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Full text of "Modern Mechanical Engineering Vol-I"

250                               HEAVY  MACHINERY
so, after part of the contraction has taken place and the grout partly settled.
With regard to the length of time required to set, this varies with the nature
of the cement; some slow-setting cements take two to three weeks to set
properly, but where ordinary cement is used in mixing the grout, and the
depth of the grout is not great, sufficient setting should take place in a week.
A simple test is to stab the surface of the grout with the tang end of a file
held in the hand; setting should be allowed till the point of the file marks
but does not enter the grout. It is usual to carry the grout up over the top
of the bottom flange of the soleplate in order to secure a good grip; in
some cases the whole of the hollow interior of the bedplate is run in solid
with grout; in such cases it is advisable to have some holes drilled previously
in the bedplate to let out the air, and allow the grout to fill the whole solidly.
When properly set, the dam of brick or wood is broken away, and the
grout projecting from the side of the bedplate is dressed off. The holding-
down bolts, which secure the turbine cylinder and bearing pedestals to the
bedplate, should be carefully examined and adjusted, and dowel-pins between
the cylinder and bedplate properly fitted. A word might usefully be added
here on the use of dowels and holding-down bolts in such cases. Due to
the wide range of temperature through which a steam turbine has to work,
i.e. from the temperature of the atmosphere, when starting up, to the tem-
perature of the steam when on loads and also on account of the high tem-
perature throughout that may be reached, if the vacuum on the condenser
is lost and the set goes over to atmospheric exhaust, the expansion and
contraction or " breathing " of the cylinder and shaft may be very consider-
able, and amount to J in. or -f in. in large sizes of plant. The amount of this
expansion can be calculated from tables of the linear coefficient of expansion
for various metals, though the actual movement may differ, in certain cases,
from the calculated amount, on account of the shape of the casting, &c.
Provision has to be made to allow this expansion to take place, and at the
same time the cylinder, bearings, Sec., have to be properly held down. It
is usual, therefore, to definitely fix a datum level from which the vertical
expansion and contraction can take place, and also to fix a definite transverse
line from which the axial expansion and contraction can take place. The
datum level for vertical expansion is naturally the top of the bedplate sup-
porting the cylinder, and it is an advantage to have this level as near as
possible to centre line of the shaft and cylinder, so as to make the expansion
or " lift " of the cylinder top half equal to the downward movement of the
cylinder bottom half, and so keep the cylinder under all conditions central,
vertically, on the turbine spindle. In some turbines the datum level is
several feet below the shaft centre line; in these cases it is usual when setting
the turbine shaft to put it some ten-thousandths of an inch high, so that,
as the cylinder expands or lifts with heat, it makes itself central on the
turbine spindle. The transverse line for fixing the datum, from which the
axial expansion takes place, is secured by means of two stout dowel-pins,
one on each side of the turbine cylinder, set half in the cylinder feet and half
in the bedplate. These dowels should be a nice tapping fit in order that,