SkyTrak 6042 Service Manual - Manual (Page 94)

Boom
3.48 Model 6042 Legacy Origin 7/02
Turning or Protruding Pins
Highly loaded chain, operating with inadequate
lubrication, can generate abnormal frictional forces
between 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 (Fig. 3-103, 1).
Chain with rotated/displaced heads (2) or abnormal
pin protrusion (3) must be replaced immediately.
(Refer to Section 3.4.5, “Boom Extend and Retract
Chains Removal and 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 must be corrected.
Figure 3-103 Inspect the Pin Head Rivets
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 must
be replaced in its entirety. (Refer to Section 3.4.5,
“Boom Extend and Retract Chains Removal and
Replacement.”)
• Fatigue Cracking - Fatigue cracks (Fig. 3-104, 4)
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 (Fig. 3-104, 5). 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
(Fig. 3-104, 4) in appearance. They generally
begin at the aperture and grow perpendicular to
the chain 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.
Figure 3-104 Inspect Plates for Cracks
MM2060
3 1
2
MM2080
4
5
MM2070