JLG G6-42A Service Manual - Manual (Page 66)

Boom
3-28 G6-42A, G9-43A, G10-43A
3.11 BOOM EXTEND AND
RETRACT CHAINS
3.11.1 Boom Chain Inspection
Under normal operating conditions the boom extend/
retract chains will need to be exposed and inspected
every
1000 hours of operation. Environmental conditions
and dynamic impulse/shock loads can drastically affect
normal operating conditions and require more frequent
inspection intervals.
Environments in which material handling vehicles
operate can vary widely from outdoor moisture to
temperature to mildly corrosive or highly corrosive
industrial atmospheres, in addition to abrasive
exposures such as sand and grit. Some effects can be
as follows:
• Moisture—Corrosive rusting reduces chain
strength by pitting and cracking.
• Temperature—Low temperature reduces chain
strength by embrittlement. Going in and out of
cold storage results in moisture from
condensation.
• Chemical Solutions or Vapors—Corrosive attack
on the chain components and/or the mechanical
connections between the chain components.
Cracking can be (and often is) microscopic.
Going from microscopic cracking to complete
failure can be either abrupt or may require an
extended period of time.
• Abrasives—Accelerated wearing and scoring of
the articulating members of the chain (pins and
plates), with a corresponding reduction in chain
strength. Due to the inaccessibility of the bearing
surfaces (pin surfaces and plate apertures),
wear and scoring are not readily noticeable to
the naked eye.
Following are some examples of dynamic shock loading
which can impose abnormal loads above the endurance
limit of a leaf chain.
• High velocity movement of load, followed by
sudden, abrupt stops.
• Carrying loads in suspension over irregular
surfaces such as railroad tracks, potholes, and
rough terrain.
• Attempting to “inch” loads which are beyond the
rated capacity of the vehicle.
The above load cycles and environmental conditions
make it impossible to predict chain life. It is therefore
necessary to conduct frequent inspections until
replacement life can be predicted.
The boom chain’s normal life expectancy can be
expressed as a maximum percent of elongation. This is
generally 3%. As the chain flexes back and forth over the
sheave, the bearing joints (pins and inside link plates)
gradually incur wear due to articulation.
3.11.2 Expose Chains for Inspection
The extend/retract chains are only partially visible
through the rear of the boom with all the sections
retracted. It is possible to see a section of the retract
chain as the boom is slowly extended. If there is ANY
question that extend/retract chains are damaged, the
chains should be removed and inspected. Refer to
Section 3.6, “Extend/Retract Chain Removal/
Installation,” for detailed information. Replace chains if
necessary.
Note: DO NOT attempt to repair a chain. Replace a
stretched or damaged chain with a new part. Always
replace both the chain and the clevis. It is recommended
that when any chain is replaced, that all chains and
clevis’ be replaced at the same time.
WARNING
Worn pins, stretched or cracked links or corrosive
environments can cause chain failure. A chain failure
could result in uncontrolled boom movement, loss of
load or machine instability.