1/22/97
two or more overlapped turns. One of these o-rings must be inserted in a
groove on the bottom of a plate in the scope, and I have not found a way
to unfold the o-ring into a circle which will lay flat. I have put a book
on it overnight and put it around a beaker slightly larger than the ID, but
neither of those is satisfactory. I don't want to deform the o-ring, so I
haven't been too rough with it. I am trying the beaker again, this time
filled with hot water. (If the cold caused the o-ring on Challanger to
harden, maybe the hot water will soften one.) Other than using so much
grease that it holds the o-ring in place--obviously not good practise--I
haven't found a way to keep the deformed o-ring in place. Has anyone out
there solved this problem? TIA.
Yours,
Bill Tivol
tivol@wadsworth.org
When our EM sevice engineer wants to "plump up" an o-ring that has been in
service for a long time, he puts it into an oven at 50 degrees C for an
hour. This seem to relax them and plump them back to round. Perhaps if you
put them in an oven with a weight on them for a while, then keep a weight on
them as you cool them down, this would flatten them enough.
Mary Mager
Electron Microscopist
Metals and Materials Eng., UBC
6350 Stores Rd.
Vancouver, B.C. V6T 1Z4
CANADA
tel:604-822-5648, fax:604-822-3619
e-mail: mager@unixg.ubc.ca
Not a problem I've faced, but how about putting it around the beaker, as
you've been doing and then putting it in the deep freeze over night. When
it comes out nice and hard, you'll probably have a minute or so to get it
into place and the bottom plate located, wait a few minutes for it to
soften and then tighten the bolts. Worth a try? ... :)
Regards,
Larry Stoter
LPS@teknesis.demon.co.uk
I can confirm that heat for restoring o-rings does work in many
instances. For many years I have successfully used hot water to
do this rather than an oven.
Regards
Robin Cross
EM Unit, Rhodes University, Grahamstown, South Africa
EURC@giraffe.ru.ac.za
Thanx for all the input. Here is a summary of that input, info
from Parker and my own observations.
The most often-mentioned method to flatten o-rings was boiling.
This method worked very well with viton. The advantages of boiling are
that the method is easy to do reproducibly, and "It also drives out a lot of
'glup,' to use the technical term. (J. Pawley)". The disadvantage is that
the water must then be removed. For viton, this is pretty easy, since that
elastomer can be baked out at ~200 C.
Another popular method was heating to 50 C in an oven. This was
not quite high enough for viton, which did not lay flat after an overnight
heating at that temp. I'm sure that heating at a somewhat higher temp
would do it, however. The advantages of the oven method are that one need
not remove water, and it is the best-controlled method for those elasto-
mers, such as buna-N and fluorocarbons, which cannot be heated above 70 C.
Following the heating process, one can cool the o-ring either slowly
or rapidly. Slow cooling works for me, but there may be situations where
shock cooling would be advantageous. In particular, to shape an o-ring to
a particular non-flat or non-round configuration, it might be good to heat,
shape, then shock-cool.
The opposite suggestion--to put the o-ring around a beaker and
freeze it in the round state--would not be applicable for my purpose.
The reassembly of the column takes so long that the o-ring would thaw and
fall out; furthermore, there would be condensation. I can imagine situa-
tions where it could be useful to shape an o-ring, freeze it, and install.
Spring clips were also suggested for holding the o-ring in place.
This also would not be suitable for my situation, but might be useful to
consider.
A caution about silicone o-rings was that they are very permeable
to He. As a result, leak-checking can give false positives for several
days.
Both viton and silicone o-rings can be baked out at ~200 C, and
that may be a good idea for a standard practise, since it will drive off
volatiles in the o-rings. Buna N and fluorocarbon cannot be heated above
70 C, and that for only a few hours. Buna N just melts, but fluorocarbon
decomposes. Ethylene-propylene is the most radiation-resistant of the
common elastomers. We use it for seals which are close to the beam, and
it remains relatively flexible under circumstances where either viton or
neoprene harden. The Parker O-Ring Handbook is a useful source of info
about many properties and applications of various available elastomers.
Since elastomers are treated by crosslinking and with additives,
and are, no doubt, optimized for particular applications, the appropriate
temps and times for particular treatments should be determined experimen-
tally, rather than relying on info from books. Some info--such as decom-
position temps--can be obtained reliably from books and used to set upper
limits for trial runs, but even in this case, it is probably better to
talk to the manufacturer before approaching these limits, since the treat-
ments may significantly change them.
As usual, the list was a great source of info. Thanx, Nestor,
for establishing and maintaining it.
Yours,
Bill Tivol
tivol@wadsworth.org