The Pyrex suspension insulator offers several
departures from conventional cap and pin design.
These departures are not alone the product of theoretical
deduction. The transparency of glass offers opportunity
to observe the correctness of calculation and theoretical
analysis which is not afforded by opaque bodies. And this
observation is not hampered by the use of equivalent
material, involving equivalent relative modulus of
resilience, etc. Observations can be made on the
materials and assemblies of the size and shape of the
finished product.
The Pyrex suspension insulator
design embodies a spirally threaded pin-hole cavity.
Considering a thin radial section, (Fig. 1) it will be
seen that the interleaved glass and alloy threads form
a series of small cantilever beams. Each alloy beam
(a) transmits a portion of the total mechanical load
to the glass beam on which it bears (b). Each little
glass beam (b) has a region in tension at the upper
surface of its base, and its strength is therefore limited
by the unit tensile strength of the glass. The small
alloy beam has a region of tension at the lower surface
of its base, and its strength is similarly limited.
Each alloy beam deflects slightly, and transmits its
load to its glass support. The ultimate strength of
each of the two beams depends on the tensile strength of
the material of which each is composed, and also of the
depth of the beam, as is well known from cantilever
beam formulae. These beams are therefor proportioned
so that they have approximately equal breaking strength,
and therefor the greatest overall mechanical efficiency
of the arrangement is secured.
By this construction, it will be
seen that a series of tension regions is substituted for
the single tension region of convention design. The
region at the top of the pin-hole is loaded in tension
no more than is the region at the center.
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The uniform
load distribution permits a greater total load to be
carried by a given surface area with the same tensile
strength of material.
The outer surface of the Pyrex insulator head similarly
consists of several alternate ridges and grooves, which
form interleaved alloy and glass beams when assembled
with the steel cap.
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Pyrex suspension insulators installed in
1930. This line insulated for 110 K.V.
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