Self loosening of prestressed bolts

Review of a Technical Paper

Blog Details Review of a technical paper titled 'Self loosening of prestressed bolts' by R. Friede and J. Lange from the Institute for Steel Structures and Materials Mechanics, Technische Universitate Darmstadt, Germany.

This paper discusses the risk of self loosening of bolts used in steel structures in the diameter range M16 to M36. Little work on self loosening has been completed on bolts this size, most of the research being conducted on bolts of M12 and below. As the paper discusses many of the anti-loosening products on the market are ineffective or not fully effective. The authors quote, in the paper, that in 2003, German regulations requiring safety elements, by which I assume they mean locking devices, were withdrawn. The authors' University is investigating ways to prevent self loosening of bolted connections for steel constructions (in the M16 - M36 size range).

The paper discusses some tests completed at the University including the effect on self loosening of M20 bolts of grip length. Essentially, the greater the grip length, the lower is the self loosening tendency of bolts. The paper quotes that for a displacement of +/- 2mm, a clamp length the bolt diameter (l/d) ratio of greater than 4.5 significantly reduces self loosening. Figure 4 from their paper is reproduced below:

Figure 4 from the paper self loosening of prestressed bolts

Above: Graph showing the effect on the loosening of M20 bolts as the grip length increases.

As other researchers have found, the way to prevent self loosening is to eliminate transverse joint movement. This may not always be possible for example, due to overloading of the joint or uncertainty of the magnitude of the loads acting on the joint. In such cases, the paper provides the following advice to reduce the risk of self loosening:

Limit the transverse displacement by using close fitting bolts i.e. limit the clearance of the hole to the bolt.
Use bolts that have a large clamp length to bolt diameter (l/d) ratio.
Use fillers (packing pieces) to increase the length of the bolt.
Increase the preload.
Use thin, or even better, no coating in the faying surfaces to reduce embedding.
Use locking features of proven efficiency.
Increase the friction under the head.

The paper does not present anything significantly new but it does provide a good discussion of the subject. The paper concludes with the sentence: "Only the combination of a good design and frequent inspections can assure the durability of the connection." There are many applications (subsea for example) for which frequent inspection is not a viable option and hence the paramount necessity of good design.

The paper was presented at the Nordic Steel Construction Conference in September 2009 and is available in Adobe PDF from: (will open in a separate window).

Related article: Vibration Loosening of Bolts

For the latest information on Bolt Science: