A utility construction company reported that a Kato CR-250Rv-7 25-ton rough terrain crane's Rated Capacity Limiter (RCL) was displaying erratic load chart values and triggering false overload alarms. During a routine HVAC unit lift at 60 percent of the rated capacity, the RCL would intermittently display a "Overload" warning and automatically cut the hoist and boom functions, forcing the operator to restart the system. The display would then show different load values for the same static lift.
The technician first verified the actual load weight using a calibrated load cell attached to the crane hook-the actual load was 5.2 tons, but the RCL displayed values ranging from 4.8 to 6.5 tons. He then inspected the "Boom Length Potentiometer" mounted at the boom's base section. The potentiometer measures the boom's extension length and provides this data to the RCL for load chart calculation. Using a digital protractor to measure the boom's actual angle and a tape measure to verify the boom's extension length, he confirmed the boom geometry was correct but the RCL was displaying an extension length 1.5 meters longer than actual.
The boom length potentiometer's rotary shaft was mounted to a cable-pulley system that converts boom extension into rotary motion. The "Drive Cable" connecting the potentiometer's pulley to the boom's mid-section had stretched by approximately 30 millimeters over years of service, causing the pulley to rotate further than it should for a given boom extension. This stretched cable was feeding an exaggerated boom length signal to the RCL.
He further inspected the "Hoist Rope Load Cell"-the tension sensor mounted at the boom head that measures the actual load weight from the hoist rope tension. The load cell's cable attachment point had shifted slightly, causing the load cell to read tension from a slightly offset angle, introducing an additional error into the load calculation.
The repair involved replacing the stretched boom length potentiometer drive cable with a new cable, re-tensioning the cable to the manufacturer's specified tension, and recalibrating the potentiometer's zero and full-extension reference points. The hoist rope load cell's cable attachment was repositioned to the correct alignment and secured with a new clevis pin. The RCL was then recalibrated using certified test weights at three boom positions. After recalibration, the RCL displayed the actual load weight to within 2 percent accuracy, and no false overload alarms occurred during subsequent lift tests.
