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Full text of "Modern Mechanical Engineering Vol-I"

FITTING AND ERECTING                        247
ably on steel packing plates at least i in. thick, placed underneath the heavier
parts, so as to leave a gap below the soleplates, which will facilitate the
insertion and drawing-out of wedges or flat packing plates used in levelling
up. The bottom portion of the turbine cylinder can then be placed in posi-
tion and levelled up, both axially and transversely.
It may be accepted as a good general rule that, before placing together
two machined faces, the faces should be lightly rubbed over with a smooth,
flat file, more particularly all round the edges of the machined faces. This
not only ensures that the faces are clean and free from burrs (particularly
round the edges of drilled holes), but it also immediately shows up any bulges
or bumps on the faces. These burrs are often caused by the links of chain
slings pressing into the machined faces, when chains are used for lifting the
castings. Instead of putting down the bedplate first, the bottom half-turbine
cylinder may be put directly on the bedplate, bolted down, and the whole
then lifted in one piece, where the capacity of the overhead crane or lifting
tackle is large enough. In the majority of steam-turbine plants, guide or
director keys are provided between the bedplate and the cylinder, which
prevent movement in a transverse or lateral direction, while allowing free
movement in an axial or vertical direction. This is done in order to allow
the cylinder to " breathe " or expand and contract with varying temperature,
without upsetting the alignment of the set. It is important, therefore, to
see that these guide or director keys are not only properly fitted, but when
fitted are secured against the possibility of working out. The turbine
bottom half-cylinder and bedplate may now be levelled up; it is often found
when levelling up large castings that if the piece be levelled, say in a trans-
verse direction, by means of a level applied to one side of the casting, and
the level be then tried in the same direction on the other side of the casting,
it will be found to be out of level. This is nearly always due to the casting
having " sprung ", due to internal strains in the casting easing themselves,
more particularly when the skin is broken by machining. Another cause
Is the manner in which the casting has been bolted or cramped down on
the boring-mill or planer-table, when being machined. If the casting has
been sprung before machining, then, when the bolts or cramps are released,
the casting will spring back, and the result will be that the machined faces
are not true. This difficulty is got over by placing a steel or stiff wood
straight-edge right across the bedplate or cylinder, and putting the level on
the straight-edge and setting the casting or piece level in this manner. The
levelling up is done by inserting steel wedges, preferably 3 in. wide, under
the soleplate, and, when wedged up, adjusting the height of the parallel
packing pieces. In this way a true surface is prepared (represented by
the top of the packing pieces), on which the bedplate may be moved laterally
without upsetting the level of the piece. The next operation is to check
the height of the centre of the turbine shaft or spindle above the engine-room
floor-level; if the centre is too low, then it will be necessary to increase the
height of all the parallel packers by the amount by which the centre is too
low. Conversely, if the centre is too high, the thickness of the packers