28.10.17

Dome build: Dome weight and wheel load.

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It has been sunny but very windy all day after the storm. So I have been getting on with capping the octagon walls with mitered 2x8s. Only one post was out of level. My stock of 2x8s is of two different sizes! 

A bare segment/gore weighs just over 6 kilos or 13lbs.  So, 16 gores = 13 x16 = 208lbs.

45x45mm horizontal struts weigh 1.1kg per meter [0.7lbs per foot]

Vertical struts: Pi x 1.6 x 8 = 25 meters for the 8 vertical runs [half circumferences] over the dome. If the vertical struts weigh about the same per meter as the horizontal struts then [say] 27.5 kg = 60lbs total.

4mm birch ply weighs about 15lbs per 5'x5' sheet. [2.25m²]
Area of a hemisphere = 2.Pi.r² = 6.284x1.6² = 16m² / 2.25 x 15 = 106lbs for the ply cladding.

Plus any reinforcement around the obs. slit and shutters + hardware + paint. Say 500lbs total for the entire dome?

Support and rotation wheel loading:  500/8 = 62.5 lbs static load per wheel.

Hard, white, 5" Polypropylene industrial wheels with ball or roller bearings and fixed forks have a load capacity of 150kg = 330 lbs.

6" PP wheels capacity is  250kg = 550lbs. The price difference between 6" & 5" wheels is 2:1.

I had ordered 18 new Rawlink 15cm/ 6" spring/glue clamps despite having badmouthed my original Rawlink clamps. Now I am delighted to report that the new Rawlink clamps are a vast improvement! They have the same shaped handles but now have nicely grippy, rubbery pads. Making them very easy to use. They also seem just as powerful but with slightly shorter jaws for better leverage when opening them.

I think the spring wire is slightly smaller diameter so it may be improved steel for greater lightness. The previously loose jaws are now properly pinned so they can't fall out like the originals. Making them excellent value compared to the pricey and much weaker Bessey clamps.

Having opened my parcel of clamps I painted the new, 2x8, wheel support ring with the Safeway, mineral/water wood treatment. I have been delighted with the very even, silvery-grey appearance of the treated timber. The bare larch floor boarding has weathered to a very similar appearance. It may not be to everybody's taste but I like the "abandoned barn" finish.

I also ordered 8 off 160x50mm nylon wheels today in pressed steel forks to support and rotate the dome. Each, individual wheel has a capacity of 350kg.  That's 770lbs! Which would easily carry the full weight of the dome all by itself. Each wheel has ball bearings for easy movement and the hard wheels should avoid flats forming if the dome isn't turned regularly.

I liked the slightly increased height of the fork to provide a marginally taller dome. At nearly 6.5" diameter I imagined these wheels would roll more easily over inconsistencies in the track than my earlier choice of 5". My eye level, when standing on the observatory floor, is now just above the intended base ring height.  So a horizontal view of the distant woods is still possible.

Shopping around online provided considerable savings. The most heavily advertised Danish dealers on Google are wholesale only. However, they do not make it remotely OBVIOUS that they will not deal with private customers! I wasted considerable time on their websites!

Click on any image for an enlargement.
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