Rigging Load Basics: Angles, Vectors, and Safety Factors

Most rigging failures are not caused by a component that was too weak. They are caused by a component that was strong enough, in a system where the geometry made it carry far more than anyone expected. Understanding angles is the whole of it.

A load is not the same as a tension

A weight hanging from a single vertical line produces a tension equal to the weight. That is the easy case, and it is the only case where the numbers are obvious. The moment a second line is introduced, or the line is not vertical, the tension in each line is no longer equal to the weight — and it is almost always larger.

The reason is that the two tensions have to combine to produce one resultant that balances the weight. If they are not parallel, part of each tension cancels the other out horizontally, which means each has to be larger to make up the vertical component. The wider the angle between the legs, the more of each tension is wasted on cancelling, and the larger both have to be.

The angle that catches people out

With two legs at a narrow included angle, each leg carries a bit more than half the load. As the angle opens, the tension climbs — slowly at first, then very fast. The figures are worth memorising because they are counter-intuitive.

More legs does not mean less tension

It is tempting to assume that four legs share a load four ways. In practice they rarely do, because the load is never perfectly centred and the legs are never exactly equal in length. The usual working assumption is that two of four legs carry the load, so a four-leg sling is planned as if it were a two-leg sling at the same angle.

Unequal leg length makes it worse. The shorter leg takes more of the load until it stretches, and if the legs are of different materials with different stiffness, the stiffer one takes a disproportionate share from the moment the load is applied.

Safety factors and why they are not arbitrary

A safety factor is the ratio between the load at which a component breaks and the load it is permitted to carry in service. Lifting hardware commonly uses five or six to one; a shackle with a breaking load of 6,000 kg may carry a working load limit of 1,200 kg.

The factor is there to absorb everything that is not in the calculation: wear, corrosion, an imperfect load distribution, a shock from a load that was not lifted smoothly, and the simple fact that the person who rated it never saw your particular arrangement. It is not a margin of comfort. It is the margin that makes a calculated number survivable.

• Static load: a steady weight, no movement. A factor of five is typical for lifting hardware.
• Dynamic load: the load moves, swings or is snatched. Apply an additional dynamic factor, commonly 1.5 to 2, on top of the static factor.
• Shock load: a fall or a sudden stop. This is what energy-absorbing elements are for, and why a taut low-stretch line is dangerous in that role.
• Fatigue: repeated cycling at a modest load can fail a component that would have survived a single larger load.

Working through an example

A 400 kg load is lifted on a two-leg sling with an included angle of 120 degrees. Each leg carries 1.00 × 400 = 400 kg. If the lifting gear is rated at five to one, each leg must have a breaking load of at least 2,000 kg, and the shackles and the anchor points must match. Now narrow the angle to 60 degrees: each leg carries 232 kg, and the requirement drops to 1,160 kg. Same load, same sling, a different angle — and a requirement that has nearly halved.

This is why an experienced rigger spends more time looking at the geometry of an arrangement than at the load chart. The chart tells you what a component can do. The geometry tells you what you are asking it to do.