# Shaping Flexible Materials...whew..non-intuitive

Hey Folks, as you know I decided to see if I could design a ventilator that really could ventilate a person for their normal Covid ICU stay of 20 days (at 15 breaths/min that would be 437,000 cycles over that 20 day interval). I think reliably and safely providing that amount of cycling in that amount of time is a tough design issue. Anyway, I wanted to use some flexible silicone rubber diaphragms to allow pressure changes in the ventilator to move parts around without the air that is changing pressure ever being in actual contact with any of the parts. This is done by allowing the silicone diaphragm to flex in and out as the pressure changes. Parts can be attached to this diaphragm so its flexing and extending action also moves the parts. Pretty cool, but good grief that is hard to do in Sketchup, at least for me. I wanted the diaphragm to have a lot of “waviness” to it so that it could do all this flexing and extending and there would be low probability that it would develop stress fractures or stress failures. I also wanted the diaphragms in Sketchup to be seen by Sketchup as true proper solids, so I could do solid subtractions and unions with them. I decided to create the cross-section of the diaphragm in 2-D to simplify how it is constructed. Took a long time to figure out it is (in cross section) an alternating sequence of smaller and larger semi-circles. I realized when the diaphragm flexes out that it is getting longer (at least the cross-section is), and yet no one is adding or subtracting any silicone from the diaphragm…this was the “Ah–HA!” moment. Of course, so as one stretched the length of the cross-section to allow the diaphragm to stretch out, then one needs to make the cross-section thinner, so that the surface area of the un-stretched cross-section is the same value as the surface area of the stretched out cross-section. Once these cross-sections are created, then you just whip them around a circle with the “Follow me” tool and they create the circular silicone rubber diaphragms, both stretched and un-stretched. Whew… I wish it just wouldn’t take me so long to figure out these most simple of concepts. I think the diaphragms are pretty to look at. Note with the cross section, you use the scale tool to stretch it along its long axis. Then you rotate it, this (after the Follow me event) will make the center of the diaphragm bulge out. As you rotate it, the square or planar outer edge and innermost edges will be rotated also, so you need to go to these areas and un-rotate their walls so they are back to square and planar to the axis of the bulging and un-bulging of the diaphragm. When you do the rotation, you will need to use your eye inspection of the shape to “ease” various sections of the curves to get the cross-section so it looks proper. Always remembering, when you think you are done, you must look at the surface area of the bulged out cross section and be sure it is essentially the same surface area of the non-bulged out cross section. Then when you do the Follow me, you will create these two diaphragms one bulged out and one flattened, but they will both have the same volume of silicone in them, and this is as it should be. I attached a 2-D export of the skp file as a jpg for folks to look at, and I’ll send the skp file to my Warehouse collection. I named my Warehouse collection Pistonrobot. I tried to show in the skp file a sequence of the steps I used to bring the bulged out cross section back to a surface area equal to the non-bulged out cross section. I understand that this is a pretty esoteric topic, but it makes for a pretty diaphragm and it gets me one step closer to presenting a ventilator that really would work properly. The name of the skp file is:
from v250 of ventilator 5th stuff remake of silicon membrane v418.skp Enjoy. Dr. Gray