Direct answer: When a load's centre of gravity is not centred between the lifting points, the legs of a polyester lifting sling assembly do not carry equal shares, and the leg on the heavier side carries more. Selection therefore starts by finding the centre of gravity and estimating how the load divides between the legs, then choosing a sling whose WLL covers the most heavily loaded leg at its actual angle. A symmetric calculation that assumes equal shares is unsafe for an offset load; the tilted geometry also changes each leg's angle and introduces load stability problems that the sling selection must address.

Why the Centre of Gravity Decides the Load Share

In a symmetric lift, the assumption of equal load share between legs is often reasonable. An offset centre of gravity breaks that assumption. The load behaves like a weight suspended from two or more points, and the reaction at each point depends on how far the centre of gravity is from it. The closer a lifting point is to the centre of gravity, the more load it takes. In a two-point lift, the load share is governed by the lever principle: if the centre of gravity sits at one-quarter of the span from one point, that point takes three-quarters of the load and the other takes one-quarter.

That ratio can be surprising. A load that "looks" balanced but is 25 percent off-centre can put 75 percent of its weight onto a single leg. If the sling was chosen on the assumption of a 50/50 split, that leg may already be overloaded before any dynamic effect or angle penalty is applied.

The Two-Leg Tilt: How Offset Loads Shift Tension

An offset centre of gravity does more than change the load shares; it also tilts the load. As the load tips toward the heavier side, the two sling legs settle into different angles, and their lengths change accordingly. A leg that becomes steeper carries its share with a smaller angle factor, while the opposite leg that becomes shallower sees a larger factor. The result is that the two effects — unequal share and unequal angle — compound on the heavily loaded, shallower leg.

This is why a lift that behaves well when lightly loaded can become unstable when fully raised: as the sling tensions, the tilt can increase, which increases the difference in leg angle, which further concentrates the load. For a polyester lifting sling assembly, this is a selection problem, not just a rigging one, because the sling on the heavy leg must have the capacity for the combined effect.

A Selection Workflow for Offset Loads

The table below sets out a practical sequence for a load with an offset centre of gravity. It is a planning aid; the final rating and configuration must be confirmed against the applicable standard and the specific lift plan.

Choosing Slings and Fittings for Off-Centre Loads

Several selection choices for the polyester lifting sling follow from the analysis. Above all, size the sling on the worst leg, not on the average. Where the imbalance is significant, a single WLL for the whole polyester lifting sling assembly is misleading; each leg of the polyester lifting sling should be checked, and if the assembly uses different sling lengths or capacities, that must be an intentional, documented decision. Keep the legs as steep as practical to reduce the angle penalty, and choose fittings — shackles, master links and hooks — rated for the load on the leg they serve, not for the total load divided by the number of legs.

Longer, purpose-matched slings also help stabilise a tilted load, because they reduce the sensitivity of the geometry to small changes in tilt. For heavier or more unbalanced loads, a double-eye sling such as this 12-tonne 7:1 double-eye webbing sling, or a rated eye-to-eye option such as this EN 1492-1 eye-to-eye flat woven sling rated to 10 t WLL, gives a clear, standards-marked starting point that can be matched to the worst-leg demand.

Load Stability: Rotation, Slippage and Settling

Uneven loads are also unstable loads. A load whose centre of gravity is above the lifting points may tend to rotate or tip; a load whose slings can slide along its length may shift during the lift and change the load shares dynamically. Both risks are managed partly by sling selection and partly by rigging: matched sling lengths to limit unwanted tilt, load restraints to prevent movement, and tag lines to control rotation. Where the centre of gravity is high or the load can settle, the plan should allow for the possibility that the geometry will change once the load is clear of the ground.

Using Spreaders and Balance Beams

A spreader beam or balance beam is often the cleanest solution to an offset load. A beam converts the lift into vertical legs at fixed, controlled positions and takes the horizontal component out of the slings, which removes the angle penalty entirely for the legs below the beam. A balance beam can be arranged so the load hangs level even with an offset centre of gravity. Both turn a difficult sling-selection problem into a straightforward vertical lift, and both are worth considering before accepting the compromise of an unbalanced two-leg arrangement.

Frequently Asked Questions

Can I just size the sling on the total load divided by the number of legs?

Not for an offset load. That method assumes equal shares, which is exactly what an offset centre of gravity breaks. The assembly should be sized on the most heavily loaded leg after the share and the angle are both applied.

How do I find the centre of gravity of a load?

For simple, uniform loads it can be estimated from geometry and density; for complex or critical loads it should be determined by calculation or by a controlled trial lift. The accuracy of the load-share estimate depends on how well the centre of gravity is known.

Does a four-leg sling fix an offset load?

Not by itself. A four-leg assembly can still distribute unevenly, and practice often assumes only three legs share the load on a rigid item. An offset centre of gravity can make even that assumption optimistic on the heavy side.

When should I use a spreader beam instead?

When the imbalance is significant, when the load must stay level, or when the legs would otherwise be forced into shallow angles. A beam removes the angle penalty for the lower legs and stabilises the lift, often making it the simpler and safer choice.

Practical Guidance for Buyers and Rigging Engineers

• Locate the centre of gravity first and estimate the load share before choosing any sling.
• Size every leg on its own worst case — share plus angle — and rate the fittings to match each leg.
• Keep legs steep and consider matched lengths or a spreader/balance beam for stability.
• Review rated double-eye and eye-to-eye options on the Polyester Flat Webbing Sling category page, and confirm WLL against the heaviest leg.
• Plan for the load to change shape or settle during the lift, and add restraints or tag lines where rotation is possible.

Offset loads punish assumptions. The fix is not a bigger sling chosen by feel; it is a short, disciplined calculation that finds the heaviest leg and then covers it. Do that, add stability control where the load can move, and an uneven load becomes an ordinary lift with an understood, documented load path.