Designing a inline piston pressure regulator - Fluid statics math model

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FighterAce
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Wed Nov 17, 2021 2:08 pm

iknowmy3tables wrote:
Wed Nov 17, 2021 1:19 pm
It would just involve adding a friction component to the equilibrium a freeze frame in time, one when it's fully open one when it's closed but it also involves an arbitrary increase in spring compression when it's open.
That's how I've been trying to do it, just to get an idea at what pressure the piston will break static friction and start moving into the closed position. The next step would probably be kicking it out of equilibrium and setting up a differential equation of the system.

I still didn't figure out the spring constants for the springs I have available to take it into consideration... not sure if going into such accuracy makes sense right now, without knowing the friction force.

The diaphragm design is something I have to look into, but the potential of catastrophic failure is kinda worrying with those.
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hectmarr
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Wed Nov 17, 2021 5:23 pm

This regulator was designed to be manufactured with inexpensive precast bronze parts, available anywhere. Does not use lathe machining for metal. The two grooves for the two o'rings are made on the domestic bench drilling machine and a small file. It can be put together in a couple of hours ...
What you are making is something much better, in terms of finish and resistance, I like it!
The regulator works very well.
I think that if the piston is twice the diameter, there will be more force produced by the air in this piston to more easily overcome the drag force of the o'ring.
Example:
Diameter = 0.6 cm
Perimeter = 1.88 cm
Area = 0.28 cm2
Pressure = 3 bar
Force = 0.84 kg

Diameter = 1.2cm
Perimeter = 3.76 cm
Area = 1.13 cm2
Pressure = 3 bar
Force = 3.39 kg
In the first case, the relationship between the drag force and the force exerted by the air on the piston is 2.
In the second case, the ratio is 4. Undoubtedly there is more energy to move the piston, and overcome the resistance of the o'ring. It is an approximation that assumes that the two o'rings are of the same thickness and that the resistance to rosing is proportional to the amount of rubber that has contact on the cylinder walls.
I incorporated a small additional spring as in the drawing, and I can regulate to less than 1 bar, without touching the main spring on the right. It works well, and it will not be necessary to redo the entire piston to be able to regulate the outlet at very low pressure
Good luck with your project and we look forward to seeing your finished work. Greetings
reg. pres.. 2.png
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Fri Nov 19, 2021 2:40 am

You can use X-shaped o-rings to reduce friction. I use them on my piston with standard o-ring compression and I can easily move the piston by pushing it with one finger.
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Sun Nov 28, 2021 12:31 pm

I just finished making the v3 based on hectmarr's design. This is the final arrangement:

Regulator nacrt-Model01-00.png

The body is 2011 Aluminum, just so it would be easier to machine. Both ports are threaded for G1/8
RegV3AlCuBiPb.jpg

Piston is 5,8mm steel rod with 3x1,5mm (6mm OD) O rings
RegV3Assembly.jpg

The test rig:
IMG_20211128_170420_4.jpg

It gives a near constant 6bar output as long as the inlet pressure is below 12bar. At anything above 12bar, the piston starts failing to move into the closed position and slowly lets the full inlet pressure into the outlet. I guess O ring friction under higher pressure is just too much...

I might make the v2 design next and see how it behaves. Maybe both designs would work best in a two stage configuration.
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Wed Dec 01, 2021 8:00 pm

Include a spring in the low pressure chamber, and a harder spring in the regulation.
The piston will move between two greater forces while the high pressure resistance of
the o'ring is constant. In this way you can regulate much more pressure. As seen in the drawing above.
By the way, your work and completion is excellent.
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