One of the locals recently suffered the loss of a Force 12 6L20
12 ft. above a 2L40 / 2L80 interlaced Yagi.
The tower was 130 ft of Rohn 45, guyed at 120 ft with a flat top
section above the last guys. The rotor was 4 or 5 ft inside the
top section with 13 or 14 ft of mast protruding out the top. The
flat plate and approximately 18 inches of two legs broke loose.
One of the welds for the top plate was alledgedly reported to be
questionable. It is not known if the stresses were analyzed.
IF you are planning to put a long mast in a tower, several steps
should be taken to ensure the safety of your design. 1 - make
sure the tower is rated for the proposed windload of the antennas,
2 - make sure the MAST is capable of handling the antenna load,
3 - place the top guys as close to the top of the tower as possible,
4 - consider placing the rotor at the top of the next LOWER section,
(approximately 8 to 10 ft inside the tower) to minimize the bending
moment on the top of the tower, and 5 - use an intermediate mast
support just above the rotor to stabilize the mast when the rotor is
removed.
de Tom N4KG
.............................
On Mon, 6 Apr 1998 00:53:12 -0700 (PDT) w7ni@teleport.com (Stan
Griffiths) writes:
>>
>>When using a long mast (say 20 ft) in a flat top section of 45G where a
>>thrust bearing is installed on the flat top and the rotor is mounted
>about 5 feet down inside the tower.... what is the preferred method of
>>supporting/guiding the mast between the rotor and the top of the
>tower:
>>
>>(1) Do nothing?
>>
>>(2) Fab up a tube/sleeve slightly larger I.D. than the mast O.D. and
>pass
>>the mast through it? If this is your preference how do you design the
>sleeve
>>support... weld it on the bottom of a Rohn accessory shlef? What do
>you use
>>for sleeve?
>>
>>(3) Install an intermediate thrust bearing? How do you mount... on a
>Rohn
>>accessory shelf? I see in the Texas Tower's catalog that rotor plates
>are
>>not designed to substitute for a flat top (rotor plates flex).... but
>is
>>this a problem on an intermediate bearing?
>>
>>
>>I would think some sort of support is needed inside the tower at
>least on a
>>temporary basis if one plans to use a tram to raise an antenna and
>the tram
>>is attached to the mast. Without such a support all the pull is on
>the top
>>bearing and it would seem the bottom of the mast would go all over
>the place
>>(and bearing would seem to be under undue stress). Other ideas for
>tramming
>>if (1), (2) or (3) not used?
>>
>>
>>Larry WI5A
>>Friendswood, TX
>
>Hi Larry,
>
>I think the use of a pipe top (45AG3) rather than a flat top should
>solve
>all those problems. If you really need a thrust bearing, it can be
>mounted
>on a rotator shelf. True, they aren't very thick but I have not seen
>one
>fail in that application.
>
>Stan w7ni@teleport.com
>
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