=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.
[Illustration: 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_).
[Illustration: (_a_) (_b_)