By Dr. Jody Muelaner
An eyebolt is a mechanical fastener with a threaded shaft and a head forming a ring. Eyebolts are used to attach an eye to a structure, through which rope, cable or shackles can be secured. A common use is to create a lifting eye so that a crane can be attached to machinery, with special purpose lifting eyes rated for their safe working load.
Low-strength eyebolts are often formed from a length of bar with the diameter of the nominal thread size. The head is simply bent into a ring from an unthreaded section of the bar. These types of eyebolts can carry axial loads reasonably well but should not be used for off-axis loads. The opening where the end of the bar is closed to complete the ring may be welded but is likely to remain a possible point of failure.
High-strength eyebolts have forged heads that normally include a substantial shoulder. The shoulder enables them to support significant off-axis loads. The ability to support off-axis loads is often important in lifting applications where lifting slings with multiple legs are used to attach to multiple lifting eyes.
When lifting machinery, it is common for an eyebolt to be screwed directly into a tapped hole in the structure. In this case, an eyebolt is required that is fully threaded all the way up to the shoulder. When an eyebolt is intended to be inserted through a hole and fastened by a nut on the other side, it is better if there is a shank. A shank is an unthreaded section of the shaft, below the shoulder, which provides a better transfer of load into the surface of the hole and is less likely to crack under a fatigue loading.
Special types of eyebolt include:
- Anchor eyebolts: Eyebolts with some form of anchor bolt to enable fastening into masonry structures.
- Screw eyes (or eye screws): The are similar to an eyebolt but with a wood screw in place of a machine screw. These are used for attaching an eye to wood or plastic structures. Small screw eyes are used to hang picture frames and curtain wire.
- Ring bolt: An eyebolt with a captive ring passing through the eye of the head, effectively forming two chain links. This can reduce bending forces on an eyebolt.
- Eye nut: An eye nut is actually a nut which serves the same purpose as an eyebolt, although they are sometimes identified as eyebolts in catalogs.
- Pigtail eye bolts: These are a formed from a length of bar, similar to ordinary low strength eyebolts, but the head is bent into a helix so that it does not fully close on itself but can continue past 360 degrees. This allows the middle of a rope to be threaded into the eye without requiring access to an end.
- Roller eyebolt: These are designed to guide a rope or cable so that it can pass smoothly with minimal friction. The head is actually an assembly with four rollers retaining the rope which passes through it.