Wednesday, May 1, 2013

Another Brilliant Idea From My Readers

Glen's comments below were pretty useful: he pointed out that had I mounted the truss block where the mirror is, it would have reduced the bending moment.  I can't mount that where the mirror cell (too many things in the way) but I can mount the truss blocks below the mirror cell (although of course, with longer tubes).  I have an intuitive sense that this would pretty well eliminate the bending of the tube, since the center of gravity would now be above where the truss blocks are.  Does this make sense to everyone?

I am assuming that I can find 1", .050" wall aluminum tubing from my local vendor.  They won't be black, but I can flat black paint them easily enough, and there won't be any shipping charges.  Better, I can have them cut me the lengths that I want, and I won't be paying for the excess that I don't need.  It increases the weight by a few ounces per tube, but that probably is not much worse than the weight of adding a coat of fiberglass to the lower tube.

So, what's everyone's opinion?  Will putting the truss blocks below the weight of the mirror on the lower cage substantially reduce this problem?

UPDATE: Another possibility is to have another ring like the last one rolled by the elves (I mean, Chicago Metal Rolled Products), but with a 1/4" wall instead of a 1/8" wall like the current one.  That would add almost six pounds to it, unfortunately, but it would be, I think, quite sufficiently stiff.  I have requested a quote from them.  Perhaps I would put more effort into cutting down the walls of the C-channel to take a couple of pounds off there -- what is in place currently has no measurable deflection.

UPDATE 2: It just occurred to me that there is another approach that doesn't involve spending a penny.  The 1" square tube that used to be used for connecting the upper and lower cages is still sitting around.  I could use these on the exterior of the lower cage (where there are still attachment holes) to increase stiffness of the tube in the very area where the bending is taking place.  Perhaps put a thin piece of aluminum plate (which I have lying around) on the inside of the tube to increase stiffness.

1 comment:

  1. The closer you can get the truss blocks to the CG of the tube, the better it's going to work, above or below, probably not a lot of difference.