TT:
First of all, the industrial bearings some of us use atop our towers are
intended for mounting with the mast horizontal, such as in manufacturing
environments. In those instances, the bearings allow the shaft they support to
rotate with zero friction against the shaft itself; the bearings and lubricant
inside the race(s) provide a low-friction means of rotation. To fully realize
the low-friction benefits of this device, the shaft must be mechanically
attached to the rotating collar via a setscrew or screws. Otherwise, the shaft
will merely rotate around within the collar and encounter damaging, or
potentially destructive, friction.
When we hams take such a bearing and mount it with the axis of rotation
vertically, as on top of a tower, the same friction risks apply, albeit to a
lesser extent. That is, unless the mast is mechanically attached to the
rotating collar, it could spin inside the collar, causing a small amount of
friction wear on its surface - same as if the mast were inside a Rohn pointy
top section.
Granted, the potential frictional wear and tear on our masts is minimal
compared to the friction encountered on a rotating shaft in an industrial
application, i.e., <1000 RPY (revs per year) in Amateur work vs 100's to 1000+
RPS (revs per sec) in industrial useage. NTL, we (some of us at least) still
install such bearings to provide lateral support for our masts during high
winds while still allowing the masts to turn freely.
I would not rely on one setscrew (typical of amateur bearings and some
industrial bearings) as the sole support for my mast and antenna dead weight.
This situation could occur when replacing a rotator on a tower that has but one
bearing plate in place - at the tower top. In this case the friction of that
setscrew (3/16 inch max diameter?) is the only force keeping the mast away from
my busy hands/wrists/arms on the rotator plate below. Not a good idea (see
yesterday's postings on this subject.)
Similarly, we seem to tell all who ask that the rotator is designed to
operate with all of the mast/antenna weight on it and on it alone. This might
be within the limits of the rotator. But we forget about the safe load limits
of the rotator's supporting shelf. As I've written here before (yesterday), my
Trylon's specs show 300# as the dead weight limit of each of its
rotator/bearing mounting plates. My 23 foot CM mast weighs about 110#; my
Bencher around 75#; other antennas and coax maybe 20#; rotator itself 10#.
Total weight on my lower (rotator) shelf is 215#, below the 300# limit.
However, I chose to distribute this load over two plates as follows. I
installed the top GP15 and D40 antennas plus coax while the mast was barely
through the top of the tower. I (actually N3RR and I) hoisted this
mast/antenna/coax assembly up to the final position above the rotator plate and
installed the rotator underneath. Then we lowered the mast onto the rotator
and tightened the rotator's u-bolts against the mast. Thus the rotator, mast,
and upper two antennas and coax were supported by the bottom rotator plate.
I next attached the back-to-back (face-to-face?) muffler clamp assembly to
the mast such that it rested snuggly against the top bearing. We hauled up the
Bencher Skyhawk and attached it to the mast about a foot above the tower top.
This weight (75# or so) was now supported by the top plate. The combined
weight of this mast/antennas/coax is thus distributed across two plates, well
below the #300 limit of either one. If I ever choose to climb the mast, I'd
add a third clamp assembly to rest against the middle bearing plate and gain a
bit more safety margin for my 200-pound frame.
I believe the collars that Alan mentions are eccentric collars that allow
for rotation of the bearing in one direction only. That is, the collar is
twisted against the shaft so that the rotating shaft will keep the collar
tight. Then the eccentric collar's setscrews are tightened against the mast to
keep it in position. However, with such an assembly, counterrotating the shaft
will try to loosen the eccentric collars, defeating its purpose. The setscrews
would be the only force holding the collar into position against the mast - why
both with eccentric collars, then?
Finally (unless you've hit DELETE by now), the set screws do have an axial
force limit. I found on one bearing manufacturer's web pages a value of some
finite amount of force along the axis of rotation that the setscrew could
withstand. I cannot recall the actual value, but it is listed. This would
correspond to the dead weight limit of a vertically-mounted Amateur bearing.
As I recall, this value was nowhere near the weight of a typical CM mast, let
alone a fully-configured mast/antennas/coax assembly.
BOTTOM LINE: Know the mechanical properties of each assembly or each piece of
material you install on your tower.
73 de
Gene Smar AD3F
> On 06/08/05 07:37 am David Robbins K1TTT tossed the following
> ingredients into the ever-growing pot of cybersoup:
>
> >> *Don't rely on the bearing's set screw to support any of the dead
> >>weight of the mast/antenna/coax.
>
> > Ummmm, it wouldn't be much of a 'thrust' bearing if the set screws couldn't
> > hold the weight of the mast.
>
> The thrust bearings I have seen are almost capable of supporting a heavy
> load, but they had no integral means of clamping the mast: there was a
> separate "collar" secured to the mast by set screws, and this collar
> then rested against the center part of the bearing.
>
> In normal operation, wouldn't the rotator itself support most of the
> downward force?
>
> Alan NV8A
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
> Weather
> Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
> and ask for Sherman, W2FLA.
>
> _______________________________________________
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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