Referring to Fig. 1, the glass ridge(s) carry
the loads imposed on them by the alloy ridges (d). Similar
reasoning will show that here, also, a greater strength may
be developed by this construction than by the conventional shape.
A study of the foregoing design
theory predicts several advantages for the Pyrex unit.
The substitution of several regions in tension for a
single one, and the more uniform loading of the material,
permit a greater load to be carried by a given weight
of the insulator shell for the same tensile stress in
pounds per square inch, or pounds per lineal inch of
surface. Due to the accurate formation of their surfaces
by steel moulds, engineering calculation of the performance
of Pyrex insulator shells is possible. Such performance
can be closely predicted from the characteristics
of the materials of the insulating shell and the alloy
supporting it.
The Pyrex insulator is assembled with antimony lead
alloy and this, together with the design produces a unit
which is less subject to change with time. Such alloys
show practically no corrosion in atmosphere of widely
varied characters.
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Pyrex suspension insulators have been on our test racks
for periods of up to two and one-half years under
indicated loads of 10,000 lbs. Additional units have been
carrying the usual service load for much longer periods
without failure.
Of utmost importance in building
long life into insulators are the means adopted for
accommodating differential thermal expansion. In this
respect, the construction of Pyrex insulators differs
from the conventional. Whereas expansion is generally
of concern, in Pyrex insulations contraction is the
factor. Factory assembly temperature is several times
any encountered in service, hence expansion forces cannot
be present in subsequent line service.
Fig. 1 shows ample clearance
over the head of the glass shell is provided by an
asbestos pad extending over the entire area above
the mechanical load-bearing grooves. Over the surfaces
of these grooves a resilient compound affords opportunity
for differential contraction without departure from the
precalculated distribution of load. The pin-hole surface
of the Pyrex insulator shell is coated with graphite
deposited from colloidal solution.
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