[Illustration: FIG. 33]
Fig. 33 shows the construction of the ordinary timber gallows frame
used at many ore mines.
Fig. 34 shows a steel =A= frame, of which the principal dimensions are
as follows: height to sheave center 48 feet; base 33 feet 10 inches by
56 feet. Legs _a_ and _b_ are made of laced channels, as are also the
central upright posts and cross-braces. The forward inclined legs are
made of =I= beams. The weight of the frame is 98,000 pounds without the
sheaves. The advantages claimed for this type of design are that it
gives a very strongly braced frame while using a minimum of material.
Also, in cases of overwinding, the cage goes over the top of the frame
without injury to the frame, and should men be overwound they would
fall only the height of the frame instead of being crushed against the
top.
=35. Square Type Without Inclined Brace.=--Fig. 35 shows a steel frame
in which the tendency to be overturned by the pull of the rope is
resisted by a nearly vertical tension leg as explained in =Art. 32=.
Each leg of the frame is built of channel bars connected by lattice
bracing, as shown, and the legs are stiffened by horizontal channel
cross-bars similarly braced and also by diagonal tie-rods, provided
with turnbuckles.
[Illustration: FIG. 35]
[Illustration: FIG. 34]
Springs are sometimes placed under the journals of the head-sheaves
to lessen the strain on the rope while starting the load; the 15-foot
head-sheaves of the Robinson deep mine at Johannesburg have locomotive
springs under the journal boxes, the actual load on each spring due to
the weight of the sheave, rope, skip, and rock being equal to about
20,000 pounds; it was estimated that the sheave would thus be lowered
by the load on it, about 3 inches, which would be equal to an action
of a spring giving motion of 6 inches at the cage. Springs can often
be used both on the rope and under the sheave in the same plant to
advantage.
[Illustration: FIG. 36]
=36. Square Type With Inclined Brace.=--Fig. 36 shows a very
substantial frame with square tower and inclined brace.
[Illustration: FIG. 37]
Its principal dimensions are as follows: height to sheave center 59
feet 6 inches; base of tower 15 feet 8 inches by 14 feet; distance of
bottom of inclined leg from vertical post 48 feet. Each end post _a_ is
composed of two channels, double-latticed. The horizontal members _b_
are =I= beams and each inclined member _c_ is made up of two angles.
The inclined leg _d_ is trussed as shown and built of channel and
angle beams, the main member being made of two channels, the incline
and base members of the truss being made up of two angles, and the
short vertical member of two channels. The center post of the tower
is similar to the end posts, except that the uprights are =I= beams
instead of channels. The frame is designed for a static weight of
16,000 pounds and for a maximum strain on the cable of 32,000 pounds.
Fig. 37 shows a frame of similar form, but in which the landing
platform is placed at a height above the surface, so that the cars
hoisted can be run off on a trestle and thus be delivered at the top of
a car, breaker, tipple, or ore house. Its principal dimensions are as
follows: height to sheave center 75 feet; base 40 feet 11¾ inches by 21
feet 8½ inches. The leg _a_ is made of two angles. The bracing leg _b_
is built of two angles. The diagonal braces _c_ are single angles. The
horizontal braces are angles or channels of various sizes depending on
the stresses.
=37.= The =head-sheave= is supported directly on top of the main frame,
as shown in Figs. 32, 34, 36, and 37, or a small superstructure _a_ is
built on top of the main frame, as shown in Fig. 38, so that the base
of the sheave journals is perpendicular to the resultant pull on the
frame, that is, to the theoretical direction of the inclined leg of the
frame if one is used.
=38.= Timber frames are usually built by the mining company from its
own designs. Steel frames are generally built by the structural steel
companies from detailed plans and designs furnished by the mining
company, or from a skeleton diagram furnished by the mining company,
giving the loads on the rope and the general conditions about the shaft
to which the frame must conform, the frame being then designed and
erected in detail by the steel company.
=39. Enclosing Head-Frames.=--Head-frames are sometimes wholly or
partially enclosed to protect them and the men from the weather. A
covering of boards is warmest. All woodwork should be painted with
fireproof paint and ample means for extinguishing fire should be
provided. A covering of corrugated sheet iron well painted on both
sides to prevent rusting is often used instead of wood and lessens the
danger of fire, but is not as warm a covering as wood.
[Illustration: FIG. 38]
=40.= In many states, it is required by law that the top of the shaft
be protected by a fence or by gates to prevent persons falling down the
shaft. This protection is secured at the sides of head-frames by extra
timbers or beams forming part of the frame, or by means of a fence
placed near the sides of the frame. The ends of the shaft are protected
by a bar placed across uprights, by gates that swing like an ordinary
door, or more generally by vertical sliding gates that are raised by
the cage when it comes to the surface and drop into place when the cage
descends. Similar gates, doors, or bars should be used at all landings
below the surface.