Lubrication is thus provided for minute movements
caused by differential thermal contraction, as is
intimate conductive electrical contact with the glass
surface. Since atmospheric temperatures nowhere
equal that of factory assembly, and since contraction
of the metallic alloy and pin is greater than that of
glass, the factor of differential thermal expansion
in the pin-hole is obviated by the characteristics of
the materials and processes employed in the manufacture
of Pyrex insulators.
The third differential
expansion amply compensated for in the Pyrex insulator,
is that obtaining axially between pin and alloy, and
glass surface. Here the greater contraction of the
alloy from assembly temperature tends to load the pin-hole
surface in a downward direction over its upper half and
in an upward direction over its lower half. The two ends
tend to contract toward the center. Adjustment of the
angular faces of the steel pin grooves compensates for
this thermal mechanical load by appropriate transfer
of the load to the alloy shell. The resultant mechanical
loading on the glass shell--thermal preloading added to
service load--is uniform over its area.
All Pyrex units are, of course,
jig assembled. Cap, pin, and glass shell, heated to
carefully controlled temperatures, are held in fixed
relation during assembly, until solidification of the
alloy.
Entrance of moisture into the space
between the pin-hole surface and the alloy is prevented
by the layer of asphalt at the bottom of the pin-hole.
Other advantages of Pyrex suspension
insulators are in general those also found in the Pyrex
one-piece pin type insulator. They have been proven by the
test of time on the lines of over 300 power companies in
the United States. The first installation dates back to
1923 when an early type of Pyrex insulator was installed on
50 kv and 66 kv lines.
|
I NON-CRYSTALLINE |
 |
Formed by the complete fusion of its constituents, the
most important of which are the oxides of silicon, boron
and alkali, the insulators are non-crystalline in structure.
The vibration of conductors and towers cannot cause the
fatigue commonly associated with bodies of crystalline
structure.
|
|
Long time tests on Pyrex suspension insulators
under loads far higher than those encountered
in service.
|
|
