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October 2004 Newsletter
The Nuts & Bolts of It

Brisbane made the AFL Grand Final, and the Cowboys nearly made the NRL Grand Final, so none of our Branches are ecstatic. Oh well bring on the Cricket.

Keeping Your Nuts On

Bolted joints have a significant advantage over welded, riveted or bonded joints, in that they can be easily dismantled. This feature however can also be a problem, when it occurs unintentionally as a result of operational conditions. Such unintentional loosening, frequently called vibrational loosening, is an important phenomenon and is widely misunderstood by Engineers.

It is widely believed that vibration causes bolt loosening, but by far the most frequent cause of loosening is side sliding of the nut or bolt head relative to the joint. This results in a relative motion occurring in the threads. If this does not occur, then the bolts will not loosen.

Tightened bolts (or nuts) rotate loose, as soon as relative motion between the male and female threads takes place. This motion creates an off torque which is related to, and proportional to the thread pitch and the preload. Once the friction under the nut or bolt head bearing surface is overcome, the off torque rotates the bolt loose.

There are three common causes of the relative motion occurring in the threads:

Bending of parts which results in forces being induced at the friction surface. If slip occurs, the head and threads will slip which can lead to loosening.
Differential thermal effects caused as a result of either temperature changes or differences in clamped materials.
Applied forces on the joint components can lead to shifting of the joint surfaces leading to bolt loosening.

Work completed during the 1960's in Germany indicated that transversely applied alternating forces generate the most severe conditions for self loosening. The result of these studies led to the design of the Junkers testing machine which allowed quantitative information to be obtained on the locking performance of self locking fasteners. As a result, a rationalisation of the variety of locking devices used by many major companies has occurred. For example, conventional spring lock washers are no longer specified on higher tensile bolts because it has been shown that they actually aid self loosening rather than preventing it.

The Nordlock Safety Washer is far more effective at maintaining a preload, and has the added advantage of resisting rotation. Engineering magazines will reveal a multitude of proprietary locking mechanisms available for fasteners. This can represent a bewildering choice. Prevailing Torque lock nuts can be sub-divided into two groupings, metallic and non-metallic.

The metallic friction locking fastener usually has a distorted thread which provides a prevailing torque; an example of this category is the "Conelock" nut. Non-metallic friction locking devices have plastic inserts which provides a thread locking function; an example being the "Nyloc" nut. The chemical locking category is adhesives, usually anaerobic, which fill the gaps between the male and female threads and bond them together. "Loctite" is an example.

To identify which category is the most suitable for an application requires a careful consideration of the application. In brief, the chemical locking category provides the greatest resistance to vibration loosening. Both these retain the nut on the bolt, but neither retain the preload, or clamp force. The Nordlock Safety Washer set actually retains the clamp load in the joint.

In general terms, the key to preventing self loosening of fasteners is to ensure that:

There is sufficient clamp force present on the joint interface to prevent relative motion between the bolt head and the joint.
The joint is designed to allow for the effects of embedding and stress relaxation.
Proven thread locking devices are specified.