Doosan D20S-5 Operation & Maintenance Manual page 133

Causes of Fork Failure Causes of Fork Failure
Improper Modification or Repair Improper Modification or Repair
Fork failure can occur as a result of a field Fork failure can occur as a result of a field
modification involving welding, flame cutting or modification involving welding, flame cutting or
other similar processes which affect the heat other similar processes which affect the heat
treatment and reduces the strength of the fork. treatment and reduces the strength of the fork.
In most cases, specific processes and techniques In most cases, specific processes and techniques
are also required to achieve proper welding of the are also required to achieve proper welding of the
particular alloy steels involved. Critical areas most particular alloy steels involved. Critical areas most
likely to be affected by improper processing are the likely to be affected by improper processing are the
heel section, the mounting components and the heel section, the mounting components and the
fork tip. fork tip.
Bent or Twisted Forks Bent or Twisted Forks
Forks can be bent out of shape by extreme Forks can be bent out of shape by extreme
overloading, glancing blows against walls or other overloading, glancing blows against walls or other
solid objects or using the fork tip as a pry bar. solid objects or using the fork tip as a
Bent or twisted forks are much more likely to break Bent or twisted forks are much more likely to break
and cause damage or injury. They should be and cause damage or injury. They should be
removed from service immediately. removed from service immediately.
Fatigue Fatigue
Parts which are subjected to repeated or fluctuating Parts which are subjected to r
loads can fail after a large number of loading cycles loads can fail after a la
even though the maximum stress was below the even though the maximum stress was below the
static strength of the part. static strength of the part.
The first sign of a fatigue failure is usually a crack The first sign of a fatigue failure is usually a crack
which starts in an area of high stress concentration. which starts in an area
This is usually in the heel section or on the fork This is usually in the heel section or on the fork
mounting. mounting.
As the crack progresses under repetitive load As the crack progresses under repetitive load
cycling, the load bearing cross section of the cycling, the load bearing cross section of the
remaining metal is decreased in size until it remaining metal is decreased in size until it
becomes insufficient to support the load and becomes insufficient to support the load and
complete failure occurs. complete failure occurs.
Fatigue failure is the most common mode of fork Fatigue failure is the most common mode of fork
failure. It is also one which can be anticipated and failure. It is also one which can be anticipated and
prevented by recognizing the conditions which lead prevented by recognizing the conditions which lead
up to the failure and by removing the fork service up to the failure and by removing the fork service
prior to failing. prior to failing.
pry bar.
epeated or fluctuating
rge number of loading cycles
of high stress concentration.
-130- -130-
Repetitive O Repetitive Overloadi verloading
 
Repetitive cycling of loads which exceeds the Repetitive cycling of loads which exceeds the
fatigue strength of the material can lead to fatigue fatigue strength of the material can lead to fatigue
failure. The overload could be caused by loads in failure. The overload could be caused by loads in
excess of the rated fork capacity and by use of the excess of the rated fork capacity and by use of the
forks tips as pry bars. Also, by handling loads in a forks tips as pry bars. Also, by handling loads in a
manner which causes the fork tips to spread and manner which causes the fork tips to spread and
the forks to the forks to twist laterally about their mountings. twist laterally about their mountings.
Wear Wear
 
Forks are constantly subjected to abrasion as they Forks are constantly subjected to abrasion as they
slide on floors and loads. The thickness of the fork slide on floors and loads. The thickness of the fork
blade is gradually reduced to the point where it may blade is gradually reduced to the point where it may
not be capable of handling the load for which it was not be capable of handling the load for which it was
designed. designed.
Stress Risers Stress Risers
 
Scratches, nicks and corrosion are points of high Scratches, nicks and corrosion are points of
stress concentration where cracks can develop. stress concentration where cracks can develop.
These cracks can progress under repetitive loading These cracks can progress under repetitive loading
in a typical mode of fatigue failure. in a typical mode of fatigue failure.
Overloading Overloading
Extreme overloading can cause permanent bending Extreme overloading can cause per
or immediate failure of the forks. Using forks of less or immediate failure of the forks. Using forks of less
capacity than the load or lift truck when lifting loads capacity than the load or lift truck when lifting loads
and using forks in a m and using forks in a manner for which they were no
designed are some common causes of designed are some common causes of overloadin
Maintenance Section Maintenance Section
ng
high
manent bending
anner for which they were not t
overloading. g.