I was at a festival at the weekend and saw a really good hub design. The hubs were made out of 150 - 200mm diameter nylon rod. Each strut was held in place by a pin and a split rung going through the hub.
It seems like a really good way of making hubs and I want to build a 3v dome but it always the hubs I''ve been afraid of. I''ve had a look at the diagrams on the site but don''t really understand how they work. I''ll need to drill holes in the side of each hub but how do I work out the angle that the holes need to be drilled at?
Any help would be great!!!
[ Comments 5 ]
That looks like it would be a pretty expensive option and probably a lot of work too. A 1m length of 150mm dia nylon rod costs about £150. It would be just enough for a 3v 3/8 dome (46 hubs)if you cut it into 20mm thick discs, but it may not be thick enough to allow for the strut axial angles (10-12 degrees) depending on the strut tube diameter.
I`m a little confused about the hole drilling issue, all the angles are shown on the diagram in the "Make your own 3v dome hubs" thread. They are colour coded to match the struts in the dome diagram further up the page.
Here`s a quick step by step which should help clarify things.
The first thing to do is to buy a 360 degree protractor from WH.Smith
Using a compass, draw a circle the same diameter as your hub tube (eg: 75mm OD) on a piece of paper. Position your 360 degree protractor on the centrepoint of the circle and mark off the 5 way hub angles at 0,72,144,216 and 288 degrees. Place your actual hub tube on the paper and transfer the angles to the bottom edge of the tube with a pen. Use a set square to extend the lines upwards and make a mark at the halfway point. If your (75mm dia) tube section is 100mm long mark it at 50mm. You will end up with 5 marks 72 degrees apart around the tube perimeter and exactly halfway up. This is where you drill the holes. The rest of the hubs are marked out and drilled in the same way except the angles and number of holes will be different.
We expect some photo`s of the finished dome :)
Thanks I've had a look at the page and it doesn't seem nearly as daunting as I first thought. I want to use the dome as a tent for a stage at festivals so the hub's need to be strong. I'm intending to use 35mm mild steel CRW tube with 1.2mm wall. The dome is ideally going to be a 3v 3/8 20m diameter, do you think that nylon rod will be strong enough for the hubs? I suppose the actual hub will need to be about 70mm thick?
Also I want to get a custom cover made...what do most people do about covering their domes?
Was the 20m diameter a typo?
The dome would be a little over 8m tall. With 4.12m strut lengths you`d need a thicker wall than 1.2mm :)
I don`t know how much the tubing weighs but the bottom hubs especially would have to be capable of handling the weight plus additional loading from the cover weight and the wind.
The total length of tubing involved is around 475m. A friend of mine in australia had a cover custom made for a 15m diameter 6v hemisphere dome and that cost $20,000..so its not going to be cheap ;)
I did a few calculations for the 20m diameter dome and as long as each base hub can handle a 700kg load it should stay intact in a 40mph wind.
If you plan to erect it on very calm days with 20mph wind maximum you can get away with 200kg per hub.
Either way it will need to be well anchored to the ground.