FALL 2016

130
THE DISTRIBUTOR’S LINK
LAURENCE CLAUS FUNDAMENTALS OF THREAD FORMING SCREWS - PART 1 from page 24
Distributors, especially those working with customers
utilizing thread forming screws or a diverse selection
of fasteners, should have one or more individuals
available to provide their customers with knowledge about
or application engineering assistance regarding thread
forming. Although this may sound like a daunting challenge
considering the wide variety of materials and selection
of thread forming screws on the market, much of the
“science” is actually the same regardless of the material
being fastened into. Once these foundational concepts
are understood, they can be applied to understanding
more specific and focused applications like thread forming
into plastics, light metals, and mild steel.
This series of articles will look at these subjects.
The remainder of this article will look at the universal
fundamentals of thread forming. Part 2 will explore thread
forming in thermoplastics and thermosets. Part 3 will
explore thread forming in mild steel and light metals.
Naturally what is presented in this series will be universal
concepts. There are always exceptions and every project
is unique onto itself, however, having
this fundamental understanding may arm
you with enough knowledge to look at
applications in a new way and provide
your customer with potential cost saving
and performance enhancing solutions.
Part 1- Fundamentals Of Thread
Forming
Figure 1 illustrates what the process
looks like when we measure applied torque
on a screw as it is being driven. Torque is
the amount of rotational or twisting force
that the screw is experiencing. On this
diagram the x-axis represents time and
the y-axis torque, so that as the screw
is turned into its mating component and
experiences friction, forming, and loading
with each turn, the graph progresses
upward and to the right. At some point
the head of the screw will seat against
the material it is clamping and no longer
be able move forward. This point is
easily distinguished on this diagram
as the inflection point between where
the graph is increasing upward along a
very shallow slope and where the graph
becomes almost vertical. This inflection
point represents the maximum torque experienced to
drive the screw in and is, therefore, referred to as the
Driving Torque. An installation does not normally end
there and continued turning of the screw results in very
minimal axial movement (although some will occur as the
joint compresses and the fastener embeds itself in the
clamped material) but significant increase in compressive
axial loading, often referred to as the Clamp Load.
Eventually the strength of one of the joint components will
exceed its limits and a failure will occur. In a traditional
bolted joint the failure is designed to occur in the bolt.
In thread forming applications, however, the material
being formed usually possesses less strength than the
fastener, often by several magnitudes. Therefore, the
most common failure mode occurs when the clamp load
exceeds the shear strength of the material being formed
and the screw strips. The resulting peak of this diagram
represents the Failure or Ultimate Torque of the joint, and
is usually called the Stripping Torque (because that is the
way most of these joints fail).
FIGURE 1: TYPICAL THREAD FORMING TORQUE BEHAVIOR
CONTINUED ON PAGE 184