=7.= Where flat ropes are used or where round ropes wind on a conical
drum, the length of rope wound or unwound is different for each turn
of the drum. With all the indicators thus far described, while the
speed with which the indicator moves is proportional to the speed at
which the drum and the drum shaft revolve, it is not proportional to
the speed of the rope when winding and unwinding on a conical drum
or on a flat rope reel. Fig. 6 (_a_) and (_b_) shows two views of a
compensating dial indicator. By means of the spiral form of sheave
_c_, the hand _d_ is made to move equal distances around the disk _e_
for equal distances of cage movement in the shaft. The rope _f_ passes
about the spiral sheave and one end is attached at the small end _g_
of the spiral, while the other end is fastened to the periphery of the
sheave _h_, which takes its motion from the drum shaft or crank-shaft
of the hoisting engine by means of the bevel gear _i_. Consequently,
while the sheave _h_ has a regular motion dependent directly on the
revolution of the hoisting drum, the pointer _d_ moves irregularly,
depending on the position of the spiral sheave _c_; that is, whether
a small or large diameter of the spiral is presented to the rope. The
rope _j_ carrying the counterweight _k_ is attached to a small circular
drum _l_ that is on the same shaft as the spiral sheave. The purpose of
this cord and counterweight is to keep the indicator line _f_ taut and
to bring the indicator back to position as the cord _f_ unwinds from
the sheave _h_.
=8.= In order that the pointer may not stand at exactly the same point
on the dial when the cage is at the top and at the bottom, and so that
the engineer may be able to distinguish between the top and the bottom
positions of the cage by the pointer, the ratio of the gearing is
usually increased by allowing one or two extra teeth on the worm-wheel.
In the example in Art. =5=, assume a ratio of 27: 1; that is, if a
worm-gear is used, the worm-wheel will have 27 teeth.
If the pitch of the teeth is ¾ inch, the circumference of the pitch
circle will be ¾ × 27 = 20.25 inches and the diameter 6.44 inches.
The pitch of the worm will, of course, be the same as that of the
wheel, and its diameter will be whatever is necessary to give
sufficient strength outside of the shaft, since it bears no relation to
the ratio of the gearing.