=2. Column Indicators.=--A very simple indicator, and one that was
formerly very commonly used, is made by inserting a pin into the
center of the end of the drum shaft and using this as a miniature
drum on which to wind and unwind a chain or cord, which corresponds
to the hoisting rope as the pin corresponds to the drum. This chain
or cord is led over a pulley placed at the top of a pair of guides,
representing the shaft, and carries at its end a weight, pointer, or
gong, representing the cage or car, as shown in Fig. 1.
[Illustration: FIG. 1]
[Illustration: FIG. 2] The different landings in the shaft are marked
on the guide; and as the pointer or gong rises and falls it indicates
the position of the cage in the shaft. If a gong is used, pointer
also may be added and the gong so arranged that it will ring at a
point some distance before the landing is reached and thus attract the
engineer’s attention. Indicators of this kind, though cheap and easily
constructed, are not reliable, for the cord and chain may stretch or
they may overlap in winding on the pin, or may bind in the pulley and
thus indicate a wrong position of the cage.
=3.= An indicator should have a positive motion and be driven by
gearing or by link belts. Fig. 2 shows a =column indicator= that
consists of a screw _a_ working inside of a slotted pipe _b_, which
may be of any length necessary. This screw is revolved by means of
the gears _c_, which are rotated by the sprocket wheel _d_. A nut _e_
travels up and down the screw _a_ and the pointer _f_ attached to the
nut indicates the position of the cage in the shaft. The pipe standard
_b_ is usually painted a dead black and the different levels may be
marked on it with chalk or white paint. Chalk marks are not safe, as
they may be tampered with and the engineer thus misled.
[Illustration: FIG. 3]
The pointer _a_, Fig. 3, is moved by the rotation of the screw shaft
_b_, which is revolved by the bevel gears _c_ and _d_. This indicator
also registers the number of hoists by means of the dials _e_, for
at each hoist the lower end of the pointer a engages a ratchet wheel
behind the two dial faces shown and thus registers on the dial.
[Illustration: FIG. 4]
=4. Dial Indicators.=--Fig. 4 shows a positive-motion indicator that is
operated as follows: A worm _a_ on the drum shaft _b_ engages with the
worm-wheel _c_ on the small shaft _d_ that is supported by the bearings
_e_. The pointer _f_ is rigidly attached to the shaft _d_ and revolves
in front of the properly marked dial _g_.
=5.= Fig. 5 shows a =dial indicator= attached to drum hoists where the
speed of rope is constant for each revolution. The wheel _a_ of this
indicator may be a worm-wheel working in a worm on the drum shaft, as
described in connection with the indicator shown in Fig. 4, or it may
be a sprocket wheel driven by a link belt from a sprocket wheel on a
drum, or it may be a gear-wheel driven directly from another gear-wheel
on the drum. The gear-wheels _b_ revolve a vertical shaft _c_ fitted
at the upper end with a worm _d_ that drives the worm-wheel _e_ placed
on the end of the pointer spindle. The different levels from which
hoisting is to be done may be painted on the dial, or better, they may
be placed on movable targets that are clamped to the dial and can thus
be moved as occasion requires.
[Illustration: FIG. 5]
EXAMPLE.--An indicator is desired for a shaft 800
feet deep at which the drum of the hoisting engine to be
used is 10 feet in diameter; what ratio of gearing must be
used so that the pointer will make one revolution during
the hoist?
SOLUTION.--The circumference of the drum is 31.42
ft. (π_D_ = 10 × 3.1416 = 31.416 ft.); hence, the
revolutions per hoist are 800 ÷ 31.42 = 25.46 revolutions.
Then, if the pointer is to make one revolution per hoist,
the ratio of the gearing will be 25.46 to 1. Ans.
=6. Special Indicators.=--One fault of nearly all indicators is that
they give a regular movement throughout the winding, and the space over
which the pointer travels is too small to enable the engineer to land
the cage accurately. Indicators have been made with a differential
motion to the pointer, the motion being greater at the time of landing
and less during the middle of the hoist. They are also made with two
pointers, one operating like the dial indicator above described and the
other remaining stationary during all the hoist but the last few feet,
when it moves around its circle.
[Illustration: (_a_) (_b_)