On 21/05/16 23:57, Robert Watzlavick 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
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