[TowerTalk] Positive pull down

[NPAlex at aol.com]

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 at 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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