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 MIL-HDBK-1038
Mobile cranes and the newer portal and floating cranes are equipped with
electronic combination radius/capacity digital readout devices for additional
operational safety. These indicators include automatic warning signals (visual
and audible) and limit switches. Load sensing is by means of a load cell
installed directly in the reeving system or a less accurate sensing of a
transverse load on the wire rope.
4.3.6.25 Spud Locks. The function of the spud locks is to positively secure the
crane's upperworks against rotation, as might be caused by strong wind gusts,
during periods of inactivity. The typical spud lock has a remotely actuated
mechanism that lowers a steel pin from the machinery deck into a pocket in the
lower roller path support structure. The older crane designs normally have only
one spud lock pocket and when it is engaged, the crane is placed in its most
stable position (with the boom parallel with the tracks) and the crane access
ladders and platforms are aligned to form a passageway between the stationary
lower structure and the rotating upperworks. The newer crane designs have two
spud lock pockets, for positioning the boom over the tracks in either direction
and the ladders and platforms are arranged to provide a passageway to the
upperworks in either locked position. The clearance between the spud lock pin and
its pocket is normally 1/8 to 1/4 inch on the diameter.
Spud lock actuators require safety interlocks to preclude damage to the
crane from unintentional lowering of the pin while the crane is rotating. The
control circuitry must be designed to disable the rotate drive when the spud lock
is not fully disengaged and must include a momentary bypass switch to permit
jogging of the rotate drive to free the spud lock pin if it is jammed due to drift
of the upperworks.
4.3.6.26 Rotate Holding Brakes. Several of the newer cranes are equipped with
rotate holding brakes in place of spud locks. These brakes are in the form of low
ratio gear reducers (installed to be back-driven and act as speed increasers) with
a brake on the high speed (low torque) shaft and a pinion in mesh with the rotate
bull gear on the low speed (high torque) shaft. A minimum of three such brakes
are recommended on a typical crane. The holding brakes allow the crane to be
stowed with the boom in any direction and there is no possibility of accidental
damage due to unintentional activation. (The designs of access ladders and
circular walkways around the rotate bearing provides access to the upperworks in
any orientation.)
4.3.6.27 Ratchet and Pawl Mechanisms. Every boom hoist must include a ratchet on
its drum and a remotely actuated pawl mounted on the hoist foundation. The
purpose of this mechanism is to ensure that there will be no downward drift of the
boom (against the holding brakes) due to the load from its own weight over
prolonged inactive periods. The control circuitry must be designed to disable the
boom hoist drive in the lowering direction when the pawl is not fully disengaged.
However, the drive must not be disabled in the raising direction so that the drum
can be rotated slightly to free and disengage a wedged pawl. The pawl operating
mechanism should include linkage which allows the pawl to maintain its position
(either engaged or disengaged) without any additional application of power.
The pawl is usually a compression member, but pawls loaded in tension are
also satisfactory. The security of the pawl/ratchet alignment (side to side) is
important to ensure full engagement and avoidance of eccentric loading. It is
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