Shackles and eyebolts19 April 2003
Derrick Bailes offers advice on specifying and procuring shackles and eyebolts
Shackles and eyebolts have been widely used in the lifting industry for decades. However, despite (or perhaps because of) the apparent simplicity and extensive use of these components, incorrect and poor specification remains a problem. Indeed it is one of the ironies of the lifting industry that it is not uncommon to see the safety of an operation involving thousands of dollars-worth of crane being compromised by an inappropriate shackle or eyebolt costing next to nothing in comparison.
As far as shackles are concerned, there are three main types for the buyer to choose from: bow (or anchor) shackles, dee (or chain) shackes and grab shackles. All comprise a body and a pin and they are sized by the diameter of the material in the body, not the pin. Normally they are forged from steel and, size for size, alloy steel shackles have a higher safe working load (SWL) than those made in higher tensile steels. Mild steel shackles are still manufactured for general engineering purposes but there is no longer a standard for such shackles for lifting purposes. In theory, the metric British Standard BS 6994 (steel shackles for lifting and general engineering purposes – Grade M(4) ) should have been available since 1988. In practice very few shackles have ever been made to this standard and its imperial predecessor, BS 3032 (higher tensile steel shackles) is still much more widely specified in the UK. Both standards have now been withdrawn by BSI although shackles to BS 3032 are still manufactured and the only current British Standard is BS 3551 (alloy steel shackles). US Federal Specifications, in the shape of RR-C-271, are increasingly popular and there is also a harmonised European Standard for grade 6 shackles, prEN 13889, at the formal vote stage. That should be published later this year. Whatever the individual preference, ensuring full compliance with the relevant standard should of course be a priority.
Working load limits
In terms of correct specification of shackles, there are three main decisions to be made: the SWL, the shape, and the pin type. The working load limit must be at least equal to the total load imposed upon the shackle. 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. It should be stressed that distributing the load right across the pin does not increase the strength of the shackle.
Selection of the right body shape depends 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. Buyers can select from large and small dee and bow shackles, obviously providing a choice in terms of the size of the internal clearances. However, size for size, the smaller option has a higher SWL. Dee shackles are generally used to join two pieces of lifting equipment, while bow shackles are typically employed where more than one attachment has to be made to the body, for example if joining two single leg slings and connecting to the hook of a crane, 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.
For dee and bow type shackles there are essentially only two types of pin available: the screwed pin type and the nut and bolt type. The screwed pin is suitable for most purposes provided there is little risk of the pin becoming unscrewed during the lifting operation. It has the advantage of keeping the shackle as a quick-to-assemble, two component item. The nut and bolt pin, together with a split retaining pin, is a four component item. The advantage of the latter, however, is that the pin cannot unscrew in service. It is therefore more suitable in applications where there may be movement which could cause the shackle to unscrew; where it remains connected for extensive periods; or in situations that do not allow the shackle to be seen easily and checked for security.
The grab shackle is effectively a dee shackle with a screwed countersunk pin. As the name suggests, it is designed for use with grabs, where the shackle must pass through a circular aperture of minimum diameter.
As with all lifting equipment, it is vital that the buyer tells the supplier about any particularly hazardous conditions or environmental issues that could compromise the long-term safety of the shackle. In particular, exposure to extreme temperature, chemical attack or other corrosive environments such as sea water should be mentioned.
Again, the need to ensure full compliance with the relevant standard cannot be over-emphasised. Buyers should not only specify the size and type required, but also the standard number; they must then check that the product supplied is stated to be in full compliance. Phrases such as “generally in compliance with” are meaningless and should set the alarm bells ringing.
In terms of safe application, the eyebolt is subject to severe limitations and a significant number of accidents occur as a result of misuse. It is therefore vital to specify a relevant standard, and observe any restrictions on use. As with shackles, British Standard 4278 specifies three main types: eyebolt with link, collar eyebolt and dynamo eyebolt.
The first two can be loaded at an angle, provided the SWL is reduced and, in the case of the collar eyebolt, it is correctly aligned. However, the latter must only be loaded in line with its axis. This is a metric standard but it also specifies eyebolts with imperial threads for situations where they must match existing threaded holes. Other national standards bodies also have specifications for collar and dynamo type eyebolts, although their proportions vary widely. Currently there is no European Standard for eyebolts but one is being developed jointly with the International Standard Organisation to replace the little-used existing ISO standard. Several manufacturers also offer alternatives to traditional eyebolts in the form of a link with a bolt connection which articulate and swivel.
Working load limits
Specifying an appropriate working load limit is clearly a key consideration. It should at least be equal to the total load imposed upon the eyebolt and it must always be borne in mind that each particular eyebolt type has its own limitations in this respect. Buyers must ensure that the correct type of eyebolt and maximum load rating is used to suit the application in question.
Eyebolts are very often screwed into threaded holes whose primary function is something other than that of a lifting point. Even when that is its primary function, the quality of the material and threadform is beyond the control of the eyebolt manufacturer. The user must therefore be careful to ensure that the fit of the eyebolt and the strength of the material are adequate for the load, particularly if the line of force is at an angle to the axis of the eyebolt.
Similar care should be taken in terms of the threadform and diameter, as mismatched threads pose a major risk to safety. The standard attempts to minimise these risks are by the shape and dimensions of the eyebolt and limiting the choice of permitted thread forms and sizes, avoiding those known to provide a low strength connection. However, not every situation can be covered so the user must still exercise caution. Eyebolts are also highly sensitive to hazardous conditions, including extreme temperature and acidic/chemical environments.
Eyebolt with link
As far as the choice of eyebolt type is concerned, the eyebolt with link can in all respects be considered as the general purpose option for use whenever the loading cannot be confined to the plane of the eye. It is designed to accept a sling hook without the need for an intermediate component and can be loaded in any direction up to the marked SWL, provided that the angle of the load to the axis of the screwthread does not exceed 15º. Beyond that, the SWL reduces as the angle increases. Eyebolts with link may be used for trunnion lifting.
The collar eyebolt is much more limited in use than the eyebolt with link. Although the SWL in axial loading is greater, it is markedly reduced in angular loading. Alignment of the eye is also very important. They may be used up to the marked SWL provided that the load is in line with the axis of the eyebolt. Collar eyebolts may also be used for non-axial loading provided that the SWL is reduced by the appropriate factors. The load should be applied within + or - 5º of the plane of the eye. The collar eyebolt is not suitable for direct connection to a hook; a shackle is usually employed for this.
Dynamo eyebolts are commonly fitted to the casing of equipment such as electric motors. They are extremely limited in their use. The SWL is lower than that of the collar eyebolt for axial loading and they cannot be used for angular loading. Loading by even 5 degrees out of the axial causes undue stress on the screwthread and shank. Dynamo eyebolts must not be used for trunnion lifting.
Finally, it is worth concluding with a point regarding the precise status of these components. This has often been the subject of confusion, especially in the case of eyebolts that are fitted to an item of equipment. Rather dubiously, some have considered such eyebolts to be a composite part of the equipment. In the UK, the Lifting Operations & Lifting Equipment Regulations (LOLER) make it clear that all shackles and eyebolts used for lifting purposes are lifting accessories. They should therefore be subject to a thorough examination by a competent person at least every six months. Regardless of the prevailing legislation, the Lifting Equipment Engineers Association (LEEA) strongly recommends that this policy is adopted by all those with responsibility for ensuring the safe condition of lifting equipment.