SPRING 2017

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THE DISTRIBUTOR’S LINK
LAURENCE CLAUS FUNDAMENTALS OF THREAD FORMING SCREWS - PART 3 from page 14
The previous discussion, although a very basic look
into more complicated metallurgy, sets the stage for our
understanding of thread forming into steel. The First Rule
of thread forming is that the screw must be stronger and
harder than the material it is forming. This is because
the threads must be able to withstand the loads exerted
on them during thread forming without breaking or
plastically deforming. This is exactly what is likely to
happen if the strength of the material to be formed is
in too close proximity of the screw threads doing the
forming. For this reason, we limit thread forming to “mild
steels” or steels that are relatively low in carbon and
have not been heat treated.
Returning to Part 1, you may recall that with any
thread forming application one of the interactions that
is of high interest to the fastener engineer is minimizing
the driving torque and maximizing the failure torque.
With this in mind, many special thread forming screw
designs for steel have been developed to optimize this
relationship. To minimize the driving torque, screws
designed for mild steel almost always have a tapered
point, sharp threads, some kind of relief, and lubrication.
The tapered point provides a gradual lead-in for the
forming threads. This provides for small incremental
steps in forming rather than attempting to move all the
material at one time. Sharp threads are also important.
It is perhaps best explained with an analogy, would you
choose a steak or butter knife to cut your favorite sirloin
steak with? Naturally, you would choose the steak knife
because it is sharper and, thus, far more efficient. In the
same way, sharp threads do a better job forming internal
mating threads than dull or flat threads do.
Almost all thread forming screws for mild steel have
some form of relief at the tip or along the entire body
length. In this instance, “relief” means that a section
of the threaded body is not fully round or is lobular
in shape. This can be accomplished with a lobulated
thread body or point, or a screw that has a flute or
series of flutes integrated into the threaded body. The
idea is that the thread forming is enhanced without a
fully round thread because the interface is only at the
lobes or high points so that the forming is accomplished
with less thread forming and thread friction torque.
This is advantageous as it effectively lowers the driving
torque. Most of these screws transition to either a fully
round thread or only superficially lobulated thread after
passing through the thread forming zone. This assists
the design in lowering the driving torque without also
sacrificing the failure torque. Additionally, most of these
screws are lubricated to reduce thread friction torque
at installation. Unfortunately, lubrication cannot be as
selectively applied as the transition feature described
just above, so that, although the lubrication factor helps
lower friction and reduce galling, it has the potential
negative effect of also lowering the failure torque.
Thread forming screws for steel are generally case
hardened. They need the high-strength and hard threads.
This solution works well for perhaps 90% of the screws
in this category. The other 10%, however, require a
level of impact toughness in application not achievable
with case hardening. A notable example of this are
the anchor screws for seat belt attachments. These
anchoring points are often fastened with thread forming
screws but must possess toughness to assure they do
not break in an accident. To accommodate these types
of applications, many thread forming screw designs for
steel can be purchased with an induction hardened point
option. In these instances, screws are through hardened,
often to Property Class 10.9 (Grade 8), and the tip
induction hardened. The induction hardened tip provides
the strength and hardness needed for thread forming,
while the rest of the screw retains the impact toughness
of a through hardened fastener.
In addition to a well-designed fastener, control of the
pilot hole is critically important. If the hole is too small,
the material will become too tightly “packed” in the joint,
resulting in very high driving torques or torsional overload
of the screw. Likewise, too large a hole will “starve”
the joint of material needed to develop well-formed and
strong mating threads. The result will be advantageous
low driving torques but unacceptably low failure torques
through stripping.
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