Hi Matt,
Your calculations are correct, but the only reason the thrust bearing in
your example reduces the bending moment on the mast is because it is 2
feet above the rotator, shortening the length of mast above it. This
would, in effect, require two more feet of tower minus the height of the
rotator. If the thrust bearing were at the same location as the top of
the rotator was, with the rotator located below, the moment on the mast
would be the same. The bending moment on the mast just below the thrust
bearing is almost exactly the same as that just above it. That, however,
is the maximum bending moment, which is the important thing as far as
the mast is concerned.
The rotator below the thrust bearing does see a bending moment, but it
is smaller that what it would see if it were on the top of the tower
without the thrust bearing. In your example, if the top of the rotator
were 2 feet below the thrust bearing, and the rotator was 1 foot tall,
the moment would be at most 500 ft-lb. It would probably be considerably
less, because of the stiffness of the clamp at the rotator. Obviously,
increasing the distance between the rotator and the thrust bearing would
reduce this further.
Like most EE students, I really hated engineering mechanics when I had
to study it in college. It's turned out to be very useful.
73,
Scott K9MA
On 12/25/2020 6:04 PM, 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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