# FreeCAD 2026 Design Contest
This is my entry to the [2026 FreeCAD design contest](https://forum.freecad.org/viewtopic.php?t=105082).
For assembly instructions see the assembly guide.

## Initial Design Choices
As the contest specifies a 5V operating voltage, and custom PCBs or more complex electronics would significantly increase development time, replication effort, and cost, I decided to limit myself to components that already ran on 5V and could immediately be used, so could be connected to a microcontroller without any special intermediate board.
That cuts the list of mechanical actuators down quite a bit. I could use a small DC motor maybe, though that is difficult(-ish) to drive bidirectionally, or maybe maybe maybe a low voltage solenoid, or, as I decided to do, a small, cheap RC servo. I think this is a pretty obvious choice, and I'd be surprised if I was the only contestant to choose this option.
To use that actuator to pinch the tube I decided on a linkage system, since I wanted to maximize the force I could apply to the tube, to then maximize the pressure I could switch.
I also decided that, for convenience of switching out the tube, the mechanism would need to be open on one side, so that no end of the tube ever needs to be fed through the mechanism (which I imagine could lead to cross contamination of the previous and current setup). 
So I wanted to convert the 180° rotation to about 1.5mm of linear actuation. I also figured the amount of force required would vary based on how far along those 1.5mm we are. Specifically I believe the last little bit will require far more force. So I decided to design a linkage system, that would reach top dead centre at full squish, to basically fully relieve the motor of force at the end of travel. This is done both in the bell crank that interfaces with the tube, and on the crankshaft that mounts to the servo. This way the valve fails further back in the stroke if the pressure is to high, wich should make failure more obvious, and since it matches the force curve better it should also work up to higher pressures. (I wasn't able to test any of this, these are really just my assumptions I built this first version on.)

## Implementation
I chose to use fairly common tools. I think 3D printers and simple hand tools (which could also be reasonably aquired if missing), are a good set for this design.

I also wanted to avoid using many different mass purchasable components. This is why all the pivots are 2x12mm pins, rather than pins of the "correct" length. There's enough space for them, and if you need to buy 100 of them regardless, and only want to build a couple of valves it's more convenient. (that, and I had them on hand myself. More convenient.)

The custom parts are all 3D printed, and most of the closer toleranced holes (the pivots mostly) should be drilled out after printing.

## Build requirements and cost
The tools I used to assemble the parts are:
* A (FDM) 3D printer (I used a modded Ender5+, it doesn't need to be very fancy)
* A vise, to press in and hold parts
* A 2mm drill to drill out the holes (I just held the drill in the vise and spun the parts)
* A screwdriver (PH1 and HEX2.5, for the plastic screws for the servo, and the M3 socket head cap screws)

The Materials per valve are
* 1 SG90 Micro-Servo (Including the screws they usually come with, one to screw in the servo horn, two to screw the servo into the frame) 10,79€/5
* 8 2x12mm Dowel pins (m6 tolerance) 9,67€/100
* 4 M3x10 socket head cap screws (DIN 912) 3,11€/20
* ~25g of PETG 3D-Printer Filamet 32,99€/1000g (I used the filament I had, which was Prusament PETG, it probably works fine with cheaper filament, but it's what I was working with)

So if you're building a lot of them the price per unit comes out to
| Count | Price per order/count of ordered parts | price per unit |
| 1x | 10,79€/5 | 2.16€ |
| 8x | 9,67€/100 | 0.78€ |
| 4x | 3,11€/20 | 0.63€ |
| 25x | 32,99€/1000 | 0.83€ |
= 4,38€/unit

If you build 5 of them (Which I think is pretty realistic) the price per unit comes out to
Price per order/number of units|price per unit|
| 10,79€/5 | 2.16€ |
| 9,67€/5 | 1.94€ |
| 3,11€/5 | 0.63€ |
| 32,99€/5 | 6.60€ |
=11.32€/unit = 13.12USD < 14USD (as of writing this)
and youd be left with with 60 dowel pins ("worth" 5.80€) and 875g of PETG (worth 28.86€). This is PPU particularly high because of the expensive filament used in the calculation.

I think a fair cost assesment is somewhere inbetween the two, since most people will have some filament, and so the 6.60€ per valve of filament are a bit over the top. I think the average maker would probably spend around 5-6€ per valve.

## Performance Assesment/Concerns/Room for improvement
I have not had the opportunity to test maximum holding and closing pressure, nor how many close/open cycles it can handle. I hope to get around to running those tests in the final week before the submission deadline. [The only test I've managed so far](https://chaos.social/@la/116727325081926518) was me blowing into the tube with the other end in a glass of water, to see if it closes and seals agains my lung pressure. 

I am particularly concerened with the requirement in the contest brief, requireing gas sealing at 0.5bar, and wear resistance.

I belive my valve is likely to hold a seal at those pressures given that the servo sees none of that force, it's directly pushed into the housing. I am however woried that it might not manage to close against that kind of pressure.

And, wear wise, the entire valve has no roller bearings. It runs entirely on dowel pins in drilled out holes. This works pretty well, but I could imagine some of those holes deforming or wearing over time, especially when running unter pressure and thereby bigger forces. I am particularly worried about the pivots of the bell crank, which take the forces of the sqished tube directly.
The second worry point for wear is the tube itself. Because of the top-dead-centre design aproach the pin rubs along the tube quite a lot. This, I imagine, could wear the tube itself down over time. Some lube would certainly go a long way there, but adding a roller bearing to the contact side of the bell crank would eliminate the problem entirely. Same would be the case for the pivot holes. This would increase the BOM by about 3-4€, which arguably would be worth it. I will have to test if the wear gets to be a problem in the first place, and then, if it turns out to be worthwhile I might add a variant of the design which solves this problem using 682 bearings (which I have, just in case, ordered already)