On 2.9.2012 4:00, Jeff Liebermann wrote:
On Sat, 01 Sep 2012 22:36:52 +0300, KBa wrote:

Just thinking of how glass ceramics may behave under bending force...

Machinable glass ceramic insulators don't behave... they break.
They're great under compression but miserable in tension. So-so in
torsion. All you have to do is make the insulator stronger than the
element it supports. If there's excess force on the element, the
aluminum element will bend rather than the insulator will break.

Element center gap is under very small vibration (wind load) and element
static load stresses the insulator center... it bends slighly depending
how elastic is the material...

I don't expect wind vibration to be much of a problem. The forces are
rather small compared to a flock of overfed birds landing on the
elements or the wind blowing at hurricane force. With a brittle
insulator, an impact with a flying hard object is a real danger.

Fiberglass have some elasticity... it's not brittle... on the other hand
there could be support limiting the bar bending or the element halves
might have more than two clamping points... so the center gap would
be steady...

You haven't supplied any numbers or dimensions, can't do any
calculations.

Hi Jeff

Ok, very good points... this design may need better tools I have in
hands for the stress checks...

But some numbers... 14MHz tapered element half, symmetric segments,
starting at center 47" (OD 1", ID 0.625"), 24" (OD 0.875",ID 0.619"),
44" (OD 0.75", ID 0.619"),36"(OD 0.625",ID 0.467") and tip 58.7" (OD
0.5", ID 0.494") AL 6082, element wall dimensions are somewhat on a
heavy side at the center segments... so is the weight a little...
Center gap 2" between element halves.

Currently planned support plate dimensions L 15.75" x W 7.87" thickness
0.393" FR4, element clamp positions 2" and 6.88" symmetric along plate
long side, so two clamps for each element half, plate located above
element for partial rain cover... Plate max size 19.6" x 9.8"...
Not yet processed, so no worries for changes...

Much appreciated if you have some dimensioning help for the center
insulator diameter and length ( minimum ) handling the stress...

I may still need to consider FR4 bar covered with shrink tube, but
ceramics are also on a map... but depending on support plate strength
and bending at gap center (or between nearest clamps 2" off the gap
center ) there might be chance to forget whole center bar in the gap...

tnx oh6io


I don't think you'll have much trouble with the static loads
(including the birds). The wind loading might be a problem depending
on design.