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Full text of "Modern Mechanical Engineering Vol-I"

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The Effects of Shrinkage in Castings

All the metals and alloys in common use shrink in cooling from the
molten state. Although the amount per foot of length may not appear large,
the fact is responsible for the deformation, the lack of homogeneity, the
weakness and the fracture of a very large proportion of the " wasters " made.
The evils arise from the different rates of cooling in large and light adjacent
masses, the very small capacity of cast metal for elongation, and the method
of its crystallization. The weakness of cast metals in tension, and their
very small percentage of elongation before fracture, when compared with
the similar physical properties of forged and rolled materials, are the causes
of these results.

The Case of Unequal Adjacent Masses.—Many designs that emanate from
the drawing office have to be modified to suit the foundryman's point of

Fig. 50.—Illustrates the Camber of Castings produced by unequal Shrinkage
view. Regarded from his aspect, the ideal casting is one in which thick-
nesses are approximately equal, with the result that all portions cool and
shrink simultaneously. The more intricate the casting and the larger the
amount of coring done, the greater is the need for preserving uniformity
of sections. Familiar examples are those of steam and motor cylinders,
in which the percentage of wasters is often rather large. In these and
other castings cores are frequently inserted, or prolonged solely to avoid
the occurrence of masses of metal in corners and angles.
But in many designs of machine and structural parts it is not practicable
to avoid great disparities in the masses of metal in parts that are contiguous.
The moulder then minimizes the evil results, first by " feeding " fresh,
hot metal into heavy masses, to prevent the formation of " draws "—hollow
places—due to internal shrinkage, and second, by uncovering the massive
section, exposing it to the air, in order to cause it to cool within about the
same period as the thinner portions adjacent. A great deal of this is done
in the case of central bosses for bedplates and heavy pulleys.
Curving, Camber.—Distortion without weakening or fracture is a common
result of unequal shrinkage. It is particularly troublesome in long and
flimsy castings, as bedplates, gutterings, and similar objects in which there
is an excess of metal, not necessarily large, on one side. The casting in cool-
ing becomes permanently concave on that side. In the group of figures,