|It’s ironic that geodesic domes are completely made up of triangles, yet the hubs used are mostly 5-star and 6-star hubs. 4-star hubs are used around the base, but they’re really just 6-star hubs with two legs cut off. 10 years ago, when Sonostar was first getting started, that’s how we did it – we just cut off two legs from some 6-star hubs to make as many 4-star hubs as were needed for the base configuration. Later, we had special molds made to make them look more aesthetic.||
4-star with the early cut-off version
But that doesn’t really answer the question. Why would anyone need a 3-legged, 3-star hub?
A black 3-star hub
|We let the R&D team take an old injection mold that had been used to make an earlier version of the ½” 6-star hubs, and they modified it to remove every other leg. They were as surprised as anyone that the mold actually worked afterwards. Then they started experimenting with the parts that came out of it, and the surprises haven’t stopped.|
Suppose you wanted to put a little bling into a Bubble Dome, and add some pentagonal windows? You need 3-way hubs to do that. By taking out the yellow struts and the 5-star hub in the middle, then replacing those with ten shorter struts and five 3-star hubs, you get a window that actually looks great and adds strength to the whole structure. The triangular windows in the Bubble Domes are good for letting in air, but the pentagonal windows let in air with style. We’re incorporating three of these into the new mini-glamping domes that will be available sometime in July. We previewed those in a recent R&D blog.
Triangular window in Bubble Dome
5-star Pentagon - no window
3-star hubs with pentagon window
The team was also intrigued by another geometric structure, technically called a dodecahedron. In laymen’s lingo, it’s a soccer ball configuration, using all 3-way connections.
|The model shows that both pentagons and hexagons have to fit together to make it round. And the sides of the pentagons have to be the same length as the sides of the hexagons – so all the struts need to be the same length.|
Would it be possible to build a dome or a sphere with all 3-star hubs? The engineers said, “No, this won’t work. The angles for the pentagon need to be 108 degrees, whereas the angles for the hexagon need to be 120 degrees. You can’t do that.” The non-engineers, said, “What the heck, let’s try it anyway. Maybe the flexibility in the PVC will make up the difference.” The engineers folded their arms and let the kids play.
Dodeca-doghouse pentagons - and hexagons
The result was not so much a defeat for the engineers, as it was a victory for the pragmatism of those with no special training.
Of course the engineers were right concerning the math – there’s 12 degrees’ difference in the angles needed. But similar to how the angles work in geodesics, there’s more than enough flexibility in the PVC pipe to handle the 12 degrees of difference. We’re calling our partial sphere made with all 3-star hubs and all 10” struts a ‘Dodeca-doghouse’, because its’ 6’ diameter makes it perfect for a full-size dog. And the dog will probably get smarter the longer he sits in it.
So what can you do with 3-star hubs? Let’s just say they open up a lot of possibilities for those with very creative minds. Sonostar sells them, even though there’s not much of a market yet. After all, it’s not R&D’s job to sell anything, it’s their job to invent stuff. And 90% of what they do never goes outside the lab. So for the R&D team, this is actually a success story!
So even though they are at 120 degrees instead of 108 degrees, can an actual dodecahedron be constructed using the 3 star hub? is the PVC that forgiving? I’m interested in working on dodecahedral domes and spheres.
You’re in luck, Phil. The biggest difference between Schedule 40 and Schedule 80 PVC pipe is the thickness of the wall. The outside diameter of both is the same. Since the standard hubs (that fit 1/2" PVC) require the pvc pipe to be inserted into the hub arm openings, it’s the outer diameter that matters. And they’re the same. Go ahead and use the thicker Schedule 80 pipe, and you’ll have a stronger dome.
Hoping you can answer the question over sched 40 and sched 80 pipe for the dome..
Up here in Canada snow loads are a little different .. and I’m wishing to use sched 80 pipe .. is this possible or ur domes are sized for sched 40 pipe only ..
Hope this finds you all well in the wonders of these covid times and everyone staying safe
March 06, 2022
Praise all things beautiful and right in the world, you’ve saved the day!! Actually, in fact saved me/us a lot of work completing my art installation… I’ve been looking for 3 years for these 🤩