[TowerTalk] FW: FW: Thrust Bearing Installation

K9MA k9ma at sdellington.us
Sat Dec 26 12:42:28 EST 2020


Yes, once you get into an overconstrained configuration and have to 
consider the stiffness of all the components, things get a lot more 
complicated. I expect mechanical designers now use finite element 
analysis for all but the simplest situations.

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 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
>> Matt
>> KM5VI
>>
>>
>> -----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.
>>
>> 73,
>> 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
>> about
>>> reducing the bending moment on the mast under wind loading.
>>>
>>> 73
>>> Matt
>>> KM5VI
>>>
>>>
>>> -----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
>> 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 at sdellington.us



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