Well, for starters, the "skyhook" is only there -- along with a row of yellow-tape flags -- to help the winch operator see what his tag line is doing. ETA: WRONG! The Chief Rigger informs me it's called the "weasel," weighs about a hundred pounds and helps keep the basket riding freely on the tagline. My scale is way off, by the way; the weasel rides up and down about 60' away from the base of the tower, while the (only middle-sized) tree is maybe 400' away. In operation, there are two fairly sharp bends in the tagline: at the weasel and at the basket.
The load line rigging is more complex than I drew; it actually runs through yet another sheave fixed to the top of the basket and back to the tower, where it is attached just below the lower of the top two sheaves.
As for the rest, why, here's a thousand words worth:I have taken some liberties in simplifying, especially the load line and hoist, but that's the basic idea. The four sheaves (it's what most people call a pulley*) closest to the tower are, of course, strapped to it.
The more tension on the tagline, the farther the basket gets from the tower, especially when it's low -- which is when it needs to be able to move farthest away. It doesn't need a lot of travel on that axis and this gives it more than enough to avoid various terrestrial-relay dishes, TV receiving antennas and the tall fence around the tower base. Interestingly, there's very little force on the tree end of the tag line; the weight of the line and weasel do a lot of the work and all it sees is the horizontal force needs to displace the load: not much.
Semi-relatedly, it appears some places omit all the hardware: Personally, I've always found 3/8" cable to be too small for comfortable unassisted hoisting and a coating of ice doesn't help any. But maybe that's just me.
* The "pulley" is really just the wheel; the housing it spins in plus a sturdy attachment loop are what makes it a sheave.
Working On A Starship
4 weeks ago