Two blocking is more likely to occur when both the main and auxiliary hoist lines are reeved
over the main boom nose and boom extension/jib nose respectively. An operator, concentrating on
the specific line being used, may telescope or lower the boom allowing the other hoist line
attachment to contact the boom or boom extension/jib nose, thus causing damage to the sheaves,
or causing the wire rope to fail, dropping the lifting device to the ground and possible injuring
personnel working below.
Caution must be used when lowering or extending the boom. Let out load line(s)
simultaneously to prevent two blocking the boom tip(s) and the hook block, etc. The closer the load
is carried to the boom nose, the more important it becomes to simultaneously let out wire rope as
the boom is lowered. Keep load handling devices a minimum of 18 in. (45.7 cm) below the boom
nose at all times.
Two blocking can be prevented. Operator awareness of the hazards of two blocking is the
most important factor in preventing this condition. An anti-two block system is intended to assist the
operator in preventing dangerous two block conditions. It is not a replacement for operator
awareness and competence.
Barricade the area where the crane is working and keep all unnecessary personnel out of
that area. DO NOT allow personnel to be under the load or boom.
Never pass loads, load handling devices, or the crane boom over people on the ground.
Never operate the crane with less than two wraps of wire rope on the hoist drum.
Never interfere with the proper functioning of operational aids or warning devices.
LOAD MOMENT INDICATING SYSTEMS.
Electronic equipment on this crane is intended as an aid to the operator.
Under NO CONDITION should it be relied upon to replace the use of capacity charts and
operating instructions. Sole reliance upon these electronic aids in place of good operating practices
can cause an accident.