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.
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.