MIL-HDBK-1038
down to the angle which puts the main hook at its maximum radius, or goes down to
horizontal. There is also a direct actuated back-up limit switch.
Each hook hoist also has a geared limit switch. It stops the motor when
the hook goes down to its lower limit, goes up to its upper limit with the boom in
the horizontal position, and goes up to its upper limit with the boom at the angle
which puts the main hook at its maximum radius. There is also a back-up limit
switch which is either an independent geared limit switch or a block actuated
limit switch. The lower limit for a hook hoist on a portal crane is usually set
to permit the hook to go to the bottom of a drydock; it would not stop the drive
if the hook touched the ground or a pier. The lower limit for a hook hoist on a
floating crane is usually set to permit the hook to touch the deck of the barge,
but can be reset so that the hook could go into the holds of a ship. Stopping the
hook at its upper limit with the boom horizontal prevents two-blocking because the
boom can never be lowered below horizontal. The upper limit for a hook with the
boom horizontal is automatically bypassed whenever the boom is at an angle which
puts the main hook at a radius less than its maximum radius; the upper limit for
the hook with the boom at the angle which puts the main hook at its maximum radius
is then established as the hook's upper limit. It is safe to do this because the
boom is prevented from being lowered beyond the angle which puts the main hook at
its maximum radius unless the hook is below its upper limit with the boom
horizontal and a keyed bypass switch is used. When a hook hoist's wire ropes are
routed parallel to the boom, there is minimal hook movement relative to the boom
due to luffing of the boom. Therefore, there would be no need for a switch to
establish an upper limit for the hook with the boom at the angle which puts the
main hook at its maximum radius. The upper limit established with the boom in the
horizontal position will provide an adequate high hook elevation at its maximum
radius.
Limit switches are not normally used with the travel or rotate motions of
a portal crane or with the rotate motion of a floating crane.
4.5.22.3 Other Portal and Floating Crane Applications. Limit switches are also
used on portal and floating cranes equipped with a spud lock to limit the upward
and downward movement of the spud and to energize and de-energize lights
indicating the engagement or disengagement of the spud. If the spud is not fully
retracted, a limit switch operating in the rotate drive control circuitry prevents
operation of the drive. Additionally, limit switches are utilized to indicate to
the operator the position of the spud, relative to the socket, when they are
within close proximity while the upperworks rotates.
Also on portal and floating cranes, limit switches are used with the boom
pawl mechanism. They are used to sense the position of the pawl so that its
engagement and release can be indicated to the operator and boom lowering is
prevented unless the pawl is released.
4.5.22.4 Condition Limits. Other types of switches are used to limit a system
operating characteristic. An example of this is the compressor in an air start
system for a diesel engine. Pressure switches are used in the controlling of the
compressor motor to sense air pressure in the accumulator.
4.5.23
Protective Devices. NCC policy is to require overcurrent protection on
cranes as addressed in Parts E and F of Article 610 of the National Electrical
Code. Both equipment and conductors must be protected. Protection must be
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