A significant advantage of a bolted joint over other joint types, such as welded and riveted joints, is that they are capable of being dismantled. This feature however, can cause problems if it unintentionally occurs as a result of operational conditions. Such unintentional loosening, frequently called vibrational loosening in much of the published literature, is an important phenomenon and is widely mis-understood by Engineers. It is important for the Designer to be aware of the bolt loosening mechanisms which can operate in order to design reliable joints. The information presented below is key information for the Designer on the theory of vibration loosening of threaded fasteners and how such loosening can be prevented.

Study of most Engineering magazines will reveal the multitude of proprietary locking mechanisms available for fasteners. For the Designer without the theoretical knowledge of why fasteners self loosen, this represents a bewildering choice. Presented below is key information, for the Designer, on why fasteners self loosen, and, how it can be prevented.

It is widely believed that vibration causes bolt loosening. By far the most frequent cause of loosening is side sliding of the nut or bolt head relative to the joint, resulting in relative motion occurring in the threads. If this does not occur, then the bolts will not loosen, even if the joint is subjected to severe vibration. By a detailed analysis of the joint it is possible to determine the clamp force required to be provided by the bolts to prevent joint slip.

Often fatigue failure is a result of the bolt self-loosening which reduces the clamp force acting on the joint. Joint slip then occurs which leads the the bolt being subjected to bending loads and subsequently failing by fatigue.

Pre-loaded bolts (or nuts) rotate loose, as soon as relative motion between the male and female threads takes place. This motion cancels the friction grip and originates an off torque which is proportional to the thread pitch and to the preload. The off torque rotates the screw loose, if the friction under the nut or bolt head bearing surface is overcome, by this torque.

There are three common causes of the relative motion occurring in the threads:
1. 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.
2. Differential thermal effects caused as a result of either differences in temperature or differences in clamped materials.
3. Applied forces on the joint 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 a testing machine which allowed quantitative information to be obtained on the locking performance of self locking fasteners. Such machines, often called Junkers machines (a video of such a machine can be seen - see the bottom of this article) in the literature - after it's inventor, have been used over the last twenty years by the major automotive and aerospace manufacturers to assess the performance of proprietary self locking fasteners. As a result, a rationalisation of the variety of locking devices used by such major companies has occurred. For example, conventional spring lock washers are no longer specified, because it has been shown that they actually aid self loosening rather than prevent it. There are a multitude of thread locking devices available. Through the efforts of the American National Standards Subcommittee B18:20 on locking fasteners, three basic locking fastener categories have been established. They are: free spinning, friction locking, and chemical locking.

The free spinning type are plain bolts with a circumferential row of teeth under the washer head. These are ramped, allowing the bolt to rotate in the clamping direction, but lock into the bearing surface when rotated in the loosening direction. The "Whizlock" is in this category.

Friction locking categories 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 "Philidas" 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 are adhesives which fill the gaps between the male and female threads and bond them together; "Loctite" is an example. Such adhesives are now available in micro-encapsulated form and can be pre-applied to the thread.

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, followed by the free spinning locking fastener. However each category has dis-advantages as well as advantages, the most suitable method being dependent upon the application.

In general terms, the key to preventing self loosening of fasteners is to ensure that:
1. There is sufficient clamp force present on the joint interface to prevent relative motion between the bolt head or nut and the joint.
2. The joint is designed to allow for the effects of embedding and stress relaxation.
3. Proven thread locking devices are specified. Specifically, thread locking compounds - such as "Loctite", flanged fasteners such as "Whizlok" or torque prevailing fasteners such as "Nyloc". In general, loose washers, of the plain or spring variety, are not generally advisable.

The self loosening of fasteners is just one aspect of bolted joint design the Designer must consider during the design process. As can be seen in the photo at the side, even if threads are completely locked together by adhesive, problems cannot be prevented if the bolt preload is insufficient to prevent joint movement. The photo shows a M12 bolt that has been partially worn away by movement.

Applying state of the art analytical analysis to prevent vibrational loosening of threaded fasteners can be complicated. To assist the Engineer in overcoming the problems associated with the use of threaded fasteners and bolted joints, Bolt Science has developed a number of computer programs. These programs are designed to be easy to use so that an engineer without detailed knowledge in this field can solve problems related to this subject.

Details of our Test Services on Fastener Vibration Testing

View our Test and Consultancy Services Brochure

View details of tests completed on the double nut locking system

View details of tests on helical spring washers

More details on this topic are available on our online training course on Bolting Technology, see www.bolting.info for further details