[TowerTalk] winch loads and tower raising fixtures.

[Jim Jarvis]

This started with Al Williams' question about winch ratings...
was amplified by a mfr comment that a 1500 lb rated winch should
do ok...and was commented on by Jim Lux, as follows:

"And now you see why you need to ask the manufacturer why they think a 1500
pound rated winch is good enough.  For instance, is there an extra pulley
pass in there, so the winch cable is doubled, in which case the winch load
is half the lifting load.  The angle might not be 45 degrees (particularly
if the far end of the tower is supported off the ground.  And, the load on
the cable will decrease as the tower comes up . "

My questions/comments:

1)  If the raising fixture isn't rated to raise the tower, why do they sell
it?
If you have to raise the tower to the point where the fixture CAN raise
it...
what good is it?   How do they propose you lift the half ton load otherwise?
    This causes me to conclude that the fixture IS rated to lift the tower
with
a rated windload of antennas on top, from a horizontal position.

2)  Comments about lifting angle and loads... I would analyse the problem
using
the center point of the tower as the lift point, just to simplify the math.
1100lbs of deadload can be assumed to locate at its center, which is N feet
out
from the hinge.  1100lbs x N = some # of ft. lbs.   The LIFTING aspect is
also
multiplied by some leverage...and will net out a deadload on the winch.  You
need
a safety margin on that...probably 30% or so.  Given real dimensions, I
could
walk you through the calculation, taking angles into account...but I think
Jim Lux
already did a good job of that.

	In point of fact, the mfr may attach the lifting cable at a point which
equals the height of the lifting fixture.  Possibly below the center of the
retracted
tower.  The loads go up quickly in that case.

3)  The REAL MYSTERY...and this is outside my experience...is whether
winches
rated at 1500lbs are so rated because that is the safe load, or whether it's
some
other figure...like maximum load capability.

n2ea
jimjarvis at ieee.org