A material handling company reported that a JCB 531-70 Loadall telehandler had developed abnormal boom behavior during boom lift and extension. When the operator lifted the boom from the fully retracted position to mid-height with a heavy load on the forks, the boom would move smoothly for part of the cycle, then "jump" or "lurch" at the specific point where the third-stage telescope section was about to begin extending, before settling back into smooth motion. The uneven motion made precise load placement difficult when positioning the fork tips at 7-meter lift heights. A rhythmic "clacking" sound was also audible from inside the boom's retracted sections during boom cycling.
The technician first understood the 531-70's boom architecture. The machine's three-stage telescopic boom extends via a combination of the main extension cylinder and an internal chain system. As the main cylinder extends the second stage, chains connected between the second and third stages simultaneously extend the third stage, keeping the extension slides synchronized at a fixed ratio. Inside each boom section, "Chain Tensioner" assemblies - spring-loaded bolt mechanisms attached to the chain ends - automatically absorb chain slack and maintain proper tension throughout the boom's travel. When a tensioner spring fails or the chains elongate beyond the tensioner's ability to compensate, the chains go slack and the different boom stages no longer extend in sync - producing the jerky behavior the operator described.
He fully retracted the boom and removed the third-stage telescope's front-end cover to access the chain ends. Inspection revealed the third-stage extension chain's left-side tensioner spring broken into three pieces - with spring coils missing and fallen into the boom cavity - leaving that tensioner unable to maintain tension. The right-side tensioner spring was intact, but the chain on that side showed visible elongation - measuring the chain's pitch across a fixed link count showed approximately 1.8 percent pitch elongation (spec maximum is roughly 1.5 percent). The chain elongation combined with the broken left-side spring had created highly asymmetric tension across the two sides of the telescope stage.
The root cause was traced to missed preventive maintenance and spring fatigue. Boom chain tensioner springs fatigue over thousands of boom-extension cycles from the constant load cycling and heat buildup inside the boom. When a tensioner spring breaks, the chain on that side is no longer tensioned, and the chain on the opposing side - still spring-loaded and under chain load - pulls unevenly, misaligning the third-stage telescope's extension. The machine's service history showed no boom-internal inspection or chain-tension check had ever been performed.
The repair involved replacing both left and right chain tensioner assemblies on the third-stage extension with new units, and replacing the extension chains on both sides with new chains, as the chains had exceeded their elongation limit. The broken spring fragments that had fallen into the boom cavity were recovered during the replacement. The tensioner springs were pre-set to JCB's specified tension, and the boom was cycled through several full extension-retraction cycles to ensure the new chains seated evenly. After the repair, the boom lifted smoothly from fully retracted to mid-height without the "lurch," the third-stage telescope extended in sync with the second stage, and the rhythmic "clacking" sound was eliminated. Boom chain tensioner inspection was added to the machine's 500-hour checklist.
