JLG sky trak mmv Service Manual page 79

Turning or Protruding Pins
Highly loaded chain, operating with inadequate lubri-
cation can generate abnormal frictional forces be-
tween pin and link plates. When chain is allowed to
operate in this condition, a pin or series of pins, can
begin to twist out of a chain, resulting in failure.
Examine the pin head rivets to determine if the "VEE"
flats are still in correct alignment (5). Chain with
rotated/displaced heads (6) or abnormal pin
protrusion (7) should be replaced immediately.
(Refer to Section 4.3.6, c. Chain Replacement .)
DO NOT attempt to repair the chain by welding or
driving the pin(s) back into the chain. Once the press
fit integrity between outside plates and pins has been
altered, it cannot be restored.
Any wear pattern on the pin heads or the sides of the
link plates indicates misalignment in the system. This
condition damages the chain as well as increases
frictional loading and should be corrected.
5
Cracked Plates
Inspect the chains very carefully, front and back as
well as side to side, for any evidence of cracked
plates. If any one crack is discovered, the chain
should be replaced in its entirety. (Refer to Section
4.3.6, c. Chain Replacement .)
It is important, however to determine the cause of the
crack before installing a new chain so the condition
does not repeat itself.
Model MMV Rev. 12/04
General Information, Specifications and Maintenance
6
7
• Fatigue Cracking - Fatigue cracks (8) are a
result of repeated cyclic loading beyond the
chain's endurance limit. The size of the load and
the frequency of its occurrence are factors which
determine when fatigue failure will occur. The
loading can be continuous or intermittent
(impulse load).
• Stress Corrosion Cracking - The outside link
plates are particularly susceptible to stress
corrosion cracking (9). Like fatigue cracks, these
initiate at the point of highest stress but tend to
extend in an arc-like path between holes in the
pin plate. 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 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 cyclic motion is required, and the plates can
crack during idle periods.
• Corrosion Fatigue Cracking - Corrosion fatigue
cracks are very similar to fatigue cracks (8) in
appearance. They generally begin at the
aperture and grow perpendicular to the chain
MM2060
pitch line. Corrosion fatigue is not the same as
stress corrosion. Corrosion fatigue is the
combined action of an aggressive environment
and cyclic stress not a static stress alone, as in
stress corrosion).
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8
MM2070
MM2080
2.63