9.04.2021 Top ring, rollers and dome base ring.

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Friday afternoon: Mostly sunny and breezy.

First I dropped a plumb line from the top of the observation slit cut-out. I wanted to be sure it was exactly in the middle of the dome. Measurements confirmed that the top ridge of the slit was indeed central. Plus the 15cm offset away from the slit to allow clear zenith views.

I then double checked the diameter of the top of the building at the outside of the upright posts with the surveyor's tape measure. 

This proved to be 4.05m Ø =  2.025m radius.
Dome Ø = 4.30m - 4.05 = 25cm difference in diameter.
A difference of 12.5cm in radius. Or about 5".

The rotation rollers have to support the dome on a reasonable, circular track. Not too near the dome skirt. Just in case anything rubs. Not too far away to avoid undue cantilevering of the quite considerable load. 

A dome is steered in rotation by vertical rollers running against the edge of the base ring. This does not guarantee absolute concentricity. So some extra clearance is desirable. Tightening the clearance of the steering rollers increases the friction dramatically.

Here I arranged a mock-up of two plywood arcs of the correct outer radius. First with 12.5cm and then 8.0cm clearance inside the arc matching the dome skirt. The rotation roller in the image shows the width of supporting steelwork of its fork mounting. The whole set-up is actually inverted in the images. The larger radius [dome] lower arc, on the right, would actually run on top of the rollers.

The dome's GRP reinforcing rib is 60mm up from the skirt and 40mm deep. So I could increase the building arc overlap slightly more. To bring the wheel track nearer the dome's skirt. This would reduce the cantilevering of the roller track away from the dome skirt.

This would be achieved by increasing the diameter of the building's, laminated, plywood top ring. I am assuming, for the moment, that the dome base ring is 20cm wide. This is both convenient and economical of plywood consumption when cutting arcs from 150x150cm sheets.

I rather like the idea of making a spaced, double base ring for the dome. With a vertical, plywood strip, spacer ring, on edge, clamped between them. This would conceal and sandwich the reinforcing rib. Providing a much stiffer, base ring arrangement for the rollers to run on. Fixing a single base ring securely would be made much more difficult by the proximity of the moulded GRP rib.

Uneven rolling surfaces lead to erratic rolling resistance as the dome rotates. Leading to "tight" spots and loose ones. As the rollers are forced to run up and down hill as the dome rises and falls on the rollers. There will also be a serious imbalance between the loads on the many rollers. Reducing the number of rollers carrying the load, at any one moment, will increase friction. I will have 14 rollers, this time. Instead of eight on the previous dome. There will be fourteen steering rollers and 14 hold down disks.

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