[TowerTalk] installing monster masts in towers(and ? twothrustbearings)

[Alan NV8A (ex. AB2OS)]

On 06/10/05 03:43 am Roger K8RI on Tower tossed the following 
ingredients into the ever-growing pot of cybersoup:

>>> Thrust bearings normally only need to handle side to side forces. The
>>
>>rotors
> 
> 
> Agreed.
> 
> 
>>can bear the weight of the antenna(s) and mast with no problem. KC1XX 
>>tells
>>me that the locking bolts should all be removed from thrust bearings. Let
> 
> 
> I'd sure hate to do that in my system.   Look at my system and tell me what 
> a half inch of side play in the  top bushing would do.  That's what I'd have 
> if I took out those bolts as there is a quarter inch clearance all the way 
> around the mast in the bearings..

>>the rotor hold the vertical load and let the thrust bearings take the
>>horizontal. My thrust bearings have no locking bolts whatsoever. No
>>problems. When I need to take the rotor out, I put some U bolts on the 
>>mast.

> There are different systems.
> Mine extends 30 feet above the top of the tower and about 14 feet below the 
> top.  The weight minus the rotator is over 600#.  The top bearing would 
> serve as a pivot point with basically a 2:1 multiplication.  That means 100# 
> side force at the top would be 200
> # of side force at the rotator.  There is also 400 # of weight above the top 
> bearing.  when it flexes that is an 800# force that would otherwise be 
> appled to the rotator.  The forces far exceed 100# at the top with 70 MPH 
> gusts and then the spring in the mast will start an oscillation.  The 
> rotator would be holding as much or more side thrust than the top bearing 
> and it's not built for that.
> 
> In 60 to 70 MPH winds which seem to be more common of late, that top mast 
> looks like a blue gill rod that just tied into a bass.  I'm amazed the 
> antennas have held together after watching it whip around.

>>   Amen to that, brother. My personal opinion is that mast vertical thrust
>>bearings are highly over-rated and don't really contribute anything of 
>>value to
>>the rotating system other than a being a nice mast bushing.

> Agreed again, but you don't want any play in them.  With the mass in my 
> system it'd destroy itself if the bolts were removed from the top bearing.

>>   IMO the amateur need for a special thrust bearing is sort of like the
>>amateur perceived need for use of  the 1-foot long "torque arms" that hams 
>>have

> If you are talking about the 18" ROHN torque arms they achor at a single 
> point and are only relatively fixed.  You end up depending on the amount of 
> torque used to tighten them in place.  The *star* system which extends less 
> is far more rigid.

>>put great value in for decades but in reality add little or nothing by its 
>>use.
>>(The Great Ham Radio Urban Legends?!?)
>>
>>   Rotators are designed to have some preload on them and their bearings.
>>Why do you think all those bearings are there in the first place?!? The 
>>T2X has
>>NINETY-SIX of them.

> A tail twister lasted a very short time on my system and the winds were not 
> all that bad either..
> 
> It went through two HDR600s in as many weeks, but that was due to the poor 
> break setup and it's ability to free wheel with no stops. You have to 
> release the brake and then apply power.  In strong winds the antennas are 
> already turning fast even if you try to operate the motor right after 
> releasing the break.  Those suckers just scream.

>>   For the reasons posted previously, a mid thrust bearing is just a total
>>waste of money. (Unless you really did need that expensive bushing - hi.)

> Again, you need to do the math.  That bearing above the rotator takes the 
> side thrust which the rotator is not designed to take. My rotator supports 
> the load, but the bearings take the side thrust which is what they were 
> designed to do.

>>   People SCREW UP bearings in different ways. If they'd have just left 
>>them
>>off, they'd have been in better shape! And that's not speaking to the bind

> In many cases I'd agree, but there are antenna systems and then there are 
> antenna systems.
> I don't think I could even work on mine without the bottom bearing.  It'd 
> take a "comealong" to center the mast to get it in the rotator No way could 
> I counter that top heavy beast by hand even on a calm day.
> 
> Now if we were talking bushings that just nicely slip fit the mast, I'd 
> agree that no bolts would be needeed with the exception of when you want to 
> do some work on the rotator.  OTOH it's easier to move the mast with a 
> comealong and lock it in place with the bearing bolts when working on the 
> rotator.
> 
> As for bind, with both bearings centered and tightened, I can loosen the 
> mast clamp in the rotator and turn the entire system with one hand (on a 
> calm day). I've done it when working on the pigtails at the top of the 
> tower. Of course this shifts the weight from the rotator to the bearings. 
> I've also had trouble just holding it with a big strap wrench in just a 10 
> to 12 MPH wind.

>>that can be easily introduced to the rotating system. And I take my 
>>hard-hat off
>>to anyone who can measure 0.01" with an instrument on top of a tower. Both 
>>of
>>you.

> You don't need to be any where near that.  1/16", or  0.065 is relatively 
> easy to do. Of course that is over 6 times the 0.01".  Having worked in a 
> shop, the 0.01" is considered a coarse measurement.
> 
> Even a steel mast with a quarter inch wall will have enough spring you'd 
> never know the 1/16th was there over three feet.  If you have the equipment 
> it matters not whether you are on top of the tower of in the work shop to do 
> the measurements.  A magnetic base or clamp to hold the indicator (or 
> ruller) is really all that is needed.  I'd not use a dial indicator as the 
> mast is not going to be true to begin.  A simple, fixed reference point and 
> pair of calipers should be suficient.
> However, given the proper equipment and a round mast almost any one whould, 
> or should,  be able to measure to three digits, not two.  It just ain't all 
> that difficult.

>>   And don't get me started about the yucky eccentric collar TB that US
>>Tower uses. They are really useless.
>>
> 
> Again we agree on this point.

Roger:

Maybe you've had somebody do the calculations and ascertain that 
everything is safe, but I definitely do not like the way your system 
sounds from your description: that's an enormous load on a long piece of 
tube sticking out the top -- and on the top section of tower above the 
uppermost guys.

I'd rather go higher with the tower itself and keep the mast to a much 
more reasonable length: 2" CM tube is almost as expensive per foot as 
additional AN Wireless mast sections,

73

Alan NV8A
Zeeland, MI