| Self loosening of prestressed bolts
Review of a Technical Paper
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I assume they mean locking devices, were withdrawn.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
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:
the transverse displacement by using close fitting bolts i.e.
limit the clearance of the hole to the bolt.
bolts that have a large clamp length to bolt diameter (l/d)
fillers (packing pieces) to increase the length of the bolt.
thin, or even better, no coating in the faying surfaces to
locking features of proven efficiency.
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
The paper was presented at the Nordic Steel Construction Conference in September 2009 and is available in Adobe PDF from: http://www.nordicsteel2009.se/pdf/106.pdf (will open in a separate window).
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