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
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