Your intuition is good Chuck. A friend in Michigan reported
rotor damage caused by supporting the weight of the mast
and antennas such that no weight was applied to the rotor.
The rotor races were damaged by the SIDE LOAD due to
the pivot action at the thrust bearing. ALL of my rotors
carry the weight of the mast and antennas to keep the
bearings centered at the bottom of the races. One of
my rotors recently quit after 25 years of service holding
up a TH6 which was preceded by a 2L Quad and 40M dipole.
The terminal strip finally disintegrated.
Tom N4KG
On Mon, 17 Dec 2001 "Chuck Lewis" <clewis@knology.net> writes:
>
> 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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>
> -----
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>
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