Alan NV8A wrote:
> On 05/24/08 04:29 am Roger (K8RI) wrote:
>
>
>> Is there a good description of the mechanics for a crank up mast on the
>> web? I'm looking at three sections or more. I've found a couple of
>>
>> <snip
>> From what I have read, you need tube rather than pipe; and
>> square-section tubing would eliminate the rotation problem.
>>
>>
Square tube comes with its own set of problems and still allows for a
great deal of movement unless *snug* fitting collars are used. When I
finish it needs to be nearly as rigid as a guyed tower, and support a
good load (both wind and weight)
>> But it seems to me that what you are trying to do is reinvent the US
>> Tower Corp.'s MA-series, or something like it.
>>
>>
Not reinvent, but recreate? Pretty much so, yes. I have most of the
equipment and I like to build so this project has two aspects. I need
the second structure and I want to build it myself. A crank up would be
much easier (and handier) to use than the guyed tower and self
supporting gets rid of a lot of *stuff* in the yard for which there is
no room. However the material itself costs more than a brand new 25G
with all accessories. So this is not an expense saving measure unless
compared to purchasing the equivalent.
BTW due to the weight and size of the required lifting cable the
clearance between sections needs to be "at least 3/8" (0.375")". That
means a 3" top section needs to nest into two sizes up, or a 4" tube
which would give a 1/2" clearance. with the next section being 5". In
this case the cable only needs to lift the 4" and 3" sections meaning it
doesn't have a terribly heavy load to get to about 50'. That's allowing
a 5' overlap at each section. But... going to 65' I'd have to add a 6"
section and now the cable would have to lift the 5" as well. That means
at least twice the weight it was previously lifting. That also puts
quite a load on the base along with a lot of momentum for the rotator.
This is the main reason the fold over is so much easier, lighter, and
less expensive to build. With schedule 40 each progressive size fits
over the previous with a pretty good fit. The Raising fixture which
also serves at the pivot for the fold over feature and the rotation
mechanism is easier to handle as well. Even that, being sturdy enough
to be self supporting without whipping around in the wind is heavy. I
think I figured near 700# for a 60' and that does not include the 20'
raising fixture which has an additional 5' extending into the concrete.
That 60 footer takes more concrete in one block than all of the concrete
used for my 100' 45G. So, even though it's home brew this is no minor
project.
73
Roger (K8RI)
>> 73
>>
>> Alan NV8A
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