the track. The adjuster is filled and topped up through a grease fitting, and a "Relief Valve" mounted on the adjuster protects the cylinder from over-pressurization - for example, when the machine strikes a rock or travels over an obstacle, the relief valve vents excess grease to prevent cylinder or track damage. When the piston seal fails, the high-pressure grease bypasses the piston internally, allowing the cylinder to retract under track-chain resistance - causing progressive track sag. When the relief valve sticks open, the grease vents externally and the track loses tension continuously.
He verified the symptom by measuring the track's sag with the machine stationary on level ground. The correct track sag for the 220X is approximately 20 to 30 millimeters measured at the midpoint between the carrier roller and the front idler. The right-side track measured 55 millimeters - well beyond specification, confirming significant loss of tension. He then pumped fresh grease into the adjuster's fitting to restore correct tension, marked the position of the idler, and re-measured after 8 hours of operation. The idler had retracted by approximately 15 millimeters - confirming grease was being lost internally or externally within the adjuster assembly.
He cleaned the adjuster assembly and pressurized the system by pumping grease while observing all external surfaces. Fresh grease was visibly escaping from the adjuster's "Relief Valve" port - confirming the relief valve was stuck open and venting grease rather than holding pressure. He removed the relief valve and disassembled it on a bench. The valve's poppet showed visible scoring on the sealing surface, and the valve's spring had taken a permanent set - the spring's free length was 3 millimeters shorter than its new specification, allowing the valve to open at a lower pressure than designed. He further removed the adjuster cylinder and bench-tested it with the relief valve removed - the cylinder's piston seal showed visible hardening and scoring, with the seal lip no longer making full contact with the cylinder bore, allowing internal bypass under pressure.
The root cause was traced to relief valve spring fatigue compounded by piston seal wear. The relief valve's spring had fatigued gradually over extended service (the machine had approximately 4,600 hours), reducing the valve's opening pressure and allowing grease to vent during normal operation. The piston seal had degraded in parallel, allowing internal bypass within the cylinder - and the combination had produced the progressive track-sag the operator described.
The repair involved replacing the track adjuster's relief valve with a new unit, replacing the cylinder's piston seal with a new OEM seal kit, and thoroughly cleaning the adjuster's internal cavity to remove all degraded grease and metallic debris. The cylinder's bore was inspected and showed minor scoring consistent with the machine's hours but was within service limits. The adjuster was refilled with fresh specified grease and the track was re-tensioned to JCB's 25-millimeter specification. The left-side adjuster was inspected for comparison (piston seal and relief valve in good condition). After reassembly, the track's tension was re-measured after 8 hours and 24 hours of loaded operation - tension loss was eliminated, the rhythmic slapping sound was gone, and the idler position remained stable. Track-adjuster relief valve and piston seal inspection was added to the machine's 1,000-hour preventive maintenance schedule.
