Rob,
Positive "pull-down" is a feature on the Tri-EX LM-470 series towers. The
key to this technique is a cable drum wide enough to hold cable for the LIFT
side and the DWN side cables. The "Lifting" cable only works on the first
moveable (second tower section) section, going up the first and back down to
the
bottom of the second section. The other sections are all lifted by
interconnecting section cables.
The DOWN cable is connected to the bottom of the TOP section, but not
directly, it should be decoupled by a spring that works in compression and is
about
12" long. This spring compensates for variation in the cable wrap rates and
helps to maintain a uniform wind tension.
When the tower is "Down", the UPCABLE is unwound, and the DWN cable is fully
wrapped on the drum. The opposite condition exists when the tower is fully
extended. The trick is to get the two cables to be synchronized in rates of
change. The slightest variation in drum diameter can effect the amount of
slop in the down cable (the up cable is always in tension). Caution, a loose
of sloppy down cable can be a problem in two ways, one is it may allow the
cable windings to wrap upon themselves (changing the effective drum diameter)
or it may allow the sections to "hang" for a few inches and then drop
suddenly - that shock isn't good for cables.
Hope this helps. BTW, US Tower system is more complex as it utilizes a
screw driven LEVEL wind feature, but still basically the same process.
Regards,
Norm W4QN
=================================================================
> On Thu, 05 Apr 2007 07:45:18 -0700
> Rob Frohne <frohro@wwc.edu> wrote:
>> Hi,
>>
>> I have two Tri-EX THD series towers and have had trouble
>>with them
>> jamming when partially erected. I am thinking of
>>putting a cable from
>> the top section to a pulley on the bottom section so
>>that I could pull
>> down if necessary if it gets jammed again. I have read
>>of a thing
>> called a "positive pull down" on some crank up towers,
>>and would like to
>> see some photos of it before I do my thing, in case I'm
>>missing
>> something, and they have a better way of doing it than I
>>am envisioning.
>> Does anyone have any photos of a "positive pull down"
>>mechanism?
>>
>> Thanks & 73,
>>
>> Rob, KL7NA
>> --
>> Rob Frohne, Ph.D., P.E.
>> E.F. Cross School of Engineering
>> Walla Walla College
>> 100 SW 4th Street
>> College Place, WA 99324
>> (509) 527-2075
>> http://www.wwc.edu/~frohro/
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