down to 50 percent of the maximum rating. In some cases, such adjustment may
involve replacement of the coil springs. Disc brakes are provided with manual
release levers and indicators of brake friction disc wear.
Disc brakes are available as separate units or may be integral with
electric motors. The shaft connections usually are made with keyed shaft-sleeve
fits. Weathertight and "washdown" designs are available for outdoor
Caliper Disc Brakes. As used on cranes, caliper disc brakes apply their
braking force at the periphery of a large disc or flange (approximately same
diameter as the wire rope drum). The flange rotates through a notch in the
housing of the brake assembly which presses opposed friction pads to both sides of
the flange. The clamping motion of the pads may be double-acting (when the
opposed pads are mounted on two individual pistons) or single-acting (when an
advancing piston acts on one side of the flange and the housing wall on the
other). For double-acting pistons, the housing is fixed and the same force is
applied to the opposed pistons; for single-acting pistons, the housing is free to
slide and equalize the force of the piston.
The caliper disc brakes are spring-applied by packs of Belleville springs
and released by a DC coil, AC pancake motor and linkage, or hydraulic pressure. A
single brake housing may carry two, four, or six opposed clamping pads, and
several housings may be mounted on the hoist foundation to clamp the brake flange.
Screw type manual release mechanisms are available.
Brake Flanges. Flanges of caliper disc brakes should be demountable (two
piece) bolted steel rings or permanent drum components with replaceable wear
segments. The contact surfaces for the brake pads must be kept clean, either by
periodic application of the brakes when the drum is rotating or by continuous
light scrubbing with a non-abrasive material.
Band Brakes. Band (or strap) brakes are available as integral components
diameter of the hydraulic motor housing and are well suited for hoist drives. The
brakes are spring-applied and hydraulically released by means of servo-valves
integrated into the electrical control system.
Band brake design is sensitive to the details of the activating lever and
the band end connections. The "simple" brake is considered uni-directional
being much more effective in one direction than in the other. The "de-energizing"
brake is equally effective in both directions, although it requires a larger
activating force than the simple brake in its active direction. The "energizing"
brake is uni-directional and develops a large braking force with a light
activating force; in fact, it can easily self-lock if the original lining is
replaced with a material that has a slightly higher friction coefficient. Brake
activation may be pneumatic, electric, or hydraulic. Custom designed band brakes
should be carefully analyzed to ensure smooth and stable braking force.
Clutches. Clutches are disc brakes, as described above in paragraph
4.4.2, except that the housing is mounted on one of the rotating shafts. Clutches
are used on built-up hoist and travel drives to couple the micro-drive to the main
drive. In this application, the clutch is de-energized (spring-released) when the
drive is operating in its normal mode, and electrically-applied (against the