If I understand correctly Jerry, you are just supporting the mast with the
top of the thrust bearing, leaving the mast free to slide up and down in the
bearing. It is serving as a guide to take the side thrust. In that case I
would expect to see nothing more than what you suspect, the only thing that
would happen is the weight might be transrered to the rotor and there should
be no problem. That's assuming the mast has a greater coefficient of
expansion than the tower. OTOH when the mast gets cold it shrinks and
might pull against the top of the rotor. Assuming the tower and mast are
both steel, even of different alloys the difference is probably very small.
In general, most rotors, at least of the ham variety can support a lot of
weight.and in most instalations the weight of the entire antenna system is
no strain on the rotor.
Although I'm not doing it that way, there is a lot to be said for using the
thrust bearing only as a guide to remove the side loads from the rotor
rather than using it to support the system weight. *Most* thrust bearings
do just that even if they are nothing more than the sleeve of the "pointed
top" tower sections. "I would think" that except in very large systems the
sleeve would be the best bet, but it needs to be a slip fit that does not
allow the mast to move from side to side..
I've been up "on top" of a pointed top tower where the top sleeve was used
as simple thrust bearing. The mast had nearly an 1/8th of an inch clearance
and supported a tribander a foot or two above the top and a vertical
(repeater antenna) about 10 feet above that. It was a windy day and it
sounded like some one was beating the top of that tower with a hammer.
After about 10 years that top sleeve was only a thin shell. I used one
"similar" to the one above, but the mast was a slip fit. After 15 years it
only displayed a little wear. It's in the top photo on
http://www.rogerhalstead.com/ham_files/tower.htm although the repeater
antenna has been removed.
I always put a seal over the top and grease zerks in the bolt holes. Even a
little grease almost eliminated that wear. OTOH up here in the north any
moisture getting into the sleeve would cause the grease to freeze in the
colder months. Possibly a dry lubricant would be better, but the grease
also served as a rust preventative. I used the plastic top from a spray
paint can with a hole just large enough for the mast. It was slid over the
mast at installation, then positioned to fit down over the sleeve and
securely "RTVd" to the mast. This kept the water out of the sleeve and
worked very well. I wish I could remember the name and call of the guy who
showed me how to do this. His first name was Lew, but his call escapes me.
I believe an earlier post mentioned a bearing that was designed to only
support weight rather than to take up the side force. Although all the
bearings I've used were designed to do both, they made a very important
point and that was the rotor alignment is critical. Remembering "Only one
size mast" will be centered in the mast clamp can make a huge difference.
Rotors such as the Hy-Gain HDR 300 and the Prosistel (Big Boy) have
provisions to move the mast clamp which allow centering different size
masts. Most bell shaped rotors do not have this capability and even though
rated for some pretty big loads they are not heavily constructed to support
side loads and could be damaged by the misalignment if a thrust bearing is
mounted close to the rotor. Undersize mast can be shimmed and pinned.
Overisze will be off center.
Depending on the length of the arm (how far the mast extends above the top
of the tower) and how far down the rotor is mounted, I would have some
concern about any side-to-side play between the mast and thrust bearing.
With a large antenna, even though the movement is small, the force of the
mast contacting the bolts is like a hammer blow. On my system, due to the
mass of the thing I feel it's necessary to keep everything tight. OTOH I
think I am going to replace the UHF and VHF beams used on FM with a single
vertical.
Roger Halstead (K8RI, EN73 & ARRL Life Member)
N833R, World's Oldest Debonair (S# CD-2)
www.rogerhalstead.com
***************
The point about mast expansion is very interesting, but am I correct in
thinking that it wouldn't produce any problem if the weight of the mast and
antennas is borne by the clamps bearing on the thrust bearings, and the mast
is not gripped by the setscrews in the thrust bearings? It seems to me the
worst that would happen is the rotor might get more load temporarily, as the
mast expansion lifts the load off the TB's, then contracts to put it back
on.
73, Jerry K3BZ
----- Original Message -----
From: Tower (K8RI)
To: towertalk@contesting.com
Sent: Thursday, July 22, 2004 5:38 PM
Subject: Re: [TowerTalk] Re: Thrust bearing
Likewise, I use two thrust bearings and unlike others here, I use the
thrust
bearings to support all the weight of the antenna system which is
substantial. Probably on the order of 600#, give or take. They maybe
crude,
but I think they are far more rugged at supporting weight than most rotors
such as the tail twister and HDR 200s and 300s.
In all the years I've used the ROHN thrust bearings I've never had one
fail.OTOH, prior to the PST61 rotors suffered very short life spans.
Nothing would hold that system including the big Hygain HDR 300s. I use
the
plural as I went through two of them in almost as many weeks.
One thing about using two thrust bearins, or even one for that matter.
The
mast is clamped in the bearing(s) and rotor. The mast changes length with
temperature. To reduce bending moment the mast should extend below the
top
of the tower about as far as it extends above the top. If the mast is
Aluminum it will expand and contract much more than the steel tower and
can
put tremendous strain on the top of the rotor, in either, or both pushing
and pulling. With Aluminum masts in Aluminum towers and steel masts in
steel towers the differential should be minimal.
Roger Halstead (K8RI, EN73 & ARRL Life Member)
N833R, World's Oldest Debonair (S# CD-2)
www.rogerhalstead.com
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
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