Shackle

A shackle is a bow of forged steel closed by a removable pin. It is how a sling meets a hoist, a point or a truss, and it is the smallest rated part in most rigging chains.

Every shackle carries its size, grade and working load limit stamped into the bow. An unmarked shackle has no rating and does not belong in a rig, however solid it looks. The common design factor for this class of hardware is five to one against breaking strength.

Two mistakes account for most shackle failures. The first is side loading: a shackle is designed to take its load in line with the bow, and loading it across the bow sharply reduces what it will hold. The second is pin substitution. A screw pin is part of the rated assembly; a bolt from a toolbox is not, and a shackle closed with the wrong pin is a shackle with no rating at all. Screw pins can also unthread under a load that moves or rotates, which is why bolt-type shackles with a nut and split pin are used where anything will be left hanging.

Orientation is the quiet one. Where several slings meet in one shackle, they belong in the bow rather than on the pin, so that the pin is not loaded sideways by the spread.

Staging and rigging — owned inventory, our own crew, and a site survey before anything is specified.

• Span set (round sling) A continuous loop of polyester in a protective jacket, used to wrap a beam or a truss — rated for how it is used, not just for what it is.
• Working load limit (WLL) The most a piece of rigging hardware may be asked to carry — typically a fifth of what it would take to break it, and halved again by convention for anything over people.
• Chain motor The electric hoist that lifts truss and scenery into the air on a loop of chain — rated by capacity, and not by itself a licence to leave a load hanging over people.