JLG 120-SXJ Service Manual - Manual (Page 59)

SECTION 4 - BOOM & PLATFORM
3121810 – JLG Lift – 4-11
d. 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 carefully after long time use and heavy
loading for this type of crack. If any cracks are
discovered, replace all chains, as seemingly
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.
e. Tight Joints: All joints in the roller chain should
flex freely. On roller chain, tight joints are usually
caused by rust/corrosion, or the inside plates
“walking” off the bushing. Limber up rusty/cor-
roded chains (after inspecting carefully) with a
heavy application 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:
1. Bent pins or plates.
2. Rusty joints.
3. Peened plate edges.
Oil rusty chains, and replace chains with bent or
peened chain components. Keep chains lubri-
cated.
f. Protruding or Turned Pins: Chains operating
with inadequate lube generate tremendous fric-
tion between the pin and plates (pin and bush-
ing on roller 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.
g. 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 than one crack can often appear on a
link plate. In addition to rusting, this condition
can be caused by exposure to an acidic or caus-
tic medium or atmosphere. Stress corrosion is
an environmentally assisted failure. Two condi-
tions 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 dur-
ing idle periods. The reactions of many chemical
agents (such as battery acid fumes) with hard-
ened metals liberate hydrogen which attacks
and weakens the metal grain structure.
TIGHT JOINTS
ABNORMAL PROTRUSION OR
TURNED PINS
ARC-LIKE CRACKED PLATES
(STRESS CORROSION)