On 2/26/14 6:30 AM, Joe Subich, W4TV wrote:
> Another way to express this is the increase in strength = 1 / the
> percentage stress reduction.
Increasing the wall size does not "increase the strength" of the
material - it reduces the stress on the mast for a given load.
Put another way it increases the load handling capacity of the mast.
In any case, it is far better to increase the diameter of the mast -
the stress in a 2.5" OD 1/4" wall mast will be only about half that
in a 2" OD, 1/4" wall mast for the same applied load. It's a shame
that the makers of antenna rotators simply don't understand that and
continue to insist on 2" mast only.
Indeed.. maybe here's an opportunity.
Say you have the rotator down in the tower and you have TWO 3" ID (or
2.5" ID) bearings above it to take the bending loads. You have an
adapter that has a 2" diameter OD that sticks into the rotator that
bolts (or shear pins, or clamps) to your larger diameter mast. Either
one of the upper bearings takes the gravity/thrust loads, or you let the
rotator carry the downforce.
Of course, just as with rotators, I'll bet everyone's antenna/mast
clamps are also designed for 2" tubing too.
what is the history of rotator designs, anyway? Did they evolve from TV
antenna rotators (where a small mast isn't a big deal) with gradually
bigger gears/housings/motors.
When did rotators start to become popular with hams? And when did they
start to be manufactured for that market? I seem to recall my
grandfather had a homebrew rotator of some sort in the 60s using a
geared down motor (probably a prop pitch), but it was modified from a
previous armstrong style using ropes and pulleys. And, as with lots of
us, I remember a pile of gear motors and synchros out behind his garage
for "some future project", although by the 70s he had a commercial
rotator up on the top of the tower.
In the 50s and 60s there were a lot of surplus gear motors from the
aircraft and DoD related industry available on the market, and even into
the 70s, but by the mid 80s, they were pretty well drying up.
I've got some old ARRL handbooks I could look through, but I'll bet the
collective hive-mind on this list probably "just knows"
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