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Full text of "Modern Mechanical Engineering Vol-I"

ESSENTIAL  MACHINES  AND  APPLIANCES         171
directed through a nozzle held by the operator to any portion of the castings.
The sand falls through gratings in the floor into a hopper, to be drawn by
an exhauster back to the sand-supply. The attendant is protected with a
helmet of felt or leather, covered in front with sheet rubber, which prevents
dust getting into the lungs, and material from striking his face. Air for
respiration enters through a hose at the top, the expired air passing out
between the lower part of the helmet and the shoulders. Glass is not used
for the eyes, since it would become obscured, but fine wire gauze instead.
Quartz sand is used, but chilled iron sand is better. It is prepared by
atomizing a stream of molten iron with jets of steam projected into a tank
of water. There are several designs of sand-blasting plants now in use,
Lifting and Transport Systems.  The calls for hoisting and
transport are incessant in the foundry. Much time will be wasted if the
provisions made for these are inefficient. The overhead travelling crane
is the best machine to install, because it will command the entire area of the
shop. Its power must be rated by that of the weight of work being done.
The most economical design is the three-motor crane, in which the motors
are respectively rated for hoisting, travelling, and cross-traversing. Cranes
of different powers are installed in different areas, to suit the work being done.
It is well to supplement these with a few swinging jib cranes located in areas
where mould parts are likely to monopolize the crane service for considerable
periods. As these are attached to the columns that support the roof, they
do not block any shop area. Very light moulding makes few demands on
cranes, and overhead tracks, from which depend pulley blocks, or light
hoists are often provided for these departments. An equally good alternative
is a light overhead traveller, worked with a dependent rope from below.
Many of these are driven electrically.
The overhead cranes transport as well as lift rapidly, taking moulding
boxes and castings along the shop, and transporting ladles of metal. But,
since their movements are confined by the shop walls, they have to work in
association with extramural tracks, entering a few feet within. These are of
standard gauge to communicate with the yard tracks. Here the question
arises of employing floor tracks throughout the length of the foundry. When
these are laid down, as they often are, the gauge is 18 in. or 24 in. But they
are only desirable in the departments that deal with the lighter castings.
Where heavy moulds are being handled, the floor tracks are of less value than
the overhead travellers, and it is difficult to keep them clear of mould parts
in the morning, when these are laid open for cleaning and coring. In long
shops, devoted to the light work, they are useful for general service, even
though a light traveller is employed.