6. Diameter of Sheave.—The size of a sheave about which a rope bends is determined generally by the size of the rope to be used, as explained under Wire Ropes in Hoisting, Part 2; but, if the rope is simply to be supported in a straight line, the space available for setting the sheave and its cost and weight usually determine the size used. The minimum allowable diameter of sheave should not be used unless it is necessary to do so, for the larger the sheave the less will be the wear of the rope due to the bending, and the longer the life of the rope, but the cost of the sheave, which increases with the size, puts a limit in the other direction.
7. Rollers and Carrying Sheaves.—Wooden or iron rollers are sometimes used for rope carriers or guides, instead of light sheaves, when the rope has merely to be supported and there is no bending of the rope, excepting the slight amount due to the sagging between the rollers. The diameter of the rollers is of little importance in such cases so far as the rope is concerned. If they are for use on a slope to keep the rope from dragging on the ground, they must be small, because the cars must run over them, and mine cars are usually made low because of restricted headroom in the mine. Rollers and carrying sheaves are fully described and illustrated in Haulage.
If a hoisting rope changes its course from a straight line, even if the deflection is only a small amount, a roller is not advisable and a sheave should be used, if possible.
CAGES
CAGES FOR VERTICAL SHAFTS
8. A cage is a carriage used for hoisting mine cars and their contents, men, timber, etc., in both vertical and inclined shafts. Cages are built of wood strengthened with iron or steel, or entirely of iron or steel.
Fig. 6
9. The cage shown in [Fig. 6] is much used in the anthracite region of Pennsylvania. It is made largely of oak strengthened with iron and the size varies to suit the shaft, being sometimes as large as 6 feet wide by 12 feet long. The general construction of the cage is evident from the figure, but several appliances that should be common to all cages in some form or other require detailed explanation.
A covering a, called a bonnet, protects persons on the cage from objects falling down the shaft, and is required by law in some States. This bonnet is made of steel plate with flanges or angle irons to stiffen it, and is usually inclined. To prevent objects of moderate size from wedging between the edge of the bonnet and the shaft lining, the former is sometimes made shorter than the cage, so that a space of about a foot is left between its lower edge and the shaft lining. A short bonnet of this character does not, however, fully protect persons on the cage. The upper part of the bonnet is fastened to the upper cross-bar of the cage by two hinges and is held up by rods b that are attached to the bonnet and have sockets at their lower ends, which fit over pins bolted to the uprights of the cage. By raising the rods from the pins the bonnet can be lowered so that pipes or long timbers may be lowered on the cage.