=35=. A =brake= is a device by means of which the motion of a hoisting
drum may be retarded or stopped. This is accomplished by friction of
the brake against the circumference of the brake wheel. There are three
types of brakes, known as _block brakes_, _post brakes_, and _strap
brakes_.
[Illustration: FIG. 20]
=36. The Block Brake.=--The =block brake=, Fig. 20, consists of one or
more wooden blocks or shoes _b_ attached to a lever having a fulcrum at
_d_, and connected by a rod to the lever _c_. Block brakes are objected
to mainly because they throw a great load on the journals of the drum
when they are applied; they cannot be relied on when there is a heavy
load on the drum, and they require the application of great force to
the lever _c_ for a given braking power. They are, however, cheap and
easily applied to a drum, and the shoe is readily replaced when worn.
=37. The Post Brake.=--The =post brake=, Fig. 21, is composed
practically of two block brakes applied at two places on the drum
diametrically opposite each other, thus equalizing the pressure on the
journals. The blocks are generally somewhat longer than in the block
brake, or about one-quarter of the circumference of the drum on each
side. In Fig. 21, _a_ is the drum; _b_ are wooden brake blocks; _c_
are the posts which in the brake shown are of massive, built-up, steel
construction; _d_ are the fulcrums on the plates _e_, which plates
are adjustable by means of the nuts _f_; by means of these nuts, the
fulcrums may be brought closer together as the wooden blocks _b_ wear
away; _g_ is a tension rod generally furnished with a turnbuckle to
adjust its length as the wooden blocks wear away. Power is applied at
the end of the bent lever _h_, as shown by the arrow.
[Illustration: FIG. 21]
The stops _i_ are adjusted so that the blocks _b_ on each side are
equally distant from the drum when the brake is off. The fulcrums _d_
should be some distance below the drum and brake ring, for if they are
too near the drum it will be difficult to swing the lower end of the
wooden blocks far enough to clear the drum.
[Illustration: FIG. 22]
=38. Improved Post Brake.=--In order to have an equal clearance at
top and bottom, and to have a more powerful leverage than in the
ordinary post brake, the posts may be made movable at both top and
bottom, Fig. 22. The tops of the posts _a a′_ are moved, as in Fig. 21,
by the tension rod _b_ and the lever _c_, the latter being connected
by rod _d_ to lever _e_. This lever is pivoted at _f_ and motion is
transmitted to the fulcrums _j_ by the link _g_, the lever _h_, and
the tension rod _i_. The back post _a_ is supported by the uprights
_k_, which are pivoted at _l_ and swing backwards and forwards like a
parallel ruler. The front post _a′_ is supported by the single upright
_m_, pivoted at _n_. The setscrews _o_ regulate the motion of the
bottom of the posts so as to give equal clearance to the bottom and top
of the posts.
An objection to both the block and the post brake is the fact that, if
the drum surface to which the brake is applied is not perfectly round,
the resistance of the brake will not be uniform when applied while the
drum is in motion.
=39. The Strap Brake.=--A =strap brake= consists of a wrought-iron
band or strap that partly encircles the drum and is connected at its
free ends to levers with which the band may be tightened on the brake
wheel and the drum thus firmly held. The iron or steel band either lies
directly against the wooden lagging of the drum or on wooden blocks
bolted to the drum; or else it has bolted to it a lining of wooden
blocks that bear on the drum when the band is tightened.
The most efficient forms of strap brakes are those in which the strap
or straps are in contact with 270° or more of the circumference of the
drum. The greater the arc of contact, the more securely is the drum
held by the brake. A single strap is sometimes used, but this is only
satisfactory with small drums, say 8 feet or less in diameter; on large
drums two straps are generally used, each extending half way around
the drum. The levers for transmitting the power from the hand lever or
treadle to the brake strap are variously arranged. In some cases, the
force is multiplied by several short levers; in others, by one long
lever. The treadle or foot-lever, however, has been replaced almost
entirely by the hand lever.
[Illustration: FIG. 23]
=40.= The simplest form of strap brake, Fig. 23, consists of a single
strap _a_, with one end anchored at _b_ and the free end attached to
the brake lever _c_. This brake acts on the same principle as the block
brake and is open to the objection that it brings an undue load on the
journals, but it is more efficient and holds the drum more firmly under
a heavy load than a block brake.
[Illustration: (_a_) (_b_) (_c_)