When the tempering has been done in such a way
that the surfaces are in compression -- the
resulting glass is more resistant to temperature
changes and to all external stresses, such as
impact. Rear windows on automobiles, locomotive
protector glasses, and "top-of-stove" cooking ware
are all "toughened," as are suspension insulators.
This treatment cannot be applied to Shell No. 2
but a similar result, in lesser degree, is obtained
by using the surface glaze with a smaller coefficient
of expansion than the underbody. This is a
"compression glaze"! Because the glaze is so thin
the toughening effect is less pronounced and not
so easy to control.
ASSEMBLY
Insulators with shells of type No. 2 have been
assembled for years, using cement as the coupling
medium. In the field the results have been good
but by no means perfect. Glass suspension
insulators, 10" in diameter, use antimonial lead
alloy because it is considered better, though
more expensive, than ordinary cement. Its
ductility, up to the point where the glass is in
equilibrium with the service stresses, insures
uniform loading. The alloy forms a perfect
cushion between the glass and the steel pin.
Recently, tempered glass suspension insulators, 9"
in diameter, have been developed.
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These use a special cement, far superior to that
ordinarily used. They meet and exceed all
requirements of a standard 15,000 pound insulator,
and are cheaper, but they are not as strong as
our 10" alloy unit.
Questions and Answers
IS TEMPERING THE SAME AS
COMPRESSION GLAZE?
No -- both seek to put the insulator surfaces in
compression, but they are as unlike as a piece of
stainless steel and a coat of paint. The paint
may have many flaws, and it may crack easily,
so that metal will rust beneath it -- but stainless
steel is non-corrosive through-out.
IS A TEMPERED INSULATOR "UNBALANCED"?
No -- the deep surface compression zones are
perfectly balanced by a tension zone still
further inside the glass, where it can do no
harm. In an insulator of type No. 2 the
change in stress from compression in the thin
glaze to tension in the underbody is likely to
be quite abrupt.
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