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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