I agree that the resistance of the thrust bearing to bending can affect
the moment on the mast. However, the moment just above the bearing is
not affected, and can be easily calculated. I think this is almost
always the maximum.
On the related topic of whether a typical thrust bearing is designed for
vertical loads, I recall taking apart a TB3 a while ago. It is clearly
designed primarily for horizontal loads. To get it apart, you remove a
plug and take out all the bearings, then the top and bottom pieces can
be separated. If I recall correctly, the bearing races are grooves in
the inner and out pieces, with semi-circular cross sections. Horizontal
loads force the bearings against the whole surface, or at least the
center part, of the races. Vertical loads concentrate that load at the
edges where the two sections meet. I wish I'd taken some photos. I
suspect there is enough vertical play in the bearing that by tightening
the bearing bolts after the vertical load is on the rotator would
minimize the vertical load on the bearing during operation.
73,
Scott K9MA
On 12/26/2020 9:07 AM, Grant Saviers wrote:
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
On 12/25/2020 16:04, maflukey@gmail.com wrote:
Thanks Scott and good point about the rotor reaction, but actually both
statements are true because the bending moment at the base of a mast,
without any intermediate horizontal support (thrust bearing), is fully
transferred to the rotor. The addition of a thrust bearing eliminates
point moment transfers to both the rotor and the bearing (but not the
tower
itself). The thrust bearing, however, does reduce peak bending
moment on
the mast.
Example as follows:
Consider a 10 ft mast with 100 lbs of horizontal wind load on an
antenna at
the top of the mast. Neglect wind load on the mast for now to keep the
calculation simple.
In the no thrust bearing case, the bending moment is maximum at the
base of
the mast and equal to 10 ft x 100 lbs = 1,000 ft lbs of torque. This
moment is fully transferred to the rotor so the rotor sees the same
1,000 ft
lbs of torque. The rotor also sees a horizontal shear reaction
force that
is equal to (100 lbs) and opposite to the direction of the wind load
at the
top of the mast.
Now if we consider a thrust bearing placed at 2 ft above the rotor,
under
the same loading condition, the 100 lb horizontal load at the top of the
mast creates a 4:1 lever arm (8 ft above the bearing divided by 2 ft
below
the bearing) that acts on the bottom of the mast. Consequently
there will
be a 100 x 4 = 400 lb horizontal shear load transferred to the rotor
in the
same direction as the load at the top of the mast. At 2ft up from the
bottom of the mast, the thrust bearing reaction will be the sum of the
horizontal loads, or 400 (bottom) + 100 (top) = 500 lb that will act
in the
opposite direction as the wind load. The peak bending moment in the
mast
will occur at the thrust bearing location and will be 8 ft x 100 lbs
= 800
ft lbs. The bending moment at the bottom of the mast will be zero
because
the wind load and the thrust bearing reaction will cancel each other
out...
(10 ft x 100 lbs) + (2 ft x -500 lbs) = 0 ft lbs. Note that the 500
lbs
reaction at the bearing is negative because it acts in the opposite
direction as the wind load.
So the thrust bearing acts to reduce the peak bending moment that
occurs in
the mast, and as you correctly point out, it eliminates the bending
moment
at the rotor. It also acts to increase the horizontal shear
reaction force
transferred to the rotor. There are no point moments transferred to the
rotor or the thrust bearing themselves. The top of the tower
however will
see the full transferred moment of the antenna load which can be
demonstrated by a similar example that includes the entire tower.
Many 73 & QSH to all
Matt
KM5VI
-----Original Message-----
From: TowerTalk <towertalk-bounces@contesting.com> On Behalf Of K9MA
Sent: Thursday, December 24, 2020 12:04 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] FW: Thrust Bearing Installation
The thrust bearing reduces the bending moment on the rotator, not the
mast.
With the rotator in the tower below the thrust bearing, there is very
little
bending moment on the rotator, just a horizontal force.
There's also, of course, a vertical force, unless that is taken by the
thrust bearing. The greater the distance between the thrust bearing and
rotator, the smaller the horizontal force on the rotator.and thrust
bearing.
73,
Scott K9MA
On 12/24/2020 3:37 AM, maflukey@gmail.com wrote:
It's typically not about the dead weight of the mast & antennas, it's
about
reducing the bending moment on the mast under wind loading.
73
Matt
KM5VI
-----Original Message-----
From: TowerTalk <towertalk-bounces@contesting.com> On Behalf Of
krgoodwin@comcast.net
Sent: Monday, December 21, 2020 3:52 PM
To: towertalk@contesting.com
Subject: [TowerTalk] Thrust Bearing Installation
Installing a thrust bearing in a tower - Dead weight (along the gravity
vector) all on the rotator or all on the thrust bearing? Seeing such
things
as sleeves for towers, I would surmise that all of the dead weight
is on
the
rotator and only off-axis loads (perpendicular to the gravity
vector) are
handled by the thrust bearing. I use two thrust bearings in my tower
which
I don't believe effects the answer to the above question. Ken K5RG
_______________________________________________
--
Scott K9MA
k9ma@sdellington.us
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