Wooden guides are always rectangular in cross-section and in the United States are usually made of yellow pine or other long-grained wood that does not splinter easily; in some localities, oak or some of the other harder woods are used. There is no fixed size for cage guides, but 4" × 4", 6" × 8", 8" × 10", and 4¼" × 11" timbers are frequently used.
The guides are firmly fastened to the shaft buntons with lagscrews or with bolts countersunk into the guide so as to be clear of the shoes, and, to secure safety with speed in hoisting, the ends of the guides must be put together with joints that are not liable to displacement and that offer no projections to the shoes in passing. The buntons to which the guides are secured must be so firmly fastened that they cannot get out of place, and the guides must be set as nearly as possible in a straight line, because if they are crooked the cage is thrown back and forth as it travels along them and this not only increases the strain on the hoisting rope and engine, but sooner or later loosens and misplaces the guide. [Fig. 23] shows a plan of a cage with the bunton A, guides B, and cage shoes C in their normal positions.
LANDING FANS OR KEEPS
24. In order to take the strain off the hoisting rope while a cage or skip is being loaded or unloaded, a mechanism to support the cage is placed at the top and at any level of the mine where loading is done, excepting at the bottom level where all that is usually required are the cross-timbers for the cage to rest on. These supports have different names in various localities, being known as fans, keeps, cage rests, landing dogs, landing chairs, wings, etc. Their use increases the safety of caging.
25. A common form of keeps is shown in [Fig. 24]. The cage a rests on four square bars of iron b, one under each corner of the cage. These bars have an eye or hub at the lower end and are keyed to the shafts d, which rest in cast-steel boxes. The levers e and f, which are also keyed to the ends of the shafts d, are connected by a rod g. Chains h prevent the fans from moving too far under the cage. When the cage is to be lowered, it is first lifted clear of the fans and the lever e is moved into the dotted position, thus moving the fans b out of the way and permitting the cage to be lowered. The inside of the fans have no projections, and the operating mechanism is such that no harm would come if they were left in the shaft and a hoist were made, as the cage would open out the fans and pass through them without any trouble. If, however, the fans are not drawn back at all the headings in the shaft when the cage is lowered, great damage results when the cage strikes the projecting fans. To avoid the possibility of such an accident, fans have been devised that fall back out of line of the shaft as soon as the weight of the cage is removed from them.
Fig. 24
26. Hydrostatic Fans.—Most fans in use are built on the same principle as those just described, although the details of their construction may vary. An objection that can be raised against them is that, with large cages and heavy loads, the jar caused by letting the cage down on such a rigid support is very hard on the cage. All cages, particularly heavy ones, suffer much more wear from being landed too suddenly than from the strains of hoisting. For this reason, it is advisable to make the upper parts as light as compatible with strength and the side pieces stronger than needed for the actual strains to which they are subjected. Hydraulic fans, [Fig. 25], have successfully overcome this trouble. The cylinder shown is one of four on which the cage rests. The eye at the lower end fits on a bar by means of which the cylinders are moved backwards and forwards similar to the motion of the fans b, [Fig. 24]. In [Fig. 25 (a)], the cage is shown as about to rest on the jaw a. As the cage settles, it pushes the plunger b downwards, but this action is resisted by oil in the cylinder at c. At first, this resistance is very slight, because the V-shaped grooves d in the plunger, which are of considerable size at the end of the plunger, allow the oil to escape freely into the upper chamber e. These grooves, however, taper down to nothing, so that the flow of oil through them decreases until none can pass except by leakage around the plunger. This allows the plunger with its load to settle slowly to the bottom, as shown in [Fig. 25 (b)].
Fig. 25