Tuesday, April 13, 2010

Pulleys rule

Warning: Today's post contains zero bike content. If you're looking for bike stuff, there's nothing to see here. Otherwise, consider it a blog tax. Guest bloggers: you're not allowed to do this. Well, maybe gNate could get away with it.

I've always like the idea of pulleys but I've never really sat down and figured out how they work. Turns out it's very simple. The piece that made it click for me is that for each pulley you add to a line, you're dividing the weight by the number of pulleys. And your multiplying the amount of line by the same amount.

So: add two pulleys to a line and you've halved the amount of weight you need to pull and doubled the amount of line your pulling.

Anyway, this entry on "How Stuff Works" lays it out nicely.

Maddie and I have been playing with pulleys the last couple days and it rules. We may use this method for entering the treehouse if we can figure out a foolproof sling/platform/harness/rope stop that any kid can use. If not, then we'll definitely keep the pulley system for hauling stuff up.

Here's a single pulley:

Simple and intuitive. You pull an armload of rope, you raise up one armload.

Here's two pulleys:

Though this is easier to lift, the complexity has increased. There are ropes everywhere and any parent worth their salt will look at that pulley there by little fingers and hair and just cringe. After we took this video, Maddie caught her finger, then little wisps of hair in the pulley. I suck.

Here's two pulleys with a seat to avoid the finger/hair-in-the-pulley issue:

Way too much complexity. It's not easy to hold on to the rope while balancing yourself on the seat. We've got some more testing and figuring to do.


michael said...

I know a few tree guys that might have some ideas on the matter.

rory said...

again. the best reason to have a kid is an excuse to make a tree house.

Anonymous said...

Pulleys are not totally unrelated to bikes, look at your deraillers.

I have a vague recollection that stringing the pull rope through an offset pulley nailed to a third point made the pulling practically easier, at least the ropes were less likely to tangle. The doublepulley setup in your link is also offset, notice.

I recall some old barn pulleys had a brake/ratchet doohickey that allowed the rope to run faster in one direction than the other (or would stop the rope) making it easier to keep control when lowering a weight. That's the more dangerous bit of using pulleys, particularly to haul bodies up and down.

You could also make a funky elevator by using offset pulleys above and fixed to the ground? Then you could have a geared crank with a ratio for easier lifting (just like a bike) and ratcheting mechanism to prevent dropping to the ground.

There's also another neat technological fix for treehouse access: google "ladder."

Philip Spohn said...

While messed about with pulleys and ropes in the barn as a kid, I learned about two nasty hazards with self-hoisting. If you inadvertently let go you go you plummet to the ground. And if you your grip slips you end up with a severe rope burn. Just saying. ;-)