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Full text of "Modern Mechanical Engineering Vol-I"

248                            HEAVY   MACHINERY
will have to be reduced, either by the use of thinner packers or by having
the packers machined. Having thus levelled, at the proper height, the bed-
plate and bottom half-cylinder, the next operation is to set the cylinder
central on the axial centre line. For this purpose it is usual to use a length
of fine piano wire, stretched tightly by means of weights between the two
outer pedestals. The wire is very carefully centred at the extreme edges
of the bearing pedestals, and the bottom half of the cylinder, plus bedplate,
is then jacked over on the packing pieces below the bedplates, so as to bring
it central on the steel wire. The final test is made with an inside micrometer,
behind which is held a piece of white paper, in order to show clearly when
the end of the micrometer is just touching the wire. This adjustment makes
the cylinder right for position sideways and vertically. Its position end-
wise is usually taken from the centre line of the turbine exhaust, and this
line, as well as the axial centre line, is determined beforehand for building
up the foundations, and is retained for definitely fixing the position of
the turbine.
When the piece is finally set, the level should again be very carefully
checked, and if necessary readjusted, before the bedplates are grouted-in.
There is a difference of opinion among engineers regarding the best time
during erection for grouting-in the bedplates.    Some men prefer to erect
the whole plant complete before doing any grouting; others prefer to grout-in
the bedplates immediately they have been finally set and checked over, and
before any weight, e.g. other portions of the plant, is put in position on
the bedplate.   The arguments used in favour of the former method are,
that if the whole plant is completely assembled first, any errors in the draw-
ings, which might make it impossible or difficult to fit the various parts of
the plant together, can be adjusted without cutting away the foundations
or undoing a lot of work made permanent.   Against this advantage must
be placed the disadvantage of liability to spring the castings and soleplates,
due to the concentration of the weight of the whole plant on the compara-
tively small area of the packing pieces between the soleplates and the founda-
tion block.   On the other hand, the number of cases where a complete plant
has to be taken up and re-erected, due to some oversight in the layout draw-
ings, is so remarkably small as to be almost negligible, and a good deal can
be said in favour of grouting-in immediately the soleplates and main struc-
ture have been assembled and checked for position.   The whole of the area of
the underside of the soleplates is thus available for distributing the weight
of the plant, and in consequence the liability to settle and get out of level
is very much reduced;  further, it is usually possible to make a very much
more satisfactory job of the grouting-in process before the whole plant is
assembled, on account of the greater freedom and space to get at the job
when the bedplates and lower parts  only of the plant are in   position.
Instead of using parallel packing pieces, which are left in and grouted-
up, some engineers  prefer to use steel wedges, about  3  in. wide and
3 or 4 in. long, tapering down in thickness from J in. to nothing.   The
wedges are driven in under the sole plates until the latter are levelled up,