ey are put on in two pieces held together by bolts; but in straight axle
engines the eccentrics are cast in a piece, and are secured on the shaft by
means of a key. The eccentric, when in two pieces, is retained at its
proper angle on the shaft by a pinching screw, which is provided with a jam
nut to prevent it from working loose. A piece is left out of the eccentric
in casting it to allow of the screw being inserted, and the void is
afterward filled by inserting a dovetailed piece of metal. Stephenson and
Hawthorn leave holes in their eccentrics on each side of the central arm,
and they apply pinching screws in each of these holes. The method of fixing
the eccentric to the shaft by a pinching screw is scarcely sufficiently
substantial; and cases are perpetually occurring, when this method of
attachment is adopted, of eccentrics shifting from their place. In the
modern engines the eccentrics are forged on the axles.

516. _Q._--How are the eccentric straps constructed?

_A._--The eccentric hoops are generally of wrought iron, as brass hoops are
found liable to break. When formed of malleable iron, one half of the strap
is forged with the rod, the other half being secured to it by bolts, nuts,
and jam nuts. Pieces of brass are, in some cases, pinned within the
malleable iron hoop; but it appears to be preferable to put brasses within
the hoop to encircle the eccentric, as in the case of any other bearing.
When the brass straps are used, the lugs have generally nuts on both sides,
so that the length of the eccentric rod may be adjusted by their means to
the proper length; but it is better for the lugs of the hoops to abut
against the necks of the screws, and, if any adjustment be necessary from
the wear of the straps, washers can be interposed. In some engines the
adjustment is effected by screwing the valve rod, and the cross head
through which it passes has a nut on either side of it, by which its
position upon the valve rod is determined.

517. _Q._--Will you describe the eccentric rod and valve levers?

_A._--In the engines in use before the introduction of the link motion, the
forks of the eccentric rod were of steel, and the length of the eccentric
rod was the distance between the centre of the crank axle and the centre of
the valve shaft; but in modern engines the use of the link motion is
universal. The valve lever in locomotives is usually longer than the
eccentric lever, to increase the travel of the valve, if