Fasteners - hdgasa

Fasteners
structures, has been replaced by the 2008 version. Light
oiling as an accepted lubrication has now been removed
in the new standard because of the inconsistent outcome
and poor lubrication quality. For example, to attain the
same pre load of 220kN for a M24 grade 10.9 bolt, a
torque of 800Nm is required when lubricated with
molybdenum disulphide, whereas with oil, 1100Nm
torque is required.
Lack of lubrication would be a problem particularly in hot
dipped galvanized applications. It has been demonstrated
that whereas the coefficient of friction starts at 0.19 at the
commencement of tightening, once preload is being
achieved, this has increased to 0.27 as a result of the
galling of the soft galvanized surface. Therefore the
required tension in the bolt will not be achieved, that is
the bolt will effectively not be holding plates in a spring
pre load condition. Under this un-lubricated condition, it
has also been found that the torsional stress can result in
the failure of the bolt at around the required pre load,
which is only 70% of the yield point before a bolt should
start deforming.
to? Torque is a misnomer because it is the pre load or
tension in the bolt which is the goal. It has already been
demonstrated above that under different coefficient
conditions, different torque is required. It is interesting
that EN 1090-2:2008, the European equivalent for DIN
18800-7, goes further by not specifying the torque values
at all. These values have to be established by testing bolt
assemblies themselves to ascertain the torque required to
obtain the specified preloads. It is also a daily
requirement on site for erectors to verify the calibration of
the wrenches in use to ensure that their torque values
match the pre load required. This is done by testing three
bolts for the diameter of bolt to be used on that day.
Finally, a further requirement of fastener manufacturers is
to conduct training to erectors explaining some of the
above. In addition to topics covered, it is also generally a
good idea to address installation best practice. This is
preventative training to ensure that reckless installation is
not creating a condition that may only manifest many
months after initial installation.
We have mentioned torque a few times and this is
generally the “talk” on site; what do we torque the bolts
R J Pietersma, Chairman, South African Fasteners Manufacturers
Association.
Volume 9 Issue 1 2012 Hot Dip Galvanizing Today 15