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The "dual-occupancy" configuration you describe is an o-ring with a backup
ring. The backup ring which may be a rectangular ring of nylon (usually
split for ease of installment) is used to eliminate the gap between the two
mating parts, in your case the gap between the tubular housing and the
endcap. Excessive gap will permit the o-ring to extrude into the gap which
may eventually cause failure of the o-ring and thus the seal. Backup rings
are used in a couple instances: when the gap is a result of oversized
machined clearances for relatively low pressure applications; and when high
pressure sealing is required with reasonably sized clearances between the
mating parts. Note the cost of machining and incidence of rejected parts
increases as the required precision of machining increases.

Since the backup ring you describe is located on the wet side of the o-ring,
it would be expected the higher pressure would have been on the inside the
cannister. If the backup ring was on the inside (dry) side of the o-ring
then the higher pressure side would have been the wet (sea) side. If you see
two backup rings in an o-ring installation, this usually means the seal was
subjected to oscillating pressure across the seal (i.e. high on one side for
a while and then high on the other). This is quite common in hydraulic systems.

If you use this housing, you would probably want to move the backup ring to
the inside side of the o-ring since the high pressure side would be the sea
side. You could remove it all togather, but this causes an abnormally wide
o-ring groove which may damage the o-ring if pressures oscillate on either
side of the o-ring causing the o-ring to translate back and forth in the
groove (i.e. abrades through friction the o-ring surface).

Hope this sheds some light,

Doug Chapman

Please respond to my private account doug@su*.ne*
'And I still owe some people some return mail so bear with me.

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