Getting shackled to safety
Shackles are critical to the integrity of lifting operations. Selecting the right one for the job will ensure the task is conducted not only efficiently but also in safety, writes Bernie Winter, from LEEA Technical Services.
Shackles are relatively simple items of lifting equipment, comprising just a body and a pin. However, they are also one of the most critical as they safely connect lifting slings, chains, ropes or other rigging hardware to loads and lifting devices, such as cranes or hoists. They are normally forged from various grades of steel, though higher quality alloy steels will give, size for size, a higher working load limit than those made in higher tensile steels. Higher tensile steel shackles, meanwhile, have a higher working load limit than those made in mild steel. In terms of shackle design, there are generally three types used for lifting: Bow, Dee and Grab.
Dee shackles are specified by all shackle standards, with some specifying both a large and small unit. The ample internal clearances in the body and jaw of a large Dee shackle make it appropriate for general engineering purposes. A small Dee shackle has moderate internal clearances in the body and jaw but, size for size, has a SWL higher than that of the large Dee. It is suitable for use with hook eyes, eyebolts, egg links, wire rope thimbles as well as for the head fittings of ships’ blocks.
Bow shackles are also specified by all shackle standards, with some specifying both large and small. As with the Dee, the large Bow’s generous internal clearances in the body and jaw make it useful for general engineering purposes. The small Bow’s moderate internal clearances in the body and jaw as well as higher SWL, size for size, than the large version means its applications are similar to those of the small Dee, as listed above.
A Grab shackle is a Dee shackle with a screwed countersunk pin. It is designed for use with grabs where the shackle must pass through a circular aperture of minimum diameter.
Shackle selection
A shackle must have a Working Load Limit (WLL) at least equal to the total load to be imposed upon it. Alloy steel shackles will be dimensionally smaller load for load than those of higher tensile steel. The shackle pin and body are designed to be of equal strength. To attain this condition, the pin normally has a slightly larger diameter than the material of the body. If the load on the pin is distributed over its length, or part of its length, the load bearing capacity of the pin is effectively increased but that of the body is not. Distributing the load right across the pin will not increase the strength of the shackle. Standard shackles are designed to take the SWL between the centre of the pin and the bearing point(s) in the crown.
The selection of the shape of the shackle body will depend on the intended use. It is desirable to use a shackle with as small a jaw opening as is consistent with adequate articulation of the connection. Dee shackles are, in general, used to join two pieces of lifting equipment. Bow shackles are, in general, used where more than one attachment is to be made to the body, or to allow freedom of movement in the plane of the bow.
The necessity for the eye of a shackle to pass through another fitting may have to be considered. Where a shackle does not have to be dismantled, or where it is required to remain captive, it may be permanently attached by forming a link or thimble round the shackle body.
Choose your pin
The choice of pin depends on the circumstances of use. Countersunk pins are used where the heads of other types might be an obstruction, such as when passing over a sheave or when the shackle has to pass close to fixed structures.
For general use, and where there is no tendency for the pin to unscrew and the shackle can be observed in use, screwed pins with eye and collar are very satisfactory and easily manipulated.
For permanent installations or where there is an inherent tendency for the pin to unscrew or when the shackle cannot be observed, pins comprising bolt, nut and split cotter pin are recommended. When maximum security is required but for other reasons this type of pin is not suitable, then refer to the manufacturer or Competent Person who will advise on alternative arrangements.
The forelock pin is not a preferred type. Its advantage is ease of assembly, but it should only be considered for specialised applications.
Many manufacturers have now adopted the current standards for shackles, but keep an eye out as some continue to make shackles to old and now withdrawn standards. Also, shackles often have a long service life but you can refer to LEEA’s Code of Practice for the Safe Use of Lifting Equipment (COPSULE), Section 19, which continues to include designs no longer produced but known to still be in service. There is plenty other useful information on Shackles, along with a comprehensive inventory of lifting equipment, in this essential free to access guide to best practice available at leeaint.com.
