The _eccentric_ is a particular form of crank, being a crank in which
the crank-pin is large enough to embrace the crank-shaft. In the
eccentric what corresponds to the crank-pin is called the sheave or
pulley. The advantage which an eccentric possesses over a crank is that
the shaft does not require to be divided at the point where the
eccentric is put on. The crank, however, has this advantage over the
eccentric, namely, that it can be used for converting circular into
reciprocating motion, or _vice versa_, while the eccentric can only be
used for converting circular into reciprocating motion. This is owing to
the great leverage at which the friction of the eccentric acts.
The chief application of the eccentric is in the steam-engine, where it
is used for working the valve gear.
To permit of the sheave being placed on the shaft without going over the
end (which could not be done at all in the case of a cranked axle, and
would be a troublesome operation in most cases) it is generally made in
two pieces, as shown in fig. 40, which represents one of the eccentrics
of a locomotive. The two parts of the sheave are connected by two cotter
bolts. The part which embraces the sheave is called the eccentric strap,
and corresponds to, and is, in fact, a connecting rod end: the rod
proceeding from this is called the eccentric rod.
The distance from the centre of the sheave to the centre of the shaft is
called the _radius_ or _eccentricity_ of the eccentric. The _throw_ is
twice the eccentricity.
The sheave is generally made of cast iron. The strap may be of brass,
cast iron, or wrought iron; when the strap is made of wrought iron it is
commonly lined with brass.
[Illustration: FIG. 40.]
EXERCISE 40: _Locomotive Eccentric._--In fig. 40 D E is the
sheave, F H the strap, and K the eccentric rod. The sheave and
strap are made of cast iron, and the eccentric rod is made of
wrought iron. (_a_) is a vertical cross section through the
oil-box of the strap; (_b_) is a plan of the end of the eccentric
rod and part of the strap. All the nuts are locked by means of
cotters. Draw first the elevation, partly in section as shown.
Next draw two end elevations, one looking each way. Afterwards
draw a horizontal section through the centre, and also a plan.
Scale 4 inches to a foot.