WHAT’S WRONG WITH THIS PICTURE?
NYLON LOCKING NUTS
The mechanical security of the
hardware and equipment aboard
your boat—from the bolts that
keep the engine’s connecting rods
attached to the crankshaft to the
screws that hold navigation lights
in place—is of the highest priority,
to be sure.
As vessel operators, we rely on
the peace of mind provided by even
the smallest fasteners, secure in the
knowledge that each nut, bolt, and
screw will perform as advertised,
regardless of the conditions under
which it operates. Provided the
fastener is chosen and installed
correctly, this is often true. In many
cases, even when there is a flaw in
the selection or installation, these
small but important parts still
soldier on without failure.
However, I’ve seen many
examples of fasteners that failed to
maintain a grip on the parts they
were tasked with holding together.
This is nearly always a result of
either insufficient torque having
been applied or use of the incorrect
size or style fastener. Additionally,
many fasteners rely on some form of
external retaining or locking device
to prevent them from loosening.
Among the most common causes
of loosening are vibration and cyclic
or reverse loading. The former is
familiar to all vessel operators, and
the havoc it can wreak on fasteners
is well known. Cyclic loading is less
well understood. It’s particularly
common in fasteners that
experience frequent “on” and
“off” loads or loads that reverse
direction, such as those in
transmission components,
flywheels, steering gear, and
running gear. Regular reversal of
load has an accelerated loosening
effect on fasteners; thus, special
care should be exercised when
installing fasteners that will be
subjected to such forces.
In the example shown here, the
fasteners are retaining what most
folks would consider an important
part: a propeller shaft. The
mechanic made an honest effort
to use what he or she perceived
to be a vibration-proof installation
in the form of nylon insert self-locking nuts. However, the nylon
portions of the nuts have failed
to engage the bolts’ threads and
thereby have been rendered
ineffective. A minimum of two
threads should stand proud
of every nut, with or without a
nylon insert, after assembly is
complete.—Steve D’Antonio
