Lloyd,
Thanks for the link - that's very good information about the moisture
and environment effects on MoS2. In the past, I've noticed that even
when I run dry N2 or He through the valve prior to operation, pretty
much all air-exposed surfaces get ice then water on them after exposure
to LN2. I guess I'm really counting on the lube just for assembly as
the PTFE seal friction on the shaft should be low enough without lube.
I disassembled the new valve the other day because I wanted to see if I
had damaged the seal during installation. The seal was fine and it was
interesting to see how the dry lube had worked its way into the seal and
shaft.
-Bob
On 05/24/2016 08:24 PM, Lloyd Droppers wrote:
Bob,
I have got better quotes from Trelleborg, but only when I am using a standard design, but the best I remember was ~$50 per seal at low quantity, but we did have to modify our design to match a seal they had on the shelf. Your prices seem about normal for bespoke seal, So I understand the drive to get the McMaster seals to work, especially for hobby projects where it is your own money. I hope that keeping the shafts very tight with no run-out works, it is at least worth a try.
For the Molykote Z i looked at my notes, and the one reference I recalled for water being an issue was for 10 micron MoS2 and bearing life if reduce by a factor of 100 in a humid enviorment vs a dry enviorment. But we are talking the difference between 5k cycles and 100k cycles so probably not a problem for a quarter turn ball valve. In addition friction coefficient is 0.02 dry and 0.08 wet with MoS2 so it will also reduce torque to keep it dry. So TL/DR is MoS2 is better that no lube even when wet, but you get even better performance if you keep the dry lubed parts and dry.
My Ref.
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19790004987.pdf
Lloyd
On Wed, May 25, 2016 at 10:16 AM, Robert Watzlavick <rocket@xxxxxxxxxxxxxx <mailto: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
