If its that kind of play that's bothering you, rather than standard ball
bearings have you had a look at angular contact ball bearings?
Its what they use in mill spindles to minimise runout and they are
designed to be loaded up like you are doing with your crush tube and the
like.
you can get cheap ones from vxb.com in stainless and probably ceramic if
you are feeling keen (though not as cheap)
On 22/05/16 08:57, 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
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