Skip to main content

Full text of "Modern Mechanical Engineering Vol-I"

See other formats

.;}                             256                              HEAVY   MACHINERY
\\l                                   any nozzles in the top half should be bolted in position and properly jointed
'j}j                                  up.    The completed top half-cylinder is then turned back into the upright
sj                                  position, and carefully lowered down into place on the bottom half.    Guide-
$1                                  rods are usually provided for guiding down the top half, and it is advisable
',ifj                                  to put strips of sheet metal -^ in. thick and 3 or 4 in. wide at several places
'';                                  round the main horizontal joint.    This is done in order to try the spindle
(!f!                             for being free before the top half is right down in position.    The spindle is
||!                            then pulled round once or twice by hand or by the crane, and if quite free,
'^                            the pieces of sheet metal can be removed from the horizontal joint, the
jj                            weight of the top half being meanwhile taken by the crane.    The top half
I                            is then lowered right down and the spindle pulled round again.    The cover
is then finally lifted, and the edges of the brass labyrinth packing strips sharp-
ened up. The setting of the thrust block, which determines the position
axially of the turbine spindle, should now be carefully adjusted, and the
axial clearances between the wheels and the fixed diaphragms very carefully
checked over at both sides of the turbine. A record should be kept of these
clearances, and if they are less than the clearances required by the particular
type of turbine being erected, it may be necessary to have the fixed diaphragms
further machined; special care should also be taken to observe and accurately
measure the clearance between the nozzles and the blades on the first wheel.
r J                                 When the spindle has been finally set, the permanent collars for securing
**                              the thrust block in position axially can be machined to the required thickness,
and either pinned on to the thrust block by two or three countersunk screws,
or left loose and tapped round into position.
Having prepared the main horizontal joint of the turbine cylinder and
spread the jointing material uniformly, the top half is lowered into position,
dowel- or steady-pins driven in to fix the relative positions of top and bottom
half-cylinders, and the bolts through the joint then put in, and either banged
up tight with a large spanner and heavy sledge-hammer, or pulled tight with
a heavy spanner over the stem of which is passed a piece of heavy pipe
several feet long, for additional leverage.
When the supply of steam is available, it is advisable to go round all
joints which reach a high temperature under working conditions, and
tighten up the bolts after the joint has been heated up. The effect of heat
causes the jointing material in the majority of cases to " give ", and this
give should be taken up on the bolts through the joint, otherwise the joint
will very probably begin to blow in a very short time.
The next operation is to erect the alternator and couple it up to the
turbine. In the very great majority of cases the alternator stator is built
in one piece, i.e. not split horizontally, the exceptions being few and far
between. The advantages of making the stator without a horizontal break
are very great, both from the electrical as well as from the mechanical stand-
point, so that even the largest alternators are made in one piece. This
means, therefore, that the generator rotor has to be threaded through the
stator, and as this is an operation which has puzzled many engineers, a
description is given of the method employed.