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