Another note: some amount of leakage for spring energized seals in dynamic
aoolications is quite common in my experience. Often the leakage will subside
after the motion stops. Also, I think I've mentioned before I've seen higher
leakage with lower pressures due to insufficient preload (at higher pressures
the seal is assisted by the fluid pressure). What surface finish do you have
on the shaft? 16 or better is typically called for.
David
On May 24, 2016, at 3:16 PM, Robert Watzlavick <rocket@xxxxxxxxxxxxxx> wrote:
When I talked to the BalSeal rep, he thought the McMaster parts might be from
their company. But since McMaster doesn't list the brand (unlike Grainger),
it may be whoever has the lowest bid. I got a quote from BalSeal from some
custom seals that had a heel you could grip to keep the OD attached and a
different material/seal lip but it was way out of my price range ($600 NRE
plus $100/seal I think). I've had such good luck with the larger McMaster
seals as static seals for my injector so I figured they might work for a
dynamic application. I can say that when I tested them with LN2, they sealed
initially then leaked upon turning so hopefully if I can keep the shaft from
wobbling around I'll have better luck. Do you have any price quotes for small
quantities (20x) of the other brands?
I had planned on testing with water prior and then drying it out without
disassembly prior to the LN2 testing. What does the water do to the Molykote
Z? I soaked the bearings in acetone for a few hours, rotating them
occasionally to get the existing lubricant out of them but I didn't
re-lubricate them. Since the motion is just intermittent, I thought I would
just run them dry. I cleaned the bearings in my LOX flowmeter the same way
and ran them dry.
-Bob
On 05/23/2016 09:46 PM, Lloyd Droppers wrote:
Looks nice Bob, and the bearing arrangement should work out. I think the
Belleville washer that other people suggested might be nice, but if
everything in the load path (shaft, bearings, nut and spacers) have the same
CTE the belleville washer shouldn't be necessary.
My main concern about the valve, if I were building it, would be the PTFE
spring energized seals. I have never been able to get the cheap mcmaster
versions (the used to be Coors-Tek brand) to work without leaking even with
good surface finishes. So if they end up leaking on you, I would recommend
spending a bit more on something from BAL or Trelleborg before you throw the
design out as not working as I have had much more luck with the higher
quality shaft seals.
For the lubrication you shouldn't certainly shouldn't use krytox, it will
freeze at cryo temperatures. The Molykote Z works fine if you keep it dry.
Speaking of dry make sure to purge the cavity with dry gas as ball bearings
really do not like ice buildup, but you have done this before and probably
knew that already.
Lloyd