Pick-and-Carry with a Loaded Tyre and a Soft Verge
You are operating a 20 t non-slewing mobile crane fitted with a pick-and-carry chart. The task is to move 2.8 t of steel plate 60 m across a yard to the fabrication shed. The route crosses a sealed apron, then a 15 m section of gravel shoulder that slopes about 4 degrees toward a drainage channel. The pick-and-carry chart for the required boom length and radius shows 3.2 t, but that figure is for level, firm ground. The plate is slung on four-leg chain slings with a 0-degree included angle at the master link. A fitter asks you to carry the load with the boom slightly luffed down to clear a pipe rack, and to keep the load about 1 m off the ground for speed.
1. Evaluate the proposed travel path and the two requests from the fitter.
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The chart figure of 3.2 t applies to level, firm ground, so the 4-degree cross-slope on the gravel shoulder is outside its basis. On a side slope the load and boom centre of gravity shift downhill, increasing the effective radius and the tipping moment, and gravel reduces traction and can rut. The margin of only 0.4 t is too thin for those conditions. I would not travel that section with the boom at the planned radius.
Luffing the boom down to clear the pipe rack increases the radius and therefore reduces capacity, so the request directly undermines the chart value. Carrying the load 1 m high is also unacceptable: a travelling load should be kept as low as practical, consistent with ground clearance and the route, so that if it swings or the crane stops suddenly it does not create a large pendulum or strike people. I would refuse both requests, keep the load low, and either find a level route, reduce the radius by repositioning the pick-up and set-down points, or use a smaller load per trip.
2. What checks and controls would you put in place before starting the move?
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Walk the route and confirm grade, surface condition, drainage, overhead lines, and clearance at both ends. Confirm the pick-and-carry chart is the correct one for the tyre configuration and that the actual radius at every point on the route, including the slope, stays within it with margin. Check tyre pressures and condition, because a soft or under-inflated tyre changes the crane's level and stability. Confirm the sling assembly is rated for 2.8 t and that the 0-degree included angle means each leg shares load evenly; check the master link and hooks are seated and latched.
Controls: designate a guide or dogger to walk the route, keep people out of the travel path and the drop zone, travel with the load low and the boom in the prescribed travel position, use a spotter for the gravel section, and stop if the surface rutts or the crane leans. Brief the fitter on why the boom is not luffed down. If the slope cannot be avoided, reduce the load or use a different machine or route.