Bob, will do.
Re o-rings. You might be swelling the o-rings up maybe? Are you using EPDM
o-rings, coz they're generally the pick for minimal swelling from gas
permeation.
I've also experienced the same sticking issues if I don't pressurize the
chamber enough ie. the design pressure for my devices is 600-950 psi. @400 psi
I'll still get reliable piston movement with no speed limitation, but at 200psi
things will become iffy if there's a bit of swelling.
As you alluded to, it's all related to o-ring friction vs the force of action
which is related to both chamber pressure and piston area differential. If you
want more force for the available designed Pc, then you can adjust the piston
diff.
Lubricants: I always use silicone for any dynamic o-rings for my devices,
because I've experienced Krytox turning from a gel to a waxy solid under high
pressure CO2. I can't remember if the same thing happens for HP compressed air.
This can sometime be a virtue for check valve sealing, but not so great for
dynamic o-rings.
Cheers,
Troy
-----Original Message-----
From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx]
On Behalf Of Robert Watzlavick
Sent: Monday, 24 June 2019 9:57 AM
To: arocket@xxxxxxxxxxxxx; Troy Prideaux <troy@xxxxxxxxxxxxxxxxxxxxx>
Subject: [AR] Re: Another variant of the pyroless release device
Cool idea - I hadn't thought of that but yeah, o-rings are cheap.
Feel free to add the pics to the "Other Devices" section of your site:
http://www.propulsionlabs.com.au/Pyroless_Release/
I have a 3D model in Pro/Engineer but I'm not very good with pretty CAD
cutaways.
I found it interesting that running the motor at the rated voltage of 3 V / 30
RPM was too fast for it to release due to o-ring friction. I noticed the
longer it
sits after assembly, the more friction the o-rings have. Immediately after
assembly you can push the pistons around with fingertip force but after it
sits
for a day, it takes about 10 lbf to move it. I polished up the sealing
surfaces
as best I could with sanding pads but it still is a fair amount of friction.
I
lubricated it with Krytox (what I had on hand) but maybe there's a better
lubricant. I followed the Parker o-ring gland design guidelines but maybe the
clearances could be opened up a bit since I'm only going to 300 psi.
-Bob
On 6/23/19 5:44 PM, Troy Prideaux wrote:
Nice work Bob, looking good!string to the device - 1 end mounted to the top piston, the other to the
BTW: if you wanted to save that blown out o-ring, you can mount a piece of
casing
body to catch it. Not something I bother with (with the o-rings being like 2c
each for me).
double-d flats of the gear motor.
Hats off,
Troy
-----Original Message-----
From: arocket-bounce@xxxxxxxxxxxxx
[mailto:arocket-bounce@xxxxxxxxxxxxx]
On Behalf Of Robert Watzlavick
Sent: Sunday, 23 June 2019 1:40 PM
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Another variant of the pyroless release device
I performed testing today on my version of Troy Prideaux's pyroless
release device. It uses a 2 inch OD cylinder with two different
sized pistons at 300 psi that when released, generates about 7 psi
and 200 lbf in the recovery bay to eject the nosecone. The concept
is similar to Troy's but has different packaging to minimize the
number of threaded parts and to support remote fill. The load on the
rotary latch mechanism is about 28 lbf which is handled by bearings on the
More details with discussion, pictures, and a video:
http://www.watzlavick.com/robert/rocket/index.html
-Bob
