Re: [PSUBS-MAILIST] Deep Flight Challengers Interior

["Andrew Goldstein" <ag@desertstar.com>]

This is another case of adding additional complexity in the hopes of solving a single specific problem, and in this case
it's even worse since the more complex design is less effective in general than the simpler one (see Jim's and Ian's previous points.)

There is a case where multiple o-rings can be of benefit, but it's not really applicable to a clamshell hatch.  On applications like
penetrators or static end-caps, having dual seals of DIFFERENT types (one face, one side for example) can be beneficial.  It adds general redundancy and the individual seals usually wont have the same failure mode.  

The two most common causes of o-ring failure that I have seen (by far) are machining errors and failure to install, as opposed to damage or aging etc.  Machining errors such as "Oh yea there is a chamfer on that drawing..." or "yes I think those gouges in the o-ring seat may be a problem..." are easily caught be a diligent inspection, or by assembly - failed pressure test - dis-assembly - diligent inspection.  Failure to install is just plain stupidity: "I told the intern to install o-rings", "These connectors always come with the o-rings installed" etc.  


Couple of other tips.

Prevco has an easy to digest list of things to consider when designing a seal http://www.prevco.com/Reference/Checklist.htm#SealDesign,  and most o-ring manufactures (like Parker http://www.parker.com/literature/ORD%205700%20Parker_O-Ring_Handbook.pdf) publish helpful design guides.  These resources while not as rigorous or complete as other texts are free.


Always destroy (Cut, burn, shred, etc.) an o-ring IMMEDIATELY when you take it out of service or suspect  it of damage.  If you don't, at best you will be throwing away two o-rings, at worst you'll be putting a damaged one back in service.

Use the correct amount of lubricant (not too much, not too little).  This is a good rule of thumb for many other situations.

-a




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