Interpolated Chart, Near-Edge Radius, and a Load Center Past 24 Inches
Your 12,000 lb telehandler carries this placarded chart for forks, no attachment, machine level, frame leveling centered: 10 ft - 8,000 lb; 12 ft - 6,500 lb; 14 ft - 5,400 lb; 16 ft - 4,500 lb. The pick is a 4,200 lb steel skid, 8 ft wide and 4 ft deep, sitting on the forks with its back against the fork face. A pump mounted on the outboard end shifts the center of gravity to about 30 in from the fork face. A footnote states all ratings are based on a 24-in load center and that the machine must be level within the frame-leveling limits. The operator says: 'I measured 13 ft from the front tires to the near edge of the skid, and 13 ft splits the difference between 12 and 14, so call it 5,950 lb. We are way under.' The 4,200 lb figure is from a certified scale ticket. The ground is compacted base course, level within 1°.
1. Is the operator's 5,950 lb interpolation valid? What number governs and why?
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No, for three reasons.
- Radius is measured to the load's center of gravity, not to the near edge. On a 4-ft-deep skid the CG sits about 2 ft behind the near edge, so his 13 ft is really about 15 ft. Radius is measured horizontally from the chart's reference point (commonly the front tire contact line) to the load CG.
- Charts are step functions, not straight lines. You read the value at the radius that applies, or at the next greater radius - never split the difference upward. At about 15 ft you fall between the 14-ft value of 5,400 lb and the 16-ft value of 4,500 lb; the governing value is 4,500 lb.
- Boom deflection grows the radius under load. The chart assumes the loaded machine, so measure with the load just clear of the ground, or add margin. So the governing capacity is 4,500 lb, and his 5,950 lb is a fiction that came from two separate measurement errors.
2. The 24-in load center footnote applies. The load CG is at 30 in. What do you do?
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Capacity falls as the load center moves outboard, so the 4,500 lb figure does not apply to this skid. The common approximation is inverse proportion: 4,500 x 24/30 = 3,600 lb, which is below the 4,200 lb load - an over-capacity pick. But an approximation is not authority: only the manufacturer's published derating for the actual load center is. So work the problem in this order. First, eliminate the condition: reposition the skid so its CG lands within 24 in of the fork face - turn it 90°, slide it back against the carriage, or re-rig so the pump end is inboard. Second, if it cannot be repositioned, reduce radius by choosing a setup position closer to the landing point and re-read the chart in the smaller bracket, then apply the load-center reduction to that number. Third, if neither works, get the manufacturer's derating in writing or use a crane. Never lift on the assumption that 'it is only 6 inches.'
3. What else must be true about the machine and setup before the chart value applies?
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The chart is conditional. All of these must hold: the machine is level within the frame-leveling limits confirmed on the indicator, with leveling centered rather than cranked to compensate for a bad setup; tire size and pressures are correct and matched on each axle; no attachment is fitted other than the one the chart covers, and if stabilizers are required by the chart they are fully deployed on firm pads; the chart is legible and matches the serial number; the load is stable on the forks, tilted back against the carriage, and secured against sliding; wind is within the manual's limit; the path of the boom and load is clear of people, structures and power lines; and no side pull or dragging is used. Travel is a separate rating and often lower than the static lift chart, so check how far the machine may move with the load. If any one condition fails, the number printed on the chart is not your capacity.