Hoisting Appliances

by International Correspondence Schools

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

If now the cage is lifted and the weight thus removed from the jaw a, the spring g pushes the plunger b outwards and allows the oil to run from e back into c.

27. Pneumatic Fans.—A pneumatic fan, shown in section in [Fig. 26], is one in which the shock of the landing is partially relieved by a cushion of compressed air. The fan is keyed at the bottom to the shaft a that rotates it, as in [Fig. 24]. The cylinder b contains the plunger c, which is kept at the top limit of its motion by the spring d. When the cage lands in the jaw e, the plunger descends, compressing the air in the cylinder b. The air escapes slowly through the ¹/₁₆-inch hole f, thus allowing the cage to settle into place with very little shock. These fans should be made of wrought-iron or cast-steel so as not to be easily broken.

Fig. 26

28. Cage Chairs.—In the case of a cage required to stop at a large number of levels, it is expensive to provide fans at each level, and to obviate this a strong steel bar or dog may be used under each corner of the cage, all four bars being connected to a lever on the cage, by means of which they can be thrown out at will so as to rest on supports provided at each level. [Fig. 27] shows Gray’s patent cage chair, which operates on this principle. The sliding bars a are connected by the cross-bars b, which are pivoted at the center and operated by the bar c through the links d. By moving the lever e into the position shown, the bars a are thrown out so as to rest in notches or on wall plates in the shaft. The springs f, through the cross-bars b, force the sliding bars a back under the cage when the lever e is released.