[TowerTalk] High VSWR

[Grant Saviers]

I've seen tests were 40% of the tension was lost in the pulley.  Ball 
bearing sheaves and some polymer sleeve bearings can substantially 
reduce friction loss.  Pretty important when Armstrong powered.  Why my 
ball bearing block set with low stretch line is "can I borrow".

The other factor is the ratio of sheave pitch diameter to wire diameter 
as this compromises wire rope strength as well as life.  16:1 ratio is 
the code minimum in many applications as that is about a 10% loss of 
strength, but every wire rope construction is different.

https://www.loosco.com/resource-library/technical-information/pulley-sheave-diameters/

Per Loosco, for the common 7x19 1/4" crankup hoist cable the "critical 
minimum" is 4.5" (18:1) and the "desirable minimum" is 6" (24:1).

No crank up I have seen meets either spec, so cable strength and life is 
degraded accordingly.  If the tower engineers designed for this loss in 
strength, then all is ok.  Crosby has more data:

https://www.thecrosbygroup.com/html/en-US/pdf/pgs/380.pdf

Grant KZ1W


On 8/31/2019 10:32 PM, Jim Thomson wrote:
> ##  any  time a winch cable  passes  through a pulley,  you can  easily  add 10%  to  the total  load.
> Smallish  diam pulleys are  the worse,  increasing  the  load  well above  10%....sometimes as  high  as  60%.
> Smallish  diam  pulleys also  put a huge  strain  on the internal strands.  You  can  have internal  strands broken...  and  not  even
> know  it.   Prelube 10-11-12 is  used  across  town at the naval  base.  4 x 55 gal  drums  on a pallet.   They pour it  onto the  winch
> cables, via an  auto  feeder system.
> 
> ## If  more  than  one  pulley  used,  factor  in at least  10%  for  EACH pulley.
> 
> ##  If  cable  clamps  are  used, instead of  nicropress  crimps, the U  bolt  has to  bear  down
> on  the  short..dead  end of the  cable.   The saddle  bears  down  on  the  live...long end  of the cable.
> Cant  saddle a dead  horse is the adage.   Dont  laugh.  I  have seen  them  installed  wrong  several times.
> 
> Jim   VE7RF.
> _______________________________________________
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