I believe Worcester offers short-stem options for cryo valves. The
drawings at
https://www.flowserve.com/files/Files/Literature/ProductLiterature/FlowControl/WorcesterControls/WCENBR1040-01.pdf
may tell you a bit about how they go about sealing things.
Henry
On 5/30/2016 8:52 AM, Robert Watzlavick wrote:
What kind of stem seal did they use? Was it the type where you have a
cone that squeezes into a sleeve?
I hadn't seen any short stem cryo models. It might be worth me getting
one to take it apart. Were these custom or off the shelf?
-Bob
On May 30, 2016, at 10:24, Ben Brockert <wikkit@xxxxxxxxx
<mailto:wikkit@xxxxxxxxx>> wrote:
To be clear, basically every vehicle I've worked on had short stem
cryo ball valves, and most of them did not leak. Torque requirements
are another issue, but it is not a universal truth that all short stem
ball valves leak out the stem.
On Monday, May 30, 2016, Robert Watzlavick <rocket@xxxxxxxxxxxxxx
<mailto:rocket@xxxxxxxxxxxxxx>> wrote:
Agree that drilling out the stem would make sense. Another option
would be to use a 1/4 shaft and 3/8 bore instead of 3/8 shaft and
1/2 inch bore as I noticed there are standard seals and bearings
made for those sizes. I could also go with an aluminum shaft (I
did in my prototype) but you have to be super careful to maintain
the surface finish and not let it get scratched. There are also
wear issues but anodization would likely help. The original
Swagelok design has an OD near the ball of 3/8 OD but the rest of
the stem is 1/4 inch. However, the original stem is also designed
to be inserted from within the ball cavity which limits its
overall length.
I sketched up a bearing-less design that still uses the PTFE
spring seals uses close-fit PTFE bushings instead of bearings. I
concluded that it might be difficult to hold tolerances on the
PTFE parts and that you would really have to load up the bushings
radially to keep the shaft from wobbling around. That's
essentially the original stem packing arrangement along with it's
high torque requirements (which leaks at cryo temps with a short
stem configuration).
I would be interested to see the the seal design on this XCOR valve:
http://aerospace.xcor.com/media/5766/fuel-valve.jpg
I was originally going to duplicate that design and I even
purchased a small air cylinder but I thought I could save some
weight and complexity by using an electric servo motor instead of
the air cylinder. I also would have had to have a separate
regulator and double-acting solenoid.
-Bob
On 05/27/2016 11:09 PM, Paul Breed wrote:
The stem limit may be rigidity against the horn turning it not
actual torque...
Also how much spring can you handle in the drive....
Rotating shafts that run aricraft controls are almost always
hollow tubes...
On Fri, May 27, 2016 at 5:41 PM, Lars Osborne
<lars.osborne@xxxxxxxxx
<javascript:_e(%7B%7D,'cvml','lars.osborne@xxxxxxxxx');>> wrote:
RE: drilling out the stem.
If you have margin on the stem yielding from torque, wouldn't
it be better in this case to reduce the OD as much as
possible? A smaller OD will reduce the friction of the stem seal.
Thanks,
Lars Osborne
On Wed, May 25, 2016 at 6:26 PM, Robert Watzlavick
<<javascript:_e(%7B%7D,'cvml','rocket@xxxxxxxxxxxxxx');>rocket@xxxxxxxxxxxxxx
<javascript:_e(%7B%7D,'cvml','rocket@xxxxxxxxxxxxxx');>> wrote:
Here are the weights of the new design (lb):
Body (6061) 0.077
Stem nut (SS) 0.005
Bearings/Sleeves (SS) 0.012
Stem (SS) 0.023
Body bolts/nuts (6061) 0.015
Servo 0.120
Ball/seats (SS/PTFE) 0.027
End caps (-6 male, 6061) 0.066
Servo hub (6061) 0.009
Total: 0.354 lb
Weighing all the items together comes out to 0.324 lb
(some scale resolution issues above). I'm not sure where
I got the 0.27 lb number from before - that may have been
without the servo. And I guess the stem didn't weigh as
much as I remembered. Brackets will add another 0.02 or
so. Switching from SS to aluminum body bolts saved a lot
of weight.
For the main LOX and fuel valves, a pin connects them
together since they are effectively on a common shaft.
So I can't drill the pin hole any deeper or the pin might
slip into one or the other stem. I could use a longer
pin but that wouldn't help the weight situation. But for
the vent valves, I could drill out the stem somewhat.
-Bob
On 05/25/2016 11:54 AM, Paul Breed wrote:
Any reason you can't drill out the center of the SS Stem?
The stuff in the middle is adding no real strength...
IE Turn it into a tube....
Leaving 0.075 wall would reduce the weight ~35%..
0.050 wall would save > 50%
On Wed, May 25, 2016 at 9:21 AM, Robert Watzlavick
<<javascript:_e(%7B%7D,'cvml','rocket@xxxxxxxxxxxxxx');>rocket@xxxxxxxxxxxxxx
<javascript:_e(%7B%7D,'cvml','rocket@xxxxxxxxxxxxxx');>>
wrote:
I'll weigh the components tonight. The heaviest
component by far is the SS stem.
-Bob
On May 25, 2016, at 10:59, Paul Breed
<<javascript:_e(%7B%7D,'cvml','paul@xxxxxxxxxx');>paul@xxxxxxxxxx
<javascript:_e(%7B%7D,'cvml','paul@xxxxxxxxxx');>>
wrote:
I'd be interested in the mass break down of your
final valve, ie weight of all the components....
I was looking at building a plug valve with
bearings to support it and
a hollow 3D printed plug....
Paul
On Wed, May 25, 2016 at 5:57 AM, Robert Watzlavick
<<javascript:_e(%7B%7D,'cvml','rocket@xxxxxxxxxxxxxx');>rocket@xxxxxxxxxxxxxx
<javascript:_e(%7B%7D,'cvml','rocket@xxxxxxxxxxxxxx');>>
wrote:
I never though to ask McMaster but they
responded quickly:
The vendor for the PTFE seal is Bal Seal
Engineering Co Inc and their part number is
100MB-012-T or X124209.
I can't find that part number in their catalogs
but the MB describes the spring. The 012 is
probably the series and T corresponds to Virgin
PTFE. Attached is the datasheet I received
from them.
I recall our conversation about leakage a
couple of years ago and my quick fix was to use
two of them and a pressurized cavity with both
U-cups facing toward the pressurization port.
That did wonders for the leakage and almost
completely eliminated it. Now that I've
improved the bearing situation, maybe it will
be better. I could add a 2nd seal in this
design but it would shrink the distance between
the bearings down. I could always make the
valve body longer if needed but that means
redesigning a few other things on the vehicle.
I'm using these shafts:
<https://www.servocity.com/html/3_8__precision_shafting.html>https://www.servocity.com/html/3_8__precision_shafting.html
and it says they are 303 SS with a 10 RMS
micron finish. There are better shafts out
there but the 303 makes it easy to cut for
threads for the nut. They tend to have some
very small nicks in them from rattling around
in the bag so I used graduated sanding pads to
polish them up:
<http://micro-surface.com/index.php/products-by-type/soft-touch-pads/micro-mesh-soft-touch-pad-variety-packs.html#>http://micro-surface.com/index.php/products-by-type/soft-touch-pads/micro-mesh-soft-touch-pad-variety-packs.html#
These go all the way up to 12000 grit so I
hopefully I am improving the surface finish -
it looks almost chrome plated when I'm done
with them.
I suspect though that due to shrinkage, the
seal locks itself around the shaft and it is
the OD of the seal turning against the bore
that becomes the new sliding surface. The bore
looks pretty smooth, mainly because I ran the
last pass by hand instead of using the mill
power feed but I would like to polish it up
some more if anybody has a good suggestion. I
thought of making my own 0.500 sanding pad but
I don't want to oval the bore or take too much
off since it is 6061-T6. I've seen rotary
grinding stones - maybe those would work
without taking too much off. I did find these
although 140 grit seems pretty low:
<http://www.artcotools.com/precision-diamond-pin-gx-141c.html>http://www.artcotools.com/precision-diamond-pin-gx-141c.html
-Bob
On 05/24/2016 10:20 PM, David Gregory wrote:
Another note: some amount of leakage for spring energized
seals in dynamic aoolications is quite common in my experience. Often the
leakage will subside after the motion stops. Also, I think I've mentioned
before I've seen higher leakage with lower pressures due to insufficient
preload (at higher pressures the seal is assisted by the fluid pressure). What
surface finish do you have on the shaft? 16 or better is typically called for.
David
