CASE CN3 - Safety Booms for Mobile Cranes, P. Malone
Failures of mobile crane booms.
Symptom: Mobile crane had a history of failing when an examination revealed
only minor damage that on the WW II vintage cranes presented no problems.
Collection of Facts: The review of mobile crane boom design indicates that
the new booms were designed using high strength steel (in the 100,000 psi
yield range). This high strength steel allowed the l/r ratio support to be
increased to maximum ratio. This higher l/r ratio means that if support
(diagonals) is lost any where along the boom cord, the l/r ratio will reach
critical proportion there by causing boom failure at very low loads.
Solution: Retire booms made of high strength steel from service if secondary
members such as diagonals as well as the main cord are damaged.
CASE CN4 - Safety of Slings and Fittings, P. Malone
Problem: Failure of wire rope sling while lifting a rigid load (barge)
resulting in damage to the crane and other equipment.
Symptom: While relocating a barge, the wire rope sling broke at an estimated
load that was well below the breaking strength of the wire rope.
Collection of Facts: Published breaking strength of the wire rope indicated
the wire rope size was correct, the sling was constructed properly, and the
sling was maintained in accordance with prescribed procedures, etc. Further
investigation revealed that the wire rope was on a fitting that reduced the
effective breaking below the safe working level.
Solution: Insure that individuals who have the responsibility for lifts are
trained in the limitations of the equipment and the fitting used in any
CASE CN5 - Capacity of Barge Mount Mobile Cranes, P. Malone
Problem: The swing gear broke during normal operation of a mobile crane while
operating a barge in moderate sea conditions.
Symptom: Routine preventive maintenance uncovered that the pinion gear of the
rotate mechanism had one tooth broken and several other damaged.
Collection of Facts: The mobile crane handbook and other pertinent documents
were reviewed. Mobile cranes are designed to operate on level ground and they
are not designed to meet AISC or other standards of this type. This means
that the normal safety factors of design are not applicable and therefore
special precaution must be undertaken when these cranes are employed other
than as specified by the manufacturer of the crane.
Solution: For mobile cranes mounted on a barge, the capacity chart must be
revised to reflect the characteristics of the barge or floating platform on
which it is to be mounted. This chart will be different for each type of
platform the crane is mounted on and also different for each crane.