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Full text of "Modern Mechanical Engineering Vol-I"

FITTING AND  ERECTING

251

while they definitely fix a line, the cylinder is free to expand along the dowels
as it gets hot. These dowels are usually placed about midway along the
cylinder length, and the expansion thus takes place in both directions from
this line. In other cases, the dowels are fixed at the exhaust end of the
cylinder, and the whole expansion thus takes place in the one direction
viz. towards the H.P. end. It is necessary, of course, to have a fixedrpoint
on the cross transverse datum line, so as to control definitely the direction
of the sideways expansion. This fixed point is formed by vertical keys at
both the H.P. and L.P. ends of the cylinder, set half in the bedplate and
half in projections from the cylinder on the vertical centre line. These
keys enable the cylinder to breathe vertically, but keep the cylinder central
sideways under all conditions, and thus compel the expansion (lateral) to
take place equally on both sides. The dowels should be made an easy tap-
ping fit, and the holes through the
cylinder feet, through which the
holding-down bolts pass, should be
at least J in. bigger in diameter than
the bolt, so as to allow the cylinder
to expand and move laterally when
heated. The bolts, therefore, have
to be of special construction; they are
known as collar or shoulder bolts,
and are shown, together with the
corresponding type of stud, in the
accompanying sketches. A special
washer is always used under the
head of the bolt or under the nut
of the stud, and it will be seen that

when the bolt is tightened hard down on the shoulder, and the length from
shoulder to under side of head is just correct, that the cylinder, while de-
finitely held down, can move or expand sideways as required. In order to
get the exact length of the bolt, from shoulder to head, the bolt is tightened
down with the collar in position, and the amount of slack between the collar
and the head is then measured with feeler gauges, and the length from
shoulder to head is then reduced by the figure obtained with the feelers,
with the exception of two thousandths, which is left on in order to provide
a very small clearance between the bolt head and washer, and thus allow the
cylinder to expand.

Before jointing up any steam-pipe,-or the valve chest to the cylinder
bottom half, they should be carefully examined for any loose material, e.g.
nuts, pieces of steel, borings, &c., that may have lodged in the steam passages;
in the case of steam-pipes it is very advisable to either draw a heavy chain
through them repeatedly, or to tap them all over the external surface with
a heavy hand hammer, or to do both, in order to loosen any scale or rust
that may have formed inside the pipes, due to the " weathering of the
hard skin on the inside of the pipes. In particular, the steam-nozzles should

Fig. 6.—Holding-down Shouldered Bolt to allow for
Expansion and Contraction of Cylinder Feet relative
to Bedplate
x, Bedplate. 2, Cylinder feet. 3, Bolt shoulder.
4, -002 in. for sliding clearance. 5, Clearance round
bolt for expansion of cylinder relative to bedplate.