Load Chart Selection When the Boom Section Is Unclear
You are asked to lift a 3.0 t generator from a truck tray onto a plinth using a non-slewing mobile crane. The crane has a telescopic boom with a jib stowed on the side, and the load chart book in the cab is worn, with two pages partly illegible. The required radius is 8 m and the boom length needed is about 14 m. One legible chart for that configuration shows 3.4 t at 8 m, but the heading refers to a boom length range you cannot read clearly. The jib is stowed, and the outriggers are fully extended on a level concrete slab. The rigger says the chart is 'close enough' because the margin is 0.4 t.
1. How do you establish the correct capacity, and why is the 0.4 t margin misleading?
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I would not use a chart I cannot read. The correct capacity comes from the manufacturer's chart for this exact crane, boom configuration, jib stowed or deployed, outrigger condition, and radius. I would stop and obtain a legible chart or the manufacturer's load chart document, confirm the crane model and serial number match it, and confirm which boom length range applies at 14 m. If the jib is stowed but its mounting affects the boom's rated capacity, that must be reflected in the chart used.
The 0.4 t margin is misleading because the illegible heading may cover a different boom length, and capacity falls as boom length and radius increase. The chart value also assumes level ground, correct tyre and outrigger setup, and no wind or dynamic effects. A margin that exists only on an unverified chart is not a margin. Until the correct chart is confirmed, the lift does not proceed.
2. What would you check on the crane and setup before accepting the chart figure?
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Confirm the outriggers are fully extended and pinned, with pads on the concrete and the crane level in both directions. Check the radius at the actual slew position, measured to the centre of the load, not estimated. Confirm the boom length and that the jib is properly stowed and secured. Check the load weight from a reliable source, add the weight of the rigging, and confirm the slings and shackles are rated for the total. Check the rated capacity limiter or indicator is set for the correct configuration and is functioning, and that the anti-two-block device is working. Confirm wind is within the chart limits and the load has no large sail area. Only then compare the total load to the verified chart figure.