=17. Skips= are self-dumping cars used for hoisting material from
shafts or slopes. In a vertical shaft, they run in guide tracks; but
in a slope they have wheels and run on a track like a car. In the
anthracite region of Pennsylvania, skips are called =gunboats=.
As the skip is not detached from the hoisting rope, time is saved at
the top over that needed to unhook and hook the cars to the rope or to
remove and place the cars on the cage. But since dumping the material
into the skip and again on the surface produces considerable fine
material, skips, or gunboats, are seldom used for any material, such
as coal, that is often lessened in value by being broken. The skip, or
gunboat, shown in Fig. 14 is closed along the top _a_ and open at the
end _b_, which is cut at about the angle of the slope in which it is
to be used, so as to remain practically level during the hoist. It is
made of sheet iron, the bottom, sides, and top being stiffened by angle
or =T= irons, and the back stiffened and protected by 3-inch planks,
backed by 3" × 6" timbers. The wheels of a skip are fixed on the axles,
which run in journal boxes, thus insuring smoother running than is
obtained with loose wheels. The details of the journal bearings, as
shown in Fig. 15, consist of three castings, the bracket _a_, which is
bolted or riveted to the gunboat, a pivot casting _b_, and the bearing
proper _c_. The bearing _c_ rests on the axle and carries, by means of
trunnions _d_, the pivot casting _b_, on the top of which is placed a
rubber cushion _e_ to lessen the shocks between the casting and the
bracket.
[Illustration: FIG. 14]
[Illustration: FIG. 15]
=18. Method of Loading Skips.=--In Fig. 16, a skip _a_ is shown in
a slope standing immediately below a level where a car _b_ is ready
to have its load dumped into the skip. Instead of dumping the mine
car directly into the skip, a bin is frequently provided at the level
station, or landing, into which the mine cars are dumped and from which
the material is loaded into the skip through suitable chutes. The
use of such bins makes the hoisting of material largely independent
of the working conditions on the levels and the hoisting can be more
systematically and satisfactorily carried on.
[Illustration: FIG. 16]
[Illustration: FIG. 17]
If the material comes to the slope as shown in Fig. 17, it is necessary
to let down a bridge _a_, on which the car runs, in order to reach the
skip. After the car is dumped, the bridge is lifted out of the way into
the dotted position, so as to leave the slope unobstructed.
=19. Method of Dumping Skips.= To dump a skip at the surface, the
tracks are extended above the slope mouth, as shown in Figs. 18 and 19,
and are arranged so that the material may be dumped directly into a bin
or into cars as desired.
In the arrangement shown in Fig. 18, the front wheel of the skip
strikes a stop _a_ and, since the bail of the skip is pivoted far down
toward the lower end, as the rope continues to pull, the rear of the
skip is raised and the material is dumped. The objection to this method
is that if the rope is slightly overwound the skip is pulled off the
track and does not then right itself on the track when the rope is
released.
[Illustration: FIG. 18]
In the Lake Superior iron and copper region, many of the dumps are
built as shown in Fig. 19. In this dump, the rails of the main track
_a_ are curved as shown at _b_; a short distance back of the beginning
of this curve, another track _c_ begins outside the track _a_ and runs
in a straight line parallel to the inclination of the hoist. The track
_c_ is of a wider gauge than _a_, and the rear wheels of the skip have
a wider tread than the front, so that they will run on _c_ while the
front wheels take the curved track until they strike the stop _d_. The
rear of the skip will thus be raised and the material dumped. There are
but two tracks in the main part of the slope.
[Illustration: FIG. 19]
[Illustration: FIG. 20]
[Illustration: FIG. 21]
In the method illustrated in Fig. 20, the rear and front wheels have
the same tread, but the rear axle is longer than the front and has
rollers _a_ on each side. These strike the track _b_, and while the
front wheels follow the curved track _c_ these rollers run on the track
_b_ and thus raise the rear end of the skip.
=20. Skip Cage.=--Where a self-dumping skip is to be used in a vertical
or highly inclined shaft and it is desired to use safety catches, the
skip _a_ is mounted in a cage or frame _b_, Fig. 21, similar to the
self-dumping cage, Fig. 11. The skip being pivoted at _c_ one side
of the center, and resting on the frame of the cage, tends to remain
upright until it reaches the dump; but for safety it is sometimes
locked in place by the latch _d_, which hooks over the pin _e_. When
near the top, the roller _f_ on the end of the latch _d_ comes in
contact with a bar that depresses the roller and thus unhooks the
latch. The roller _g_ enters and travels along the guide rails _h_,
tipping the skip. There are two rollers _g_, one on either side of
the skip. The nose _i_ is temporarily caught on the roller _j_, thus
stopping the movement of the skip sidewise and away from the upright
guide.