[TowerTalk] Strength of installation raising cable for the LM470?

[Roger (K8RI) on TT]

When installing the LM470 using the raising fixture, the retracted tower 
is raised from horizontal to vertical.  The cable attaches to the tower 
roughly half way out presenting one whale of an initial load on the 
cable and winch.

Does anyone have the strength listed for the original cable? It's 
roughly a quarter inch galvanized steel. Unfortunately the strength for 
this size and kind of cable varies widely over more than 2 to 1.

I'm planning on replacing the single straight pull with a double pulley 
(rated at 32 kN = 7,193.8# ) 
http://www.convertunits.com/from/kN/to/pounds   at the tower and a 
single pulley back at the raising fixture.  This means I'm going to need 
a longer cable.  The existing cable isn't quite long enough to make the 
4 trips between the tower and raising fixture. Fortunately the new 
method reduces the stress on the cable by a LOT.  The cables available 
at the hardware store would likely be strong enough and they do carry a 
selection of strengths, although my preference is for using one of the 
established cable suppliers on the net.

BTW: As for cable flexing there is the drum on the winch, the original 
pulley on the raising fixture plus the 3 additional pulleys between the 
tower and raising fixture.  Fortunately these extra pulleys are only 
used when installing, taking the tower down, or laying it down for 
inspections.  I would add that the larger, pulleys rated for  32kN and 
up, get expensive.  I did find a couple rated for 2000# at the hardware 
store, but when I went back for more the strongest the had was only 
rated for 800#  A pair of single pulleys can be used in place of the 
double pulley and the pair of those pulleys rated for 2000# was about a 
quarter the list price for the Aluminum pulleys rated at 32 kN.

The multiple pulleys do reduce the linear stress on the cable, but each 
time that cable goes around a pulley the strands slide back and forth 
against each other, increasing wear on the cable.

I haven't been able to find the strength of the original cable. Searches 
on the net have become extremely difficult. Regardless of what search 
engine I use, it take me through multiple layers of other search 
engines, changing what used to take a minute or two into a long, arduous 
selection process.

As for rusting, or corrosion those raising pulleys don't have to be left 
in place as they see so little use.  Just oil them well before putting 
them in storage.

73

Roger (K8RI)


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