A precast concrete supplier reported that a Genie GTH-844 telehandler's boom would not extend past 50% travel when carrying a 3-ton precast stair unit. Without a load, the boom extended normally to full reach. When retracting under load, the boom juddered violently and produced a loud, metallic clanking from the boom section. The lift (vertical) function operated normally.
The technician recognized the symptoms of a chain-driven telescoping boom. The GTH-844's middle boom section is extended and retracted by a pair of heavy-duty leaf chains routed around a fixed idler and a moving tensioner idler. When the boom is unloaded, the chains only need to overcome the boom's own weight. Under load, the chains must overcome the load's weight plus friction.
He inspected the extension chains. The leaf chains were visibly slack on the boom's upper run-there was approximately 40 millimeters of sag (spec is 15 millimeters). The chains had stretched beyond the adjustment limit due to 6,000 hours of heavy-load extension cycles without lubrication. He further inspected the "Chain Tensioner Idler Pin." The idler pin was deeply grooved and worn oval, allowing the idler pulley to wobble under tension. The combination of the stretched chains and worn idler pin meant the tensioner could not maintain even chain tension. When the boom was loaded, the slack chains allowed the middle boom section to "free-fall" slightly against the load's inertia, slamming into the tensioner's stop (the clanking judder), and the excessive chain flex consumed the hydraulic cylinder's stroke without actually extending the boom.
The repair involved replacing both the extension and retraction leaf chains with new chains and fitting a new tensioner idler pin and bushing. The chains were lubricated with a penetrating chain oil and tensioned to the specified 15-millimeter sag under a no-load condition. The hydraulic cylinder was inspected for scoring-the cylinder rod was clean and undamaged. After reassembly, the boom extended smoothly to full reach under a full 3-ton load, and the retract judder disappeared.
