[TowerTalk] FW: FW: Thrust Bearing Installation
grants2 at pacbell.net
Sat Dec 26 10:07:56 EST 2020
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
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.
On 12/25/2020 16:04, maflukey at 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
> -----Original Message-----
> From: TowerTalk <towertalk-bounces at contesting.com> On Behalf Of K9MA
> Sent: Thursday, December 24, 2020 12:04 PM
> To: towertalk at 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.
> Scott K9MA
> On 12/24/2020 3:37 AM, maflukey at gmail.com wrote:
>> It's typically not about the dead weight of the mast & antennas, it's
>> reducing the bending moment on the mast under wind loading.
>> -----Original Message-----
>> From: TowerTalk <towertalk-bounces at contesting.com> On Behalf Of
>> krgoodwin at comcast.net
>> Sent: Monday, December 21, 2020 3:52 PM
>> To: towertalk at 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
>> as sleeves for towers, I would surmise that all of the dead weight is on
>> 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
>> I don't believe effects the answer to the above question. Ken K5RG
>> TowerTalk mailing list
>> TowerTalk at contesting.com
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