I had the same reaction when I read it, and also the same question regarding
editorial review of QST articles.
Furthermore, the previous hint, from the same submitter, has us drilling a
hole in the mast just above the thrust bearing in order to transfer vertical
loads to the bearing so we don't have to rely on the jam-nutted bolts
normally used through the bearing upper collar. I can't think of a worse
place to weaken a mast: right at the point of maximum bending stress. Sure,
we've argued over drilling and pinning masts to rotator upper brackets, but
that's in a relatively safe spot. I can see it now...hundreds of ten-foot
lengths of two inch water pipe being drilled for a 3/8" bolt right at the
prime failure point.
This brings up a pet peeve: Why are we so determined to remove the vertical
load from rotors unless it exceeds the makers specs? I'm an EE, not a stress
guy, so my credentials are questionable, but my intuition tells me that
rotors DEPEND on some vertical preload to obtain lateral (radial?) load
reaction from their bearing races, and to maximize bearing life. Sure, each
rotor design will treat this a little differently, but their specs will
account for it, and they all seem to be clear on the allowable vertical load
for the size of the rotor. My Rohn "thrust" bearing is set up with the
jam-nutted screws just clear of the O.D. of the mast, so the "thrust"
bearing is reacting against RADIAL thrust loads and reducing the overturning
load applied to the rotor (NOT the weight of the mast plus antenna). My
total "stack" weight is well within the allowable vertical load as specified
by the rotor manufacturer; and is constant, so there was no good reason to
reduce it. On the other hand, wind-induced overturning moments could
conceivably exceed the capability of the rotor, and the "thrust" bearing is
there to add additional protection against excessive bending loads at the
rotor. The bending load becomes zero-ed out by the "thrust" bearing, and the
rotor races are subjected to lateral (radial) forces instead. Having a
vertical pre-load ensures that these races have a chance to do their job.
When I check things at the top, I make sure the bearing collar will rotate
around the mast and that the jam-nutted screws are still just clear of the
mast OD. AND...there are never any alignment problems.
Now...had I placed a BIG stack up there, with a vertical load which exceeded
the allowable for the chosen rotor, a thrust bearing that shared some of the
vertical load would have been appropriate. In that case, however. the
hint/kink we're bashing becomes even scarier!
Chuck, N4NM
----- Original Message -----
From: "Tom Rauch" <w8ji@akorn.net>
To: <towertalk@contesting.com>
Sent: Monday, December 17, 2001 12:28 PM`
Subject: [TowerTalk] Hink and Kinks
>
> Re: Add Safety and Comfort Jan QST page 67
>
> This seems like a death wish!
>
> I can't imagine standing on a rubber hose that was slit and installed
> over a tower rung....even if it is RTVed in place!! All it needs to do
> is spin, and woops...there go your feet out from under you
>
> Does this concept bother anyone else, or am I paranoid about
> having a solid foothold?
> 73, Tom W8JI
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