Interesting approach Bob, thanks for sharing. I agree that a
Belleville washer would be a good addition to maintain preload. You
also may want a dorky little clip that goes over the nut after it's
torqued and keeps it from backing off with rotation of the stem. The
clip being missing on repeatedly rebuilt pneumatic valves is
responsible for about 80% of stem seal leaks on test stands that I've
seen.
Instead of the two sleeves you could use a frusto-conical spacer (or a
thick ring with appropriate lips) inside that loaded the outer race of
the upper bearing into the inner race of the lower bearing to drop the
parts count by 1. It would guarantee that the load on the races was
identical between the bearings, whereas in your setup that's only true
if the sleeves are exactly identical in length. But making that part
might be more complex than making the two tube spacers.
Oh, and it'd be loading between a spinning and non-spinning part. So
that's probably a nonstarter... you'd have to add a little thrust
bearing.
Ben
On Sun, May 22, 2016 at 5:57 AM, Robert Watzlavick
<rocket@xxxxxxxxxxxxxx> wrote:
I was never really satisfied with my semi-custom cryo ball valves (see
aRocket archives around 2014-11-24). These valves use the ball and body
seals from a Swagelok SS-62T6 but with a new aluminum body, stem, and stem
seals. I had some stem leakage due to shaft wobble that I was able to
address by adding a 2nd spring energized PTFE seal and pressurizing the seal
cavity but I had the feeling I was approaching it the wrong way. Several
folks made good suggestions including Peter who provided some sketches of
alternate configurations. I had wanted to get the bearing closer to the
seal so I spent some time designing and fabricating an alternate arrangement
the past few weeks. One thing I discovered is that even with two bearings
on the shaft, there is still an excessive amount of free play until the
bearings have some axial preload. Apparently, bearings (at least standard
ones) have to have a minimum amount of preload on them just to meet their
specifications. I set up a test fixture with two bearings separated by
about 0.5 inches and even with the outer races held tight in a collet, I
could wiggle the shaft up to about 0.005 inches by pushing sideways with my
finger. After some of reading about bearing design, I modified the valve
body and stem to use 0.0005 inch slip fits and then added two sleeves
between the bearings. The preload is applied to the inner races through an
inner sleeve (recommended for maximum stiffness) and the outer sleeve is
used to help keep the bearings aligned with each other and to press them
into the housing without damage. Here are some sketches of the new design:
http://www.watzlavick.com/robert/rocket/rocket1/drawings/ball_valve_3_assy-annotated.pdf
http://www.watzlavick.com/robert/rocket/rocket1/drawings/ball_valve_body_3-nodims.pdf
http://www.watzlavick.com/robert/rocket/rocket1/drawings/ball_valve_stem_3-nodims.pdf
When I assembled it, applying a small amount of preload using the nut
reduced the shaft wobble a factor of 10 (0.005 inches down to 0.0005) with a
fair amount of force from my uncalibrated finger. From that standpoint, the
design is much better than the previous one. One improvement would be to
use a spring washer between the preload nut and the upper bearing. When I
test it with LN2, we'll see how well the single seal holds up at cryo temps.
I was trying to keep the overall dimensions the same as the previous valves
so there is only room for a single seal. The 0.500 bore in the aluminum
valve body will shrink about 0.002 inches while the seal OD will shrink
about 0.009 so the squeeze on the OD of the seal will go from 0.020 to about
0.010 (still some squeeze though). Ideally I would use a seal that has a
heel on the OD to ensure it stays attached to the wall but those are very
expensive in small quantities. The difference in thermal expansion
coefficients between the aluminum body and stainless steel bearing/shaft
will tend to increase the grip on the bearings so hopefully that won't lock
them up. Also, instead of Krytox for lubricating the shaft and seal, I used
Molykote-Z to see if that makes a difference.
After I test it in a few weeks with LN2, I'll report back on how well it
works.
-Bob