On 6/1/2012 10:16 PM, Jim W7RY wrote:
> You probably are going to put more stress on the tower by adding the
> additional weight and wind load of the extra parts Vs. cutting the brace.
Even without the rotator mounted outside I'd still have the mast and
antennas. The extra wind load from the mast and the rotator outside the
tower (at 20') is very little. I could get by with only one mast
section, but that would only remove 60# and it'd put the rotator at a
very inconvenient height. Each piece of DOM has a 1/4" thick end cap
welded in place so each section is sealed with rust preventative inside.
Each section is 24'. Both DOM and structural steel tube come in 24'
lengths. The tower is only 50' with the mast extending 10 to 12' above
the top of the tower.
> In other words... Cut the brace.
Not a chance. Too crude. Besides the side load from chain tension is
spread over several feet and it's symmetrical over that area about 3 to
5 feet long, depending on where I locate the bearings. Also if the
tower went down for some reason, my insurance company's first question
would be "Why did you cut a brace out". I'd replace the tower before
cutting a brace.
Actually, this is supposed to be temporary and replaced with a home
brew, motorized crank up with an extendible mast. Now that's going to
take some calculations.
The vertical load from 48' of DOM is about 120# . The side load is also
about 120# but is spread between two bearings that will be some where
between 3 and 5 feet apart. The side plates to mount the rotator and
side thrust bearing are not very heavy. The rotator will be pretty much
down out of the wind and protected by the West end of the shop. The
weight and wind loadings are well within the ratings of the tower. OTOH
once the brace is cut I have no specific strength figures to base any
calculations on even if It is likely well within the tower ratings,
> Jim W7RY
> -----Original Message-----
> From: K8RI
> Sent: Friday, June 01, 2012 6:17 PM
> To: firstname.lastname@example.org
> Subject: [TowerTalk] Getting your moneys Worth
> A while back I mentioned trying to fit a Han IV into a 25G and not
> wanting to cut a brace. As the mast will extend about 12' above the top
> of the tower, I did not consider mounting inside the short section with
> the flat top plate to be desirable either. Sooo... I finally decided to
> mount the rotator externally and run a two section mast down through the
> tower. That will leave the rotator 20 feet above ground level and 30
> feet below the flat top plate...give or take a tad. I should be able to
> stand on the roof and work on it.
> The rotator will be mounted on a 3/16ths (0.1875" for those working in
> decimal) steel plate with a 2" stub mast out the top and through a
> thrust bearing. There will be a sprocket on the mast out of the rotator
> and another on the mast running up through the tower with bearings
> above and below the sprockets. There will be 2 idlers. As this is 80P
> chain it's 1" between centers for the rollers. The first digit is the
> number of 1/8ths while the second indicates standard (0) chain. Chain
> comes in 10' lengths and 10' of 80P just fits in one of the larger "IF
> it fits it ships" boxes.
> I haven't weighed it but I'd guess that box ran some where between 20
> and 30#
> BTW, Thanks to all those who came up with suggestions on locations to
> find parts that were far cheaper than the big supply houses. 10' of 80P
> can run $130 to $140, but through surplus sales in Texas it was (IIRC
> $40 shipped to my door) Those sprockets are massive too. Problem now,
> other than making the mounting plates for the rotator and bearings is I
> have to ream out the ID of the large sprockets to fit the mast which is
> 2" DOM and the ID of the bearings is 2" for an interference fit. The
> idlers just fit over a 3/4" bolt.
> I think the top of the tower in
> http://www.rogerhalstead.com/ham_files/skyhook.htm is driven by 80P
> chain. The sprocket in the second photo from the top appears to be for
> 80P which looks small in comparison and I'm just using a 2" mast. That
> top section is 3 feet across each side.
> Roger (K8RI)
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