Rigging practice: hitches, sling angles and load control

Choosing sound rigging is only half the job. The way slings are attached to the load, the angles they form and the way the load is guided and landed decide whether that rigging carries its rated load or fails well below it. Many dropped loads involve slings that passed inspection that morning but were used in the wrong hitch, at too flat an angle or across a sharp edge.

This lesson explains the basic hitches, how sling angles multiply the tension in each leg, how unequal load sharing arises, how to protect slings from edges, and how to control and land a load safely. The calculations show why the rules exist. On the job, the rigger uses the manufacturer's tag and rigging tables, and complex arrangements are designed by a qualified person.

The three basic hitches

• Vertical hitch: a single sling leg runs straight from the hook to a lifting point. The sling carries the full load and its rated load is the vertical rating on the tag. A single vertical leg gives no control over rotation, so it suits loads with one central lifting eye.
• Choker hitch: the sling wraps around the load and passes through its own eye or a shackle, tightening as it is loaded. Chokers grip bundles and round loads, but the tight bend at the choke point weakens the sling. Choker ratings are lower than vertical ratings, typically around 75 to 80 per cent for wire rope and synthetic slings when the choke angle is 120 degrees or more, and lower still if the choke is pulled down to a sharper angle.
• Basket hitch: the sling passes under or through the load with both eyes on the hook. With both legs vertical, a basket carries up to twice the vertical rating, but only if the D/d ratio is adequate and the load is balanced. As soon as the legs spread apart at an angle, the capacity falls.

Bridle hitches use two, three or four legs from a master link to separate lifting points. They give good control, but the load is only shared evenly if the legs are the same length, the lifting points are symmetrical about the centre of gravity and the load is rigid. With three or four legs on a rigid load, it is common practice to assume that only two legs carry the load unless an engineer shows otherwise.

Sling angles multiply tension

When a sling leg is at an angle, it pulls both up and sideways. Only the upward part supports the load, so the tension in the leg must be greater than its share of the weight. For a load shared equally by \( n \) legs, each at angle \( \theta \) measured from the horizontal, the tension in each leg is:

The factor \( 1/\sin\theta \) is the sling angle factor. It is 1.0 when the legs are vertical and grows quickly as the angle flattens.

Angles can be measured directly with an angle finder or worked out from the sling length and the vertical height from the hook to the lifting point: \( \sin\theta = H/L \). That relationship lets a rigger check the angle with a tape measure.

Flat sling angles also create a horizontal force that squeezes the load between the lifting points. Thin-walled loads, long bundles and fabricated frames can buckle or bend under that force. A spreader beam keeps the slings vertical and takes the squeezing force in the beam instead of the load.

Unequal load sharing

Equal tension in every leg is the exception, not the rule. Whenever the centre of gravity is not centred between the lifting points, the legs nearer to it carry more. For two vertical legs, the share of each leg is found by taking moments about the other lifting point.

Here \( a \) is the horizontal distance from lifting point A to the centre of gravity and \( b \) is the distance from the centre of gravity to point B.

When slings of slightly different lengths are used in a bridle, the shorter one takes more load and may carry nearly all of it on a rigid load. Matched sets of slings, adjustable chain legs or turnbuckles help balance the load.

Edges, softeners and protection

A sharp edge can cut a synthetic sling under load in an instant and can crush and break the wires of a wire rope. Steel beams, plate, precast concrete corners and the edges of crates are all hazards. Edge protection, also called softeners, includes manufactured corner protectors, wear sleeves, split pipe and padding. It must be strong enough for the load and stay in place as the sling tightens. Wood blocks and rags often slip out and give false confidence.

Surface protection is different from edge protection. A sleeve that protects a painted finish from scuffing may not protect the sling from being cut. Check that any protector is rated for cut resistance if it will be used on a sharp edge.

Hooking up

• Seat sling eyes in the bowl of the hook, never on the tip, and close the latch.
• Avoid putting more than two sling eyes directly on a hook; use a shackle or master link to gather several legs.
• Keep the included angle between legs at the hook within the limits set by the hook and sling manufacturers.
• Make sure shackle pins are fully engaged and, for screw pin shackles, tightened and if necessary moused so they cannot unscrew.
• Do not twist, knot or shorten slings by tying them; use the correct length.
• Never wrap the hoist rope itself around a load as a sling.

Controlling the load

A suspended load can rotate, swing and drift with the wind. Tag lines, usually made of non-conductive rope, let workers guide the load from a distance. They should be attached before the lift, long enough that workers can stay outside the fall zone, and held so they can be released instantly. Workers should never wrap a tag line around a hand or body. Push poles and hands-free load-control devices let workers guide loads without touching them at all.

Tag lines can also add risk. Near energized lines, even a dry rope can track current if it becomes wet or dirty, and a tag line that snags on equipment can pull a worker toward the load. Alberta's guidance, for example, notes that tag lines should not be used where they increase the danger, such as near live conductors, moving machinery or mobile equipment. The lift plan should say whether tag lines are used and who holds them.

Landing and unhooking

Landing is one of the most dangerous phases of a lift because people are close to the load. Prepare the landing area in advance: firm, level, clear, with dunnage or cribbing placed so slings can be pulled out after the load is down. Lower the load slowly the last part of the way, stop just above the landing point, position it with tag lines or push poles, then set it down. Check that the load is stable before slackening the slings, and never pull a sling out from under a load with the crane, which can flip the load or whip the sling.