If the inner sleeve is slightly shorter than the outer sleeve (0.002 or
so), it will allow the inner races to be pulled together just a bit
relative to the outer races, the same as shown here (duplex back-to-back
configuration, solid preload):
http://www.nmbtc.com/bearings/engineering/preload/
The only purpose for the inner sleeve is to keep from accidentally
applying too much preload. In my bench tests, just a slight tightening
of the nut took the motion down from 0.005 to 0.00005 so I think the
design is working as intended (at least at room temp).
The seal is allowed to float up and down a bit (typically 0.010 as
recommended by BalSeal) and the shim washer between the seal and lower
bearing provides a flat surface for the seal to push against so it
doesn't extrude into the lower bearing cage. The seals are very fragile
so I don't want to apply any downward force to them (up is obviously OK
as that is how they are designed to work).
The sketches with the internal threads are interesting but that makes it
a lot more difficult to install the sleeve without damaging it. I tried
numerous times to get an aluminum foil sleeve to work for installation
but gave up. I think the smooth bore is the key for successful
installation.
I wanted to put the seal closer to the ball as you suggest but the
distance between the bottom of the bore and the body seals (not shown in
the cross section) is just barely over 0.050 so there's no room to put
the seal closer. I like your 3rd sketch with the seal right on the ball
- essentially a face seal. It wouldn't necessarily have to be angled
like that - the U sections could point out horizontally. I couldn't
find any off-the-shelf PTFE face seals, only shaft seals, so it might
have to be a custom part.
-Bob
On 05/24/2016 01:32 PM, Peter Fairbrother wrote:
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
The inner sleeve prevents the preload nut from doing anything useful. It just tightens the distance between the bearings - which you don't really care much about.
Tightening the nut actually _lessens_ the force on the seal ... and increases the useless torque needed to turn two unnecessarily-squashed-together bearings.
I think you need to ignore the top bearing, and just worry about the lower bearing, which should be pressed against the seal with a belleville washer. You will need some kind of screw adjustment on the body (tap the body, don't thread the stem) to do that.
sorry no, that's not clear.
you have two seperate issues, the force on the stem and the force on the seal. There is only one adjustment needed, the force on the seal.
The top bearing can just sit there, floating up-and-down on the stem, with no up-and-down force on it, only concerned with keeping radial position.
The lower bearing should take the up-and-down forces on the stem. Within reason, it doesn't need to be adjustable in position to do that. upwards motion can be mechanically constrained, and downwards motion can be constrained by the seal.
so, from the top we have: top bearing (floating), space, nut screwed into body with shaft running through it. bearing between nut and washer, belleville washer, seal. The bottom bearing should perhaps be angular contact.
The seal is still too far from the ball.
- Peter Fairbrother
(in a hurry, will try and look closer this weekend - I think I can do better than this: