Good point. If you actually need a moving-surfaces seal that's reliable
at cryo temperatures, the only good option I'm aware of is the various
spring-activated PTFE seals - the PTFE retains a bit of flex, but not
enough to reliably seal a moving surface by itself; some flavor of
embedded stainless steel spring provides the main sealing force.
(the other) Henry
On 4/25/2016 3:59 PM, Troy Prideaux wrote:
Henry said:
The bigger issue is the one Ben mentioned: make sure the materials are
okay for cryo use. In particular, ordinary steel is flat out unacceptable
for anything that *might* get cryo cold, because it goes brittle. Brass,
bronze, stainless, nickel alloys are all generally okay. And most
elastomers stop being elastic at cryo temperatures, so things like seals
generally need to be Teflon aka PTFE (which retains some flexibility).
Good advice, however, I'm not sure I'd call anything "flexible" at LOX
temperatures. From my understanding, PTFE is quite hard at those temps, but
does retain some toughness ie doesn't suffer the same brittleness that other
room temperature plastics and rubbers suffer.
Minor point: Telfon (a trade name) is aka (commonly) PTFE, but it's also
important to point out there are many different materials that fall under
the tradename of "Teflon" of which PTFE is only one of and there's also a
multitude of materials that aren't labelled as "Teflon" but have similar
properties to that family - eg. PCTFE which is also used in cryogenic
sealing applications.
Troy.
