[Illustration: FIG. 45.]
295. One of the most useful applications of the screw is met with
in the common bolt and nut, shown in Fig. 45. It consists of a
wrought iron rod with a head at one end and a screw on the other, upon
which the nut works. Bolts in many different sizes and forms represent
the stitches by which machines and frames are most readily united.
There are several reasons why the bolt is so convenient. It draws the
parts into close contact with tremendous force; it is itself so strong
that the parts united practically form one piece. It can be adjusted
quickly, and removed as readily. The same bolt by the use of washers
can be applied to pieces of very different sizes. No skilled hand is
required to use the simple tool that turns the nut. Adding to this that
bolts are cheap and durable, we shall easily understand why they are so
extensively used.
296. We must remark in conclusion that the bolt owes its utility to
friction; screws of this kind do not overhaul, hence when the nut is
screwed home it does not recoil. If it were not that more than half
the power applied to a screw is consumed in friction, the bolt and the
nut would either be rendered useless, or at least would require to be
furnished with some complicated apparatus for preventing the motion of
the nut.
LECTURE X.
_THE WHEEL AND AXLE._
Introduction.—Experiments upon the Wheel and
Axle.—Friction upon the Axle.—The Wheel
and Barrel.—The Wheel and Pinion.—The
Crane.—Conclusion.