Matt,
Got the image, thanks and nice work. Very informative about some of the
issues. The three bearing study would also help settle a lot of
debates. Were real bearing models developed? The design that was
developed for radial float (done in some machine tooling to float
reamers) of the rotator would be interesting.
My thinking re mast length is most towers have the largest yagi close to
the top to minimize its load on the tower. Given the usual 20ft mast,
that leaves a lot to add VHF/UHF, etc. A recent TT post asked "what
should I do with the extra N feet?"
Your analysis makes it more interesting that UST made the tube mount on
the HDX really stiff with the double top plates.
I was involved in FEA way back for designing high performance disk
drives. Autodesk Fusion360 CAD/CAM now includes it, so I should try
some modeling. A lot easier with all the assistance now in the FEA
software. Ridiculously cheap to run on a PC rather than a Cray
supercomputer back then.
73 and HNY,
Grant KZ1W
On 12/27/2020 03:42, maflukey@gmail.com wrote:
Hi Grant & Happy Holidays. Sorry to mislead anyone. I lowered the
mast in my example because I'm not sure I ever met a ham that welded 2'
more mast on the end to compensate for a thrust bearing installation -
hi hi. The original point I was making was relative to radial and
axial loads which I think has been pretty well ventilated at this
point. Beyond statics 101 I have run a lot of finite element stress
analysis for complete tower installations and even more analysis on mast
installations. Typically there just isn't enough load to create
significant statically indeterminate stresses of significance at that
location in the tower... below is a FE model zoomed in on the top of a
crank-up beyond tower leg failure... I’m not sure if the reflector will
handle HTML image. Peak stress at the leg connection of the rotor
plate is only about 9000 psi at tower failure loading (which occurs
lower in the tower). Displacement of the rotor plate relative to the
top plate is less than 10 mils. It’s all typically pretty stiff here
relative to typical peak loads if that is what you are trying to get a
handle on.
Regarding a 3^rd mast support, I have done that scenario too on a
project a few years back and I know we ran a FE model of it. I’ll see
if I can find the file. I recall that the key to getting it to work
was to float the rotor in resilient bushings while restricting it’s
rotation (obviously). I think you made a point about the alignment of
3 constraint locations being problematical and I agree with you.
Thanks for your insight and response.
Best 73
Matt
KM5VI
-----Original Message-----
From: TowerTalk <towertalk-bounces@contesting.com> On Behalf Of Grant
Saviers
Sent: Saturday, December 26, 2020 9:08 AM
To: 'Tower Talk' <TowerTalk@contesting.com>
Subject: Re: [TowerTalk] FW: FW: Thrust Bearing Installation
Matt,
Your statics 101 analysis is misleading. You lowered the load above the
tower by 2 ft, At 10ft load height, the mast would still have
1000ft-lb bending moment at the thrust bearing in the analysis, and no
reduction as claimed. Others pointed this out.
While instructive, it's also bit misleading and more complex since mast
support bearings have resistance to bending forces - they aren't a
friction-less point pivot of simple mechanics. The resistance is higher
when there is axial preload on an their commercial angular contact
bearing designs. The mast support bearing really isn't an unconstrained
pivot and is limiting the moment below when the mast is loaded by the
wind forces.
For a tower top tube radial bearing ala UST crank ups and Rohn tube tops
there is much higher resistance, depending on the plate thickness,
welding, and tube properties. The two separated tube mounting plates on
an HDX589 make that resistance to bending very high. Thus, the moment
that is transferred to anything below will be substantially lowered or
close to zero depending on the design. In this case the stiffness of
the tower matters as I posted. If the tower bends (it does, how much?)
then the result is different.
For top bearings with pillow block ball bearings in spherical (self
aligning) or cylindrical blocks or thick polymer etc. radial bearings,
then there are different properties.
Except for the tower bending situation, I think what happens below the
top support will have very little influence on the mast bending moment
just above a real support. However, the forces on the tower and rotator
can be redistributed.
The thread start mentioned a concern about the benefit of a third
support of the mast inside the tower. So, that question hasn't been
tackled. I think that is a big challenge for a number of reasons and it
doesn't fit into a statics 101 tool bucket.
Grant KZ1W
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