Maintenance Section
-117-
Causes of Fork Failure
Improper Modification or Repair
Fork failure can occur as a result of a field modification
involving welding, flame cutting or other similar processes
which affect the heat treatment and reduces the strength
of the fork.
In most cases, specific processes and techniques are
also required to achieve proper welding of the particular
alloy steels involved. Critical areas most likely to be
affected by improper processing are the heel section, the
mounting components and the fork tip.
Bent or Twisted Forks
Forks can be bent out of shape by extreme overloading,
glancing blows against walls or other solid objects or
using the fork tip as a pry bar.
Bent or twisted forks are much more likely to break and
cause damage or injury. They should be removed from
service immediately.
Fatigue
Parts which are subjected to repeated or fluctuating loads
can fail after a large number of loading cycles even
though the maximum stress was below the static strength
of the part.
The first sign of a fatigue failure is usually a crack which
starts in an area of high stress concentration This is
usually in the heel section or on the fork mounting.
As the crack progresses under repetitive load cycling, the
load bearing cross section of the remaining metal is
decreased in size until it becomes insufficient to support
the load and complete failure occurs.
Fatigue failure is the most common mode of fork failure.
It is also one which can be anticipated and prevented by
recognizing the conditions which lead up to the failure
and by removing the fork service prior to failing.
Repetitive Overloading
Repetitive cycling of loads which exceeds the fatigue
strength of the material can lead to fatigue failure. The
overload could be caused by loads in excess of the rated
fork capacity and by use of the forks tips as pry bars. Also,
by handling loads in a manner which causes the fork tips
to spread and the forks to twist laterally about their
mountings.
Wear
Forks are constantly subjected to abrasion as they slide
on floors and loads. The thickness of the fork blade is
gradually reduced to the point where it may not be
capable of handling the load for which it was designed.
Stress Risers
Scratches, nicks and corrosion are points of high stress
concentration where cracks can develop. These cracks
can progress under repetitive loading in a typical mode of
fatigue failure.
Overloading
Extreme overloading can cause permanent bending or
immediate failure of the forks. Using forks of less capacity
than the load or lift truck when lifting loads and using forks
in a manner for which they were not designed are some
common causes of overloading.
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