MIL-HDBK-1038
operation. On a newer AC powered crane, the second one is required to be a block
actuated control circuit type limit switch operating a line contactor located on
the line side of the directional contactors or flux vector controller requiring
resetting of the hoist control circuit by a keyed bypass switch prior to resuming
operation. The keyswitch is required to be spring returned to normal. A geared
control circuit type hoist lower limit switch is also required. The primary upper
limit switch setting is required to preclude activation of the secondary upper
limit switch at full speed and no load. Bridge and trolley limit switches may be
required.
With the NCC limit switch arrangement on newer portal or floating cranes,
luffing hoist limit switching is required to prevent raising the boom
above the angle which puts the main hoist hook at its minimum operating radius, to
prevent lowering the boom below the angle which puts the main hoist hook at its
maximum operating radius unless the operator uses a key-operated bypass switch, to
prevent lowering the boom beyond horizontal, and to bypass the main and other hook
hoist upper limits with a horizontal boom whenever the boom is at or above the
angle which puts the main hoist hook at its maximum operating radius. A boom-
actuated limit switch is also required as an emergency back-up of the geared upper
limit switch, by causing power interruption to the hoist motor through a means
different from that used by the geared limit switch (for example, by opening a
line contactor). The operator would have the capability to bypass this switch and
lower (not lift) the boom with the emergency back-up switch tripped. Main hoist
limit switching on newer portal and floating cranes is required to prevent raising
the hook beyond its maximum elevation with the boom at the angle which puts the
main hoist hook at its maximum operating radius, to prevent raising the hook
beyond its maximum elevation with a horizontal boom unless automatically bypassed
whenever the boom is at or above the angle which puts the main hoist hook at its
maximum operating radius, and to prevent lowering the hook beyond its minimum
elevation with boom at the angle which puts the main hoist hook at its minimum
operating radius. An independent geared limit switch or a block-actuated limit
switch is required to provide an upper limit as emergency back-up of the geared
upper limit switch by causing power interruption to the hoist motor. Power
interruption must be by means different from that used by the primary geared limit
switch (for example, by opening a line contactor). The operator retains the
capability to bypass this switch and lower (not lift) the hook with the emergency
back-up switch tripped.
When a hook hoist's wire ropes are routed parallel to the boom, there is
minimal 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.
Other hook hoist's limit switching is required to function in the same
manner as the main hoist limit switching. When the boom is at the angle for the
main hook hoist maximum operating radius, all hook hoists are required to be at
their maximum operating radii coincidentally.
5.5.5.13 Pendent Pushbutton Stations. The pendent pushbutton station is required
to be suspended from the crane by a small stainless steel wire rope strain lead
which is required to be 1/8 inch (minimum). The pendent cable is required to be
type SOO. Minimum conductor size is required to be No. 14 AWG. The pushbutton
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