Maintenance Section
-125-
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.
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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.
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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.
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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.
