297. The mechanical powers discussed in these lectures may be grouped
into two classes,—the first where ropes or chains are used, and the
second where ropes or chains are absent. Belonging to that class in
which ropes are not employed, we have the screw discussed in the
last lecture; and the lever discussed in Lecture VIII.; while among
those machines in which ropes or chains form an essential part of the
apparatus, the pulley and the wheel and axle hold a prominent place. We
have already examined several forms of the pulley, and we now proceed
to the not less important subject of the wheel and axle.
298. Where great resistances have to be overcome, but where the
distance through which the resistance must be urged is short, the
lever or the screw is generally found to be the most appropriate means
of increasing power. When, however, the resistance has to be moved a
considerable distance, the aid of the pulley, or the wheel and axle,
or sometimes of both combined, is called in. The wheel and axle is the
form of mechanical power which is generally used when the distance is
considerable through which a weight must be raised, or through which
some resistance must be overcome.
[Illustration: FIG. 46.]
299. The wheel and axle assumes very many forms corresponding to
the various purposes to which it is applied. The general form of
the arrangement will be understood from Fig. 46. It consists of an
iron axle B, mounted in bearings, so as to be capable of turning
freely; to this axle a rope is fastened, and at the extremity of the
rope is a weight D, which is gradually raised as the axle revolves.
Attached to the axle, and turning with it, is a wheel A with hooks in
its circumference, upon which lies a rope; one end of this rope is
attached to the circumference of the wheel, and the other supports a
weight E. This latter weight may be called the power, while the weight
D suspended from the axle is the load. When the power is sufficiently
large, E descends, making the wheel to revolve; the wheel causes the
axle to revolve, and thus the rope is wound up and the load D is raised.
300. When compared with the differential pulley as a means of raising
a weight, this arrangement appears rather bulky and otherwise
inconvenient, but, as we shall presently learn, it is a far more
economical means of applying energy. In its practical application,
moreover, the arrangement is simplified in various ways, two of which
may be mentioned.
301. The capstan is essentially a wheel and axle; the power is not in
this case applied by means of a rope, but by direct pressure on the
part of the men working it; nor is there actually a wheel employed, for
the pressure is applied to what would be the extremities of the spokes
of the wheel if a wheel existed.
302. In the ordinary winch, the power of the labourer is directly
applied to the handle which moves round in the circumference of a
circle.
303. There are innumerable other applications of the principle which
are constantly met with, and which can be easily understood with a
little attention. These we shall not stop to describe, but we pass on
at once to the important question of the relation between the power and
the load.