28.03.2021 Weighing up the shutter slides.

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Sunday 28th 40F, heavy overcast with rain forecast.

After searching for hours online I have ordered two pairs of heavy duty drawer slides. 50cm long with full 50cm travel. Claimed over 110kg load capacity per pair. I can't weigh the shutters until they are cut out and reinforced with plywood ribs. So decided to play it safe again. The GRP averages between 8 and 10mm thick. 

In theory, both sets of slides will support half the load. I still find it difficult to believe the same [half] load is carried by the top pair of slides. In fact they must do. It is simply that the positions of each pair have over 2m difference in height. So they should be equal load carriers if there is no spring in the shutters between their slides. 

The shutters must rest on both slides in a completely relaxed state before being fixed with screws. The shutters then become two short beams supported equally at either end. "Shorter" because of the horizontal spacing between the supports. The diagonal distance between the supports has no bearing [sic] on weight distribution.

The shutters on my 3m dome had to resist considerable spring. I even had to use crowbars to lift the shutters high enough to fit the drawer slides after fitting them to the top slides first. 

The new dome shutters will be measured without any lower restraint. I shall support the tops and see how far down the relaxed shutters reach. Then I shall know where to place the lower slides on the shutters. The slides position will already have been fixed on the the dome by its construction. Both at the top and the bottom. The cannot be placed anywhere else. So the vertical position of the bottom slides on the shutters is the only variable.

Note how the radius of the shutters exactly matches the dome in the drawing. They are both made from exactly the same mould. So cannot be any different. Which works in my favour. Because the relaxed shutters will have slightly greater radius due to being a beam, supported only at each end. Provided, of course, that they are not sprung between the slides. 

The spherical form of the spare segment, from which the shutters will be cut, will probably have some effect on longitudinal stiffness. Though the narrowness of the strips may be the real factor. Around 50-52cm wide, allowing for the twin overlaps over the plywood shutter ribs. A little over 3.4metres long. All shutter dimension are yet to be decided. A decent cover over the top [drawer] slides will really help protect them and also avoid rain blowing in.

The shutter ribs will be carefully cut [scribed] to match the dome on the inside curve. With the same radius as the shutters on the outside. The closer the inner curve is to the dome surface the better the shielding from wind, snow and rain when the shutters are closed. As the only physical movement is lateral these ribs will be moving away from the dome. So no close clearance issues should arise.

There will be a slight difference in curvature across the width of the shutters. I hope to minimize wind drag using this subtle advantage by producing a snug fit. There may even be a cosmetic advantage to spherical shutters. Though the outer ribs may need to be chamfered to fit squarely inside the shutters. [i.e. at right angles.]  

All this detail can only be fully discovered once I have the strips cut out from the spare roof segment. The lower the profile of the shutters the more insensitive are they to wind direction.  Bulky shutters can readily cause the dome to rotate away from the wind if it rolls very easily. This happened several times with my present dome.

"Make haste slowly" is my best advice to myself. I daren't waste the spare, top segment's, totally unique and valuable raw materials. Making silly mistakes in measuring or cutting would be hideously expensive! The shutters must obviously match the needs of the observation slit. Chicken or egg? Which comes first? I had better see the drawer slides before making any final decisions to start cutting!

It started raining around lunch time. So I gave up trying to see the green laser lines in daylight. Even overcast is not dim enough to see them on the green dome. I had raised the laser onto the top of a folding stepladder with an action camera clamp. It didn't help.

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