At the end of the day, a shackle has a single purpose: to attach a sling to a hook. However, there are two main styles of shackle that are commonly used, and thinking of them as interchangeable is among the most common mistakes seen on site.
Bow shackles feature a wide and rounded body. The curvature allows bow shackles to take loads from several directions without the sling legs bunching up or pin rubbing against the leg. For this reason, they are best used when the angle of the load is not completely straight or when more than one sling leg is used. Bow shackles are best used for multi-leg slings, basket hitches, and any lifting operation that may swing or shift.
Dee shackles are shaped as a D, with straight sides, forming a narrow shackle. Because of the lack of curvature in a dee shackle, they are best used for single and in-line pulls. For this reason, dee shackles may often be rated to carry more than bow shackles of the same pin size. However, once a dee shackle is used for anything outside of an in-line pull, the rating on the shackle may no longer be trusted. Deviating from an in-line pull may cause the working load limit of the shackle to decrease significantly, and it may even reduce by more than 50%, and the shackle may fail without any warning.
This part is tricky. The shackle has no warning for incorrect usage. It actually has a lower margin than indicated, and may only show a burden of carrying the load when it chooses to. A Dee shackle that is used for a multi-leg system, because it was the only one available, is a quieter and more common cause of overload than one may think.
The type of pin plays a major role as well. For quick jobs where the shackle is used repeatedly, a screw pin shackle is optimal because the pin can be quickly removed and re-inserted. For jobs where the shackle remains rigged for longer, a bolt type shackle is optimal. It is more time consuming to put in place, but cannot work loose or unscrew due to vibration.
Shackles that are used for lifting should comply with EN 13889 and should have their size, manufacturer’s mark, and working load limit stamped or tagged. If any of these markings are missing or damaged, a shackle should be taken out of service. A shackle that has no markings is not a cheaper option. It is an unknown option, and an unknown rating is not worth the price of a new piece of lifting gear.
When deciding between the two shapes, consider whether the pull is in-line or not. A dee should be used for single sling legs pulling directly vertical. A bow should be used for multiple sling legs, in-line pulls that are not directly vertical, or loads which will shift during the lift. In fact, deciding which shape to use will improve the safety of the lift more than almost any other piece of equipment in the rigging.
The size of the shackle is just as important as the shape. The size of the shackle should not be determined by the weight of the load, but by the size of the sling or fitting. Shackles that are too large will too large of a pin shackle will move in the fitting, causing it to wear uneven. This uneven wear will not be obvious after a short inspection. If there is any doubt, it is better to use a sizing chart than to size by eye. It will be better to size a shackle appropriately than to have to replace a shackle for having the inappropriate size mid job.