[TowerTalk] TB3 Postmortem

Mon May 6 11:37:52 EDT 2019

The TB3 did last for almost 30 years.

73,
Scott K9MA

On 5/5/2019 22:38, Grant Saviers wrote:
> A great list of why a bored plastic plate as a sleeve radial bearing 
> is better than the so called "thrust" bearings.  Note that US Tower 
> crank-ups use a piece of galvanized tubing as a radial bearing at the 
> tower top.
>
> Grant KZ1W
>
> On 5/5/2019 9:50 AM, Bob Shohet, KQ2M wrote:
>> Hi Scott,
>>
>> Your post brings up some important points.
>>
>> First, when using any thrust bearing (TB), it is important to realize 
>> that the TB may not be machined perfectly so that the angle of the 
>> holes for the setscrews may be slightly different, so when tightened, 
>> can cause the antenna mast in the rotator to be slightly out of 
>> alignment when compared to vertical. This can become a problem 
>> especially with a heavy antenna and a heavy mast!
>>
>> Second, often the clamps on the rotator shelf (on which the TB is 
>> mounted) or on the top flat tower section are also slightly out of 
>> alignment when compared to vertical so that the combination forces 
>> the mast holding the antenna to NOT be vertical.  This can make is 
>> easier for the antenna and rotator to bind in the TB and constrict 
>> rotation.  You would notice this even more on very windy days or when 
>> there is ice which adds additional weight to the entirety of what is 
>> being rotated and adds to the forces fighting each other.  There is 
>> only so much binding that the rotator can overcome and often it will 
>> help to degrade the ball bearings and the motor much more quickly 
>> than would otherwise occur.
>>
>> Third, most people make the mistake of tightening the mast clamps in 
>> the rotator before rotating the antenna to see if there is anything 
>> if misaligned and if the antenna can rotate freely.  This ensures 
>> that the TB is most likely to bind, especially if it is on a 
>> sidemount which may be misaligned in and of itself.
>>
>> So there are at least four types of potential misalignments that can 
>> account for binding of rotation over and above any existing issues 
>> with the rotator or the TB.
>>
>> I have had a lot of experiences with these various binding forces.  
>> It is quite amazing how the complex combinations of the slightest 
>> differences in misalignments of fractions of an inch here and there 
>> can be the difference between an antenna that freely rotates with a 
>> happy rotator and thrust bearing vs an antenna that frequently binds 
>> with a worn TB and damaged rotator.
>>
>> I???m inclined to think that turning the antenna every so often and 
>> leaving in different positions should help to spread the wear and 
>> deter the formation of any ???grooves???.  But a better alternative 
>> is to make sure that the rotator that you are using is significantly 
>> higher rated than what you are attempting to turn and that everything 
>> is aligned as close to perfectly level as possible with the 
>> tightening of all bolts and clamps done in several stages with 
>> rotation observed in between to ensure that further tightening is not 
>> causing any problems while the person doing the tower work is still 
>> on the tower and observing everything from a few feet away before 
>> finally making the final tightening.  In some cases I have had to 
>> leave extra ???play??? in the TB screws to adjust for the 
>> misalignments using the TB more as a ring to hold the mast in place 
>> as opposed to being tight.
>>
>> It also helps to have the antenna balanced for weight right at the 
>> point where it is clamped to the mast.  All of my yagis are perfectly 
>> balanced for weight
>> at the boom to mast clamp because to do otherwise not only makes it 
>> harder and less safe to work on them but also because it puts 
>> additional stress on the TB and the rotator even while fixed in place 
>> and may cause them to wear unevenly and fail more easily.
>>
>> 73
>>
>> Bob KQ2M
>>
>>
>> From: K9MA
>> Sent: Sunday, May 05, 2019 11:05 AM
>> To: towertalk at contesting.com
>> Subject: Re: [TowerTalk] TB3 Postmortem
>>
>> True, but the rotator distributes the load among a much larger number of
>> ball bearings. The shape of the races in the TB3 make it clear it was
>> designed primarily for radial loads, not axial.  Also, an "in tower"
>> rotator can be removed without taking the whole antenna system down.
>>
>> Another thought occurred to me: With the thrust bearing bolts tightened
>> down, the system is overconstrained. That means that very high stresses
>> could occur if the two bearings systems aren't perfectly true as they
>> fight each other.
>>
>> 73,
>> Scott K9MA
>>
>>
>> On 5/5/2019 06:46, mike repinski via TowerTalk wrote:
>>>    Then the rotator gets the little marks. Can't win.
>>>       -----Original Message-----
>>> From: K9MA <k9ma at sdellington.us>
>>> To: towertalk <towertalk at contesting.com>
>>> Sent: Sat, May 4, 2019 9:39 pm
>>> Subject: [TowerTalk] TB3 Postmortem
>>>
>>> After nearly 30 years of holding up my TH7 and D40, the thrust bearing
>>> was pretty rough. The T2X never had any trouble turning it, but I took
>>> it apart to see what was going on. I found the surfaces of the two
>>> aluminum pieces deeply indented by the ball bearings. It's scrap metal
>>> now, but I wonder if the long periods I left the antenna in one 
>>> position
>>> might have made things worse. Would it have helped to just turn it once
>>> a week or so? Or is such failure inevitable? After all, once the
>>> bearings create the slightest indentation, they're going to always
>>> settle into those positions, making them worse. I suppose one could
>>> periodically take it down and smooth out the surfaces, but who is going
>>> to bother with that? It lasted almost 30 years, so I got my money's 
>>> worth.
>>>
>>> Another possibility: Would it be better to leave the bearing set screws
>>> slightly loose, so the rotator takes the vertical load?
>>>
>>> 73,
>>>
>>> Scott K9MA
>>>
>>
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-- 
Scott  K9MA

k9ma at sdellington.us