[Illustration: FIG. 39]
=42.= In hoisting, there is more or less danger of overwinding or
lifting the cage too far, and dashing it against the top of the
head-frame, or if the top is open the cage may be pulled entirely
over the top. =Detaching hooks= are intended to prevent this. Several
varieties of such hooks are made, which differ from each other only
in their smaller details. In all of them, detachment is effected by
passing the rope through a circular hole in an iron plate or through an
iron cylinder, the diameter of which is sufficient to allow the upper
portion of the hooks to pass through when passing upwards, but the
lower portion is made larger and so arranged that when this larger part
strikes the plate the upper portion is forced open and the hoisting
rope released. After the upper part has been thus opened, it is too
large to pass back through the opening and the plate and the cage is
therefore held suspended. Fig. 39 shows such a hook. It consists of two
outside fixed plates slightly narrower at the top than the diameter
of the hole in the disengaging plate _h_. Between the frame plates
_a_ are two inner plates _b_ that move about a strong pin _c_ passing
through both plates _a_ and _b_, but near the bottoms there are two
projections _d_ to prevent the hook from passing entirely through the
hole. The winding rope is attached to the top shackle _e_ and the cage
to the lower shackle _f_. When the two movable plates _b_ are closed as
tightly as possible at the top about the pin of the shackle _e_, they
are secured by a copper pin _g_. In case of overwinding, when the hook
passes into the hole of the disengaging plate _h_, the two projections
_k_ on plates _b_ are pressed inwards, shearing off the copper pin _g_
and allowing the plates _b_ to turn about the central bolt _c_, thus
releasing the shackle _e_. The plates _b_ are then in such a position
that the projections _l_ on them cannot pass down through the hole.
The cage then hangs by the hooks from the disengaging plate, and the
rope passes on. An objection raised against this hook is that, being
constructed of plates, there is considerable surface in contact between
the moving parts, and unless they are regularly taken apart and oiled,
there is danger of their rusting firmly together.
In England, detaching hooks are used quite commonly, and also in
certain parts of the Central Basin in the United States, but they have
not yet been generally adopted throughout the United States.
=43.= It is claimed by many that such devices inspire the engineer
with a misleading feeling of security; that they are more or less
complicated in construction and so need care, and destroy the
simplicity of the plant; that they may be the direct cause of accident
by introducing new elements of danger; that they add to the cost; and
that they are not thoroughly reliable. Again, it is held that the
surest prevention of overwinding is obtained by the employment of
a sober, reliable, and competent engineer, who is held personally
responsible for overwinding accidents; by having a good brake and an
engine thoroughly under the control of the engineer; by a reliable
method of indicating the position of the cage; by sufficient height
to head-sheaves to allow of considerable hoisting over and above that
necessary for landing.