SUMMER 2018
Distributor's Link Magazine Summer 2018 / Vol 41 No3
Distributor's Link Magazine Summer 2018 / Vol 41 No3
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154<br />
THE DISTRIBUTOR’S LINK<br />
BENGT BLENDULF FASTENER FRICTION – FRIEND OR FOE? from page 94<br />
FIGURE 4<br />
The impact on the tightening torque is enormous if<br />
we are basing it on the first example (Fig 3) and switching<br />
to the second (Fig 4). This can be done by accident or by<br />
purpose.<br />
If we want to reach a preload of nominal mean 22.7<br />
kN and our k-factor is 0.31 the torque value would be<br />
70.4 Nm according to figure 3. If the fastener is according<br />
to our example in figure 4, the required torque value<br />
would be only 38.6 Nm to reach the 22.7 kN. If we by<br />
mistake put the higher torque value on the fastener with<br />
coating the preload would be 120 kN or more than twice<br />
the tensile strength of the fastener. In that case the<br />
friction coefficient is not a friend but a foe. The fastener<br />
would fail long before we reached the projected torque<br />
value. I would suggest that you get the Torque Book<br />
for Fasteners from IFI if you are dealing with tightening of<br />
fasteners. It is not a perfect book, but will give you some<br />
control of the tightening issues. Our goal must be to get<br />
the best possible preload on the joint without killing the<br />
fastener.<br />
Let me also give you a few words about torsional<br />
stresses in the fastener. It would make an article by itself<br />
so let me just give you couple of hints about torsion.<br />
When you have a cylindrical body of any kind and you twist<br />
it, its capacity of carrying load through its axis is limited<br />
by the twisting. The limiting factor here is the friction<br />
between the internal and external threads in the fastener.<br />
If we can limit the coefficient of friction in the threads we<br />
will also limit the torsional effect. So, a good lubrication<br />
of the fastener is always a positive and “friendly” thing to<br />
do, just make sure that it is properly understood by the<br />
people holding the torque wrench.<br />
BENGT BLENDULF<br />
GUY AVELLON WHAT FASTENER DISTRIBUTORS NEED TO KNOW ABOUT SUBSTITUTIONS from page 96<br />
Because of RoHS, the gold colored cadmium multiple bolt connection are torqued to the same value,<br />
plated nuts were discontinued. These nuts produced the higher strength fasteners will produce less clamp<br />
a consistent and low torque-tension relationship. The load than the others. Not the same clamp load, but less.<br />
replacement silver colored nuts are zinc with a wax Arguments have ensued with mechanical engineers and<br />
coating, which requires a greater torque for assembly. metallurgists about all steels (fasteners) have the same<br />
Therefore, using a cadmium torque on a zinc coated nut slope on a tensile graph. This is true, but not during<br />
produced lower clamping loads.<br />
assembly where instead of all tension, there is torsion.<br />
Bolts: Aside from the thread length differences Simply stated; it takes more force to stretch a higher<br />
and some head dimensional differences, many ASTM grade than a lower grade. Therefore, if you stretch the<br />
bolts may be substituted among themselves or with SAE higher grade fastener to the same amount or distance as<br />
bolts. However, regardless of the grade, make sure all of the lower grade, the higher grade has not been stretched<br />
the fasteners in a multiple assembly joint are the same the same distance and will consequently not produce<br />
grade and torqued the same.<br />
the same clamp force but instead, a lower clamp force<br />
than the lower bolt grade. The difference is torque, not<br />
This is by far the most important item to remember: tension.<br />
If a multiple bolt connection mixes different strength Always ask to be sure. A phone call is less costly<br />
grades in the assembly, even if all the fasteners in a than a law suit.<br />
GUY AVELLON