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

[Roger K8RI on Tower]

> In a message dated 6/8/2005 7:29:19 PM Pacific Daylight Time,
> towertalk-request at contesting.com writes:
>
>
>>  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 Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
www.rogerhalstead.com
>    Hey - that was fun!
>
> Cheers,
> Steve    K7LXC
> TOWER TECH
> Steve    K7LXC
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