Sling Angle and Capacity on a Skewed Pick
You are rigging a 4,800 lb skid with four lifting points at the corners. The crane hook is directly above the load's geometric center, but the skid's center of gravity sits 18 in off-center toward one end, so the two slings on that end carry more tension than the other two. Your crew proposes four single-leg wire rope slings rated 3.0 tons each in a vertical hitch, choked to the lifting eyes. The horizontal spread between opposite lifting points is 8 ft, and the vertical distance from hook to lifting eyes is 6 ft. The load must be lifted, swung 90 degrees, and set on a pad 6 in higher than its current position, with no tag line currently rigged.
1. Determine whether the proposed four-leg sling arrangement is adequate, showing the tension calculation for the worst-loaded leg.
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Step 1 - Leg angle. Half the horizontal spread is 4 ft, vertical is 6 ft, so the sling length is sqrt(4^2 + 6^2) = 7.21 ft. The angle from horizontal is atan(6/4) = 56.3 degrees, and the angle from vertical is 33.7 degrees.
Step 2 - Share of load. With a symmetric four-leg pick, only two legs are assumed to share the load for capacity purposes (a common conservative rule because a four-leg sling rarely shares load evenly). But this load is also off-center, so the end nearer the CG carries more. Treat the load as a beam: the CG is 18 in from center over an 8 ft (96 in) span, so the near-end pair carries a reaction of 4,800 x (48 + 18)/96 = 3,300 lb, and the far pair 1,500 lb. The near-end pair is two legs, but assume one leg takes the whole 3,300 lb as the worst case.
Step 3 - Tension. Vertical tension 3,300 lb divided by cos(33.7 degrees) = 3,300 / 0.832 = 3,966 lb, roughly 1.98 tons per leg in the worst case.
Step 4 - Capacity check. The 3.0 ton rating is for a vertical single-leg hitch. A choker reduces capacity (commonly to about 75-80 percent of vertical), so effective capacity is roughly 2.25-2.4 tons. The worst leg at 1.98 tons is close to that limit, with no margin for dynamic load, and the assumed two-leg sharing is optimistic given the off-center CG.
Conclusion: Do not proceed as proposed. Reconfigure to a four-leg bridle with the hook repositioned over the CG, add a spreader bar to reduce the included angle, or use higher-rated slings. Verify actual sling ratings against the manufacturer's chart for the specific hitch and D/d ratio.
2. Identify the rigging and communication deficiencies in the plan and state what must change before the lift.
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Deficiencies:
- No tag line is rigged, yet the load must swing 90 degrees. Without tag lines the load can rotate or swing uncontrolled, and the signal person cannot arrest it.
- The hook is not over the CG, so the load will tilt when picked. That changes the sling angles and load distribution from the assumed values, making the calculation unreliable.
- Setting on a pad 6 in higher means the load must be raised above the set point and lowered precisely; the plan has no allowance for the extra height or for the landing zone being clear.
- No mention of a pre-lift meeting or a designated signal person, and no agreed hand signals or radio channel.
Required changes before the lift:
- Reposition the crane or use a bridle so the hook is over the CG, or accept the tilt only after recalculating.
- Rig two tag lines, one on each end, with personnel outside the fall zone and never under the load.
- Designate one signal person and one backup; agree on hand signals and radio channel; the operator takes signals from one person only.
- Confirm the landing pad is level, clear, and capable of supporting the load, and verify the boom length and radius for the higher set point.
- Hold a pre-lift meeting covering the plan, the load weight, the rigging, and the exclusion zone.
3. The crew wants to lift with only two of the four slings to save time. Evaluate this and state the conditions under which a two-leg pick would be acceptable.
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Evaluation: Removing two slings does not remove the load; it concentrates it. With a two-leg pick on an off-center CG, one leg can carry most of the load, and the load will rotate about the lifting axis. The tension in the heavily loaded leg can exceed the vertical share by a large factor depending on the angle. With the same 56.3 degree angle from horizontal, a leg carrying 3,300 lb of vertical load sees about 3,966 lb of tension, so the 3.0 ton rating is approached and the choker derating is exceeded.
Conditions for an acceptable two-leg pick:
- The slings are rated for the actual tension at the actual angle, with the choker derating applied and margin for dynamic effects.
- The hook is over the CG, or the tilt is calculated and controlled.
- The load is stable and will not rotate or flip when lifted.
- The sling angles are known and within the manufacturer's limits, and the D/d ratio at the choker is acceptable.
- The lift plan and the signal person are briefed on the change.
A time saving of minutes is not worth an uncontrolled load. If the four-leg bridle is already rigged and correct, use it.