Group 34, Uprights
34-6-4 • Upright Chain Inspection, Adjustment, and Replacement SM 765, Nov ’06
• 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 (pin hole) but tend to extend
in an arc-like path between the holes in the pin
plate.
Arc-like cracks in plates are a sign of stress corrosion.
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 fail-
ure. Two conditions must be present: a corrosive
agent and static stress. In the chain, static stress is
present at the pin hole due to the press fit pin. No
cyclic motion is required, and the plates can crack
during idle periods. The reactions of many chemi-
cal agents (such as battery acid fumes) with hard-
ened steel can liberate hydrogen which attacks and
weakens the steel grain structure.
For this same reason, never attempt to electroplate
a leaf chain or its components. The plating process
liberates hydrogen, and hydrogen embrittlement
cracks will appear. These are similar in appearance
to stress corrosion cracks.
If a plated chain is required, consult Clark. Plated
chains are assembled from modified, individually
plated components which may reduce the chain
rating.
• Corrosion Fatigue - Corrosion fatigue cracks are
very similar (in many cases identical) to normal
fatigue cracks in appearance. They generally begin
at the pin hole and move perpendicular (90°) to the
chain pitch line.
Corrosion fatigue is not the same as stress corro-
sion. Corrosion fatigue is the combined action of
an aggressive environment and a cyclic stress (not
a static stress alone, as in stress corrosion).
Ultimate Strength Failure
This type of failure is caused by overloads far in excess of
the design load.
Broken plate caused by overload.
Tight Joints
All joints in leaf chain should flex freely. Tight joints
resist flexure and increase internal friction, thus increasing
chain tension required to lift a given load. Increased ten-
sion accelerates wear and fatigue problems.
If lubrication does not loosen a tight joint, the chain may
have corrosion and rust problems or bent pins and must
be replaced.
See Section 3 for detailed chain stretch, length, and ten-
sions checks.
Chain Length Adjustments
WARNING
!
An upright or carriage can move unexpect-
edly:
• Do not walk or stand under raised forks
• Keep clear of load and carriage when mak-
ing any check or adjustment
• Keep your arms and fingers away from
moving parts of the upright.
• Block the carriage or upright when work-
ing with the components in a raised posi-
tion.
• Do not reach through open areas of the
upright.
• Never attempt to move or align the rails by
hand. Use a prybar.
Failure to follow these warnings can result in
serious injury.
To Next Page
Loading...
