SECTION 2 - GENERAL
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2-7
Fatigue Cracks:
Fatigue is a phenomenon that affects most
metals, and is the most common cause of chain plate failures.
Fatigue cracks are found through the link holes, perpendicular
(90 degrees)
from the pin in-line position. Inspect chains care
-
fully after long time use and heavy loading for this type of
crack. If any cracks are discovered, replace all chains, as seem
-
ingly sound plates are on the verge of cracking. Fatigue and
ultimate strength failures on JLG Lifts are incurred as a result of
severe abuse as design specs are well within the rated lifting
capacity of these chains.
Tight Joints:
All joints in the leaf chain should flex freely. On
leaf chain, tight joints are usually caused by rust/corrosion, or
the inside plates “walking” off the bushing. Limber up rusty/
corroded chains
(after inspecting care fully)
with a heavy appli
-
cation of oil
(preferably a hot oil dip)
. Tap inside “walking”
plates inward; if “walking” persists, replace the chain. This type
of problem is accelerated by poor lubrication maintenance
practice, and most tight joint chains have been operated with
little or no lubrication. Tight joints on leaf chain are generally
caused by:
a. Bent pins or plates.
b. Rusty joints.
c. Peened plate edges.
Oil rusty chains, and replace chains with bent or peened chain
components. Keep chains lubricated.
Protruding or Turned Pins:
Chains operating with inade
-
quate lube generate tremendous friction between the pin and
plates (pin and bushing on leaf chain). In extreme cases, this
frictional torque can actually turn the pins in the outside
press-fit plates. Inspect for turned pins, which can be easily
spotted as the “V” flats on the pin heads are no longer in line.
Replace all chains showing evidence of turned or protruding
pins. Keep chains lubricated.
Stress Corrosion Cracking:
The outside link plates, which are
heavily press-fitted to the pins, are particularly susceptible to
stress corrosion cracking. Like fatigue cracks, these initiate at
the point of highest stress (aperture) but tend to extend in an
arc-like path, often parallel to the rolling grain of the material.
Also, more then one crack can often appear on a link plate. In
addition to rusting, this condition can be caused by exposure
to an acidic or caustic medium or atmosphere. Stress corrosion
is an environmentally assisted failure. Two conditions must be
present; corrosive agent and static stress.
In the chain, static stress is present at the aperture due to the
press fit pin. No cycle motion is required and the plates can
crack during idle periods. The reactions of many chemical
agents (such as battery acid fumes) with hardened metals lib
-
erate hydrogen which attacks and weakens the metal grain
structure.
Chain Anchors and Sheaves:
An inspection of the chain must
include a close examination of chain anchors and sheaves.
Check chain anchors for wear breakage and misalignment.
Anchors with worn or broken fingers should be replaced. They
should also be adjusted to eliminate twisting the chain for an
even load distribution.
Inspect the sheaves, sheave bearings, sheave grooves and pins
for extreme wear, replace as necessary. A worn sheave can
mean several problems, as follows:
a. Chains too tight.
b. Sheave bearings/pin bad.
c. Bent/misaligned chains.
TIGHT JOINTS
ABNORMAL PROTRUSION OR
TURNED PINS
ARC-LIKE CRACKED PLATES
(STRESS CORROSION)
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