Block brakes are usually run dry, but in band brakes and post brakes
with ample surfaces and proper leverage the wood may be occasionally
slightly oiled with black oil, which greatly adds to the durability of
the blocks without unduly lessening the power of the brake.
=41.= A two-strap brake is shown in Fig. 24. One end of each strap _a_,
_b_ is fastened to the pedestal _c_ by either of the methods shown in
Fig. 24 (_a_), (_b_), and (_c_). In the method shown in Fig. 24 (_a_)
and (_b_), the forgings _d_, _d′_, drawn out to the form of bolts, are
riveted to the ends of the straps and passed through a casting _c_ that
is secured to the foundation. The object in giving one bolt to one
strap and two bolts to the other strap is to allow the straps to pass
each other and yet have their lines of action intersect. The bolts are
fastened to _c_ by four nuts on each bolt, i. e., two principal nuts
and two locknuts. This gives a means of adjustment in the length of the
strap to take up the wear.
A second method of securing or anchoring the back ends of the straps is
shown at (_c_). In this case, a wrought-iron angular piece is riveted
to each strap, and these pieces are passed over the bolt _e_ that takes
the place of the casting of the former arrangement. Nuts are used, as
shown, to adjust the straps for wear. The bolt should be short and
stiff, so as to be well able to stand up to its work when the drum is
moving or tending to move in the direction shown by the arrow.
When the brake is applied, the friction between the brake strap and
the circumference of the brake wheel produces a great strain on the
pedestal _c_, which must be securely anchored.
The front ends of the straps are worked into eyes, as shown at _f_,
and by these eyes and suitable pins passing through them the ends are
fastened to the brake lever _g_. This lever is supported on and rotates
about a pin _h_, so that when the braking force is applied at _i_, in
the direction of the arrow, the brake lever rotates, pulling down on
strap _a_ and up on strap _b_; and, if the straps are held firmly at
the back end, the more force that is applied at _i_ the tighter will
the drum be gripped by _a_ and _b_.
The ends of the straps should be brought in as close to the drum as is
practicable, both front and back, so as to give the greatest amount of
contact between the drum and the straps and to get the best effect from
the force applied. The springs _j_ are used with straps that are not
stiff enough to clear the drum when the brake is released.
=42.= The rotation of the drum may assist or retard the action of the
lever in applying the drop brake. For instance, if, in Fig. 23, the
drum revolves in the direction indicated by the arrow, the pull of
the drum at the brake strap is in the same direction as the pull of
the lever when applying the brake and the action of the lever is then
assisted by the motion of the drum. On the other hand, if the drum
is revolving in the opposite direction, it opposes the action of the
lever and a greater force must be applied to the lever to overcome
this opposing pull of the drum. Hence, in the case of strap brakes, if
possible, that end should be anchored which will cause the revolution
of the drum to assist the lever in applying the brake and throw the
strain on the anchor bolt instead of on the lever.
[Illustration: FIG. 25]
=43.= If a brake is required to work with the drum running in either
direction, there are several ways of bringing the strain due to the
load on the anchorage in whichever way the drum runs. One of the
simplest of these is shown in Fig. 25, where _a_ is a drum with a strap
brake _b_ embracing nearly the entire circumference; _c_ is a lever
bar that is attached to the ends of the brake strap by pins _d_ and
_e_, which work in the slots _f_ in the iron anchor plates _g_. One
anchor plate is on each side of the lever, and both are bolted to the
foundation. If the band is kept of the proper length, then, no matter
which way the drum is turning, the pull of the drum will come on the
anchorage, and the pull on the lever need be only sufficient to take
up the slack end of the band. To illustrate: If the drum is turning in
the direction indicated by the arrow, the pin _e_ holding the lower end
of the band will be on the bottom of its slot and the pin _d_ will be
free in its slot and engaged in tightening the slack end of the band
through the motion of the lever _c_. Were the drum running the other
way, the pin _d_ connected with the upper half of the band would move
to the upper end of its slot and take the main load, while the pin _e_
at the lower end of the band would only have to take up the slack.
The outer, or long, end of the lever moves downwards in all cases to
tighten the band. Provision must be made to lift the band clear of the
drum when slack, but no anchorage other than at _g_ should be attempted.
[Illustration: FIG. 26]
=44. The Differential Brake.=--The differential brake has both ends
of the brake strap attached to short lever arms operated by the brake
lever, but these arms are of different lengths and are so arranged that
as the longer arm tightens the brake strap the shorter arm yields and
loosens the strap. The tightening, however, is more than the loosening
or yielding and, as a result, the brake band is tightened about the
brake wheel. The form of the lever arm is immaterial so long as the
differential principle is retained, that is, that the shorter arm
yields when the longer pulls, when the brake is thrown into action.
This principle is illustrated in Fig. 26. In this brake, no provision
is made for anchoring either end of the brake strap, but the entire
load is thrown on the lever arms _a_ and _b_. These lever arms are
connected with the arm _c_, which revolves on the same shaft _d_ and
is operated by the reach rod _e_. The revolution of the drum is thus
resisted by the shaft _d_.
This brake is self-acting when the drum revolves so as to pull on the
shorter arm, as indicated by the arrows; that is, the motion of the
drum helps to set the brake when the latter is once applied. When,
however, the drum revolves in the opposite direction, the action of
the brake is opposed, instead of being assisted, by the motion of the
drum. As a consequence, this particular form of brake is not adapted to
hoisting drums that revolve in opposite directions at each alternate
hoist. Differential brakes are not generally used.
[Illustration: FIG. 27]
=45. Power for Brakes.=--For small drums and light loads, the brakes
are usually applied by hand power through suitable lever connections.
The force that a man can exert can be multiplied indefinitely by
levers and combinations of levers; but while the force is multiplied,
the distance through which it can act is divided in the same ratio. A
certain amount of motion is required to free the brake band from the
drum, when the brake is off; this, then, limits the leverage that a
man can use. Suppose, for instance, that with a strap brake the band
moves from the drum ½ inch, thus increasing the diameter 1 inch, or the
circumference about 3 inches. Then, supposing that a man can exert his
force to advantage through 3 feet, or 36 inches, the available leverage
is ³⁶/₃ = 12. That is, if a man can pull 50 pounds on his hand lever,
he can exert 50 × 12 = 600 pounds circumferentially on the brake band,
with simple levers. If any form of differential levers is used, the
ratio by which the force applied at the hand lever can be increased
will be considerably larger. A diagram will explain this more clearly.
=46.= In Fig. 27, _a_ is the hand lever, with a fulcrum at _b_ and a
pin at _c_ by which it takes hold of a reach rod or connection _d_.
This rod is connected to the end _h_ of the brake lever _e_, which is
connected by pins at _f_, _g_ to the brake bands. If the leverage of
the hand lever _a_ is made 6 to 1, that is, if
_ab_ 6
----- = --- ,
_cb_ 1
and a force of 50 pounds is applied at _a_, a pull of 300 pounds will
be exerted at the pin _c_ and, consequently, along the rod _d_ to the
end of the brake lever _e_. Then, if the brake lever is made with a
ratio of 4 to 1, that is, if
_eh_ 4 _eh_
----- = --- = ---- ,
_eg_ 1 _ef_
a pull of 300 pounds × 4 = 1,200 pounds will be exerted at the pin _f_
or _g_. This total pull must be divided equally between the arms _eg_
and _ef_, giving 600 pounds pull on each. According to the principle of
the lever, the distances through which these forces act are inversely
proportional to the forces acting. It is assumed that the brakeman can
exert the force of 50 pounds through 36 inches; if this is the motion
of the end of the hand lever _a_, one-sixth of this, or 6 inches, will
be the motion at _c_ and, therefore, at _h_; one-fourth of 6 inches or
1½ inches will be the motion at _f_ and _g_; that is, _f_ will increase
its half of the brake band 1½ inches in circumference, and _g_ will do
likewise with its half, making the total circumference 3 inches more,
or the diameter 1 inch more, and thereby moving the band away from the
drum ½ inch radially. The levers are all shown in mid-position to make
the figure more simple, but the relative leverages remain the same at
all points in the motion.
This is an example of simple levers, but the force applied at the hand
lever may be increased in a much greater ratio by the use of a device
known as a _differential lever_.
[Illustration: FIG. 28]
=47. The Differential Lever.=--The principle of the operation of the
=differential lever= with which a constantly increasing force can
be applied to the brake strap is illustrated in Fig. 28. Let _a o_
represent a straight lever whose fulcrum is at _o_; and let the reach
rod be attached at _e_. In this position, if
_a o_ 6
------ = --- ,
_e o_ 1
the effective lever is 6 to 1. If, now, the lever is moved through
30° to the position _b o_, the force applied at _a_ moves through the
distance _a b_, and the reach rod through the horizontal distance _k
f_, so that the effective leverage is increased a small amount _e k_
and the ratio of the arms becomes
_a o_
------- .
_k o_
When the lever is moved another 30° to the position _c o_, the reach
rod moves a distance _i g_, which is less than _k f_, so that the
effective leverage is increased by the amount _k l_ and the ratio of
the arms becomes
_a o_
------ .
_l o_
Again, moving the lever 30° more to the position _d o_, the reach rod
moves through the still shorter distance _j h_, which is less than
_i g_, and the effective leverage becomes very great. It is evident
from this that the farther the lever is moved toward _d_ the greater
becomes the effective leverage. In practice, it would be impossible
to move the lever through the entire quadrant to advantage, and there
would also be more movement of the reach rod at the beginning of the
stroke and less at the end than is needed to produce the desired effect.
[Illustration: FIG. 29]
From the principle just given, it is plain that, if _p o_, Fig. 28,
represents a brake lever with the reach rod attached at _q_, a smaller
pull will be exerted on the brake band if the lever is moved to the
position _b o_ than would be exerted if a lever were moved through the
same angle from _b o_ to _d o_. The movement from _p o_ to _b o_ is a
convenient and easy one for the engineer to make, while the movement
from _b o_ to _d o_ is inconvenient. To overcome the inconvenience and
still to obtain the advantage of this latter movement, the differential
lever shown in Fig. 29 is used. By means of an arm placed on the lever,
the point of attaching the reach rod is at _l_ instead of _p_; hence,
when the handle _r b_ is moved to the position _s b_, the point _l_
moves to _m_, thus securing a greater and gradually increasing pull
with the easier movement of the handle.
A differential lever may be advantageously used in connection with any
band or post brake and on a drum running in either direction. Such
levers are considered by many preferable to the differential brake.
=48. Power Brakes.=--Large drums and heavily loaded drums cannot be
controlled by hand-power brakes, and in such a case some other form of
power, such as steam, compressed air, or water, must be used.
[Illustration: FIG. 30]
Fig. 30 shows, in outline, how such power is applied. The movements
of the hand lever _A_, instead of being directly communicated to the
lever operating the brake, merely control the valve _v_ connected
with the cylinder _a_. By means of this valve, steam, compressed air,
or water is admitted to either end of the cylinder and this moves
the piston in the direction necessary to apply or release the brake.
There are a number of varieties of such power brakes, differing in
structural details, but the action of all is essentially the same.
With steam or air power, the brake would be applied with its full
force almost instantaneously, thus subjecting the various parts of the
mechanism to very severe and objectionable strains, unless the valves
were modified so as to regulate the admission of the steam or air. One
method of controlling this action is the use of a valve that requires
a long travel to give it a full opening. Such a valve can be opened a
little, so as to allow the steam to leak through and thereby increase
the pressure in the cylinder gradually. As the motion is difficult to
regulate, a better method is by means of a floating valve, described in
_Hoisting_, Part 1.
=49. Crank Brake.=--In addition to the brake applied to the drum and
intended for use mainly in emergencies, many hoisting engines are also
fitted with a strap brake applied to the crank-disk. In some states,
crank-brakes are required by law. In order to give a large bearing
surface, the crank-disk is made very large.
HOISTING
(PART 4)