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 <mailto: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
<rocket@xxxxxxxxxxxxxx <mailto: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
<rocket@xxxxxxxxxxxxxx <mailto: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 <paul@xxxxxxxxxx
<mailto: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
<rocket@xxxxxxxxxxxxxx <mailto: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
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#
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
-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
