Overload Alarm During a Long Travel Move
A 25 t capacity double-girder overhead crane with a 22 t load cell and overload alarm is moving a 21 t machined casting along a 40 m runway. The load is slung on a four-leg chain sling at 60 degrees included angle, and the crane's load display reads 23.5 t. The overload alarm has not sounded, but the display is above the rated capacity. The operator is halfway along the runway, and the load is 3 m above the floor over an aisle used by forklifts. The operator's mate says the display is 'always a bit high' and tells the operator to keep going. The crane has a 5 t auxiliary hoist that is not in use.
1. Is the crane overloaded, and what should the operator do right now?
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The display reading of 23.5 t exceeds the 25 t? No - 23.5 t is below 25 t, but the question is whether the true load is higher. The four-leg sling at 60 degrees included angle (30 degrees from vertical) increases leg tension: each leg tension = (W/4)/cos30 = W/4 x 1.155, so the vertical load on the hook is still W plus sling weight. The sling angle does not increase hook load, but it does increase leg tension, which matters for the sling rating, not the crane rating.
So on the numbers, 23.5 t is under 25 t, but the margin is only 1.5 t and the display may be inaccurate. The operator should not rely on the mate's reassurance. Actions:
- Stop travel over the forklift aisle and lower the load to a clear, barricaded area if safe to do so, or hold position with the aisle cleared.
- Check the load cell calibration and compare against a known mass or the casting's verified weight.
- Check the sling for rating at 60 degrees and for damage.
- Do not continue until the actual load is confirmed and the lift is within rated capacity with margin.
If the true load exceeds 25 t, the lift must be replanned, possibly using the auxiliary hoist only if it is rated for the share, or a larger crane.
2. How does the sling angle affect the rigging, and what should be verified before the move resumes?
Read the worked answer
At 60 degrees included angle, each leg is 30 degrees from vertical. Leg tension = (W/4)/cos30 = 0.289W per leg. For W = 21 t, each leg carries about 6.06 t. If the chains are rated 5 t WLL in a straight lift, they are overloaded even though the crane is not. The rigging must be rated for the leg tension at the actual angle, and the angle must be measured, not assumed.
Before resuming, verify:
- Actual load mass from a verified source or calibrated load cell.
- Sling leg rating at the measured angle, including any derating for the configuration (e.g. choke, basket, edge protection).
- Master link and hooks rated for the total load and the angle.
- Load stability and CoG - the casting must hang level and not rotate.
- Edge protection on the machined casting to prevent chain damage.
- Exclusion zone under the travel path, with forklift traffic stopped.
If any item cannot be verified, the load is lowered to a safe location and the lift is replanned.