[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
More information about the TowerTalk
mailing list