Eyebolts are used in a wide variety of applications to provide lifting points on loads. Sometimes the hole they are screwed into is there specifically for the eyebolt. Alternatively a hole which is primarily intended for some other purpose, such as a stud, can be utilised.
Perhaps the most common application is an eyebolt located centrally in the top casing of an electric motor. These eyebolts usually have an eye large enough to engage with a hook of similar capacity thereby providing an easy connection to a small hoist or a single leg sling.
They are designed for axial loading only, for example, along the axis of the screw thread. They usually have a small diameter collar between the eye and the screw thread. However this is not sufficient to support the eye if it is loaded at an angle to the thread axis. They are called dynamo eyebolts which reflects their original application in early power generators.
Prior to the introduction of a British Standard, dynamo eyebolts were manufactured to a wide range of ‘commercial’ patterns of varying quality. However their typical applications were such that they were usually only used a few times during the lifetime of the equipment they were screwed into. As such cost was a more important consideration than performance characteristics like fatigue life.
Although dynamo eyebolts are a relatively low cost item, the limitation on axial loading is obviously a considerable disadvantage as, for many applications, two or more lifting points are required. The answer is an eyebolt with a large collar between the eye and the screw thread. With the collar screwed firmly down onto a machined face, it can support the eye and prevent any bending of the screw thread. By also reducing the size of the eye, it keeps the leverage to a minimum. This design is called a collar eyebolt and can be considered as the general purpose eyebolt.
The underside of a collar eyebolt has a thread run-out at the top of the screw thread and an undercut of the collar. Small radius curves blend the transitions between the screw thread, plain shank, undercut and collar. These avoid unnecessary stress concentrations which can lead to fatigue failure and the undercut ensures that the collar can sit fully down onto a machined face.
Although a collar eyebolt is designed for angular use it is nevertheless necessary to align the eye to the line of pull. A tolerance of ±5º is acceptable but it is a matter of luck whether this will be achieved when first screwed in because the eyebolt manufacturer has no control over the start point of the threaded hole. Therefore it is permissible to shim up to half the screw pitch to achieve the correct orientation.
This brings us to some details about the screw thread itself. The current British Standard for eyebolts is BS 4278 and it specifies coarse series metric threads. Eyebolts are made from higher tensile steel and, if screwed into the same strength material, would need only to engage for a length equal to that of a full nut. However it is possible and even likely that the material they are screwed into will be considerably weaker. The screwed length is therefore extended to compensate. Also the eyebolt manufacturer has no control over the tolerances of the screwed hole, hence the restriction to coarse series threads.
The screw thread required for a particular application is of course dictated by the threaded hole. New eyebolts are required to fit old equipment which may have imperial threads. Standard eyebolts can have other screw threads and BS 4278 does allow for imperial threads for replacement purposes. If the hole is not specifically for an eyebolt, its primary purpose will dictate the thread used which may, for example, be a fine series thread. If the tolerances between the eyebolt and hole are not tight enough, a fine thread may not be strong enough. So, whilst this does not rule out the use of fine threads, it does place greater responsibility on the user to ensure a good fit.
A final point for the user to check is that the thread forms of the eyebolt and hole are the same. Unfortunately, certain sizes of metric, UNC and Whitworth threads will engage with each other and appear to be compatible. However they are, in effect, hanging on by their fingertips and not strong enough to take the load. When the current British Standard was written, an exercise was carried out to identify these possible mismatches and they were eliminated from the standard range of sizes. Unfortunately the standard cannot control this problem entirely and certainly not when special sizes and thread forms are specified.
Stresses
When used at an angle, the working load of a collar eyebolt must be reduced to take account of the different stresses imposed. For the BS 4278 collar eyebolt, the reduction factors are: 0.63 for angles up to 15º to the vertical; 0.4 for angles up to 30º; 0.25 for angles up to 45º. These reductions are considerably more than those which apply to slings.
The logic behind them is that the force on an eyebolt increases due to two factors: the geometry as in a sling, and the additional effect of loading it at an angle to its axis. These reduction factors are now being challenged by a new standard which has just been approved jointly by ISO and CEN. Whereas the current British Standard is fully dimensioned, the new standard is a performance standard with a limited dimensional envelope for compatibility purposes only. It allows manufacturers to design eyebolts capable of working to the same angle factors as used for slings. As this standard is very new and not yet implemented into national standards, it remains to be seen how many manufacturers will take advantage of it. I hope many do as it is a significant step forward. Meanwhile, for the foreseeable future, products to the current standard are likely to be the most widely available.
Collar eyebolts can be used as trunnions, that is when the load is applied at right angles to the thread axis. Typically the eyebolt is screwed into the side of the load rather than the top. The eye must be aligned with the line of pull, as explained above, and the rating is reduced to 0.25 of the marked working load.
The eye of a collar eyebolt is too small to accept a hook so a shackle is always needed as the first item of any connection. However BS 4278 does offer a third option and that is the eyebolt with link.
The origin of this eyebolt is the swivels which were once widely used in traditional ships’ derrick rigs. One of the two forgings used to make the swivel is, in effect, a very compact form of collar eyebolt. The eye is just large enough to accept a link of comparable grade and strength. The link can articulate in any direction within the eye.
In combination with the very compact eye, this allows it to be loaded in any direction, removing the need to align the eye to the line of pull. For a given thread size the axial working load is less than that of a collar eyebolt but the reduction factors for angular use are better. They can be used up to 15º to the vertical without any reduction and the other factors are 0.8 for angles up to 30º and 0.63 for angles up to 45º. Unfortunately, because of their origin, they are only available in a small range of sizes.
From time to time, I have seen collar eyebolts permanently fitted with a welded link. Properly done, this is an acceptable alternative to using a shackle. However they should not be confused with the standard eyebolt with link. Their rating and the alignment requirements are still those of a collar eyebolt.
Whilst on the subject of variations, another two I see are the eyenut and bownut. Essentially they are the same thing except for the shape of the eye, one being circular and the other more elongated similar to a bow shackle. They are in effect female dynamo eyebolts although they are designed for pipe hanger and similar applications rather than lifting. There was a British Standard for them but it is now withdrawn.
The standard specified mild steel although other grades may be commercially available. My personal view is that I would avoid using them, although I acknowledge that there may be applications where there is no better alternative. If they are used, they should only be loaded axially and clearly the stud they are screwed onto is a critical component in the connection.
Traditionally eyebolts were often fitted to their load and left in place for life, being regarded as part of the load. However modern practice is to treat detachable lifting points as lifting accessories. As such most countries require that they are periodically inspected or thoroughly examined. Therefore good practice is to remove eyebolts, plug the holes and put the eyebolts into storage until needed. This often considerably reduces the quantity required and opens up other options.
In general, the best alternative to eyebolts is the modern swivel link. Although there is not a specific standard for these, several reputable manufacturers make them to their own designs. The designs vary but essentially they all incorporate a link large enough to accept a comparable hook, connected through a bearing to a bolt which screws into the load.
The bearing allows for 360º rotation which removes any alignment problems and the link will tilt to the line of force. Some have a latch to lock the bolt to the link thereby allowing the link to be used as a lever to tighten or slacken the bolt when fitting. They are a portable lifting accessory designed to be easily fitted and removed.
Swivel links are considerably more expensive than eyebolts but their advantages can easily outweigh their initial cost which is, in any case, often low compared to the value of the machinery they are to lift.
About the author
Derrick Bailes is technical consultant (formerly chief executive) for the Lifting Equipment Engineers Association, 3 Osprey Court, Kingfisher Way, Hinchingbrooke Business Park, Huntingdon, PE29 6FN, tel: +44 (0)1480 432 801, fax: +44 (0)1480 436 314, email: info@leea.co.uk
