A shackle is a U- or O-shaped piece of metal secured with a bolt, pin or spring that connects items together. An anchor shackle is a specific kind of shackle that is used to secure a moving object to a fixed object. The shape of the anchor shackle allows the moving object to push and pull from several directions without creating a shearing force on the device. These shackles have many industrial uses, but are especially common on boats. There they do everything from holding rigging lines to securing the boat's actual anchor.
From a load standpoint, a shackle is a solid loop of metal. In order to be a true shackle, there cannot be any openings in the loop that makes up the fastener. Since the shackle would be of little use if it were simply a solid loop of material, it has an opening called a gate that is able to be closed. There are several methods used for closing the gate in the shackle in order to effectively create a single piece.
There are two common fastening methods for most shackle bolts or pins. For these shackles, the bolt or pin makes up a portion of the loop itself. When they are open, the bolt or pin that holds them shut is typically removed from the shackle entirely. When in use, a bolted shackle has a set of threaded loops on each side of the opening that allows the bolt to hold the shackle shut. A pin shackle is very similar, but the pin simply slides through the opening and is secured on both sides.
Spring shackles have either a spring or a tension system that allows a user to push the shackle open without removing the connecting piece. The gate opens by pressing into the shackle; that way, a load cannot accidentally push the gate outwards. While both bolt and pin systems are commonly used to fasten an anchor shackle, spring systems are not.
Anchor shackles connect to a fixed object and a moving one. The gate on the shackle is generally set to face the fixed object, and the bulk of the shackle faces the moving one. An anchor shackle is usually round; the rounder the shackle, the better it is at balancing the forces on it. The round shape allows the moving load to pull from any direction but still have the same relative force applied to the anchor shackle. A round shackle is generally lighter-duty than a U-shaped shackle of the same weight, but it can withstand changes in directional force that would damage the U-shaped version.