MIL-HDBK-1038
brake on its side for a vertical drum axis because the linkage is too loose to
keep the shoes from dragging or applying unevenly on the brake wheel. The
required modifications for this brake orientation include replacement of the plain
linkage pins with a type that has a rolling ball on the bottom end and a machined
hard flat plate to support the linkage pins and thus maintain the linkage and
shoes in proper alignment. Whenever possible, the drum shaft axes should be
horizontal to avoid such modifications.
Manual release mechanisms are available on all shoe brakes and should
always be provided on cranes. The mechanism must be of the maintained force type
that is, the brake may be in the released position only while the manual force
is being applied. Locking type of release mechanism, such as over-the-center or
screw type are prohibited. On hoists, the manual release mechanism must be able
to modulate the brake release force to permit the lowering of a suspended load by
gravity in a positive, controlled manner.
4.4.1.1
Combination Hydraulic Shoe Brakes. Mechanical-electrical shoe brakes can
be equipped with a hydraulic activating system. With this arrangement, the brake
operating sequence is: spring-applied, electrically released, hydraulically re-
applied (by foot pedal). These brakes are used on travel drives which have a
drift (coasting) mode. When the brake is electrically released and the crane is
coasting, the operator can modulate the speed or the deceleration rate by the
force on the brake foot pedal. The need for routing the hydraulic lines between
the operator's cab and the brake restricts these brakes to crane structures on
which the operator's cab is mounted the most common being the bridge of an OET
crane (or the trolley, if the cab is mounted on it) and machinery deck of a portal
crane. (Because of the complexity of this brake design, NCC recommends a separate
hydraulic shoe brake for these applications.)
4.4.1.2
Electro-Hydraulic Shoe Brakes. Electrically activated shoe brakes can
also be applied through an intermediate hydraulic system. The elements of the
system are the electric circuit, electric motor driving a hydraulic pump,
hydraulic cylinder, and mechanical linkage with the brake shoes. These brakes are
used most often on the travel drives of portal and container cranes. The electric
circuitry from the operator's cab is routed through slip rings or a festoon system
to the brake assembly. The electric activating system does not provide modulating
control but the relatively gradual application of the full brake torque softens
the stops.
4.4.2
Disc Brakes. Disc brakes use several alternating stationary and rotating
discs (pressure plates and friction discs) to develop a high friction (braking)
force from the pressure of a number of symmetrically arranged coil springs. All
discs are free to slide axially the stationary set inside the housing (usually
on pins) and the rotating set on the spline of the drive shaft. Being very
compact and enclosed, disc brakes have less thermal capacity than the shoe brakes.
Design details of the standard commercial disc brakes have been refined so that
there is virtually no physical contact and heat generation between discs when the
brake is in the released position. (Oil immersed designs are available for
increased thermal capacity, but should be avoided because of the additional
complexity.) The most common activation is by AC solenoid, but direct acting DC
magnet designs are available. The brakes are completely enclosed by cylindrical,
flange connected, housings. The housings may be cast aluminum, ductile cast iron,
or cast steel. The brake torque can be adjusted
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