=10.= The outer part, or =shell=, of a drum _a_, Fig. 7, is supported
on rims _b_, and these rims are connected by arms or spiders _c_ with
the hubs _d_. The brake rings _e_ are for the band brakes, of which
there may be one or two. The part _a_ may be lagged with strips of wood
bolted to the rims _b_, the heads of the bolts being countersunk. Fig.
8 shows the detailed dimensions of a drum 8 feet in diameter having a
4-foot face designed to carry heavy loads and a large amount of rope.
The shell is of boiler plate and the spiders of cast-steel.
[Illustration: FIG. 7]
=11.= The shell may be cast in one piece for small drums or built up
in sections for large drums, as in Figs. 7 and 8. The shell may have a
smooth surface, Fig. 8, or it may have grooves, Fig. 7, for the rope
to lie in as it is wound on the drum. On an iron drum without grooves,
the rope will chafe sidewise; and furthermore if the rope winds on a
hard flat surface it bears here and there on a single wire and tends to
flatten, causing internal wear between the wires; while, in the case
of a rope winding in a groove, it is supported on about one-quarter
of its circumference, bringing many more wires to bear on the drum
and dividing the pressure between them. A wooden-lagged drum causes
less wear on a rope than an ungrooved iron-shell drum, as grooves are
gradually worn in the lagging, but is not so good as a grooved iron
drum. It is not good practice to allow a rope to wind on itself, and
the drum should be long enough to take the full length of the rope
required for the hoist. At least two turns of the rope should be on
the drum when the load is at the bottom, as the friction between the
rope and drum thus greatly lessens the strain coming on the rope at
the point where it is fastened to the drum. Allowance for two or three
additional turns of the rope should also be made so that the cage may
be hoisted above the landing.
[Illustration: FIG. 8]
The shell usually has a flange at each end, as shown in Figs. 7 and 8,
but it may have a flange at one end only, or may be without flanges
entirely. If, however, the flanges are not used, the drum must be extra
long to prevent the rope running off the end. If the drum is very long,
a third spider is added midway between the other two to stiffen it
against collapse.
EXAMPLE.--Find the length of a drum 6 feet 3 inches in
diameter necessary to hold 1,000 feet of 1¼-inch wire-rope.
SOLUTION.--The diameter from center to center of the rope
when wound on the drum is 6 ft. 3 in. plus 1¼ in., or 6
ft. 4¼ in., which is equal to 19.96 ft. (approximately
20 ft.) of circumference. Then, to wind 1,000 ft. will
require ¹,⁰⁰⁰/₂₀ = 50 turns on the drum. Allowing two
turns of the rope to protect the fastening and three
turns in case of overwinding, gives fifty-five turns to
be allowed for on the drum. If the drum is of iron with
grooves turned in it, ¼ in. must be left between adjacent
parts of the rope, or 1½ in. from the center of one turn
to the center of the next. Then, 55 × 1½ = 82½ in. plus ¾
in. at each end = 84 in., or 7 ft. for the length of the
drum between the flanges. Ans.
If the drum has wooden lagging, clearance need not be
allowed between two adjacent coils of rope, as in this
case the rope winds against itself and so takes up only 1¼
in. It will then be 55 × 1¼ in. = 68¾ in., or 5 ft. 8¾ in.
long (say 5 ft. 9 in.). Ans.