SPRING 2024

94
THE DISTRIBUTOR’S LINK
GUY AVELLON WHAT DISTRIBUTORS NEED TO KNOW ABOUT HOW FASTENERS ARE TIGHTENED IN MAINTENANCE ASSEMBLIES from page 26
This would mean that at the same torque, the
cadmium plated lock nuts would produce more tensile
load on the fastener, with the potential of stretching the
fastener into yield, than the zinc plated lock nut. The
connection would be compromised. Some maintenance
shops and those performing field repair work will use
a micrometer adjustable torque wrench. This is also
referred to as a ‘click-type’, where the torque value
is numerically adjusted on the handle. The wrench
will produce an audible ‘click’, which can also be felt,
when the set torque value is reached. At this point, the
operator stops turning the wrench. However, there is no
provision for preventing the operator to continue turning
the wrench and applying more torque to the fastener
which will increase the load on the fastener joint. There
are special adjustable torque wrenches available that
have a cam in the head that will prevent over tightening.
Realistically, most of the maintenance performed
on fasteners is done without using a torque wrench but
with the ‘calibrated elbow’. And, many times using either
the torque wrench or elbow, the tightening procedure is
incorrect.
Torque charts have been printed as guides to
achieving the proper tension on the bolt but it may not
be as perfect for the connection or overall joint. We can
achieve adequate tension on one bolt, but what about
multiple fasteners in the joint? What about environmental
FIGURE 1
conditions? Is it a rigid joint or compressible joint?
A common practice in the maintenance industry is
to calculate the torque to produce a clamp load that was
75% below proof load. Proof load being approximately 9%
below yield gave the operator a good margin of safety to
avoid stretching the fastener into yield. However, there
are many variables to consider which can affect the
clamp load by over ±50%.
In Figure 1 we have the ideal clamp load with 25%
safety factor.
Figure 2 represents an example of where a small
installation torque error will result in a wide clamp load
variance. This wide range of a variance is not good for a
multiple bolting assembly.
FIGURE 2
So, the man with the wrench must be consistent.
But the joint will never be consistent with material
hardnesses, flatness, smooth surfaces, burrs, rust, etc.
Then, after tightening, all clamping forces will decrease
as soon as the wrench stops turning and the joint relaxes
from the compression. Typically, a joint relaxes anywhere
between 5-15%, depending on whether the head of the
bolt or nut is turned.
As the joint enters service, the joint may relax even
more which is why critical joints are retightened shortly
after the initial installation, then again after experiencing
service loads.
CONTINUED ON PAGE 95