SPRING 2011

170 THE DISTRIBUTOR’S LINK
FASTENER STRENGTH - REAL OR NOT? continued from page 8
“plated”, some call out “dry” or “oiled”. Some others
refer to a “torque coefficient” (actually the K-factor or
nut-factor) of 0.20 for plain fasteners or 0.15 (!) for
plated. This is not good enough information and can be
down-right dangerous if applied to a critical joint.
A proper calculation of a tightening torque (taught in
my classes) must be based on a K-factor taking into
account thread friction, bearing surface friction (also
area), thread pitch and the nominal fastener size. The
biggest problem is to determine friction data, since there
are not just one standard type of lubricant (like oil) but
also very effective coatings, waxes, molybdenum
disulfide, PTFE, graphite and others that will have much
lower (and more consistent) coefficients of friction than
oils and greases. Also, a “dry” fastener may be
tightened against a “dry” surface, but be threaded in to
a tapped hole with some residual tapping oil present.
That condition would not match up with the simplified
approach in a typical torque chart.
If we do a reasonably good job creating a K-factor
based on reality, we could with confidence use the well
known “short formula” to find a torque value to get it right;
T=K x d x Fp
where
T = Torque or moment of force (Nm, lbfft, etc.)
K = see above
d = nominal diameter of fastener (mm, inch)
Fp= target mean preload force (N, lbf, etc.)
Please note that some (most) torque charts neglect
the desired preload level.
Friction coefficients
There is, unfortunately, no easy way to determine
friction coefficients. The German design guide line VDI
2230 (the best available in the world) lists various
friction classes based on material surfaces and types of
lubricants or no lubricants. The guide line suggests that
we try to be in friction class B meaning coefficients 0.08
– 0.16. The lower range means that we use very effective
lubricants like MoS2, graphite, PTFE and similar, the
upper range being less effective lubricants like oils and
greases.
wrench (dial or clicker) will typically give a tension scatter
around +/-20 %, even if the wrench itself is calibrated to
have an accuracy of +/- 2 % or better. The worst
performer is the noisy impact wrench that will give
tension scatters around +/- 60! Don’t let your service
station tighten the lug nuts on your car with these
horrible tools, it is always better to keep the wheels on
your car instead of out in the woods. If a joint is designed
to be tightened with a calibrated torque wrench and
someone uses an impact wrench, the scatter will either
break the fastener at the upper scatter or not produce
enough load at the lower end.
Torsional stresses
How does torsion (twisting) affect the loading
capacity of a bolt/screw? I mentioned earlier the data in
our various standards showing proofing loads and
tensile load levels. Those numbers are valid only for
testing with a straight pull in a tensile testing apparatus.
When we tighten (avoid the term torquing, we may
tighten by means of torque) a fastener (nut on a bolt or
a screw in a blind hole) typically 90 % of the
moment/torque is wasted by overcoming friction. About
40 % of the torque value is for the friction between the
internal and external threads. With this resistance, the
bolt/screw will actually experience a twisting motion with
torsional stresses being introduced in the fastener. The
fastener is, in fact, a “glorified” rubber band. This
twisting could substantially lower the capacity of the
fastener to carry an axial load.
This is why we should never use the table values in
the standards as they are listed, but instead modify
them with the influence of the torsional “wind-up”.
Factoring in torsion we will establish a new proofing load
level that properly reflects the actual fastener strength
as tightened. The way to predict the influence of torsion
is described in figure 2.
Tightening
All tightening equipment will produce some scatter in
the resulting tension in the joint. A well calibrated torque
please turn to page 172